ORIGIN OF PODIFORM CHROMITITE, A NEW MODEL

Podiform chromitites have been interpreted as the result of harzburgite/melt reaction and related melt mixing in the upper mantle. The discovery of ultrahigh-pressure (UHP) minerals, especially diamond, coesite and moissanite, in some podiform chromitites and host peridotites, raises many questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated Many chromite grains in these bodies contain inclusions of forsterite and pyroxene. Forsterite inclusions have Fo numbers of 97-99 and NiO 1.11-1.29 wt%. Mg# of clinoproxene inclusions are 96-98 and those of orthpyroxene 96-97. X-ray studies show that the olivine inclusions have very small unit cells and short cation-oxygen bond distances, suggesting crystallization at high pressure. However, nodular and disseminated chromites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94-96 and lower NiO 0.35~0.58 wt%. In addition, Mössbauer spectroscopy shows that Fe3+/∑Fe=0.42 is higher in chromite from massive ore than from the nodular and disseminated ores, which have Fe3+/∑Fe=0.22. Disseminated chromites also show systematic changes in olivine and spinel composition from the dunite envelop to the massive ore, indicating melt-rock reaction. These observations suggest that the formation podiform chromitites is multi-stage process. Massive ores form first deep in the mantle under low ambient ƒO2 from partial melts of the host peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these chromites. Then, as the massive chromite rises to the shallower levels in the mantle it partially reacts with their host peridotite to form the nodular and disseminated chromitite ores that may develop by interaction with boninitic melts in a suprasubduction zone environment as proposed previously.