Pristine helium from the Karoo mantle plume within the shallow asthenosphere beneath Patagonia.

Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3He/4He isotope ratios (6 ± 1 RA) lower than those of mid-ocean ridge basalts (8 ± 1 RA). Otherwise, basalts from oceanic islands related to hotspots often have high 3He/4He ratios (>10 RA), suggesting a deep and pristine undegassed mantle source. Here we present a striking high-3He/4He component (up to 27.68 RA) recorded by spinel-facies mantle xenoliths from Patagonia. Remarkably, the highest ratios were found in a long-lived trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks: the Deseado and the North Patagonian massifs. The mantle xenoliths with notably high-3He/4He ratios are inferred to be fragments of the shallow asthenosphere rising through the eroded and rejuvenated thin lithosphere. The pristine helium component is derived from the western margin of the Karoo mantle plume, related to the initial stages of the Gondwana fragmentation.

A cold seep triggered by a hot ridge subduction.

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise the assumption that these anomalous heat flow values are also promoted by the eastern flank of the currently subducting Chile Rise. Part of the rift axis is located just below the toe of the wedge, where active deformation and vigorous fluid advection are most intense, enhanced by the proximity of the young volcanic chain. Our results provide valuable information to current and future studies related to hydrothermal circulation, seismicity, volcanism, gas hydrate stability, and fluid venting in this natural laboratory.

Discovery of new hydrothermal activity and chemosynthetic fauna on the Central Indian Ridge at 18°-20° S.

Indian Ocean hydrothermal vents are believed to represent a novel biogeographic province, and are host to many novel genera and families of animals, potentially indigenous to Indian Ocean hydrothermal systems. In particular, since its discovery in 2001, much attention has been paid to a so-called 'scaly-foot' gastropod because of its unique iron-sulfide-coated dermal sclerites and the chemosynthetic symbioses in its various tissues. Despite increasing interest in the faunal assemblages at Indian Ocean hydrothermal vents, only two hydrothermal vent fields have been investigated in the Indian Ocean. Here we report two newly discovered hydrothermal vent fields, the Dodo and Solitaire fields, which are located in the Central Indian Ridge (CIR) segments 16 and 15, respectively. Chemosynthetic faunal communities at the Dodo field are emaciated in size and composition. In contrast, at the Solitaire field, we observed faunal communities that potentially contained almost all genera found at CIR hydrothermal environments to date, and even identified previously unreported taxa. Moreover, a new morphotype of 'scaly-foot' gastropod has been found at the Solitaire field. The newly discovered 'scaly-foot' gastropod has similar morphological and anatomical features to the previously reported type that inhabits the Kairei field, and both types of 'scaly-foot' gastropods genetically belong to the same species according to analyses of their COI gene and nuclear SSU rRNA gene sequences. However, the new morphotype completely lacks an iron-sulfide coating on the sclerites, which had been believed to be a novel feature restricted to 'scaly-foot' gastropods. Our new findings at the two newly discovered hydrothermal vent sites provide important insights into the biodiversity and biogeography of vent-endemic ecosystems in the Indian Ocean.