Microstructural evolution in deforming olivine-serpentine aggregates at subduction zones conditions using in-situ X-ray tomography

Abstract:

Serpentinized peridotite is one of the key material controlling the flow and dynamics of the subducting slabs. Antigorite, the high-temperature variety of serpentine, coexists with olivine in serpentinized peridotite at subduction zones. It is generally accepted that antigorite mechanical strength is lower than olivine at geological strain rates. This implies that deformation localization is expected to occur in antigorite rather than in olivine, and antigorite may influence the overall strength of the aggregate. Such aggregate may display two main end-member mechanical and microstructural behaviours: strong olivine minerals form a load-bearing framework that contains spaces filled with weaker antigorite; weak antigorite minerals govern the bulk rheology of the aggregate by forming an interconnected weak layer, while the stronger olivine minerals form clasts.

We report new insights on the microstructural and mechanical behaviour of antigorite + olivine aggregates as proxy for partially serpentinized peridotite being experimentally deformed during torsion experiments at high pressures (HP, > 2 GPa) and high temperatures (HT, 400-600°C). Experiments are coupled with in-situ time-resolved X-ray tomography on the PSICHE beamline at SOLEIL synchrotron. HP-HT in-situ X-ray tomography is a powerful tool to track the microstructural developments, quantify connectivity, development of preferred microstructural directions and phase morphology. Our results suggest that the total connectivity of antigorite increases with the twisting angle transferred to the samples with antigorite minerals forming, in some cases, clear interconnected weak layers in the aggregate. In order to link our tomography observations to plastic properties of the phases, post-mortem electron microscopy analysis is performed on the recovered samples. Olivine crystals show lattice bending of some olivine clasts indicating that local deformation is accommodated by intracrystalline low-temperature plasticity.