Theoretical prediction of phase relationships in planetary mantles

Abstract

Calorimetric and phase equilibrium data have been used to establish an internally consistent thermodynamic data base for the silica‐undersaturated part of the system Na2O‐FeO‐CaO‐MgO‐Al2O3‐SiO2. This data base has been used to calculate the stability fields of assemblages involving the phases: olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (sp), plagioclase (plag) and garnet (gt) at high pressures and temperatures. In the six‐component system all these phases are solid solutions; their mixing properties were also obtained from phase equilibrium and calorimetric data. In the model system CaO‐MgO‐Al2O3‐SiO2 (CMAS), the stable assemblages are ol‐opx‐cpx‐plag at pressures less than 8 kb at 1300°C and less than 6 kb at 900°C. Above these pressures, spinel replaces plagioclase, until pressures of 16 kb at 1300° and 14 kb at 900°C are reached. At higher pressures, garnet lherzolite (ol+opx+cpx+gt) is stable. Stable phase assemblages have also been calculated for sodium and iron‐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.