Sr, Nd, and Pb isotope geochemistry of Tertiary and Quaternary alkaline volcanics from West Germany

Mantle-derived alkaline magmas from the Quaternary East and West Eifel volcanic fields (West Germany) and a range of basalts from various Tertiary Provinces in West Germany show considerable variation in their Nd and Sr isotopic compositions. East Eifel mafic magmas (basanites, leucitites) and derivatives thereof (tephrites, phonolites) are, with the exception of two nephelinites, very similar in 87Sr/86Sr (0.70456-0.70472) but show a significant range in 143Nd/144Nd (0.51263-0.51273). In contrast, West Eifel melilitite-nephelinites, leucitites, and melililites (F-group magmas), and olivine-nephelinites and basanites (ONB-group), define a steeply inclined array in a Sr-Nd isotope variation diagram extending from 0.7039 to 0.7045 and from 0.51285 and 0.51267, for 87Sr/86Sr and 143Nd/144Nd respectively. ONB-magmas form a separate low 87Sr/86Sr-high 143Nd/144Nd group. These data combined with analyzed Tertiary lavas define an array extending from E-type MORB down to estimated bulk silicate earth values. Lead isotopic compositions of Eifel volcanics are rather radiogenic compared to other continental alkaline rocks and are positively and negatively correlated with 143Nd/144Nd and 87Sr/86Sr, respectively. An evaluation of possible contaminants demonstrates that crustal assimilation has not played a major role in determining the isotopic compositions of the lavas. Isotopic variations thus document heterogeneity on a variety of scales within the Eifel mantle source. Estimated source Sm/Nd ratios, in conjunction with measured isotopic compositions, suggest that mixing between enriched and depleted mantle materials, which occurred sometime during the past 1.1 Ga, resulted in the production of the heterogeneous proto-Eifel source. This ''mixing'' event may have occurred in conjunction with Tertiary magmatism in central Europe. (orig.)