ANHYDROUS PHASE EQUILIBRIA IN EARTHS UPPER MANTLE

Abstract

Phase equilibrium relations are computed in five different chemical compositions appropriate for a primitive upper mantle (estimated), a peridotite, an olivine-basalt, an olivine-tholeiite, and a quartz-tholeiite in the pressure range of 1 bar to 100 kb and in the temperature range of 500 to 1500 °C. The mineralogical phase assemblages change from peridotite and lherzolite to garnet granulites, eclogites and kyanite-quartz-eclogites. The small pressure-temperature field of stability of plagioclase-garnet-granulite in the primitive compositions extends at 600 °C from less than 3 kb to more than 15 kb in the quartz-tholeiite composition with intermediate pressure values in other basalts. Continental crust with composition similar to quartz-tholeiite remains as plagioclase-garnet-granulite changing to eclogite only below a depth of 50 km. Calculated adiabats in the peridotite compositions show that diapirs must ascend from a depth of at least 300 km to become partially molten in the low-velocity zone. Various shallower depths of partial melting are possible for material ascending from depths corresponding to the oceanic geothermal gradient. Actual pressure-temperature densities of rocks have been calculated from the mineral densities computed for the equilibrium assemblages on the oceanic geothermal gradient. While there are no important changes found in the peridotite density down to a pressure of 100 kb, there are important inflections in the pressure-density curve in quartz-tholeiite which relate to the increasing conversion of plagjoclase to jadeite component in clinopyroxene and a sharp rise in density due to the quartz-coesite phase transformation.