SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-8-93-2017Arrested development – a comparative analysis of multilayer corona textures
in high-grade metamorphic rocksOgilviePaulaGibsonRoger L.roger.gibson@wits.ac.zaSchool of Geosciences, University of the Witwatersrand, P O WITS, Johannesburg 2050, South AfricaRoger L. Gibson (roger.gibson@wits.ac.za)6February201781931359July201625July201621December20165January2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/8/93/2017/se-8-93-2017.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/8/93/2017/se-8-93-2017.pdf
Coronas, including symplectites, provide
vital clues to the presence of arrested reaction and preservation of partial
equilibrium in metamorphic and igneous rocks. Compositional zonation across
such coronas is common, indicating the persistence of chemical potential
gradients and incomplete equilibration. Major controls on corona mineralogy
include prevailing pressure (P), temperature (T) and water activity (aH2O) during formation, reaction duration (t) single-stage or
sequential corona layer growth; reactant bulk compositions (X) and the extent of metasomatic exchange with the surrounding rock;
relative diffusion rates for major components; and/or contemporaneous deformation
and strain. High-variance local equilibria in a corona and
disequilibrium across the corona as a whole preclude the application of
conventional thermobarometry when determining P–T conditions of corona
formation, and zonation in phase composition across a corona should not be
interpreted as a record of discrete P–T conditions during successive
layer growth along the P–T path. Rather, the local equilibria between
mineral pairs in corona layers more likely reflect compositional partitioning
of the corona domain during steady-state growth at constant P and T.
Corona formation in pelitic and mafic rocks requires relatively dry,
residual bulk rock compositions. Since most melt is lost along the
high-T prograde to peak segment of the P–T path, only a
small fraction of melt is generally retained in the residual post-peak
assemblage. Reduced melt volumes with cooling limit length scales of
diffusion to the extent that diffusion-controlled corona growth occurs. On
the prograde path, the low melt (or melt-absent) volumes required for
diffusion-controlled corona growth are only commonly realized in mafic
igneous rocks, owing to their intrinsic anhydrous bulk composition, and in
dry, residual pelitic compositions that have lost melt in an earlier
metamorphic event.
Experimental work characterizing rate-limiting reaction mechanisms and their
petrogenetic signatures in increasingly complex, higher-variance systems has
facilitated the refinement of chemical fractionation and partial
equilibration diffusion models necessary to more fully understand
corona development. Through the application of quantitative physical
diffusion models of coronas coupled with phase equilibria modelling utilizing
calculated chemical potential gradients, it is possible to model the
evolution of a corona through P–T–X–t space by continuous,
steady-state and/or sequential, episodic reaction mechanisms. Most coronas in
granulites form through a combination of these endmember reaction mechanisms,
each characterized by distinct textural and chemical potential signatures
with very different petrogenetic implications. An understanding of the
inherent petrogenetic limitations of a reaction mechanism model is critical
if an appropriate interpretation of P–T evolution is to be inferred from
a corona. Since corona modelling employing calculated chemical potential
gradients assumes nothing about the sequence in which the layers form and is
directly constrained by phase compositional variation within a layer, it
allows far more nuanced and robust understanding of corona evolution and its
implications for the path of a rock in P–T–X space.
Introduction
Fundamental to the study of metamorphic rocks is the application of
equilibrium thermodynamics in the understanding of the development of a
mineral assemblage within evolving pressure (P), temperature
(T) and chemical potential regimes. In an equilibrated assemblage,
the chemical potentials of all components are equal throughout the
equilibrium volume. However, different rates of intergranular diffusion for
major and trace components limit the capacity of a rock to fully eliminate
gradients in chemical potentials and attain equilibrium on both micro- and
macro-scales (Fisher, 1977; Joesten, 1977; Fisher and Lasaga, 1981; Foster,
1981; Ashworth and Birdi, 1990; Carlson, 2002; White et al., 2008; White
and Powell, 2011). A more realistic model of partial equilibrium, i.e.
equilibrium for some components and not for others, is likely to be attained
in a rock. In a sense partial equilibrium is fortuitous, since evidence of
disequilibrium preserved in reaction textures reveals basic physico-chemical
reaction dynamics operating during metamorphism that are obscured if a rock
equilibrates completely. However, partial disequilibrium also compromises
petrographic and geothermobarometric evidence as records of the metamorphic
evolution of a rock and can lead to erroneous interpretations (Mueller et
al., 2010, 2015; White and Powell, 2011). Thus, an
understanding of how partial equilibrium manifests itself petrographically and
chemically is critical if we are to appreciate both the limitations and
opportunities it affords in petrogenetic studies.
The most obvious manifestation of partial equilibrium is reaction textures
comprising coronas and symplectites. The spatially segregated phases
preserved within these reaction textures are the best petrographic evidence
available to study the evolution of chemical potential gradients
governing the reorganization of components within a rock with changing
P–T–X (composition) conditions (e.g. White et al., 2008;
Štípská, et al., 2010; White and Powell, 2011; Baldwin et al.,
2015). The disequilibrium commonly preserved in coronas and symplectites
does not, however, preclude the application of equilibrium thermodynamics in
modelling and interpreting those textures; it only invokes a reconsideration
of the appropriate equilibration volume in which chemical potential
gradients are absent (White and Powell, 2011). Within any reaction texture,
on an appropriate scale, chemical equilibrium exists, and attendant chemical
potentials may be determined for a given P and T within
the local bulk composition dictated by the equilibration volume. This
concept of local equilibrium was first introduced by Korzhinski (1959) and
has been the premise upon which all studies of reaction textures are
predicated.
In this paper, we present an analysis of more than 50 metamorphic corona
textures developed in high-temperature granulite facies rocks (Tables A1
and A2 in the Appendix) and discuss two contrasting modelling methodologies used in
interpreting the evolution of these textures. There are innumerable examples
of reaction fronts and replacement textures encountered in lower-temperature
metamorphic rocks where the role of a discrete fluid phase is critical in
the development of the texture. An exhaustive review of low-temperature
replacement textures is not attempted here. Rather, we focus on the kinetics
of reaction mechanisms and processes with particular reference to those
applicable in the granulite P–T regime. The review concludes with
an appraisal of efforts employing equilibrium thermodynamics and calculated
phase diagrams to model corona textures and assesses their significance and
limitations when used to infer the P–T–X evolution of a metamorphic
rock (White et al., 2008; Štípská et al., 2010; Baldwin et al.,
2015).
Reaction kinetics
Metamorphic reactions are initiated when a pre-existing mineral assemblage
becomes unstable owing to changing P–T–X conditions. Chemical
equilibrium is re-established by adjustment of chemical constituents into a
new mineral assemblage coupled with a requisite change in phase compositions
in higher-variance equilibria. New minerals typically grow as a layer or
sequence of layers between reactant phases. This layer succession forms a
reaction rim most commonly observed as a corona in granulite facies rocks.
Processes involved during metamorphic reaction include (a) nucleation of
product minerals; (b) transport of components to the reaction interface
through the reactant by volume or intragranular diffusion; (c) dissolution
of the reactant phases; (d) transport of components across the reaction
interface along grain boundaries or through melt; (e) incorporation of the
diffusing components into the product surface through precipitation; and (f) the rate of supply or removal of heat (Fisher, 1977; Joesten, 1977; Brady,
1983; Foster, 1986; Tracey and McLellan, 1985; Carlson, 2002; Dohmen and
Chakraborty, 2003; Mueller et al., 2010; Abart and
Petrishcheva, 2011; Abart et al., 2012). Where reaction rate is primarily
governed by nucleation and precipitation, the reaction is referred to as
interface-controlled. Where reaction rate is constrained by rates of
component diffusion, it is termed transport-controlled. Mueller et al. (2010)
stress that the serial nature of these processes means that the slowest of
them is the limiting constraint on overall reaction rate and extent as a
function of both temperature and time.
A model predicting the relative importance of either interface or diffusion
controls on a particular reaction rate with respect to P and T was
derived by Dohmen and Chakraborty (2003). Although only defined for mineral
exchange reactions in the presence of a fluid phase, they derive a reaction
mechanism map for the determination of the rate-limiting step in any reaction
based on relative dominance of either interface-controlled or
transport-controlled mechanisms. Employing a thermodynamic model predicated
on Fick's laws governing chemical mass transfer, Abart and Petrischeva (2011)
demonstrate that, during initial rim growth, reaction is interface-controlled
and gradually becomes diffusion- or transport-controlled as the reaction
proceeds. Abart et al. (2012) augmented this thermodynamic model to include
chemical segregation within a reaction front as a rate-limiting reaction
mechanism during the growth of reaction rims with lamellar internal structure
or symplectites. These models have been substantiated by a vast body of
rigorous experimental work constraining the kinetics of reaction rim growth
(e.g. Farver and Yund, 1996; Yund, 1997; Fisler et al., 1997; Milke et al.,
2001; Watson and Price, 2002; Milke and Heinrich, 2002; Milke and Wirth,
2003; Abart and Schmidt, 2004; Schmid et al., 2009; Götze et al., 2010;
Keller et al., 2008; Niedermeier et al., 2009; Dohmen and Milke, 2010; Keller
et al., 2010; Gardés et al., 2011; Gardés and Heinrich, 2011; Joachim
et al., 2011a, b; Mueller et al., 2012; Helpa et al., 2014, 2015; Jonas et
al., 2015; Abart et al., 2016). Phenomenological models of disequilibrium
elemental and isotopic compositions produced experimentally with incomplete
diffusive element exchange (e.g. Mueller et al., 2008, 2012; Watson and
Mueller, 2009) may be utilized to constrain the textural, isotopic and
compositional evolution of mineral assemblages and infer timescales of
reaction duration (Lasaga, 1983; Ague and Baxter, 2007; Niedermeier et al.,
2009; Mueller et al., 2015). Ongoing experimental work aimed at
characterizing rate-limiting reaction mechanisms and their petrogenetic
signatures in more complex, higher-variance systems is imperative to allow
further refinement of partial equilibration models necessary to fully
understand coronas. Reviews of the experimental basis for kinetic theory,
reaction mechanisms and the petrogenetic implications of this work are
provided by Putnis (2009), Dohmen and Milke (2010), Mueller et al. (2010,
2015), and Zhang (2010).
Diffusion and corona growth
Diffusion is a consequence of the random motion of atoms, ions or molecules
within a host reference frame (Mueller et al., 2010; Zhang, 2010). This
random motion may result in a net diffusive flux when the concentration (or,
rather, chemical potential) of a component is not uniform in that reference
frame (e.g. a reaction rim). The resultant diffusive mass transport in one
dimension is governed by Fick's laws. Fick's first law relates diffusive
component flux to component diffusivity in the presence of a concentration
or chemical potential gradient:
J=-D∂C∂x.
In Eq. (1), J is the diffusive mass flux (a vector), D is the
diffusion coefficient (or diffusivity), C is the concentration of a
component (in mass per unit volume), x is distance, and
∂C/∂x is the concentration
gradient. Diffusivities are measures of the rate of component transport or
diffusion. Values of diffusion coefficients (typically in square metres per second)
are dependent on temperature, pressure, composition, and the physical state
and structure of the phase (Zhang, 2010). The time dependence of diffusive
mass transport (again in one dimension) and evolving concentration gradients
are determined by Fick's second law:
∂C∂t=D∂2C∂x2.
Additional influences on evolving concentration gradients and diffusive flux
in natural systems, which include the effect of bulk flow of the reaction
framework, are quantified through extensions to Fick's laws (Mueller et al.,
2010; Zhang, 2010).
Phenomenological models of corona formation through diffusion-controlled
reaction employ a combination of Fick's first law in conjunction with
equilibrium thermodynamics (Fisher, 1970; Joesten, 1977; Foster, 1981).
Fick's first law forms the basis for the first of these equations, which
relates component fluxes Ji to chemical potential gradients dμi/dx such that for each component i=1 to S,
Ji=-∑j=1SLijdμjdx.Lij are phenomenological coefficients for diffusion in a multi-component
system (Joesten, 1977). Straight coefficients (i=j) relate diffusion of
component i to the chemical potential gradient of i, and
the cross coefficients (i≠j) relate diffusion of component i
to the chemical potential gradient of j. Joesten (1977) and Fisher (1970) assume that the contribution of terms involving cross coefficients is
negligible.
The Gibbs–Duhem equation relates component chemical potential gradients in
the coronas to each other in the presence of a mineral k, in which
the molar content of component i is nik, such that in a layer
containing k,
∑i=1Snikdμidx=0.
A final equation relates the flux change between layers r-1 and
r to the stoichiometric coefficients vkr of phases
k in the reaction at boundary r, such that for mass
conservation and local mass balance at boundary r,
Jir=Jir-1+dε′dt∑k=1ϕnikvkr.
In Eq. (5), ϕ is the number of phases in layer r and ε′
is the modified form of the reaction progress variable ε
appropriate to layer growth (Ashworth and Sheplev, 1997). The factor
dε′dt in Eq, (5) converts
nikvkr (moles of component per mole of reaction progress) to
Jir in its true units (mol component m-2 s-1).
Intrinsic to any corona formation model is mass balance. An overall reaction
may be reconstructed using the measured proportion of phases to derive an
open-system reaction, with boundary fluxes representing metasomatic
interaction with the surrounding rock. The overall reaction may be
summarized by a mass balance for each component i.
∑k=1ϕvknik=0
In a closed system, the summation is over all minerals k. In an open
system, the metasomatic fluxes at the end boundaries are treated as “dummy”
phases with unit stoichiometric coefficients. An expression for overall
corona model reaction affinity was derived by Ashworth and Sheplev (1997).
The reaction affinity (i.e. Gibbs energy of reaction, -ΔG) is expressed as a function of phase compositions, phase
proportions (vk), layer thicknesses and chemical potential gradients
across all layers in Eq. (6):
-ΔG=∑k=1ϕvk∑i=1Snik∑r=1qk-1hr∗dμidxr∗.
Flatter chemical potential gradients reduce the (-ΔG) accordingly, such that (-ΔG)
approaches 0, indicative of greater extent of equilibration. In contrast,
large chemical potential gradients over thicker layers will cause the
product (-ΔG) to deviate further from 0,
suggesting suppressed equilibration.
These equations predicate the steady-state diffusion-controlled models
developed for spatially segregated reaction products in multilayer coronas
arranged in order of increasing or decreasing chemical potential (Fisher,
1977; Joesten, 1977; Mongkoltip and Ashworth, 1983; Foster, 1986; Grant,
1988; Johnson and Carlson, 1990; Carlson and Johnson, 1991; Ashworth and
Birdi, 1990; Ashworth et al., 1992, 1998; Ashworth and Sheplev, 1997; Markl et
al., 1998; Dohmen and Chakraborty, 2003; Gardés et
al., 2011). As changing P and T induces incipient reaction
between contiguous metastable reactants, components will start to migrate
between the reactants. If the major components display variable
intergranular diffusivities, they will be partitioned into a continuum of
compositional subdomains, or incipient “effective bulk compositions”, in
each of which local equilibrium is attained with its own unique chemical
potentials. The width of the corona and each of its layers will be dictated
by the different length scales of diffusion for each component. A layered
corona assemblage develops, across which transient chemical potential
gradients exist, which drive diffusion through the layers. With prolonged
reaction or enhanced intergranular diffusion, component flux through the
corona layers equalizes chemical potentials at all points in the corona.
Local incipient bulk compositions of subdomains should gradually expand with
mass transfer across layers and approach the final steady-state effective
bulk composition for the corona as a whole. Equilibrium is attained when no
chemical potential gradients exist for any components, despite the spatial
segregation of corona phases in layers.
The interpretation of corona textures has traditionally been a primary
diagnostic tool for inferring metamorphic P–T–t paths and, hence,
tectonics (Whitney and McLelland, 1973; Grew, 1980; Joesten, 1986; Droop,
1989; Clarke et al., 1989; Ashworth et al., 1992; White and Clarke, 1997;
Norlander et al., 2002; White et al., 2002; Kelsey et al., 2003; Johnson et
al., 2004; Tsunogae and Van Reenen, 2006; Zulbati and Harley, 2007; Hollis et
al., 2006). Diffusion-constrained conditions may arise on both the prograde
and retrograde paths, but, most commonly, coronas are thought to have formed
during retrogression from peak P–T conditions as low-variance equilibria
are crossed. The topology of the inferred low-variance equilibria with
respect to the peak assemblage has commonly been used to constrain a
retrograde P–T path (Harley, 1989). The inherent assumption of
disequilibrium between reactants and corona products was elegantly questioned
in a study by White et al. (2002) on metapelites from the Musgrave Block in
Australia. Phase equilibria modelling employing pseudosections in KFMASHTO
demonstrated that corona textures could realistically be developed in a peak,
high-variance assemblage that remains in equilibrium but undergoes large
changes in mineral modes as the P–T path tracks through the phase field.
Thus, it may not be necessary to invoke crossing of low-variance equilibria
and disequilibrium to explain corona textures. Indeed, the amount of
decompression required to generate the equilibrium reaction texture described
by White et al. (2002) was comparatively minor and may well have been
overestimated by earlier researchers (Harley, 1989). Similarly, incomplete
reaction may not be assumed in coronas where the cessation of textural
development reflects the consumption of melt, in which case the reaction
responsible has gone to completion (White and Powell, 2011).
Whilst there is a general understanding of the processes that induce corona
formation (e.g. Harley, 1989; White et al., 2002, 2008; Johnson et al.,
2004), the mechanism for corona development is obscured since the final
steady-state configuration of corona layers observed in a rock reflects the
complex evolution of chemical potential relationships with P, T and bulk
composition. These same complexities must also govern metamorphic processes
on the prograde path, albeit on larger length scales. However, greater melt
or fluid volumes and increasing temperatures on the prograde path facilitate
equalization of chemical potentials through accelerated diffusion in the
assemblage, such that only the spatial sequestration of phases (for example,
between melt-rich leucosomes and melt-poor mesosomes) attests to the
compositional partitioning of the rock and attendant chemical potential
gradients that must have prevailed during diffusion-controlled reaction
(White et al., 2004). In coronas, transient disequilibrium is frozen in the
rock as reaction textures. Coupled with experimental work, they present the
best petrographic evidence available to us to allow the study of the
evolution of chemical potential gradients governing the reorganization of
components within a rock with changing P–T–X conditions (e.g. White
et al., 2008).
Corona growth models
Two endmember corona formation models have evolved in the last 4 decades to
explain the development of multilayered coronas, namely,
single-stage, steady-state (e.g. Ashworth and Sheplev,
1997) and discontinuous, sequential (Joesten, 1986; White
and Clarke, 1997) diffusion-controlled growth. The formation mechanism for an
individual corona is typically predominantly governed by either of these two
endmember models. Since each endmember model is governed by reaction
processes which limit their petrogenetic significance, determining the extent
to which a particular formation mechanism applies when studying granulite
corona evolution is thus critical when using them to infer information
regarding the P–T–X path for a rock (White and Powell, 2011).
This growth model attributes corona development to diffusion-controlled
reaction mechanisms at constant pressure and temperature, utilizing local
equilibrium and chemical potential gradients across each layer and the
corona as a whole (Fig. 1). The spatial segregation of phases into layers
reflects the relative mobility of components owing to variable intergranular
diffusivities rather than distinct P–T conditions. All layers in
the reaction bands coexist contemporaneously with infinitesimal thickness at
the incipient stages of reaction. Layer thickness increases with reaction
duration and no change to a corona layer sequence occurs. Chemical potential
gradients evolve toward a steady-state and final configuration balancing the
rate of production and consumption of each component within each layer
(Korzhinskii, 1959; Joesten, 1977; Mongkoltip and Ashworth, 1983; Foster,
1986; Grant, 1988; Johnson and Carlson, 1990; Carlson and Johnson, 1991;
Ashworth and Birdi, 1990; Ashworth et al., 1992, 1998; Ashworth and Sheplev, 1997;
Markl et al., 1998).
Chemographic relationships and chemical potential saturation
surfaces for local transient equilibria at corona boundaries during incipient
stages of single-stage, steady-state diffusion-controlled corona growth
(after Joesten, 1977). (a) Original phases (A and D) initially at
equilibrium under P1 and T1, with bulk composition indicated by the
circle. (b) Under new P and T conditions (P2, T2),
reaction progress becomes diffusion-controlled. The corona domain is
partitioned into a continuum of compositional subdomains, or incipient
effective bulk compositions (triangle, square), each with unique chemical
potentials, in which local equilibrium is attained. (c) Ternary
G–X surface, in which local equilibria are separated by chemical potential
differences. (d) The chemical potential saturation surface for each
of the local phase assemblages. (e) Projection of the saturation
surface on the μcomp1–μcomp2 plane. Chemical
potential gradients between local equilibria drive the diffusion of components
from one compositional domain to another until chemical potentials are
equalized and equilibrium is attained.
Figure 1 illustrates incipient stages of single-stage, steady-state corona
formation chemographically and in chemical potential space by considering
two phases (A and D) initially at equilibrium under P1
and T1, with bulk composition indicated by the circle
(Fig. 1a). If under new P and T conditions
(P2, T2), reaction rate is diffusion-controlled,
relative differences in intergranular diffusivities partition the original
bulk composition (circle) into two endmember, non-overlapping, local bulk
compositions (square, triangle, Fig. 1b). The resulting product mineral
assemblage forms layers that are spatially segregated but in local
equilibrium and comprise the mineral assemblage stabilized in each local
effective bulk composition (Fig. 1b). A ternary G–X surface (Fig. 1c) indicates that the tangent planes to the minimum free-energy assemblages
have different orientations, and, accordingly, components have different
chemical potentials in each assemblage. The coexistence of two local
juxtaposed equilibria buffers the chemical potentials of diffusing
components across the coronas (Joesten, 1977). Figure 1d represents the
associated isothermal–isobaric chemical potential saturation surface for
each of the local phase assemblages (modified after Joesten, 1977). Each
local bulk composition, represented by a three-phase assemblage, is
invariant in chemical potential space at constant P and T.
The invariant assemblage ABC (triangle) lies at a higher chemical potential
for component 3 and lower chemical potentials for components 1 and 2 than
does the invariant assemblage BCD represented by the square. A projection of
the saturation surface on the μcomp1–μcomp2 plane more clearly indicates the difference between chemical
potentials for each local equilibrium (Fig. 1e). Maintenance of these local
equilibria requires that chemical potential gradients must exist across each
layer and, thus, that the system as a whole is in disequilibrium, which
drives diffusion of components from one compositional domain to another.
Chemical potential differences across each layer adjust to steady-state
values that balance the rates of production and consumption of each
component within the layer (Joesten, 1977). Chemical potential gradients for
rapidly diffusing components may be eliminated across the corona, whilst
those for the slowest-moving components (typically Al and Si, e.g. Ashworth
and Sheplev, 1997) are maintained, establishing partial equilibrium.
Open-system, single-stage, steady-state diffusion-controlled growth of prograde corona layers between olivine and
plagioclase (modified after Johnson and Carlson, 1990). (a) With
incipient reaction, different rates of intergranular diffusion for major
components manifest themselves as spatially segregated layers. The corona domain is
partitioned into a continuum of compositional subdomains or incipient
effective bulk compositions in which local equilibrium is attained, each with
unique chemical potentials. Fe, Mg and Si released from olivine diffuse down
chemical potential gradients toward plagioclase, whereas Na, Ca, Al and Si
released from plagioclase diffuse toward olivine. Layers comprising the
slowest diffusing species (Al) adjoin the most aluminous reactant.
(b) Reactions occur at layer boundaries, and layers expand as
diffusion progresses. The width and composition of each corona layer depend
on the relative fluxes of the diffusing elements. Minor spinel clouding
occurs in reactant plagioclase as Ca and Si diffuse preferentially into the
reaction band, creating an Si deficiency in reactant plagioclase. Phases in the
diagrams are labelled using Kretz (1983) mineral abbreviations.
Continued corona evolution entails the growth of a layer assemblage at the
expense of its neighbour (Joesten, 1977). The relative diffusive fluxes of
components in adjacent layers determine which mineral phases are consumed
and produced at each layer boundary, as well as the reaction stoichiometry
(Joesten, 1977; Fisher, 1977). All mineral layers grow simultaneously, by a
set of partial reactions at the layer interfaces liberating and consuming
components in appropriate proportions to account for mass balance in the
overall system (Joesten, 1977, 1986; Fisher, 1977). The only layer that
grows at both contacts is the layer that initially contained the original
reactant interface (Joesten, 1977, 1986). Fisher (1973)
demonstrated that diffusion will automatically tend to shift potentials
toward values such that the flux differences at every point in a corona
balance local reactions, thereby establishing a steady-state configuration.
Growth of coronas will decelerate and eventually cease when either diffusive
transport becomes inefficient or chemical potential gradients are erased
and/or intergranular diffusivities are reduced with cooling during
retrogression (Joesten, 1977; Fisher, 1977; Ashworth and Sheplev, 1997).
The corona in Fig. 2 is a schematic reconstruction of those described by
Johnson and Carlson (1990) from metagabbros in the Adirondack Mountains that
they interpreted as a natural example of this corona formation mechanism. A
primary igneous assemblage involving contiguous olivine and plagioclase
(Fig. 2a) becomes unstable during granulite facies metamorphism and is
replaced by a new stable assemblage (Fig. 2b) involving orthopyroxene,
clinopyroxene, plagioclase and garnet, i.e. Ol|Opx + Cpx|Pl|Grt|Pl
(reactants in italics). Variable relative rates of intergranular diffusion
manifest themselves as spatially segregated product layers, depending on the
diffusional length scale of each component, and the corona domain is
partitioned into a continuum of compositional subdomains or incipient
effective bulk compositions in which local equilibrium is attained, each
with unique chemical potentials (Fig. 1). Asymmetric composition profiles
for species are established across product bands reflecting variable
intergranular diffusivities, e.g. Al content in product bands increases
toward the Al-rich reactant. Fe, Mg and Si released from olivine diffuse
down chemical potential gradients toward plagioclase, whereas Na, Ca, Al and
Si released from plagioclase diffuse toward olivine. Reactions occur at
layer interfaces and layers expand as element flux progresses (Fig. 2b).
Inherent in the model is that the product mineral assemblage does not change
as reaction proceeds. With time, chemical potentials and fluxes approach
steady-state values. Mg, Ca, Na and Al migrate into the corona and Fe and Si
move out from the corona. Minor spinel occurs in reactant plagioclase as Ca
and Si diffuse preferentially into the reaction band, creating an Si
deficiency that stabilizes spinel in relict reactant plagioclase (Johnson
and Carlson, 1990).
Multi-stage sequential layer development in a
corona between olivine and plagioclase formed in response to changing P and
T along the P–T path shown in Fig. 2 (after Griffin, 1972).
(a) Original olivine and plagioclase react to form orthopyroxene and
clinopyroxene. (b) Clinopyroxene breaks down to form a less
Tschermakitic composition with plagioclase and spinel.
(c) Clinopyroxene reacts with orthopyroxene, spinel and plagioclase
to produce garnet. (d) Orthopyroxene reacts with spinel and
plagioclase to produce omphacite, garnet and quartz. (e) Omphacite
decomposes to clinopyroxene and plagioclase.
Sequential diffusion-controlled corona growth
This corona growth model involves successive, stepwise, growth of layers,
leading to overprinting and partial re-equilibration of younger layers as new
equilibria are encountered on either the prograde or retrograde path. These
changes are typically triggered by changing P and/or T but can also be
triggered through changing component fluxes through the corona as a function
of evolving local effective bulk compositions (e.g. Griffin, 1972; Griffin
and Heier, 1973; Joesten, 1986; Droop, 1989; Indares, 1993; White et al.,
2002; Johnson et al., 2004; Štípská et al., 2010; Baldwin et
al., 2015). In contrast to the single-stage, steady-state model, the internal
layer configuration of the corona reaction band evolves with time as new
layers develop and old layers are resorbed. Relative diffusion fluxes and
attendant chemical potential differences shift and evolve from one
steady-state configuration to another under new P–T–X conditions.
Sequential corona development with changing P and T has
been demonstrated in prograde coronas found in mafic rocks between olivine
and plagioclase by Griffin (1972) and Mork (1986). Griffin (1972) derived a
sequential model for corona formation that involved cooling from
temperatures in excess of the dry basalt solidus (> 1200 ∘C), between 0.8 and 1.1 GPa, and the crossing of univariant
equilibria (Figs. 3 and 4). Initially, olivine and plagioclase crystallized
at point A, but, as the rock cooled, it was buried and followed the path
delineated by the arrow in Fig. 4. At point B, the olivine and plagioclase
reacted to produce Tschermakitic clinopyroxene (Cpx I) and aluminous
orthopyroxene (Opx I; Fig. 3a). As the rock tracked through P–T
space from B to C (Fig. 4), the clinopyroxene (Cpx I) exsolved spinel and
anorthite to form a less Tschermakitic clinopyroxene (Cpx II; Fig. 3b). This
clinopyroxene was partly consumed at point C (Fig. 4) to produce garnet and
a jadeitic clinopyroxene (Cpx III; Fig. 3c). Further cooling into the
eclogite facies produced omphacitic clinopyroxene and garnet with lesser
quartz at point D (Fig. 3d). Finally, decompression on exhumation induced
the exsolution of the jadeite component from omphacite to yield diopside
(Cpx IV) and plagioclase towards point E (Figs. 3e and 4).
P–T grid indicating univariant equilibria crossed during cooling
to produce the sequence of reactions in Fig. 1 (after Griffin, 1972).
Sequential corona development may also occur at constant P and
T through changes in the effective element fluxes across the corona
band. A multilayer corona may evolve in a steady or quasi-stationary state
controlled by diffusion (single-stage, steady-state growth) and then
subsequently modify through retrograde reaction between two adjacent layers
at constant P and T through changing composition of the
effective equilibration volume as the composition of a reactant evolves with
protracted reaction. Brady (1977) and Vidale (1969) introduced a
modification to the steady-state model that was used to explain variability
in coronal layer development between the same reactants by Johnson and
Carlson (1990). Vidale (1969) modelled the development of calc-silicate
bands in a system with a waning availability of certain components. According
to his model, rapidly diffusing components in a reaction band will
eventually eliminate their chemical potential gradients. The chemical
potentials of these rapidly diffusing components in the phases comprising
the corona assemblage are then equivalent to those in matrix phases outside
the corona band. As the number of components exerting a diffusive control on
the reaction is reduced, so mineral phases are lost from the band (Vidale,
1969; Brady, 1977). This manifests itself as “cannibalization” of corona layers
comprising the rapidly diffusing components. The original steady state is
modified as the system enters a transient state that will evolve through
time toward a new steady state with constant chemical potential gradients.
Johnson and Carlson (1990) employed the sequential development model to
explain the variability in corona product assemblages developed between
plagioclase and olivine in a mafic granulite from the Adirondack Mountains
(plagioclase (Ol|Opx|Cpx|Pl|Grt|Pl – Fig. 5). As the reactant
plagioclase was gradually depleted in Ca and Si, it was converted from
labradorite to andesine + spinel (Fig. 5a). This modification of the
chemical potentials of Ca and Si by equilibria outside of the corona band
manifests itself as the destabilization and subsequent cannibalization of first
the plagioclase corona layer and then the clinopyroxene layer (Fig. 5a, b),
as the system evolved toward a new steady-state scenario with constant
chemical potential gradients. According to Johnson and Carlson (1990), all
corona bands were initially plagioclase- and clinopyroxene-bearing but then
evolved to different final configurations with greater or lesser
cannibalization of these phases, depending on the availability of Ca and Si
in the surrounding phases. Where the olivine grain adjoins the spinel-poor
plagioclase (originally less calcic, An43), both product plagioclase
and clinopyroxene have been consumed and the orthopyroxene is in contact
with garnet (Fig. 5b, c). In contrast, where olivine is adjacent to
spinel-rich reactant plagioclase (originally more calcic, An56), corona
plagioclase and clinopyroxene are retained (Fig. 5c).
Multi-stage
sequential layer corona layer development at constant P and T in response
to waning boundary fluxes of rapidly diffusing components from the reactants
into the corona in an open system (after Johnson and Carlson, 1990).
(a) Initial steady-state layer configuration for an
olivine–plagioclase corona. (b) Depletion of Ca and Si in the
reactants leads to the consumption of plagioclase and then
(c) clinopyroxene in transient states. The system gradually evolves
toward a new steady state. Cannibalization of corona plagioclase and
clinopyroxene is more enhanced where the original reactant is Ca-poor (top
right, An38).
Multi-stage
sequential corona layer development between plagioclase and olivine owing to
varying component fluxes across the corona bands and, later, owing to
decompression (modified after Indares, 1993). Corona layer growth in
panels (a–c) occurs under constant high P and T, initially from
discrete reactions between reactants and then subsequently between individual
corona layers as component fluxes vary across the corona. The formation of
the plagioclase layer in panel (d) is ascribed to decompression.
Detailed reaction mechanisms are discussed in the text.
Sequential layer development in a corona through variation in P,
T and changing bulk composition of the corona reaction volume was
invoked by Indares (1993) to explain coronas between olivine and plagioclase
in an olivine gabbro from the Shabogamo Intrusive Suite, eastern Grenville
Province (Ol|Opx|Cpx|Pl|Grt|Pl – Fig. 6). Initially, calcic
plagioclase reacted with olivine to form orthopyroxene and garnet coronas at
high P and T, under eclogite facies conditions (Fig. 6a).
The relative difference in intergranular diffusivities of components results
in two distinct corona layers, grading from Al-rich garnet adjacent to
plagioclase to (Al-poor) orthopyroxene adjacent to olivine. Excess Al in the
plagioclase was accommodated by the formation of corundum (Fig. 6a). At the
same pressure and temperature, the garnet layer grew by reaction between
calcic plagioclase and corona orthopyroxene in a local effective bulk
composition different from that which produced the initial corona
orthopyroxene and garnet, which included olivine (Fig. 6b). Continued
reaction generated excess Si and Al in the reactant plagioclase, which
reacted with corundum to form kyanite (Fig. 6b). In Fig. 6c, the reactant
plagioclase is relatively enriched in Na through the two former reactions.
Na then diffused out of plagioclase and reacted with corona orthopyroxene
and garnet to form omphacite. In response, more kyanite formed in the
plagioclase to accommodate excess residual Si and Al. With subsequent
exhumation and decompression, corona garnet reacted with kyanite and
corundum in plagioclase to form spinel and more calcic plagioclase (Fig. 6d). In addition, garnet reacted with omphacite and some excess Si to
produce intervening plagioclase.
The sequential development of symplectites in pelitic rocks has been
elegantly modelled using calculated phase diagrams involving chemical
potentials for coupled spinel + plagioclase symplectites and monomineralic
plagioclase coronas after kyanite (Ky|Spl + Pl|Pl|Qtz+Fsp) by
Štípská et al. (2010) and Baldwin et al. (2015). With
isothermal decompression from peak conditions, kyanite was no longer stable, and a zoned monomineralic plagioclase layer formed between the kyanite and
matrix with quartz in excess and only Al considered immobile. As the
plagioclase layer evolved, the diffusion of Si through the plagioclase layer
from the matrix was retarded and the local equilibrium volume encompassing
the kyanite and plagioclase layer contact became a silica-deficient one. The
chemical potential of SiO2 at the kyanite contact was accordingly
lowered sufficiently to stabilize spinel in a symplectite intergrowth with
plagioclase.
Controls on corona development in granulites
Of all the substantive literature references to corona textures, only a few
relate to compositions that are neither pelitic or mafic. Table A1
presents details of prograde coronas in the literature, whereas Table A2
comprises a selection of the more numerous references to coronas formed
during retrograde re-equilibration. Selected coronas from mafic and pelitic
rocks are schematically illustrated in Figs. 7 and 8, respectively. The
assemblages and microstructure in coronas in both pelitic and mafic rocks
vary considerably depending on (a) metamorphic conditions (P, T and
water activity – aH2O), (b) inferred formation mechanism through either
steady-state and/or sequential layer development, (c) reactant compositions,
(d) reaction kinetics, and (e) the amount of deformation or strain intensity
on either the prograde or retrograde path.
Common corona textures developed in mafic granulites.
(a) Prograde corona developed between olivine and plagioclase during
burial following shallow intrusion in the southwestern Adirondacks, New York
(after Whitney and McLelland, 1973). Garnet is not present in this corona
owing to low inferred pressures during corona reaction. There is no variation
in XMg of pyroxenes. (b) A retrograde corona developed
between olivine and plagioclase in an olivine metagabbro from northeast
Scotland (after Mongkoltip and Ashworth, 1983). The presence of amphibole
suggests higher aH2O than in more anhydrous domainal compositions
where only clinopyroxene is stable. Al content and XFe of Opx and
Hbl increase toward Pl reactant. (c) Retrograde corona developed
between garnet and clinopyroxene during a static thermal event with the
intrusion of numerous granite plutons in the Llano Uplift, Texas (after
Carlson and Johnson, 1991). The presence of hornblende implies relatively
high aH2O during reaction. Both hornblende and plagioclase are
asymmetrically zoned across the corona band. Plagioclase becomes less calcic
(An35 to An18) and amphibole Fe / Mg and Al / Si ratios
decrease toward omphacite. (d) Retrograde corona developed between
garnet and clinopyroxene from the Snowbird tectonic zone, western Canadian
Shield (after Baldwin et al., 2004). The restricted distribution of
hornblende in this corona compared to that in panel (c) suggests a
less hydrous bulk corona composition. Marked zonation in plagioclase occurs
from An91 adjacent garnet to An44 at clinopyroxene margin.
(e) Prograde corona developed between plagioclase and orthopyroxene
during deformation-enhanced reaction in a dolerite towards a shear zone
(after White and Clarke, 1997). Garnet exhibits asymmetric zonation as
XAlm, XPrp and XGrs increase toward Pl.
Garnet zoning diminishes toward shear zone. (f) Prograde corona
developed between plagioclase and orthopyroxene in a mafic granulite from
Yenisey Ridge, Siberia (after Ashworth et al., 1998). Layer 1 garnet
(Grt1) is zoned: Fe increases and Ca decreases (XGrs:
0.24–0.21; XAlm: 0.54–0.60) toward layer 2. A slight
compositional perturbation across layer 1 is thought to mark the initial
Pl–Opx boundary. In layers 3 and 4, Ca in garnet is almost constant, with
higher Fe and lower Mg than in layer 1. No systematic zonation is observed in
pyroxene. Non-equilibrium thermobarometric estimates for corona formation are
740 ± 20 ∘C and 9.5 ± 0.7 kbar (Ashworth et al.,
2001).
Sectoral complexity in corona textures developed in pelitic
granulites. (a) Complex corona between kyanite and gedrite (after
Norlander et al., 2002). No compositional variation in any corona phases was
observed. Conditions of formation constrained at < 5 kbar and
∼ 750 ∘C with TWQ and conventional thermobarometers.
(b) Common complex corona developed after garnet and quartz (after
Hollis et al., 2006). No systematic variation is described in corona
products. (c) Complex sectoral corona between garnet, biotite and
quartz. Monomineralic plagioclase is constrained to the corona immediately
adjacent to biotite. Similarly, blocky orthopyroxene occurs only in the
corona sectors where garnet reacts with quartz (after Kelsey et al., 2003).
Cordierite XMg varies across symplectite increasing toward
orthopyroxene in general. No variation in orthopyroxene composition is
observed. (d) Symplectite-dominated corona developed between biotite
and K-feldspar (after Bruno et al., 2001). Where biotite reacts with quartz,
monomineralic garnet comprises the corona. Elsewhere, a complex,
symplectite-dominated corona comprising garnet, quartz and phlogopite occurs
where biotite and feldspar react. Corona garnet is weakly zoned.
(e) Monomineralic sillimanite and orthopyroxene developed after
sapphirine and quartz (after Ellis, 1980, and Grew, 1980).
(f) Retrograde spinel–garnet symplectite replacing peak garnet
during post-peak decompression (after White et al., 2002). This corona
develops in response to changing modes in a high-variance equilibrium
assemblage. No univariant reaction is crossed. (g) Prograde complex
corona comprising spinel–cordierite symplectite and leucocratic biotite,
K-feldspar and plagioclase after andalusite (after Johnson et al., 2004).
XMg of cordierite decreases toward biotite (0.55–0.51) with no
variation in spinel composition. Cordierite moat formation occurs during an
andalusite melting reaction consuming quartz and biotite, followed by
continued breakdown of andalusite to cordierite–spinel symplectite in
SiO2 deficient domains. (h) Sectoral replacement of kyanite by
plagioclase + spinel symplectite and zoned monomineralic plagioclase.
Where primary garnet abuts kyanite, the symplectite is not developed, and
kyanite is replaced by low-Ca garnet enclosed by unzoned plagioclase (After
Štípská et al., 2010).
Pressure, temperature and aH2O
Pressure, temperature and aH2O conditions determine which
mineral phases form within the corona. In olivine gabbros or troctolites
from the Adirondack Highlands, coronal assemblages vary from
Ol|Opx + Cpx|Grt|Pl in the northeast (Johnson and Carlson, 1990 – Fig. 5a) to
Ol|Opx|Cpx + Spl|Pl in the southwest (Whitney and McLelland, 1973 – Fig. 7a), with
the presence of garnet in the former being attributed to higher pressures
towards the northeast. In the Newer Basic Intrusion of NE Scotland, the
coronal assemblage Ol|Opx|Hbl + Spl|Pl is observed (Mongkoltip and Ashworth,
1983 – Fig. 7b). In this case, hornblende is favoured over clinopyroxene
under higher aH2O conditions. Similarly, the dominance of
hornblende in the corona assemblage between garnet and clinopyroxene
described in Carlson and Johnson (1991) (Fig. 7c) versus the restriction of
pargasite to the layer closest to garnet in the coronas described by Baldwin
et al. (2004) (Fig. 7d) is attributed to higher aH2O in the
former corona compositional domain.
In metapelites, coronas after sapphirine and quartz comprise the sequence
Spr|Sil|Opx|Qtz
at higher pressures but Spr|Sil|Opx + Crd|Qtz at lower pressures and temperatures
and/or higher aH2O conditions (e.g. Lal et al., 1987).
Coronas after gedrite and kyanite from the Thor–Odin Dome in British
Columbia comprise the sequence Ged|Crd|Crd + Spl symplectite|Crd + Crn symplectite|Ky (Norlander et al., 2002 – Fig. 8a). The lower-temperature
equivalent corona (assuming minimal bulk compositional differences) is
Ged|Crd|St|Ky,
which is seen in the Errabiddy metapelitic granulites in Western Australia
(Baker et al., 1987).
Sequential versus single-stage corona formation mechanism
Corona assemblages are also governed by the mechanism by which they formed,
i.e. either in a single-stage, steady-state event, as sequential layers in
response to varying pressure, temperature or component fluxes into the
reaction volume, or by a mechanism intermediate between these two endmember
formation models. Most coronas listed in Tables A1 and A2 appear to be
interpreted via the single-stage, steady-state model, but models in which
sequential growth dominates are invoked commonly. Determining which model of
corona formation is applicable in a specific context is commonly difficult
but vital if information on the P–T path is to be gleaned correctly
from the corona (White and Clarke, 1997). This is critically evident in
contrasting interpretations of the coronas formed between olivine and
plagioclase in metagabbros from Risør, Norway (Joesten, 1986; Ashworth,
1986). Joesten (1986) cited textural evidence and the diffusional
instability of any closed-system, steady-state diffusion model for the
coronas in support of a model involving a primary magmatic origin for the
coronas, followed by secondary annealing. He suggested that cuspate
olivine-orthopyroxene contacts, thickening of orthopyroxene layers at narrow
terminations of olivine grains, irregular contacts between
orthopyroxene–spinel and amphibole–spinel layers, and heterogeneity in the
corona assemblage developed depending on the adjacent magmatic phase (i.e.
either plagioclase, amphibole or clinopyroxene) are all inconsistent with a
diffusion-controlled origin. These features were thought to be more likely a
result of olivine dissolution in a melt, followed by the sequential growth
of corona layers with cooling at magmatic temperatures above the
olivine–plagioclase stability field. Joesten (1986) proposed that these
primary magmatic coronas were diffusionally unstable and that they were
spontaneously partially to completely annealed on cooling.
In contrast, Ashworth (1986) suggested the Risør coronas formed by
single-stage, steady-state diffusion-controlled replacement of plagioclase
and olivine with an open-system modification to mass balance model
constraints. Textural evidence apparently inconsistent with a diffusion
model was attributed to locally variable kinetic controls on reaction
mechanism, for example, epitaxial growth of tabular amphibole on magmatic
grains versus heterogeneous nucleation at reactant contacts. Ashworth (1986)
did not address the sectoral heterogeneity of the coronas nor the irregular
contacts between amphibole–spinel and orthopyroxene–spinel layers. However,
it is conceivable that variation in the bulk composition of the
equilibration volume – both spatially and temporally as reaction proceeded –
may account for such heterogeneity (e.g. Johnson and Carlson, 1990).
Alternative sequential models of corona formation, invoking varying
P, T and/or boundary fluxes, may similarly have important
implications for the reconstruction of P–T paths. For the same corona
textures between olivine and plagioclase in the New York Adirondacks (Figs. 2, 5 and 6), three different P–T paths were constructed by
Griffin (1972), Johnson and Carlson (1990), and Indares (1993) based
on their inferences about the drivers behind the corona reactions, namely,
changing pressure and temperature (Griffin, 1972; Joesten, 1986), changing
component fluxes (Johnson and Carlson, 1990), or a combination of all three
parameters (Indares, 1993). Mass balance constraints and compositional
zonation within each corona assemblage were cited in each case in support of
the adopted model.
Criteria for the identification of single-stage, steady-state layer growth
include mineral zonation and a marked spatial organization of product
reaction bands such that each layer represents a non-overlapping volume in
compositional space (Joesten, 1977; Fisher, 1977), all arranged in an
orderly sequence of increasing or decreasing chemical potential (Fisher,
1977). If the corona has not attained equilibrium, asymmetric composition
profiles in minerals within a corona layer and in the corona as a whole are
consistent with chemical potential gradients induced by relative differences
in intergranular diffusion rates of components at approximately constant
P–T conditions (Indares, 1993; White and Clarke, 1997). In
contrast, a sequential corona model predicts symmetric, radial zoning of
phases with respect to grain boundaries. Mass balance constraints commonly
preclude the formation of an intervening layer by reaction between two
initially contiguous layers in a sequential model. This necessitates the
diffusion of requisite components from outside the limits of the immediate
equilibration volume within a single-stage, steady-state diffusional regime.
Even so, evidence may be equivocal, and it may not be possible to exclusively
establish single-stage, diffusion-controlled multilayer corona growth from
stepwise, sequential growth in response to changing P–T conditions
or component fluxes. In these cases, the corona formation mechanism likely reflects a combination of both endmember corona models. Tectonic
context and structural data might provide independent constraints on the
relative contributions of either endmember model to the overall corona
formation mechanism. Ultimately, clarification is best attained by modelling
the spatial arrangement of textures in a series of chemical potential phase
diagrams, which allow the full range of possible textural configurations,
given different formation mechanisms, to be evaluated (White and Powell,
2011; Štípská et al., 2010; Baldwin et al., 2015).
Reactant compositions
The compositions of local reactants principally determine the effective bulk
composition of the corona, with a minor degree of open-system communication
with matrix. The most obvious manifestation of local compositional control
on corona configuration is demonstrated by the three main types of coronas
observed in mafic rocks, where metasomatic exchange with the enclosing rock
is minimal and the corona bulk composition is principally determined by the
reactants. Local corona bulk compositions comprising orthopyroxene,
clinopyroxene, plagioclase and garnet form after olivine and plagioclase
(Ol|Opx|Cpx|Pl|Grt|Pl – Figs. 2, 3 and 5). More
aluminous, hydrous corona bulk compositions after garnet and clinopyroxene
stabilize amphibole, plagioclase and orthopyroxene
(Grt|Prg|Pl|Cpx/Opx|Cpx – Fig. 7c, d). Commonly, clinopyroxene
reacts with plagioclase to yield clinopyroxene (with or without
orthopyroxene), quartz and garnet coronas (Cpx|Cpx/Opx|Qtz|Grt|Pl –
Fig. 7e, f).
Markl et al. (1998) described coronas after fayalite and K-feldspar or
plagioclase (Fa|Opx|Grt + Opx|Pl/Kfs), in which the layer thicknesses,
product proportions and their compositions vary systematically depending on
whether plagioclase or K-feldspar is the reactant. Carlson and Johnson (1991) described a corona after garnet and quartz in a metagabbro from the
Llano Uplift in Texas comprising the layer sequence
Grt|Pl + Mgt|Opx + Aug|Qtz. In metapelites, coronas after garnet and quartz typically
yield a coronal assemblage of Grt|Crd + Opx|± Pl|Opx|Qtz (Hollis et al., 2006 – Fig. 8b). The presence of augite,
plagioclase and magnetite in a metagabbro corona may be attributed to
significantly more calcic garnet (∼ 8 wt % CaO) with a
higher XFe than typical pelitic garnets. Van Lamoen (1979) and
Nishiyama (1983) reported coronas after olivine and plagioclase in metamafic
rocks and conclusively demonstrated a correlation between the compositions
of reactant olivine and product orthopyroxene.
Sectoral development in complex coronas is perhaps the most obvious
manifestation of reactant compositional control on corona mineralogy and
morphology. Kelsey et al. (2003) described sectoral development of coronas
around garnet in pelitic granulites from the Mather Paragneiss in the Rauer
Group, Antarctica (Fig. 8c). In these granulites, garnet is enclosed by a
complex corona that comprises Grt|Crd + Opx
symplectite|Opx|Qtz where garnet was
initially adjacent to quartz and Grt|Crd + Opx
symplectite|Pl|Bt where it was initially
adjacent to biotite. These corona sectors appear to define unique, highly
localized effective bulk compositions. The sharp changes in mineral
proportions between sectors attests to the limited degree of chemical
communication between the Grt+Bt and
Grt+Qtz compositional domains. Bruno et al. (2001)
described coronas after biotite and quartz or feldspar, in which corona
mineralogy varies around a single biotite grain from
Bt|Grt|Qtz where biotite abuts
quartz to Bt|Grt|Grt + Qtz|Phg + Qtz|Kfs where biotite
is adjacent to K-feldspar and Bt|Grt|Grt + Jd|Pl where plagioclase encloses biotite (Fig. 8d). Štípská et al. (2010) noted complex radial and sectoral
heterogeneity in coronas after kyanite (Fig. 8h). Where kyanite is enclosed
by plagioclase–K-feldspar–quartz matrix, it is replaced by a reasonably
uniform corona comprising Ky|Pl + Sp ± Crn
symplectite|Pl|Matrix. The monomineralic
plagioclase layer is strongly zoned with respect to anorthite content,
grading from XAn=0.45 to 0.20 adjacent to the matrix. Locally,
where kyanite abuts garnet from the peak assemblage, the plagioclase–spinel
symplectite is absent and a thin Ca-poor garnet monomineralic layer is
rather developed, which is in turn enclosed by unzoned monomineralic
plagioclase. Štípská et al. (2010) ascribed the antipathetic
occurrence of the garnet corona layer and the spinel + plagioclase
symplectite to higher FeO and MgO chemical potentials in the equilibration
volume encompassing both garnet and kyanite as a reactant, which stabilized
garnet in the calculated product phase equilibria.
Reaction kinetics – diffusion
The spatial array of corona product bands and the presence or absence of
associated symplectite is a function of relative intergranular diffusivities
of major system components. Typically, Al and Si are relatively immobile
compared to more rapidly diffusing components such as Fe, Mg and, to a lesser
extent, Ca (e.g. Johnson and Carlson, 1990; Carlson and Johnson, 1991;
Ashworth and Birdi, 1990; Ashworth et al., 1992; Ashworth and Sheplev, 1997).
In natural coronas that are inferred to have formed in a single-stage,
steady-state diffusion-controlled scenario, typically limited Al diffusion
manifests itself as both modal and phase compositional zonation in the
corona; i.e. Al-rich minerals occur in layers closest to the aluminous
reactant, and, within these layers, the corona minerals exhibit asymmetric
zonation in compositional profiles, e.g. y(Opx) increases toward the
Al-rich reactant. Since Fe and Mg typically diffuse more rapidly than Al,
ferromagnesian minerals tend to segregate into layers farthest from the
aluminous reactant. XFe varies across the corona depending on
relative diffusion length scales of Fe and Mg. Coronas after sapphirine and
quartz in metapelites (Ellis, 1980 – Fig. 8e) and between sillimanite and
orthopyroxene (Kriegsman and Schumacher, 1999; Table A2) demonstrate spatial
segregation of aluminous corona layers (sillimanite and sapphirine,
respectively) from more Fe- and Mg-rich corona products (orthopyroxene and
cordierite, respectively). Coronas after garnet and clinopyroxene in more
mafic bulk compositions segregate into pargasite adjacent to garnet and
orthopyroxene + plagioclase adjacent to clinopyroxene (Baldwin et al.,
2004 – Fig. 7d).
Diffusion-controlled reaction rates arise most commonly on the retrograde
P–T path (Table A2) in melt-depleted, residual bulk rock compositions.
In metapelites, coronal reaction textures are commonly attributed to
near-isothermal decompression following peak conditions on a clockwise
P–T path (e.g. coronas after garnet and quartz; Kelsey et al., 2003 –
Fig. 8c) or to near-isobaric cooling (e.g. coronas after sapphirine and
quartz; Grew, 1980 – Fig. 8e). White et al. (2002), however, urge caution in
inferring large amounts of decompression and cooling along the retrograde
path to produce corona textures; phase equilibria modelling of spinel-bearing
symplectites after garnet from an Fe-rich pelitic granulite in the Musgrave
Block, Australia (Fig. 8f), suggested to them that coronas might develop on
any number of retrograde P–T path trajectories through a high-variance
field in which the mode of garnet is decreasing while that of the corona
products is increasing. Thus, large amounts of decompression are not required
to produce coronas and symplectites after garnet, and, hence, estimates of
decompression from other terranes (e.g. Harley, 1989) may well have been
overestimated.
Coronas developed on the prograde path (Table A1) are far less common than
coronas that form during retrogression (Table A2), owing largely to more
prolonged reaction duration, the presence of a melt or fluid that promotes
greater length scales of diffusion, and/or deformation on the prograde path.
Thus, the diffusion-constrained conditions on the prograde path suitable for
corona growth likely occur where deformation is largely absent (e.g. White
and Clarke, 1997 – Fig. 7e), in low aH2O mafic rocks
(Ashworth et al., 1998 – Fig. 7f; Johnson and Carlson, 1990 – Fig. 2) or
melt-depleted pelitic rocks, or where the rate of change in pressure and
temperature occurs sufficiently fast such that diffusion rates are exceeded.
Typically, the latter scenario arises in contact aureoles characterized by
rapid heating and cooling (Johnson et al., 2004 – Fig. 8g; Mcfarlane et
al., 2003; Ings and Owen, 2002; Barboza and Bergantz, 2000; Wheeler et al.,
2004; Daczko et al., 2002; Dasgupta et al., 1997; Joesten and Fisher, 1988),
but it can also occur in shock-heated rocks within large impact structures
(Gibson, 2002; Ogilvie, 2010).
Deformation and strain
High-strain intensities have been shown experimentally to enhance
equilibration (Delle Piane et al., 2007; Heidelbach et al., 2009; Götze
et al., 2010; Keller et al., 2010; Helpa et al., 2015). Deformation is
thought to enhance diffusion-controlled reaction rates through inducing
defects which act as additional diffusion pathways, e.g. dislocations and or
new subgrain boundaries (Helpa et al., 2015). Experimental work is supported
by field observations. White and Clarke (1997) described coronas developed
after orthopyroxene and plagioclase in a dolerite adjacent to a shear zone
in the western Musgrave Block, Australia (Fig. 7e). Towards the shear zone,
coronas diminish in complexity until complete equilibration and
recrystallization is attained in the highest-strain domains within the shear
zone. Koons et al. (1987) documented similar findings in a quartz diorite
from the Sesia Zone, Western Alps, whilst Smit et al. (2001) described
enhanced replacement of garnet by, and deformation of,
orthopyroxene + cordierite symplectite approaching bounding shears zones in
the Limpopo Belt, South Africa. With increasing deformation, equilibrium
domains progressively approach that of the bulk rock composition without any
discernable change in pressure and temperature. White and Clarke (1997)
attributed this enhanced equilibration in high-strain domains to a
combination of a reduction in grain size with an attendant increase in
intergranular area, accelerated intracrystalline diffusion and nucleation,
and increased permeability and aH2O.
Conditions of corona formation
Thermobarometric estimates for the average P–T conditions of corona
formation in mafic and pelitic granulites are depicted in Fig. 9. In most
cases, average P–T conditions exceed the wet solidus for their
respective bulk rock compositions. The few exceptions plotting below the
solidus may be attributed to retrograde compositional resetting with
cooling. Figure 9 is consistent with corona formation in granulite facies
rocks that have an intrinsically low aH2O bulk rock
composition (e.g. mafic granulites) and/or have undergone a degree of melt
loss. Under these conditions, intergranular diffusion limits reaction rate
and the extent of equilibration, especially when melt is absent in
coarse-grained assemblages. Retrograde corona development is likely
constrained to the high-T, suprasolidus, heating portion of the P–T
path immediately following peak T. Since most melt is lost at or
near peak conditions (White and Powell, 2002), only a fraction of melt is
retained in the restitic post-peak assemblage, and since diffusion in melts
is much more efficient than on dry grain boundaries (Zhang, 2010), element
mobility diminishes markedly in the absence of a melt phase. Reduced melt
volumes thus limit length scales of diffusion during cooling to the extent
that diffusion-controlled corona growth occurs. On the prograde path, the
low/absent melt volumes required for corona growth are only commonly
realized in mafic igneous precursors, owing to their intrinsic anhydrous
bulk composition, and in dry, restitic pelitic compositions that have lost
melt in an earlier metamorphic event. White and Powell (2011) distinguish
two types of coronas formed either on the prograde or retrograde paths,
namely, progressive or non-progressive coronas. Progressive coronas develop on the
same P–T path as the assemblage that they replace, in response to a
smooth change in P–T conditions from those that produced the peak
assemblage (e.g. Johnson et al., 2004; Hollis et al., 2006; Kelsey et al.,
2003). Non-progressive coronas develop in a separate P–T event to
those that generate the peak assemblage (e.g. Johnson and Carlson, 1990;
Gibson, 2002; McFarlane et al., 2003).
Summary of P–T conditions of formation for coronas reviewed in
this study. (a)P–T conditions for prograde coronas.
(b)P–T conditions for retrograde coronas. In general,
conditions of corona formation occur above the wet solidus for each
respective bulk composition. The few coronas that plot at lower temperatures
than the wet solidi may be subject to retrograde diffusional resetting of the
thermometers and, in reality, may have formed at higher suprasolidus
temperatures. Error bars are for the range of each estimate. BWS: wet
basalt solidus; GWS: wet granite solidus; GDS: dry granite solidus
and BDS: dry basalt solidus. Solidi were digitized in P–T space from the geosciences resource database available at
http://www.geosci.usyd.edu.au/users/prey/Granite/Granite.html.
Corona microstructure
Corona microstructure in prograde and retrograde coronas for which data are
available is summarized in Figs. 10 and 11. The average maximum corona
layer thickness in mafic prograde coronas is 475 µm (range: 70–1000 µm; n=19), and the average maximum vermicule length is 118 µm
(range: 50–300 µm; n=19). Pelitic prograde coronas are
characterized by an average maximum corona thickness of 496 µm
(range: 75–1500 µm; n=13) and an average maximum vermicule length
of 115 µm (range: 10–300 µm; n=13). Thus, mafic and
pelitic prograde coronas do not differ significantly with respect to maximum
corona layer thickness and vermicule length. However, pelitic prograde
coronas developed in contact metamorphic aureoles appear to exhibit greater
maximum corona layer thicknesses (> 500 µm) compared to
regional pelitic prograde coronas (Fig. 10a).
Variation in prograde corona microstructure in mafic and pelitic
bulk rock compositions. (a) Variation in maximum corona thickness in
prograde coronas. (b) Variation in maximum vermicule length in
prograde coronas. Hatched bars are prograde coronas from contact aureoles.
Each corona reference is tagged by a code (e.g. WM73) which correlates with
the detailed characteristics of each corona in the Tables included in
Appendix A.
Variation in retrograde corona microstructure in mafic and pelitic
bulk rock compositions. (a) Variation in maximum corona thickness in
retrograde coronas. (b) Variation in maximum vermicule length in
retrograde coronas.
Magnitude of compositional zonation in product corona bands. Hatched
fields indicate pelitic bulk rock compositions; unhatched fields are mafic.
(a)XMg variation in product phases.
(b) Variation in Al content in orthopyroxene across each corona.
(c) Garnet zonation across each corona. (d) Plagioclase
zonation across coronas where it is documented.
Most retrograde coronas described in the literature occur in pelitic bulk
compositions (Table A2; Fig. 11). Pelitic retrograde coronas are
characterized by an average maximum corona thickness of 571 µm
(range: 100–3000 µm; n=28) and an average maximum vermicule
length of 147 µm (range: 20–500 µm; n=28). The average
maximum corona layer thickness in mafic retrograde coronas is 262 µm
(range: 80–500 µm; n=5), and the average maximum vermicule length is
27 µm (range: 10–40 µm; n=5). Whilst retrograde pelitic
coronas do not differ significantly from prograde pelitic coronas in terms
of width and vermicule length, retrograde mafic coronas are distinctly
narrower and show significantly reduced vermicule length relative to
prograde mafic coronas (Fig. 11). The latter most likely reflects greater
length scales of melt-enhanced diffusion along the prograde path. A similar
relative paucity of melt may explain the difference in corona thickness and
vermicule length in retrograde mafic coronas compared to retrograde pelitic
coronas.
Internal compositional zonation in coronas
Complex compositional zonation is commonly observed in coronas (Fig. 12).
Fully equilibrated coronas, where no compositional zonation or chemical
potential gradients exist, are rare. In the population of coronas studied,
only 30 % were fully equilibrated, of which 60 % were in pelitic bulk
compositions. Commonly, coronas exhibit asymmetric zonation across the band
as a whole, reflecting variable length scales of diffusion for major
components during single-stage, steady-state growth (e.g. Ashworth et al.,
1998 – A98; Johnson et al., 2004 – J04; Fig. 12). Less commonly, radial
zonation occurs within a product layer or vermicule from the band
centre/vermicule core to the rim, indicative of sequential corona growth
(e.g. Zulbati and Harley, 2007 – ZH07; Fig. 12). The maximum magnitude of
zonation in XMg of orthopyroxene across a corona band in the
coronas reviewed is 0.08 (Kriegsman and Schumacher, 1999 – K99; Osanai et
al., 2004 – O04; Fig. 12) and 0.07 in cordierite (Baker et al., 1987 –
BKS87; Fig. 12). Unfortunately, Al content in orthopyroxene is expressed as
y(Opx), AlIV and Al wt % in the literature commonly without
accompanying raw analyses, so that these values cannot be recomputed to a
single formulation of Al in orthopyroxene to aid comparison. Maximum
asymmetric zonation magnitude with respect to y(Opx) is 0.08 in Hollis et
al. (2006 – H06; Fig. 12); 0.13 with respect to AlIV (a.p.f.u.
– atoms per formula unit) (Brandt et al., 2003 – BKO03; Fig. 12); and 0.05 with respect to
recalculated molecular proportion (Hisada and Miyano, 1996 – H96; Fig. 12).
Maximum magnitude of zonation in garnet is 0.22 for XGrs (White
and Clarke, 1997 – WC97; Fig. 12), 0.18 for XAlm (Indares, 1993
– I93; Fig. 12) and 0.17 for XPrp (Koons et al., 1987 – K87;
Fig. 12). Maximum magnitude in plagioclase zonation (ΔXAn)
is 0.42 (Baldwin et al., 2004 – B04; Fig. 12).
Product phase zonation makes the application of quantitative thermobarometry
exceptionally difficult. In some instances, corona product phases in local
equilibrium adjacent to a reactant possess low enough variance to apply a
conventional thermobarometer. For example, Baldwin et al. (2004) obtained
P–T conditions of corona formation from Grt–Opx–Pl–Qtz equilibria
using garnet rim and orthopyroxene–plagioclase symplectite compositions in
direct contact. Some authors have applied conventional thermobarometers to
spatially segregated phases in a corona that are not in direct contact
(e.g. Perchuk et al., 2002; Brandt et al., 2003). This approach is only
valid if there is no variation in phase composition across the corona band
and chemical potential gradients do not exist.
Ashworth et al. (1998) derived a non-equilibrium extension to conventional
thermobarometry based on open-system, steady-state diffusion modelling of
coronas that has been successfully employed to estimate P–T
conditions of formation of asymmetrically zoned coronas (Ashworth et al.,
2001). Unfortunately, non-equilibrium thermobarometry, like conventional
thermobarometry, is very sensitive to uncertainties in compositional data
and prone to underestimating peak temperatures of formation because of
retrograde resetting upon cooling. The preferred thermobarometric technique
for coronas entails phase equilibria modelling in THERMOCALC (e.g. Baldwin
et al., 2015), where modes and phase compositions are used to jointly
constrain a field of equilibration in P–T–X space. THERMOCALC
allows the modelling of corona textures in chemical potential space (White
et al., 2008; White and Powell, 2011; Štípská et al., 2010 and
Baldwin et al., 2015) facilitating direct comparison of the observed phase
zonation and spatial array of layers across a corona in which chemical
potential gradients prevail with predicted compositions at a range of
temperatures and pressures.
Modelling of coronas
Diffusion modelling of metamorphic reactions began in earnest with the
foundational work of Thompson (1959) and Korzhinskii (1959), who
demonstrated that infinitesimally small regions of rock can attain local
equilibrium in the presence of chemical potential gradients for all or some
components. This meant that even if the system is in disequilibrium as a
whole, with gradients in chemical potentials of components in the
intergranular medium, it is nevertheless possible to relate the mineral
assemblage at any point to the chemical potentials at that point.
Korzhinskii (1959) devised a graphical method for plotting a saturation
surface in chemical potential space that allowed the determination of relative
chemical potential differences across a series of layers (Fig. 1). This
method facilitated an understanding of how layer sequences would evolve as
components diffuse down chemical potential gradients. The limitation of
Korzhinskii's technique is that many diffusion paths from one reactant to
another are possible in the chemical potential diagram, such that more than
one possible layer sequence could evolve for a particular P–T
condition (Nishiyama, 1983). The advances in thermodynamic formulations of
phases required to model these relationships would only be developed by
researchers in later decades (Powell and Holland, 1988,
1990; Holland and Powell, 1998,
2003, 2011; Powell et al., 1998, 2005) and even then only
readily applied to coronas using the appropriate activity–composition
relationships through pioneering studies by White et al. (2008),
Štípská et al. (2010) and Baldwin et al. (2015). In the
interim, researchers modelled coronas through a quantitative physico-chemical
modelling approach, in which component fluxes and chemical potential
gradients required to reproduce observed corona layers configurations were
derived assuming reaction was driven and governed by minimization of Gibbs
free energy.
Quantitative physical modelling of coronas
The quantitative physical modelling of coronas is premised on the fact that,
in layered reaction products, mineral layers grow by reaction at their
contacts and the stoichiometries of the layer contact reactions are
determined by the relative diffusion fluxes of components within the layer.
Component fluxes and chemical potential differences across each layer attain
steady-state values as a function of the rate of production and consumption
of phases in the layer (Fisher, 1975; Dohmen and Chakraborty, 2003). Joesten (1977) combined the approaches of Fisher (1975) and Korzhinskii (1959) into
a hybrid methodology that allowed the prediction of a unique sequence of
mineral layers produced by steady-state diffusion for a given choice of
phenomenological coefficients in an isochemical system. Joesten's model is
based on three fundamental assumptions: first, diffusing components are in
local equilibrium with contiguous minerals at every point in a corona,
despite the fact that the corona as a whole is in disequilibrium; second,
component fluxes and chemical potential gradients remain constant at each
point in the corona in a steady state throughout its evolution; and third,
all components are considered to be conserved within the reaction band;
i.e. there is no communication with a system beyond the boundaries of the
reaction bands (the system is closed).
Joesten's model required the simultaneous solution of the three sets of
equations defined previously (Eqs. 3, 4 and 5), independently relating
component fluxes to chemical potential gradients in a layer, chemical
potential gradients to each other in the presence of a mineral with a
particular composition, and the flux change between layers to reaction
coefficients at layer boundaries (e.g. Ashworth and Sheplev, 1997). It is
possible to evaluate the stability of a multilayer reaction band for a
postulated set of intergranular diffusion coefficients if the compositions
of the phases in each band are known. The model predicts the relative widths
of layers in the reaction band, modal proportions of phases within each
layer, component fluxes across layers and reaction stoichiometry at layer
boundaries.
Early attempts to model corona textures using Joesten's formalism focussed
on corona reaction bands formed between olivine and plagioclase in
metagabbros (e.g. Nishiyama, 1983; Joesten, 1986; Grant, 1988). This early
work was hindered by the closed-system constraint in Joesten's model. For
example, Grant (1988) was unable to produce enough Ca from the observed
reactant plagioclase to accommodate all the Ca in the corona reaction band.
Furthermore, the failure of Joesten's model to account for hydrous corona
products, such as hornblende, from anhydrous plagioclase and olivine
reactants led researchers to embrace an open-system, metasomatic modification
to Joesten's model. An open-system modification was introduced by Johnson
and Carlson (1990) and Ashworth and Birdi (1990). Material balance
calculations allowed them to determine the external component fluxes across
the outer boundaries of the corona, thereby accommodating open-system
communication with the enclosing matrix. Johnson and Carlson (1990) and
Carlson and Johnson (1991) introduced external boundary flux equations to
model open-system behaviour. Ashworth and Birdi (1990) treated metasomatic
fluxes at corona boundaries as theoretical “phases” with “negative”
compositions where components were lost from the system and “positive”
compositions where they entered into the corona system. The open-system
studies of Johnson and Carlson (1990) and Carlson and Johnson (1991)
accommodated gradual changes in the composition of the reactants and
external fluxes throughout corona evolution, thus manifesting themselves as variable
product assemblages.
Open-system diffusion models for coronas had much more success in explaining
corona development in a variety of different bulk compositions, from mafic
rocks to metapelites, than the earlier isochemical models (Johnson and
Carlson, 1990; Carlson and Johnson, 1991; Ashworth and Birdi, 1990; Ashworth
et al., 1992, 1998; Ashworth, 1993; Ashworth and Sheplev, 1997). Ashworth (1993) noted that, although the overall extent of reaction
was constrained by highly mobile components with large diffusive fluxes, the
actual spatial arrangement of minerals in coronas appears to be strongly
controlled by those components with lower diffusivities, particularly Al and
Si. He noted that, in all cases, an Al-rich layer (commonly symplectitic)
was located adjacent to the most aluminous reactant, grading into an Al-poor
layer adjacent to the less aluminous reactant, and both separated by a
“transitional” layer of intermediate contents of Al (Fig. 13).
Ashworth and Birdi (1990) compared the Al / Si ratio in aluminous reactants
and the adjacent symplectite for a number of coronas using an isocon diagram
(Fig. 14). The isocon plot suggested that total Al and Si (strictly
A1O3/2 and SiO2, since the components used are oxides) included
within the phases in the symplectite appear to be “inherited
stoichiometrically” from the adjacent reactant. Any mismatch between the Al / Si
ratio of the reactant and individual phases comprising the symplectite is
accommodated by proportional growth of symplectite phases in the appropriate
ratio such that cumulatively the Al / Si ratio is retained. Ashworth and Birdi (1990) proposed that this was a consequence of low diffusivities of Al and
Si relative to Mg and Ca. According to them, any mismatch between the Al / Si
ratio of the symplectite and reactant implies open-system behaviour for
these components. Thus, the endmember scenario involving near-complete
open-system behaviour for Al and Si would be a monomineralic reaction band
in which mismatch in the Al / Si ratio is greatest. Mongkoltip and Ashworth (1983)
ventured still further that the occurrence of two immobile components is a
necessary condition for symplectite formation. This assertion agreed with
the metasomatic equilibrium theory of Korzhinskii (1965), which states that
any divariant equilibrium assemblage of n phases contains at least
n inert or immobile components. Assessing open- or closed-system
behaviour for Al and Si is critical in deciding which assumptions are
realistic when determining the overall reaction. If Al and Si are preserved
in the symplectite, then closure to Al and Si can be used to constrain the
system of simultaneous equations defining the overall reaction, such that it
is not underdetermined. If this assumption is not valid, a constant volume may
have to be assumed (Carlson and Johnson, 1991).
Sketch of a typical corona developed between plagioclase and olivine
in metagabbros (after Ashworth, 1993). As reaction proceeds, layers grow by
diffusion along grain boundaries of requisite components down concentration
gradients to layer boundaries where they are consumed in the production of
product phases. Al is considered to be the most immobile diffusing species,
since Al concentration gradients are most marked. Al exerts the greatest
control on segregation of corona products in bands, from the most Al-rich
symplectite adjacent to plagioclase to Al-poor orthopyroxene adjacent to
olivine.
Isocon plot of Al / Si ratios in symplectites and the adjacent
reactant plagioclase. The isocon line represents Al / Si ratios that are
preserved exactly between reactant and products. Any deviation from this line
indicates a degree of open-system behaviour. In general, analysed
symplectites from the literature plot above the isocon line, suggesting that
the Al / Si ratio is lower in the product symplectite than it is in the
reactant plagioclase; i.e. the corona system is losing Al to the external
system relative to Si with prolonged reaction.
The first thermodynamic treatment of conservation of volume during diffusion
metasomatism was undertaken by Carmichael (1987). Carmichael challenged the
assumption that pressure remains constant during irreversible diffusion
metasomatism. During reaction, there is a tendency for the boundary between
two juxtaposed reactants to be displaced perpendicular to the interface
between the reactants at a magnitude corresponding to the change in volume
of solid phases of the reaction. If there is any mechanical resistance to
this displacement, constant volume replacement is approached. Carmichael (1987) was able to model a field of nonhydrostatic stress induced by
the migration of the boundary between reactants. The stress field is oriented in
a manner which opposes the displacement and strain accompanying the
migration of the boundary. The stress field may be dissipated by either rock
deformation or secondary mass transfer out of the reacting volume. According
to Carmichael's model, the secondary mass transfer may be so efficient as to
eliminate the induced stress caused by boundary migration, such that the
original interface between reactants remains undisplaced. This realization
allows reasonable approximations to be made for the original boundary
between reactants (and the relative proportions of reactants involved in
reaction) such that an overall reaction may be derived.
In this context, the spacing of lamellae or vermicules in symplectites
reflects a balance between diffusive energy dissipation and grain-boundary
energy. Ashworth and Chambers (2000) derived a theory quantifying this
relationship employing both non-equilibrium thermodynamics and the principle
of the maximum rate of energy dissipation. Accordingly, the spacing of lamellae
in a symplectite for a particular reaction is a function of the reaction
rate (i.e. reaction front velocity), diffusion coefficient of the
slowest-diffusing components and the width of the reaction front:
λ=Lδv3,
where λ is lamellae spacing, L is the Onsager coefficient, δ is the reaction front width and v is the reaction
rate.
The finest symplectitic intergrowths (closest lamellae spacing) are
predicted to occur when reaction rates greatly exceed diffusion coefficients
for the slowest-diffusing species for a particular reaction front width.
Despite advances in diffusion metasomatic modelling of coronas in the early
1990s, success was still limited in that commonly more than one stable
layer sequence was computable for the same inputs. Sheplev et al. (1991,
1992a, b) presented a criterion to determine which non-unique solution is
more thermodynamically stable compared to others. The criterion was
formalized by Ashworth and Sheplev (1997) and extended so as to obtain a
measure of the affinity of reaction, or, rather, departure from equilibrium,
preserved in the corona. A final refinement to the open-system diffusion
model for coronas was derived by Ashworth et al. (2001), in which ratios of
the affinity of independent endmember reactions modelled for a corona are
compared to ratios calculated from an internally consistent thermodynamic
database (Holland and Powell, 1998). The pressure and temperature at which the
ratio of model endmember reaction affinities and real endmember reaction
affinities approach the same value is considered to represent the closure
pressure and temperature below which the corona remained inert to reaction.
This allowed quantitative estimates of pressure and temperature of the formation
of minerals in disequilibrium to be made.
Calculated phase equilibria modelling
A limitation of the quantitative physical modelling of coronas outlined
above is that solid solutions and the gradational shifts in phase
composition within a band cannot practically be accounted for in the
modelling (White and Powell, 2011; Baldwin et al., 2015). In the last decade,
advances in phase equilibria modelling have allowed geologically realistic
corona compositional systems to be modelled in P–T–X
(Johnson et al., 2004) and chemical potential space (White et al., 2008;
Štípská et al., 2010; White and Powell, 2011; Baldwin et al.,
2015). It is possible to predictively model corona evolution with changing
effective bulk composition through progressive metasomatic exchange of
components with the external matrix in a rock and/or partitioning of the
corona effective bulk composition with reduced length scales of component
diffusion on cooling (e.g. Johnson et al., 2004; White et al., 2008;
Štípská et al., 2010; Baldwin et al., 2015). For completeness
and clarity, all component chemical potentials referred to in this section
apply to those within the phases in the local equilibria under
consideration.
Chemical potential relationships governing the development of a
corona after kyanite (after Štípská et al., 2010). All component
chemical potentials referred to apply to those within the phases in the local
equilibria under consideration. (a) Calculated
μ(SiO2)–μ(CaO) diagrams in the NCKAS system for the matrix (red
lines) and the kyanite boundary (light blue lines). Gradients in the chemical
potentials from the matrix to the kyanite–plagioclase boundary are
represented by a vector in μ(SiO2)–μ(CaO)–μ(Na2O)
space. (b) Superimposed μ(MgO) and μ(FeO) variations on the
μ(SiO2)–μ(CaO)–μ(Na2O) vector from
panel (a): (i) for the matrix, (ii) for the plagioclase–kyanite
boundary and (ii) inside kyanite. The topology shows garnet and orthopyroxene
fields, while spinel is metastable. Garnet compositional isopleths x(Grt)
are plotted within the garnet stability field. The arrow is a vector
coincident with the x(Grt) = 70 isopleth, where
x(Grt) = Fe / (Fe + Mg) ×100. (c) Phase
topology obtained by manual combination of the calculated phase relations
along a slice at approximately fixed μ(MgO) /μ(FeO) (along x(g)=70) from panel (b), with the calculated phase relations along the
vector μ(SiO2)–μ(CaO)–μ(Na2O) from panel (a),
contoured with compositional isopleths ca(pl). The dashed arrow shows a path
from kyanite across garnet and plagioclase towards the matrix.
(d)μ(FeO)–μ(MgO) diagrams along the
ca(Pl) = 45 line calculated at 800 ∘C and 5.5 kbar.
SiO2 and Al2O3 are immobile. Fields are labelled with
Al2O3–SiO2 bar diagrams and contoured for x(Grt), x(Spl)
and ca(Pl). The grey ellipse shows regions of
plagioclase–spinel symplectite where mineral compositions correspond to
observed values (ca(Pl) = 35–45 mol % and
x(Spl) = 60–63).
One of the most robust and elegant applications of chemical potentials in
constraining corona textural and compositional evolution in P–T–X
space is that undertaken by Štípská et al. (2010). These
researchers modelled coronas developed after kyanite in a quartzofeldspathic
gneiss from the Bohemian Massif (Fig. 8h). Phase equilibria modelling
entailed an initial estimate of overall P and T conditions prevailing
using a conventional P–T pseudosection in
NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) (Štípská et al., 2010). For the purpose of phase equilibria
modelling in chemical potential space, it is necessary to reduce the number
of components treated, based on assumptions considering their inferred
relative mobility. Štípská et al. (2010) ranked components in
the corona according to a hierarchy of mobility or relative diffusivities in
which slowest diffusing components are considered effectively immobile (i.e.
chemical potential gradients are static and cannot change during reaction);
other components are considered mobile (their chemical potential gradients
vary on the scale of the corona); and some components are treated as
completely mobile (their chemical potentials do not vary across the corona
and are superimposed by the matrix). Accordingly, Štípská et
al. (2010) were able to reduce the model compositional system to NCKFMAS.
Prior to their consideration of the ferromagnesian minerals in the corona,
Štípská et al. (2010) modelled the monomineralic plagioclase
moat in NCKAS, with the further assumption that K2O is completely
mobile and Al2O3 is immobile with static potentials; i.e. it is
treated as an extensive variable in terms of phase composition. The chemical
potentials for the matrix edge of the corona correspond to those for the
equilibrated peak assemblage and the corona plagioclase composition in local
equilibrium with matrix (i.e. An20) (Fig. 15a). The chemical
potentials for the metastable kyanite corona contact were derived by
modifying μ(Na2O) at the matrix contact until the
kyanite–plagioclase boundary with An45 appears on the phase diagram
(Fig. 15a). In Fig. 15a, the chemical potential relations at the
kyanite–matrix boundary are overlain in μ(CaO)–μ(SiO2) space, and local equilibrium potentials
are indicated. Since the values of μ(Na2O), μ(CaO) and μ(SiO2) differ between the two equilibria, a
chemical potential gradient is established and is represented by the vector
in Fig. 15a. For equilibrium to be attained throughout the corona, chemical
potentials must be equalized everywhere by diffusion. If element transport
is constrained, these chemical potential gradients persist as stranded
gradients (Baldwin et al., 2015).
Štípská et al. (2010) modelled the presence or absence of a garnet layer in
the corona by superimposing μ(FeO) and μ(MgO) variations on the vector in μ(Na2O)–μ(CaO)–μ(SiO2) space
obtained in Fig. 15b. The authors calculated μ(FeO)–μ(MgO) diagrams for the matrix boundary, kyanite boundary
and midway between them with respective μ(Na2O)–μ(CaO)–μ(SiO2) dictated by
the vector constrained in NCKAS space (Fig. 15a). The observed composition
of garnet (XFe=0.70) defines a corresponding vector in
μ(FeO) and μ(MgO) space (Fig. 15b). Štípská
et al. (2010) manually constructed a phase diagram by combining the phase
relations along the XFe=0.70 vector in μ(FeO)–μ(MgO) space with those corresponding in
μ(Na2O)–μ(CaO)–μ(SiO2) space (Fig. 15c). Two observed chemical potential paths were
proposed to account for garnet-present and garnet-absent coronas that
reproduced the known spatial array and composition of phases. They suggest
that the chemical potential path required to produce garnet requires the
μ(FeO) and μ(MgO) potentials to be boosted
relative to those in local equilibrium with the matrix. This is consistent
with the spatial association of original matrix garnet in the corona, such
that the μ(FeO) and μ(MgO) potentials are
locally augmented, thereby stabilizing a garnet layer in the coronas in local equilibrium with kyanite (Štípská et al., 2010).
Modelling of the development of the plagioclase–spinel symplectite required
that SiO2 also be treated as immobile (Štípská et al.,
2010). Constrained SiO2 diffusion from the matrix toward kyanite across
the plagioclase moat induced a silica-deficient effective local bulk
composition at the plagioclase–kyanite boundary, thus lowering the local
SiO2 chemical potential sufficiently to stabilize spinel (assuming
corundum was unable to nucleate). As a consequence, both SiO2 and
Al2O3 chemical potentials are treated as quasi-stationary; i.e.
they are modelled as the coupled extensive composition variables. As a
consequence, phase fields in μ–μ space are
labelled with Al2O3–SiO2 bar compatibility diagrams.
Štípská et al. (2010) proceeded to model the requisite chemical
potentials for the symplectite stability initially in μ(Na2O)–μ(CaO)–μ(SiO2) space. They
derived a vector in chemical potential space between the symplectite contact
with the plagioclase moat and the kyanite boundary (Fig. 15d) that accounted
for the plagioclase composition within the symplectite. However, the
restricted stability limits of spinel in μ–μ space at the modelled conditions of post-peak conditions led
Štípská et al. (2010) to infer that the spinel-bearing symplectites
must have formed during subsequent decompression after plagioclase moat
formation, as the spinel stability field is far broader at lower pressures
for the same potentials.
Similarly, Baldwin et al. (2015) modelled spinel–plagioclase,
sapphirine–plagioclase and corundum–plagioclase symplectites after kyanite
in a quartzofeldspathic granulite gneiss from the Athabasca granulite
terrane, Snowbird tectonic zone, Canada. These researchers, like
Štípská et al. (2010), deduced that the spinel–plagioclase
symplectites must be metastable with respect to the corundum-bearing
alternative. Assuming corundum was unable to nucleate, they were able to
account for spatial relationships and compositions observed in the
symplectites over a wide range of P–T conditions and plagioclase
compositions. Crucially they were able to deduce that, without the
application of chemical potential phase diagrams suggesting otherwise, such
reaction textures may occur over a wide range of P–T conditions and
extreme caution must be exercised in inferring P–T conditions of
retrograde metamorphism from them.
Štípská et al. (2010) and Baldwin et al. (2015) conclusively
demonstrate that the use of chemical potentials is imperative and
unavoidable when investigating coronas. Previous researchers (Johnson et al.,
2004; Tajčmanová et al., 2007; Ogilvie, 2010) have attempted to
model corona textures without the chemical potential phase diagrams. These
authors invoked an equilibrium volume comprising the corona, with or without
a matrix contribution, which they assumed to be effectively closed-system
during textural development. Accordingly, corona growth involved a
redistribution of chemical components within the limits of the equilibrium
volume. This approach might account for some of the phases within the
corona but fails to account for the non-linear exchange of components both
within local equilibria across the corona but also external metasomatic
exchange with the enclosing matrix during corona evolution.
Tajčmanová et al. (2007) tried to circumvent this problem by
constructing a T–X section to model the compositional partitioning,
owing to variable diffusion of components across the corona and predicted
phases. Similarly, Ogilvie (2010) attempted to model shifts in corona phase
compositions and modes through the inferred exchange of components between
the corona effective bulk compositions and the external matrix through a
T–X section involving pure reactants on one axis and pure matrix as
the other axis. The fundamental problem with both these approaches, as noted
by White and Powell (2011), is that at best, it is only possible to account
for observed assemblages in a qualitative generalized sense. This is because
the high variance of the phase fields from the T–X section or
P–T pseudosection predicts that stable phases should be present in the
coronas that are not actually observed. This can only be treated by
considering some components as mobile and removing them from the bulk
composition utilized to model the corona. Crucially, the manner in which the
chemical potentials evolve through P–T space involves non-linear
changes in chemical potentials and local effective bulk compositions. Since
P–T pseudosections are constrained at a static bulk composition and
a T–X section can only model linear changes in bulk composition, by
their nature they are not flexible enough to allow modelling of the
intricacies of corona development either owing to variable external
component flux into the corona (for example, by melt ingress or loss) or
variable multi-component length scales of diffusion.
Concluding remarks
Evidence of partial equilibrium, preserved in coronas, allows us to examine
fundamental processes governing reaction mechanism, rates and extents of
equilibration in metamorphic (and, more rarely, igneous) rocks. Mechanisms
of corona formation have been reviewed, i.e. single-stage, steady-state diffusion-controlled vs. sequential development. Determining which model of
corona formation is most applicable in the corona study (and/or relative
contribution of each endmember model to the overall reaction mechanism) is
critical, since both models have limitations in the information that may be
gleaned from them in petrogenetic studies. A comprehensive review of
prograde and retrograde coronas for mafic and pelitic bulk rock compositions
from both regional and contact aureole terranes reveals that major controls
on corona mineralogy include P, T and aH2O
during formation, mechanism of formation, reactant bulk compositions and
extent of metasomatic exchange with the surrounding rock, relative diffusion
rates for major components, and associated deformation and strain. In
general, corona formation occurs under granulite facies conditions, in low
aH2O and/or melt-depleted, bulk rock compositions (Fig. 9).
With respect to corona microstructure, prograde coronas in pelitic rocks
developed in contact metamorphic aureoles exhibit greater maximum corona
thickness than those in regional coronas (Fig. 11a). Mafic and pelitic
prograde coronas do not differ significantly with respect to maximum corona
layer thickness and vermicule length; however, corona thickness and maximum
vermicule length in retrograde mafic coronas are significantly smaller than
both retrograde pelitic coronas and prograde mafic coronas, which likely
attests to the role of melt in enhancing length scales of diffusion during
corona formation (retrograde mafic rocks are more likely to be melt-poor and
anhydrous). Increased maximum layer thickness and vermicule length in
prograde mafic coronas compared to retrograde mafic coronas (Fig. 11) may
reflect greater length scales of diffusion in potentially more melt-rich
bulk compositions with protracted reaction along the prograde path. Prograde
pelitic coronas do not differ significantly from retrograde pelitic coronas
with respect to microstructure (Fig. 11), owing to the intrinsically more
hydrous pelitic bulk compositions and capacity to generate
diffusion-enhancing melt during decompression.
High-variance local equilibria in a corona and disequilibrium across the
corona as a whole preclude the application of conventional thermobarometry
when determining P–T conditions of corona formation. Although
tempting, the asymmetric zonation in phase composition across a corona,
indicative of single-stage, steady-state diffusion-controlled formation should not be interpreted as a record of discrete P–T conditions
during successive layer growth along the P–T path. Rather, the
local equilibria between mineral pairs in corona layers reflect
compositional partitioning of the corona domain during steady-state growth
at constant P and T. A non-equilibrium extension of
conventional thermobarometry derived by Ashworth et al. (2001) should be
used with phase equilibria modelling in THERMOCALC to constrain P–T
evolution of coronas (e.g. Ogilvie, 2010).
Through the application of equilibrium thermodynamics on an appropriate scale
(i.e. that of local equilibrium – Korzhinksi, 1959; Thompson, 1959), corona
evolution can be modelled either through quantitative physico-chemical
diffusion modelling (Johnson and Carlson, 1990; Carlson and Johnson, 1991;
Ashworth and Birdi, 1990; Ashworth et al., 1992, 1998; Ashworth, 1993;
Ashworth and Sheplev, 1997) or calculated phase equilibria involving chemical
potentials (White et al., 2008; Štípská et al., 2010; White and
Powell, 2011; Baldwin et al., 2015). While the former allows quantification
of reaction affinity and chemical potential gradients across coronas bands,
it is unable to practically accommodate variation in phase composition within
a band. Moreover, it assumes that corona layer configuration formed during
one continuous, single-stage diffusion-controlled process; i.e. component
flux between local equilibria across all bands in the corona was controlled
by chemical potential gradients on that scale. In contrast, forward modelling
utilizing calculated chemical potential gradients to account for corona phase
compositions and layer array assumes nothing about the sequence in which the
layers form, and, since the chemical potential gradients prevailing are
constrained by observed phase compositional variation within a layer, it
allows a far more nuanced yet robust understanding of corona evolution and
the implications for the path followed by a rock in P–T–X space. When
combined with evolving physical diffusion models predicated on the
experimental investigation of diffusion and nucleation in higher-variance
systems (e.g. Jonas et al., 2015; Mueller et al., 2015), temporal resolution
will be afforded to phase equilibria models, seamlessly integrating corona
evolution in P–T–X space with time.
Data availability
All data utilized in this study are included in Tables A1 and A2 in the
Appendix.
Summary of prograde corona occurrences in the literature.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsJC90Olivine metagabbroAdirondack Mountains, New YorkOl-PlOl|Opx|Cpx|Grt|PlOl|Opx + Cpx|Pl|Grt|PlCorona thickness: 250 µm; vermicule length: 4–50 µm; vermicule spacing: 8 µm (measured from backscattered electron images); vermicule shape: columnar Opx and Grt (cannibalized Pl and Cpx – vermicular); orientation: columnar grains oriented perpendicular to grain boundariesComplete – no variation in composition of corona product phases.Assuming pressure of 8 kbar, Grt–Cpx (Ellis and Green, 1979) thermometer: northern Adirondacks: 881 ∘C; southern Adirondacks: 708 ∘Cisobaric cooling, anticlockwiseSingle-stage – gradual exhaustion of Pl as a reactantOpen: L ratios not constrained tightlyFormation at high pressure and low aH2O. As Ca is depleted in reactant plagioclase, product plagioclase and clinopyroxene are “cannibalized”. Geochronological evidence negates a magmatic origin and cooling from igneous temperatures at high pressures as a cause of reaction and, instead, invokes a much younger superimposed metamorphic event (cf. Whitney and McLelland, 1973).N83Olivine metagabbroMt Ikoma, Osaka, JapanOl-PlOl|Opx|Hbl + Spl symp|PlCorona thickness: 50–300 µm; Opx: < 30 µm; Hbl + Spl: < 60 µm; vermicule length: – ; vermicule spacing: – ; vermicule shape: Hbl – fibrous rods and needles; Spl – vermicular; orientation: vermicules and rods perpendicular to layer boundariesDisequilibrium – no systematic variation is observed.Amphibolite facies – no quantitative thermobarometryNot specifiedSingle-stageClosed: Lii/LSiSi > 1 and Lii/LSiSi > LAlAl/LSiSiJ04MetapelitePhepane Dome, Bushveld Complex AureoleAnd-Bt + matrixAnd|Crd + Spl symp|Crd|Kfs + Pl +Bt leucosome|35 mol %fringe Bt+15 mol %matrix Crd, Kfs, Qtz, BtCorona thickness: Crd + Spl symp < 1 mm; Crd: < 0.5 mm; vermicule length: 0.01–0.25 mm; vermicule spacing: – ; vermicule shape: Crd – granoblastic-polygonal; Spl – vermicular; vermicules perpendicular to layer boundariesDisequilibrium. XMg of Crd decreases toward Bt (0.55–0.51); no variation in spinel composition.700–725 ∘C, 3 kbar (P–T–X relationships from pseudosection)ClockwiseSequentialNoneCrd moat formation during And + Bt melting reaction consuming quartz, followed by continued breakdown of And to Crd + Spl symplectite in SiO2 deficient domains.WC97DoleriteWestern Musgrave Block, AustraliaCorona 1: Opx–Pl corona 2: Cpx–Pl corona 3: Cpx–Pl corona 4: Ilm-PlCorona 1: Opx|Cpx|± Pl2|Grt|Plcorona 2: Cpx1|Cpx2|Grt|Plcorona 3: Cpx|Cpx2|Hbl|Plcorona 4: Ilm|Bt|Grt + Bt symp|Grt|PlCorona thickness: corona 1: 0.25 mm; corona 2: < 0.5 mm; corona 3: < 0.2 mm; vermicule length: < 10–125 µm; vermicule spacing: – ; vermicule shape: columnar Opx and subhedral, elongate Grt and Hbl oriented perpendicular to layer boundariesNo systematic variation in composition of Hbl, Pl (An20–25) and Cpx. Garnet asymmetric zoning: XAlm, XPrp and XGrs increase toward Pl (XGrs=0.18–0.24 to grossular peaks of up to XGrs=0.4). Grt zoning diminishes toward shear zone.T∼ 750 ∘C and 12–14 kbar (core and rim compositions used in average P–T mode in THERMOCALC based on two assemblages: Grt, Pl, Cpx, Qtz and Grt, Pl, Hbl, Qtz–Hybrid single-stageNoneEquilibration is enhanced in high-strain domains via a reduction in grain size leading to an increase in intergranular area, enhanced intracrystalline diffusion and nucleation, permeability, and fluid access (White and Clarke, 1997).I93Olivine gabbroShabogamo Intrusive Suite, eastern Grenville ProvinceCorona 1: Ol-Pl corona 2: Opx–PlCorona 1: Ol|Opx + Cpx|Pl|Grt|Plcorona 2: Opx|Cpx|Grt|PlCorona thickness: corona 1: < 1 mm; corona 2: < 4 mm; vermicule length: – ; vermicule spacing: – ; vermicule shape: granoblastic-polygonal with no preferred orientationAsymmetrical zonation in Grt (XAlm: 0.4–0.58; XPrp: 0.17–0.24) from Pl toward Opx. Layers more calcic in cores. No systematic zoning in Cpx. Opx homogeneous.T∼ 700–800 ∘C and 16 kbar (core and rim compositions used) by Grt–Cpx–Pl–Qtz thermobarometryClockwise with steep isothermal decompressionSequential––
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsJF88Chert/calc-silicate nodules in marbleChristmas Mountains, Texas (aureole enclosing alkali gabbro)Cal-QtzCal|Wo|Qtz (102–125 m from gabbro) Cal|Tilleyite|Wo|Qtz (23–15 m from gabbro) Cal|Spurrite|Wo|Qtz
(13–0 m from gabbro)No compositional variation in corona products possible.No pressure estimate. Thermal gradient (from numerical models of cooling pluton) ranges from 600 ∘C (115 m from contact) to 1030 ∘C (at contact)–Single-stageClosed: LCaCa/LSiSi=42 and LCaCa/LCO2 > 1Numerical modelling of diffusion-controlled mineral growth in the aureole yielded kinetic coefficients for non-isothermal processes.ABE92Basic orthogneissJotun Nappe Complex, NorwayOl-PlOl|Opx/Tlc|Hbl|Hbl + Spl symp|Ep + Hbl + Spl symp|Ep + Ts,+ St + Spl symp|Ep + Ts + Ky symp|PlCorona thickness: < 160 µm; vermicule length: not resolvable; vermicule spacing: – ; vermicule shape: granoblastic-polygonal/ interlobate epidote, spinel + hornblende needles/rods; orientation: needles weakly oriented perpendicular layer boundariesDisequilibrium. Hbl Al / Si ratios decrease toward olivine. No systematic variation in Fe / Mg ratio of Spl and Hbl (ΔAl = 3.13–3.74 a.p.f.u; ΔMg = 2.17–2.97 a.p.f.u).Epidote–amphibolite facies. No quantitative thermobarometryIsothermal decompressionSingle-stageOpen: LFe2+, LMg > LFe3+≥LCa > LAl≥LSiSt + Ky and Ky + Hbl stable in H2O-undersaturated conditionsK87Quartz dioriteSesia Zone, Western AlpsBt-MatrixBt|Ms|Grt|Matrix (Jd+Qtz+Zo)Corona thickness: 100 µm; vermicule length: < 50 µm; vermicule spacing: – ; vermicule shape: granoblastic-polygonalDisequilibrium. XAlm and XPrp increase and XGrs decreases in garnet toward biotite (XAlm: 0.6–0.73; XPrp: 0.10–0.17; XGrs: 0.30–0.10).Eclogite faciesClockwiseSingle-stage–Domainal equilibration (extent of which is determined by the amount of deformation) and wide variation in composition of peak phases precludes application of thermobarometryWM83Metagabbro, metatroctoliteAdirondack Mountains, New York.Ilm-PlIlm|± Bt|± Hbl|± Grt|PlCorona thickness: < 100 µm; garnet vermicule length: < 50 µm; vermicule spacing: – ; vermicule shape: columnar Opx and Bt oriented perpendicular layer boundariesNo systematic zoning observed.700–800 ∘C and 8 ± 1 kbar from equilibrium assemblages in host rocksNo P–T path suggestedSequential–Open system invoked for mass balance. Polymetamorphic history, with a high-pressure event following low-pressure metamorphism.B01GranodioriteDora–Maira Massif, Western AlpsCorona 1: Bt–Qtz; corona 2: Bt–Kfs; corona 3: Bt–PlCorona 1: Bt|Grt|Qtz; corona 2: Bt|Grt|Grt + Qtz|Phg + Qtz|Kfs; corona 3: Bt|Grt|Grt + Jd|PlCorona thickness: corona 1: 5–40 µm corona 2: layer 1: 10–120 µm; layer 2: 60 µm; layer 3: 100 µm; corona 3: layer 1: – ; layer 2: < 100 µm; garnet vermicule length: 2–50 µm; vermicule spacing: – ; vermicule shape: vermicular garnet to rod-like Phg; orientation: perpendicular layer boundariesDisequilibrium. Corona 1: garnet weakly zoned (no more detail provided). Corona 2: garnet weakly zoned (no more detail provided). Corona 3: garnet asymmetrically zoned with Ca increasing and Fe + Mg decreasing toward Pl.Eclogite facies. Minimum conditions of 650 ∘C and 24 kbar, based on comparison between measured and modelled equilibrium compositions of garnet.ClockwiseSingle-stage–Sectoral corona development around biotite depending on immediately adjacent phase. Each corona type represents different P–T conditions at which that corona-forming reaction is overstepped.
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsA98Metabasic granuliteYenisey Ridge, SiberiaPl-Pgt (now Opx + Cpx)Pl|Grt|Grt + Qtz|Cpx + Qtz + Grt|Pgt (Opx + Cpx exsolution)Corona thickness: layer 1: 440 µm; layer 2: 150 µm; layer 3: 300 µm; vermicule length: 10–00 µm; vermicule spacing: – ; vermicule shape: granoblastic-polygonal/interlobate and locally needle/rod-like pyroxenes; orientation: noneDisequilibrium. Layer 1 Grt is zoned: Fe increases and Ca decreases (XGrs: 0.24–0.21; XAlm: 0.54–0.60). In layers 3 and 4, Grt XGrs is constant, while XFe is higher than in layer 1. No systematic zonation in pyroxene observed.Layer 4 Grt–Opx and Grt–Opx pairs yield 614–635 ∘C at 6–10 kbar (retrograde diffusional resetting). Al content in Opx yields poorly constrained, slightly higher temperatures. Layer 4 Grt, Opx and Pl reactant yields a pressure estimate of 5.8–7.5 kbar (Grt–Opx–Pl–Qtz; Bhattacharya et al., 1991).ClockwiseSingle-stageOpen: LFe≈LMg≈LCa > LAl≈LSiThe geobarometry is compromised by chemical potential gradients between phases in layer 4 and Pl reactant.WM73MetagabbroAdirondacks, New YorkOl-PlSouthwestern Adirondacks: Ol|Opx|Cpx + Spl symp|PlCorona thickness: < 0.25 mm; vermicule length: 0.01–0.25 mm; vermicule spacing: – ; vermicule shape: vermicular symplectite and columnar Opx laths; orientation: strongly oriented perpendicular to layer boundariesNo variation in XMg of pyroxenesSouthwestern Adirondacks: marginally > 8 kbar and 800 ∘CClockwiseSingle-stage?––A01Hornblendite xenoliths in marbleIvrea Zone, northern ItalyHornblendite – marbleHbl+Grt+Cpx|Cpx| Grt + Cpx|Scp + Cpx|CalCorona thickness: layer 1: 1–4 cm; layer 2: 3–12 cm; layer 3: < 2 cm; vermicule length: ∼ 1 mm; vermicule spacing: – ; vermicule shape: granoblastic-interlobate except at layer boundaries where vermicular symplectite occurs; preferred orientation: vermicular symplectite strongly oriented perpendicular to layer boundaries, otherwise noneDisequilibrium. XMg (0.8–0.55) and Tschermak's content in Cpx decreases toward Cal. No variation in Grt composition.700–900 ∘C and 7–10 kbar (independent estimates of peak conditions from pelites)–Single-stageOpen: a number of mass balance scenarios proposed based on qualitative evidence to constrain boundary fluxes. LSiSi/LCaCa > 2.5 and LAlAl/LCaCa < 10 and LMgMg/LCaCa > 1Abart et al. (2001) relaxed the constant volume or closure to certain components constraints to evaluate the overall reaction. Instead, they constrained a range of mass balance scenarios for which major element fluxes across boundaries were solved.BB88Metapelite and noriteHoggar–Iforas granulite unit, MaliCorona 1: Grt–Sil; corona 2: Grt–QtzCorona 1: Grt|Crd|Crd + Spl|Sil St and younger euhedral Grt appear to replace Crd in a younger post-corona event; corona 2: Grt|Opx + Pl symp|Grt2+ Qtz|QtzCorona thickness: corona 1: < 500 µm; corona 2: < 700 µm; vermicule length: 5–50 µm; vermicule spacing: – ; vermicule shape: vermicular symplectite and granoblastic-polygonal/interlobate monomineralic layers; preferred orientation: vermicular symplectite weakly oriented perpendicular to layer boundaries, otherwise noneCorona 1: no zoning in Crd or Spl. Garnet XFe decreases rimward. corona 2: no systematic zonation. Opx: XFe=0.65–0.70, XMg=0.29–0.32 and XCa=0.01–0.02.550–650 ∘C and 4.5–5.7 kbar (thermobarometry using the assemblage Grt–Spl–Crd–Bt–Pl)Clockwise isothermalSequential–The first stage of corona growth involves breakdown of Grt to form Crd + Spl symplectites in the metapelites and Opx + Pl symplectites in the norites. Replacement of Crd + Spl by younger Grt and Sil, as well as the replacement of Opx and Pl symplectite by Grt + Qtz, suggests renewed burial. Thermobarometry yields P–T conditions of the latter event. P–T path reflects Eburnean decompression, followed by pan-African burial.
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsC06MetapeliteHuangtuling granulite, northern Dabie Orogen, eastern ChinaCorona 1: Bt–Pl; corona 2: Opx–Pl; corona 3: Grt–Qtz + liq; corona 4: Bt–Pl + QtzCorona 1: Bt|Grt + Qtz|Pl; corona 2: Opx|Grt + Qtz + Bt2+ Pl2|Pl; corona 3: Grt|Crd + Opx ± Bt ± Pl ± Spl symp|Qtz+Melt; corona 4: Bt |Crd + Opx + Qtz|PlCorona thickness: 100–200 µm; vermicule length: 5–50 µm; vermicule spacing: – ; vermicule shape: vermicular symplectite; preferred orientation: vermicular symplectite weakly oriented perpendicular to layer boundariesCoronas 1 and 2: XGrs increases toward Pl (from 2–3 to 3–4 mol %); Opx: AlVI=0.05–0.07 a.p.f.u. and XMg=0.59–0.63; garnet: Ti = 0.24–0.28 a.p.f.u. and AlVI=0.12–0.17 a.p.f.u.; corona 3: Opx: AlVI=0.08–0.12 a.p.f.u. and XMg=0.63–0.65; biotite: XMg=0.71–0.76; no indication of symmetry in zonation.Coronas 1 and 2: 690–790 ∘C and 7.7–9.0 kbar; corona 3: 900–920 ∘C and 4.3–4.7 kbar; corona 4: 860–880 ∘C and 4.0–4.4 kbar (average PT, THERMOCALC)ClockwiseSingle-stage–Coronas formed at unique P–T conditions under steady-state conditions during multi-stage metamorphic history. Coronas 1 and 2: decompressive cooling on clockwise path (2000 Ma); coronas 3 and 4: renewed burial of granulites (220 Ma) on a second clockwise prograde path. Corona phase compositions broadly determined by composition of local reactants.I07UltramaficSefuri Mountains, NW Kyūshū, JapanOl-PlCorona 1: Ol|Opx|Pl; corona 2: Ol|Hbl + Opx|Hbl + Spl symp|PlCorona thickness: corona 1: < 70 µm; corona 2: < 400 µm; vermicule length: – ; vermicule spacing: – ; vermicule shape: corona 1: columnar Opx; corona 2: granoblastic-polygonal layer 1; Layer 2 comprises vermicular symplectite weakly oriented perpendicular to layer boundariesAl cation total decreases (1.21–0.48; 23 Os) and XMg increases (0.88–0.91) in Hbl toward Ol in layer 1. XAl in Opx increases toward Pl (0.06–0.11) in layer 1.600–700 ∘C, 5 kbar (Hbl-Opx thermometry on mineral pairs from layer 2 – employing Gibbs method for Fe-Mg exchange between Hbl and Opx)Not describedSingle-stage–Open-system removal of MgO from the local corona volume, stabilizes Opx in the coronaMCC03MetapeliteMakhavinekh Lake plutonGrt–Qtz + FspGrt|Crd + Opx|Pl|Opx|Qtz+FspCorona thickness: < 50 µm (6 km from pluton) to > 1000 µm (adjacent to pluton); vermicule length: < 10 µm (6 km from pluton) to 250 µm (adjacent to pluton); vermicule spacing: – ; vermicule shape: elongate and vermicular furthest from contact becoming more equant toward pluton; orientation: rods strongly aligned perpendicular to garnet substrateAl2O3 wt % and XMg of Opx increases toward reactant garnet. Magnitude of difference dependent on distance to pluton. Max ΔAl wt % = 0.8 wt %; Max ΔXMg=0.05. Overall Opx more Fe-rich and Al-rich toward pluton. Intracrystalline zonation toward lower Al wt % and XMg values at margins of vermicules.Grt–Opx Al-solubility thermometry: contact: 785–875 ∘C at 5 kbar At 6 km: 650–750 ∘CContact aureoleSingle-stage–Progressive replacement of garnet toward pluton contact
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsIO02Pelitic and mafic granulitesTaylor Brook gabbro complexCorona 1: Sil–Grt–Qtz; corona 2: Grt–QtzCorona 1: Grt|Spl + Crd|Qtz+Sil+Grt; corona 2: Grt|Pl|Opx|QtzCorona thickness: corona 1: < 500 µm; corona 2: < 100 µm; vermicule length: corona 1: < 300 µm; corona 2: < 50 µm; vermicule spacing: – ; vermicule shape: corona 1: tortuous, contorted lamellar symplectite; corona 2: granoblastic-polygonal; orientation: lamellae weakly aligned perpendicular to garnet substrateNo zonation describedGrt–Opx–Pl–Qtz thermobarometry on corona phases in metabasite yields a P–T estimate of 4.4 kbar at 615 ∘C. Coronas in tonalitic gneiss yield a slightly higher P and T of 4.7 kbar at 645 ∘C.Contact aureoleSingle-stage––BB00MetapeliteMafic Complex contact aureoleCrd–KfsCrd|Bt + Sil + Qtz |KfsCorona thickness: < 200 µm; vermicule length: < 30 µm; vermicule spacing: > 5 µm; vermicule shape: vermicular symplectiteNo zonation described–Contact aureole–––WMP04MetapeliteRoss of Mull contact metamorphic aureole, ScotlandCorona 1: Ky–Bt–Qtz; corona 2: Grt–Qtz–MsCorona 1: Ky|Crd + Ms|Qtz+Bt; corona 2: Grt|Crd + Bt|Qtz+MsCorona thickness: corona 1: < 350 µm; corona 2: < 1000 µm; vermicule length: corona 1: – ; corona 2: < 20 µm; vermicule spacing: – ; vermicule shape: corona 1: fibrous intergrowth; corona 2: elongate, anhedral biotite laths in cordierite; orientation: vermicules weakly aligned perpendicular to reactant substrateZonation not described; apparently in equilibriumP–T estimates based on relative stability of phases on P, T gridsContact aureole––=L04MetatroctoliteBuck Creek ultramafic body, North Carolina Blue RidgeOl-PlOl|Opx|Cpx +Spl symp|PlOl|Opx|Cpx + Hbl symp|PlOl|Opx|Cpx + Spr symp|PlCorona thickness: 0.5–0.75 mm; vermicule length: – ; vermicule spacing: – ; vermicule shape: vermicular symplectite and elongate or columnar Opx laths; orientation: strongly oriented perpendicular to layer boundariesEquilibrium. No variation in composition.700–900 ∘C and > 9 kbar–Single-stageClosed: no diffusion modelling performed. SVD (singular value decomposition) analysis concludes Hbl of primary metamorphic origin – not retrograde after Cpx.Overall corona reaction modelled by SVD matrix technique. Possible mass balance reactions are generated by SVD technique. A successful model is one that reproduces a sensible overall reaction with residuals that match expected analytical errors (Lang et al., 2004). Closed-system approximation possibly invalid.
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsD02Mafic granofelsArthur River complex at Mt Daniel, Fiordland, New ZealandHbl-CzoHbl|Cpx + Ky + Qtz + Pl|CzoCorona thickness: < 500 µm; vermicule length: < 60 µm; vermicule spacing: – ; vermicule shape: irregular, elongate, euhedral laths of Ky embedded in Cpx and Qtz; orientation: no preferred orientationNo zonation – equilibriumThe assemblage garnet, kyanite, plagioclase and quartz yielded pressure estimates of 13.2 kbar and temperature estimates of 700 ∘C using conventional thermobarometer of Newton and Perkins (1982).Contact aureole–––D97MetapeliteChimakurthy mafic-ultramafic complex aureole, Eastern Ghats Belt, IndiaSpl–CrdCorona 1: Spl|Grt + Sil + Crd|Crd; corona 2: Grt|Opx + Sil + SplCorona thickness: corona 1: < 75 µm; corona 2: < 230 µm; vermicule length: corona 1: < 15 µm; corona 2: < 10 µm; vermicule spacing: – ; vermicule shape: corona 1: irregular, lenticular sillimanite needles in cordierite intergrown with “spongy” garnet; corona 2: rod-like intergrowth of Opx, Sil and Spl; orientation: orthopyroxene vermicules aligned perpendicular to layer boundariesNo zonation describedP–T grid constraints on pressure: 5–6 kbar with cooling from 1000 ∘CContact aureole–––
JC90: Johnson and Carlson (1990); N83: Nishiyama (1983); J04:
Johnson et al. (2004); WC97: White and Clarke (1997); I93: Indares (1993);
JF88: Joesten and Fisher (1988); ABE92: Ashworth et al. (1992); K87: Koons et
al. (1987);WM83: Whitney and McLelland (1983); A98: Ashworth et al. (1998);
B01: Bruno et al. (2001); WM73: Whitney and McLelland (1973); A01: Abart et
al. (2001); BB88: Boullier and Barbey (1988); C06: Chen et al. (2006); I07:Ikeda et al. (2007); MCC03: McFarlane et al. (2003); IO02: Ings and
Owen (2002); BB00: Barboza and Bergantz (2000); WMP04: Wheeler et al. (2004);
L04: Lang et al. (2004); D02: Daczko et al. (2002); D97: Dasgupta et
al. (1997).
Summary of retrograde corona occurrences in the literature.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsCJ91MetagabbroLlano Uplift, central TexasCorona 1: Grt - Qtz; corona 2: Grt–OmpCorona 1: Grt|Pl + Mgt|Opx + Aug|Qtz; corona 2: Grt|Pl + Prg symp|Mg-Hbl|Pl + Mg-Hbl + Opx symp|OmpCorona 1: corona thickness: Pl = 60 µm; Opx + Hbl = 20 µm; vermicule length: < 5–10 µm; vermicule spacing: – ; vermicule shape: Pl + Mgt – granoblastic-polygonal; Opx + Aug – columnar; orientation: columnar Opx + Aug perpendicular layer boundaries; corona 2: corona thickness: 200 µm; vermicule length: < 2–40 µm; vermicule shape: granoblastic-polygonal Pl; vermicular/rod-like Hbl and Opx; orientation: weakly perpendicular to layer boundariesDisequilibrium. Pl less calcic and amphibole Fe / Mg and Al / Si ratios decrease toward quartz/omphacite (An35-An18).–Isothermal decompressionSingle-stageOpen: Grt–Qtz: LFe > LMg≥LCa≥LAl≥LSi; Grt–Omp: LNa≈LFe > LMg≥LSi≥LCa≥LAlCPG89MetapeliteMac. Robertson Land, AntarcticaCorona 1: Crd–Spl; corona 2: Spl-Matrix (Grt + Sil + Qtz + Kfs); corona 3: Ilm-GrtCorona 1: Spl|Mgt|Sil|Crd; corona 2: Spl|Grt + Sil + Crd|Grt+Sil+Qtz+Kfs; corona 3: Ilm|Sil|GrtCorona thickness: corona 1: 200 µm; corona 2: < 120 µm; corona 3: 30–120 µm; grain size: < layer thickness; grain shape: granoblastic-polygonal; orientation: none––Isobaric cooling following isothermal decompressionSingle-stage––K03MetapeliteMather Paragneiss, Rauer Group, AntarcticaCorona 1: Grt–Qtz; corona 2: Grt–Bt; corona 3: Sil–OpxCorona 1: Grt|Crd + Opx symp|Opx|Qtz; corona 2: Grt|Crd + Opx symp|Pl|Bt; corona 3: Sil|Crd|OpxCorona thickness: < 0.5 mm; grain size: –; grain shape: –; orientation: symplectite phases perpendicular to layer boundariesDisequilibrium. Crd XMg varies across symplectite – generally not systematically, but XMg may increase toward Opx. No variation in Opx composition.750–800 ∘C and 7–8 kbar (modal P–T–X relationships from pseudosections)Decompressive cooling on clockwise pathSingle-stage––WPC02Fe-rich metapelitesMusgrave Block, Central AustraliaGrt-Matrix (Sil, Qtz, Kfs, Bt)Grt|Spl + Qtz + Grt|Matrix; Grt encloses Spl in coronaCorona thickness: < 0.5 mm; grain size: 0.05–0.2 mm; grain shape: granoblastic-polygonal/interlobate; orientation: noneEquilibrium.800–850 ∘C and 5.5–6.0 kbar (P–T–X relationships from pseudosections)Clockwise path with minor decompression dominated by coolingSequential–Coronas develop in response to changing modes in a high-variance equilibrium assemblage. No univariant reaction is crossed. Garnet is still stable. Implies that decompression implied by this texture may have been overestimated in other terranes (Harley, 1989).
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsKS99Mg-rich metapelitesHighland Complex, Hakurutale, Sri LankaCorona 1: Grt–Qtz; corona 2: Grt–Qtz; corona 3: Grt–Bt; corona 4: Sil–Opx/Grt (SiO2 deficient local bulk composition)Corona 1: Grt|Opx + Sil symp|Opx|Qtz; corona 2: Grt|Opx + Crd ± Spr symp|Opx|Qtz; corona 3: Grt|Opx + Pl ± Crd symp|Bt; corona 4: Sil|Crd ± Spr symp|Crd|Opx/GrtCorona thickness: corona 1: < 0.5 mm; grain size: corona 1: coarse granoblastic-polygonal grains < 0.2 mm; symplectite vermicules: 10–50 µm; grain shape: symplectite: vermicular; monomineralic layers: granoblastic-polygonal; orientation: symplectite vermicules weakly aligned perpendicular to layer boundariesDisequilibrium. Corona 1: Opx: XMg=0.76–0.84; XAl=0.09–0.15; corona 2: Opx: XMg=0.76–0.83; XAl=0.12–0.17. No systematic variation noted.810 ∘C and 7.5 kbar. TWQ P–T estimate on Grt, Qtz, Opx, Crd assemblage. Assumed Grt + Qtz in equilibrium with products.Clockwise path with isothermal decompressionSingle-stage–N02Mafic, aluminous pelitesThor–Odin Dome, Shuswap metamorphic core complex, British ColumbiaGed–KyGed|Crd|Crd + Spl sympl|Crd + Crn sympl|KyCorona thickness: layer 1: < 350 µm; layer 2: < 900 µm; layer 3: pseudomorphs kyanite ; vermicule length: 25–350 µm; vermicule spacing: 20 µm; vermicule shape: vermicular symplectite; granoblastic-polygonal/interlobate in Crd moat; orientation: vermicular symplectite weakly oriented perpendicular to layer boundaries, otherwise noneEquilibrium. No variation in any phases noted.P < 5 kbar and T∼ 750 ∘C. Equilibria constrained with TWQ and conventional thermobarometers.Rapid isothermal decompression on a clockwise pathSingle-stage–H06MetapeliteLeverburgh Belt, South Harris, ScotlandGrt–QtzGrt|Crd + Opx symp|Opx|QtzCorona thickness: layer 1: < 200 µm; layer 3: < 50 µm; vermicule length: 10–50 µm; vermicule spacing: 25 µm; vermicule shape: rod-like symplectite; columnar Opx in layer 2; orientation: vermicular symplectite weakly oriented perpendicular to layer boundariesDisequilibrium. No systematic variation described. Opx: y(Opx) = 0.02–0.08; XMg=0.71–0.78; cordierite: XMg=0.89–0.92P∼ 9 kbar and T∼ 870 ∘C.Isothermal decompression on anticlockwise pathSingle-stage–G80MetapeliteEnderby Land, AntarcticaSpr–QtzSpr|Sil|Opx|QtzCorona thickness: < 0.5 mm–7 ± 1 kbar; 900 ± 30 ∘CIsobaric cooling on anticlockwise pathSingle-stage––B04Troctolitic gabbroSnowbird tectonic zone, western Canadian ShieldGrt–CpxGrt|Opx + Pl symp|Spl + Cpx + Opx symp|OmpGrt|Prg + Pl|Pl|Opx + Pl|OmpCorona thickness: layer 1: < 70 µm; layer 2: 20 µm; layer 3: 50 µm; vermicule length: < 50 µm; vermicule spacing: –; vermicule shape: elongate Prg laths; euhedral Opx rhombs; orientation: oriented perpendicular to layer boundariesDisequilibrium. Marked zonation in Pl (An91 adjacent Grt to An44 at corona margin). No variation in amphibole, orthopyroxene or clinopyroxene compositions documented.850–855 ∘C at 10–12 kbar (two pyroxene thermometry); 810 ∘C and 12 kbar (Grt–Opx–Pl–Qtz equilibria – TWQ; Grt rim and symplectite compositions)Isothermal decompression on clockwise pathSingle-stage?–Corona mineralogy depends on availability of H2O to form amphibole. The use of corona plagioclase composition in TWQ thermobarometry may not be valid.
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsACK07MetapeliteNeocene volcanic province, El Hoyazo, SpainGrt-Matrix (Bt + Sil + Pl)Grt|Spl + Crd + Kfs + Melt (glass)|Matrix; Pl rinds separate Spl from Crd.Corona thickness: < 150 µm; vermicule length: < 50 µm; vermicule spacing: –; vermicule shape: euhedral Spl and granoblastic-interlobate Crd; orientation: no preferred orientationEquilibrium. No systematic variation in spinel or cordierite composition.820 ± 50 ∘C, 4.5 ± 0.6 kbar (ternary feldspar thermometry, Grt–Crd thermobarometry, GASP barometry)Rapid decompression during eruption followed by isobaric coolingSingle-stage/sequential–Plagioclase rind between cordierite and spinel is attributed to isobaric cooling post-eruption.T06MetapeliteMather Paragneiss, Rauer Group.Grt Grt-Melt Sil–Opx Grt–Qtz Grt–Cpx + HblGrt|Spr + Opx + Crd symp Grt|Bt + Crd symp|MeltSil|Spr + Crd symp|OpxGrt|Crd + Opx symp|Opx|QtzGrt|Opx1+ Pl|Opx2+ Pl + Mgt|Cpx+HblCorona 5: corona thickness: layer 1: < 2 mm; layer 2: 1 mm; grain size: < 0.5 mm; grain shape: Opx1 is vermicular, Opx2 is euhedral and columnar; orientation: symplectite vermicules strongly aligned perpendicular to layer boundariesNo variation in corona cordierite observed. Symplectitic orthopyroxene: XMg=0.71–0.72; 4.1–4.2 Al2O3 wt %. Symplectitic Spr: Si = 1.36–1.39 a.p.f.u.Two groups of post-peak P–T estimates: 980–1010 ∘C (Grt–Opx thermobarometry) and ∼ 10 kbar; ∼ 800 ∘C and ∼ 7 kbar (Grt–Bt thermometry).Clockwise path with near isothermal decompression under ultra-high T followed by decompression cooling(1–4) Single-stage; (5) sequential–G72Olivine pod in anorthositeSognefjorden and Bergen, NorwayOl-PlType 1: Ol|Opx|Cpx|Grt|Cpx + Spl|Pl; type 2: Ol|Opx|Cpx|Grt + Cpx + Spl|Pl; type 3: Ol|Cpx|Grt + Cpx + Spl|Pl; type 4: Ol|Cpx|Pl|Grt|PlCorona thickness: type 2: Opx: 6–15 mm; Cpx: 4–6 mm; Grt: 7–15 mm; type 3: Opx: 1 cm; Cpx: 5 mm; Grt: 5–8 mm; type 4: Opx: 3 mm; Cpx: 1 mm; Grt: 2 mm; grain size: – ; grain shape: – ; orientation: –Disequilibrium. Type 2 corona: Bergen: Opx strongly zoned – Al content and XFe increase from olivine toward Pl. Garnet strongly zoned – Ca decreases and Mg + Fe increases towards Cpx. Sognefjorden: all phases homogeneous. Type 3 corona: all phases homogeneous, except garnet, where Ca decreases toward Cpx. Type 4 corona: Mg and Ca increase in Cpx toward plagioclase layer. Ca decreases in the garnet layer toward plagioclase layer.1000 ∘C and 12 kbar (Grt–Cpx thermometry); cooling to 500 ∘CCooling from peak igneous T at high pressureSequential–Coronas 1–4 are considered to represent transient, increasingly more evolved stages of corona development on the P–T path.MA83Olivine metagabbroNE ScotlandOl-PlOl|Opx|Hbl|Hbl + Spl/An symp|PlCorona thickness: Opx: 70 µm; Hbl: 30 µm; Hbl + Spl: 100 µm; Spinel vermicule length: 0.5–5µm (max 30 µm); vermicule spacing: – ; vermicule shape: subhedral, columnar Hbl + Opx; vermicular spinel; preferred orientation: perpendicular layer boundariesDisequilibrium. Al content and XFe of Opx and Hbl increase toward Pl reactant. Opx: Alcat=0.03–0.07; XFe=0.26–0.28. Hbl: Alcat=2.72–2.77; XFe=0.28–0.26.Amphibolite facies. No quantitative thermobarometry.Cooling from peak igneous TSingle-stageOpen: LFe≥LMg≥LCa > LAl > LSi
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsG88MetagabbroCentral Gneiss Belt, western Grenville Province, Ontario.Ol-PlOl|Opx|Cpx|± Amph |Grt|PlCorona thickness: overall 250–500 µm; vermicule/grain size: not resolvable; vermicule spacing: not resolvable; vermicule shape: granoblastic-polygonal Grt, columnar Cpx and Opx; preferred orientation: Opx and Cpx oriented perpendicular to layer boundariesDisequilibrium. Opx and Cpx zoned w.r.t. Al – increases from 1.4 to 1.8 wt % toward Pl. Variable Si and Al in amphibole (±2 wt %). Al / Si ratio decreases in the symplectite toward olivine. Garnet compositions are homogeneous.8–10 kbar and 700–750 ∘C from equilibrium assemblages is in host gneissesCooling from peak igneous T at high pressureSingle-stage – with back reaction and re-equilibrationClosed: cross-coefficient terms LCaMg and LCaFe introduced to stabilize garnet in symplectite. Semi-quantitative results only with LFe2+≈LMg > LAl > LSi and LMg/LNa < 6–8. LCa/LAl < 1.5–2.0. LMg/LCa < 4–6.The closed-system assumption forced Grant to assume a “fictive” non-stoichiometric plagioclase composition to accommodate excess Ca required to stabilize Cpx-bearing coronas. The garnet layer was not included in the single-stage model since it was believed to post-date the main corona.J86Troctolitic gabbroRisør, NorwayOl-PlOl|Opx|Opx + Spl symp|Hbl + Spl symp|PlOl|Opx|Hbl|Hbl + Spl symp|PlCorona thickness: Opx layer: 72–105 µm; Opx + Spl: 10–0 µm; Hbl + Spl: 110–205 µm; vermicule length: Spl: 1–2 µm; vermicule spacing: – ; vermicule shape: Opx and Hbl: columnar; Spl: needles/rods; orientation: Spl rods perpendicular layer boundariesEquilibrium – no variation described.–Primary coronas magmatic in origin, followed by secondary solid-state annealingSequentialClosed: none stableDiffusional instability of primary coronas drives secondary annealing to produce stable secondary solid-state coronas. Remodelled successfully by Ashworth (1986), consistent with emplacement into regional metamorphic terranes and cooling from igneous temperatures.WM73MetagabbroAdirondacks, New YorkOl-PlAdirondack Highlands: Ol|Opx|Cpx|Pl|Grt (± Cpx ± Hbl)|PlCorona thickness: overall 500–700 µm; vermicule/grain size: not resolvable; vermicule spacing: not resolvable; vermicule shape: granoblastic-polygonal Grt; columnar Cpx and Opx; preferred orientation: Opx and Cpx oriented perpendicular to layer boundariesJd/Ts in Cpx decreases from Ol toward Pl (ascribed to formation under different P–T conditions in sequential model). No variation in XMg of pyroxenes.Adirondack Highlands: ∼ 8 kbar and 800 ∘CCooling from peak igneous temperaturesSequential–Close association of metagabbros with anorthosite argues against depression of relatively buoyant crust to depths sufficient to explain prograde formation of coronas. Garnet-bearing coronas in the northern Adirondacks reflect higher pressures during corona formation. Absence of garnet locally in coronas is attributed to kinetic nucleation constraints.D89MetapeliteCentral Zone, Limpopo Belt, ZimbabweCorona 1: Grt–Krn; corona 2: Crn-Krn; corona 3: Crn-GedCorona 1: Grt|Ged + Spl|Ged + Spr|Spr + Crd|Krn; corona 2: Crn|Spl|Spr + Crd|Krn; corona 3: Crn|Spl|Spr + Crd|Crd |GedCorona thickness: corona 1: layer 1: < 200 µm; layer 2: ∼ 250 µm; layer 3: < 200 µm; corona 2: < 750 µm; corona 3: layer 1: < 1000 µm; layer 2: 500 µm; layer 3: < 700 µm; grain size: <maximum layer thickness; grain shape: symplectite: vermicular; monomineralic layers : granoblastic-polygonal; orientation: symplectite vermicules radially aligned in sectors (not always perpendicular to layer boundaries)Equilibrium: no variation described in any phases.700–800 ∘C; 3.5–5 kbar (MASH equilibria: Grt–Spr–Spl–Crd thermobarometry)Isothermal decompressionSequential––
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsLAL87MetapelitePaderu, Southern IndiaCorona 1: Spr–Qtz; corona 2: Spl–Qtz; corona 3: Spr-Sil; corona 4: Opx-MeltCorona 1: Spr|Sil|Opx/Crd|Qtz; corona 2: Spl|Spr|Sil|QtzSpl|Spr|Opx|QtzSpl|Sil|Opx|QtzSpl|Spr|Sil|Grt/Opx|Qtz; corona 3: Spr|Opx|Sil; corona 4: Opx|Bt + Qtz symp|Melt–No systematic variation recorded for any corona phases.900 ± 60 ∘C and 6.5 ± 0.7 kbar to 760 ± 50 ∘C and 5.0 ± 0.6 kbar (Grt–Opx thermobarometry) and relationships on the FMASH petrogenetic gridDecompressive cooling on clockwise pathSingle-stage–Variation in corona mineralogy dependent on topology of petrogenetic grid and local variation in bulk composition (μFeO, μFe2O3 and μH2O)O04MetapeliteKon Tum Massif, Central VietnamGrt–QtzGrt|± Prp ± Spl symp|Crd + Opx symp|Opx|± Pl ± Kfs |QtzCorona thickness: symp layer: < 400 µm; mono Opx: < 100 µm; vermicule length: 20–100 µm; vermicule spacing: 10–30 µm; vermicule shape: symplectite: vermicular; monomineralic Opx: columnar/blocky; Spl: needles/rods; orientation: weakly oriented perpendicular to layer boundariesDisequilibrium. Zonation noted but not described. Symplectite Opx: XMg=0.62–0.70; 5.8–7.6 Al2O3 wt % Crd: XMg=0.82–0.84; Spl: XMg=0.67–0.71; mono Opx: XMg=0.67–0.71; 4.7–5.3 Al2O3 wt %.Cooling from 9 kbar, 1000 ∘C to 6.5 kbar, 800 ∘C from FMASH petrogenetic grid, XAl and XMg contoursIsothermal decompression on clockwise pathSequential–Assumed preserved metastable equilibria in corona represent discrete P–T conditions.T04MetapeliteGanguvarpatti, Southern IndiaCorona 1: Grt-Matrix; corona 2: Grt–QtzCorona 1: Grt|Opx ± Spr ± Spl|Matrix (composite symplectite); corona 2: Grt|Opx + Crd|QtzCorona thickness: corona 1: < 300 µm; corona 2: layer 1: < 1000 µm; grain size: 50–100 µm; grain shape: symplectite: needle-like Spr rods; monomineralic layers: granoblastic-polygonal; orientation: symplectite Spr vermicules strongly aligned radially w.r.t layer boundaries, Opx vermicules perpendicular to layer boundariesNear equilibrium. Minor asymmetrical zonation in cordierite described with highest XMg values adjacent to garnet (XMg: 0.85–0.89). Spr XMg: 0.73–0.75. Spl XMg: 0.41–0.44. Opx in corona 1: Al2O3 contents 7.6–8.2 wt %. Opx in corona 2: Al2O3 contents 6.9–7.5 wt %.Crd + Opx symp: 950–990 ∘C, > 8 kbar; Crd ± Spr ± Spl symp: 850–900 ∘C, 8 kbar (XMg and XAl Opx thermobarometry, Hensen and Harley, 1990)Steep decompressive cooling on clockwise pathSingle-stage––ZH07MetapeliteVestfold Hills, East AntarcticaCorona 1: Pl-Opx; corona 2: Grt–PlCorona 1: Pl1|Pl2|± Kfs|Grt2b+ Qtz|Opx; corona 2: Grt|Grt2a+ Qtz|Pl2Corona thickness: corona 1: layer 1: ? ;layer 2: < 30 µm; layer 3: < 170 µm; corona 2: layer 1: < 100 µm; grain size: 5–100 µm; grain shape: symplectite: granoblastic; monomineralic layers: granoblastic-polygonal; orientation: symplectite vermicules weakly aligned perpendicular to layer boundariesGrt2a zoned radially w.r.t. Ca, Fe and Mg. Alm65–68 Grs10–15 Prp19–21 Sps1–2, with XMg of 0.21–0.24.600–680 ∘C, 6–8 kbar (Garnet-Opx thermometry, GAFS barometry)Isobaric coolingSingle-stage––
Continued.
TagBulk compLocationReactantsCorona product assemblageCorona thickness, vermicule length, vermicule spacingEquilibrationP, T of corona formationInferred P–T pathLayer growth modelDiffusion modelCommentsM98Fayalite granitesLofoten Islands, NorwayCorona 1: Fa/Mgt–Pl; corona 2: Fa/Mgt–KfsCorona 1: Fa|Opx|Grt + Opx|Pl/Kfs; corona 2: Fa|Opx|Amph|Pl/Kfs (Opx + Grt layers thinner and Opx:Grt in Opx + Grt layer is higher adjacent to Kfs than Pl); Mgt|Grt + Opx|FspMgt|Amph|FspCorona thickness: corona 1: layer 1: < 50 µm; layer 2: < 20 µm; grain size: 2–20 µm; grain shape: symplectite: granoblastic; monomineralic layers : polygonal granoblastic; orientation: symplectite vermicules aligned perpendicular to layer boundariesDisequilibrium. Garnet only exhibits zonation, becoming more calcic (XGrs: 0.09–0.15) and magnesian (XMg: 0.95–0.97) toward feldspar.780–840 ∘C and 4–10 kbar (Grt–Cpx–Opx–Pl thermobarometry). Large variation in pressure is due to variation in garnet composition across the corona.Cooling from peak igneous temperaturesSingle-stageOpen-system model with constant volume constraint: LFe > LSi > LMg > LK > LNa≫LCa > LMn > LAlMarkl et al. (1998) is first study to constrain relative diffusion coefficients of major components in granulite facies granitic rocks.BPS87MetapeliteErrabiddy, Western AustraliaCorona 1: Grt–Ged; corona 2: Ky–GedCorona 1: Grt|Crd + Ged2|Ged; corona 2: Ky|St|Crd|GedCorona thickness: < 100 µm; grain size: 20 µm; grain shape: symplectite: granoblastic; monomineralic layers: granoblastic-polygonal; orientation: symplectite vermicules aligned perpendicular to layer boundariesCorona 1: XFe in Crd decreases from 0.17 near Grt to 0.1–0.13 adjacent to Ged. Ged XFe 0.37–0.47; AlIV 1.3–2.2. corona 2: XFe in Crd increases toward Ged from 0.10 to 0.16. Ged XFe: 0.26–0.36.600–650 ∘C, 4–6 kbar (P–T–X phase equilibria)Clockwise reheating of originally high-grade rocks, followed by isothermal upliftSingle-stage–Variations in the proportions of cordierite and staurolite are directly related to the XFe and Ca composition of gedrite reactant.BKO03MetapeliteEpupa Complex, NW NamibiaGrt–QtzGrt|Crd + Opx|Pl|Opx|QtzCorona thickness: layer 1: < 250 µm; layer 2: < 70 µm; grain size: 20–250 µm; grain shape: symplectite – lamellar; orientation: symplectite vermicules aligned perpendicular to layer boundariesOpx XAl increases toward Grt (Altot: 0.12–0.25). XMg of Opx and Crd decrease toward Qtz (0.66–0.52 in Opx; XFe 0.87–0.81 in Crd). No zonation in Pl.Stage 1 corona formation: 940 ± 30 ∘C and 8 ± 2 kbar (Grt–Opx thermometry and Grt–Opx–Pl–Qtz barometry on Grt rim, layer 2 Pl and layer 3 Opx); stage 2 corona growth: 800 ± 60 ∘C and 6 ± 2 kbar (Grt–Opx thermometry and Grt–Opx–Pl–Qtz barometry)Post peak decompression, followed by near isobaric cooling – clockwise retrograde pathSequential–Increase in Al content in Opx toward Qtz inconsistent with diffusion-controlled growth; i.e. Al diffusion is rate-limiting. Either Al diffused anomalously quickly in this instance or the Crd + Opx symplectites formed at lower temperature. Thermobarometry potentially applied to disequilibrium compositions.TM85EmeriesCortland Complex, New YorkSpr–QtzSpr|Opx|Sil|QtzCorona thickness: < 60 µm; grain size: < 60 µm; grain shape: granoblastic-polygonal; no preferred orientationDisequilibrium. Homogeneous Fe + Mg in Opx. Al content in Opx increases toward Spr.––Sequential–Partial equilibrium attained by Fe + Mg, i.e. inferred to be the most rapidly diffusing components such that chemical potential gradients were eliminated
CJ91: Carlson and Johnson (1991); CPG89: Clarke et al. (1989);
K03: Kelsey et al. (2003); WPC02: White et al. (2002); KS99: Kriegsman and
Schumacher (1999); N02: Norlander et al. (2002); H06: Hollis et al. (2006);
G80: Grew(1980); B04: Baldwin et al. (2004); ACK07: Álvarez-Valero et
al. (2007); T06: Tong and Wilson (2006); G72: Griffin (1972); MA83:
Mongkoltip and Ashworth (1983); G88: Grant (1988); J86: Joesten (1986); WM73:
Whitney andMcLelland (1973); D89: Droop (1989); LAL87: Lal et al. (1987);
O04: Osanai et al. (2004); T04: Tamashiro et al. (2004); ZH07: Zulbati and
Harley (2007); M98: Markl et al. (1998); BPS87:
Baker et al. (1987); BKO03: Brandt et al.(2003); TM85: Tracey and
McLellan (1985); TVR06: Tsunogae and Van Reenen (2006); HM96: Hisada and
Miyano (1996).
The authors declare that they have no conflict of
interest.
Acknowledgements
Funding from the National Research Foundation Scarce Skills Scholarship and
Rated Researcher Programmes is gratefully acknowledged. Tim Johnson and
Thomas Mueller are thanked for insightful reviews.
Edited by: M. Heap
Reviewed by: T. Mueller and T. Johnson
References
Abart, R. and Petrishcheva, E.: Thermodynamic model for reaction rim growth:
Interface reaction and diffusion control, Am. J. Sci., 311, 517–527, 2011.
Abart, R. and Schmid, R.: Silicon and oxygen self-diffusion in enstatite
polycrystals: the Milke et al. (2001) rim growth experiment revisited,
Contrib. Mineral. Petr., 147, 633–646, 2004.
Abart, R., Schmud, R., and Harlov, D. E.: Metasomatic coronas around
hornblendite xenoliths in granulite facies marble, Ivrea zone, N Italy,
I: Constraints on component mobility, Contrib. Mineral. Petr., 141, 473–493,
2001.
Abart, R., Petrishcheva, E., and Joachim, B.: Thermodynamic model for growth
of reaction rims with lamellar microstructure, Am. Mineral., 97, 231–240,
2012.
Abart, R., Svoboda, J., Jerabek, P., Karadeniz, E. P., and Habler, G.: Interlayer
growth kinetics of a binary solid-solution based on the thermodynamic
extremal principle: application to the formation of spinel at
periclase-corundum contacts, Am. J. Sci., 316, 309–328, 2016.
Ague, J. J. and Baxter, E. F.: Brief thermal pulses during mountain building
recorded by Sr diffusion in apatite and multicomponent diffusion in garnet,
Earth Planet. Sc. Lett., 261, 500–516, 2007.
Álvarez-Valero, A. M., Cesare, B., and Kriegsman, L. M.: Formation of
spinel-cordierite-feldspar-glass coronas after garnet in metapelitic
xenoliths: reaction modelling and geodynamic implications, J. Metamorph.
Geol., 25, 305–320, 2007.
Ashworth, J. R.: The role of magmatic reaction, diffusion, and annealing in
the evolution of coronitic microstructure in troctolitic gabbro from Risor,
Norway: a discussion, Mineral. Mag., 50, 469–473, 1986.
Ashworth, J. R.: Fluid-absent diffusion kinetics of Al inferred from
retrograde metamorphic coronas, Am. Mineral., 78, 331–337, 1993.
Ashworth, J. R. and Birdi, J. J.: Diffusion modelling of coronae around
olivine in an open system, Geochim. Cosmochim. Ac., 54, 2389–2401, 1990.
Ashworth, J. R. and Chambers, A. D.: Symplectic reaction in olivine and the
controls of intergrowth spacing in symplectites, J. Petrol., 41, 285–304,
2000.
Ashworth, J. R. and Sheplev, V. S.: Diffusion modelling of metamorphic
layered coronas with stability criterion and consideration of affinity,
Geochim. Cosmochim. Ac., 61, 3671–3689, 1997.
Ashworth, J. R., Birdi, J. J., and Emmett, T. F.: A complex corona between
olivine and plagioclase from the Jotun Nappe, Norway, and the diffusion
modelling of multimineralic layers, Mineral. Mag., 56, 511–525, 1992.
Ashworth, J. R., Sheplev, V. S., Bryxina, N. A., Kolobov, V. Y., and
Reverdetto, V. V.: Diffusion-controlled corona reaction and over-stepping of
equilibrium in a garnet granulite, Yenisey Ridge, Siberia, J. Metamorph.
Geol., 16, 231–246, 1998.
Ashworth, J. R., Sheplev, V. S., Khlestov, V. V., and Ananyev, A. A.:
Geothermometry using minerals at non-equilibrium: a corona example European,
J. Mineral., 13, 1153-1161, 2001.
Baker, J., Powell, R., Sandiford, M., and Muhling, J.: Corona textures
between kyanite, garnet and gedrite in gneisses from Errabiddy, Western
Australia, J. Metamorph. Geol., 5, 357–370, 1987.
Baldwin, J. A., Bowring, S. A., Williams, M. L., and Williams, I. S.:
Eclogites of the Snowbird tectonic zone petrological and U-Pb
geochronological evidence for Paleoproterozoic high-pressure metamorphism in
the western Canadian Shield, Contrib. Mineral. Petr., 147, 528–548, 2004.
Baldwin, J. A., Powell, R., White, R. W., and Štípská, P.: Using
calculated chemical potential relationships to account for replacement of
kyanite by symplectite in high pressure granulites, J. Metamorph. Geol., 33,
311–330, 2015.
Barboza, S. A. and Bergantz, G. W.: Metamorphism and Anatexis in the Mafic
Complex Contact Aureole, Ivrea Zone, Northern Italy, J. Petrol., 41,
1307–1327, 2000.
Boullier, A. M. and Barbey, P.: A polycyclic two-stage corona growth in the
Iforas Granulitic Unit (Mali), J. Metamorph. Geol., 6, 235–254, 1988.
Brady, J. B.: Metasomatic zones in metamorphic rocks, Geochim. Cosmochim.
Ac., 41, 113–125, 1977.
Brady, J. B.: Intergranular diffusion in metamorphic rocks, Am. J. Sci.,
283A, 181–200, 1983.
Brandt, S., Klemd, R., and Okrusch, M.: Ultrahigh-Temperature Metamorphism
and Multistage Evolution of Garnet-Orthopyroxene Granulites from the
Proterozoic Epupa Complex, NW Namibia, J. Petrol., 44, 1121–1144, 2003.
Bruno, M., Compagnoni, R., and Rubbo, M.: The ultra-high pressure coronitic
and pseudomorphous reactions in a metagranodiorite from the Brossasco-Isasca
Unit, Dora-Maira Massif, western Italian Alps: a petrographic study and
equilibrium thermodynamic modelling, J. Metamorph. Geol., 19, 33–43, 2001.
Carlson, W. D.: Scales of disequilibrium and rates of equilibration during
metamorphism, Am. Mineral., 87, 185–204, 2002.
Carlson, W. D. and Johnson, C. D.: Coronal reaction textures in garnet
amphibolites of the Llano Uplift, Am. Mineral., 76, 756–772, 1991.
Carmichael, D. M.: Induced stress and secondary mass transfer: thermodynamic
basis for the tendency toward constant-volume constraint in diffusion
metasomatism, in: Chemical Transport in Metasomatic Processes, C 218, NATO
Adv. Study Inst., Ser., Dordrecht, the Netherlands, 239–264, 1987.
Chen, Y., Ye, K., Liu, J. B., and Sun, M.: Multistage metamorphism of the
Huangtuling granulite, Northern Dabie Orogen, eastern China: implications for
the tectonometamorphic evolution of subducted lower continental crust, J.
Metamorph. Geol., 24, 633–654, 2006.Clarke, G. L., Powell, R., and Guiraud, M.: Low-pressure granulite facies
metapelitic assemblages and corona textures from MacRobertson Land, east
Antarctica: the importance of Fe2O3 and TiO2 in accounting for
spinel-bearing assemblages, J. Metamorph. Geol., 7, 323–335,1989.
Daczko, N. R., Stevenson, J. A., Clarke, G. L., and Klepis, K. A.: Successive
hydration and dehydration of high-P mafic granofels involving
clinopyroxene-kyanite symplectites, Mt. Daniel, Fiordland, New Zealand, J.
Metamorph. Geol., 20, 669–682, 2002.
Dasgupta, S., Ehl, J., Raith, M. M., Sengupta, P., and Sengupta, P.:
Mid-crustal contact metamorphism around the Chimakurthy mafic-ultramafic
complex, Eastern Ghats Belt, India, Contrib. Mineral. Petr., 129, 182–197,
1997.
Delle Piane, C., Burlini, L., and Grobety, B.: Reaction-induced strain
localization: torsion experiments on dolomite, Earth Planet. Sc. Lett., 256,
36–46, 2007.
Dohmen, R. and Chakraborty, S.: Mechanism and kinetics of element and
isotopic exchange mediated by a fluid phase, Am. Mineral., 88, 1251–1270,
2003.
Dohmen, R. and Milke, R.: Diffusion in polycrystalline materials: grain
boundaries, mathematical models, and experimental data, Rev. Mineral.
Geochem., 72, 921–970, 2010.
Droop, G. T. R.: Reaction history of garnet-sapphirine granulites and
conditions of Archaean high-pressure granulite-facies metamorphism in the
Central Limpopo Mobile Belt, Zimbabwe, J. Metamorph. Geol., 7, 383–403,
1989.
Ellis, D. J.: Osumilite-sapphirine-quartz granulites from Enderby Land,
Antarctica: P-T conditions of metamorphism, implications for
garnet-cordierite equilibria and the evolution of the deep crust, Contrib.
Mineral. Petr., 74, 201–210, 1980.
Ellis, D. J. and Green, D. H.: An experimental study of the effect on Ca upon
garnet-clinopyroxene Fe-Mg exchange equilibria, Contrib. Mineral. Petrol.,
71, 13–22, 1979.
Farver, J. and Yund, R.: Volume and grain boundary diffusion of calcium in
natural and hot-pressed calcite aggregates, Contrib. Mineral. Petr., 123,
77–91, 1996.
Fisher, G. W.: The application of ionic equilibria to metamorphic
differentiation: an example, Contrib. Mineral. Petr., 29, 91–103, 1970.
Fisher, G. W.: Non-equilibrium thermodynamics as a model for
diffusion-controlled metamorphic processes, Am. J. Sci., 273, 897–924, 1973.
Fisher, G. W.: The thermodynamics of diffusion-controlled metamorphic
processes, in: Mass Transport Phenomena in Ceramics, Plenum Press, New York,
USA, 111–122, 1975.
Fisher, G. W.: Nonequilibrium thermodynamics in metamorphism, in:
Thermodynamics in Geology, D. Reidel, Boston, USA, 381–403, 1977.
Fisher, G. W. and Lasaga, A. C.: Kinetics of Geochemical Processes, Rev.
Mineral., 8, 171–185, 1981.
Fisler, D. K., Mackwell, S. J., and Petsch, S.: Grain boundary diffusion in
enstatite, Physics and Chemistry of Minerals, 24, 264–273, 1997.
Foster, C. T.: A thermodynamic model of mineral segregations vin the lower
sillimanite zone near Rangeley, Maine, Am. Mineral., 66, 260–277, 1981.
Foster, C. T.: Thermodynamic models of reactions involving garnet in a
sillimanite/staurolite schist, Mineral. Mag., 50, 427–439, 1986.Gardés, E. and Heinrich, W.: Growth of multilayered polycristalline
reaction rims in the MgO-SiO2 system, part II: modelling, Contrib.
Mineral. Petr., 162, 37–49, 2011.Gardés, E., Wunder, B., Wirth, R., and Heinrich, W.: Growth of
multilayered polycrystalline reaction rims in the MgO-SiO2 system,
part I: experiments, Contrib. Mineral. Petr., 161, 1–12, 2011.
Gibson, R. L.: Impact-induced melting in Archaean granulites in the Vredefort
Dome, South Africa I.: Anatexis of metapelitic granulites, J. Metamorph.
Geol., 20, 57–70, 2002.Götze, L. C., Abart, R., Rybacki, E., Keller, L. M., Petrishcheva, E.,
and Dresen, G.: Reaction rim growth in the system
MgO–Al2O3–SiO2 under uniaxial stress, Contrib. Mineral.
Petr., 99, 263–277, 2010.
Grant, S. M.: Diffusion models for corona formation in metagabbros from the
western Grenville Province, Canada, Contrib. Mineral. Petr., 98, 49–63,
1988.Grew, E. S.: Sapphirine+quartz association from Archaean rocks in Enderby
Land, Antarctica, Am. Mineral., 65, 821–836, 1980.
Griffin, W. L.: Formation of eclogites and coronas in anorthosites, Bergen
Arcs, Norway, Geol. Soc. Am. Mem., 135, 37–63, 1972.
Griffin, W. L. and Heier, K. S.: Petrological implications of some corona
structures, Lithos, 6, 315–335, 1973.
Harley, S. L.: The origins of granulites: a metamorphic perspective, Geol.
Mag., 126, 215–247, 1989.
Heidelbach, F., Terry, M. P., Bystricky, M., Holzapfel, C., and McCammon, C.:
A simultaneous deformation and diffusion experiment: quantifying the role of
deformation in enhancing metamorphic reactions, Earth Planet. Sc. Lett., 278,
386–393, 2009.
Helpa, V., Rybacki, E., Abart, R., Morales, L. F. G., Rhede, D.,
Jeřábek, P., and Dresen, G.: Reaction kinetics of dolomite rim
growth, Contrib. Mineral. Petr., 167, 1–14, 2014.
Helpa, V., Rybacki, E., Morales, L. F. G., and Dresen, G.: Influence of
stress and strain on dolomite rim growth: a comparative study, Contrib.
Mineral. Petr., 170, 1–21, 2015.Hensen, B. J. and Harley, S. L.: Graphical analysis of P-T-X relations in
granulite facies metapelites, in: High Temperature Metamorphism and Crustal
Anatexis, chap. 3, edited by: Ashworth, J. R. and Brown, M., 19–56. Unwin Hyman,
London, UK, 1990.
Hisada, K. and Miyano, T.: Petrology and microthermometry of aluminous rocks
in the Botswanan Limpopo Central Zone: evidence for isothermal decompression
and isobaric cooling, J. Metamorph. Geol., 14, 183–197, 1996.
Holland, T. and Powell, R.: An improved and extended internally consistent
thermodynamic dataset for phases of petrological interest, involving a new
equation of state for solids, J. Metamorph. Geol., 29, 333–383, 2011.
Holland, T. J. B. and Powell, R.: An internally consistent thermodynamic data
set for phases of petrological interest, J. Metamorph. Geol., 16, 309–343,
1998.
Holland, T. J. B. and Powell, R.: Activity-composition relations for phases
in petrological calculations: an asymmetric multicomponent formulation,
Contrib. Mineral. Petr., 145, 492–501, 2003.
Hollis, J. A., Harley, S. L., White, R. W., and Clarke, G. L.: Preservation
of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure
kyanite-bearing granulites, South Harris, Scotland, J. Metamorph. Geol., 24,
263–279, 2006.
Ikeda, T., Nishiyama, T., Yamada, S., and Yanagi, T.: Microstructures of
olivine-plagioclase corona in meta-ultramafic rocks from Sefuri Mountains, NW
Kyushu, Japan, Lithos, 97, 289–306, 2007.
Indares, A.: Eclogitized gabbros from the eastern Grenville Province:
textures, metamorphic context, and implications, Can. J. Earth Sci., 30,
159–173, 1993.
Ings, S. J. and Owen, J. V.: “Decompressional” reaction textures formed by
isobaric heating: an example from the thermal aureole of the Taylor Brook
Gabbro Complex, western Newfoundland, Mineral. Mag., 66, 941–951, 2002.
Joachim, B., Gardés, E., Abart, R., and Heinrich, W.: Experimental growth
of ackermanite reaction rims between wollastonite and monticellite: evidence
for volume diffusion control, Contrib. Mineral. Petr., 161, 389–399, 2011a.Joachim, B., Gardés, E., Velickov, B., Abart, R., and Heinrich, W.:
Experimental growth of diopside + merwinite reaction rims: the effect of
water on microstructure development, Am. Mineral., 97, 220–230, 2011b.
Joesten, R.: Evolution of mineral zoning in diffusion metasomatism, Geochim.
Cosmochim. Ac., 41, 649–670, 1977.
Joesten, R.: The role of magmatic reaction, diffusion and annealing in the
evolution of coronitic microstructure in troctolitic gabbro from Risor,
Norway, Mineral. Mag., 50, 441–467, 1986.
Joesten, R. and Fisher, G. W.: Kinetics of diffusion-controlled mineral
growth in the Christmas Mountains (Texas) contact aureole, Geol. Soc. Am.
Bul., 100, 714–732, 1988.
Johnson, C. D. and Carlson, W. D.: The origin of olivine-plagioclase coronas
in metagabbros from the Adirondack Mountains, New York, J. Metamorph. Geol.,
8, 697–717, 1990.
Johnson, T. E., Brown, M., Gibson, R. L., and Wing, B.: Spinel-cordierite
symplectites replacing andalusite: evidence for melt-assisted diapirism in
the Bushveld Complex, South Africa, J. Metamorph. Geol., 22, 529–545, 2004.
Jonas, L., Mueller, T., Dohmen, R., Baumgartner, L., and Putlitz, B.:
Transport-controlled hydrothermal replacement of calcite by Mg-carbonates,
Geology, 43, 779–783, 2015.
Keller, L. M., Wirth, R., Rhede, D., Kunze, K., and Abart, R.: Asymmetrically
zoned reaction rims: assessment of grain boundary diffusivities and growth
rates related to natural diffusion-controlled mineral reactions, J.
Metamorph. Geol., 26, 99–120, 2008.
Keller, L. M., Götze, L. C., Rybacki, E., Dresen, G., and Abart, R.:
Enhancement of solid-state reaction rates by non-hydrostatic stress effects
on polycrystalline diffusion kinetics, Am. Mineral., 95, 1399–1407, 2010.
Kelsey, D. E., White, R. W., Powell, R., Wilson, C. J. L., and Quinn, C. D.:
New constraints on metamorphism in the Rauer Group, Prydz Bay, east
Antarctica, J. Metamorph. Geol., 21, 739–759, 2003.
Koons, P. O., Rubie, D. C., and Fruch-Green, G.: The Effects of
Disequilibrium and Deformation on the Mineralogical Evolution of Quartz
Diorite During Metamorphism in the Eclogite Facies, J. Petrol., 28, 679–700,
1987.
Korzhinskii, D. S.: Physicochemical Basis of the Analysis of the Paragenesis
of Minerals, Consultants Bureau, New York, USA, 142 pp., 1959.
Korzhinskii, D. S.: The theory of systems with perfectly mobile components
and processes of mineral formation, Am. J. Sci., 263, 193–205, 1965.
Kretz, R.: Symbols for rock-forming minerals, Am. Mineral., 68, 277–279,
1983.
Kriegsman, L. M. and Schumacher, J. C.: Petrology of sapphirine-bearing and
associated granulites from central Sri Lanka, J. Petrol., 40, 1211–1239,
1999.Lal, R. K., Ackermand, D., and Upadhyay, H.: P-T-X relationships
deduced from corona textures in sapphirine-spinel-quartz assemblages from
Paderu, southern India, J. Petrol., 28, 1139–1168, 1987.
Lang, H. M., Wachter, A. J., Peterson, V. L., and Ryan, J. G.: Coexisting
clinopyroxene/spinel and amphibole/spinel symplectites in metatroctolites
from the Buck Creek ultramafic body, North Carolina Blue Ridge, Am. Mineral.,
89, 20–30, 2004.
Lasaga, A. C.: Geospeedometry: an extension of geothermometry, in: Kinetics
and equilibrium in mineral reactions, edited by: Saxena, S. K., Springer, New
York, USA, 81–114, 1983.
Markl, G., Foster, C. T., and Bucher, K.: Diffusion-controlled olivine corona
textures in granitic rocks from Lofoten, Norway: calculation of Onsager
diffusion coefficients, thermodynamic modelling and petrological
implications, J. Metamorph. Geol., 16, 607–623, 1998.McFarlane, C. R. M., Carlson, W. D., and Connelly, J. N.: Prograde, peak, and
retrograde P-T paths from aluminium in orthopyroxene: High-temperature
contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Nain
Plutonic Suite, Labrador, J. Metamorph. Geol., 21, 405–423, 2003.
Milke, R. and Heinrich, W.: Diffusion-controlled growth of wollastonite rims
between quartz and calcite: comparison between nature and experiment, J.
Metamorph. Geol., 20, 467–480, 2002.
Milke, R. and Wirth, R.: The formation of columnar fiber texture in
wollastonite rims by induced stress and implications for diffusion-controlled
corona growth, Phys. Chem. Miner., 30, 230–242, 2003.
Milke, R., Wiedenbeck, M., and Heinrich, W.: Grain boundary diffusion of Si,
Mg, and O in enstatite reaction rims: a SIMS study using isotopically doped
reactants, Contrib. Mineral. Petr., 142, 15–26, 2001.
Mongkoltip, P. and Ashworth, J. R.: Quantitative estimation of an open-system
symplectite-forming reaction: restricted diffusion of Al and Si in coronas
around olivine, J. Petrol., 24, 635–661, 1983.
Mork, M. B. E.: Coronite and eclogite formation in olivine gabbro (Western
Norway): reaction paths and garnet zoning, Mineral. Mag., 50, 417–426, 1986.
Mueller, T., Baumgartner, L. P., Foster Jr., C. T., and Roselle, G. T.:
Forward modeling of the effects of mixed volatile reaction, volume diffusion,
and formation of submicroscopic exsolution lamellae on calcite-dolomite
thermometry, Am. Mineral., 93, 1245–1259, 2008.
Mueller, T., Watson, E. B., and Harrison, T. M.: Applications of diffusion
data to high-temperature earth systems, Rev. Mineral. Geochem., 72,
997–1038, 2010.
Mueller, T., Cherniak, D., and Watson, B.: Interdiffusion of divalent cations
in carbonates: experimental measurements and implications for timescales of
equilibration and retention of compositional signatures, Geochim. Cosmochim.
Ac., 84, 90–103, 2012.
Mueller, T., Massonne, H. J., and Willner, A. P.: Special Collection:
Mechanisms, Rates, and Timescales of Geochemical Transport Processes in the
Crust and Mantle. Timescales of exhumation and cooling inferred by kinetic
modeling: An example using a lamellar garnet pyroxenite from the Variscan
Granulitge, Am. Mineral., 100, 747–759, 2015.
Niedermeier, D., Putnis, A., Geisler, T., Golla-Schindle, U., and Putnis, C.:
The mechanism of cation and oxygen isotope exchange in alkali feldspars under
hydrothermal conditions, Contrib. Mineral. Petr., 157, 65–75, 2009.
Nishiyama, T.: Steady diffusion model for olivine-plagioclase corona growth,
Geochim. Cosmochim. Ac., 41, 649–670, 1983.
Norlander, B. H., Whitney, D. L., Teyssier, C., and Vanderhaeghe, O.: Partial
melting and decompression of the Thor-Odin dome, Shuswap metamorphic core
complex, Canadian Cordillera, Lithos, 61, 103–125, 2002.
Ogilvie, P.: Metamorphic Studies in the Vredefort Dome, Unpublished PhD
thesis, University of Witwatersrand, Johannesburg, South Africa, 772 pp.,
2010.
Osanai, Y., Nakano, N., and Owada, M.: Permo-Triassic ultrahigh-temperature
metamorphism in the Kontum Massif, central Vietnam, J. Miner. Petrol. Sci.,
99, 225–241, 2004.
Perchuk, L. L., Tokarev, D. A., van Reenen, D. D., Varlamov, D. A., Gerya, T.
V., Sazonova, L. V., Fel'dman, V. I., Smit, C. A., Brink, M. C., and
Bisschoff, A. A.: Dynamic and Thermal History of the Vredefort Explosion
Structure in the Kaapvaal Craton, South Africa, Petrology, 10, 395–432,
2002.
Powell, R. and Holland, T. J. B.: An internally consistent thermodynamic
dataset with uncertainties and correlations: 3: application methods, worked
examples and a computer program, J. Metamorph. Geol., 6, 173–204, 1988.Powell, R. and Holland, T. J. B.: Calculated mineral equilibria in the pelite
system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O), Am.
Mineral., 75, 367–380, 1990.
Powell, R., Holland, T. J. B., and Worley, B.: Calculating phase diagrams
involving solid solutions via non-linear equations, with examples using
THERMOCALC, J. Metamorph. Geol., 16, 577–588, 1998.
Powell, R., Guiraud, M., and White, R. W.: Truth and beauty in metamorphic
mineral equilibria: conjugate variables and phase diagrams, Can. Mineral.,
43, 21–33, 2005.
Putnis, A.: Mineral replacement reactions, Rev. Mineral. Geochem., 70,
87–124, 2009.
Schmid, D. W., Abart, R., Podladtchikov, I., and Milke, R.: Matrix rheology
effects on reaction rim growth II: coupled diffusion and creep model, J.
Metamorph. Geol., 27, 83–91, 2009.
Sheplev, V. S., Kolobov, V. Yu., Kuznetsova, R. P., and Reverdatto, V. V.:
Analysis of growth of zonated mineral segregation and characteristics of mass
transfer during metamorphism. 1. Theoretical model in a quasi-stationary
approximation, Soviet Geol. Geophys., 32, 1–12, 1991.Sheplev, V. S., Kuznetsova, R. P., and Kolobov, V. Yu.: Analysis of growth of
zonated mineral segregations and characteristics of mass transfer during
metamorphism. 2. The system SiO2-Al2O3-MgO-NaCaO, Russ. Geol.
Geophys., 33, 73–80, 1992a.
Sheplev, V. S., Kuznetsova, R. P., and Kolobov, V. Yu.: Analysis of growth of
zonated mineral segregations and characteristics of mass transfer during
metamorphism. 3. The model of steady diffusions, Russ. Geol. Geophys., 33,
46–52, 1992b.
Smit, C. A., Van Reenen, D. D., Gerya, T. V., and Perchuk, L. L.: P–T
conditions of decompression of the Limpopo high-grade terrain: record from
shear zones, J. Metamorph. Geol., 19, 249–268, 2001.
Štípská, P., Powell, R., White, R., and Baldwin, J.: Using
calculated chemical potential relationships to account for coronas around
kyanite: an example from the Bohemian Massif, J. Metamorph. Geol., 28,
97–116, 2010.
Tajčmanová, L., Konopásek, J., and Connolly, J. A. D.:
Diffusion-controlled development of silica-undersaturated domains in felsic
granulites of the Bohemian Massif (Variscan belt of Central Europe), Contrib.
Mineral. Petr., 153, 237–250, 2007.
Tamashiro, I., Santosh, M., Sajeev, K., Morimoto, T., and Tsunogae, T.:
Multistage orthopyroxene formation in ultra-high temperature granulites of
Ganguvarpatti, southern India: implications for complex metamorphic evolution
during Gondwana assembly, J. Miner. Petrol. Sci., 90, 279–297, 2004.
Thompson, J. B.: Local equilibrium in metasomatic processes, in: Researches
in Geochemistry, Wiley, New York, USA, 427–457, 1959.
Tong, L. and Wilson, C. J. L.: Tectonothermal evolution of the ultrahigh
metapelites in the Rauer Group, east Antarctica, Precambrian Res., 149,
1–20, 2006.
Tracy, R. J. and McLellan, E. L.: A natural example of the kinetic controls
of compositional and textural equilibration, in: Advances in physical
geochemistry 4, Springer-Verlag, New York, USA, 118–137, 1985.Tsunogae, T. and Van Reenen, D. D.: Corundum + quartz and Mg-staurolite
bearing granulite from the Limpopo Belt, southern Africa: Implications for a
P-T path, Lithos, 92, 576–587, 2006.
Van Lamoen, H.: Coronas in olivine gabbros and iron ores from Susimäki
and Riuttamaa, Finland, Contrib. Mineral. Petr., 68, 259–268, 1979.
Vidale, R.: Metasomatism in a chemical gradient and the formation of
calc-silicate bands, Am. J. Sci., 267, 857–874, 1969.
Watson, E. B. and Mueller, T.: Non-equilibrium isotopic and elemental
fractionation during diffusion-controlled crystal growth under static and
dynamic conditions, Chem. Geol., 267, 111–124, 2009.Watson, E. B. and Price, J. D.: Kinetics of the reaction MgO Al2O3→ MgAl2O4 and Al-Mg interdiffusion in spinel at 1200 to
2000 ∘C and 1.0 to 4.0 GPa, Geochim. Cosmochim. Ac., 66,
2123–2138, 2002.
Wheeler, J., Mangan, L. S., and Prior, D. J.: Disequilibrium in the Ross of
Mull Contact Metamorphic Aureole, Scotland: a Consequence of
Polymetamorphism, J. Petrol., 45, 835–853, 2004.
White, R. W. and Clarke, G. L.: The Role of Deformation in Aiding
Recrystallization: an Example from a High-pressure Shear Zone, Central
Australia, J. Petrol., 38, 1307–1329, 1997.
White, R. W. and Powell, R.: Melt loss and the preservation of granulite
facies mineral assemblages, J. Metamorph. Geol., 20, 621–632, 2002.
White, R. W. and Powell, R.: On the interpretation of retrograde reaction
textures in granulite facies rocks, J. Metamorph. Geol., 29, 131–149, 2011.White, R. W., Powell, R., and Clarke, G. L.: The Interpretation of reaction
textures in Fe-rich metapelitic granulites of the Musgrave Block, central
Australia: Constraints from mineral equilibria calculations in the system
K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3,
J. Metamorph. Geol., 20, 41–55, 2002.
White, R. W., Powell, R., and Halpin, J. A.: Spatially-focussed melt
formation in aluminous metapelites from Broken Hill, Australia, J. Metamorph.
Geol., 22, 825–845, 2004.
White, R. W., Powell, R., and Baldwin, J. A.: Calculated phase equilibria
involving chemical potentials to investigate the textural evolution of
metamorphic rocks, J. Metamorph. Geol., 26, 181–198, 2008.
Whitney, P. R. and McLelland, J. M.: Origin of coronas in metagabbros of the
Adirondack Mts., N.Y., Contrib. Mineral. Petr., 39, 81–98, 1973.Whitney, P. R. and McLelland, J. M.: Origin of biotite-hornblende-garnet
coronas between oxides and plagioclase in olivine metagabbros, Adirondack
region, NY, Contrib. Mineral. Petr., 82, 34–41, 1983.
Yund, R. A.: Rates of grain boundary diffusion through enstatite and
forsterite reaction rims, Contrib. Mineral. Petr., 126, 224–236, 1997.
Zhang, Y.: Diffusion in minerals and melts: theoretical background, Rev.
Mineral. Geochem., 72, 5–59, 2010.
Zulbati, F. and Harley, S. L.: Late Archaean granulite facies metamorphism
in the Vestfold Hills, East Antarctica, Lithos, 93, 39–67, 2007.