| Abstract |
Mayotte island is part of the Comoros Archipelago in the Indian Ocean. Late Pleistocene volcanic activity led to the formation of two phonolitic maars above the coral reef, which subsequently formed a small island named Petite-Terre. Prominent gas emissions, but no thermal springs, have been observed for a long time in Petite-Terre. In 2018-2020, a seismic crisis and a submarine eruption revealed a submarine volcanic chain, with an important volcanic center located about 10 km ESE of Petite-Terre. Based on abundant geological, geochemical, geophysical and bathymetric data collected during the crisis, a new conceptual model of the geothermal system is proposed. It links the gas emissions observed on land to the nearby submarine volcanic center by the development of a deep geothermal reservoir, where downward seawater infiltrations are mixed with upward high-temperature gases from the underlying magmatic chamber. Gas geothermometers suggest temperature of 250-300°C at depth. Magnetotelluric surveys reveal a resistivity pattern which is consistent with a 500-m-thick reef limestone invaded by seawater, acting as a barrier for the ascent of the liquid phase of geothermal fluids to the surface. Below, a conductive horizon interpreted as a clay cap approximately 1000 ± 500 meters thick overlies a less conductive horizon, which may correspond to a reservoir formation with fluid outflows rising from the distant deep reservoir and migrating along the regional fault network. This new conceptual model should be validated soon by the exploration drilling to be carried out by Albioma to produce local electricity, reducing dependence on fossil fuels. |