半封闭硝酸溶解体系ICP-MS快速测定辉钼矿的Re-Os年龄及Re含量

摘要

Re-Os isotope method is a powerful tool for dating ore deposits,but digesting samples with the Carius tube is complicated and dangerous. A simple and quick method to determine the age of molybdenite Re-Os and Re content has been established. 5 - 15 mg of molybdenite samples were digested with 3 mL HNO3 in a 10 mL colorimetric cylinder. Molybdic acid in solution was removed by a centrifuge,the supernatant of the solution was then diluted to an appropriate volume. M187 / M185 ratio of the resulting solution was determined directly by Inductively Coupled Plasma-Mass Spectrometry(ICP-MS). According to the principle of radioactive decay,it can be inferred that the mass ratio is positively correlated with the age. The Re-Os age of the sample can be calculated by comparing with the M187 / M185 ratio of the molybdenite standard material(JDC and HLP). The Re content in the sample can be calculated on the basis of 185 Re counting using the relative method. In order to test the reliability of this method,a series of molybdenite samples,whose ages vary from 13. 26 Ma to 2130 Ma,were analyzed. The resulting ages were generally identical to the reference ages within 0. 36% - 7. 42% deviations. The samples with older ages yield better results because the 187 Os is more radiogenic. This method,without spike addition,can avoid the trouble of sealing and opening the Carius tube and the process of separating and purifying Re and Os,which significantly improves the experimental efficiency. This simple and quick method is highly suitable for the preliminary analysis of molybdenite Re-Os ageing.%Re - Os 法能够直接厘定内生金属矿床成矿时代,但是封闭 Carius 管法化学流程复杂,且有一定的危险性.本文建立了一种简便快速测定辉钼矿 Re - Os 年龄及其 Re 含量的方法.用 3 mL 浓硝酸在 10 mL 比色管中溶解 5 ~ 15 mg 辉钼矿样品,将溶液中的钼酸沉淀分离后稀释定容,直接采用质谱测量 187 和 185 质量数比值(M187 / M185).利用辉钼矿 Re - Os 年龄标准物质 JDC 和 HLP 的 M187 / M185 值与其年龄的正相关线性关系计算未知辉钼矿样品的 Re - Os 年龄,并通过185 Re 计数利用相对法计算 Re 含量.本方法应用于测量 13. 26 ~ 2130 Ma 的辉钼矿,Re - Os 年龄的测定值与推荐值的相对偏差多数在 0. 36% ~ 7. 42% 之间,由于放射性积累较多,长年龄样品测量的准确度较高.与传统 Carius 管法相比,该方法不需要加入稀释剂,省去了封闭和打开 Carius 管环节以及 Re - Os 分离纯化流程,适合于辉钼矿 Re - Os 年龄的初步分析.