用SHRIMP测定锆石U-Pb年龄的工作方法

用SHRIMP测定锆石U-Pb年龄涉及锆石单矿物分选、样品靶制备、U-Pb同位素组成测定、原始数据处理、数据解释等多方面。锆石分选中要避免不同样品的交叉污染,要尽量将岩石中所有的锆石分选出来;淘洗岩石粉末时要避免丢失细小的锆石,因细小的锆石更重要。制作样品靶时,粘贴到样品靶的锆石应反映岩石中锆石的全貌。数据处理中,要修正或剔除异常数据、评估拟参加加权平均计算的每个数据权重的合理性。通常用测得的众数年龄的加权平均值作为侵位年龄或喷发年龄会老于实际年龄,应使用样品中最年轻的锆石年龄来限定岩浆侵位或喷发年龄。

SHRIMP zircon U-Pb age measurement: Sample preparation, measurement, data processing and explanation

SHRIMP zircon U-Pb age measurement involves sample collection, zircon separation, mount preparation, analysis of U-Pb isotope composition and data processing as well as explanation. It is important to avoid cross-contamination of various samples in zircon separation, and all kinds of zircons in a rock sample should be separated out, with special attention paid to the finest grains because of their probable minimum age. Zircons mounted in a mount should report features of the population of a sample. In data processing, original data from SHRIMP should be firstly checked in order to find errors and abnormalities, and they should be amended based on the interpolation method of mathematics. In the calculation of weighted average, the rationality of the weight of a U-Pb age should be judged. Generally, it may be older than actuality if the weighted average of modal ages is used as the age of emplacement or eruption of a magma; therefore, it is more reasonable to use the minimum age of zircons in a sample to confine the emplacement or eruption age of a magma.