Matrix rheology effects on reaction rim growth II: coupled diffusion and creep model


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Matrix rheology effects on reaction rim growth II: coupled diffusion and creep model
Journal of Metamorphic Geology
Schmid D.W., Abart R., Podladchikov Y.Y., Milke R.
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Chemical reactions and phase changes generally involve volume changes.
In confined settings this will cause a mechanical deformation of the
matrix that surrounds the reaction sites where the volume change takes
place. Consequently, mineral reactions and the mechanical response of
the rock matrix are coupled. A companion paper in this issue illustrates
this coupling with experiments where quartz and olivine react to form
enstatite reaction rims under ambient conditions of 1 GPa and 1000
degrees C. It has been demonstrated that for identical run conditions,
the thickness of the reaction rims depends on whether quartz grains are
embedded in an olivine matrix or olivine grains are included in a quartz
matrix. The experimental conditions, the nature of the results, and the
large volume change of the reaction (-6%) leave only viscous creep as a
viable matrix response to the reaction progress. A model is developed
for this reaction, which combines diffusion of chemical components
through the growing rim and viscous creep of the matrix. The resulting
rate law for reaction rim growth in spherical geometry shows that the
progress rate is proportional to the reaction overstepping and
controlled by the slower of the two competing processes; either
diffusion or creep. If diffusion is rate limiting the usual linear
proportionality between rim growth and results. However, if viscous
creep is rate limiting, then the reaction rates are reduced and may
become effectively creep controlled. With respect to the experiments in
the companion paper it is inferred that the effective viscosity of the
two matrix materials, i.e. polycrystalline quartz and olivine, differ by
approximately one order of magnitude with the quartz being the stronger
one. The absolute values of the inferred viscosities correspond well to
published flow laws. The rheological properties of natural rocks are
well within the parameter range for which significant mechanical control
on reaction rim growth is expected. This implies that for the
interpretation of natural reaction rims and corona structures both
diffusion and mechanical control need to be considered. In addition the
mechanical effect also needs to be considered when interdiffusion
coefficients are retrieved from rim growth experiments. This should also
be considered for geospeedometry analyses. Furthermore, the control on
reaction rate because of slow creep of the matrix is expected to be even
more important, compared to the experiments, under colder crustal
conditions and may contribute substantially to the frequent observation
of only partially completed reactions. We suggest that this phenomenon
is referred to as `mechanical closure', which may be an important
mechanism in the kinetic displacement of the boundaries between the
stability fields of phase assemblages.
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09/10/2012 20:50
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