Magma ascent and lava flow field emplacement during the 2018–2021 Fani Maoré deep-submarine eruption insights from lava vesicle textures.

• Textural analysis was performed on basanitic deep submarine lavas from the 2018–2021 Fani Maoré eruption. • Pillow selvages are either highly vesiculated (max 50%) or very massive (<1%). • Heterogeneous textures result from ascent and effusion rate variations with time. • Fani Maoré lavas experienced different degrees of degassing and outgassing. • This submarine lava flow fields was dominantly tube-fed due to high effusion rates (11–200 m3/s). The 2018–2021 Fani Maoré submarine eruption (offshore of Mayotte, Mozambique Channel) extruded a bulk volume of ∼6.5 km3 of basanite magma onto the seafloor at a depth of 3300 m, with effusion rates ranging from 150 to 200 m3/s in the first year of the eruption, to less than 11 m3/s in the final months. Six oceanographic campaigns provided a large sample set covering the entire flow field at high spatial and temporal resolution. These samples allow us to precisely track syn-eruptive degassing processes through quantification of textural parameters including porosity, pore connectivity, vesicle number density (N V) and vesicle size distributions (VSD). Three different textural facies have been distinguished. (1) Vesicular lavas (average porosity of 35%) display unimodal VSDs, high N V (14–214 mm−3), and small and spherical vesicles. (2) Lavas with intermediate porosities (25%) have scarce small vesicles, VSDs shifted towards larger vesicles, and low N V (0.2–39 mm−3). (3) Dense lavas with low porosities (14%) display bimodal VSDs distribution, a dominant mode of small vesicles, and low N V (0–87 mm−3). The early phase of activity (Phase 1, June 2018 – May 2019) built the main edifice and was fed by rapid ascent and closed-system degassing of volatile-rich magma ascending from a deep reservoir to the seafloor (Facies 1). Distal samples collected from lava flows emitted during Phase 2, between June and July 2019, show large and irregular shape vesicles mostly related to bubble growth and coalescence, and outgassing during emplacement (Facies 2). These lavas are interpreted to be emplaced during extension of a lava tube system which began to develop during Phase 1. The final phase (Phase 3, August 2019 – January 2021) was associated with lava effusion located at the northwest lava flow front, 6 km from the summit. Phase 3 involved a more degassed magma due to the increase in the length of the magma pathway (Facies 3). Phase 3 lavas were also extremely outgassed and associated with construction of a new complex lava flow field with tumuli and multiple ephemeral vents (lava breakouts). The heterogeneous textures within the studied samples reflect changing ascent and effusion rates with time, leading to emplacement of lava flows which varied depending on the degree of degassing and effusion rate. We conclude that emplacement of the Fani Maoré large submarine lava flow fields developed through extensive and prolonged tube systems this being supported by the high effusion rates. [ABSTRACT FROM AUTHOR]