Syn-kinematic mineral reactions play an important role for the mechanical
properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new
phases) may lead to grain size reduction, producing fine-grained
polymineralic mixtures, which have a strongly reduced viscosity because of
the activation of grain-size-sensitive deformation processes. In order to
study the effect of deformation–reaction feedback(s) on sample strength, we
performed rock deformation experiments on “wet” assemblages of mafic
compositions in a Griggs-type solid-medium deformation apparatus. Shear
strain was applied at constant strain rate (10-5 s-1) and constant
confining pressure (1 GPa) with temperatures ranging from 800 to
900 ∘C. At low shear strain, the assemblages that react faster
are significantly weaker than the ones that react more slowly, demonstrating
that reaction progress has a first-order control on rock strength. With
increasing strain, we document two contrasting microstructural scenarios:
(1) the development of a single throughgoing high-strain zone of
well-mixed, fine-grained aggregates, associated with a significant weakening
after peak stress, and (2) the development of partially connected, nearly
monomineralic shear bands without major weakening. The lack of weakening is
caused by the absence of interconnected well-mixed aggregates of
fine-grained reaction products. The nature of the reaction products, and
hence the intensity of the mechanical weakening, is controlled by the
microstructures of the reaction products to a large extent, e.g., the amount
of amphibole and the phase distribution of reaction products. The samples
with the largest amount of amphibole exhibit a larger grain size and show
less weakening. In addition to their implications for the deformation of
natural shear zones, our findings demonstrate that the feedback between
deformation and mineral reactions can lead to large differences in
mechanical strength, even at relatively small initial differences in mineral
composition.
Introduction
Mafic rocks constitute a large part of the oceanic crust and may be one of
the main components of the lower continental crust (Rudnick and
Fountain, 1995). The major constituents of mafic rocks, i.e., pyroxene and
plagioclase, are mechanically strong minerals that show crystal plastic
deformation only at high temperatures in natural systems (e.g.,
Rutter and Brodie, 1985; Brodie and Rutter,
1985, 1992). However, there are abundant
examples of strongly deformed mafic rocks, even at relatively low
temperatures, but invariably these rocks show metamorphic retrogression
(Rutter and Brodie, 1985). For instance, concomitant
deformation and metamorphism are observed along oceanic detachments, where
deep levels of the oceanic mafic crust are exhumed
(Harigane et al.,
2008; Schroeder and John, 2004). Strongly
sheared meta-gabbros are also present in exhumed subduction belts
(Imon et al., 2004;
Shelley, 1994;
Soret et al., 2019) or in
large-scale transcurrent shear zones (Jolivet and Miyashita,
1985; Shelley, 1994). In most cases, the
decreasing temperature conditions during deformation result in coeval
mineral reactions, often causing strong grain size reduction. The coupling
between deformation and reaction is therefore essential to understand the
process of strain localization, as observed in mafic mylonites
(Brodie et al., 1992).
The pyroxenes typically deform by crystal plasticity at high temperatures
and high stresses (e.g.,
Borg and Handin, 1966;
Coe, 1970; Coe and
Kirby, 1975; Bystricky and Mackwell, 2001;
Bystricky et al., 2016). Mechanical
data from high-temperature deformation experiments with mafic rocks are
relatively scarce (e.g., Dimanov et al., 2003,
2007;
Dimanov and Dresen,
2005; Marti et al., 2017, 2018;
Mansard et al., 2020), but the existing
studies indicate that weakening processes are dependent on the “deformation
history”. The study of mylonitic deformation of natural mafic rocks
provides insights, at small scale, into the deformation mechanisms and
strain localization processes and, at large scale, into the strength of the
lower crust (e.g., Rutter and Brodie, 1992;
Kanagawa et al., 2008). There are
two main mechanisms of rock deformation in the viscous deformation regime:
(1) dislocation creep (crystal plasticity; e.g.,
Paterson, 2013) and (2) diffusion creep. The latter
includes grain-scale diffusion creep, whereby diffusive mass transfer either
occurs through the volume or phase boundaries of individual grains (e.g.,
Wheeler, 1992) and is the main strain-producing process, and
diffusion-accommodated grain boundary sliding (GBS), whereby diffusive mass
transfer adjusts grain shapes and asperities during cohesive, frictionless
sliding (e.g., Ashby and Verrall, 1973;
Paterson, 1990; Langdon, 2006, and
references therein). If a fluid is involved and material is dissolved and
reprecipitated, the process is often referred to as
dissolution–precipitation creep (DPC). Grain boundary sliding and diffusive
mass transfer are both always involved, so the terms diffusion creep and
DPC are collective terms involving GBS and diffusive mass transfer.
Diffusion creep is a grain-size-sensitive (GSS) deformation mechanism and
may operate at low or high temperature, as well as typically at low stresses
(e.g., Paterson, 1995).
Many researchers have pointed out the close relationship between strain or
reaction-dependent grain size reduction and the activation of GSS creep in a
variety of mafic assemblages (e.g., Kruse
and Stünitz, 1999; Kenkmann and
Dresen, 2002; Baratoux et al.,
2005; Kanagawa et al., 2008;
Mehl and Hirth, 2008; Menegon
et al., 2015; Okudaira et al., 2015;
Degli Alessandrini et al.,
2017). For this reason, grain size reduction is recognized as one of the
most significant mechanisms that controls rheological properties (e.g.,
Elyaszadeh et al., 2018;
Brodie and Rutter, 1987;
Bercovici and Ricard, 2012;
Montési, 2013,
Platt, 2015). Grain-size-controlling processes
usually include dynamic recrystallization (e.g., Schmid,
1982; Brodie and Rutter, 1987; Behrmann, 1985;
Fliervoet and White, 1995;
Vissers et al., 1997) and/or
metamorphic reactions (e.g., Rubie, 1983;
Fitz Gerald and Stünitz, 1993;
Stünitz and Fitz Gerald, 1993;
Newman et al., 1999), but while dynamic
recrystallization is only considered to have transient mechanical effects
(Brodie and Rutter, 1987), a small grain size can be
stabilized by new phases in phase mixtures wherein grain growth is inhibited
by the pinning of grain boundaries (e.g., Olgaard and
Evans, 1986, 1988;
Fliervoet et al., 1997;
Herwegh et al., 2011;
Herwegh and Berger, 2004). Furthermore, phase separation
and compositional layering commonly form or develop during crystal plastic
deformation (dislocation creep) of minerals. During metamorphic reactions
and nucleation of new phases, minerals are spatially rearranged so that
fine-grained mixed-phase zones and polyphase shear bands may develop (e.g.,
Stünitz and Tullis,
2001; De Ronde et al., 2004,
2005;
Kilian et al., 2011;
Platt, 2015;
Mansard et al.,
2018, 2020). Such a spatial rearrangement
controls the bulk strength of the rock, particularly when these phases have
a large mechanical contrast. In particular, the interconnection of weak
materials is necessary to induce a significant drop in bulk strength (e.g.,
Jordan, 1988; Handy, 1994;
Dell'Angelo and Tullis, 1996;
Holyoke and Tullis, 2006a,
b).
The principal objective of this contribution is to study the effect of
initial rock composition on the feedback processes between reaction and
deformation. To do so, we have performed rock deformation experiments on
“wet” assemblages of plagioclase–pyroxene assemblages in a Griggs-type
solid-medium deformation apparatus. As representative of the lower crust,
the starting material was composed of plagioclase (labradorite; plag) and
either Mg-rich orthopyroxene (opx) (from peridotite) or Fe-rich opx (from a
granulite-facies anorthosite) in order to investigate the effect of
different mineral compositions on rock deformation. In this system, the opx
deformation properties are assumed to be the same for Mg- and Fe-rich opx.
We also performed deformation experiments on amphibole (amph) + plag and
pure amph assemblages at similar conditions to extend the study to typical
amphibolite facies conditions. All these assemblages were deformed to
varying amounts of strain, including at the early stages of deformation.
These early stages can be challenging to access when studying natural cases
because of successive overprints of deformation stages. In this
contribution, we suggest that viscous strain localization is primarily
dependent on the ability of minerals to react and that the feedback between
deformation and mineral reactions can lead to large differences in
mechanical strength and deformation processes.
MethodsExperimental proceduresStarting material and sample preparation
We have performed a series of shear deformation experiments in two
Griggs-type deformation apparatuses at the University of Tromsø (Norway)
and at the University of Orléans (France). Experiments were conducted on
mineral powders separated from natural materials. Four different starting
materials were prepared from different mineral sources: (1) gem-quality
labradorite (An60–Ab38–Or2) from Sonora (Mexico) mixed with
orthopyroxene (Wo1–En88–Fs11) from Damaping peridote (China),
here referred to as Mg-opx; (2) labradorite
(An55–Ab44–Or1) mixed with orthopyroxene
(Wo2–En62–Fs36) from Hidaka granulite (Japan), here
referred to as Fe-opx; (3) amphibole (Mg-rich hornblende; composition
available in Table 1) from the Massif Central (France) mixed with Sonora
labradorite; and (4) pure Mg-rich hornblende. The pre-separated minerals
were crushed in an alumina mortar, then sieved to <100µm,
handpicked, and finally sorted in a distilled water column to obtain grain
sizes between 10 and 20 µm. Powders were mixed in a 50:50 vol. % ratio
in acetone using an ultrasonic stirrer to avoid density and/or grain size
separation (De Ronde et al.,
2004, 2005).
Chemical compositions of plagioclase, pyroxene, and amphibole
starting materials.
Sonora plag Damaping Mg-opx wt. % oxidesIons per 8Owt. % oxidesIons per 6OSiO253.872.43455.711.929Al2O329.411.5663.810.155CaO11.680.5650.420.016Na2O4.060.3560.070.005K2O0.460.0270.020.001MgO0.090.00632.511.678TiO20.080.0030.070.002FeO0.380.0147.210.209MnO0.050.0020.180.005Total100.084.972100.014.000An60 Wo1 Ab38 En88 Or2 Fs11 Hidaka plag Hidaka Fe-opx wt. % oxidesIons per 8Owt. % oxidesIons per 6OSiO254.712.44852.861.952Al2O329.281.5441.000.044CaO11.420.5481.230.049Na2O4.940.4290.060.004K2O0.220.0130.020.001MgO0.010.00122.411.234TiO20.000.0000.210.006FeO0.390.01522.550.696MnO0.090.0030.460.014Total100.065.000100.804.000An55 Wo2 Ab44 En62 Or1 Fs36 Massif Central amph wt. % oxidesIons per 23OSiO243.996.646Al2O39.911.765CaO11.121.800Na2O1.670.489K2O0.470.091MgO10.872.448TiO21.210.138FeO17.792.248MnO0.380.000Total97.4115.625Magnesio-hornblende
To perform experiments, we used a conventional solid-salt, non-coaxial
(“general shear”) sample assembly with alumina pistons
(Précigout et al., 2018). After adding
0.1 wt. % of distilled H2O, the powder was placed between alumina
forcing blocks along a 45∘ precut so that a shear zone
∼ 1 mm thick is formed when the deformation experiment starts
(Fig. 1). The assembly was wrapped into a nickel foil 25 µm thick
and then inserted into a weld-sealed platinum jacket. NaCl pieces were used
as solid confining medium for both the inner and outer furnace assembly.
The temperature was measured by S-type (Pt / Pt–Rh) thermocouples centered on
the sample. Readers are invited to refer to
Pec et al. (2012b) and
Précigout et al. (2018) for further
details and descriptions of sample assemblies and experimental protocols
employed.
Drawing of the sample assembly in the Griggs-type apparatus.
Experiments and mechanical data processing
Deformation experiments were conducted at a constant shear strain rate of
∼ 2 × 10-5 s-1 to varying amounts of shear strain
(see Table 2 for a summary of experimental conditions), at temperatures of
800, 850, and 900 ∘C, and at a confining pressure of 1 GPa. To bring
the samples to the desired pressure–temperature (P–T) conditions, both the
σ1 and σ3 pistons are alternatingly advanced
between steps of increasing temperature. At the desired P–T conditions, a
period of hydrostatic hot-pressing was applied, and the deformation was
started by advancing the σ1 piston first through the lead piece
(“lead run-in”) to bring it into contact with the upper forcing block (hit
point). During the lead run-in stage, the sample is maintained in a more
or less isostatic stress state, as the lead protects the sample from
becoming deformed. Two series of experiments on Mg-rich opx-bearing
assemblages have been performed: one series with a short run-in period
and a second one with a longer period (Table 2).
List of experiments and experimental conditions. PS: peak
stress, D: deformed samples to varying amounts of shear strain. A cross is
added to the samples for which the forcing blocks started to slip at the sample
interface. τpeak: differential stress at peak, τflow:
steady-state differential stress, τend: differential stress at
end of experiment, γ: shear strain, th0: thickness initial of
the shear zone, thf: final shear zone thickness, t: time before hit
point.
At the end of experiments, samples were quenched to 200 ∘C within
2 to 3 min (∼ 150 to 300 ∘C per minute) so that the
deformation microstructures and grain size were preserved. Subsequently, the
force and confining pressure were decreased simultaneously to room pressure
and temperature conditions. During initial stages of the decompression, the
differential stress is kept above the confining pressure (∼ 100 to 200 MPa) to reduce the formation of unloading cracks.
Experimental data were acquired and recorded using catman®
Easy and then processed after the experiment with a MATLAB-based program
following the “rig” program of Matej Pec (Pec et al., 2016; available at
https://sites.google.com/site/jacquesprecigout/telechargements-downloads, last access: 21 March 2020).
The hit point is defined by intersecting the run-in and loading curve
tangent lines. After this point, several corrections are then applied to
consider the rig distortion and changes in thickness and surface due to the
sample compaction and piston overlap, respectively. The corrected mechanical
data are represented in stress vs. strain graphs from the hit point.
Analytical methods
Double-polished thick sections (∼ 150–200 µm) of the
starting materials were prepared for Fourier transform infrared (FTIR) analysis. Thick sections were
prepared from mineral powders for the Mg-rich opx + plag sample and from a
natural section of mylonite for the Fe-rich opx + plag sample. For each
sample, only grains larger than 100 µm were used for FTIR analysis.
Infrared absorption spectra were collected for Mg-opx, Fe-opx, and plag using
a Nicolet 6700 FTIR instrument at the ISTO (Orléans, France); 128 scans
were acquired for each spectrum at a resolution of 4 cm-1 with a spot
size of 40 × 40 µm2. Only grain interiors were analyzed
by FTIR. The integrated areas of the absorption bands measured between 3750
and 3000 cm-1 were used to calculate the H2O contents
using the calibration of Bell et al. (1995) for opx and of Johnson
and Rossmann (2003) for plag.
After the experiments, samples were cut parallel to the shear direction and
impregnated under vacuum with low-viscosity epoxy to prepare thin sections.
Sample microstructures were analyzed using a scanning electron microscope
(SEM – TESCAN MIRA 3 XMU) at ISTO-BRGM (Orléans, France). All SEM
analyses have been performed on carbon-coated (20 nm thickness) thin
sections at 12–15 kV and a working distance of ∼ 8 mm. Mineral
compositions were collected using a CAMECA SX Five electron microprobe
analyzer (EPMA) at ISTO-BRGM (Orléans, France). We adopted the following
analytical condition: an acceleration voltage of 12–15 kV, a beam current
of ∼ 6 nA, and a beam diameter of ∼ 1µm.
Thin sections were additionally polished with colloidal silica and then
coated with a thin carbon coat ∼ 2 nm thick for electron
backscatter diffraction (EBSD) analysis. The EBSD analyzes were carried out
using an EDAX PEGASUS EDS/EBSD system and the OIM DC 6.4 software
(manufacturer EDAX; Mahwah, USA) at ISTO-BRGM (Orléans, France). The
operating conditions involved an accelerating voltage of 20–25 kV and a
working distance of 15–18 mm. Post-acquisition treatments, which include
plotting equal-area lower-hemisphere pole figures of amphibole lattice
preferred orientation (LPO), were performed using the open-source MTEX
toolbox for MATLAB. For the definition of a grain, five adjacent indexed points
were required. Texture strength is expressed through the J index and M index
(Bunge,
1982; Skemer et al., 2005).
Microstructural analysis
SEM backscattered electron (BSE) images were used to produce manually
digitized grain maps. Grain sizes were measured from these grain maps by
using the public domain software ImageJ (http://rsb.info.nih.gov/ij/, last access: 11 March 2020). By extracting the area-equivalent diameter
from these maps, the grain size is defined as the diameter of equivalent
circular diameter dequ=2×areaπ. From these grain maps, the grain shape preferred
orientation (SPO) and the modal proportion of phases can be analyzed. In
the case in which grain boundaries are indistinguishable within amphibole
aggregates, EBSD maps have been processed with MTEX to determine the grain
size.
In order to estimate the proportion of phases we used SEM BSE images to
produce manually digitized grain maps with the illustrator software when it
was possible to distinguish the grain boundaries (e.g., Fe-rich opx + plag
assemblages). From these phase maps we could separate the phases with the
ImageJ software and estimate their proportion. This is how the amount of
amphibole in the Fe-rich opx + plag assemblages is estimated. When it was
impossible to distinguish the grains individually, we drew areas that
corresponded either to a set of grains of the same phase or to several
phases that could not be separated. Some grains are too small to be
separated with enough confidence from other grains. For this reason, we have
included all reaction products together and have not separated the
amphibole from the plagioclase2 and the pyroxene2 in the Mg-rich opx + plag assemblages.
ResultsMechanical data
Depending on the starting material, the mechanical data differ significantly
with respect to each other (Fig. 2). At 850 and 900 ∘C, the
Mg-rich opx + plag samples (Fig. 2b–c) are characterized by a pronounced
peak stress at shear strains of less than γ∼ 1,
whereas the Fe-rich opx + plag ones do not show a pronounced peak stress
but a steady-state flow after yield or, at 900 ∘C, a constant
strain-hardening behavior (Fig. 2d). At 800 ∘C, the peak stresses
of all opx + plag samples are above the Goetze criterion (Δσ≤Pconf), which provides an empirical upper
limit for viscous creep. Above this limit, samples are expected to deform by
brittle mechanisms (Kohlstedt et al., 1995). The abrupt
stress drop in Fe-rich opx + plag samples at 800 ∘C and short
run-in samples at 800 and 850 ∘C indicates that slip has occurred
at the interface between the sample and one forcing block (Fig. 2b, d;
557NM, 559NM, and 538NM). The slip is confirmed by sample microstructures
and suggests that peak stress might have been higher without the slip event.
In contrast, two Mg-rich opx + plag samples at 900 ∘C show a
pronounced strain weakening after peak stress. In one case, the sample falls
substantially below the Goetze criterion as a result of slip along the
forcing block interface and stabilizes around 800 MPa (OR49NM). In the other
case, the sample weakens continuously after peak stress and reaches almost
steady state for γ∼ 6.5 (OR41NM). One way to vary the
time of reaction in different experiments is to keep the sample at
hydrostatic conditions at given pressure and temperature before starting to
deform it (long or short run-in periods of the piston). In the experiments
with longer run-in periods, the Mg-rich opx + plag assemblages weaken
systematically after peak stress. They reach stresses lower than 400 MPa at
850 and 900 ∘C, corresponding to ∼ 64 % (OR38NM) and ∼ 78 % (OR34NM) of the peak stress
before reaching a quasi-steady-state shear stress near γ∼ 6.5 for OR34NM and γ∼ 7.7 for
OR38NM (Fig. 2c). At 850 ∘C, the Fe-rich opx + plag sample
behavior is no longer comparable to that of the mixed Mg-rich assemblages,
as no weakening occurs after peak stress and a quasi-steady-state shear
stress is reached at low shear strain (γ∼ 2). At
900 ∘C, the sample remains weak but hardens continuously until
γ∼ 5, unlike the other experiments (Fig. 2d).
Mechanical data. (a) Terms used to describe the different stages
of an experiment. (b–c) Differential stress (MPa) versus shear strain
(γ) showing the mechanical behavior of the Mg-rich opx + plag
assemblages deformed at temperatures ranging from 800 to 900 ∘C,
at constant confining pressure of 1 GPa, and at a strain rate of 10-5 s-1. The difference between (b) and (c) is related to the different
duration of the “run-in” section, i.e., time spent at P–T conditions before
the hit point. Mechanical data for Fe-rich opx + plag (d) as well as amph + plag
and pure amph (e) assemblages are also plotted in stress–strain graphs. opx:
orthopyroxene, pl: plagioclase, amph: amphibole.
Compared to the opx + plag assemblages, the amph + plag and pure amph
assemblages do not reach the same strength, regardless of the deformation
temperature (Fig. 2e). At 800 ∘C, the amph + plag assemblage
reaches a peak stress of ∼ 563 MPa and then slightly weakens
to ∼ 483 MPa at γ∼ 5.8. The pure amph
assemblages deformed at 800 and 900 ∘C show stress–strain curves
with a significant weakening after peak stress at γ∼ 0.5 to 1.0. However, the sample at 800 ∘C documents a peak stress
of ∼ 450 MPa higher than the sample deformed at
900 ∘C.
Our set of experiments reveals two distinct types of mechanical behavior:
one that shows a pronounced weakening after high peak stress (Mg-rich opx
+ plag and pure amph assemblages) and one without weakening (Fe-rich opx
+ plag and amph + plag assemblages) or even hardening (Fe-rich sample at
900 ∘C, 532NM; Fig. 2d) but that deforms at considerably lower
stresses (all Fe-rich samples; Fig. 2d).
Mineral reactions and microstructures
Mineral reactions occur pervasively in deformed portions of the samples –
there is a clear correlation between strain and reaction progress (Mansard et al., 2020). The pervasive
occurrence of mineral reactions induces substantial changes in the grain size
and spatial distribution of phases. This is particularly prominent in the
development of shear bands within the deformed assemblages. For the sake of
clarity, the term “bulk shear zone” refers to the whole sample deformed
between the two alumina forcing blocks, while the term “shear bands”
refers to a localized zone of variable thickness of high shear strain
accommodation within the bulk shear zone. In addition, the term
“high-strain zone” is also used to refer to the domain of coalescence of
fine-grained shear bands that are connected as a more or less single zone
through the bulk shear zone.
In the two main systems studied, the Fe-rich and Mg-rich opx + plag
assemblages, the opx deformation features are the same. In these deformed
assemblages, similar reaction products are observed, with the formation of
amphibole, pyroxene, plagioclase, and, to a minor extent, quartz.
opx1+plag1+H2O→amph+opx2+plag2±cpx±qtz
In these assemblages the mineral compositions were analyzed if the grain
size was sufficiently large because in mixture zones, individual grains are
typically too small to be analyzed. Thus, the very small grain size
precludes an exact compositional analysis of the actual reaction products.
The Mg-rich opx + plag assemblages deformed to high shear strain are
characterized by the development of low-strain zones and a high-strain zone
in the center of the bulk shear zone (Fig. 3a). Mineral reactions are
localized within this single connected zone that traverses the sample from
one interface of the forcing block to the other (at 850 ∘C; Fig. 3a) as fine-grained mixed zones and C-geometry shear bands subparallel to
the shear plane (or forcing block interface; Figs. 4a–c, 5a–f). The
C-type shear bands are mainly composed of fine opx2, plag2, and
amph that are present as equant grains; their identification is
possible by EDS analysis in the SEM (Fig. 4). The original large grains of
plag1 and opx1 form porphyroclasts embedded in a mixture of
reaction products (Fig. 3a). In contrast to the high-strain zone, the
reaction products in the low-strain zones usually occur as coronas or rims
surrounding opx1 clasts (Fig. 4d–e). The main difference between the
assemblages deformed at 850 and 900 ∘C is the
degree of strain localization. At 850 ∘C, strain is highly
localized in a ∼ 250–300 µm wide single zone, while strain is
more distributed throughout the sample at 900 ∘C, and therefore
local strain appears lower. Otherwise, the microstructural features are
similar to those of the high-strain zone (Fig. 4f–g).
Distribution of reaction products after deformation in the Mg-rich
opx + plag (a), Fe-rich opx + plag (b), and amph + plag (c)
assemblages. For each assemblage, a manually digitized overview of the shear
zone is associated with a zoomed part of it. The mechanical data associated
with these assemblages are also represented in stress–strain graphs.
SEM BSE images representative of microstructures observed in
deformed Mg-rich opx + plag assemblages; opx2 and plag2 are the
main reaction products. (a–b) At 850 ∘C, mineral reactions are
mainly localized in the high-strain zones in the form of fine-grained mixed
zones. (c) The original plag1 almost completely disappears. (d) In
low-strain zones, the reaction products appear as coronas around the
original opx1 and as aggregates; (e) opx1 is locally fractured and
filled with reaction products. (f–g) Similar microstructures are observed at
900 ∘C, although deformation is less localized compared to the
assemblage deformed at 850 ∘C. opx: orthopyroxene, pl:
plagioclase, amph: amphibole.
At the scale of the bulk shear zone, both the Fe-rich opx + plag and amph
+ plag assemblages are banded and show a locally developed, nonconnected
mylonitic foliation characterized by the development of amph-rich shear
bands and tails at opx1 porphyroclasts (Fig. 3b–c). These bands wrap
around the original and large opx1 porphyroclasts in Fe-rich opx + plag assemblages (Fig. 3b) and in amph + plag assemblages (Fig. 3c). The
reaction products in shear bands alternate with aggregates of original
grains. The mylonitic foliation is better defined in the amph + plag
assemblage because of more pervasive deformation and strain localization
(Fig. 3c). The shear bands are more heterogeneously distributed in the
Fe-rich opx + plag assemblage deformed at 850 ∘C (Fig. 3b).
This results in the development of an anastomosing network of amph-rich
shear bands that are less connected (Fig. 3b), without forming any
throughgoing high-strain zone (Fig. 3a).
The reaction products in the Fe-rich opx + plag assemblages contain less
opx2 than the Mg-rich opx + plag assemblages, and amph + plag2
represents the main reaction products (Figs. 5g–j, 6). From site to site in the
Fe-rich opx + plag sample deformed at 850 ∘C, a variable
proportion of opx and plag has reacted. Mineral reactions preferentially
occur in strongly deformed areas and form only locally interconnected
separate aggregates and shear bands consisting of amph + opx2/ cpx or
plag (Fig. 6a–b), while in less deformed areas, the reaction products
typically occur as thick rims at grain boundaries of extensional sites of
opx1 and plag1 clasts (Fig. 6c). The amph grains are elongated and
typically form tails extending from opx1 porphyroclasts (Fig. 6b, c).
At 900 ∘C, the Fe-rich opx + plag sample is more homogeneously
deformed with a similar distribution of reaction products as observed at
850 ∘C (Fig. 6d–e).
SEM BSE images representative of the different small-scale
microstructures encountered in the assemblages and their respective manually
digitized phase maps. Please note that the resolution is too low to resolve
the full extent of phase mixing. opx: orthopyroxene, cpx:
clinopyroxene, pl: plagioclase, amph: amphibole, qtz: quartz, gt:
garnet, zo: zoisite.
SEM BSE images representative of microstructures documented in
deformed Fe-rich opx + plag assemblages. Amph and plag2 are the main
reaction products. At 850 ∘C, the deformation is heterogeneously
distributed, and a strain gradient is clearly apparent at sample scale.
(a–b) In strongly deformed parts, the amph appears as partially connected
shear bands, while in the other parts (c) it appears as thick coronas around
the opx1 and plag1 clasts. (e–f) At 900 ∘C, the deformation is
more homogeneously distributed, and the reaction products appear as reaction
coronas. opx: orthopyroxene, cpx: clinopyroxene, pl: plagioclase,
amph: amphibole.
The amph + plag assemblage (Figs. 5k–l, 7a–c) deformed at
800 ∘C produced a large quantity of reaction products composed of
amph2, plag2, clinopyroxene (cpx), and minor zoisite (zo). There
is a spatial relationship between amph and cpx, the latter occurring as
small grains, predominantly around amph porphyroclasts (amph1) mixed
with small grains of new amph2 (Fig. 7a–b). Unlike Fe-rich opx + plag
assemblages, the shear bands involve several phases (amph and cpx) in the
amph + plag assemblages (Fig. 5k–l). Regarding the pure amph assemblages
deformed at 800 and 900 ∘C (Figs. 5m–n, 7d–e), mineral reactions
are homogeneously distributed with the formation of amph2, cpx and
minor zo, quartz (qtz), and garnet (grt). Reaction induces the development of
mixture zones of amph2 and cpx and unmixed zones of grt, zo, and qtz
(Fig. 5m–n). No melt was detected in the microstructures.
SEM BSE images representative of microstructures shown in deformed
amph + plag and pure amph assemblages. (a, b, c) Nucleation of amph-rich
layers in the amph + plag assemblage deformed at 800 ∘C. (d–e)
The pure amph assemblages deformed at 800 and 900 ∘C show the development of fine-grained mixture zones of amph2 and cpx.
opx: orthopyroxene, cpx: clinopyroxene, pl: plagioclase, amph:
amphibole, gt: garnet, zo: zoisite.
Chemical composition
The chemical composition of new grains is systematically different compared
to that of the original ones. In each assemblage, the original plag1
composition is An55–60. These grains are rimmed by more
albite-rich plag2 (An38–45 in Fe-rich opx + plag samples, An48–55
in Mg-rich opx + plag samples; Figs. 4, 5, 7, 8a–a′). The fine grains in
mixed zones are also more albite-rich than the starting plag1 (Fig. 8a–a′). For the opx2, the ferrosilite content increases with respect to
the original opx1, regardless of the original opx1 composition
(Fig. 8b). The XMg ratio ranges between ∼ 0.85 and
∼ 0.89 in the Mg-rich opx2 and between ∼ 0.64 and ∼ 0.68 in the Fe-rich opx2 (with XMg = Mg / (Mg+Fe2+)). The newly formed cpx in the amph + plag and
pure amph assemblages has an augite composition (Fig. 8b). It is worth
mentioning that the composition of the main reaction products (orthopyroxene
and plagioclase) in the Fe-rich opx + plag assemblages are farther away
from the starting composition compared to those of the Mg-rich opx + plag
assemblages.
Mineral composition plots. Plagioclase compositions in the Fe-rich
opx + plag (a) and Mg-rich opx + plag (b) assemblages plotted on the
ternary diagram of orthoclase (KAlSi3O8), albite
(NaAlSi3O8), and anorthite (CaAlSi3O8). (c) Pyroxene
compositions plotted on the ternary diagram of wollastonite
(Ca2Si2O6), enstatite (Mg2Si2O6), and
ferrosilite (Fe2Si2O6). The chemical composition of plagioclase
and pyroxene is divided into three subsets: clast core, clast rim, and fine
grains. (d) Classification of amphibole in a graph of Mg / (Mg + Fe2+) versus
Si content for the case of Ca ≥ 1.5, (Na + K)A < 0.5, and CaA < 0.5.
The composition of the original amph used in both amph + plag and pure
amph experiments is not constant and varies in the magnesio-hornblende field
(Fig. 8c). The newly formed amph for all assemblages has a composition
ranging from magnesio-hornblende to tschermakite (Fig. 8c). There are
nonetheless four chemically distinct populations of new amph, depending on
the composition of the starting mixture, as shown in the plot of Si vs. XMg
(Fig. 8c).
OH content and thermodynamic modeling
Fe-rich opx and plag grains from Hidaka granulite show a broad and
asymmetric infrared (IR) water absorption, with a maximum amplitude at ∼ 3580–3590 cm-1 (Fig. 9). The average H2O contents (ppm
H2O by weight), calculated between 3750 and 3000 cm-1,
are 451 ± 35 ppm for Fe-rich opx and 226 ± 24 ppm for plag. In contrast,
there are no H2O IR absorption bands for Mg-rich opx grains from
Damaping peridotite and plag grains from Sonora (Fig. 9).
Representative Fourier transform infrared (FTIR) spectra of
orthopyroxene and plagioclase starting materials. m: average water
content.
Pseudo-sections for the Mg-rich opx + plag and Fe-rich opx + plag
assemblages were calculated for different H2O contents in the
simplified but representative system of our experiments
(Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O) using the
Perple_X 6.6.8 package
(Connolly, 2009) combined with the updated
database of Holland and Powell (1998) and the
following solution model: amphibole
(Dale et al., 2005), orthopyroxene
(Powell and Holland, 1999), garnet, clinopyroxene (Holland
and Powell, 1998), and feldspar
(Newton
et al., 1980).
At the experimental P–T conditions, plag, opx, amph, qtz, cpx,
and grt are expected to be stable phases in the Fe-rich opx + plag
assemblage; plag, opx, amph, and qtz are expected to be stable phases in the Mg-rich opx + plag
assemblage. The modeling of the Fe-rich opx + plag assemblage agrees
rather well with the observed reaction product assemblage, whereas the
modeling of the Mg-rich opx + plag assemblage does not match the observed
reaction product assemblage.
Initial shear localization
In the Mg-rich opx assemblages, shear deformation is initially localized
between the boundaries of original opx1 and plag1, where the
nucleation of fine-grained tails of mixed phases that define the general
shear foliation occurs (Fig. 10a). Everywhere, the new grains of opx2
are pervasively mixed with plag2 and amph grains (Fig. 10a). In
contrast, the initial strain localization in the Fe-rich opx + plag
assemblages predominantly forms amph + less opx2 shear bands or tails
extending from opx1 porphyroclasts. Amph + opx2 nucleates at
boundaries between opx1 and plag, preferentially at extensional ends of
opx1 grains, defining σ-type tails that stay connected to the
original opx1 grains (Figs. 6b, 10b). In the samples of amph + plag
starting material, the reaction products cpx + amph2+ plag2+ zo nucleate at the boundaries between original amph1 and plag1
grains (Figs. 8a–c, 10c). Amph2+ cpx tends to form mixed layers that
surround and stay connected to amph1 porphyroclasts (Figs. 7a, b, 10c),
while plag2+ zo forms separate aggregates that tend to surround the
amp2+ cpx aggregates (Figs. 8c, 10c). Both types of reaction products
organize into an anastomosing network of thin amph2+ cpx and
plag2+ zo in which layers become progressively more parallel to the
shear plane (Fig. 10c).
SEM BSE images of incipient nucleation and shear localization.
(a) In the Mg-rich opx + plag assemblages, fine-grained tails of mixed
phases nucleate at the edges of original grains. In the Fe-rich opx + plag
(b) and amph + plag (c) assemblages, the nucleation is fairly monophase.
The new grains tend to organize into an anastomosing network of thin amphibole
and zoisite (orange triangle). opx: orthopyroxene, pl: plagioclase,
amph: amphibole, zo: zoisite.
To summarize, two types of reaction products and related microstructures
form in the two different starting material assemblages: (1) in the Mg-rich
opx + plag samples, fine-grained phase mixtures are produced by nucleation
of reaction products and localize into more or less contiguous bands
approximately parallel to the forcing block interface, i.e., the shear
plane. (2) In the Fe-rich opx + plag and amph + plag samples,
predominantly amph + opx2 and plag2+ zo shear bands and tails
develop from opx1 porphyroclasts and stay connected with these. While
fine-grained reaction products in case (1) produce C-type shear bands, the
sample fabrics of case (2) develop S–C'-type shear band geometries in
strongly deformed areas, the C' bands of which are predominantly formed
amph and plag2.
Abundance and grain size of reaction products
The transition from low-strain to high-strain zones in the Mg-rich opx + plag assemblages is accompanied by a significant grain size reduction (mode
of the distribution as the dominant grain size) from ∼ 15 µm to
∼ 0.2 µm (Fig. 11a). The high-strain zones concentrate most of the
reconstituted material with roughly ∼ 65 % and locally more than
∼ 80 % of reaction products. This proportion is substantially higher
compared to that in the low-strain zones (∼ 25 %), where no mixed-phase layers develop. Similarly, in the mixed-phase reacted zones of the
original pure amph assemblages, the new grains are very small (∼ 0.6 µm) and they represent nearly ∼ 61 % of the assemblage (Fig. 11b). The amph grain size is also reduced in Fe-rich opx + plag
assemblages compared to the starting material (Fig. 11c), but the reaction
products are not as pervasively mixed and show a grain size that is
approximately 1 order of magnitude larger (1.9 µm) than that in the
Mg-rich opx + plag samples (0.2 µm; Fig. 11a). In the 850 ∘C sample, the reaction products are largely connected to opx1
porphyroclasts, and their proportion increases with increasing proximity to
higher-strained portions of the shear zone (∼ 26 %). In these Fe-rich
opx + plag assemblages, amph is the main reaction product (∼ 17 %).
In contrast, the quantity of reaction products at 900 ∘C remains
roughly constant at approximately ∼ 29 % and there is no strain
gradient. Finally, in the amph + plag starting material, the nucleation in
amph-rich shear bands is more extensive (∼ 36 %) and the grain size
of cpx reaction products decreases to ∼ 1.3 µm (Fig. 11d).
Grain size distribution reported as a histogram of grain size
versus density for different category of grains in the Mg-rich opx + plag
(a), pure amph (b), Fe-rich opx + plag (c), and amph + plag (d)
assemblages. Overall, there is a significant and systematic reduction in
grain size between the original grains and the reaction products. The lognormal distribution curves are fit for each grain size distribution. The
modal proportion (vol. %) of reaction products and original grains of each
assemblage is also reported.
Analysis of SPO and amphibole LPO
Amph formed during deformation of Fe-rich opx + plag assemblages exhibits
a distinct SPO oriented at ∼ 30∘ to the shear direction (Fig. 12a–b). These amph aggregates occur as reaction rims around original
opx1 grains and form in strongly deformed areas partly interconnected
to aggregates that define S–C' fabrics (Fig. 12c). Plag2 and
opx2 also reveal a well-defined SPO similarly oriented to amph. The
preferred orientation of amph forms an angle of ∼ 30∘ with
the shear plane, slightly greater than that of plagioclase and original opx
(Fig. 12a–b). In addition, the amph reaction seams in high-stress sites of
opx porphyroclasts are significantly thinner compared to those in low-stress
or extensional sites, indicating that amph grows preferentially in strain
shadows. In the higher-strained portions of the shear zone deformed at
850 ∘C, the foliation defined by amph-dominated layers rotates
and is now well-oriented subparallel to the boundaries of C'
shear bands or the shear plane (Fig. 12c). Within these shear bands, amph
grains show a moderate LPO with [001] axes aligned subparallel to the
boundaries of C' shear bands and poles (100) normal to the shear plane (Fig. 12c–d). In the mixed zones of Mg-rich opx + plag samples, the fine grains
are characterized by equant grain shapes with an aspect ratio of 1.23 (Fig. 13) and a weak preferred orientation either parallel or at 45∘ to
the shear plane (Fig. 13).
SPO and amph LPOs in the Fe-rich opx + plag assemblages. (a–b)
Rose diagrams of the SPO of plag1, opx1, and amph (a: 533NM – 850 ∘C, b: 532NM – 900 ∘C). (c) Strongly deformed part
of the Fe-rich opx + plag assemblage deformed at 850 ∘C. (d)
Amph LPOs are shown for the (100) plane, (010) plane, and [001] axis with
respect to the shear direction, considering one point per grain. Number of
data points: 365; step size: 100 nm; average grain size of amphibole: 4.1 µm.
pfJ: pole figure texture index, MUD: multiple of uniform distribution,
opx: orthopyroxene, pl: plagioclase, amph: amphibole.
Rose diagram of the SPO of undifferentiated reaction products in
fine-grained mixed zones in the Mg-rich opx + plag assemblages. opx:
orthopyroxene, pl: plagioclase, amph: amphibole.
DiscussionNucleation, grain size reduction, and phase mixing
In this study, the deformation of two-phase assemblages is accompanied by
the nucleation of new grains with (1) systematic differences in composition
between new grains and parent grains (Fig. 8), (2) significant grain size
reduction (Fig. 11), and (3) a new spatial arrangement of reaction products into fine-grained mixture zones (Fig. 5). Based on these
observations, we infer that grain size reduction and phase mixing both
result from the nucleation of new phases (e.g.,
Kruse and Stünitz, 1999;
Kenkmann and Dresen, 2002;
Kilian et al., 2011;
Linckens et al., 2015;
Platt, 2015;
Précigout and Stünitz, 2016;
Mansard et al., 2018). Despite
common features, our samples have developed two sets of microstructures
distinguished mainly by the size and spatial arrangement of reaction
products and the degree of phase mixing. We discuss these differences below.
In the Mg-rich opx + plag assemblages, a strain gradient is clearly
observed and expressed by the development of low- and high-strain zones
(Figs. 3a, 4). This is particularly notable in the sample deformed at
850 ∘C. The transition from a low- to high-strain zone is
accompanied by drastic grain size reduction and an increase in reaction
products within localized mixed zones by a factor of more than 2 (Fig. 11a).
The grain size of reaction products in the mixed high-strain zones
(∼ 0.2 µm) is about 2 orders of magnitude smaller than
the opx grain size of the starting material and the low-strain regions
(∼ 10–20 µm; Fig. 11a). Similarly, an extensive
nucleation of reaction products within mixed-phase zones is documented in
the pure amph starting material (Fig. 7d–e). The reaction products are also
very small, far below 1 µm (∼ 0.6 µm; Fig. 11b). As
documented in Mansard et al. (2020), such a
correlation between deformation, mineral reactions, and related grain size
reduction in Mg-rich assemblages has been also observed in experimentally
deformed fine-grained gneiss (Holyoke and
Tullis, 2006a, b), plagioclase + olivine samples (De Ronde et
al., 2005; De Ronde and Stünitz, 2007), and plagioclase + pyroxene samples (Marti et al., 2018).
In the Fe-rich opx + plag assemblages, amph is by far the more abundant
product, and there is less phase mixing (Fig. 5g–j). The reaction products
consist mainly of thick rims around the opx1 (Fig. 6c) or
amph-dominated tails and shear bands (Fig. 6a–b). The abundance of reaction
products increases with increasing proximity to strongly deformed portions
of the shear zone. The reaction products have a grain size about 1 order
of magnitude smaller than the starting material (∼ 1.9 µm) but 1 order of magnitude larger than the mixed reaction products in
the Mg-rich opx + plag assemblages (Fig. 11). Similarly to the Fe-rich opx
+ plag assemblages, amph-rich shear bands have developed in the amph + plag assemblages (Fig. 7a–c), and the grain size, reduced by about 1 order
of magnitude compared to the starting material, is 1 order of magnitude
larger than in the mixed zones in the Mg-rich opx + plag assemblages (Fig. 11).
Two types of major microstructures can be distinguished: (1) in one case,
intense grain size reduction by 2 orders of magnitude is produced by
nucleation of reaction products in pervasive and layered phase mixtures
(Mg-rich opx + plag and pure amph starting materials); (2) in the other
case, the reaction products are less pervasively mixed and develop
aggregates that are dominated by amph extending from opx porphyroclasts.
These aggregates have a grain size 1 order of magnitude larger than (1)
and may form C' shear bands or tails connected to opx
porphyroclasts.
Deformation processes
As the reaction products in Mg-rich opx + plag samples occur in
pervasively mixed high-strain zones and layers with equant grains of a size
far below 1 µm, the dominant deformation mechanism is inferred to be
one of grain-size-sensitive creep. The absence of well-developed layering of
separate phases and absence of strong elongation of individual grains during
GSS creep suggest that grain boundary sliding makes the dominant kinematic
contribution to the finite strain (Rachinger sliding; e.g.,
Langdon, 2006). Such a deformation mechanism should probably be
termed diffusion-accommodated grain boundary sliding (GBS) or, more
generally, as a fluid has been present and solution precipitation is the
probable transfer mechanism, dissolution–precipitation creep (DPC). The
pronounced weakening of the samples in combination with localization of
strain made strain rate stepping tests problematic in our samples, so
the deformation mechanism is primarily identified based on microstructures.
At first, the microstructures are similar to other cases in which such
deformation mechanisms have been identified (e.g.,
Marti et al., 2017, 2018;
De Ronde et al., 2005;
Stünitz and Tullis,
2001; Holyoke and Tullis, 2006a,
b;
Tasaka et al., 2016,
2017;
Getsinger and Hirth, 2014). The transition
from low- to high-strain zones in the Mg-rich opx + plag assemblages marks
the transition from a two-phase aggregate with strong phases (starting
material) potentially deforming by dislocation creep to a material deforming
by grain-size-sensitive mechanisms, including DPC and/or GBS (e.g.,
Boullier and Gueguen, 1975; Kerrich et al.,
1980; Schmid, 1982; Brodie and Rutter, 1987;
Kilian et al., 2011), whereby
samples deform by low bulk stresses (Fig. 2).
The spatial arrangement of mineral phases in mixed aggregates by nucleation
impedes grain growth (e.g., Olgaard and Evans, 1986,
1988). As documented by
Mansard et al. (2020), the mixing of
mineral phases is homogeneous and starts from peak stress, in favor of GBS
accommodated by DPC (e.g.,
Fliervoet et al., 1997;
Kilian et al., 2011;
Linckens et al., 2011,
2015). The original plag1 almost
completely disappeared in the high-strain zones of Mg-rich samples, while
the original opx1 remains as small clasts embedded in fine-grained mixed
zones (Figs. 3a, 4a–c). Although the opx1 is still present, its
proportion, aspect ratio, and size decrease compared to that in the
low-strain zones (Fig. 4). These microstructures suggest that opx1
grains act as rigid particles affected by dissolution and, together with
plag1, represent a source of elements required for the development of
the ductile fine-grained zone deformed by diffusion-accommodated GBS.
In the Fe-rich opx + plag samples, the deformation mechanism is also
assumed to be predominantly DPC, as demonstrated by the fact that (1) the
samples deform at lower stresses well below the Goetze criterion, and (2)
the microstructures that consist of elongate mineral aggregates of reaction
products dominantly grow in the extension direction, shear bands, or
dilatant sites (Figs. 2, 6, 12). The nucleation of reaction products,
anisotropic growth, and local dilatancy could also require the operation of
GBS (Lifshitz sliding; Paterson, 1990;
Langdon, 2006). The grain sizes of the reaction products in the
Fe-rich opx + plag samples are larger, resulting in higher flow stresses
compared to the final stresses – after weakening – of the Mg-rich opx + plag samples (Figs. 2, 11; e.g., Mansard et
al., 2020).
In addition, the amphibole fabric displays a fairly moderate but consistent
LPO with [001] axes aligned subparallel to the boundaries of C' shear bands
(Fig. 12). This type of LPO is typical for naturally deformed amphibole
(e.g., Berger and Stünitz, 1996). It is
generally accepted that significant LPO of minerals is attributed to viscous
deformation dominated by dislocation creep (e.g., Nicolas and
Christensen, 1987; Knipe, 1989;
Wenk and Christie, 1991). However, an increasing
number of studies have found that LPOs can develop without the dominance of
dislocation creep, as shown in olivine (e.g.,
Sundberg and Cooper, 2008;
Miyazaki et al., 2013,
Précigout and Hirth, 2014),
plagioclase (e.g., Barreiro et al.,
2007), and amphibole (e.g., Getsinger and Hirth, 2014). The amph
LPOs presented here are similar to the ones documented in experimentally
deformed amphibolite by Getsinger and Hirth (2014) and to
those of natural samples deformed at lower crustal conditions (e.g.,
Berger and Stünitz, 1996;
Getsinger et al., 2013;
Menegon et al., 2015;
Okudaira et al., 2015). These studies
have demonstrated that oriented grain growth of pyroxenes and amphibole can
lead to consistent amphibole–pyroxene LPO, even though diffusion creep is
the dominant deformation mechanism. We thus interpret the presence of
amphibole LPO in our samples as resulting from oriented growth with the
fastest growth direction (c axis) in the flow direction. This supports an
important contribution of metamorphic re-equilibration, chemical transport,
and hence DPC in the development of amph mylonitic foliation in the Fe-rich
and amph + plag assemblages (e.g., Bons
and Den Brok, 2000; Berger and Stünitz,
1996).
Formation of polyphase vs. monophase shear bands and implications for
the degree of rheological weakeningPolyphase shear bands
In Mg-rich opx + plag assemblages at 850 ∘C, the mixture zones
are strongly connected to form a single high-strain zone that traverses the
sample through the center from one interface of the forcing block to the
other (Fig. 3). The deformation microstructures at early stages of our
experiments suggest that phase mixing occurs at phase boundaries with the
nucleation of small equant grains in polyphase aggregates and layers (Fig. 10a). These polyphase aggregates, into which strain is partitioned, are
composed of opx2, plag2, and amph. It has been documented by many
authors (e.g., Kruse and Stünitz,
1999; Kenkmann and Dresen, 2002;
Linckens et al., 2015) that phase
mixing can be produced by the nucleation of new phases within clast tails.
The effect of mixing on grain size is significant as grain growth is impeded
by the nucleation of second phases and preserves a small grain size in the
mixture zones (Fig. 11a), producing stable microstructures (e.g.,
Olgaard and Evans, 1986; 1988;
Olgaard, 1990;
Stünitz and Fitz Gerald, 1993; Herwegh
and Berger, 2004; Warren and Hirth, 2006;
Farla et al., 2013). The opening of cavities and the
formation of dilatant sites in which material can nucleate (e.g.,
Fusseis et al., 2009;
Platt, 2015;
Menegon et al., 2015;
Précigout and Stünitz, 2016;
Précigout et al., 2017;
Gilgannon et al., 2017) may be additional factors
to promote mixing and lead to the stabilization of small grain sizes.
Amphibole-dominated σ tails and shear bands
In the Fe-rich opx + plag samples, the majority of reaction products
occurs in strongly deformed portions of the shear zones, indicating a close
relationship between deformation and mineral reactions (Fig. 3b). These
portions are not characterized by the development of intensely mixed zones
with C-type mylonite geometries, as in the Mg-rich opx + plag assemblages.
Instead, deformation and reaction have induced the formation of shear bands
or, more frequently, σ tails at the tip of elongated amph or amph
+ opx2 (Figs. 5, 6). These tails stay connected with the original
porphyroclasts, forming S–C or S–C' fabrics. Similarly, the
amph + plag assemblage is banded and shows a subhorizontal mylonitic
foliation characterized by the development of amph-rich shear bands (Fig. 3c). These shear bands have a similar geometry as those of the amph + opx2 bands and σ clasts of the Fe-rich opx + plag
assemblages.
Effects of shear band interconnection on the degree of rheological
weakening
Shear bands connect differently depending on the composition of the starting
material. In the Mg-rich opx + plag samples, strain tends to produce mixed
and connected fine-grained bands with C-type geometry in high-strain zones
(Figs. 3a, 4, 10a, 11a). This gave rise to the development of a single
throughgoing high-strain zone that probably contributed to strongly weakening
the samples after peak stress (Figs. 2, 3a). And the samples are so weak that
we consider the fine-grained aggregate of reaction products to be
sustaining almost the whole sample strength. In contrast, the Fe-rich opx + plag
and amph + plag mixtures tend to form clusters and only locally connected
amph-rich σ tails at porphyroclasts (S–C- or S–C'-type geometries;
Figs. 3b–c, 5g–l, 6, and 10b). When the shear bands and tails of clasts are
only partially connected or nonconnected, there is no peak stress and hence no
subsequent weakening (Fig. 3b; Fe-rich opx + plag assemblages). This
feature may be caused by two aspects: (1) the absence of highly connected
aggregates of reaction products and (2) the fact that reaction products
have a larger grain size and are less intensely mixed so that GSS creep
occurs at lower strain rate. The connectivity of the reaction products
appears to also be affected by their geometry; the fine-grained C-type bands
are parallel to the shear plane and seem to connect much more easily than
the local S–C- and S–C'-type tails and shear bands.
The fact that the connectivity of weak zones has a major effect on the bulk
sample strength has been documented by, e.g., Pec et al. (2012a, b, 2016),
Palazzin et al. (2018), and
Richter et al. (2018) and is
definitely a major rheological factor in these mafic samples. However, in
addition to the connectivity, we also documented different microstructures
in the weak parts of each sample, pointing to a difference in GBS–DPC
mechanisms. The less intensely mixed layers, tails, and shear bands in the
Fe-rich assemblages display higher aspect ratios of the reaction products,
i.e., more fiber-like grains (Figs. 6c, 10b, 12a–b). These microstructures
constitute a type of DPC that tends more towards grain-scale DPC, whereby
ideally (in the end-member case) the grain shapes would reflect the finite
strain, and grain boundary sliding is more a type of Lifshitz sliding.
We cannot consider the Fe-rich opx + plag samples to be representative of the
end-member case, but they approach this situation far more than the
fine-grained, well-mixed mylonitic bands of equant grains in Mg-rich opx + plag samples (Figs. 4a–b, 13), wherein DPC is probably dominated by GBS or
Rachinger sliding. The combination of relatively fast strain rates in the
fine-grained layers and strong connectivity of the reaction product zones
causes the pronounced weakening in the Mg-rich opx + plag samples,
emphasizing the importance of grain size for material strength
(Mansard et al., 2020). Such expected higher
strain rates in aggregates dominated by Rachinger sliding have been pointed
out by Paterson (1990).
OH speciation and concentration in the starting material
Our absorption spectra have been compared with reference spectra available
for plagioclase (Johnson and
Rossmann, 2003, 2004;
Johnson, 2006) and orthopyroxene (Skogby, 2006)
in the literature. While the IR spectra we obtained in plagioclase and opx are
very similar to one another, they strongly differ from the reference
spectra, which correspond to structural OH or H2O, i.e., to molecules
with specific position and orientation in the structure of the host mineral.
The spectra in plagioclase bear the largest similarities to reference
measurements of fluid inclusions and alteration products
(Johnson and Rossman, 2004), i.e., OH
or H2O molecules not structurally bound to the host solid. This
interpretation is also favored by the very similar shape of the spectra in
plag and opx (Fig. 9), whereas structural OH or H2O in these minerals
yields different spectra.
According to this interpretation, the calibration coefficient for the
H2O content should be different from that of the mineral-specific ones
we have used. However, using the general coefficient from
Paterson (1982) would not significantly affect the
estimates of H2O concentration in the starting material: the integrated
absorption coefficient used in Fig. 9 is 80 600 L / (mol H2O cm2) for
opx (Bell et al., 1995) and 107 000 L / (mol H2O cm2) for plag, i.e., relatively similar to the estimate
of 82 200 L / (mol cm2) for quartz and other silicates
(Paterson, 1982).
Bearing in mind these limitations, the amount of H2O initially present
in the minerals of the Fe-rich system is of the order of 451 ppm for
pyroxene and 226 ppm for plagioclase, whereas it is effectively zero in the
Mg-rich system. The total amount of H2O in pyroxene and plagioclase in
the Fe-rich system is ∼ 677 ppm, i.e., more than half with
respect to the addition of H2O to the powder. The initial amount of
H2O estimated by FTIR is a lower bound as only grain interiors were
analyzed by FTIR, while crushed polycrystalline aggregates include grain
boundary area with additional adsorbed H2O
(Palazzin et al.,
2018). In summary, it is estimated that the total amount of H2O in the
Fe-rich system is ∼ 50 % higher than that in the
Mg-rich system.
Role of H2O availability in reaction
In the Fe-rich opx + plag samples, a large amount of amph is formed with a
minor amount of opx2, whereas in the Mg-rich opx + plag samples, the
nucleation of a new opx2 is more abundant. The thermodynamic modeling of the
Fe-rich opx + plag system has produced a result similar to that of
Okudaira et al. (2015) as an extension to higher pressures and temperatures that is rather consistent with the observed phase compositions, although the
predicted garnet was not observed, and cpx is of lower abundance than
predicted by the model. However, it was not possible to model the observed
reactions in the Mg-rich opx + plag system in terms of phase compositions.
The reason for the inadequate modeling is most likely the somewhat
inadequate activity–composition relation in the Mg-rich system. Due to this
issue, it is impossible to calculate differences in free energy between the
two systems as a possible explanation for the different reaction kinetics.
Another difference between the two starting materials is the higher content
of H2O in the Fe-rich system. Considering that amphibole contains 2 wt. %
of H2O, the amount of H2O required to form 17 % of amphibole would be 3400 wt. ppm, which is a bit higher than the total amount of H2O present in the
sample. Our estimation of 17 % amphibole content is also rough. In any case, it is probable that a large amount, if not all, of
the H2O is used up by amphibole formation, even though the total amount
of H2O in the Fe-rich system is greater to begin with. Conversely, the
amount of amphibole formed in the Mg-opx + plag assemblage is subordinate
so that, despite a lower total amount of H2O in the starting material, there
is probably free H2O present in the Mg-rich system. It could be speculated
that the presence of more H2O in the grain boundary region could lead to a
more disperse nucleation of reaction products in the Mg-rich system.
Influence of reaction on material strengthBehavior at the onset of deformation: peak strength
Considering that the rheological behavior is strongly controlled by the
reaction products, it is inferred that the Fe-rich opx + plag assemblages
do not develop a peak stress behavior and initially deform at lower stresses
than the Mg-rich opx + plag assemblages because in the Fe-rich assemblages
reaction products nucleate faster than in the Mg-rich assemblages at early
stages of the experiment (Figs. 2b–d, 14). We do not have samples of the same
low strain for Mg-rich and Fe-rich compositions, but OR61 and 557NM (Fig. 2)
can be compared as approximately similar strain samples. The Mg-rich
material shows a slightly lower initial quantity of reaction products. The
faster nucleation of reaction products in Fe-rich samples is consistent with
the recent study of Mansard et al. (2020), showing that for samples wherein
reaction products nucleate at an early stage with respect to the onset of
deformation, the peak strength is lowered. The system that reacts faster is
the Fe-rich one, which contains a greater concentration of H2O. This
water is present inside the grains of the starting material, whereas the
added water is located along the grain boundaries. These differences suggest
that the presence of H2O in inclusions or aggregates inside grains may
have triggered the onset of hydration reactions so that, possibly, the
higher initial content in H2O and its location inside grains in the
Fe-rich system may have had a positive effect on the kinetics of reaction.
Microcracking is common in opx grains, so H2O initially stored in the
grains may easily become available for reactions during deformation, as
observed in Palazzin et al. (2018). The reactions commenced earlier in
the Fe-rich system, presenting a weakening agent in the early stages of the
deformation. The fact that sample strength is directly related to H2O
content, such as in H2O weakening in olivine and quartz, is very unlikely
because crystal plasticity does not contribute significantly as a
deformation mechanism.
Schematic mechanical and microstructural evolution of the
deformed Mg-rich opx + plag and Fe-rich opx + plag assemblages. Rp: reaction product, opx: orthopyroxene, pl: plagioclase, amph:
amphibole.
Behavior at large strain
During later stages of our experiments, the highest proportion of reaction
products and the smallest grain size are documented for the Mg-rich opx + plag samples (Figs. 11, 14), which also record a far more pronounced
weakening with respect to the Fe-rich samples (Figs. 2, 14). The phase mixing
and fine grain sizes of the reaction products in Mg-rich opx + plag
samples cause greater strain partitioning and suggest a faster nucleation
rate after peak stress, even though the Fe-rich opx + plag samples react
faster at early stages of experiment (Figs. 8, 14). Deformation and reaction
products are strongly localized and connected in high-strain zones of the
Mg-rich opx + plag assemblages (Figs. 3a, 10a), whereas the reaction
products in the Fe-rich opx + plag samples are poorly connected or nonconnected
(Figs. 3b, 10b). In the case in which the viscosity of the reaction products is
very low (fine-grained mixture zones in the Mg-rich opx + plag) compared
to the starting material, the reacting domains tend to connect much better
during the deformation (Fig. 14). It has been demonstrated that deformation
enhances the kinetics of mineral reactions
(De Ronde and Stünitz, 2007;
Mansard et al., 2020) so that the
significant localization of deformation in the Mg-rich opx + plag samples
may account for the more advanced reaction progress in those samples. In
contrast, if the reaction products are stiffer – in Fe-rich opx + plag
samples, amphibole appears to be one of the strongest silicates
(Brodie and Rutter, 1985;
Berger and Stünitz, 1996) – or harden
during the reaction (see Fe-rich opx + plag at 900 ∘C; Fig. 2d), the feedback effect of enhancing reaction kinetics appears to be
limited and no weakening is observed (Fig. 2). Therefore, the viscosity of
the Fe-rich opx + plag and amph + plag assemblages is related to their
ability to transform and connect.
The reason why the reaction products in Mg-rich samples characterized by
fine-grained layers that do not stay attached to the original opx1
clasts is not clear yet, but this might be related to the difference in the
amount and distribution of H2O. Indeed, in the Fe-rich system, the
reaction products are dominated by a single-phase material, i.e., amphibole
(Fig. 6). In contrast, although the very small grain size did not allow for any
quantitative estimates of the Mg-rich reaction products, much more phase
mixing occurs in the latter. The difference in the nature and proportion of
reaction products between Fe- and Mg-rich systems is a major control factor
for their later resistance evolution during connectivity and reaction
progress. This situation illustrates that substantially more work is needed
to understand the relationship between mineral reaction and mechanical
behavior.
Geological application
According to the results of this study, the rheological behavior of
two-phase mixtures of mafic composition is dependent on mineral reactions,
as reactions can trigger strain localization and weakening, even at low
shear strain. In two different starting materials, the reactions coupled
with dissolution–precipitation creep (DPC) in combination with grain
boundary sliding allow the mafic assemblage to be deformed. Without mineral
reactions, the mafic assemblage of pyroxene and plagioclase would be too
strong to be deformed by dislocation creep at the applied strain rates
(Mansard et al., 2020). This situation is the
same in naturally deformed rocks: below ∼700 ∘C, pyroxene
and plagioclase assemblages do not show significant crystal plastic
deformation (e.g., Brodie and Rutter, 1985,
1992). The partly extensive deformation of mafic
rocks at lower or higher metamorphic grades appears to be dependent on
mineral reactions, which can induce the development of rheologically weaker
phases, on the formation of fine-grained aggregates deforming by GSS
creep, or on both (e.g., Brodie and Rutter, 1985;
Brodie et al., 1992;
Handy and Stünitz, 2002;
Brander et al., 2012;
Okudaira et al., 2017;
Degli Alessandrini et al.,
2017).
At low shear strain and identical P–T conditions, the small difference in
mineral reactions between the Fe-rich opx + plag and Mg-rich opx + plag
assemblages induces clear differences in mechanical strength at peak stress
and during subsequent weakening. These observations are crucial as shear
zones are known to represent preferential regions for mineral reactions to
occur, either owing to deformation (e.g., Brodie, 1980)
or to the presence of fluids (e.g.,
Etheridge et al., 1983) during
protracted shearing (e.g., Brodie, 1980;
Philippot and Kienast, 1989;
Newman et al., 1999). Thus, local spatial
variations in mineral reactions can induce gradients in deformation due to
sufficient differences in the mechanical behavior to localize strain (e.g.,
Holyoke and Tullis, 2006a,
b). In addition, our observations suggest
that the mechanical strength at peak stress is controlled by the ability of
the material to react and connect to the weak material, not necessarily
by the strength of the reaction products. In fact, the Fe-rich opx + plag
assemblages produce intrinsically stronger reaction products (more
amphibole) than the fine-grained mixed aggregates and layers in the Mg-rich
opx + plag assemblages, and yet Fe-rich opx + plag assemblages are
weaker at low shear strain because they react faster. It is speculated that
this difference in reaction kinetics is in part controlled by the amount of
H2O present. The higher H2O content in the Fe-rich opx + plag
assemblages may have had a positive effect on the kinetics of reaction. This
aspect of the influence of H2O availability on the strength of
materials is important because shear zones have long been recognized as a
preferred locus for fluid migration (e.g.,
Austrheim, 1987;
Newton, 1990;
Selverstone et al.,
1991; Gueydan et al., 2004;
Angiboust et al.,
2011), particularly due to the development of syn-kinematic porosity
resulting from dilatancy at grain boundaries (Tullis
et al., 1996), creep cavitation (Fusseis et
al., 2009; Menegon et al., 2015;
Précigout and Stünitz, 2016;
Précigout et al., 2019), or from dehydration reactions and associated
fracturing (Plümper et al., 2017).
Our results suggest that small initial differences in mineral reactions in
natural materials can trigger strain localization and lead to large
differences in mechanical strength, regardless of the intrinsic strength of
the reaction products. These observations mainly apply to the initiation and
early stages of shear zone formation, as once the incipient stages of strain
localization and weakening are initiated, additional mechanisms are involved
and control the development of the shear zones in space and time.
The development of natural shear zones is commonly arranged into distinctive
monophase layers (e.g., Berthé et al., 1979;
Gapais, 1989) and/or
polyphase mixtures (e.g.,
Fliervoet et al., 1997;
Keller et al., 2004;
Warren and Hirth, 2006; Kanagawa et al., 2008;
Raimbourg et al., 2008;
Okudaira et al., 2015) that may be found
in the same shear zone as alternating layers (e.g.,
Kenkmann and Dresen, 2002;
Kilian et al., 2011;
Oliot et al., 2014;
Mansard et al., 2018). In our
case, the progressive deformation of two-phase assemblages also shows the
development of (1) a strongly localized polyphase high-strain zone associated
with a significant weakening after peak stress (Mg-rich assemblages) and
(2) only locally connected σ-clast bands that do not induce major
weakening (Fe-rich opx + plag and amph + plag assemblages). Strain
localization into reaction-derived fine-grained mixtures is also well-documented in nature with the dominance of diffusion-accommodated GBS and/or
DPC mechanisms (e.g., Newman et al., 1999;
Handy and Stünitz, 2002;
Kanagawa et al., 2008;
Okudaira et al., 2015) and is expected to
promote major rheological weakening (e.g.,
Fliervoet et al., 1997;
Svahnberg and Piazolo, 2013;
Warren and Hirth, 2006;
Platt, 2015). The development of amph-rich
σ-shaped aggregates and layers, notably through dissolution and
precipitation processes, is also commonly documented in middle to
lower crustal banded mylonites (e.g.,
Berger and Stünitz, 1996;
Imon et al., 2002, 2004;
Baratoux et al., 2005;
Getsinger et al., 2013;
Elyaszadeh et al., 2018;
Giuntoli et al., 2018).
When applied to natural shear zones, our results suggest that (1) the
ability of minerals to react controls the portions of rocks that deform, and
(2) minor chemical and mineralogical variations (including the H2O
available for reaction) can considerably modify the strength of deformed
assemblages. Then, significant strain localization and partitioning at high
shear strain, expressed by the development of interconnected high-strain
zones (Mg-rich opx + plag), illustrate that fine-grained polyphase mixed
zones can become weaker than coarse-grained, poorly connected ones (Fe-rich
opx + plag and amph + plag). These results emphasize the importance of
strain partitioning, grain size reduction, and interconnectivity of weak
material as primary controls of lithospheric strength. This also suggests
that the rheology of mafic rocks, which constitute a large part of the
oceanic crust and may be one of the main components of the lower continental
crust, cannot be summarized as being rheologically controlled by monophase
materials (e.g.,
Dimanov and Dresen,
2005).
Conclusions
Shear deformation experiments with monophase and polyphase assemblages show a
clear relationship between deformation and mineral reactions. A greater
volume of reaction products is documented with increasing strain. This study
shows that the premise of mineral reaction and strain localization has a
major impact on sample strength because it conditions the resistance of the
assemblage at low shear strain. Indeed, the Fe-rich opx + plag assemblages
deform at lower stresses than the Mg-rich opx + plag assemblages because
they react faster at early stages of experimentation. Thus, chemically similar
assemblages may have significantly different strength development depending
on the ability of these minerals to react. It is suggested that the
availability of H2O, among other factors such as mineral composition,
has a positive effect on the kinetics of reaction. With increasing strain,
there are two very contrasting pathways in the material evolution, controlled
by the properties of the reaction: in one case (Fe-opx + plag and amph +
plag assemblages), the reaction products (mostly amphibole) have elongated
grain shapes, a larger grain size, and poor connectivity. As a result, they
do not show significant weakening. In such a case, the reaction products are
only partially connected, the material deforms by grain-scale DPC, and
the strength of the bulk sample is stable so the feedback effect of deformation
on the reaction is limited. In the other cases (Mg-rich opx + plag and pure
amph assemblages), the fine-grained, intensely mixed reaction products of
equant grains weaken considerably during the experiment and end up much
weaker than the initial material. The mechanically weak zone of reaction
product deforms by GBS-dominated DPC and tends to form an interconnected
zone that leads to the weakening of the bulk sample, and the drastic
increase in the proportion of reaction products with strain suggests a large
feedback effect. Overall, the value of the initial peak stress (that is
responsible for determining where strain will start localizing) and the large
weakening associated with the formation of fine-grained products (that
determines whether shear bands thicken or not) seem to be mainly controlled
by the chemical reactions.
Data availability
Experimental data were processed using a MATLAB-based program inspired by
the “rig” program of Dr. Matej Pec (Pec et al., 2016) and available at
https://sites.google.com/site/jacquesprecigout/telechargements-downloads, last access: 5 August 2020.
Author contributions
NM, HS, HR, and JP designed the experiments. NM and JP carried them out. NM carried out the various
analyses after the experiments (e.g., SEM, EPMA). AP worked on
the thermodynamic modeling part, while LN contributed to the
FTIR data acquisition. NM prepared the paper with
contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work has received funding from the European Research Council (ERC)
under the Seventh Framework Programme of the European Union (ERC Advanced
Grant, grant agreement no. 290864, RHEOLITH), Labex VOLTAIRE
(ANR-10-LABX-100-01), and the project FluPrism (CNRS INSU, funding
program SYSTER). We gratefully acknowledge the help provided by Patricia Benoist and Ida Di Carlo for analytical support, Sylvain Janiec for the
preparation of thin sections, and Kai Neufeld for EBSD data acquisition. The
authors wish to thank Mark Zimmerman for generously providing the Damaping
peridotite and Laurent Arbaret for the Massif Central amphibolite material.
We also appreciate thorough and constructive reviews by Luca Menegon and
Jolien Linckens. We would like to thank Florian Fusseis for editorial
handling.
Financial support
This research has been supported by the European Research Council (ERC) under the Seventh Framework Programme of the European Union (ERC Advanced Grant, grant agreement no. 290864, RHEOLITH), Labex VOLTAIRE (grant no. ANR-10-LABX-100-01), EquipEx PLANEX (grant no. ANR-11-EQPX‐0036), and the project FluPrism (CNRS INSU, funding program SYSTER).
Review statement
This paper was edited by Florian Fusseis and reviewed by Luca Menegon and Jolien Linckens.
ReferencesAngiboust, S., Agard, P., Raimbourg, H., Yamato, P. and Huet, B.: Subduction
interface processes recorded by eclogite-facies shear zones (Monviso, W.
Alps), Lithos, 127, 222–238, 10.1016/j.lithos.2011.09.004, 2011.Ashby, M. and Verrall, R.: Diffusion-accommodated flow and
superplasticity, Acta Metall., 21, 149–163,
10.1016/0001-6160(73)90057-6, 1973.Austrheim, H.: Eclogitization of lower crustal granulites by fluid migration
through shear zones, Earth Planet. Sci. Lett., 81, 221–232,
10.1016/0012-821X(87)90158-0, 1987.Baratoux, L., Schulmann, K., Ulrich, S., and Lexa, O.: Contrasting
microstructures and deformation mechanisms in metagabbro mylonites
contemporaneously deformed under different temperatures (c. 650 C and c.
750 C), Geol. Soc. London, Spec. Publ., 243, 97–125,
10.1144/GSL.SP.2005.243.01.09, 2005.Barreiro, J. G., Lonardelli, I., Wenk, H. R., Dresen, G., Rybacki, E., Ren,
Y., and Tomé, C. N.: Preferred orientation of anorthite deformed
experimentally in Newtonian creep, Earth Planet. Sci. Lett., 264,
188–207, 10.1016/J.EPSL.2007.09.018, 2007.Behrmann, J. H.: Crystal plasticity and superplasticity in quartzite; A
natural example, Tectonophysics, 115, 101–129,
10.1016/0040-1951(85)90102-7, 1985.Bell, D. R., Ihinger, P. D., and Rossman, G. R.: Quantitative analysis of
trace OH in garnet and pyroxenes, Am. Mineral., 80, 465–474,
10.2138/am-1995-5-607, 1995.Bercovici, D. and Ricard, Y.: Mechanisms for the generation of plate
tectonics by two-phase grain-damage and pinning, Phys. Earth Planet. Inter.,
202–203, 27–55, 10.1016/J.PEPI.2012.05.003, 2012.Berger, A. and Stünitz, H.: Deformation mechanisms and reaction of
hornblende: examples from the Bergell tonalite (Central Alps),
Tectonophysics, 257, 149–174, 10.1016/0040-1951(95)00125-5,
1996.Berthé, D., Choukroune, P., and Jegouzo, P.: Orthogneiss, mylonite and
non coaxial deformation of granites: the example of the South Armorican
Shear Zone, J. Struct. Geol., 1, 31–42,
10.1016/0191-8141(79)90019-1, 1979.
Bons, P. D. and Den Brok, B.: Crystallographic preferred orientation
development by dissolution-precipitation creep, J. Struct.
Geol., 22, 1713–1722, 2000.Borg, I. and Handin, J.: Experimental deformation of crystalline rocks,
Tectonophysics, 3, 249–367, 10.1016/0040-1951(66)90019-9, 1966.Boullier, A. M. and Gueguen, Y.: SP-Mylonites: Origin of some mylonites by
superplastic flow, Contrib. Mineral. Petrol., 50, 93–104,
10.1007/BF00373329, 1975.Brander, L., Svahnberg, H., and Piazolo, S.: Brittle-plastic deformation in
initially dry rocks at fluid-present conditions: Transient behaviour of
feldspar at mid-crustal levels, Contrib. Mineral. Petrol., 163,
403–425, 10.1007/s00410-011-0677-5, 2012.Brodie, K. H.: Variations in mineral chemistry across a shear zone in
phlogopite peridotite, J. Struct. Geol., 2, 265–272,
10.1016/0191-8141(80)90059-0, 1980.
Brodie, K. H. and Rutter, E. H.: On the Relationship between Deformation and
Metamorphism, with Special Reference to the Behavior of Basic Rocks, pp.
138–179, Springer, New York, NY, 1985.Brodie, K. H. and Rutter, E. H.: The role of transiently fine-grained
reaction products in syntectonic metamorphism: natural and experimental
examples, Can. J. Earth Sci., 24, 556–564, 10.1139/e87-054, 1987.Brodie, K. H., Rutter, E. H., and Evans, P.: On the structure of the
Ivrea-Verbano Zone (northern Italy) and its implications for present-day
lower continental crust geometry, Terra Nov., 4, 34–40,
10.1111/j.1365-3121.1992.tb00448.x, 1992.
Bunge, H.-J.: Texture analysis in materials science?: mathematical methods,
Butterworths, London, ISBN: 978-0-408-10642-9, 1982.Bystricky, M. and Mackwell, S.: Creep of dry clinopyroxene aggregates, J.
Geophys. Res.-Solid Earth, 106, 13443–13454, 10.1029/2001JB000333,
2001.Bystricky, M., Lawlis, J., Mackwell, S., Heidelbach, F., and Raterron, P.:
High-temperature deformation of enstatite aggregates, J. Geophys. Res.-Solid
Earth, 121, 6384–6400, 10.1002/2016JB013011, 2016.Coe, R. S.: The thermodynamic effect of shear stress on the ortho-clino
inversion in enstatite and other coherent phase transitions characterized by
a finite simple shear, Contrib. Mineral. Petrol., 26, 247–264,
10.1007/BF00373203, 1970.Coe, R. S. and Kirby, S. H.: The orthoenstatite to clinoenstatite
transformation by shearing and reversion by annealing: Mechanism and
potential applications, Contrib. Mineral. Petrol., 52, 29–55,
10.1007/BF00378000, 1975.Connolly, J. A. D.: The geodynamic equation of state: What and how,
Geochemistry, Geophys. Geosystems, 10, Q10014,,
10.1029/2009GC002540, 2009.Dale, J., Powell, R., White, R. W., Elmer, F. L., and Holland, T. J. B.: A
thermodynamic model for Ca-Na clinoamphiboles in
Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-O for petrological calculations, J.
Metamorph. Geol., 23, 771–791, 10.1111/j.1525-1314.2005.00609.x,
2005.Degli Alessandrini, G., Menegon, L., Malaspina, N., Dijkstra, A. H., and
Anderson, M. W.: Creep of mafic dykes infiltrated by melt in the lower
continental crust (Seiland Igneous Province, Norway), Lithos, 274–275,
169–187, 10.1016/j.lithos.2016.12.030, 2017.Dell'Angelo, L. N. and Tullis, J.: Textural and mechanical evolution with
progressive strain in experimentally deformed aplite, Tectonophysics,
256, 57–82, 10.1016/0040-1951(95)00166-2, 1996.De Ronde, A. A. and Stünitz, H.: Deformation-enhanced reaction in
experimentally deformed plagioclase-olivine aggregates, Contrib. Mineral.
Petrol., 153, 699–717, 10.1007/s00410-006-0171-7, 2007.De Ronde, A. A., Heilbronner, R., Stünitz, H., and Tullis, J.: Spatial
correlation of deformation and mineral reaction in experimentally deformed
plagioclase-olivine aggregates, Tectonophysics, 389, 93–109,
10.1016/j.tecto.2004.07.054, 2004.De Ronde, A. A., Stünitz, H., Tullis, J., and Heilbronner, R.:
Reaction-induced weakening of plagioclase-olivine composites,
Tectonophysics, 409, 85–106, 10.1016/j.tecto.2005.08.008, 2005.Dimanov, A. and Dresen, G.: Rheology of synthetic anorthite-diopside
aggregates: Implications for ductile shear zones, J. Geophys. Res.-Solid
Earth, 110, 1–24, 10.1029/2004JB003431, 2005.Dimanov, A., Lavie, M. P., Dresen, G., Ingrin, J., and Jaoul, O.: Creep of
polycrystalline anorthite and diopside, J. Geophys. Res.-Solid Earth,
108, 2061, 10.1029/2002JB001815, 2003.Dimanov, A., Rybacki, E., Wirth, R., and Dresen, G.: Creep and
strain-dependent microstructures of synthetic anorthite–diopside
aggregates, J. Struct. Geol., 29, 1049–1069,
10.1016/J.JSG.2007.02.010, 2007.Elyaszadeh, R., Prior, D. J., Sarkarinejad, K., and Mansouri, H.: Different
slip systems controlling crystallographic preferred orientation and
intracrystalline deformation of amphibole in mylonites from the Neyriz
mantle diapir, Iran, J. Struct. Geol., 107, 38–52,
10.1016/j.jsg.2017.11.020, 2018.Etheridge, M. A., Wall, V. J., and Vernon, R. H.: The role of the fluid phase
during regional metamorphism and deformation, J. Metamorph. Geol., 1,
205–226, 10.1111/j.1525-1314.1983.tb00272.x, 1983.Farla, R. J. M., Karato, S.-I., and Cai, Z.: Role of orthopyroxene in
rheological weakening of the lithosphere via dynamic recrystallization,
P. Natl. Acad. Sci. USA, 110, 16355–16360,
10.1073/pnas.1218335110, 2013.Fitz Gerald, J. and Stünitz, H.: Deformation of granitoids at low
m∼ tamo∼∼ ic grade. I: Reactions
and grain size reduction, Elsevier Sci. Publ. B.V, 221, 269–297,
10.1016/0040-1951(93)90164-F, 1993.Fliervoet, T. F. and White, S. H.: Quartz deformation in a very fine grained
quartzo-feldspathic mylonite: a lack of evidence for dominant grain boundary
sliding deformation, J. Struct. Geol., 17, 1095–1109,
10.1016/0191-8141(95)00007-Z, 1995.Fliervoet, T. F., White, S. H., and Drury, M. R.: Evidence for dominant
grain-boundary sliding deformation in greenschist- and amphibolite-grade
polymineralic ultramylonites from the Redbank Deformed Zone, Central
Australia, J. Struct. Geol., 19, 1495–1520,
10.1016/S0191-8141(97)00076-X, 1997.Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M., and De Carlo, F.:
Creep cavitation can establish a dynamic granular fluid pump in ductile
shear zones, Nature, 459, 974–977, 10.1038/nature08051, 2009.
Gapais, D.: Shear structures within deformed granites: Mechanical and
thermal indicators, Geology, 17, 1144–1147, 1989.Getsinger, A. J. and Hirth, G.: Amphibole fabric formation during diffusion
creep and the rheology of shear zones, Geology, 42, 535–538,
10.1130/G35327.1, 2014.Getsinger, A. J., Hirth, G., Stünitz, H., and Goergen, E. T.: Influence
of water on rheology and strain localization in the lower continental crust,
Geochem. Geophys. Geosy., 14, 2247–2264,
10.1002/ggge.20148, 2013.Gilgannon, J., Fusseis, F., Menegon, L., Regenauer-Lieb, K., and Buckman, J.: Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing, Solid Earth, 8, 1193–1209, 10.5194/se-8-1193-2017, 2017.Giuntoli, F., Menegon, L., and Warren, C. J.: Replacement reactions and
deformation by dissolution and precipitation processes in amphibolites, J.
Metamorph. Geol., 36, 1263–1286, 10.1111/jmg.12445, 2018.Gueydan, F., Leroy, Y. M., and Jolivet, L.: Mechanics of low-angle
extensional shear zones at the brittle-ductile transition, J. Geophys. Res.-Solid Earth, 109, 1–16, 10.1029/2003JB002806, 2004.Handy, M. R.: Flow laws for rocks containing two non-linear viscous phases:
A phenomenological approach, J. Struct. Geol., 16, 287–301,
10.1016/0191-8141(94)90035-3, 1994.Handy, M. R. and Stünitz, H.: Strain localization by fracturing and
reaction weakening – a mechanism for initiating exhumation of
subcontinental mantle beneath rifted margins, Geol. Soc. London, Spec.
Publ., 200, 387–407, 10.1144/GSL.SP.2001.200.01.22, 2002.Harigane, Y., Michibayashi, K., and Ohara, Y.: Shearing within lower crust
during progressive retrogression: Structural analysis of gabbroic rocks from
the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc
basin, Tectonophysics, 457, 183–196, 10.1016/j.tecto.2008.06.009,
2008.Herwegh, M. and Berger, A.: Deformation mechanisms in second-phase affected
microstructures and their energy balance, J. Struct. Geol., 26,
1483–1498, 10.1016/J.JSG.2003.10.006, 2004.Herwegh, M., Linckens, J., Ebert, A., Berger, A., and Brodhag, S. H.: The
role of second phases for controlling microstructural evolution in
polymineralic rocks: A review, J. Struct. Geol., 33, 1728–1750,
10.1016/j.jsg.2011.08.011, 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, 10.1111/j.1525-1314.1998.00140.x, 1998.Holyoke, C. W. and Tullis, J.: Formation and maintenance of shear zones,
Geology, 34, 105–108, 10.1130/G22116.1, 2006a.Holyoke, C. W. and Tullis, J.: Mechanisms of weak phase interconnection and
the effects of phase strength contrast on fabric development, J. Struct.
Geol., 28, 621–640, 10.1016/j.jsg.2006.01.008, 2006b.Imon, R., Okudaira, T., and Fujimoto, A.: Dissolution and precipitation
processes in deformed amphibolites: an example from the ductile shear zone
of the Ryoke metamorphic belt, SW Japan, J. Metamorph. Geol., 20,
297–308, 10.1046/j.1525-1314.2002.00367.x, 2002.Imon, R., Okudaira, T., and Kanagawa, K.: Development of shape- and
lattice-preferred orientations of amphibole grains during initial
cataclastic deformation and subsequent deformation by
dissolution-precipitation creep in amphibolites from the Ryoke metamorphic
belt, SW Japan, J. Struct. Geol., 26, 793–805,
10.1016/j.jsg.2003.09.004, 2004.
Johnson, E. A.: Water in nominally anhydrous crustal minerals: Speciation,
concentration, and geologic significance, in: Water in Nominally Anhydrous
Minerals, vol. 62, pp. 117–154, Walter de Gruyter GmbH, 2006.Johnson, E. A. and Rossmann, G. R.: The concentration and speciation of
hydrogen in feldspars using FTIR and 1H MAS NMR spectroscopy, Am. Mineral.,
88, 901–911, 10.2138/am-2003-5-620, 2003.Johnson, E. A. and Rossman, G. R.: A survey of hydrous species and
concentrations in igneous feldspars, Am. Mineral., 89, 586–600,
10.2138/am-2004-0413, 2004.Jolivet, L. and Miyashita, S.: The Hidaka Shear Zone (Hokkaido, Japan):
Genesis during a right-lateral strike-slip movement, Tectonics, 4,
289–302, 10.1029/TC004i003p00289, 1985.Jordan, P.: The rheology of polymineralic rocks - an approach, Geol.
Rundschau, 77, 285–294, 10.1007/BF01848690, 1988.Kanagawa, K., Shimano, H., and Hiroi, Y.: Mylonitic deformation of gabbro in
the lower crust: A case study from the Pankenushi gabbro in the Hidaka
metamorphic belt of central Hokkaido, Japan, J. Struct. Geol., 30,
1150–1166, 10.1016/j.jsg.2008.05.007, 2008.Keller, L. M., Abart, R., Stünitz, H., and De Capitani, C.: Deformation,
mass transfer and mineral reactions in an eclogite facies shear zone in a
polymetamorphic metapelite (Monte Rosa nappe, western Alps), J. Metamorph.
Geol., 22, 97–118, 10.1111/j.1525-1314.2004.00500.x, 2004.Kenkmann, T. and Dresen, G.: Dislocation microstructure and phase
distribution in a lower crustal shear zone – An example from the Ivrea-Zone,
Italy, Int. J. Earth Sci., 91, 445–458, 10.1007/s00531-001-0236-9,
2002.Kerrich, R., Allison, I., Barnett, R. L., Moss, S., and Starkey, J.:
Microstructural and chemical transformations accompanying deformation of
granite in a shear zone at Miéville, Switzerland; with implications for
stress corrosion cracking and superplastic flow, Contrib. Mineral.
Petrol., 73, 221–242, 10.1007/BF00381442, 1980.Kilian, R., Heilbronner, R., and Stünitz, H.: Quartz grain size reduction
in a granitoid rock and the transition from dislocation to diffusion creep,
J. Struct. Geol., 33, 1265–1284, 10.1016/j.jsg.2011.05.004, 2011.Knipe, R.: Deformation mechanisms – recognition from natural tectonites,
J. Struct. Geol., 11, 127–146, 10.1016/0191-8141(89)90039-4,
1989.Kohlstedt, D. L., Evans, B., and Mackwell, S. J.: Strength of the
lithosphere: Constraints imposed by laboratory experiments, J. Geophys. Res.-Solid Earth, 100, 17587–17602, 10.1029/95JB01460, 1995.Kruse, R. and Stünitz, H.: Deformation mechanisms and phase distribution
in mafic high-temperature mylonites from the Jotun Nappe, southern Norway,
Tectonophysics, 303, 223–249, 10.1016/S0040-1951(98)00255-8,
1999.Langdon, T. G.: Grain boundary sliding revisited: Developments in sliding
over four decades, J. Mater. Sci., 41, 597–609,
10.1007/s10853-006-6476-0, 2006.Linckens, J., Herwegh, M., Müntener, O., and Mercolli, I.: Evolution of a
polymineralic mantle shear zone and the role of second phases in the
localization of deformation, J. Geophys. Res.-Solid Earth, 116, B06210,
10.1029/2010JB008119, 2011.Linckens, J., Herwegh, M., and Müntener, O.: Small quantity but large
effect - How minor phases control strain localization in upper mantle shear
zones, Tectonophysics, 643, 26–43, 10.1016/j.tecto.2014.12.008, 2015.Mansard, N., Raimbourg, H., Augier, R., Précigout, J., and Le Breton, N.:
Large-scale strain localization induced by phase nucleation in mid-crustal
granitoids of the south Armorican massif, Tectonophysics, 745,
10.1016/j.tecto.2018.07.022, 2018.Mansard, N., Stünitz, H., Raimbourg, H., and Précigout, J.: The role
of deformation-reaction interactions to localize strain in polymineralic
rocks: Insights from experimentally deformed plagioclase-pyroxene
assemblages, J. Struct. Geol., 134, 104008, 10.1016/j.jsg.2020.104008, 2020.Marti, S., Stünitz, H., Heilbronner, R., Plümper, O., and Drury, M.:
Experimental investigation of the brittle-viscous transition in mafic rocks
– Interplay between fracturing, reaction, and viscous deformation, J.
Struct. Geol., 105, 62–79, 10.1016/j.jsg.2017.10.011, 2017.
Marti, S., Stünitz, H., Heilbronner, R., Plümper, O., and Kilian, R.: Syn-kinematic hydration reactions, grain size reduction, and dissolution–precipitation creep in experimentally deformed plagioclase–pyroxene mixtures, Solid Earth, 9, 985–1009, https://doi.org/10.5194/se-9-985-2018, 2018.Mehl, L. and Hirth, G.: Plagioclase preferred orientation in layered
mylonites: Evaluation of flow laws for the lower crust, J. Geophys. Res.,
113, B05202, 10.1029/2007JB005075, 2008.Menegon, L., Fusseis, F., Stünitz, H., and Xiao, X.: Creep cavitation
bands control porosity and fluid flow in lower crustal shear zones, Geology,
43, 227–230, 10.1130/G36307.1, 2015.Miyazaki, T., Sueyoshi, K., and Hiraga, T.: Olivine crystals align during
diffusion creep of Earth's upper mantle, Nature, 502, 321–326,
10.1038/nature12570, 2013.Montési, L. G. J.: Fabric development as the key for forming ductile
shear zones and enabling plate tectonics, J. Struct. Geol., 50, 254–266,
10.1016/j.jsg.2012.12.011, 2013.Newman, J., Lamb, W. M., Drury, M. R., and Vissers, R. L. M.: Deformation
processes in a peridotite shear zone: reaction-softening by an
H2O-deficient, continuous net transfer reaction, Tectonophysics, 303,
193–222, 10.1016/S0040-1951(98)00259-5, 1999.Newton, R. C.: Fluids and shear zones in the deep crust, Tectonophysics,
182, 21–37, 10.1016/0040-1951(90)90339-A, 1990.Newton, R. C., Charlu, T. V., and Kleppa, O. J.: Thermochemistry of the high
structural state plagioclases, Geochim. Cosmochim. Acta, 44, 933–941,
10.1016/0016-7037(80)90283-5, 1980.
Nicolas, A. and Christensen, N. I.: Formation of anisotropy in upper mantle
peridotites: A review, pp. 111–123, American Geophysical Union (AGU),
1987.Okudaira, T., Jeřábek, P., Stünitz, H., and Fusseis, F.:
High-temperature fracturing and subsequent grain-size-sensitive creep in
lower crustal gabbros: Evidence for coseismic loading followed by creep
during decaying stress in the lower crust, J. Geophys. Res.-Solid Earth,
120, 3119–3141, 10.1002/2014JB011708, 2015.Okudaira, T., Shigematsu, N., Harigane, Y., and Yoshida, K.: Grain size
reduction due to fracturing and subsequent grain-size-sensitive creep in a
lower crustal shear zone in the presence of a CO2-bearing fluid, J. Struct.
Geol., 95, 171–187, 10.1016/j.jsg.2016.11.001, 2017.Olgaard, D. L.: The role of second phase in localizing deformation, Geol.
Soc. London, Spec. Publ., 54, 175–181,
10.1144/GSL.SP.1990.054.01.17, 1990.Olgaard, D. L. and Evans, B.: Effect of Second-Phase Particles on Grain
Growth in Calcite, J. Am. Ceram. Soc., 69, C-272–C-277,
10.1111/j.1151-2916.1986.tb07374.x, 1986.Olgaard, D. L. and Evans, B.: Grain growth in synthetic marbles with added
mica and water, Contrib. Mineral. Petrol., 100, 246–260,
10.1007/BF00373591, 1988.Oliot, E., Goncalves, P., Schulmann, K., Marquer, D., and Lexa, O.:
Mid-crustal shear zone formation in granitic rocks: Constraints from
quantitative textural and crystallographic preferred orientations analyses,
Tectonophysics, 612–613, 63–80, 10.1016/j.tecto.2013.11.032, 2014.Palazzin, G., Raimbourg, H., Stünitz, H., Heilbronner, R., Neufeld, K.,
and Précigout, J.: Evolution in H2O contents during deformation of
polycrystalline quartz: An experimental study, J. Struct. Geol., 114,
95–110, 10.1016/J.JSG.2018.05.021, 2018.Paterson, M. S.: The determination of hydroxyl by infrared adsorption in
quartz, silicate glasses and similar materials., Bull. Mineral., 105,
20–29, 10.3406/bulmi.1982.7582, 1982.Paterson, M. S.: Superplasticity in Geological Materials, MRS Proc., 196, 303,
10.1557/proc-196-303, 1990.
Paterson, M. S.: A Granular Flow Approach to Fine-Grain Superplasticity, in:
Plastic Deformation of Ceramics, pp. 279–283, Springer USA, 1995.
Paterson, M. S.: Materials Science for Structural Geology, 1st ed.,
Springer, New York, 2013.Pec, M., Stünitz, H., Heilbronner, R., Drury, M., and de Capitani, C.:
Origin of pseudotachylites in slow creep experiments, Earth Planet. Sci.
Lett., 355–356, 299–310, 10.1016/J.EPSL.2012.09.004, 2012a.Pec, M., Stünitz, H., and Heilbronner, R.: Semi-brittle deformation of
granitoid gouges in shear experiments at elevated pressures and
temperatures, 38, 200–221, 10.1016/j.jsg.2011.09.001, 2012b.Pec, M., Stünitz, H., Heilbronner, R., and Drury, M.: Semi-brittle flow
of granitoid fault rocks in experiments, J. Geophys. Res.-Solid Earth,
121, 1677–1705, 10.1002/2015JB012513, 2016.Philippot, P. and Kienast, J. R.: Chemical-microstructural changes in
eclogite-facies shear zones (Monviso, Western Alps, north Italy) as
indicators of strain history and the mechanism and scale of mass transfer,
Lithos, 23, 179–200, 10.1016/0024-4937(89)90004-2, 1989.Platt, J. P.: Rheology of two-phase systems: A microphysical and
observational approach, J. Struct. Geol., 77, 213–227,
10.1016/j.jsg.2015.05.003, 2015.Plümper, O., Botan, A., Los, C., Liu, Y., Malthe-Sørenssen, A., and
Jamtveit, B.: Fluid-driven metamorphism of the continental crust governed by
nanoscale fluid flow, Nat. Geosci., 10, 685–690, 10.1038/ngeo3009,
2017.Powell, R. and Holland, T.: Relating formulations of the thermodynamics of
mineral solid solutions: Activity modeling of pyroxenes, amphiboles, and
micas, Am. Mineral., 84, 1–14, 10.2138/am-1999-1-201, 1999.Précigout, J. and Hirth, G.: B-type olivine fabric induced by grain
boundary sliding, Earth Planet. Sci. Lett., 395, 231–240,
10.1016/j.epsl.2014.03.052, 2014.Précigout, J. and Stünitz, H.: Evidence of phase nucleation during
olivine diffusion creep: A new perspective for mantle strain localisation,
Earth Planet. Sci. Lett., 455, 94–105,
10.1016/j.epsl.2016.09.029, 2016.Précigout, J., Prigent, C., Palasse, L., and Pochon, A.: Water pumping in
mantle shear zones, Nat. Commun., 8, 15736, 10.1038/ncomms15736, 2017.Précigout, J., Stünitz, H., Pinquier, Y., Champallier, R., and
Schubnel, A.: High-pressure, High-temperature Deformation Experiment Using
the New Generation Griggs-type Apparatus, J. Vis. Exp., 134, e56841,
10.3791/56841, 2018.Raimbourg, H., Toyoshima, T., Harima, Y., and Kimura, G.: Grain-size
reduction mechanisms and rheological consequences in high-temperature gabbro
mylonites of Hidaka, Japan, Earth Planet. Sci. Lett., 267, 637–653,
10.1016/j.epsl.2007.12.012, 2008.Richter, B., Stünitz, H., and Heilbronner, R.: The brittle-to-viscous
transition in polycrystalline quartz: An experimental study, J. Struct.
Geol., 114, 1–21, 10.1016/j.jsg.2018.06.005, 2018.Rubie, D. C.: Reaction-enhanced ductility: The role of solid-solid
univariant reactions in deformation of the crust and mantle, Tectonophysics,
96, 331–352, 10.1016/0040-1951(83)90225-1, 1983.Rudnick, R. L. and Fountain, D. M.: Nature and composition of the
continental crust: A lower crustal perspective, Rev. Geophys., 33, 267,
10.1029/95RG01302, 1995.
Rutter, E. H. and Brodie, K. H.: The Permeation of Water into Hydrating
Shear Zones, Adv. Phys. Geochem., 4, 242–250, 1985.
Schmid, S. M.: Microfabric studies as indicators of deformation mechanisms
and flow laws operative in mountain building, Mt. Build. Process., edited by: Hsu, K. J., Academic Press,
95–110, 1982.Schroeder, T. and John, B. E.: Strain localization on an oceanic detachment
fault system, Atlantis Massif, 30∘ N, Mid-Atlantic Ridge,
Geochem. Geophy. Geosy., 5, Q11007, 10.1029/2004GC000728, 2004.Selverstone, J., Morteani, G., and Staude, J.-M.: Fluid channelling during
ductile shearing: transformation of granodiorite into aluminous schist in
the Tauern Window, Eastern Alps, J. Metamorph. Geol., 9, 419–431,
10.1111/j.1525-1314.1991.tb00536.x, 1991.Shelley, D.: Spider texture and amphibole preferred orientations, J. Struct.
Geol., 16, 709–717, 10.1016/0191-8141(94)90120-1, 1994.Skemer, P., Katayama, I., Jiang, Z., and Karato, S.: The misorientation
index: Development of a new method for calculating the strength of
lattice-preferred orientation, Tectonophysics, 411, 157–167,
10.1016/J.TECTO.2005.08.023, 2005.Skogby, H.: Water in natural mantle minerals I: Pyroxenes, in: Water in
Nominally Anhydrous Minerals, vol. 62, pp. 155–168, Walter de Gruyter
GmbH, 2006.
Soret, M., Agard, P., Ildefonse, B., Dubacq, B., Prigent, C., and Rosenberg, C.: Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy, Solid Earth, 10, 1733–1755, 10.5194/se-10-1733-2019, 2019.Stünitz, H. and Fitz Gerald, J. D.: Deformation of granitoids at low
metamorphic grades: II. Granular flow in albite rich mylonites,
Tectonophysics, 221, 299–324, 10.1016/0040-1951(93)90164-F, 1993.Stünitz, H. and Tullis, J.: Weakening and strain localization produced
by syn-deformational reaction of plagioclase, Int. J. Earth Sci., 90,
136–148, 10.1007/s005310000148, 2001.Sundberg, M. and Cooper, R. F.: Crystallographic preferred orientation
produced by diffusional creep of harzburgite: Effects of chemical
interactions among phases during plastic flow, J. Geophys. Res.-Solid Earth,
113, B12208, 10.1029/2008JB005618, 2008.Svahnberg, H. and Piazolo, S.: Interaction of chemical and physical
processes during deformation at fluid-present conditions: A case study from
an anorthosite-leucogabbro deformed at amphibolite facies conditions,
Contrib. Mineral. Petrol., 165, 543–562,
10.1007/s00410-012-0822-9, 2013.Tasaka, M., Zimmerman, M. E., and Kohlstedt, D. L.: Evolution of the
rheological and microstructural properties of olivine aggregates during
dislocation creep under hydrous conditions, J. Geophys. Res.-Solid Earth,
121, 92–113, 10.1002/2015JB012134, 2016.Tasaka, M., Zimmerman, M. E., Kohlstedt, D. L., Stünitz, H., and
Heilbronner, R.: Rheological Weakening of Olivine + Orthopyroxene
Aggregates Due To Phase Mixing: Part 2. Microstructural Development, J.
Geophys. Res.-Solid Earth, 122, 7597–7612, 10.1002/2017JB014311,
2017.Tullis, J., Yund, R., and Farver, J.: Deformation-enhanced fluid distribution
in feldspar aggregates and implications for ductile shear zones, Geology,
24, 63–66, 10.1130/0091-7613(1996)024<0063:defdif>2.3.co;2, 1996.Vissers, R. L. M., Drury, M. R., Newman, J., and Fliervoet, T. F.: Mylonitic
deformation in upper mantle peridotites of the North Pyrenean Zone (France):
implications for strength and strain localization in the lithosphere,
Tectonophysics, 279, 303–325, 10.1016/S0040-1951(97)00128-5,
1997.Warren, J. M. and Hirth, G.: Grain size sensitive deformation mechanisms in
naturally deformed peridotites, Earth Planet. Sci. Lett., 248,
423–435, 10.1016/j.epsl.2006.06.006, 2006.Wenk, H.-R. and Christie, J. M.: Comments on the interpretation of
deformation textures in rocks, J. Struct. Geol., 13, 1091–1110,
10.1016/0191-8141(91)90071-P, 1991.Wheeler, J.: Importance of pressure solution and coble creep in the
deformation of polymineralic rocks, J. Geophys. Res., 97, 4579,
10.1029/91JB02476, 1992.