Quaternary volcanism in the Eifel, Germany, is characterized by K-rich, primitive mafic magmas, considered to be derived by partial melting of a phlogopite-bearing lithospheric mantle source (Wilson and Downes, 1991). The magmas have exhumed abundant mantle xenoliths (e.g. Lloyd et al., 1991; Witt-Eickschen et al., 1993; Kempton, 1988) which provide a direct sample of the mantle lithosphere beneath the region allowing us to evaluate this hypothesis.
The mineralogy, petrography, and radiogenic isotope composition of a suite of xenoliths from a quarry near Gees, Germany (Lloyd et al., 1991) has been examined in detail. The xenoliths include anhydrous, coarse and fine grained lherzolites, wehrlites, harzburgites, and dunites as well as two large clinopyroxene megacrysts and one phlogopite-bearing xenolith. The Pb, Sr, and Nd isotope compositions, determined on hand-picked, leached clinopyroxenes, show that all of the mantle sampled has been enriched isotopically.
When Sr, Nd, and Pb isotopes are examined simultaneously, xenolith data are distinct from basalt data (Wörner et al., 1986) (Fig. 1). The relationship between radiogenic isotope composition and mineralogy supports previous suggestions (Witt-Eickschen et al., 1993; Kempton, 1988; Stosch and Lugmair, 1988) that at least two episodes of mantle enrichment have occurred. The first of these completely reset the isotope composition of the mantle beneath Gees, increasing its Sr and Pb isotope ratios, and decreasing its Nd isotope ratio. The subsequent episode reduced the Sr, while increasing the Nd and Pb isotope ratios. Based on studies of mantle xenoliths throughout Europe (Wilson et al., 1994; Rosenbaum et al., subm.), we interpret the Eifel data set as evidence of subduction modification of an initially depleted mantle protolith during the Hercynian (I), followed by enrichment of the modified protolith by asthenosphere-derived silicate melts, in general similar in isotopic composition to those of sodic basalts from elsewhere in Europe (Wilson and Downes, 1991; Cebriá and Wilson, 1995) (II) (Fig. 1). A coarse-grained lherzolite preserves the strongest stage (I) influence. The dunite and harzburgites have the highest Pb concentrations and radiogenic Pb isotopic compositions suggesting enrichment by melts from a HIMU-like reservoir during the second event.
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Fig. 1: Sr, Nd, and Pb isotope data for leached clinopyroxenes from mantle xenoliths from Gees, Germany. Basalt data are from this study and Wörner et al. (1986).