Refertilization of Jurassic oceanic peridotites from the Tethys Ocean — Implications for the Re–Os systematics of the upper mantle

The influence of melt migration and solid state mixing of pyroxenites on Re–Os isotopic and major element systematics of oceanic mantle rocks have been studied in serpentinized spinel lherzolites and associated spinel and spinel–garnet pyroxenite layers from the Totalp ultramafic massif near Davos (eastern Swiss Alps). The ultramafic body originally was emplaced and serpentinized on the Jurassic Liguria–Piedmont ocean floor and was little modified during the Alpine orogeny. Field and petrographic observations indicate that refertilization of the peridotites occurred by migration of melt between abundant websterite layers and the peridotites, as well as by mechanical stretching and thinning of websterite layers down to the subcentimeter scale, thus producing fertile lherzolite compositions. Osmium concentrations in the peridotites are in the range observed for normal upper mantle peridotites, whereas Re concentrations in some peridotites are substantially higher than typical mantle values (up to 0.76 ppb). Initial Os isotopic compositions and Re/Os ratios in the peridotites range from subchondritic to suprachondritic (γOs (160 Ma) between −5.5 and +7.2). Re/Os, γOs (160 Ma) and Re concentrations correlate with fertility indicators such as Al2O3 and Na2O, suggesting limited influence of serpentinization on Re–Os systematics of the peridotites. These correlations extend beyond estimates for primitive upper mantle (PUM) compositions suggesting that depletion by partial melting cannot have been the only process producing the variations. The pyroxenites generally have higher Re (up to 3.2 ppb Re), but lower Os concentrations than lherzolites, and suprachondritic initial γOs (+9.0 to +497). Websterites with suprachondritic γOs (160 Ma) alternate with peridotites with chondritic or even subchondritic 187Os/188Os, indicating substantial small-scale Os isotopic heterogeneity. The combined petrographic observations and contrasting compositions of peridotites and pyroxenites indicate that the correlations of γOs (160 Ma) with fertility indicators reflect mixing of peridotites with a long-term depletion history and Re-enriched melts. The pyroxenites may represent residues of eclogites that melted during asthenospheric upwelling or could be cumulates which precipitated from asthenospheric melts. These data provide new evidence that the lower section of the suboceanic lithospheric mantle contains fertile components that are enriched in Re and radiogenic Os. Melt migratio