Recycled crustal carbon in the Austral–Cook mantle: clues from CO₂ and noble gases in fluid inclusions

Hotspot magmatism in Polynesia represents a unique natural laboratory for investigating the origin of mantle carbon and the role of recycled crustal materials in plume-related settings. Here, we present new carbon (δ¹³C) and noble gas isotope data for fluid inclusions (FIs) hosted in mantle xenoliths and volcanic rocks from selected Polynesian localities, including Savai‘i and Ta‘u Island (Samoa), Moorea Island (Society Islands), and Tubuai Island (Austral–Cook Islands). The Society samples display δ¹³C values ranging from −10.83‰ to −3.12‰, overlapping the MORB (Middle Oceanic Ridge Basalt) upper mantle range, whereas the Tubuai xenoliths exhibit significantly heavier compositions, between −1.3‰ and +1‰, coupled with elevated CO₂/³He ratios. Helium isotopes further distinguish the regions, with Samoan samples being characterized by high 3He/4He values (up to 13.4 Ra, where Ra is the atmospheric He isotope ratio), indicating contributions from a relatively undegassed lower mantle source, while Society (lava) and Tubuai (xenoliths) exhibit lower than MORB ratios (6-7 Ra). The heavy δ¹³C values observed in Tubuai are interpreted as evidence for crustal carbon recycled into the Austral-Cook mantle. In contrast, the Samoa FIs suggest subsequent isotopic re-setting during pervasive silicate metasomatism. These results demonstrate that mantle carbon isotope signatures are controlled not only by crustal recycling, but also by the sequence and style of metasomatic processes affecting the lithospheric mantle, which are strictly dependent on the Earth’s geodynamics.