Diagenetic alteration of fossil reef corals may have severe effects on the reliability of ²³⁰Th/U-ages. The widely applied criteria introduced to test for the reliability of coral ²³⁰Th/U-ages are often not sufficient to identify all altered ages. The combination of ²³⁰Th/U- with ²³¹Pa/U-dating has been suggested as a promising method to identify altered ages. Furthermore, ²²⁶Ra/²³⁰Th ratios can provide information about the diagenetic history during the last 10 000 years. Here we present a method to chemically separate U, Th, Pa and Ra from the same aliquot of a carbonate sample. The isotope ratios of our mixed U–Th–Pa–Ra spike are calibrated using a secular equilibrium material treated in the same way as the samples during chemical separation and mass spectrometric analysis. This approach does not only circumvent corrections for the radioactive decay of the short-lived ²³³Pa spike, but also enables us to estimate the reproducibility of the spike calibration. The relative standard deviation (RSD) of the spike ratios is 0.27% for ²²⁹Th/²³⁶U, 4.2% for ²²⁸Ra/²³⁶U and between 0.6 and 4.0% for ²³³Pa/²³⁶U. The RSDs of the final ratios are 1.2% for ²³¹Pa/²³⁵U and 3.4% for ²²⁶Ra/²³⁰Th. We assess the individual sources of uncertainty (e.g., mass spectrometric corrections, counting statistics, and spike calibration) in detail and quantify their contribution to the total uncertainties of the final isotope ratios. Most corrections contribute only moderately to the final uncertainties. However, in the case of a large abundance of natural isotopes in the spike, this correction can have a large influence on both the ratios and their uncertainties. Another parameter affecting the final uncertainties and the reproducibility of the method is the sample size and, thus, the concentration of the natural isotopes. This discussion provides useful guidelines for future applications and can be adjusted to the individual requirements of a specific user.

中文翻译：

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碳酸盐的U，Th，Pa和Ra同位素比的化学分离和MC-ICPMS分析†

化石珊瑚的成岩变化可能会对²³⁰ Th / U年龄的可靠性产生严重影响。为测试²³⁰ Th / U年龄珊瑚的可靠性而引入的广泛应用的标准通常不足以识别所有变化的年龄。有人建议将²³⁰ Th / U-和²³¹ Pa / U的日期结合起来作为鉴定年龄变化的有前途的方法。此外，²²⁶ Ra / ²³⁰这些比率可以提供有关最近一万年的成岩史的信息。在这里，我们提出一种从碳酸盐样品的相同等分试样中化学分离U，Th，Pa和Ra的方法。我们的混合U–Th–Pa–Ra尖峰的同位素比率是使用长期平衡材料校准的，该材料在化学分离和质谱分析过程中的处理方式与样品相同。这种方法不仅可以避免对短时²³³ Pa尖峰的放射性衰变进行校正，还可以使我们估计尖峰校准的可重复性。²²⁹ Th / ²³⁶ U的峰值比率的相对标准偏差（RSD）为0.27 ％，²²⁸ Ra / ²³⁶的峰值比率为4.2％U为²³³ Pa / ²³⁶ U时介于0.6和4.0％之间。最终比率的RSD为²³¹ Pa / ²³⁵ U时为1.2％和²²⁶ Ra / ²³⁰ Th时为3.4％。我们评估不确定性的各个来源（例如，质谱校正，计数统计和加标校正），并量化它们对最终同位素比的总不确定度的贡献。大多数校正只会对最终的不确定性产生适度的影响。但是，在尖峰中存在大量自然同位素的情况下，此校正可能会对比率及其不确定性产生很大影响。影响最终不确定性和方法可重复性的另一个参数是样品大小，因此是自然同位素的浓度。该讨论为将来的应用提供了有用的指导，并且可以根据特定用户的具体要求进行调整。