Chromium-bearing phases in the Earth’s mantle: Evidence from experiments in the Mg2SiO4–MgCr2O4 system at 10–24 GPa and 1600 °C

Phase relations in the system Mg2SiO4–MgCr2O4 were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the forsterite-magnesiochromite system to derive a P–X phase diagram and synthesize chromium-bearing phases, such as garnet, wadsleyite, ringwoodite, and bridgmanite of a wide compositional range. Samples synthesized at 10 GPa contain olivine with small chromium content and magnesiochromite. Mg2SiO4 wadsleyite is characterized by the pressure-dependent higher chromium solubility (up to 7.4 wt% Cr2O3). The maximal solubility of chromium in ringwoodite in the studied system (~18.5 wt% Cr2O3) was detected at P = 23 GPa, which is close to the upper boundary of the ringwoodite stability. Addition of chromium to the system moves the boundaries of olivine/wadsleyite and wadsleyite/ringwoodite phase transformations to lower pressures. Our experiments simulate Cr-rich phase assemblages found as inclusions in diamonds, mantle xenoliths, and UHP podiform chromitites.