Th and U Recycling Through Subduction; Noble Gases and Pb Isotope Evidence

Abstract:

HIMU is one of mantle components that has been recognized to form by subduction and subsequent storage of ancient oceanic crust in the mantle. We use noble gas and Pb isotopic compositions of HIMU basalts to constrain age of reservoir formation and recycling of K, U, and Th. It has been confirmed that subaerial and submarine Austral Islands HIMU basalts show low ³He/⁴He relative to MORB, indicating the HIMU reservoir involving either U-enriched or He-depleted material. Moreover, fractionation-corrected ⁴He/⁴⁰Ar* of the HIMU basalts is higher than the ⁴He/⁴⁰Ar* production ratio in the mantle. This suggests that K/U of the HIMU component converted from ⁴He/⁴⁰Ar* is approximately 3000, and hence lower than the canonical mantle K/U of 13000. These facts are best explained by a model that the HIMU component originates from subducted oceanic crust that preferentially lost He and K relative to U and Th during subduction.

The St. Helena HIMU basalts in the Atlantic show systematically higher ²⁰⁷Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb than the Austral Islands HIMU basalts in the Pacific. The Pb isotope evolution model suggests that both HIMU components formed around 2 Ga; however, the HIMU component beneath Atlantic is about 0.3 Ga older than that beneath Pacific. We interpret the age difference as that the HIMU reservoir for St. Helena involved old subducted materials more abundant than that for Austral Islands during the long-term accumulation of subducted materials. Using the same Pb evolution model, Th/U deduced from the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb relationship (3.3-3.7) is higher than that of present-day altered MORB (<2), Th/U of which is lowered by addition of U from oxic hydrothermal fluids. This suggests that either the HIMU precursor was fresh MORB, or more likely, hydrothermally altered MORB with minimal U enrichment in the less oxic environment in the Archean or early Proterozoic.