In-situ stresses distribution and deformation mechanisms in eclogites at ultrahigh pressure

Eclogites compose the majority of the subducted oceanic crust at great depth, with garnet and clinopyroxene as major phases. High stress concentration could exist in the UHP eclogites, with a mechanical contrast between garnet and clinopyroxene that leads to complex microstructures, between brittle and ductile deformation. Coexistence of frictional and viscous regime in such two-phase aggregates raise the question of the competitivity between phases in leading deformation. The fracturation of garnet in natural rocks has been interpreted as related to seismicity in the lower crust and oceanic crust at the interface plate in subduction zones (Trepmann and Stöckhert, 2002, Angiboust et al., 2012, Hawemann et al., 2019), but the question remains if such features can also be produced at lower strain rates (Yamato et al., 2019, Rogowitz et al., 2023).

In order to understand the effect of hard- vs. weak mineral fraction on eclogite mechanical properties, stresses distribution and deformation mechanisms of synthetic eclogites were experimentally investigated under deep subduction zones conditions. Samples were deformed under ultrahigh pressures (3 to 5 GPa), high temperature (820°C) and constant strain rate (1 x 10^-5 – 2.5 x 10^-5 s^-1), using X-rays diffraction to measure in-situ stresses during deformation in each phase in garnet-clinopyroxene aggregates, with various garnet fraction. Back-scattered electron (BSE), electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM) with automated crystal orientation mapping (ACOM) was used on the recovered samples, in order to determine deformation mechanisms from the micrometric to the nanometric scale.

In our experiments, deformation was accommodated by a mix of brittle and intracrystalline plastic mechanisms, as proposed or observed in previous studies at lower pressures (e.g. Yamato et al., 2019, Rogowitz et al., 2023). Cataclastic flow and dynamic recrystallization are observed. The distribution of stresses in the phases and variations in stress levels depend on garnet vs. pyroxene fraction in the samples. Differential stresses are greater in garnet than pyroxene and stresses increase with increasing % vol. garnet. Phase fraction impact the mechanical behavior, i.e. fracturation of each phase and deformation accommodation mechanisms vary. In this semi-brittle regime each phase is rheologically active and contributes to the deformation of the aggregate except at the lowest pyroxene fraction.

Our experiments together with last studies (e.g. Yamato et al., 2019, Rogowitz et al., 2023), indicate that frictional deformation of eclogites is not limited to seismic strain rate (i.e. > 1 s^-1) but can occur at strain rate around 10^-5 s^-1 and slower with a high amount of garnet. The grain size reduction mechanisms observed could allow a switch to grain size sensitive mechanisms like grain boundary sliding.  Questions still remain about the extrapolation of such mechanical distribution and fracturation in deep subduction zones.