First-principles study of the elastic properties of aluminous phases and silica at high temperature and pressure

Seismological studies of the Earth's lower mantle reveal the presence of two nearly antipodal regions beneath Africa and the central Pacific, with lower than average shear wave velocities (-3% to -5%) and higher than average densities (+0.5%), indicating that they are district from normal mantle. These large low shear-wave velocity provinces (LLSVPs) cover almost half of the outer core surface and represent about 3% of the Earth's volume, making them the most prominent features in the lower mantle. Despite their large scale, there is still much debate about their origin and composition, in particular, whether they are a thermal, chemical or thermo-chemical anomaly. The reported sharpness of the boundaries of large low shear velocity provinces suggests that they are at least in part compositional anomalies. One potential explanation is that they comprise the relics of subducted slabs, and thus have mid-ocean ridge basalt (MORB). The high temperature and pressure elastic properties of MORB phases, needed to estimate the seismic properties of MORB mineral assemblages are unknown. Here we report the high temperature and pressure elastic properties of several aluminous phases and the α-PbO2 phase of SiO2, which comprise about 20 percent of MORB material each. These should provide further constraints to aid the interpretation of seismic observations, contributing to the thermal vs compositional anomaly debate.