Geochemical and Sr-Nd isotopic characteristics and pressure-temperature estimates of mantle xenoliths from the French Massif Central: Evidence for melting and multiple metasomatism by silicate-rich carbonatite and asthenospheric melts

Ultramafic xenoliths from Mont Briançon, Ray Pic and Puy Beaunit in the French Massif Central show variable mineral compositions that indicate a residual origin after various degrees of partial melting of a fertile peridotite. Furthermore, trace element and Sr-Nd isotopic variations of clinopyroxenes indicate mixing processes between depleted mantle and enriched components such as asthenospheric melt and silicate carbonatite melt. Pyroxene geothermometer and CO₂ geobarometer estimates are 860-1060°C at 0.92-1.10 GPa for Mont Briançon, 930-980 ^oC at 0.89-1.04 GPa for Ray Pic and 840-940 ^oC at 0.59-0.71 GPa for Puy Beaunit. From south to north, the xenoliths show the following trends: (1) deeper to shallower origin; (2) more depleted mineral compositions, suggesting higher degrees of partial melting; and (3) more enriched isotopes and trace elements, indicating a mixing process with a silicate-rich carbonatite melt characterized by high H₂O and K₂O, possibly during Variscan subduction.