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 Na 2O-CaO-FeO-MgO-Al 2O 3-SiO 2 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 Na 0.8Ca 0.3(Fe,Mg) 2.25Al 0.5Si 4.15O 12. 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.