Theoretical prediction of phase relationships in planetary mantles

Calorimetric and phase equilibrium data were used to establish an internally consistent thermodynamic database for the Si-undersaturated part of the system Na₂O-FeOnCaO-MgO-Al₂O₃-SiO₂. This database has been used to calculate the stability fields of assemblages involving olivine, orthopyroxene, clinopyroxene, spinel, plagioclase, and garnet at high P and T. In the six-component system CaO-MgO-Al₂O₃-SiO₂ the stable assemblages are ol-opx-cpx-plag at P < 8 kbar at 1300ºC and <6 kbar at 900ºC. Above these P, spinel replaces plagioclase until P of 16 kbar at 1300ºC and 14 kbar at 900ºC are reached. At higher P, garnet lherzolite (ol + opx + cpx + gt) is stable. Stable phase assemblages have also been calculated for Na- and Fe-bearing compositions postulated as possible martian mantles. The McGetchin-Smyth composition would not contain orthopyroxene and a low-pressure assemblage of plag+ol+cpx+sp would be replaced by gt+ol+cpx+sp at pressures above 13 kb. In contrast, the Morgan-Anders composition has stability fields of plagioclase Iherzolite (P < 8 kb) and garnet lherzolite (P > 8 kb) and a small field of spinel Iherzolite at about 6 kb at temperatures below 900°C.