Abstract
The podiform chromitites in the Tibetan ophiolites have been thought to have experienced metamorphism corresponding to depths at the top of the mantle transition zone (MTZ). However, little is known about the phase relations of natural podiform chromitite in the MTZ, as well as the implications for processes related to formation of the ultra-high pressure (UHP) chromitite. An experimental study is presented to constrain the phase relations of natural chromitite at pressures and temperatures of 11–18 GPa and 1400–1600 °C. The experimental results show that the chromitite transforms into phase assemblages composed of chromite + eskolaite + garnet for pressures at the base of the upper mantle, and chromite + eskolaite + garnet + MC-phase at the top of the MTZ. Chromitite is replaced by a CaTi2O4-structured phase (CT-phase) + eskolaite + garnet in the middle of the MTZ. The MC-phase has an average composition of (Mg, Fe)3[(Cr, Al)2-2x-4/3y(Mg, Fe)xSix+y]O6 (x ≈ 0.2, y ≈ 0.3), with a cubic space group of Fd3m. It is suggested that the MC-phase is a new silicon-bearingMg- and Cr-rich phase, with idealized stoichiometry of Mg3Cr2O6. The observation of the MC-phase in a natural rock system is helpful for understanding the incorporation of Mg and Cr minerals in the MTZ, and helps to constrain the related geological processes that controlled the formation of the Tibetan chromitites.
Published Version
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