Helium isotope geochemistry of some volcanic rocks from Saint Helena

Abstract

3He/ 4He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, including in vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited 3He/ 4He ratio at St. Helena is 4.3–5.9 R A. Helium isotope disequilibrium is present within the phenocrysts, with lower 3He/ 4He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to α-particle implantation from the surrounding (U + Th)-rich lavas. A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave 3He/ 4He= 7.1 R A . 3He/ 4He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to 3He/ 4He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low 206Pb/ 204Pb ). Overall, the He Sr Pb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere. 3He/ 4He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R A). This suggests that the recycled materials were injected into the mantle within the last 10 9 yrs.