First-Principles Study of the Elastic Properties of Aluminous Phases at High Temperature and Pressure

Abstract:

Seismic studies report the presence of regions below Africa and the Pacific, which exhibit shear-wave velocity reductions of about 3 percent. These so-called large low shear velocity provinces (LLSVPs), cover approximately half of the core-mantle boundary, making them an important interface between the core and lowermost mantle. The fact that they exhibit different seismic properties indicates that they are different from surrounding mantle, either in temperature and/or composition. 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 seismic properties of MORB phases, needed to compare with seismic observations, are unknown. Here we report the high temperature and pressure elastic properties of several aluminous phases, which comprise about 20 percent of MORB material. In particular, we have looked at MgAl₂O₄, NaMg₂Al₅SiO₁₂, and KMg₂Al₅SiO₁₂, with the calcium ferrite (CF) and new aluminous (NAL) structure. These should provide further constraints to aid the interpretation of seismic observations, contributing to the thermal vs compositional anomaly debate.