Vesicle Morphologies of Popping Rocks: Implication for degassing processes during the 2018-2021 Mayotte submarine eruption

Between July 2018 and January 2021, the submarine eruption of Fani Maoré volcano, offshore Mayotte (Mozambique Channel), has been unprecedentedly well monitored with several oceanic cruises that provided a large number of samples (obtained by sea floor dredges) from the deep volcanic activity (3,300 m). The unique spatial and temporal sampling of the main edifice, flanks and distal ponded lava flows allows us to precisely track the vesicle morphologies and syn-eruptive degassing processes. This is an exceptional situation as deep-submarine eruptions remain poorly known due to the limited access to deep sites. We quantified textural parameters such as porosity, vesicle connectivity and vesicle size distributions (VSD) of these unusually gas-rich lavas, also known as popping rocks. Three different textures can be distinguished. The most vesicular (average of 35% vesicles) lava fragments display a unimodal distribution, and large vesicles are almost absent. Samples with intermediate porosities (average of 24% vesicles), show a bimodal distribution, with a dominant mode of large vesicles and a subordinate mode of small vesicles. Denser samples have the lowest porosity (average of 18% vesicles) characterized by a bimodal distribution, with a dominant mode of small vesicles. Samples with the highest and intermediate porosities were collected at lava flows emitted from the main eruptive vent during the early phase of the volcanic activity between July 2018 and August 2019. Both textures contain a population of small vesicles centred around the same mode suggesting a common origin, mostly related to the sudden gas-rich magma decompression. By contrast, the population of large vesicles is only present in pillow fragments located at the edge of the lava flows. These vesicles may be the result of bubble growth and coalescence during lava flow emplacement. Finally, the densest samples are only observed in lava flows that originated from a secondary fissure located on the Northwest, during the late phase of the volcanic activity between August 2019 and January 2021. This very poorly vesicular lava may result from a different magma batch (also supported by petrographic study) that was more degassed. Comparison with existing dataset from surface basaltic lava flows also revealed that most of our samples contain a significant amount of isolated vesicles (up to 18 vol%) emphasizing that popping rocks are able to retain gas at seafloor pressures. The detailed analysis of pillow lava texture allow us to make a clear distinction between lava flows erupted by Fani Maoré before and after August 2019. Our results also provide information on the eruption dynamics such as vesiculation processes and could be used to better constrain the rheology of the basanite melt for the Fani Maoré eruption.