胶南榴辉岩矿物氧同位素平衡及其Sm-Nd年代学制约

Oxygen isotope equilibrium between the minerals of eclogites from South Shandong and constraints on Sm- Nd geochronology

对苏鲁地体中的胶南榴辉岩进行了矿物氧同位素分析，并与同一手标本矿物的 Sm- Nd内部等时线定年和 Nd- Sr同位素分布进行了对比。研究表明，石榴子石与绿辉石之间的氧同位素平衡与否能够对矿物 Sm- Nd同位素体系的平衡状况和内部等时线定年结果的有效性给予直接制约。合理的石榴子石＋绿辉石 Sm- Nd内部等时线年龄产于两矿物之间达到并在峰变质条件下保持氧同位素平衡的样品中，而两矿物之间处于氧同位素不平衡的样品不能给出正确的 Sm- Nd内部等时线年龄。同一矿物在手标本尺度出现显著的 O- Nd- Sr同位素不均一性，据此对这些元素在石榴子石和绿辉石中的扩散速率顺序进行了估计，不仅得到了与实验扩散系数相吻合的结果，而且由此估计出在峰变质条件下达到矿物内部同位素均一化所需要的时间应大于 10 Ma。

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.