Structural Stability of Natural Magnesiochromite at High-Temperature-Pressure Conditions

The podiform chromitites in the Luobusha ophiolite have been thought to experience a very deep formation, but the maximum depth is still an open issue. Here, we have investigated the structural stability of natural magnesiochromite using the synchrotron-based powder X-ray diffraction and diamond anvil cells up to 48.6 GPa and 2 450 K. The results have shown that spinel-type magnesiochromite first decomposes into corundum-type ‘Cr₂O₃’ + B1-type ‘MgO’ at 11–14 GPa and 1 250–1 450 K, then modified ludwigite (mLd)-type ‘Mg₂Cr₂O₅’ + corundum-type ‘Cr₂O₃’ at 14.3–20.5 GPa and 1 300–2 000 K, and finally CaTi₂O₄-type phase at 24.5 GPa. During the quenching procession from high-temperature-pressure conditions, the mLd-type phase appeared again and was kept at ambient conditions. We also obtained the isothermal equation states of spinel-type and CaTi₂O₄-type phases, revealing the composition effect on their elasticities. Based on the updated results, we propose chromitites could not experience pressure exceeding ∼14.3 GPa (approximate maximum depth ∼400 km) in the subduction-recycling genesis model.