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

Abstract

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a submarine 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 Maoré, one tephri-phonolite 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 to 0.99 log10 Pa⸱s), phonolites the highest (1.75 to 3.10 log10 Pa⸱s), while the viscosity of the tephri-phonolite falls in between (0.89 - 1.97 log10 Pa⸱s). Viscosity measurements at undercooled temperatures have only been performed for one phonolite melt because Raman spectroscopy showed nanolites within the basanite and tephri-phonolite glass samples. The phonolite has a viscosity of 10.19 to 12.30 log10 Pa⸱s at 1058 to 986 K. Comparison with existing empirical models revealed discrepancies up to 2.0 log units with our experimental measurements. This emphasizes (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 the variations in viscosity as a function of temperature and chemical composition for alkaline magmas. The presented new measurements indicate that, at eruptive temperatures between 1050 °C and 1150 °C, the anhydrous, crystal- and bubble-free basanite melt is very fluid with a viscosity around 2.6 log10 Pa⸱s whereas the anhydrous phonolite crystal- and bubble-free melt at eruptive temperatures ranging from 800 to 1000 °C has a viscosity around 6 - 10 log10 Pa⸱s. These new viscosity measurements are essential to define eruptive models and to better understand the storage, transport and ascent dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface.