The uniform and low ³He/⁴He ratios of HIMU basalts as evidence for their origin as recycled materials

Abstract

Several hypotheses have been proposed for the origin of the group of lavas having the isotopic signature known as ‘high μ’ (HIMU, where μ= ²³⁸U/²⁰⁴Pb)^1,2,3,4; these explanations have invoked processes involving recycled oceanic crust and sediment, metasomatically enriched subcontinental lithosphere, or intra-mantle metasomatism^{1,2,3,4,5,6,7,8,9,10,11,12}. Here we present helium isotope analyses of HIMU basalts, with ages of 10–18 Myr, from three islands of the Cook–Austral Archipelago in the southern Pacific Ocean. We find that the HIMU samples have a relatively uniform and low ³He/⁴He ratio of 6.8 ± 0.9 R_A compared with mid-ocean-ridge basalt, whereas samples of other enriched-mantle lavas from this region have more variable and higher signatures. The consistency of our HIMU results with those obtained from previous analyses of HIMU lavas at St Helena¹³ in the Atlantic Ocean lead us to conclude that a relatively low and uniform ³He/⁴He ratio represents a general characteristic of the mantle source region for HIMU lavas. Also, the uniform ³He/⁴He ratio (in both space and time) suggests that recycled oceanic crust and/or sediments are present in the source region for HIMU lavas, as it seems less likely that the other candidate processes, invoking metasomatism, would produce such consistent values.