Highly siderophile elements (PGE, Re and Au) in mantle xenoliths from the West Eifel volcanic field (Germany)

Abstract

Mantle peridotite xenoliths ranging in modal composition between fertile lherzolites and modally metasomatized harzburgites, and magmatic pyroxenite xenoliths from the West Eifel and Vogelsberg volcanic fields (Germany) have been analyzed for their whole rock major and minor elements, six platinum-group elements PGE (PGE: Os, Ir, Pt, Ru, Rh, Pd), Re and Au, rare earth elements (REE), and several other trace elements (e.g., As, Ta). The bulk rock Ir concentrations range from 0.23 to 9.07 ng/g (total PGE contents range between <5.9 and >43.2 ng/g), exceeding the range previously found in mantle rocks. Two pyroxenite samples show large variabilities in their PGE concentrations, with Ir contents of 0.23 and 4.30 ng/g. No systematic dependence of the contents of PGE on the degree of fertility of the host rock was found. Three harzburgitic samples (1343, 589, 211) with significantly higher concentrations of all PGE in comparison to samples of similar fertility reflect the mobility of these elements in a metasomatized mantle. Approximately unfractionated PGE patterns (with the exception of Re) of the enriched samples indicate that the PGE were apparently transported with a single phase. The Al 2O 3 vs. 187Os/ 188Os correlation for Eifel xenoliths reported by Schmidt and Snow [Contrib. Mineral. Petrol. 143 (2002) 694] indicates that massive redistribution of Os must have occurred recently. The simultaneous enrichment of some samples in Os, As and light REE and other incompatible lithophile elements suggests a single S-, As- and PGE-containing phase that was responsible for the metasomatism.