Seminar Romain Sauvalle

Abstract

Most of Earth’s magmatism is concentrated at plate boundaries and is well explained by the plate tectonic model. However, a significant fraction of volcanic activity occurs in intraplate settings, where its origin remains debated. Since the seminal work of Morgan (1971), mantle plumes have been widely invoked to explain oceanic island volcanism, supported by numerous geochemical and geophysical observations.

The Comoros archipelago represents a classic example of oceanic island volcanism. Recent submarine volcanic activity east of Mayotte between 2018 and 2021 has renewed interest in the geodynamic context of this region. However, recent studies have increasingly questioned the role of a chemically distinct mantle plume in driving magmatism beneath the Comoros.

Noble gas isotopes provide powerful tracers of mantle sources and are particularly sensitive to plume-influenced signatures. Here, we present the first step-crushing noble gas analyses of pristine, glassy, gas-rich basanitic lavas erupted at depths exceeding 3,000 m from the Fani Maoré submarine volcano. The resulting noble gas isotopic systematics are broadly MORB-like, with the notable exception of significant excesses in the radiogenic isotope 4He

These observations argue against the involvement of a classical mantle plume beneath the Comoros archipelago. Instead, they support a model in which magmatism is tectonically triggered, potentially related to reactivation and thinning of an old, heterogeneous oceanic lithosphere. Preferential melting of distributed mantle heterogeneities, as inferred from the noble gas isotopic signatures of the Fani Maoré lavas, may explain the extreme geochemical characteristics observed in some intraplate volcanic provinces worldwide, without requiring the contribution of a deep mantle plume.