OSMIUM-ISOTOPE RATIOS OF PLATINUM-GROUP MINERALS ASSOCIATED WITH ULTRAMAFIC INTRUSIONS - OS-ISOTOPIC EVOLUTION OF THE OCEANIC MANTLE

Osmium-isotope ratios were determined by an ion microprobe on the individual platinum-group minerals (PGM) from placers, which are associated with ultramafic intrusions of late Precambrian to Tertiary age. Unlike Os-isotope ratios in large layered mafic intrusions, these Os-187/Os-186 ratios are low, and within a narrow range from 0.99 to 1.12, which is attributed to the occurrences of the intrusions. There was no opportunity to incorporate old crustal Os because of the small sizes of the intrusions and the mode of emplacement into the upper crustal level. In addition, the interaction with the host volcanic rocks of similar age, if any, would not have seriously affected the Os-187/Os-186 ratios of the peridotites. While different phases of PGM in one grain have similar Os-187/Os-186 ratios, there is a significant variation in a given district. The variation is attributed to a long-term heterogeneity in Re/Os ratios of the oceanic upper mantle. The lowest value in each area is lower than the value expected from the evolution of bulk Earth composition. The lowering may be due to primordially low Re/Os ratios in the mantle or preferential removal of Re by partial melting to form the continental crust. The former model is rejected because most chondrites have higher Re/Os ratios than type C1 and the core-mantle separation would not have lowered Re/Os ratios. The low Os-187/Os-186 ratios are, therefore, attributed to the extraction of continental crust by preferential removal of Re from the mantle through partial melting. The model is consistent with the depleted nature of oceanic peridotites (positive epsilon(Nd), negative epsilon(Sr), and low Re/Os ratios). Calculations of Os-187/Os-186 ratios of the mantle residue suggest that the observed data are in accordance with a model involving the extraction of approximately 2% melt by fractional fusion from the mantle of C1 chondritic composition at approximately 2.0 Ga. If the bulk Earth has higher Re/Os ratios, as proposed by Martin [1], then the observed data require much larger degrees of partial melting or older mean age of partial melting at approximately 3.0 Ga. The variation in Re/Os ratios in each area is ascribed to the various degrees of mixing of the depleted mantle source and more fertile material. This may be due to local heterogeneity in the mantle, "marble-cake" like mantle [2] or replenishment of Re and Os from a more fertile source by sulphide (+CO2) in the mantle. Generally higher Os-187/Os-186 ratios found in this study compared with those from the southern African sub-continental lithosphere [3] may be attributed to the nature of the latter which is more isolated than the oceanic mantle from the rest of the mantle. The occurrence of S in the mantle may influence Re-Os isotope variations and the behavior of S may decouple the Re-Os system from other lithophile radiogenic isotope systems.