Experimental insights on storage depth of Fani Maoré magmas (Mayotte)

From May 2018 to late 2021, Mayotte experienced a major volcanic crisis that produced ~6.5 km³ of nearly aphyric (<5 wt.% crystals) evolved basanitic magma (4–5 wt.% MgO) and led to the discovery of a new offshore volcanic edifice, Fani Maoré, located ~50 km east of the island. The emitted basanite is characterized by the presence of Olivine (Fo₇₀), Titanomagnetite and Apatite assemblage which dominated throughout the eruption. Yet, in some phases of the eruption, the main mineral cargo also contained minor amounts of reversely zoned ferrous olivine from Fo₅₅ cores to Fo₇₀ rims, associated with ilmenite crystals partially resorbed and overgrown by titanomagnetite at their margins. Thermobarometric constraints derived from clinopyroxene antecrysts identified in the basanite indicate that the eruption was supplied by two magmatic reservoirs located at deep mantle conditions (≥37 km), in close agreement with the depths of volcano-tectonic seismicity recorded throughout the crisis. Yet, these inferred depths contrast with the origin of the dominant Ol+Mt+Ap assemblage which, according to previous studies, was formed during syn-eruptive ascent. Further, interaction of the dominant basanitic magma with a more evolved tephri-phonolitic reservoir at 17 ± 6 km could explain the reverse zoning observed in some olivines. Based on the disparity between reservoir location and mineral assemblage crystallization conditions and the large uncertainties of thermobarometric tools (100–400 MPa and ±50°C for Pressure and Temperature respectively), we conducted high-pressure crystallization experiments using a piston-cylinder apparatus at pressures of 0.7–1.3 GPa and temperatures of 1050–1100°C, exploring seismically defined magma storage depths and intensive parameters (H₂O + CO₂ contents and oxygen fugacity). Experimental results demonstrate that the transition from Olivine-dominated to clinopyroxene-bearing assemblages occurs between 0.7 and 1 GPa (~21–30 km), with clinopyroxene stable only at higher pressures. However, clinopyroxene is absent from the magmatic paragenesis and only one antecryst has been described. This inescapably implies pre-eruptive magmatic storage at depths ≤21 km, where Olivine, Titanomagnetite and Apatite are stable. The Mg/Fe ratios of experimental olivine crystals show a strong dependence on temperature and oxygen fugacity, providing robust constraints on magma storage conditions. Best-fit conditions for the final storage episode of the evolved basanite are ~1075°C and pressures ≤0.7 GPa at ~FQM buffer.  At odds with previous allegations, we show here that our experiments successfully reproduce the chemical evolution of the rocks, indicating that crystallisation processes within a single reservoir remain possible. This storage depth, obtained from our experiments, contrasts with seismicity and thermobarometry that locate a reservoir at >37 km depth. This contrast implies at least two distinct levels of storage within the plumbing system, which may be related to a seismicity gap between 12 and 25 km beneath the volcano observed during the eruption. These results constrain the architecture and dynamics of the magmatic plumbing system feeding one of the largest recent submarine volcanic events.