Assessing the hazard of potential submarine eruptions at the Horseshoe complex through diffusion chronometry

Submarine volcanoes make up ~75% of volcanism on Earth, yet they are one of the least explored volcanic features. Our understanding of these eruptions is limited due to challenges in monitoring these often remote volcanos with summits deep underwater. Nonetheless, consequences of their eruption can pose significant risks to the local economy and society (e.g. tsunamis) [1]. The recent (2018-2020), unanticipated submarine eruption of Fani Maoré ~50 km East of Mayotte island (Comoros Archipelago) [2] demonstrates this. Monitoring of the area increased only after its eruption, leading to the discovery of a submarine volcanic chain near the island of Mayotte. After the Fani Maoré eruption ended, seismicity and CO2 fluid emissions still continues below the so-called Horseshoe complex [3,4]. This complex is composed of recent phonolitic pyroclastic cones and lava flows, a number of them are of Holocene age and significant volume. Thus, it poses a critical risk to the >320000 inhabitants of Mayotte, as it is only ~10 km from the island [5]. Scenarios for future increased unrest and eruptive activity need to be considered. To contribute to this, we use diffusion chronometry in olivine to determine the maximum interaction times of Holocene magmas with peridotite and gabbro xenoliths in the Horseshoe phonolites. We find extremely short diffusion times on the order of minutes to hours (