Deformation of olivine-orthopyroxene aggregates at high pressure and temperature: Implications for the seismic properties of the asthenosphere

Abstract

The effect of pressure, temperature and composition on the development of crystal preferred orientations (CPO) and seismic properties of olivine-orthopyroxene aggregates were investigated using samples containing olivine and 12.5, 25 and 50vol.% of orthopyroxene. The samples were deformed in simple-shear at a constant strain-rate of 10−4s−1 with total shear strains between 0.5 and 1.3, at pressures of 3, 5 and 8GPa and temperatures of 1300, 1400 and 1500°C, respectively. Olivine's CPO vary as a function of the orthopyroxene content. All samples have their olivine [010] axes normal to the foliation. Samples with 12.5 and 25% orthopyroxene have their [001] axes parallel to the lineation (B-type), whereas the samples with 50% orthopyroxene have their [100] axes oriented parallel to the lineation (A-type). At 3GPa, we propose that olivine CPO may result from a variation between two types of diffusion accommodated grain boundary sliding (difGBS) mechanisms. At higher pressure, the relative contribution of difGBS and dislocation related mechanisms depends on the volume of secondary phases. For low orthopyroxene contents, dislocation related mechanisms prevail and induce the development of B-type CPO, whereas for higher amount of orthopyroxene difGBS controls the deformation and leads to A-type CPO. Orthopyroxene's CPO strength increases with increasing pressure and temperature and is characterized by the concentration of [100] and [010] axes normal to the foliation and [001] close to the lineation. The seismic properties show that deformation in pyroxene-poor and rich peridotites are consistent with the seismic anisotropy observed in intraplate regions where the mantle flow is horizontal. Conversely, only pyroxene-rich peridotites deformed through difGBS could explain the Vsh/Vsv<1 observed below mid-oceanic ridges.