碳酸盐地层中Sr同位素实验关键问题的讨论

Discussions on key problem of Sr isotope analysis in carbonate strata

减少或消除放射性成因Sr的影响是碳酸盐地层Sr同位素研究技术之关键。碳酸盐地层的Sr同位素研究,是建立在样品不含放射性成因Sr的基础上。离子交换柱不具有消除放射性成因^(87) Sr影响的功能,只能减少或除去Ca^(2+)、Ba^(2+)等阳离子,以便在质谱分析中提高Sr同位素的电离概率,获取更高的灵敏度,同时消除^(87) Rb(同质异位素)对^(87)Sr测定值的干扰。在质谱分析时,一旦发现样品中含Rb,就必然存在放射性成因^(87)Sr的影响,导致测定值偏高。鉴于一些碳酸盐地层内含有富Rb的硅酸盐和其他矿物组分,盲目使用氢氟酸、硝酸、高氯酸等强酸溶解样品,有可能严重地影响到Sr同位素(^(87)Sr/^(86)Sr)的测定值。实验证明,采用选择性溶解技术,用醋酸或浓度较低的稀盐酸萃取碳酸盐,可以减少或排除硅酸盐及其他富含放射性成因Sr的矿物的影响。

Reduction or elimination of 87Sr originated from 87 Rb decay is the key to strontium isotope study on carbonate strata. The strontium isotope study on carbonate strata is based on that there is no S7Sr from ST Rb decay after the strata were formed. The ion exchange column is no function for removing STSr influence from the radioactive origin and can only remove Rb＋ , Ca2＋ , Ba2＋ and so on, meanwhile, improves Sr ionization efficiency and avoids 87Rb influence for 87Sr test. Once the 87Rb signal occurs in isotope mass determination, it is likely that the 87Sr influence comes from the radioactive origin of other no-carbonate minerals in the samples. Careless use of hydrofluoric acid, hydrogen nitrate and perchloric acid in the dissolution of samples could severely affect the values of 87Sr/S6Sr determination because some Rb-enriched silicate and other minerals, according to previous studies, could be found in the carbonate strata. Recently, a selectively dissolving technique, as used by acetic acid or hydrochloric acid with low concentration, can effectively extract pure carbonate components and eliminate the 87Sr influence minerals. The application of this method successful. from the radioactive origin in the Rb-enriched silicate to complex marine carbonate analysis proves to be successful.