Laboratory Duplication of Phase Equilibria Observed in Natural Garnet Lherzolites

Abstract

High pressure experiments have been carried out on two synthetic ultramafic compositions and also on natural mineral mixes at 30-40 kbar and 950-l,500°C. Olivine, orthopyroxene, clinopyroxene, and garnet coexist in the majority of runs. Because of the close similarity in chemical systems, the experimental results can be directly applied to natural garnet lherzolites. The natural garnet lherzolites show wider variety in mineral chemistry than the run products, indicating very wide ranges of pressure-temperature for their equilibration. However, mineral compositions found in some runs closely reproduce the compositions of corresponding minerals in some natural garnet lherzolites from Lesotho and Montana kimberlites and from Alpe Arami lherzolite. Thus, the phase equilibria observed in some natural lherzolites can be successfully duplicated in laboratory at appropriate P-T conditions. As a consequence, pressure and temperature estimates for these rocks would be most accurate. Though the equilibria among the four minerals are not inconsistent with general features found in simplistic chemical systems, the estimates often differ significantly from those based on geothermometers and barometers obtained in the simple systems; the Lesotho pyroxene geotherm should be located, at least in part, at lower temperatures and pressures than the previous proposal. It was also found that Fe-Mg partition coefficient among the four minerals is most sensitive between garnet and clinopyroxene and least between olivine and orthopyroxene.