New Insights on the Rheology of Olivine Deformed under Lithospheric Temperature Conditions

Abstract:

Rheology of mantle rocks at lithospheric temperatures remains poorly constrained, since most experimental studies on creep mechanisms of olivine single crystals ((MgFe)₂SiO₄, Pbnm) and polycrystalline olivine aggregates were performed at high-temperatures (T >> 1200^oC). In this study, we report results from deformation experiments on oriented single crystals of San Carlos olivine and polycrystalline olivine aggregate at temperatures relevant of the uppermost mantle (ranging from 800^o to 1090^oC) in tri-axial compression. The experiments were carried out at a confining pressure of 300 MPa in a high-resolution gas-medium mechanical testing apparatus at various constant strain rates (from 7 x 10^-6 s^-1 to 1 x 10^-4 s^-1). Mechanical tests show that mantle lithosphere is actually weaker than previously inferred from the extrapolation of high-temperature experiments. In this study, we present characterization of dislocation microstructures based on transmission electron microscopy and electron tomography. It is shown that below 1000°C, dislocation activity is restricted to [001] glide with a strong predominance of {110} as glide planes. We observe recovery mechanisms which suggest that the mechanical properties observed in laboratory experiments represent an upper bound for the actual behavior of olivine under lithospheric mantle conditions. Moreover, the drastic reduction in slip system activity observed questions the ability of deforming olivine aggregates in the ductile regime at such temperatures. We show that ductility is preserved thanks to the activation of alternative deformation mechanisms in grain boundaries involving disclinations.