Phase assemblage and stability of pyroxenite at Lower-Mantle conditions

We investigated a mixture of natural pyroxenites (MIX1G [Hirschmann et al., Geology, 2003]) in the laser-heated diamond-anvil cell at Lower-Mantle conditions (pressures between 25 and 60 GPa and temperatures above ~2000 K) in order to determine the phase assemblage and stability of this composition and to evaluate its suitability as a candidate deep ocean island basalt (OIB) source or widespread Lower Mantle component. Quenched samples were characterized by in-situ high-pressure synchrotron-based x-ray diffraction. Recovered samples were also analyzed for composition by energy-dispersive x-ray spectroscopy on a scanning electron microscrope in thick-section and thin-film modes. At these Lower-Mantle conditions, pyrolite-like compositions yield a three-phase assemblage (Mg, Fe, Al)SiO3 orthorhombic perovskite, (Mg, Fe)O magnesiowüstite and CaSiO3 calcium perovskite. This pyroxenite, on the other hand, shows an excess of silica, evident with the identification of stishovite, as well as a sodium, aluminum-rich phase in addition to (Mg, Fe, Al)SiO3 orthorhombic perovskite and CaSiO3 calcium perovskite phases. Whereas pyrolite compositions yield a density ~2% lower than Preliminary Reference Earth Model (PREM), our estimates suggests MIX1G pyroxenite is ~1% denser than PREM. Present mineral physics data therefore suggest a Lower Mantle composition different from the upper mantle but not so enriched as pure pyroxenite.