Effect of grain‐scale pressure variations on garnet growth: a numerical approach

Abstract

Grain-scale pressure variations have recently been investigated due to its significant effect on the observed microstructures and chemical redistribution in metamorphic rocks. In this study, the impact of grain-scale pressure variation on a growing garnet grain is investigated. We model prograde garnet growth at fluid-saturated conditions and sufficiently low temperature (<650°C) to avoid modification of the inner part of the grain by diffusion. The method uses the Perple_X computer program package combined with Matlab Ò scripts allowing for a heterogeneous fluid pressure distribution around the growing grain. It is shown that the pressure variation around the grain may result in an asymmetric chemical zonation. Motivated by the result, we apply the model to selected published prograde paths to evaluate the magnitude of the pressure variation that is necessary for the development of the asymmetric zoning preserved in the samples. The magnitude of such a pressure gradient in the selected samples may correspond up to 1 kbar. Finally, it is demonstrated that the modelling of asymmetric chemical zoning in garnet under a pressure gradient may improve the precision of thermobarometric constraints. The numerical model evaluates the effect of the pressure gradient on a growing grain and thus can serve as a benchmark tool in future development of fully coupled models involving rock deformation and mineral reactions.

Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?

“…Pressure gradients that can exist on an outcrop scale due to heterogeneous distribution of fluids and consequent rock weakening, can also exist on a grain scale (Tajčmanova et al, 2014(Tajčmanova et al, , 2015Wheeler, 1987;Zhong et al, 2017) when fluid pathways may be controlled by grain boundaries and local stresses by the proximity of hard minerals in a softer matrix (Vrijmoed and Podladchikov, 2015). The observation made in the eclogite finger described here, that low pressure granulite pyroxenes (with Type T1 textures of amphibole lamellae in the pyroxenes) are preserved adjacent to high pressure omphacite raises the same problem as to whether this represents metastable retention of a granulite fragment due to limited fluid access or whether the limited fluid access generates a local pressure gradient.…”

3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

“…Spalla et al 2000Spalla et al , 2005Hobbs et al 2010;Chapman et al 2019) or to local instabilities due to diffusion processes (e.g. Pearce and Wheeler 2010) or stress-driven reaction heterogeneities (Schmalholz 2014;Tajčmanová et al 2014;Zhong et al 2017). Detailed multiscale correlation among superposed fabric elements and metamorphic assemblages is known to be of first-order importance to infer relationship between fabric and metamorphic gradients (e.g.…”