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

Strontium isotopes are a powerful geochemical indicator for tracing the sources and ages of ore-forming 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).

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.

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.

(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.

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.