Seismic anisotropy of the lithospheric mantle beneath Marie Byrd Land, West Antarctica: Constraints from peridotite xenoliths

Constraining the seismic structure of the West Antarctic mantle is important for understanding its viscosity structure, and thus for accurately predicting the evolution of the West Antarctic Ice Sheet.  Seismic anisotropy, which is the dependence of seismic velocities on the propagation and polarization direction of seismic waves, is a valuable tool for understanding mantle deformation and flow.  We provide petrological and microstructural data from a suite of 44 spinel peridotite xenoliths entrained in Cenozoic (1.4 Ma) basalts of 7 volcanic centers located in Marie Byrd Land, West Antarctica.  Equilibration temperatures obtained from three different calibrations of the two-pyroxene geothermometer and the olivine-spinel Fe-Mg exchange geothermometer range from 780°C to 1200°C, calculated at a pressure of 1500 MPa.  This range of temperatures corresponds to extraction depths between 39 and 72 km, constraining the source of the xenoliths within the lithospheric mantle above the low velocity zone modelled by seismic studies.

The Marie Byrd Land xenoliths are fertile with average clinopyroxene mode that ranges between 15 and 24%.  Based on their modal composition, xenoliths are predominantly classified as lherzolites (n=30), with lesser occurrences of harzburgite (n=4), wehrlite (n=3), dunite (n=3), olivine websterite (n=1), websterite (n=1), and clinopyroxenite (n=2).  Petrological data suggest that the xenoliths have been affected by various degrees of partial melting as well as by reaction with silicate melts or fluids.  For example, clinopyroxenes in the more fertile lherzolites and wehrlites show a constant TiO₂ concentration at 0.65 wt% and 0.8 wt% over a range of olivine Mg# values, while TiO₂ decreases rapidly with increasing Mg#, down to 0.01 wt% in the more refractory harzburgites and dunites.  The observed trend is interpreted to indicate a refertilization process.  Microstructures also indicate multiple episodes of reactive melt percolation under either static conditions or during the late stages of deformation.  Pyroxenes may enclose rounded olivine grains in crystallographic continuity with neighbouring grains, cross-cut the subgrain boundaries of olivine grains, or show an interstitial habit, either forming cuspate-shaped grains in olivine triple junctions or films along olivine-olivine grain boundaries.  Olivine shows a range of crystallographic preferred orientation (CPO) patterns, including the A-type, axial-[010], axial-[100], and B-type.  Pyroxenes have weaker but not random CPOs with [001] axes having similar orientation to olivine [100] axes in the majority of the xenoliths.  Calculated P and S waves anisotropy is variable (2–12%) and increases with olivine fraction but decreases with both increasing ortho- or clinopyroxene content.  P-wave anisotropy is correlated with the strength of olivine CPO expressed with the M-index and increases with increasing strength of the orthopyroxene CPO, but seems to be less correlated with the strength of the clinopyroxene CPO.