The competing effects of olivine and orthopyroxene CPO on seismic anisotropy

Abstract

Orthopyroxene is the second most abundant mineral in the upper mantle and, like olivine, can form crystallographic preferred orientations (CPO) during mantle deformation that may influence mantle seismic anisotropy. Here we use a compilation of 21 peridotites in which both olivine and orthopyroxene CPOs are preserved to document the range of orthopyroxene CPO types and to explore their effects on seismic anisotropy of peridotites. In addition to four previously published orthopyroxene CPO types, this study establishes three new CPO types, two of which result in orthopyroxene fast axes aligned parallel to the lineation and the direction of mantle shear/flow. We calculate bulk elastic tensors for each sample from the measured olivine and orthopyroxene CPOs, and use those in turn to calculate Vp and Vs velocities and anisotropy percentages. In order to explore the relative influence of olivine and orthopyroxene on anisotropy, these properties were calculated for a range of possible modal percentages, from 100% olivine to 100% orthopyroxene. While the effect of orthopyroxene CPO appears to be a negligible influence on the orientation of bulk seismic fast axes, the presence of increasing orthopyroxene decreases the anisotropic signal overall. However, at very low olivine modal percentages, olivine and orthopyroxene can be mutually destructive, where the addition of olivine to a mostly orthopyroxene rock can actually decrease anisotropy.