Fig. 1: Map and description of the studied area.

a Location of the Comoros archipelago and a bathymetric map of the eastern slope of Mayotte Island (France). b Zoom on the Fani Maoré volcano area. Red lines show the dredging lines corresponding to the investigated samples, with ending arrows indicating dredge directions. c List of the studied samples, with dredging locations and bulk rock compositions29. The ratio of Non-Bridging Oxygens to Tetrahedrally coordinated cations (NBO/T) is also displayed for each sample (cf. Supplementary Data 1).