Experimental evidence for the shallow production of phonolitic magmas at Mayotte

Abstract

Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km$^{3}$ of basanitic magma. The eruption occurred along a WNW–ESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (${\le }$12–15 km) yields a phonolitic residual liquid containing up to 3–4 wt% $\mathrm{H}_{2}\mathrm{O}$ after ${\ge }$65 wt% of an assemblage of olivine$+$plagioclase$+$amphibole$+$clinopyroxene$+$biotite$+$magnetite$+$ilmenite$+$apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6–8 km depth, while a shallower differentiation (${\le }$4–5 km) results in the production of liquids with trachyte–benmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in $\mathrm{SiO}_{2}$–$\mathrm{Al}_{2}\mathrm{O}_{3}$ but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.

Keywords

• Mayotte
• Phase equilibria
• Phonolite
• Iron-enrichment
• Basanite