Mössbauer spectroscopy studies of the valence state of iron in chromite from the Luobusa massif of Tibet: implications for a highly reduced deep mantle

Mossbauer spectroscopy was applied to study the valence state of iron in chromite from massive, nodular and disseminated podiform chromitite ores of the Luobasa ophiolite massif of Tibet. The results show that Fe 3+ ⁄S Fe = 0.42 in chromite from massive ore, and Fe 3+ ⁄S Fe = 0.22 in chromite from nodular and disseminated ores. The massive ore records traces of ultra high pressure mineralogical assemblages, such as diamond inclusions in OsIr alloys, exsolution lamellae of coesite and diopside in chromite, inclusions of metal-nitrides, native iron and others, which suggests a strongly reducing environment. In contrast, chromite from nodular and disseminated ore contains abundant low-pressure OH-bearing mineral inclusions whose formation requires a more oxidizing environment. The high value of Fe 3+ ⁄S Fe in the reduced massive ore is explained by crystallographic stabilization of Fe 3+ in a high- pressure polymorph of chromite deep in the upper mantle despite low ambient fO2 conditions. The presence of high-pressure phases within the massive chromitite ore requires that the latter, together with its host peridotite, was transported in the solid state from a highly reduced deep mantle environment to shallow depths beneath an ocean spreading centre. It is suggested that in the low-pressure environment of the spreading centre, the deep-seated, reduced, massive chromitites partially reacted with their host peridotite in the presence of hydrous melt, yielding the nodular and disseminated chromitite ores. The preponderance of evidence suggests that the latter interaction involved boninitic melts in a supra- subduction zone environment as proposed previously.