Helium isotopic systematics of oceanic islands and mantle heterogeneity

Abstract

Isotopic variations in oceanic igneous rocks provide important constraints on models of oceanic mantle structure. Of particular interest is the global negative correlation between ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, which has been used to estimate ‘bulk earth’ values^1–3 for ⁸⁷Sr/⁸⁶Sr, ⁸⁷Rb/⁸⁸Sr and ¹⁴³Nd/¹⁴⁴Nd. Simple two-reservoir models have failed to explain all the isotopic variations, however, because of the complicated trends in Pb isotopes^4–6. This has led to suggestions that recycled oceanic crust or sediments must be considered in these models^7–9. We report here the results of helium isotopic analyses in basaltic phenocrysts from the islands of Gough and Tristan da Cunha. Because basalts from the islands lie near bulk earth on the Sr–Nd correlation diagram³, the study was initiated to characterize the helium isotopic signature of this component. Whereas the ³He in mantle gases is mostly primordial, the ⁴He is primarily radiogenic, having been produced by decay of ²³⁸U, ²³⁵U and ²³²Th. High ³He/⁴He ratios in igneous rocks therefore indicate primordial volatiles^10,11. We believe that the present results are inconsistent with the notion that the mantle beneath Gough and Tristan da Cunha is primitive or undepleted relative to mid-ocean ridge basalt (MORB). Helium isotopic results on basaltic glasses and phenocrysts from the rift zone of Kilauea confirm the previously reported high values from this area^12–15. We also report new analyses from Loihi Seamount (40km south-east of Kilauea), which does seem to be derived from a more primitive source. When these data are combined with values for MORBs (from ref. 16) and plotted with respect to ⁸⁷Sr/⁸⁶Sr, the observed trends offer insight into the different source regions for oceanic island basalts and the nature of mantle heterogeneity.