SOURCES OF OCEAN ISLAND BASALTS - A REVIEW OF THE OSMIUM ISOTOPE EVIDENCE

The Os isotope system provides insight into the origin of mantle sourc e heterogeneity which is complementary to that provided by the incompa tible element isotope systems of Sr, Nd and Pb. The Os isotope system is both an extremely sensitive tracer of crustal contamination and the only isotope system which can clearly distinguish between contaminati on of magmas in the crust vs. contamination in the lithospheric mantle . Os isotopes therefore provide important constraints regarding whethe r basalts are recording plume or lithospheric signatures. Basalts infe rred to be recording mantle plume signatures indicate that the plume s ources contain an enriched component which is more radiogenic in Os th an primitive upper mantle. While this might be attributed to recycled crustal material, the expected correlations between Os and Pb isotopic signatures are largely absent. An alternative possibility is that man tle plumes contain a component of lower mantle which is radiogenic in Os. The Os isotopic compositions of plumes may be further enriched in some cases by the addition of recycled crustal components which also p roduces distinctive signatures in the incompatible element isotope sys tems. HIMU basalts, which have both extremely radiogenic Os and Pb iso topic signatures, can be produced by the addition of 15-25% recycled o ceanic crust to a plume source already slightly enriched in Os due to a radiogenic lower mantle component. The origins of EMI and EMII mantl e sources are currently less well constrained by Os isotopic signature s, but might be attributed to recycling of oceanic crust plus pelagic sediment, and metasomatized subcontinental lithospheric mantle, respec tively.