Lithospheric mantle evolution beneath the Eifel (Germany): Constraints from Sr-Nd-Pb isotopes and trace element abundances in spinel peridotite and pyroxenite xenoliths

The Pb isotope compositions of amphiboles and clinopyroxenes in spinel peridotite and pyroxenite mantle xenoliths from the intra-plate Quaternary volcanic fields of the Eifel province (Germany) are strongly correlated with their Sr-Nd isotope and trace element compositions. High-temperature anhydrous xenoliths from a depth of around 60 km have trace element and Sr-Nd-Pb isotope compositions similar to the depleted source of mid-ocean ridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearing xenoliths from shallower depths ( < 45 km) provide evidence for three temporally distinct episodes of mantle metasomatism in the subcontinental lithosphere: (1) aqueous fluids from an isotopically enriched (EM-like) mantle reservoir caused amphibole formation during deformation in the shallow continental lithospheric mantle and may be subduction related, probably associated with the last major tectonic event that influenced the area (Hercynian orogeny). (2) During a second phase of mantle metasomatism the EM-like lithospheric mantle was affected by melts from an ancient, HIMU-like (high time-integrated μ = ²³⁸ U/²⁰⁴ Pb) mantle source. The HIMU-like component introduced by these fluids had a much more radiogenic Pb isotope composition than the asthenospheric source of the widespread Cenozoic magmatism in Europe and may be linked to reactivation of ancient subducted crustal domains during the Hercynian orogeny or to early Cretaceous deep-sourced mantle plumes. (3) During a brief final stage the heterogeneously enriched EM-HIMU subcontinental lithosphere was locally modified by basaltic melts migrating along fractures and veins through the upper mantle as a consequence of the Cenozoic Eifel volcanism. Although a DMM component is completely lacking in the metasomatic fluids of the metasomatic episodes 1 and 2, the vein melts of episode 3 and the Cenozoic Eifel lavas require mantle sources containing three end-member components (DMM-HIMU-EM). Thus, mobilization of the more depleted mantle material occurred at the earliest in the Tertiary, contemporaneously with the development of the extensive rift system and main melt generation in Europe. Alternatively, the variety of Sr-Nd-Pb isotope signatures of the metasomatic agents may have been produced by melting of isotopically distinct mantle domains in a heterogeneous uprising mantle plume.