Effects of melt percolation on the Re–Os systematics of peridotites from a Paleozoic convergent plate margin

The effects of melt percolation on the Re–Os systematics of peridotites were studied in garnet- and spinel-bearing high-temperature peridotite bodies in the southern Bohemian massif (lower Austria). The mantle rocks occur in the high-grade core of a Carboniferous collision zone. Age constraints, occurrences of calc–alkaline rocks, and the trace element and isotopic composition of garnet pyroxenite layers in the peridotites indicate that the latter must have originated in the hanging wall mantle of a late Devonian–early Carboniferous subduction zone. The garnet pyroxenites are cumulates and their Eu anomalies, radiogenic initial 187Os/188Os (γOsi up to 450), 87Sr/86Sr (up to 0.709), and unradiogenic 143Nd/144Nd (ϵNdi as low as −4.8) were probably inherited from melts that contained slab-derived components. Because the isotopic signatures of the melts were substantially different from normal mantle, these peridotites are particularly suitable for a study of the effects of melt percolation. Layered dunite–pyroxenite rocks resemble replacive dunites in ophiolites, and may have acted as high permeability channels for Mg-rich melts. The dunite–pyroxenite rocks are characterized by suprachondritic γOsi (7.2 to 13.9), indicating addition of radiogenic Os from the melts. Assuming the dunites originally had a composition similar to harzburgites at the same outcrop (γOsi of −2.7 to −3.9), the change in γOsi requires melt/rock ratios between 10 and 400. Osmium and Cr are depleted in the dunites by up to 50–70% compared to normal peridotites, and enriched in associated orthopyroxenites (up to 3.8 ppb Os and 5700 ppm Cr), indicating substantial transfer of peridotitic Os and Cr into melt. Preferred incorporation of Os and Cr into Mg-rich pyroxenites suggests a coupling of the chemical behavior of Os and Cr in Mg-rich igneous silicate systems. Rhenium abundances are uniformly low in these rocks, presumably because of low Re and sulfur contents in the melts. Melt percolation in depleted lherzolites and harzburgites in the vicinity of pyroxenite layers is indicated by Sr–Nd isotopic data and REE patterns. γOsi in these rocks (−5.5 to +0.5) suggest no or only minor addition of radiogenic Os and low melt/rock ratios near 1. However, correlations of mildly incompatible elements such as Al with Re abundances and negative correlation of 143Nd/144Nd with Re in lherzolites suggest refertilization of previously depleted peridotites, with concurrent addition of significant amounts of Re. Metasomatic addition of radiogenic Os at high melt/rock ratios, and Re at low melt/rock ratios, produced large shifts in the Re–Os model ages of the peridotites.