Primordial and recycled noble gases in the Cook-Austral HIMU mantle: Insights into the onset of volatile subduction

Ocean island basalts (OIBs) sample multiple distinctive geochemical reservoirs in the mantle and provide rich information on the generation and preservation of mantle compositional heterogeneities. Cook-Austral OIBs with HIMU (high-mu, where mu=238U/204Pb) signatures tap into a mantle reservoir that hosts ancient altered oceanic crust recycled to the deep mantle via plate tectonics as well as a less degassed component. Noble gas isotope and elemental systematics are powerful tools for investigating the origin and the evolution of mantle geochemical reservoirs, providing insights into accretion, early and long-term mantle degassing, and injection of atmospheric volatiles into the mantle. We present new high precision He-Ne-Ar-Xe isotope data measured in olivine phenocrysts from HIMU basalts from the islands of Raivavae and Tubuai in the Cook-Austral archipelago. We confirm the existence of a less-degassed Ne component in OIB samples with strong HIMU lithophile isotope signatures. The large sample quantity and detailed sample processing enabled us to resolve moderate Xe isotopic excess from pervasive atmospheric contamination from gas-poor subaerial samples. Multi-dimensional insights from a full suite of noble gases indicate that subduction left significant long-term compositional imprints on the HIMU mantle source: Cook-Austal HIMU 4He*/40Ar* may be elevated, reflecting fractionation of the K/U ratio during slab dehydration reactions. Injection of surface-derived Ar and Xe into Earth's interior via effective gas retention in hydrous phases including serpentine was potentially initiated earlier than similar to 2.5 Gyrs ago. The introduction of mantle heterogeneities through survival of heavy noble gases in downgoing slabs, and the subsequent preservation of these recycled signatures in the mantle since the late Archean or early Paleoproterozoic provide key thermal and dynamical constraints regarding the style of plate tectonics and mantle convection over billion-year timescales.