Oxygen isotope equilibrium between the minerals of eclogites from South Shandong and constraints on Sm- Nd geochronology
ZHENG Yong- fei, WANG Zheng- rong, LI Shu- guang, LI Long (Laboratory for Chemical Geodynamics, Department of Earth and Space Sciences,University of Science and Technology of China, Hefei 230026, China)
The oxygen isotope composition of minerals were measured for eclogites from Sulu terane in South Shangdong. The same minerals were also analysized for the purpose of Sm- Nd isochron dating and Nd- Sr isotope partitioning. The results show a direct correspondence in equilibrium or disequilibrium state between oxygen and Sm- Nd isotope systems. In other words, when oxygen isotope equilibrium at eclogite- facies conditions are observed between omphacite and garnet, the Sm- Nd mineral isochron can give reasonable age; unreasonable Sm- Nd isochron age are yielded when there is oxygen isotope disequilibrium between the minerals. It appears that the state of oxygen isotope equilibrium between the minerals can place a valid constraint on the equilibrium state of mineral Sm- Nd isotope system. There is significant O- Nd- Sr isotope heterogeneities within the same minerals in the scale of a specimen. The relative rates of O, Nd and Sr diffusion in garnet and omphacite are empirically estimated accordingly. Not only the obtained sequence of diffusion rates are concordant with experimentally determined data, but also it is reckoned that the time of isotope homogenization within the minerals is greater than 10 Ma under the conditions of peak metamorphism.