Characterization of the metasomatic agent in mantle xenoliths from Devès, Massif central (France) using couple in situ trace-element and O, Sr and Nd isotopic compositions

Spinel lherzolites and harzburgites from Mont-Briançon and Marais de Limagne in the Devès volcanic district display coarse-grained to porphyroclastic microstructures and the modal content of volatile-bearing phases increases with the degree of deformation. Clinopyroxene and/or spinel are partly or totally reacted to amphibole. The coupled interpretations of trace-element, rare earth element (REE) and O-Sr-Nd data for clinopyroxene and amphibole indicate that the metasomatized mantle beneath Devès is a mixture of depleted and enriched mantle associated with an alkaline, high field strength element poor, and light rare earth element, U- and Th-rich carbonate-rich silicate fluid-melt metasomatic agent. Oxygen isotope and REE data for clinopyroxene-amphibole pairs indicate a (La/Yb)_N enrichment related to an increasing metasomatic agent/rock ratio.