A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.