Melt Influx and the Effects on Os-Sr-Nd Isotopic Systems and Platinum Group Elements in Jurassic Oceanic Mantle (Totalp Massif, Swiss Alps)

University Of Maryland, College Park, College Park MD

Investigators

Abstract

EAR 0309810 Becker Melt ascending in the mantle interacts with peridotite wall rock in various ways. Movement in fractures may result in dissolution of peridotite minerals and precipitation of pyroxenes and garnet or spinel in the fractures. Percolative flow in dunite channels appears to dissolve pyroxenes and Al-phases and result in precipitation of olivine. In other environments, diffuse refertilization of previously depleted peridotite may occur. The effects of these interaction processes on abundance patterns of platinum group elements (PGE) and Re, and possibly on radiogenic 187-osmium in peridotites have not been studied in a systematic way. Results of a pilot study suggest that the Jurassic Totalp ultramafic massif in the Swiss Alps, a lithospheric fragment of the Ligurian ocean basin, is a suitable site to study these effects. The Re-Os, Rb-Sr and Sm-Nd isotopic systematics will be studied, and PGE and sulfur concentration data will be obtained and compared to major element variations in order to confirm the hypothesis that melts with suprachondritic 187-Os refertilized the spinel peridotites. Whole rock data will be supplemented by laser ablation ICP-MS work on PGE in sulfides from grain boundaries and inclusions in silicates to test for micro-scale equlibration. Concordant pyroxenite layers in this massif are precipitates of mafic melts and show subchondritic to highly suprachondritic Os isotopic ratios. This study will evaluate the distribution of Os isotopic ratios in the pyroxenites, particularly, whether suprachondritic Os isotopic ratios predominate in spinel websterites, the most ubiquitous pyroxenite. The radiogenic Os isotopic ratios may reflect a high-Re reservoir in the convecting mantle, most likely recycled oceanic crust. Isotopic and concentration profiles across layered pyroxenites and peridotites will yield constraints on the scale of isotopic homogenization for Os in the presence of silicate and sulfide melts in the mantle. This work will improve our understanding of chemical effects of ascending melts on surrounding mantle in the transition region between asthenosphere and lithosphere.