W isotope compositions of oceanic islands basalts from French Polynesia and their meaning for core–mantle interaction

Abstract

In order to understand the behavior of W during magmatic differentiation and to examine possible W contributions from the core, the W abundances and isotope compositions were obtained for oceanic island basalts (OIB) from French Polynesia, where a whole-mantle–scale mantle upwelling has been documented using global seismic tomography. An additional reason for analyzing OIB in this particular region is the occurrence of HIMU (high time-integrated 238U/ 204Pb or high µ) OIB, which was proposed to tap the mantle source with minor contributions of recycled sediments with high W abundance. The W abundances of HIMU OIBs from Mangaia, Rurutu, and Tubuai and of EM1 OIBs from Rarotonga show five-fold variation from 0.2 ppm to 1.0 ppm, which is attributed to result from variation in degrees of melting, fractionation and accumulation of olivine and clinopyroxene phenocrysts, and source heterogeneities. The W abundances of HIMU OIBs lower than those of EM1 OIBs at a given MgO content is consistent with an idea that HIMU magma sources were contaminated by recycled sediments to lesser extent compared to EM1-magmas. The W abundance in an inferred primary HIMU magma and plausible degrees of melting (1–3%), as estimated based on incompatible element concentrations, enable us to deduce the W abundance in the sources of HIMU OIBs (2–15 ppb). The W abundance in the HIMU source is likely to be somewhere between that in the depleted mantle and that in primitive mantle, suggesting that severe contamination with recycled sediment is unlikely. All analyzed OIB samples have W isotopic composition identical to that of the bulk silicate Earth. The mixing calculation demonstrates that any contribution of core material to the source of HIMU OIBs is no greater than 0.6%.