Viscosity of crystal-free silicate melts from the active submarine volcanic chain of Mayotte

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Fani Maore ', was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Fani Maore ', one tephriphonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature in the air. At super-liquidus temperatures, basanites have the lowest viscosity (0.11-0.34 to 0.99-1.16 log10 Pa center dot s), phonolites the highest (1.75-1.91 to 3.10-3.89 log10 Pa center dot s), while the viscosity of the tephriphonolite falls in between (0.89-1.97 log10 Pa center dot s). Near the glass transition, viscosity measurements were performed for one phonolite melt because obtaining pure glass samples for the basanite and tephriphonolite compositions was unsuccessful. This is due to the formation of nanolites upon quench as evidenced by Raman spectroscopy. The phonolite viscosity ranges from of 10.19 log10 Pa center dot s at 1058 K to 12.30 log10 Pa center dot s at 986 K. Comparison with existing empirical models reveals an underestimation of 1.2 to 2.0 log units at super-liquidus and undercooled temperatures, respectively, for the phonolite. This em-phasizes (i) the lack of data falling along the alkaline basanite-to-phonolite magmatic differentiation trend to calibrate empirical models, and (ii) the complexity of modeling viscosity variations as a function of temperature and chemical composition for alkaline compositions. The new measurements indicate that, at eruptive tem-peratures between 1050 degrees C and 1150 degrees C (1323-1423 K), the oxidized, anhydrous, crystal-free and bubble-free basanite melt have a viscosity around 2.6 log10 Pa center dot s. In contrast, the anhydrous phonolite crystal-and bubble -free melt would have a viscosity around 6-10 log10 Pa center dot s at expected eruptive temperatures, from 800 to 1000 degrees C (1073-1273 K). Considering that both basanite and phonolite lavas from the Mayotte submarine vol-canic chain contain <6% crystals and a significant amount of water (1-2.3 wt% and 0.8-1.2 wt%, respectively), such viscosity values are probably upper limits. The new viscosity measurements are essential to define eruptive models and to better understand the storage and transport dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface.