EVIDENCE FOR IMMISCIBILITY IN MAJORITE GARNET FROM EXPERIMENTS AT 13-15 GPA

Abstract

The compositions of coexisting aluminous garnet, clinoenstatite and either jadeitic clinopyroxene or a second (silica-rich) garnet were experimentally determined in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2 at 1450–1650°C and 13–15 GPa, using a split-sphere anvil apparatus (USSA-2000). The pyroxene content of the aluminous garnet increases from 50% at 13 GPa to 100% at 15 GPa. For bulk compositions with a high Na/Ca ratio, the trends of the observed variation in the compositions of both garnet and clinopyroxene exhibit a major change with increasing pressure at 14 GPa. This is consistent with the stabilization of a second, silica-rich garnet, which replaces jadeitic clinopyroxene in the assemblage with aluminous garnet and clinoenstatite. The transformation from clinopyroxene to silica-rich garnet is almost isochemical and occurs near the composition Na0.8Ca0.3(Fe,Mg)2.25Al0.5Si4.15O12. Due to the observed immiscibility in the garnet solution, the transformation from pyroxene to garnet could produce a sharp seismic discontinuity in the Earth’s mantle at 400 km depth.