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 (<5 ± 1 h) for pyroclastic samples, which implies rapid ascent of magma from the MOHO (~20 km). For lavas, these times are significantly longer in the range of days to months (5 ± 2 to 163 ± 17 days). While the longest times in lava flows are attributed to continued diffusion during post emplacement cooling, shorter times provide minimum ascent rates. These are 7.7 ± 4.2 m/s for explosive eruptions and 0.016 ± 0.008 m/s for effusive ones. Hence, the final warning signals of impending eruptive activity would only be detected a few hours to days before eruption. This underscores the necessity to have efficient preventive risk reducing strategies well emplaced by the time very early-warning signs of potential unrest are detected through the local Volcanological and Seismological Monitoring Network of Mayotte - REVOSIMA [6].

[1] Gusman et al. (2022) Pure Appl. Geophys., 179, 3511-3525. [2]  Feuillet et al. (2021), Nat. Geosci., 14, 787-795. [3]  Thivet et al. (2023) Chem. Geol.,618, 121297. [4]  Lavayssière & Retailleau (2023) Volcanica, 6, 331-344. [5]  Puzenat et al. (2022) C. R. Geosci., 354, 81-104. [6]  REVOSIMA: https://www.ipgp.fr/en/observation/national-hosted-infrastructures/revosima/