Stable isotope compositions of the Penninic ophiolites of the Kõszeg-Rechnitz series

Abstract

The ophiolitic rocks of the easternmost Penninic unit, the Kõszeg-Rechnitz series, were analyzed for their H, C and O stable isotope compositions. Serpentinite, gabbro, blueschist, talc deposits, ophicarbonates, as well as calcite and inclusion fluids from quartz segregation veins were analyzed in order to determine the effects of different metamorphic events on the stable isotope compositions. The oxygen isotope compositions have a wide range depending on rock type and locality. Gabbro and serpentinite of Bienenhütte (Bernstein Window) have preserved mantle-like δ 18O values (5.9 to 6.3‰; all values are in ‰ relative to V-SMOW), whereas the serpentinite of Glashütten and Rumpersdorf (Kõszeg-Rechnitz Window) and the silicate minerals of the ophicarbonate rocks show a strong 18O-enrichment (up to 16.2‰). The 18O-enrichment may have been induced by low-temperature serpentinization or interaction with 18O-rich fluids that had been in equilibrium with sedimentary rocks. Contrary to the O isotope compositions, the H isotope compositions seem to be homogeneous in the entire series, with D values of −63 ± 7‰. Only some serpentinite rocks were depleted in D (down to −106‰), usually regarded as a result of interaction with meteoric water infiltrating during late-stage metamorphism. The meteoric water infiltration was rather limited, as even samples taken directly from slickensides within serpentinite bodies preserved isotopic compositions close to those of the bulk series. H and O isotope compositions of fluids mobilized in the metasedimentary rocks of the Penninic unit during the main metamorphic stage were determined by analyzing inclusion fluids and calcites in quartz-carbonate veins. The isotope compositions indicate interaction between these fluids and the ophiolite series, although relative deuterium enrichment has been preserved in the ophiolitic rocks. The strong D-enrichment characteristic for oceanic crust that has experienced high-temperature interaction with seawater was not detected. However, the H isotope compositions obtained for the Kõszeg-Rechnitz series indicate that subduction of the Penninic oceanic crust and the associated devolatilization may have been potentially responsible for mantle metasomatism, resulting in H isotope compositions of about −40‰, similar to the range determined from mantle-derived amphibole megacrysts (Demény et al. 2005). To conclude, the present dataset is discussed in the light of earlier studies on the formation of the Sopron leucophyllite.