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

Abstract

Initial 187Os/ 188Os (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 ( 187Os/ 188Os i up to 0.686). The main factor controlling 187Os/ 188Os appears to be the extent of Os exchange of the parent melts with the host peridotites. The radiogenic 187Os/ 188Os i 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. An isotopic and chemical profile across a modally layered pyroxenite indicates substantial Os isotopic disequilibrium on the centimeter scale, while Nd and Sr isotopes are equilibrated or nearly equilibrated. This observation and the Os concentration gradient suggest that sulfide melts cannot be mobile in these lithologies, and under these conditions, otherwise isotopic and concentration gradients should have been erased by fast Os diffusion in sulfide melt. The Os isotopic profile is inconsistent with a simple diffusion model. Rather, it suggests that the pyroxenites formed from multiple batches of magma with differing 187Os/ 188Os. Reopening of pyroxenitic dikes likely plays an important role in the preservation of pristine slab signatures in melt and fluid that would be lost with extensive interaction with peridotite.