Isotope evolution in the HIMU reservoir beneath St. Helena: Implications for the mantle recycling of U and Th

HIMU (high- μ ; 238 U/ 204 Pb) is a mantle reservoir that has been thought to form by subduction and subsequent storage of ancient oceanic crust and lithosphere in the mantle. In order to constrain the processes that acted on subducted materials over several billion years, we present precise Pb–Sr–Nd–Hf–He isotopic data together with 40 Ar/ 39 Ar and K/Ar ages of HIMU lavas from St. Helena in the Atlantic. Clinopyroxene separates were analyzed together with whole-rock samples to better describe the geochemical characteristics of the HIMU component. Although isotopic variations are small in the St. Helena lavas (20.6–21.0 for 206 Pb/ 204 Pb) between 12 and 8 Ma, the younger lavas have more HIMU-like isotopic compositions than the older lavas. The mixing arrays defined by these lavas are remarkably similar to those observed in HIMU lavas from Austral Islands in the Pacific, suggesting that the two HIMU reservoirs located in different mantle domains are characterized by similar isotopic compositions with radiogenic 206 Pb/ 204 Pb and 208 Pb/ 204 Pb, enriched Nd and Hf isotopes, depleted Sr isotopes, and radiogenic 3 He/ 4 He. However, there is a significant difference between the St. Helena and Austral Islands lavas in 207 Pb/ 204 Pb. The St. Helena lavas show systematically higher 207 Pb/ 204 Pb for a given 206 Pb/ 204 Pb. Lead isotope evolution models suggest that both HIMU reservoirs formed around 2 Ga; however, the HIMU reservoir for St. Helena is about 0.3 Ga older than that for Austral Islands. The relation between 206 Pb/ 204 Pb and 208 Pb/ 204 Pb could reflect the time-integrated κ ( 232 Th/ 238 U) in the components. The HIMU components for St. Helena and Austral Islands have κ values between 3.3 and 3.7, which are intermediate between the present-day fresh mid-ocean ridge basalts (MORB; 2.6–3.2) and the chondritic silicate Earth (∼4). This is consistent with the model that the HIMU precursor is subducted oceanic crust created around 2 Ga from depleted upper mantle, in which κ monotonously decreased from the chondritic to the present-day values since late Archean or early Proterozoic, because of enhanced U recycling from the Earth’s surface back to the mantle in response to the increasing oxygen levels in the hydrosphere. Moreover, the fact that the HIMU components have much higher κ than the present-day hydrothermally altered MORB (0.2–2) suggests that either the HIMU precursor was an unaltered ancient oceanic crust, or more likely, an altered oceanic crust with minimal U enrichment by hydrothermal fluids in the less oxic marine environment of the late Archean or early Proterozoic. The unradiogenic 87 Sr/ 86 Sr of the HIMU components also suggests formation of ancient oceanic crust altered with hydrothermal fluids having much lower 87 Sr/ 86 Sr in that eon than at present, followed by removal of Rb from it by subduction dehydration.