Effects of Retrograde-Zoning of Garnet in Eclogite on Nd and O Isotopic Equilibrium between High-Pressure Minerals

Compositional zoning of garnet was observed in a strongly deformed eclogite associated with a foliated peridotite at Raobazhai in North Dabie. CaO concentration of garnet decreases from core to rim, whereas its MnO concentration increases, suggesting a retrograde origin of such Ca Mn zoning and a fast rate of Ca and Mn diffusion in garnet. The tie line of garnet + omphacite from this eclogite gave a Sm Nd age of 187±5 Ma, which is significantly less than Sm Nd isochron ages of 221±5 Ma to 228±3 Ma and 210±6 Ma to 214±6 Ma for ultrahigh pressure eclogites in South Dabie and North Dabie, respectively. This younger Sm Nd age is interpreted to be resulted from a decrease in 143 Nd/ 144 Nd ratio of the retrograde rim in garnet. Oxygen isotope analysis of omphacite and garnet shows less fractionation values than equilibrium fractionation values between them at temperature of 500 to 900 ℃, suggesting a oxygen isotope disequilibrium. This study demonstrates that the retrograde zoning of garnet in eclogite could result in significant disequilibrium of Nd and O isotopes between omphacite and garnet.