English abstract
Podiform chromitites have been interpreted as cumulates formed via melt/harzburgite reaction and subsequent melt mixing within the upper mantle. Recent finding of ultrahigh-pressure (UHP) minerals such as diamond from some podiform chromitites has, however, seriously required us to reconsider the whole framework of podifrom chromitite genesis. The UHP podiform chromitites are characterized by presence of frequent silicate lamellae in chromian spinel and only PGE (platinum-group elements) alloys as platinum-group minerals. The UHP chromitites were possibly formed by deep recycling of the ordinary low-pressure podiform chromitites. Diamonds could be formed by reduction of CO2 metasomatically supplied to chromian spinel in advance. Silicate lamellae in chromian spinel of UHP chromitites were possibly derived from primary silicate inclusions (pyroxenes, pargasite and Na phlogopite), commonly found in the low-pressure chromitite. They were decomposed/partially melted and resolved in high-pressure chromian spinel during compression/heating during downward transportation, and were exsolved as silicate lamellae on decompression/cooling during uprising. PGE sulfides commonly found in the low-pressure chromitite may have been decomposed to PGE alloys and sulfur-rich melt/fluid that were removed outside. We should systematically re-examine all podiform chromitites that have been ever documented, but our preliminary examination indicates that both concordant and discordant chromitites from Oman ophiolite are of low-pressure origin, containing primary pargasite inclusions in chromian spinel. More thorough characterization of the UHP chromitite may enable us to place constraints on the style of mantle convection that provided the MORB source possibly comprising the UHP chromitite. Possible presence of ringwoodite as one of UHP minerals in chromitite may favor the two-layer mantle convection for supplying the source mantle that formed the oceanic lithosphere.
