Oxygen and hydrogen isotope geochemistry of ultrahigh-pressure eclogites from the Dabie Mountains and the Sulu terrane

Abstract

The oxygen and hydrogen isotope compositions of mineral separates have been determined for ultrahigh-pressure (UHP) eclogites from Shuanghe in the eastern Dabie Mountains and from Donghai in the western Sulu terrane, East China. The results show a large variation in δ 18O values of garnet and omphacite (−2.6 to +7.0‰ for Shuanghe and −10.4 to +4.8‰ for Donghai) but a small range in phengite δD value (−104 to −73‰). Oxygen isotope equilibrium has been preserved between the eclogite minerals and thus records the metamorphic temperatures of 550–730°C for the Shuanghe eclogites and 650–750°C for the Donghai eclogites. These not only demonstrate that the UHP rocks acquired the unusual δ 18O values prior to eclogite-facies metamorphism by interaction with 18O-depleted fluids, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O depletion in the eclogites. Ancient meteoric water is assumed to exchange oxygen isotopes with the eclogite precursors on the continental crust prior to plate subduction. The extremely low δ 18O values (−10 to −9‰) and δD values (−104 to −100‰) for the Qinglongshan eclogite may represent the oxygen and hydrogen isotope compositions of ancient meteoric water at some earlier time than subduction. The survival of the oxygen and hydrogen isotopic signature of meteoric water in the UHP eclogites indicates that the eclogites resided at mantle depths only for a short time, otherwise the extremely 18O-depleted eclogites would be re-equilibrated isotopically with the mantle due to diffusion and recrystallization. This suggests restricted fluid mobility and limited crust–mantle interaction during the UHP metamorphism. The consistency of oxygen isotope temperatures between different mineral pairs in this study suggests relatively rapid cooling and ascent for the UHP eclogites in the early stage of their exhumation. However, there are differential exchanges of oxygen and hydrogen isotopes in hydroxyl-bearing minerals (and rutile) with retrograde fluid during exhumation, which has not only resulted in lower oxygen isotope temperatures for mineral pairs containing zoisite and rutile, but also disequilibrium and reversed hydrogen isotope fractionations between phengite, amphibole and zoisite.