Slab-derived osmium and isotopic disequilibrium in garnet pyroxenites from a Paleozoic convergent plate margin (lower Austria)

Initial Os-187/Os-188 (350 Ma) of cumulate garnet pyroxenites enclosed in high-temperature peridotite massifs from a Paleozoic convergent plate margin (lower Austria) is slightly suprachondritic to highly radiogenic (Os-187/Os-188(i) up to 0.686). The main factor controlling Os-187/Os-188 appears to be the extent of Os exchange of the parent melts with the host peridotites. The radiogenic Os-187/Os-188; of the least reacted parent melts is most easily explained by an Os flux from the slab, although low Os concentrations suggest that the magnitude of this flux may be rather small. Sediment-like Sr-Nd-Pb isotopic compositions and negative Eu anomalies in plagioclase-free pyroxenites indicate that the Os was derived from the sediment section in the slab. Osmium abundances in the pyroxenites correlate well with Ni and Cr, indicating compatible behavior during pyroxene and garnet fractionation, likely because of coprecipitating sulfide. Rhenium abundances in the pyroxenites do not correlate with mildly incompatible elements such as Yb and are obviously not controlled by silicate phases. Except for two samples with high Re. Re appears to have partitioned preferentially into the melt, which, judging from low sulfur and scarce sulfides in most pyroxenites, may have been undersaturated in sulfur.