Global Variability in the Platinum-group Element Contents of Komatiites

Abstract

Platinum-group element (PGE) concentrations in komatiites of similar age but different petrological affinity, lithofacies type and emplacement style are remarkably similar when the effects of olivine fractionation and accumulation are accounted for. All komatiites display a slight enrichment of PPGE (Rh, Pt and Pd) over IPGE (Ir, Ru and Os) attributable to limited retention of IPGE in their mantle sources, coupled with saturation in Ir–(Os) alloys. Significant differences and trends exist, however, including subtle provinciality in Pt/Pd ratios, and in particular a systematic linear increase in komatiite PGE contents with age from the oldest at >3·5 Ga to the late Archaean group at c. 2·7 Ga, regardless of petrogenetic type. There is a linear secular trend of increasing Pt at a given MgO content and in Pt/Ti from 3500 Ma to 2700 Ma across all komatiite types. Barberton-type komatiites younger than 3000 Ma are generally slightly depleted in Pt and Pd relative to Munro-type. Munro-type komatiites from 3200–3500 Ga greenstone belts of the east Pilbara Craton of Western Australia are strongly depleted in PGE relative to younger belts, but similar to Barberton-type komatiites of the same age. Munro-type and Barberton-type komatiites from the 2900 Ma greenstone belts of the Youanmi Terrane in the Yilgarn Craton have similar PGE contents. Age, rather than komatiite type, appears to be the dominant control. These observations lead us to the hypothesis that the age control is related to a secular variation in the PGE content of komatiite mantle source regions. We envisage that Early Archaean komatiites could have been derived from plumes sourced in the deep mantle, which had been largely stripped of its highly siderophile element budget during core segregation and as yet incompletely replenished in PGE by the Late Veneer. Late Archaean komatiites have higher PGE contents as a result of progressive mixing-in of Late Veneer material over the period between the Late Heavy Bombardment (4·3–3·9 Ga) and 2·7 Ga, at which time the mantle had become effectively homogeneous with respect to PGE. The relative constancy of gamma-osmium values in komatiites throughout the Archaean is consistent with this hypothesis.