On the Archean vs. Proterozoic age of the HIMU mantle component : New 33S/32S, 34S/32S, 36S/32S-data from Saint-Helena glasses.

English Abstract:

In order to better address mantle sulfur isotope variability, we report on the only two available HIMU basalt glasses dredged on Josephine seamount, in the close vicinity of Saint-Helena. Together with several HIMU localities in the south Pacific (including Mangaia), St. Helena exhibits a HIMU signature. The two lavas show similar Sr-, Nd- and Pb-isotope compositions and trace element patterns compositions compared with previous data. Both are enriched in d³⁴S > 0.5‰ vs CDT and do not display any significant mass-independent signature, with ∆³³S ~ 0.014±0.010 and ∆³⁶S ~ 0.040±130‰. Importantly ∆³³S and ∆³⁶S values are within error of the MORB mantle, at +0.010 ± 0.005‰ and -0.071 ± 0.047‰ respectively. Trace elements ratios (e.g. Cl/K ~ 0.045) show that assimilation of either altered oceanic crust or seawater played a negligible role in accounting for the S-isotope characteristics of these lavas. Despite their large variations in S-contents and major element compositions, the samples show similar S-isotope compositions suggesting a minimal role for either degassing or sulfide segregation. These results contrasts with those obtained on Mangaia sulfide inclusions in olivine phenocrysts (with ∆³³S and d³⁴S as low as -0.35‰ and -11‰ respectively) and suggests that the HIMU component is likely isotopically heterogeneous for sulfur and Pb-isotopes. The ∆³³S and ∆³⁶S rather support a Proterozoic recycled component for the source of Saint-Helena melts.