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 {Mathematical expression} in the presence of chlorite involve minerals which are relatively aluminous compared with those at low {Mathematical expression}. 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. © 1990 Springer-Verlag.