Layered or interconnected ? In-situ connectivity and topology from X-ray tomography on deformed serpentine+olivine aggregates

The polymineralic nature of most rocks induce changes in deformation mechanisms with respect to those observed in monomineralic aggregates, and challenges our understanding of the feedbacks between microstructure and rheology. In-situ information from high pressure experiments, such as X-ray tomography and X-ray diffraction, offer the opportunity of quantifying the fabric and stresses, and their evolutions, in polymineralic rocks, under high pressures and high temperatures relevant for the deep earth. Here we present results relevant to the deformation of serpentinized peridotites. Interconnected weak layers (IWL) of serpentine can cause morphological anisotropy and strain localization in serpentinized peridotite, with important implications for the mechanical properties of the lithosphere. We quantify the morphological anisotropy, topology, and interconnectivity of serpentine, in serpentine + olivine aggregates, under torsion. We use in-situ X-ray absorption-contrast tomography at pressures of ca. 4 GPa and temperatures 300-400°C. At shear strains γ >= 4 and ~10 vol. % serpentine fraction, the topology of the serpentine clusters becomes simple, with few interconnections between long isolated serpentine clusters. Conversely, for ~20 vol. % serpentine, the clusters increase in length and topological complexity, resulting in large interconnected serpentine network for γ > 4. This study reveals how serpentinized peridotite with ~20 vol.% serpentine can develop a deformation-induced IWL of serpentine, where strain can preferentially localize. IWL of serpentine may not happen when the serpentine content is ~10 vol. % because of the formation of a serpentine disjoint network. The results will be put in perspective with published in-situ stresses distribution within various serpentine+olivine aggregates, deforming under similar pressures and temperatures. These experiments participate to set the ground for exploring the deformation distribution, stresses and fabric evolution in polymineralic rocks, using in-situ information.