The nature of young vein metasomatism in the lithosphere of the West Eifel (Germany): Geochemical and isotopic constraints from composite mantle xenoliths from the Meerfelder Maar

Trace element compositions of amphibole and clinopyroxenes from composite mantle xenoliths crosscut by micaceous hornblendite veinlets that served as conduits for melts moving through vein systems in the upper mantle have been investigated by ion-microprobe analysis to evaluate the process of vein metasomatism in the lithosphere beneath the West Eifel (Germany). The geochemical and Sr-Nd isotopic characteristics of the veinlets suggest that these are products of megacryst precipitation of melts of the same composition as West Eifel alkali basalts. Their geochemical imprint on the host peridotite is constrained by the evolution of compositional gradients in trace element contents within a small-scale, 0.5-1.0 cm wide transition zone at the veinlet-host contact. Highly LREE-enriched, HSFE-poor clinopyroxenes identified in same host rocks have Sr-87/Sr-86 (0.7041) and Nd-143/Nd-144 ratios (0.5125) different from those of veinlets and West Eifel basalts, and provide evidence for a metasomatizing agent before melt injection. Trace element modelling of zoning profiles developed in a single amphibole gram of the transition zone shows that the veining event manifested in the composite xenoliths was rapid and represents the brief final stage of multiple enrichment processes, probably as a consequence of the Quaternary Eifel volcanism.