Possible recycled origin for ultrahigh-pressure chromitites in ophiolites

Abstract

Podiform chromitites have been interpreted as cumulates by harzburgite/melt interaction and related melt mixing at the upper mantle to the Moho transition zone. Recent discovery of diamond and other ultrahigh-pressure (UHP) minerals from some podiform chromitites, especially those from Tibet, however, has raised a question about the depth of their formation. These UHP chromitites are possibly of deep recycling origin; they had been originally formed at the upper mantle before sinking down to deeper mantle, and upwelling again to the shallowest mantle by convection. Diamond is formed by reduction/oxidation of fluidal carbon species (e.g., CO2 or CH4) obtained during the convection history, and has survived oxidation because of strong encapsulation in metal alloys further included by chromian spinel. Exsolved silicates (diopside and coesite) in chromian spinel from some UHP chromitite are possibly derived from hydrous mineral inclusions in chromian spinel formed at low pressures, which have been decomposed/molten during sinking and solved in the UHP chromian spinel phase. Both the UHP chromitites and ordinary low-pressure ones could be present in the upper mantle derived from the mid-oceanic ridge, one of the main ends of the mantle upwelling flow. The mantle recycling issue unraveled through chromitite thus can be one of the targets of deep oceanic mantle drilling including the Mohole.