Low δ¹⁸O eclogites from the Kokchetav massif, northern Kazakhstan

Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole-grain CO₂-laser fluorination methods. Systematic analyses yield extremely low δ¹⁸O for eclogites, as low as -3.9‰ for garnet; these values are comparable with those reported for the Dabie-Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ¹⁸O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high-temperature isotopic equilibration at 500 800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak-metamorphic oxygen isotopic signatures. A possible geological environment to account for the low-δ¹⁸O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ¹⁸O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.