Coesite and clinopyroxene exsolution lamellae in chromites: In-situ ultrahigh-pressure evidence from podiform chromitites in the Luobusa ophiolite, southern Tibet

We report in-situ petrological evidence of deep mantle origin from podiform chromitites in the Luobusa ophiolite, southern Tibet. Analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the Luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite, which indicate an ultrahigh-pressure origin of over 3 GPa (> 100 km deep). The presence of these lamellae, coupled with abundant micro-inclusions of clinopyroxene, requires high solubility of SiO2 and CaO in the host chromite, and suggests a precursor of chromite, a CaFe2O4-structured high-pressure polymorph, stable at pressures over 12.5 GPa (> 380 km deep). These nano-scale observations and geological occurrence indicate that the mantle peridotite under the Tibetan mid-ocean ridge was transported from the deep mantle (at least 100 km, probably more than 380 km deep) by the mantle convection. It implies that the root of mantle upwelling has a much deeper origin than previously believed.