High stress concentration could exist in UHP eclogites, with a mechanical contrast between garnet and clinopyroxene leading to complex microstructures, between brittle and ductile deformation. Coexistence of frictional and viscous regime in two-phase aggregates raise the question of the competitivity between phases in leading deformation. Question remains about the fracturation of garnet [1-4].

Stresses distribution and deformation mechanisms in synthetic eclogites were experimentally investigated, under deep subduction zones conditions, in order to understand the effect of hard vs. weak mineral fraction on eclogite mechanical properties. Samples were deformed under ultrahigh pressures (3-5 GPa), temperature of 820°C and constant strain-rate (10-5 to 2.5x10-5 s-1), using X-rays diffraction to measure in-situ stresses in each phase of garnet-clinopyroxene aggregates, with various garnet fraction.

Electron microscopy (BSE, EBSD, TEM orientation mapping) on recovered samples shows that deformation was accommodated by a mix of cataclastic flow and dynamic recrystallization. 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 garnet fraction. 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 [3-4], 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.

[1]Angiboust et al., 2012, [2]Hawemann et al., 2019, [3]Yamato et al., 2019, [4]Rogowitz et al., 2023