Lava Rheological Evolution and Flow Emplacement During the Deep Submarine Fani Maoré Eruption (Mayotte)

Abstract

The 2018–2020 lava flow field that built the submarine Fani Maoré volcano (Mayotte, north Mozambique Channel) at a water depth of 3,300 m was extensively sampled. We use the petrological analysis of samples collected at high spatial and temporal resolution to provide information on the textural evolution of the submarine lavas and its impacts on lava rheology, and thus on flow emplacement. Although the crystal content is lower than 10%, we observed two populations of crystals: (a) the micro-phenocrysts which mostly display skeletal shapes suggesting rapid growth upon magma ascent most likely due to decompression-driven crystallization, (b) the microlites resulting from post-emplacement groundmass crystallization. Combining both petrological and geochemical analyses, and applying rheological models, we show that lava viscosity increases from 158 ± 21 to 608 ± 134 Pa·s considering rigid bubbles or from 36 ± 5 to 485 ± 130 Pa·s considering deformable bubbles. Together with the waning ascent velocity and the decreasing effusion rates, this correlates with the evolution of lava flow emplacement dynamics, from long sheet lava flows to shorter flows exhibiting pillow mounts and multiple breakouts.