EXPERIMENTAL-DETERMINATION OF THE SPINEL PERIDOTITE TO GARNET PERIDOTITE INVERSION AT 900-DEGREES-C AND 1,000-DEGREES-C IN THE SYSTEM CAO-MGO-AL2O3-SIO2, AND AT 900-DEGREES-C WITH NATURAL GARNET AND OLIVINE

Hydrothermal reversals of the univariant reaction orthopyroxene + clinopyroxene + spinel to garnet + olivine in the system CaO-MgO-Al2O3-SiO2 have been made in the range 900 °-1,000 ° C without intervention of hydrous phases. At 1,000 ° C the equilibrium is bracketed at 16.0±0.5 kbar, and, at 900 ° C, at 15.0±0.5 kbar. A flat but not well-constrained dP/dT slope is inferred. The composition of the garnet has been determined as (Ca3Al2Si3O12)0.15 (Mg3Al2Si3O12)0.85 at 900 ° C and 1,000 ° C. A dP/dT slope of 4.5±3 bars/K at 1,000 ° C, calculated from enthalpy of solution data, is within the experimental error range and agrees with the flat calculated slopes of Obata (1976) and Herzberg (1978). The large curvature of the reaction in the low-temperature range predicted by these authors is established. Hydrothermal reversal runs on a mixture of natural pyropic garnet and forsteritic olivine and their low-pressure breakdown products were made at 900 ° C. Above 12.5 kbar, garnet + olivine are stable. Below 9.5 kbar, garnet + olivine react to pyroxenes +spinel. Between these pressures, the probable stable assemblage on the bulk composition used is garnet + olivine + orthopyroxene. Thus, the peridotite transition has only been determined within a three-kbar interval. It is inferred from the experimental data that garnet peridotites occurring in amphibolite-granulite gneiss terrains could be formed by deep-crustal metamorphism; a subcrustal origin of the garnet-olivine association is not necessary. Seismic velocity increases at depths of 60-90 km in oceanic lithosphere, which have been considered by some to correspond to the onset of garnet in peridotite, are much deeper than the experimentally determined transition interval at 30-40 km at 900 ° C. © 1979 Springer-Verlag.