Experimental constraints on the formation of the Tibetan podiform chromitites

The discovery of diamonds and highly reduced minerals in podiform chromitites, which have generally been interpreted as magmatic rocks formed from partial melts of upper mantle peridotites under low-pressure conditions, has raised many questions about the origin of these enigmatic bodies. In order to provide experimental constraints on the formation and emplacement of podiform chromititesin ophiolites, we carried out a number of multi-anvil experiments in the magnesiochromite + SiO2 system at temperatures of 1000-1600 degrees C and pressures of 5-15 GPa. The experimental results demonstrate that magnesiochromite is stable up to 14 GPa and decomposed into eskolaite (Cr2O3) together with a quenchable modified ludwigite-structured phase [(Fe, Mg)(2)(Al, Cr)(2)O-5] at higher pressures, thus placing an approximate maximum depth for chromite crystallization and/or metamorphism. This depth corresponds to the top of the mantle transition zone (MTZ) at 410 km. The ludwigite-structured post-chromite phase has significant implications for understanding phase transformations and Cr incorporation/partitioning of minerals in the MTZ. On the basis of our results, we propose a multi-stage model for the formation of podiform chromitites that incorporates the geochemical, textural and mineralogical features of these bodies. (C) 2015 Elsevier B.V. All rights reserved.