Theoretical Computation of Mineral Assemblages in Pyrolite and Lherzolite

Using the thermochemical data base of Helgeson et al. (1978), missing data on several phases have been calculated from experimental phase equilibrium relations in the systems MgO–Al2O3–SiO2, CaO–MgO–Al2O3–SiO2, CaO–MgO–FeO–SiO2, diopside–jadeite–quartz, spinel–olivine, biotite–garnet and clinopyroxene–garnet. These data have been used to compute mineral assemblages (including modes and compositions of coexisting phases) in the system CaO–MgO–Al2O3–FeO–Na2O–SiO2 with pyrolite and lherzolite compositions over a broad pressure (up to 50 kb) and temperature (up to 1500 °C) range. Generalized isopleths (lines of equal composition) of A12O3 in pyroxenes, Ca in clinopyroxene and the pressure–temperature dependence of garnet–clinopyroxene distribution coefficient (Fe2+–Mg) have been graphically displayed. Depending on the temperature, the pressures of transition of a plagioclase bearing basic rock to spinel–peridotite and of the latter to garnet–peridotite lie in the ranges of 4 to 10 and 9 to 25 kb respectively.