He–Ne–Ar isotopic constraints on the mantle source of the Fani Maoré paroxysm: implications for plume involvement beneath the Comoros

The origin of intraplate magmatism remains a subject of active debate. Recent volcanic activity offshore Mayotte, the easternmost volcanic island of the Comoros archipelago, has reignited extensive scientific investigations aimed at better understanding the geodynamic context of the region. Two main hypotheses are generally invoked to explain the observed volcanic activity: one involves a mantle plume, while the other attributes the magmatism primarily to tectonic processes, possibly linked to the offshore prolongation of the East African Rift System. However, both models present limitations. Noble gases are powerful tracers for distinguishing between these scenarios, as mantle plumes typically exhibit distinct isotopic signatures. We conducted step-crushing analyses on pristine glassy samples from the most recent (2018–2021) submarine volcanic edifice of Mayotte named Fani Maoré. The noble gas isotopic systematics of the Fani Maoré lavas are strikingly similar to a MORB-like mantle, casting doubt on a significant chemical contribution from a mantle plume. Instead, we favor a tectonically triggered, shallow origin, for the magmatism in the Comoros. Initiation of rifting in the region, coupled with edge-driven convection at the base of the lithosphere, can induce low-degree partial melting in the upper mantle without the thermal contribution of a mantle plume. The Comorian lithosphere is old and metasomatized, potentially hosting fertile heterogeneities. Preferential melting of these heterogeneities can explain the non-MORB-like lithophile isotopes while preserving MORB-like noble gas signatures. This mechanism provides a coherent explanation for intraplate magmatism in the region and could be applicable to other oceanic intraplate magmatic provinces.