OPHIOLITIC CHROMITITES: ORIGINS AND IMPLICATIONS FOR MANTLE DYNAMICS

Podiform chromitites, which are ubiquitously found in the Moho transition zone (MTZ) to upper mantle of ophiolites, have been interpreted as an in-situ magmatic cumulate related with melt/harzburgite interaction and subsequent melt mixing within melt conduits in the upper mantle. Our consensus on the podiform chromitite genesis, however, has been changed since the discovery of ultrahigh-pressure (UHP) chromitites (= chromitites that contain ultrahigh-pressure minerals, such as diamond) from Tibet (e.g., Robinson et al., 2004).

Ordinary podiform chromitites exhibit some features indicating a low pressure origin; the melt/peridotite interaction, a key reaction for chromitite genesis, is associated with incongruent melting of opx of wall harzburgite at low P, and chromian spinel, an essential constituent, enclose hydrous mineral inclusions, especially pargasites, which are stable only at low P. On the other hand, the Tibetan UHP chromitites show an essentially high-pressure origin; their chromian spinel displays exolutions of coesite (one of UHP minerals) and clinopyroxene (Yamamoto et al., 2009), excluding the possibility of xenocrystal origin of UHP minerals. Some features of UHP chromities can be accomplished by deep recycling of shallow cumulus chromitites; diamonds are formed by reduction/oxidation of fluidal C-rich phases, and the silicate lamellae are derived from the silicate inclusions magmatically trapped by spinel. PGE alloys, the only PGM in the Tibetan UHP chromitites, are formed by desulfurization of PGE sulfides, which are common in ordinary podiform chromitites of shallow origin.

Two types of chromitites, discordant and concordant, were examined in the Oman ophiolite. The discordant chromitite, which is non-deformed and cuts deformation structures of host harzburigte, contains spinels with plenty of hydrous mineral inclusions but without silicate lamellae. The concordant chromitite chromitite, parallel to host harzburgite foliation, contains spinels with silicate lamellae and very minute (< 10 µ) globular hydrous mineral inclusions. They are essentially different in origin, but neither of them are of UHP origin. There are chromitites and chromitites in ophiolites.