动态多接收TIMS方法高精度测定岩石标准物质的锶同位素组成

High Precision Strontium Isotope Measurement of Rock Standard Materialsby Multi-dynamic TIMS

铷-锶(Rb-Sr)同位素体系的放射性同位素衰变规律可以用于分析成矿物质来源和确定成矿时代,对于理解矿床的形成过程、确定矿床的成因以及对进一步找矿都具有十分重要的作用。多接收表面热电离质谱(MC-TIMS)是当前地质样品高精度锶同位素组成分析的首选技术,但法拉第杯之间的杯系数差异严重制约了仪器测量的精确度和准确度。本文采用动态多接收校正方法开展了TIMS的高精度锶同位素分析,通过蒙特卡洛模拟和实际测量论证了这一方法在提高锶同位素分析精确度和准确度方面的有效性。结果表明,采用动态多接收校正方法能够有效地消除杯系数的影响,不仅能确保测量的准确度,还能提高2~3倍的测量精确度,实现优于8ppm的仪器长期测试精度。地质样品的化学前处理采用AG 50W-X8树脂和Sr特效树脂两柱可以实现分离。淋洗曲线实验表明该分离流程具有很好的普适性,全流程空白小于150pg,锶回收率≥95%。采用本方法对13种具有不同岩性和锶含量的国家标准物质进行了高精度锶同位素分析测定,其中9种为首次报道,丰富了该系列标样的锶同位素数据库,为更广泛的地质应用提供有力支撑。

BACKGROUND:Strontium isotopes are a powerful geochemical indicator for tracing the sources and ages of oreforming materials.TIMS is internationally recognized as the“gold standard”for determining Sr isotopic compositions,however,its analysis accuracy is severely limited by the attenuation of Faraday cup efficiency.How to effectively eliminate the influence of Faraday cup efficiency change is the key to improving the accuracy of Sr isotopes measured by TIMS.In addition,matrix reference materials are crucial for validating measurements on geological samples.Therefore,it is necessary to calibrate the Sr isotope composition of new geological reference materials to replace the unavailable standards(e.g.,USGS).OBJECTIVES:To develop a high-precision Sr isotope analysis method using multi-dynamic TIMS and accurately calibrate a set of GBW reference materials with varying sample matrices.METHODS:Samples were completely digested by the high-pressure bomb method.Complete separation of Sr from sample matrices was accomplished through a two-stage column separation method consisting of ion exchange resin(AG 50X-12)and extraction resin(Sr spec).Sr isotopes were measured using a multi-collector TIMS and collected in a three-lines cup configuration.The internal normalization method was used to correct for instrument mass bias,and the multi-dynamic collection method was employed to mitigate the effects of Faraday cup efficiency drift.RESULTS:(1)Significant Faraday cup deterioration(up to 160μg/g on C cup)was observed during an 8 months Sr isotope analytical session.Nevertheless,the results from the Monte Carlo simulation indicate that the multi-dynamic collection method can eliminate 99.6%of the cup coefficient effect.Moreover,long-term testing of NBS987 shows that employing the multi-dynamic collection method results in an instrumental precision of 8μg/g,which is 2-3 times more accurate than traditional static collection methods.Overall,results from both theoretical predictions and practical testing confirmed that the multi-dynamic collection method can effectively eliminate the effects of the cup effect drift.(2)The Sr recovery of the leaching experiments for BCR-2 and BHVO-2 with AG 50W-X8 resin column were 99.16%and 98.91%,respectively.The total Sr recovery of the two-stage columns was as high as 95%,thus preventing any potential Sr isotope fractionation resulting from Sr losses during the column separation process.Furthermore,the total blank throughout the entire procedure was no higher than 150pg for Sr,which was negligible compared to the large sample size.(3)High precision Sr isotope compositions were determined for 13 geological reference samples with various sample matrices,resulting in 87Sr/86Sr ratio measurements ranging from 0.704078 to 0.807402.Among these results,GBW07104,GBW07105,GBW07106,and GBW07108 were found to be consistent with previously reported values within the uncertainties and the other nine reference materials were reported herein for the first time.CONCLUTIONS:The cup effect can significantly impact the Sr isotope measured by MC-TIMS,but it can be effectively mitigated by using a multi-dynamic collection method.Furthermore,independent test results demonstrate the uniformity of the Sr isotope composition in these GBW standards,rendering them suitable for both quality control and interlaboratory comparison purposes.