HYDROGEN AND OXYGEN-ISOTOPE COMPOSITIONS OF ECLOGITES FROM THE DABIE MOUNTAINS AND GEODYNAMIC IMPLICATIONS

Heterogeneous delta(18)O values as low as -2.6 parts per thousand to 7.0 parts per thousand are observed for ultrahigh pressure eclogites from the Dabie Mountains in East China. Oxygen isotope equilibrium has been approached between the eclogite minerals, suggesting that the ro cks would have acquired the unusual delta(18)O values prior to ultrahi gh pressure metamorphism by interaction with O-18-depleted fluid. delt a D values of hydroxyl-bearing are between -51 parts per thousand and -83 parts per thousand, precluding the possibility of paleoseawater in volvement. The only likely fluid is ancient meteoric water that exchan ged oxygen isotopes with the eclogite precursor (a kind of basaltic ro cks) formerly resident on the continental crust. This suggests a crust al recycling process in the suture zone of plate subduction. Because s ilicate minerals undergo rapid oxygen isotope. exchange at mantle pres sures, preservation of the isotopic signature of meteoric water in the eclogites indicates limited crust-mantle interaction and thus a short residence time (<20 Ma) when the plate containing the eclogite precur sor was subducted to mantle depths. The agreement in oxygen isotope te mperatures for different mineral pairs suggests a rapid cooling and as cent process for the eclogites subsequent to their formation at mantle depths.