Extensive CO 2 degassing in the upper mantle beneath oceanic basaltic volcanoes: First insights from Piton de la Fournaise volcano (La Réunion Island)

In spite of its major role on the atmospheric volatile budget, climate, and tracking magmatic transfers, mantle (CO2) degassing below volcanoes is still poorly understood. Most of the studies on this scientific topic lack constraint on the CO2 concentration of primary melts, the depth at which it starts degassing, and the extent of this process in the mantle. In this study of Piton de la Fournaise (PdF) volcano, we couple geochemistry of low solubility gases (He, Ar, CO2, δ13C) in fluid inclusions (FIs) and petro-chemistry of magmatic inclusions on a set of olivine and clinopyroxene crystals from basalts and ultramafic enclaves.We constrain basaltic melt degassing at PdF over a large pressure range (from 4 GPa up to the surface). Based on CO2-He-Ar systematics, we infer that extensive degassing occurs already in the upper mantle (4–1 GPa) and it is favored by multiple steps of magma ponding and differentiation up to the mantle-crust underplating depth (0.4 GPa). Thus, we calculate that basaltic melts injected at crustal depth (