Effects of melt percolation on the Re-Os systematics of peridotites from aPaleozoic convergent plate margin

The effects of melt percolation on the Re-Os systematics of peridotites wer e 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, occ urrences of calc-alkaline rocks. and the trace element and isotopic composi tion of garnet pyroxenite layers in the peridotites indicate that the latte r must have originated in the hanging wall mantle of a late Devonian-early Carboniferous subduction zone. The garnet pyroxenites are cumulates and the ir Eu anomalies, radiogenic initial Os-187/Os-188 (gamma OSi up to 450). Sr -87/Sr-86 (up to 0.709), and unradiogenic Nd-143/Nd-144 (epsilon Nd-i as lo w as -4.8) were probably inherited from melts that contained slab-derived c omponents. Because the isotopic signatures of the melts were substantially different from normal mantle, these peridotites are particularly suitable f or a study of the effects of melt percolation. Layered dunite-pyroxenite ro cks resemble replacive dunites in ophiolites, and may have acted as high pe rmeability channels for Mg-rich melts. The dunite-pyroxenite rocks are char acterized by suprachondritic gamma Os-i (7.2 to 13.9), indicating addition of radiogenic Os from the melts. Assuming the dunites originally had a comp osition similar to harzburgites at the same outcrop (gamma Os-i of -2.7 to -3.9), the change in gamma Os-i requires melt/rock ratios between 10 and 40 0. Osmium and Cr are depleted in the dunites by up to 50-70% compared to no rmal peridotites, and enriched in associated orthopyroxenites (up to 3.8 pp b Os and 5700 ppm Cr). indicating substantial transfer of peridotitic Os an d Cr into milt. Preferred incorporation of Os and Cr into ME-rich pyroxenit es suggests a coupling of the chemical behavior of Os and Cr in Mg-rich ign eous silicate systems. Rhenium abundances are uniformly low in these rocks, presumably because of low Re and sulfur contents in the melts. Melt percol ation in depleted Iherzolites and harzburgites in the vicinity of pyroxenit e layers is indicated by Sr-Nd isotopic data and REE patterns. gamma Os-i i n these rocks (-5.5 to +0.5) suggest no or only minor addition of radiogeni c Os and low melt/rock ratios near 1. However, correlations of mildly incom patible elements such as Al with Re abundances and negative correlation of Nd-143/Nd-144 With Re in Iherzolites 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. (C) 2001 Elsevier Science B.V. All rights reserved.