Ab initio density determination of subducting basalt

High pressure and temperature experiments on glasses of compositions representative of mid-ocean ridge basalt (MORB) have demonstrated that, at temperatures and pressures corresponding to areas just below the transition zone, the basaltic portion of a subducting slab partitions into four or five phases: stishovite, Mg-perovskite, Ca-perovskite, and an aluminous, sodic phase with the Ca-ferrite structure, and possibly a new hexagonal aluminous phase (NAL). While thermoelastic properties of the first three phases have been well constrained throughout the mantle using various experimental and computational methods, resolution of the structures and thermoelastic properties of the Ca-ferrite and NAL minerals have been complicated by their complex solid solutions. Determination of the density profiles of these phases is important in understanding the buoyancy of subducting MORB as it approaches the transition zone. We present a computational structure determination using USPEX (Universal Structure Predictor: Evolutionary Xtallography) for end members of the Ca-ferrite and NAL phases. Thermoelastic parameters from ab initio Molecular Dynamics (MD) are then used to create density profiles for both minerals.