Carbonatite metasomatism overprinted by silicate melt metasomatism in the mantle beneath the West Eifel volcanic field

Abstract. The texture and mineral compositions of mantle xenoliths from the West Eifel volcanic field (WEVF) reveal three distinct mantle events. The first, identified in LREE-depleted and isotopically depleted anhydrous (type Ib) xenoliths, resulted from partial melting of fertile mantle around 2 Ga. The second event, marked by the formation of Ti-poor, LREE-rich clinopyroxene, amphibole, and phlogopite in hydrous type Ia xenoliths, is attributed to mantle metasomatism. The third event, evidenced by LREE-depleted and MREE-enriched clinopyroxene in olivine–clinopyroxenite veins and in more evolved hornblendite and phlogopite–clinopyroxenite veins that occur in type Ia xenoliths, is associated with the passage of primitive alkaline mafic magma related to the Quaternary host magmatism. Previous studies of mantle xenoliths have proposed that metasomatic agents could include both silicate and carbonatite melts. The trace element signatures in the studied xenoliths, particularly the Ti / Eu and LaN / YbN ratios and the large negative Zr and Hf anomalies, support the involvement of carbonatite melts in the formation of Ti-poor, LREE-enriched clinopyroxene and amphibole in type Ia xenoliths and clinopyroxene–amphibole–phlogopite veins in the earlier metasomatic event. The subsequent silicate melt metasomatism overprinted this carbonatite metasomatism, producing Ti-rich clinopyroxene and amphibole. Trace element data and diffusion modelling suggest that the silicate metasomatism was not uniformly distributed, resulting in compositional heterogeneity in individual grains and xenoliths. Diffusion-controlled mixing between the earlier carbonatite-associated clinopyroxene and the silicate melt equilibrium phase is proposed to explain this heterogeneity. The complex interplay of metasomatic agents resulted in highly heterogeneous mantle compositions on a grain-to-grain and xenolith-to-xenolith scale.