New Thermodynamic Models and Calculated Phase Equilibria in NCFMAS for Basic and Ultrabasic Compositions through the Transition Zone into the Uppermost Lower Mantle

Phase equilibria calculated with thermocalc for simplified mantle ultrabasic and basic bulk compositions in Na2O-CaO-FeO-MgO-Al2O3-SiO2 (NCFMAS) at depths down to the Transition Zone and uppermost lower mantle are presented here for the first time. This capitalizes on a new equation of state for solid phases and ties in deep mantle phases (wadsleyite, ringwoodite, akimotoite, garnet, MgSi-perovskite, CaSi-perovskite, cf phase, ferropericlase, nal, corundum and stishovite) to the thermodynamic dataset used at lower pressures in thermocalc. Activity-composition relationships for these complex phases are formulated that combine the end-member properties, allowing such phase equilibria to be calculated. The results are presented in the form of P-T and P-X pseudosections showing the dependence of mineral assemblage on conditions of formation and composition in basic and ultrabasic compositions, and also in terms of modal proportion and mineral composition diagrams. Such thermodynamic calculations will allow quantitative assessment of the mineral assemblages in the mantle, as well as in subducting lithospheric slabs. The results will be a key input into geophysical modelling, allowing questions such as the controls on the penetration of subducted slabs into the lower mantle to be addressed. The calculated mineral assemblages can also help in the interpretation of deep mantle phase inclusions trapped within diamonds. Calculations are made more realistic than in pre-existing models by inclusion in this study of a chemically complex nal phase as well as inclusion of a wide range of substitutions in the cf phase and in both the Mg- and Ca-perovskite phases.