A New mantle source contributing to volcanism in the Indian Ocean

Intra-plate volcanism is commonly attributed to mantle plumes originating from deep-seated therm0-chemical anomalies that rise buoyantly through the mantle and puncture the overlying lithosphere. These long-lived magmatic systems can persist for tens of millions of years, producing age-progressive volcanic chains and, in some cases, interacting with mid-ocean ridges to generate off-axis volcanism. The Rodrigues Ridge, and more generally the Mascarene Islands in the Indian Ocean have traditionally been interpreted within this framework as the result of interaction between the Central Indian Ridge and the Réunion hotspot. Here, we present a new geochemical and geochronological investigation of volcanic rocks from Rodrigues Island, the subaerial expression of the Rodrigues Ridge, which challenges this classical model. Compared with published data from the Mascarene islands, our major, trace element and Sr-Nd-Pb isotopic analyses reveal systematic deviations from compositions expected for simple mixing between depleted mid-ocean ridge mantle and Réunion plume-derived melts. Instead, Rodrigues subaerial lavas, along with the intermediate and younger volcanic series of Mauritius Island; record the contribution of a third, geochemically distinct mantle source whose signature lies in the focus zone (FOZO) of Ocean Island Basalts isotopic compositions. This additional component requires the involvement of material derived from another deep mantle source. The most suitable candidate providing this plume-related material is the Mascarene Basin asthenospheric reservoir (MBAR), a low shear velocity zone in the asthenosphere beneath the Mascarene Basin identified by seismic tomographies described in Barruol et al. (2019). Moreover, on-axis volcanism in the Central Indian Ridge —and thus recent— have already been linked to the influence of the MBAR (Vincent et al., 2024). K-Ar geochronology combined with geochemistry allows us to constrain the timing of its contribution to the magmatism of the western Indian Ocean to the last ~4 million years. These results highlight the complexity of mantle plume–ridge interactions and suggest that the Indian Ocean upper mantle is fed by multiple plume sources whose contributions may overlap in space and time. Our study emphasizes the need to reconsider the upper mantle architecture beneath off-axis volcanic ridges and sheds light on the dynamics of plume dispersal within the asthenosphere.