Thermoelastic properties of the “new aluminous” (NAL) phase

Mid-Ocean Ridge Basalt (MORB) is formed via seafloor spreading at mid-ocean-ridges. It is one of the most important rocks in the Earth’s crust. Upon subduction and exposure to high pressures and temperatures (PT), the high Al₂O₃ content in MORB gives rise to a “new aluminous phase” (NAL) responsible for up to 25 w% of its composition. This phase has hexagonal structure, 21 atoms/cell, and is a complex solid solution with chemical formula XY₂Z₆O₁₂, where X is a large monovalent or divalent cation, e.g., Na+ or Ca2+, Y is a mid-sized cation, e.g., Mg2+, and Z is Al3+ and/or Si4+. Chemical constraints on its composition may reduce the number of end-member compounds to less than twenty. Using ab initio calculations, we model the iron-free NAL phase by considering seven end-members. We perform Quasiharmonic (QHA) calculations to address composition dependent thermodynamic and thermoelastic properties of this phase. These results are essential to investigate the seismic signature of subducted MORB crust into the deep mantle.