Oxygen Isotope Composition in Olivine and Melts from Cumulates of the Yoko-Dovyren Layered Massif, Northern Transbaikalia, Russia

Oxygen isotope composition was studied in 33 monomineralic fractions of olivine from plagioclase lherzolite, dunite, troctolite, and olivine gabbronorite of the Yoko-Dovyren layered massif. The δ18O values of the least altered rocks not contaminated with crustal materials range within 5.8 ± 0.2‰ (n = 27). These values for the dunite contaminated with carbonate material are notably higher and average at 6.2 ± 0.3‰. This is similar to characteristics of the Bushveld and Jinchuan mineralized complexes. Using the COMAGMAT-5 model, the temperatures of equilibrium of 98% crystals with late portions of the residual melts were evaluated at 1131–1266°C (1176 ± 34°C on average). These estimates are viewed as the closing temperatures of the cumulate systems with respect to the efficient exchange of 18O and 16O isotopes between olivine and mobile melt/fluid. For the uncontaminated systems, the δ18O values of the parental melt calculated using the modeled temperatures are 6.6 ± 0.2‰, and are 7.1 ± 0.3 ‰ for melts with signatures of crustal contamination. Such relations are consistent with insignificant (no more than a few percent) contamination of the parental magmas with carbonate materials from the host rocks.