Osmium isotopic systematics of melilitites from the Tertiary Central European Volcanic Province in SW Germany

Abstract

Combined Os–Pb–Sr–Nd isotopic compositions of melilitite samples from SW Germany provide new constraints on the origin of melts derived by low degrees of mantle melting in the course of Tertiary–Quaternary volcanism in Central Europe. The first Os isotopic compositions reported here show an unusually wide range in initial 187Os/ 188Os isotopic ratios of 0.119 to 0.225. The low ratios overlap those in the lithospheric mantle and the highly radiogenic ratios exceed those in ocean island basalts. Pb and Sr isotopic compositions show small variations ( 206Pb/ 204Pb=19.19–19.57; 87Sr/ 86Sr=0.7034–0.7039) whereas Nd isotopes ( ε Nd 3.4–4.2) indicate melting of overall uniform sources. A positive correlation between initial 187Os/ 188Os and 207Pb/ 204Pb isotopic ratios is interpreted as a mixing trend between melts with low and lithospheric mantle-like 187Os/ 188Os isotopic ratios and an old crustal component. Thus, we interpret the low 187Os/ 188Os isotopic ratios in some of the samples as primary mantle source-derived signatures and propose that the melilitites originated from the lithospheric mantle. Independent evidence for an lithospheric mantle source is provided by earlier reported trace element data indicating melting of metasomatized sources with residual K-bearing phases whose stability is confined to the lithospheric mantle rather than thermal plume. A contribution from deeper mantle sources is not required to explain the data; however, the lack of Os data from other more typical basalts of the Central European Volcanic Province (CEVP) precludes an evaluation of the extent of plume involvement. Remarkable characteristics of the melilitites are their high Re/Os and U/Pb ratios. It follows that overprinting of the lithospheric mantle by melts derived by low degrees of melting of metasomatized sources will eventually produce domains with evolved Os and Pb isotopic compositions as required as source material for basalts of high 238U/ 204Pb (HIMU) affinity.