• Schmidt, Gerhard

Abstract

In order to investigate the behaviour of highly siderophile elements (HSE) and Os isotopes during melt extraction and metasomatism, mantle peridotite xenoliths from the West Eifel and Vogelsberg volcanic fields have been analyzed for their whole rock HSE, REE, Ta, As etc. by using a nickel sulfide fire assay collection procedure followed by instrumental neutron activation analysis (INAA) for HSE and INAA for REE, As and some other trace elements. No systematic dependence of the contents of PGE on the degree of fertility of the host rock was found. The bulk rock Os concentrations range from <0.1 to 6.47 ng/g, far outside the range usually found in mantle rocks from worldwide occurrences. The covariance between LREE and Ta enrichment and the concentrations of PGE (e.g., Sm vs Os) recorded by these harzburgites gives evidence that LREE-metasomatism is directly connected with the mobility of PGEs and As by fluids or melts. The loss of the complete budget of PGE from some xenoliths seems to indicate melt percolation that may have removed intergranular sulfides and reduced the PGE contents. Thus different types of metasomatic processes can significantly modify the PGE chemistry of mantle rocks obscuring the original signature. The presence of a plume-like structure down to 400 km in depths beneath the Eifel may cause thermal erosion at the base of the lithosphere. Ascending plume melts could be responsible for heating, partial melting and metasomatism, resulting in enhanced PGE mobility in the overlying mantle. Samples from the anhydrous peridotite suite (Ib) are highly variable in their Os isotopes, ranging from subchondritic (187Os/188Os = 0.1236) to suprachondritic values ( 187Os/188Os = 0.1420). LREE (e.g., Sm up to 0.82 μg/g), Os (up to 6.47 ng/g) and As (up to 250 ng/g) in the hydrous suite (Ia) show a strongly inverse correlationwith Os isotope ratios demonstrating that metasomatism can significantly modify the Os isotope chemistry of mantle xenoliths. Samples with Os contents >1.5 ng/g from the hydrous suite have relatively unradiogenic Os isotope compositions (187Os/188Os = 0.1208-0.1237) and Al 2O3 – Os isotopic systematics consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of theoverlying crust (~1.5 Ga) and from European peridotite massifs, e.g., Ronda. A model Re-depletion age of~1.6 Ga is based on eight hydrous peridotite samples from Dreiser Weiher, Meerfelder Maar and the Vogelsberg volcano (this study) and on three anhydrous samples from Dreiser Weiher by Meisel et al. [5]. One anhydrous sample from our study together with harzburgitic samples from Gees analysed by Cohen et al. [6] would imply an much older model Re-depletion age of ~2.7 Ga [7].

Topics

• impedance spectroscopy
• nickel
• mobility
• extraction
• melt
• activation
• trace element
• neutron activation analysis