Fractionation of trace and platinum-group elements during metamorphism of komatiitic chromites from the early Archean Gorumahishani greenstone belt, Singhbhum Craton (eastern India)

It is well established that the major and minor element contents of chromites are subject to change during greenschist amphibolite facies metamorphism. During upper metamorphism, chromite can be completely converted chrome magnetite. However, not all elements affected same degree, concentrations +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable modification. The degree which trace elements, platinum-group (PGE), has been closely examined. compositions textures from komatiites Gorumahishani greenstone belt Singhbhum Craton (India) have experienced a range metamorphic conditions facies, providing opportunity study changes composition with grade. Five types altered identified komatiitic suite rocks in ~120-km-long belt. type-I non-porous characterized by least modified cores. These mostly present northern Maharajgunj-Tua Dungri section where show metamorphism greenschist-amphibolite transition facies. type-II type-III porous found southern Kapili up mid-amphibolite Type-IV type-V ferritchromit magnetite, respectively, entire central cores grains relatively higher mobile Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr Al)], Fe3+/R3+ (R3+ = Fe3+ Cr3+ Al3+) ratios than their respective rims. Significantly immobile Ti V) relative magnetite rims varieties other sections might due under higher-grade (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals concentration Ru Rh diffusion these normal spinel structure towards bivalent octahedral sites inverse processes. Os, Ir, Ru, related facilitated associated sulphide or mineral alloy phases. calculated partition coefficients Sc, Ti, V, Mn, Ni, Ga, assuming equilibrium parental melt also suggest variable effects when compared global experimental empirical values natural samples.