the Creative Commons Attribution 4.0 License.

the Creative Commons Attribution 4.0 License.

# Post-entrapment modification of residual inclusion pressure and its implications for Raman elastic thermobarometry

### Xin Zhong

### Evangelos Moulas

### Lucie Tajčmanová

Residual pressure can be preserved in mineral inclusions, e.g.
quartz-in-garnet, after exhumation due to differential expansion between
inclusion and host crystals. Raman spectroscopy has been applied to infer
the residual pressure and provides information on the entrapment temperature
and pressure conditions. However, the amount of residual pressure relaxation
cannot be directly measured. An underestimation or overestimation of residual
pressure may lead to significant errors between calculated and actual
entrapment pressure. This study focuses on three mechanisms responsible for
the residual pressure modification: (1) viscous creep; (2) plastic yield; (3) proximity of inclusion to the thin-section surface. Criteria are provided to
quantify how much of the expected residual pressure is modified due to these
three mechanisms. An analytical solution is introduced to demonstrate the
effect of inclusion depth on the residual pressure field when the inclusion
is close to the thin-section surface. It is shown that for a quartz-in-garnet
system, the distance between the thin-section surface and inclusion centre needs
to be at least 3 times the inclusion radius to avoid pressure release.
In terms of viscous creep, representative case studies on a quartz-in-garnet
system show that viscous relaxation may occur from temperatures as low as
600–700 ^{∘}C depending on the particular
pressure–temperature (*P*–*T*) path and various garnet compositions. For quartz
entrapped along the prograde *P*–*T* path and subject to viscous relaxation at
peak *T* above 600–700 ^{∘}C, its residual pressure after
exhumation may be higher than predicted from its true entrapment conditions.
Moreover, such a viscous resetting effect may introduce apparent
overstepping of garnet nucleation that is not related to reaction affinity.

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During metamorphism, the growth of porphyroblasts often results in the
entrapment of solid or fluid inclusions, which then provide important
information about the rock's history
(e.g.
Farber et al., 2014; Yardley and Bodnar, 2014; Ferrero and Angel, 2018). Due
to the differences in the elastic compressibility and thermal expansion
coefficient between the inclusion and host, residual inclusion pressures may
be preserved after exhumation
(e.g.
Rosenfeld and Chase, 1961; Gillet et al., 1984; Zhang, 1998; Angel et al.,
2015). The residual pressure can be inferred by Raman shift based on
experimental calibrations, e.g. quartz inclusions (Liu
and Mernagh, 1992; Schmidt and Ziemann, 2000). This allows the application
of Raman thermobarometry to infer the entrapment pressure and temperature
(*P*–*T*) conditions
(e.g.
Ashley et al., 2014; Bayet et al., 2018; Enami et al., 2007; Izraeli et al.,
1999; Kohn, 2014; Spear et al., 2014; Taguchi et al., 2019a; Zhong et al.,
2019a). Existing models that link residual pressure and entrapment *P*–*T*
conditions are based on elastic rheology and often assume an infinite host
radius (Rosenfeld and
Chase, 1961; Van Der Molen and Van Roermund, 1986; Guiraud and Powell, 2006;
Angel et al., 2017b). Despite these simplifications, recent experimental
works have been successfully performed to compare the measured residual
pressure with modelled residual pressure under well-controlled *P*–*T* conditions
for synthetic samples with a quartz-in-garnet system
(Thomas and Spear, 2018; Bonazzi
et al., 2019).

Although many studies using Raman spectroscopy reported maximal residual
pressure close to the predictions from elastic models
(e.g. Ashley et al., 2014; Enami
et al., 2007; Zhong et al., 2019a), a large amount of inclusion pressure
estimates are lower than theoretically predicted by the elastic model
(Korsakov et
al., 2009; Kouketsu et al., 2016; Yamamoto et al., 2002). The modification
of inclusion pressure can be due to various reasons and a systematic
investigation is critical to better apply Raman thermobarometry to natural
samples. Meanwhile, Raman thermobarometry has been employed to investigate
the amount of overstepping for garnet growth by comparing the *P*–*T* constraints
from phase equilibria and elastic thermobarometry
(Spear et al., 2014; Castro and Spear,
2017; Wolfe and Spear, 2017). Particularly, when comparing the determined
paleopressures based on phase equilibria and elastic barometry using
a quartz-in-garnet system, careful examination of the amount of residual
quartz pressure modification due to the creep of garnet host becomes
critical.

When a mineral inclusion maintains residual pressure, differential stress is concomitantly developed around the inclusion on the host side to maintain mechanical equilibrium (e.g. Zhang, 1998; Tajčmanová et al., 2014). The host mineral may experience viscous creep, which is manifested by the dislocation structures (Chen et al., 1996; Yamamoto et al., 2002; Taguchi et al., 2019b). Furthermore, the host mineral may also experience rate-independent plastic yield when the differential stress exceeds the yield criterion (e.g. Zhang, 1998). In the mechanics literature, plastic deformation is commonly considered as any inelastic deformation (time-dependent and time-independent) (e.g. Kachanov, 1971). In this work, we distinguish between viscous creep, i.e. the rate-dependent inelastic deformation, and the rate-independent plastic flow. Mechanical models show that both viscous creep (dislocation or diffusion creep of host) and plastic flow during decompression and cooling can cause a significant inclusion pressure drop (Dabrowski et al., 2015; Zhang, 1998). This would lead to an underestimate of residual inclusion pressure (Zhong et al., 2018) (Fig. 1). Meanwhile, during the thin-section preparation, mineral inclusions are positioned into proximity towards the thin-section surface (Fig. 1). The thin-section surface is stress-free and may elastically release the residual pressure (Mindlin and Cheng, 1950; Seo and Mura, 1979; Zhong et al., 2019b). It is of petrological interest to study how deep the inclusion needs to be in order to preserve the residual pressure. Experimental works and numerical simulations with the finite-element method have been performed to test the safe inclusion depth (inclusion radius less than one-third of host radius) so that the residual inclusion pressure can be preserved for the application of Raman barometry (Campomenosi et al., 2018; Mazzucchelli et al., 2018).

In this contribution, we systematically investigate the following mechanisms for residual inclusion pressure modification: (1) viscous creep of the host materials, (2) plastic yield within the host, and (3) pressure release due to the proximity of inclusion towards the thin-section surface. For the first and second purposes, a 1-D visco-elasto-plastic mechanical model is developed in a radially symmetric spherical coordinate frame. The derived system of equations is non-dimensionalized to extract the key parameters that control the amount of viscous relaxation and plastic yield. For the third mechanism, a simple analytical solution for the residual inclusion pressure field close to the thin-section surface is introduced based on the existing work of Seo and Mura (1979). The analytical solution demonstrates the effect of the inclusion depth that controls the amount of pressure release. This solution applies to the case where the inclusion possesses the same elastic moduli as the host. The inclusion is initially subject to an arbitrary hydrostatic pressure in an infinite host and its pressure is released as it moves towards a stress-free thin-section surface. In comparison, for a natural quartz-in-garnet system, numerical solutions are applied to investigate the safe distance that causes negligible pressure release due to the presence of the thin-section surface (stress-free boundary). In this study, both inclusion and host are treated as elastically isotropic as an assumption to put focus on the effect of these three mechanisms on preserved residual pressure. The effects of elastic anisotropy for commonly encountered quartz inclusion have been studied experimentally and theoretically by, e.g., Murri et al. (2018) and Campomenosi et al. (2018) and are discussed at the end.

## 2.1 Governing equations

We develop a 1-D mechanical model with spherical symmetry that is based on the conservation of mass and momentum. In 1-D radially symmetric spherical coordinate frame, mechanical equilibrium is expressed as follows:

where *τ*_{rr} is the radial component of deviatoric stress (Pa), *P* is
pressure (Pa), and *r* is radial coordinate (*m*). We apply the Maxwell
visco-elasto-plastic rheology as follows:

where ${\dot{e}}_{rr}$ is the radial components of the deviatoric strain rate
(s^{−1}) composed of elastic, viscous (rate-dependent) and plastic
(rate-independent) parts. The elastic and viscous strain rates are expressed
as

where the dot above ${\dot{\mathit{\tau}}}_{rr}$ denotes time derivative, *G* is shear
modulus (Pa), and *η* is viscosity (Pa s). The non-Newtonian
(effective) viscosity is expressed as follows:

where *A* is the temperature-dependent pre-factor and *n* is the stress
exponent (e.g. Dabrowski et al., 2015, Eq. 10). The plastic strain rate is
obtained by using the Tresca yield criterion (e.g. Kachanov,
1971):

where *C* is plastic yield strength (Pa) and *τ*_{tt} is the tangential
component of deviatoric stress. Due to spherical symmetry, we have ${\mathit{\tau}}_{tt}=-\mathrm{1}/\mathrm{2}{\mathit{\tau}}_{rr}$. Applying the plastic flow law (e.g.
Vermeer and De Borst, 1984), we get

where *λ* is the plastic multiplier (s^{−1}), which provides the
amount of plastic strain (rate) that guarantees the yield criterion is not
exceeded, and *δ* is the sign of *τ*_{rr}. For isotropic
materials, the pressure (negative mean stress) can be expressed as a
function of volume and temperature via the equation of state (EoS), and its time
derivative is as follows:

where *β* is compressibility (Pa^{−1}), *α* is the thermal
expansion coefficient (K^{−1}), and $\dot{T}$ is the rate of temperature change
(K s^{−1}). Temperature is treated as homogeneous within the inclusion–host system.
Einstein summation is used here for the volumetric strain rate
(${\dot{\mathit{\epsilon}}}_{kk}={\dot{\mathit{\epsilon}}}_{rr}+\mathrm{2}{\dot{\mathit{\epsilon}}}_{tt}$,
where due to spherical symmetry the two tangential strain rates are equal).
No viscous or plastic volumetric strain is considered. This assumption is a
good approximation for non-porous, crystalline materials (e.g.
Moulas et al., 2019).

Substituting Eqs. (3) and (6) into Eq. (2) and applying first-order finite
difference in time to Eqs. (2) and (7) (i.e. ${\dot{\mathit{\tau}}}_{rr}=\frac{{\mathit{\tau}}_{rr}-{\mathit{\tau}}_{rr}^{o}}{\mathrm{\Delta}t}$ and
$\dot{P}=\frac{P-{P}^{o}}{\mathrm{\Delta}t}$), we can explicitly express
*τ*_{rr} and *P* as

where $Z=\frac{\mathrm{\Delta}tG}{\mathrm{\Delta}tG+\mathit{\eta}}$ is the
viscoelastic coefficient, Δ*t* is the time increment, ${\mathit{\tau}}_{rr}^{o}$ is the radial component of deviatoric stress in the previous time
step, *P*^{o} is the pressure in the previous time step. If the yield criterion
in Eq. (5) is exceeded (*F*>0), the plastic multiplier must be adjusted to
drive *F* to zero. This can be achieved by substituting the deviatoric
stress Eq. (8) into Eq. (5), and we let *F*=0. Therefore, we obtain *λ* as
follows:

## 2.2 Non-dimensionalization

The variables in the above equations can be scaled to derive non-dimensional
parameters for better understanding the behaviour of the inclusion–host
system. This is done by choosing the following six parameters to
non-dimensionalize the system of equations: the temperature drop in the
host-inclusion system Δ*T*, the inclusion radius *R*, the
time of the *P*–*T* path *t*^{∗}, the host's viscosity pre-factor
*A*_{h}, the host's plastic yield strength *C*_{h}, and
the expected pressure perturbation *P*_{exp} that is given as
follows:

where Δ*P* and Δ*T* are the confining pressure
and temperature drops from entrapment to the Earth's surface, *β*_{i}
and *β*_{h} are the compressibility of inclusion and host, *α*_{i} and *α*_{h} are the thermal expansion coefficients of inclusion
and host, and *G*_{h} is the shear modulus of host. The number
*P*_{exp} is the expected amount of residual inclusion pressure
after exhumation assuming linear thermo-elasticity and an infinite host
(Zhang, 1998). It is noted that this is
not the actual final residual inclusion pressure but merely a scale to
non-dimensionalize the stress (and pressure). By choosing *P*_{exp}
as the stress scale, the inclusion residual pressure is expected to be
between 0 and 1 for a case of cooling and decompression. This pressure scale
allows convenient quantification for the amount of pressure modification due
to viscous creep and plastic yield. The involved physical variables are
scaled as shown below:

where the overhead bars indicate dimensionless properties. Substituting these scaling equations into Eqs. (1), (8), and (9), we get

where dimensionless viscosity, viscoelastic coefficient, and plastic multiplier are expressed as

Two dominant dimensionless numbers emerge after non-dimensionalization. They
are the Deborah number *De* and dimensionless yield strength *C*^{∗} defined
as follows:

The Deborah number (*De*) is the ratio between the characteristic
viscous relaxation time (${A}_{\mathrm{h}}/{P}_{\mathrm{exp}}^{n}$) and model
duration (*t*^{∗}) (Reiner, 1964). If *De*>1, the
system behaves in an elastic manner, and if $\mathit{\text{De}}<\mathrm{1},$ viscous creep
becomes important. The pre-factor of viscosity is temperature-dependent. By
choosing the pre-factor *A*_{h} at peak temperature, one can
directly use *De* to estimate the maximal amount of viscous relaxation. This
is especially suitable for the process of isothermal decompression in many
high-pressure (HP) rocks.

The dimensionless yield strength *C*^{∗} characterizes the ability of a
host mineral to plastically yield and a high *C*^{∗} implies that the
material is less prone to plastic yield given the amount of residual
inclusion pressure *P*_{exp}. The viscosity of different mineral
phases may vary by orders of magnitude, and the plastic yield strength of
different minerals may also vary by several factors. Therefore, these two
dimensionless numbers have a dominant effect on the amount of inclusion
pressure modification due to viscous relaxation and plastic yield.

## 2.3 The numerical approach for the visco-elasto-plastic model

The dimensionless viscosity (Eq. 16), viscoelastic coefficient (Eq. 17), and
plastic multiplier (Eq. 18) can be substituted into pressure equation (Eq. 14) and deviatoric stress equation (Eq. 15). Together with mechanical
equilibrium equation (Eq. 13), they form a system of three equations with
three unknowns, namely $\stackrel{\mathrm{\u203e}}{{v}_{r}}$, $\stackrel{\mathrm{\u203e}}{{\mathit{\tau}}_{rr}}$, and $\stackrel{\mathrm{\u203e}}{P}$. The
numerical model is based on a finite-difference scheme over a 1-D staggered
grid (on the numerical stencil, see, e.g., Gerya, 2010,
chap. 7). The initial pressure $\stackrel{\mathrm{\u203e}}{P}$ is set at the beginning of the
numerical model. If upon entrapment, the inclusion and host possess the same
hydrostatic pressure, the deviatoric stress $\stackrel{\mathrm{\u203e}}{{\mathit{\tau}}_{rr}}$ is zero in
the inclusion and host. If pressure heterogeneity exists upon entrapment, the
deviatoric stress of the host ($\stackrel{\mathrm{\u203e}}{{\mathit{\tau}}_{rr}}$) needs to be
precalculated with the elastic model $\stackrel{\mathrm{\u203e}}{{\mathit{\tau}}_{rr}}=-({P}_{\mathrm{inc}}-{P}_{\mathrm{host}})/{\stackrel{\mathrm{\u203e}}{r}}^{\mathrm{3}}$ to ensure that
mechanical equilibrium is satisfied at the beginning of the model
(*P*_{inc} is the initial inclusion pressure and
*P*_{host} is the initial host pressure).

For predefined *P*–*T* path, the inclusion–host system is loaded by the increment
of confining pressure and temperature. Both temperature and far-field
pressure are obtained directly from the *P*–*T*–*t* path as prescribed. Temperature is
treated as homogeneous in the model and the new pressure is set as the outer
boundary value. Because viscosity, the viscoelastic coefficient, and the plastic
multiplier are functions of deviatoric stress (viscosity is also a function
of temperature as prescribed by the *P*–*T* path), the system of the mechanical
equations is nonlinear. We solve for the three variables ($\stackrel{\mathrm{\u203e}}{{v}_{r}}$,
$\stackrel{\mathrm{\u203e}}{{\mathit{\tau}}_{rr}}$, and $\stackrel{\mathrm{\u203e}}{P}$) using an iterative (Picard) method. Within
the iteration loop, an elastic test stress is first evaluated by letting
$\stackrel{\mathrm{\u203e}}{\mathit{\lambda}}=\mathrm{0}$ so that the prediction for the yield function $\stackrel{\mathrm{\u203e}}{F}$
is obtained. If $\stackrel{\mathrm{\u203e}}{F}<\mathrm{0}$, no plastic yield occurs and $\stackrel{\mathrm{\u203e}}{\mathit{\lambda}}$
remains zero. Otherwise the prediction of the yield function is positive and
$\stackrel{\mathrm{\u203e}}{\mathit{\lambda}}$ is computed based on Eq. (18). The calculated $\stackrel{\mathrm{\u203e}}{\mathit{\lambda}}$
is then substituted back into Eq. (15) to adjust the amount of plastic strain
rate. This will drive $\stackrel{\mathrm{\u203e}}{F}$ to zero (on the plastic yield surface).
After the Picard iteration loop, the residuals of the three Eqs. (13), (14), and (15) are minimized below ca. 10^{−12}.

The elastic moduli are updated based on pressure and temperature conditions from tabulated lookup tables within the iteration. The lookup tables are pre-computed based on EoS. We used the EoS for quartz crystal from Angel et al. (2017a) and the EoS for pyrope, grossular, and almandine crystals from Milani et al. (2015). The detailed expressions of EoS can be found in the EoSFit7c software documentation (Angel et al., 2014). The EoS for spessartine is from Gréaux and Yamada (2014). The compressibility and thermal expansion coefficient for garnet are averaged based on the molar percentage of garnet endmembers. The shear moduli of garnet endmembers are from Bass (1995). The numerical model has been benchmarked using the analytical solution with elastic, non-Newtonian viscous rheology in Zhong et al. (2018). The numerical benchmark for elasto-plastic rheology is performed by using the analytical solution of Zhang (1998) (see Supplement).

## 3.1 Residual pressure affected by viscous or plastic flow

The solutions of the system of equation (Eqs. 13, 14, 15) are obtained using
the elasticity of a quartz-in-garnet system. The host radius is set to be 50
times the inclusion radius to make boundary effects negligible. Temperature
is treated as homogeneous in space. At the beginning of the model, a
pressure perturbation within the inclusion is prescribed, and the far-field
host maintains zero confining pressure. The pre-factor of viscosity is fixed
as temperature does not vary in this case. The amount of inclusion pressure
relaxation is systematically investigated for the two inelastic deformation
mechanisms (i.e. viscous creep and plastic yield) as a function of *De* and
*C*^{∗}. The results are shown in Fig. 2 with the purpose of
systematically demonstrating how much the initially prescribed residual
pressure can be reduced due to viscous creep and plastic yield. This diagram
may assist petrological investigations because *De* and *C*^{∗} can be
evaluated based on experimental rock deformation data for different
minerals. The Deborah number can be evaluated using the experimental flow law of a single crystal, e.g. garnet
(Karato
et al., 1995; Wang and Ji, 1999), as given in the next section. The plastic
yield strength is evaluated using microhardness test data (see Discussion
below for details). The thickness of the plastic yield region is plotted as
contours. The thick grey contour represents the onset of plastic yield
starting from the inclusion–host interface and propagating towards the host
side (Fig. 2). Based on the amount of inclusion pressure relaxation, three
regimes are distinguished.

The *elastic regime* takes place when *De* and *C*^{∗} are higher than 1. Under these
circumstances, no viscous relaxation and plastic yield occurs. The residual
inclusion pressure is close to the expected residual pressure
(*P*_{inc}≈*P*_{exp}). This regime is the most
suitable for the application of Raman thermobarometry.

The *viscous regime* dominates when *De* is lower than 1, and *C*^{∗} is above the plastic
onset shown by the thick grey contour. In this case, the main mechanism
responsible for the inclusion pressure relaxation is the viscous creep. The
effect of stress exponent on the amount of viscous relaxation is also
significant. In general, a higher stress exponent delays pressure relaxation
(cf. Dabrowski et al., 2015). As the viscosity of
natural minerals is low at high-temperature conditions, the viscous regime
may be reached at a high temperature, which leads to the relaxation of residual
pressure.

The *plastic regime* prevails when *C*^{∗} is lower than 1 and *De* is above 1. Under
this circumstance, the residual pressure is not significantly relaxed by
viscous creep, but by plastic yield. In general, the radius of the plastic yield
region is positively related to the amount of residual pressure release.

It is noted here that although viscous relaxation and plastic yield of the
host mineral have the same effect in reducing the residual inclusion
pressure after exhumation, there is a fundamental difference between them.
Viscous relaxation is time-dependent (*De* includes time), which means that the
residual pressure will vanish given an infinite amount of time. Plastic yield
refers to a time-independent process, and it will only limit the amount of
deviatoric stress supported by the host mineral. If the yield criterion is
reached, plastic strain (rate) in the host immediately occurs, which leads to
the inclusion pressure release. Both viscous relaxation and plastic yield
are irreversible; as a consequence, if the inclusion-host system is placed back in the
original entrapment condition, the stress state would be different.

## 3.2 Viscous relaxation of garnet host

Assuming that the thin-section surface is sufficiently far away from a
quartz inclusion and no plastic yield occurs around the quartz inclusion,
only viscous creep may contribute to the modification of residual pressure.
Here, we show the effect of viscous relaxation, particularly influenced by
the temperature, on the preserved residual pressure. Using *De* as a criterion
to estimate the amount of viscous relaxation, we show the relationship
between temperature, inclusion pressure, and relaxation time given *De*=1 (see
Eq. 19) in Fig. 3. The flow law of garnet from
Wang
and Ji (1999) is applied. The pre-factor *A* of the effective viscosity (Eq. 4) is as follows:

where *B*=exp (40.1) in s^{−1} and *g*=32. The stress exponent
*n*=3. Geometric correction based on experimental set-up (simple or pure shear)
was not applied. The melting point *T*_{m} of pyrope-rich garnet, grossular, and spessartine is from the Table 1 in Karato et al. (1995). For almost pure almandine, the garnet melting point is found to be
1588 K from Mohawk Garnet Inc., which is slightly higher than 1570 K for
almandine-rich (Alm_{0.68}Prp_{0.20}Grs_{0.12}) garnet in
Karato et al. (1995).

^{1} Data reported in
Whitney et
al. (2007).
^{2} Data reported in Wong and Bradt (1992). The
reported data for calcite and dolomite are averaged from the applied load
and azimuthal angle from [$\mathrm{10}\stackrel{\mathrm{\u203e}}{\mathrm{1}}\stackrel{\mathrm{\u203e}}{\mathrm{1}}$].
^{3} Data reported in Smedskjaer et al. (2008).
^{4} Data reported in Yuan et al. (2017)
^{5} Data reported in Dekker and Rieck (1974). The
reported data are averaged from the applied load at [110] and [100].

As an example (Fig. 3), for a quartz inclusion possessing 0.5 GPa residual
pressure maintained at 650 ^{∘}C, significant viscous relaxation is
expected during 1 Myr for almandine-rich garnet host based on the applied
flow law. This temperature becomes higher (700 ^{∘}C) for pyrope-rich
garnet. If the residual pressure is used to recover the entrapment pressure
given that the temperature experienced by the garnet-host system was higher
than 650–700 ^{∘}C, an underestimate of the entrapment
pressure may potentially occur.

In Fig. 4, synthetic retrograde *P*–*T* paths from eclogite and amphibolite-facies
metamorphic conditions are prescribed with different peak temperatures. The
entrapment *P*–*T* conditions for the three synthetic *P*–*T* paths are along an elastic
isomeke, which is the isopleth where no relative elastic interaction exists
between inclusion and host. Thus, the residual inclusion pressure would be
the same if the inclusions were entrapped along the same isomeke and they
were purely elastic. By involving the viscous rheology of the garnet host,
different residual inclusion pressures are predicted. For the *P*–*T* path starting
at 800 ^{∘}C, 2 GPa, the quartz inclusion pressure is predicted to be less
than 0.2 GPa. This reduced value of the residual pressure is then used to
determine the apparent entrapment pressure (Fig. 4b). In Fig. 4b, it is
shown that for the entrapment pressure within eclogite-facies conditions at
700 ^{∘}C, and by using a purely-elastic model, a value of entrapment
pressure is inferred that is approximately 10 % less than the actual
value. The amount of underestimation of entrapment pressure increases to
30 % when the entrapment temperature reaches 800 ^{∘}C. These values are
conservative estimates since the total exhumation time is set to 1 Myr.
Longer residence at high-temperature conditions would result in larger
modifications of the residual pressure.

For amphibolite-facies entrapment conditions, the residual pressure that is
preserved in the quartz inclusion is significantly lower compared to the
case where the entrapment occurred at eclogites-facies conditions. As shown
in Fig. 4d, ca. 5 % and 20 % underestimates of true entrapment pressure
are predicted depending on whether the entrapment occurred at 700 ^{∘}C or 800 ^{∘}C, respectively. Similarly, the amount of underestimation will be
larger if the duration of exhumation is longer than 1 Myr.

## 3.3 Pressure relaxation along the prograde *P*–*T* path and apparent overstepping

The pressure relaxation problem becomes more complicated when the quartz
inclusion is entrapped not at the peak *P*–*T* conditions but along the prograde
*P*–*T* path. In this case, viscous relaxation occurs also along the prograde *P*–*T* path
and the pressure difference between host and inclusion will relax with time
and increasing temperature. This effect starts before the rock reaches the
peak *P*–*T* conditions. Two synthetic *P*–*T* paths are illustrated in Fig. 5. In Fig. 5a, the quartz is entrapped in the almandine-garnet host at 400 ^{∘}C, 1 GPa, and further experiences eclogites-facies *P*–*T* conditions. During the
prograde path, the quartz inclusion will develop underpressure
(e.g. Angel et al., 2015, Fig. 5),
which will also be subject to viscous relaxation over geological time. The
quartz pressure starts to converge towards the garnet host pressure at
*T* > 600 ^{∘}C. Nearly complete viscous resetting is observed when
the system is brought up to 800 ^{∘}C. The prograde time is set to 1 Myr or
10 Myr to compare the amount of viscous relaxation as a function of time in
Fig. 5.

An alternative scenario is considered where the rock may also stay at the
peak *P*–*T* conditions before decompression occurs. A synthetic clockwise *P*–*T* path
reaching eclogite-facies metamorphic conditions is constructed as shown in
Fig. 6. The quartz inclusion is entrapped in the garnet host at 400 ^{∘}C, 0.6 GPa, which is considered to be along the entrance of the garnet
stability field. Subsequently the system is brought to 700–750 ^{∘}C, 1.8–1.9 GPa, conditions and stays there for 5 Myr.
Afterwards, the retrograde *P*–*T* path takes 10 Myr. Two different *P*–*T* paths of quartz
inclusions are constructed based on the implemented elastic and
visco-elastic rheologies. Interestingly, the residual pressure of the
inclusion that was subject to viscous relaxation is significantly higher (by
0.2 GPa) than the prediction of the pure elastic model as shown by the black
dashed curve (0.14 GPa). The apparent entrapment pressure is calculated
using the predicted residual pressure for the inclusion whose host
experienced viscous relaxation. A large discrepancy exists between the
apparent entrapment pressure (ca. 1 GPa at the entrapment *T* 400 ^{∘}C) and
the true entrapment pressure (0.6 GPa). The overall overestimate of true
entrapment pressure (0.6 GPa) is about 0.3–0.4 GPa, which may
potentially be interpreted as overstepping the garnet growth or nucleation.

Despite the importance of viscous or plastic relaxation in the post-entrapment modification of pressure, residual pressure measurements may be different when the inclusions are closer to the thin-section surface (Enami et al., 2007). When a pressurized mineral inclusion in an infinite host under mechanical equilibrium is moved towards the thin-section surface, its pressure is released and the pressure distribution within the inclusion becomes heterogeneous. Mindlin and Cheng (1950) provided a closed-form analytical solution of stress field inside and outside a spherical inclusion with thermal strain in a semi-infinite host. The analytical solution has been generalized to ellipsoidal inclusion (Seo and Mura, 1979). Substantial mathematical investigations have also been done in deriving the analytical solution of the elastic field for inclusion in half-space (e.g. Tsuchida and Nakahara, 1970; Aderogba, 1976; Jasiuk et al., 1991). Although the analytical formulations for individual stress components of inclusion are non-trivial, here, we show that the formula for the pressure distribution of a pressurized inclusion can be significantly simplified (detailed derivations are provided in the Appendix):

where *P*_{ini} is the initial inclusion pressure in an infinite host
under mechanical equilibrium, $\stackrel{\mathrm{\u203e}}{L}$ is the scaled inclusion depth
($\stackrel{\mathrm{\u203e}}{L}=L/R$) and ${\stackrel{\mathrm{\u203e}}{R}}_{\mathrm{2}}=\sqrt{{x}_{\mathrm{1}}^{\mathrm{2}}+{x}_{\mathrm{2}}^{\mathrm{2}}+({x}_{\mathrm{3}}+L{)}^{\mathrm{2}}}/R$ is a function of position in Cartesian coordinate system (Fig. 7), and *ν* is the Poisson ratio of the inclusion and host. It is emphasized that in
this situation the inclusion and host possess the same elastic moduli.

The released inclusion pressure due to proximity to the thin-section surface is plotted in Fig. 7b and c using Eq. (22). Pressure release is concentrated at the top of the inclusion, while the bottom of the inclusion is subject to minimal pressure releases (< 10 %). Interestingly, the top of the inclusion is subject to negative pressure (expansion) when the inclusion is very close to the thin-section surface (see, e.g., the case of $\stackrel{\mathrm{\u203e}}{L}=\mathrm{1.1}$). Based on the analytical solution, the safe inclusion depth to avoid residual pressure release is ca. $\stackrel{\mathrm{\u203e}}{L}=\mathrm{2.5}$ (the amount of pressure release is less than 3 % within the entire inclusion). Here, the simple analytical solution in Eq. (22) can precisely model the inclusion's residual pressure due to stress release at the thin-section surface, where the inclusion possesses the same elastic moduli as the host. In a natural mineral inclusion–host system, the inclusion and host possess different elastic properties. As a case study, the stress fields of quartz-in-almandine and almandine-in-quartz systems are numerically modelled using a finite-difference (FD) thermo-elastic model (model benchmarks are provided in the Supplement). These examples are chosen to investigate two endmembers: elastically stiffer host (quartz-in-almandine in Fig. 8a) and softer host (almandine-in-quartz in Fig. 8b). Pressures at three points within the inclusion (top, centre, and bottom) are contoured as a function of $\stackrel{\mathrm{\u203e}}{L}$. The pressures evaluated at these three localities based on the analytical solution in Eq. (22) are also shown by the dashed curves for comparison with numerical solutions. With decreasing distance to the thin-section surface, the heterogeneity of the pressure field increases. It is shown that the pressure release is less significant in elastically stiffer host (garnet) than in elastically softer host (quartz).

It is shown that the difference between the analytical and the numerical solution due to the difference in elastic moduli becomes significant when the inclusion depth is shallow. The analytical and solutions are similar when evaluated at the bottom point at any depth (Fig. 8). For a quartz-in-garnet system, the analytical solution overestimates the amount of pressure release (Fig. 8a). Assuming 3 % pressure release as acceptable for the application of Raman barometry, the analytical solution yields a safe distance of ca. $\stackrel{\mathrm{\u203e}}{L}=\mathrm{2.0}$ for the bottom and centre point, while the numerical solution yields ca. $\stackrel{\mathrm{\u203e}}{L}=\mathrm{1.5}$. For the top point, the safe distance of ca. $\stackrel{\mathrm{\u203e}}{L}=\mathrm{2.5}$ based on the analytical solution is again higher than the prediction of ca. $\stackrel{\mathrm{\u203e}}{L}=\mathrm{2.0}$ based on a numerical solution. The difference in safe distance between analytical and numerical solution is due to the presence of elastically stiffer garnet host.

Differential stress (|*σ*_{1}−*σ*_{3}|) is also shown in
Fig. 8b. High differential stress at the host appears when the inclusion is
close to the thin-section surface. Differential stress may also exist inside the
inclusion, but it is in general smaller than that of the host. For a quartz-in-garnet system, the differential stress forms a “ring”-shaped
pattern with a peak at the surface. The differential stress may reach up to
3 times the expected residual pressure. This may potentially trigger
plastic failure at the thin-section surface. However, for the garnet-in-quartz
system, such pattern is not observed even if the inclusion depth is shallow.

## 5.1 What may cause the residual pressure modification?

The mechanisms investigated here, i.e. visco-plastic flow of the host and
proximity of the inclusion to the thin-section surface, can all be responsible for
the modification of the residual inclusion pressure. The amount of
inclusion pressure change due to these mechanisms is controlled by
the Deborah number (*De*), dimensionless yield strength (*C*^{∗}), and
dimensionless depth ($\stackrel{\mathrm{\u203e}}{L}$). In the examples of
quartz-in-garnet systems, the residual pressure is considered to be sealed
in a perfectly elastic garnet host. Based on our study, the presence (radius)
of a plastic yield region and preserved residual inclusion pressure are
dominated by dimensionless yield strength $({C}^{\ast}={C}_{\mathrm{h}}/{P}_{\mathrm{exp}}$) as shown in Fig. 2. Strength *C*_{h} can be
converted from hardness test data using the formula below
(e.g. Evans and Goetze,
1979):

where *H* is the measured microhardness and *C*_{g} is a constant accounting
for the indenter's geometry in the experiment. Taking *C*_{g}=3
(Evans and Goetze, 1979), the
yield strength of garnet host is between 4.4 and 5 GPa at room conditions
(Whitney et
al., 2007), which leads to ${C}^{\ast}=\mathrm{4.4}\sim \mathrm{5}$ given residual inclusion
pressure *P*_{exp}=1 GPa. This suggests that plastic
yield does not occur in an idealized scenario of isotropic, spherical quartz
inclusion entrapped in an infinite garnet host. However, such an ideal scenario
is highly improbable in natural samples. Localized plastic yield may still
occur due to the following reasons: (1) elevated differential stress when the
inclusion is close to the thin-section surface (“ring”-shaped pattern in Fig. 8); (2) stress concentration at the corners of the quartz inclusion
(Whitney et al., 2000); (3) anisotropic elastic
deformation of the quartz inclusion (e.g. Murri
et al., 2018); (4) pre-fractures or weakness in garnet host before the
entrapment of quartz inclusions that leads to the localization of
dislocations. Although our model does not predict exact conditions for
plastic yield due to the above possibilities, it gives a lower bound for the
strength and provides information on what type of host mineral phase cannot
be used for Raman barometry. Plastic yield strength of some common
rock-forming minerals measured in hardness tests are compiled and provided
in Table 1. As an example, given *P*_{exp}=1 GPa, the
dimensionless yield strength of calcite host is ca. 0.6 and dolomite is ca. 1.5 (Wong and Bradt, 1992). This implies that
plastic flow will most likely affect the residual pressure
*P*_{exp} in the calcite host but not in dolomite host.

After thin-section preparation, the inclusion pressure may be (partially) released. The dimensionless depth can be evaluated by performing depth-step scan analysis with Raman spectroscopy in order to observe if the pressure gradually decreases towards the thin-section surface (Enami et al., 2007). For a quartz-in-garnet system, to avoid significant pressure release (> 3 %) in the bottom half of the inclusion, the dimensionless depth needs to be above at least 1.5 (Fig. 8). To avoid significant pressure release in the entire quartz inclusion, the dimensionless depth needs to be above ∼2. Therefore, we recommend a safe dimensionless depth of 2–2.5 (from the surface to the centre of the inclusion) for quartz-in-garnet Raman barometry (see also Mazzucchelli et al., 2018). For a 30 µm thick thin section, the maximal radius of an inclusion is ca. 6 µm (12 µm in diameter) located at the mid-point of the thin section. In practice, it is difficult to precisely measure the depth of an inclusion, and it is uncommon that an inclusion is located right in the middle of a thin section. Therefore, it is ideal to choose smaller inclusions or prepare a thicker thin section for measurement (Campomenosi et al., 2018; Mazzucchelli et al., 2018).

For commonly used quartz-in-garnet Raman barometry, our results show that
below 550–600 ^{∘}C, the effect of viscous relaxation can
be negligible. Above ca. 650–750 ^{∘}C, the effect of
viscous relaxation needs to be taken into account depending on a particular
*P*–*T* path, garnet composition, and timescale (Figs. 3, 4). This is similar
to the empirical estimate ca. 750 ^{∘}C in Walters and
Kohn (2014). It is also shown that the preserved residual pressure may even
increase due to viscous relaxation if viscous resetting occurs at peak *P*
condition (Fig. 6). This is simply because viscous creep does not only relax
the overpressure in the quartz inclusion but also the underpressure that
develops along the prograde *P*–*T* path. Meanwhile, the amount of viscous relaxation is
time-dependent (*De* is a function of the operating time of viscous
relaxation). Thus, the above temperature criterion for Raman barometry
applies only for exhumation lasting at a million years' timescale. A higher
temperature criterion for Raman barometry (e.g. ∼1000 ^{∘}C
for garnet host at high pressure close to the coesite–quartz transition) is
applicable for more rapid exhumation, e.g. xenolith ascent carried by magma
(Zhong et al., 2018) or garnet synthesis experiments that
lasts hours or days (Thomas and
Spear, 2018; Bonazzi et al., 2019).

## 5.2 Implications to garnet overstepping

Quartz-in-garnet Raman barometry has been used to determine the entrapment
pressure, i.e. garnet nucleation or growth conditions, and compared it to the *P*–*T*
conditions determined based on phase equilibria or classical chemical
thermobarometry (Castro and Spear, 2017; Spear et
al., 2014). As has been shown in Fig. 6, viscous resetting occurs when the
inclusion–host system is brought to high temperature (> 600–700 ^{∘}C). Even if the quartz inclusion is entrapped
at lower *P*–*T* conditions, e.g. the garnet entrance conditions, the preserved
residual inclusion pressure may still be significantly higher than predicted
from the actual entrapment *P*–*T* conditions using a pure elastic model. In this
case, erroneous results may emerge if one uses the relaxed residual quartz
inclusion pressure to determine the entrapment pressure. In case of
significant viscous resetting at peak *T* conditions followed by decompression,
as in the case of some HP rocks, apparent garnet growth overstepping will be
inferred (see Fig. 6b). In that case, care must thus be taken to interpret
the discrepancy between the results of quartz-in-garnet Raman barometry and
phase equilibria. As shown in the example with a synthetic clockwise *P*–*T* path
(Fig. 6), ca. 3–4 kbar apparent overstepping is predicted by
considering viscous resetting at peak *T* condition. The amount of apparent
overstepping will be even larger if the exhumation process happens faster
(current model assumes 10 Myr decompression time).

We first presented a 1-D visco-elasto-plastic model to study the
inclusion–host system undergoing a prograde or retrograde *P*–*T* path. The
non-dimensionalization of the governing equations yields two controlling
parameters: the Deborah number (*De*) and dimensionless yield strength (*C*^{∗}), which control the amount of pressure drop due to viscous relaxation and
plastic yield. Both *De* and *C*^{∗} must be higher than 1 to avoid
underestimating the residual pressure. Subsequently, a simplified analytical
solution for inclusion pressure (Eq. 22) close to a stress-free thin-section
surface is presented based on the existing analytical solution from
Seo and Mura (1979). It is suggested that the distance between
the thin-section surface and inclusion must be higher than 2–3 times the inclusion radius to avoid stress release.

The relevance of our presented visco-elasto-plastic model to
quartz-in-garnet elastic barometry has been systematically studied. Although
plastic yield is not expected for garnet host due to its high yield
strength, the residual inclusion pressure preserved in quartz inclusion can
be partially modified at high temperature due to time-dependent viscous
creep. It is shown that above 650–700 ^{∘}C over a Myr timescale, viscous creep of garnet host may partially reset the quartz pressure.
This may have important implications for the determination of entrapment
pressure of quartz inclusion. Additionally, this may also cause apparent
overstepping of garnet growth; thus care must be taken when applying
quartz-in-garnet barometry at rocks which experienced high temperatures
(> 600–700 ^{∘}C).

Here, we introduce a simplified formula for the pressure distribution of an
initially pressurized inclusion in an infinite host moved toward a stress-free
surface based on the existing analytical solution of Seo and
Mura (1979). A Cartesian coordinate system is employed as shown in Fig. 7.
The full stress tensor *σ*_{ij} of inclusion loaded with eigenstrains
is represented as follows (Seo and Mura, 1979):

While for the host, stresses are given below:

where the indices of *x*_{i} ($i=\mathrm{1},\mathrm{2},\mathrm{3}$) are in Cartesian coordinate frame
following the order of *x*, *y*, and *z*(see Fig. 7) and *ε*^{∗} is
the isotropic eigenstrain that is expressed as the difference of volumetric
strain between inclusion and host assuming that they are not bounded by each
other. The elliptic integrals *ψ* and *ϕ* are expressed below:

where $\mathit{\lambda}={R}_{\mathrm{1}}^{\mathrm{2}}-{R}^{\mathrm{2}}$ for host, *λ*=0 for inclusion, and
${R}_{\mathrm{1}}=\sqrt{{x}_{\mathrm{1}}^{\mathrm{2}}+{x}_{\mathrm{2}}^{\mathrm{2}}+({x}_{\mathrm{3}}-L{)}^{\mathrm{2}}}$.

where $\mathit{\lambda}={R}_{\mathrm{2}}^{\mathrm{2}}-{R}^{\mathrm{2}}$ for both host and inclusion and ${R}_{\mathrm{2}}=\sqrt{{x}_{\mathrm{1}}^{\mathrm{2}}+{x}_{\mathrm{2}}^{\mathrm{2}}+({x}_{\mathrm{3}}+L{)}^{\mathrm{2}}}$. Here, we focus on the stress experienced by the inclusion and derive a simplified form for the pressure of inclusion. For the inclusion, the elliptic integrals are derived:

The normal stresses in the inclusion are

By substituting *ψ* and *ϕ* into the equations above, the normal
stresses can be obtained. In deriving the pressure, i.e. negative mean
stress, many terms in Eqs. (A7)–(A9) can be cancelled out. A
simplified form is obtained as follows:

The pre-factor $\frac{\mathrm{4}{\mathit{\epsilon}}^{\ast}\left(\mathrm{1}+\mathit{\nu}\right)G}{\mathrm{3}\left(\mathrm{1}-\mathit{\nu}\right)}$ is in fact the initial pressure of the inclusion in an infinite
host loaded by the eigenstrain *ε*^{∗} under mechanical
equilibrium. Therefore, we may simplify Eq. (A10) as follows:

where *P*_{ini} is the inclusion pressure in an infinite host loaded
by eigenstrain *ε*^{∗} under mechanical equilibrium before
moving it close to the thin-section surface. The equation can be
non-dimensionalized by using *R* as a length scale shown below:

The analytical solution for pressure in the mineral inclusion subject to an
initial residual pressure *P*_{ini} is obtained. When the inclusion
is far from the thin-section surface ($\stackrel{\mathrm{\u203e}}{L}\to +\mathrm{\infty}$, and ${\stackrel{\mathrm{\u203e}}{R}}_{\mathrm{2}}\to +\mathrm{\infty}$), the actual residual pressure approaches the expected residual
pressure based on a classical elastic model (*P*_{inc}→*P*_{ini}). Another potential application of the solution in Eq. (A12)
is for benchmarking numerical solutions. Due to the simplicity of the
pressure expression, it is particularly suitable for quick validation of
numerical models, e.g. the finite-difference model presented in the Supplement.

The MATLAB code to reproduce the results of a quartz-in-garnet system is available upon request.

The supplement related to this article is available online at: https://doi.org/10.5194/se-11-223-2020-supplement.

XZ designed the numerical or analytical model and wrote the MATLAB code. All the co-authors contributed to discussion and wrote the manuscript together.

The authors declare that they have no conflict of interest.

This work is supported by MADE-IN-EARTH ERC starting grant (no. n.335577) to Lucie Tajčmanová and Swiss National Science Foundation (P2EZP2_172220) to Xin Zhong. We thank Viktoriya Yarushina and an anonymous reviewer for helpful comments that greatly improved the quality of this work. Taras Gerya and Susanne Buiter are acknowledged for editorial handling.

This research has been supported by the European Research Council (grant no. n.335577) and the Swiss National Science Foundation (grant no. P2EZP2_172220).

This paper was edited by Taras Gerya and reviewed by Viktoriya Yarushina and one anonymous referee.

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- Abstract
- Introduction
- 1-D mechanical model with visco-elasto-plastic rheology
- Inclusion pressure modification due to visco-plastic deformation of host
- Inclusion pressure modification due to proximity to the thin-section surface
- Discussion
- Conclusions
- Appendix A
- Code availability
- Author contributions
- Competing interests
- Acknowledgements
- Financial support
- Review statement
- References
- Supplement

- Abstract
- Introduction
- 1-D mechanical model with visco-elasto-plastic rheology
- Inclusion pressure modification due to visco-plastic deformation of host
- Inclusion pressure modification due to proximity to the thin-section surface
- Discussion
- Conclusions
- Appendix A
- Code availability
- Author contributions
- Competing interests
- Acknowledgements
- Financial support
- Review statement
- References
- Supplement