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 ¿C 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 lherzolite (P8 kb) and a small field of spinel lherzolite at about 6 kb at temperatures below 900 ¿C.