Formation of pyroxenite layers in the Totalp ultramafic, massif (Swiss Alps) - Insights from highly siderophile elements and Os isotopes

Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet-spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition. Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial gamma(Os) (160 Ma) of -2 to +27. Osmium, Ir and Ru. abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated. Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic Os-187/Os-188 with initial gamma(Os) (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic Os-187/Os-188 compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts. Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt-rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic Os-187 and Os-186, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt-Re-Os composition to account for the coupled suprachondritic Os-186-Os-187 signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume. (c) 2009 Elsevier Ltd. All rights reserved.