Current limitations to the understanding of Re–Os behaviour in subduction systems, with an example from New Britain

Existing data for subduction-related magmas reveal a considerable diversity in both Re and Os concentrations and osmium isotope ratios. Unfortunately, with current knowledge, deciphering the relative roles of mantle, subducting slab and crust in generating these signatures remains a difficult undertaking. Fractional crystallisation imparts a significant influence on both Re and Os abundances which, in the case of Re, can be corrected for, although the extent to which Re volatility influences measured concentrations remains to be firmly established. With these provisos in mind, Re does appear to be more mobile in ‘fluid-dominated’ low-K tholeiite-type arcs. A compilation of existing isotope data strongly suggests that Os isotope signatures in continental arcs are compromised by interactions within the crust rendering them largely unsuitable for the estimation of subduction fluxes. Within intra-oceanic arcs, taking lavas from New Britain as an otherwise well-constrained example, estimation of the local upper mantle isotopic composition remains a major impediment to the quantification of slab-derived Os fluxes.