Sources of ocean island basalts: A review of the osmium isotope evidence

Abstract

The Os isotope system provides insight into the origin of mantle source heterogeneity which is complementary to that provided by the incompatible 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 contamination of magmas in the crust vs. contamination in the lithospheric mantle. Os isotopes therefore provide important constraints regarding whether basalts are recording plume or lithospheric signatures. Basalts inferred to be recording mantle plume signatures indicate that the plume sources contain an enriched component which is more radiogenic in Os than 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 mantle 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 produces distinctive signatures in the incompatible element isotope systems. HIMU basalts, which have both extremely radiogenic Os and Pb isotopic signatures, can be produced by the addition of 15–25% recycled oceanic crust to a plume source already slightly enriched in Os due to a radiogenic lower mantle component. The origins of EMI and EMII mantle sources are currently less well constrained by Os isotopic signatures, but might be attributed to recycling of oceanic crust plus pelagic sediment, and metasomatized subcontinental lithospheric mantle, respectively.