Molecular-Level Mechanism of Phosphoric Acid Digestion of Carbonates and Recalibration of the 13C-18O Clumped Isotope Thermometer
Molecular-Level Mechanism of Phosphoric Acid Digestion of Carbonates and Recalibration of the 13C-18O Clumped Isotope Thermometer
复制标题
磷酸消解碳酸盐的分子水平机制及13C-18O团块同位素温度计的重新校准
DOI：
10.1021/acsearthspacechem.9b00307
复制
发表时间：
2020-03-19
影响因子：
3.4
通讯作者：
Liu, Yun
中科院分区：
文献类型：
--
作者：
Zhang, Siting;Liu, Qi;Liu, Yun
自稳定同位素地球化学开始以来，碳酸盐岩的磷酸消解反应产生的动力学同位素效应（KIE）就得到了很好的研究，但其分子水平的机制尚不清楚。重要的是，碳酸盐C-13-O-18块状同位素测温方法的有效性还需要进一步研究，该方法在很大程度上依赖于磷酸消化处理。由不同群体校准的现有Delta(47)-T关系是不相容的，并在社区中造成了严重的混乱。本文提出了碳酸盐岩磷酸消解反应分子水平机理的新模型。这个新模型指出，磷酸消化过程有三个平行的途径，可以解释不同组在略有不同的实验条件下获得的Delta(47)-T关系的很大一部分差异。具体来说，它为25、70和90摄氏度的反应提供了不同的团块同位素富集因子，而不是以前的理论研究。在绝对参考框架（ARF）处理下，这些因素对于获得可比的Delta(47)-T关系非常重要。结合利用非调和修正等高阶理论处理重新计算的平衡团块同位素分馏因子，建立了新的理论Delta(47)-T关系。这种新的关系不再是一条单线，而是一个范围，由于三个平行路径的个体贡献的变化而有很小的变化，反映了实验程序和条件的微小差异。这些校准线的斜率与90℃时消化线的斜率接近，但明显小于25℃时的斜率。基于线性数学分析，可以清楚地识别出25℃实验中引起重O同位素富集的未知同位素效应。我们推测，这一隐性因素是导致坡度上升的原因。通过严格控制的实验，可以在很大程度上缩小90℃下这些校准线的分布范围，为构建理想的团块同位素温度计提供了良好的基础。采用磷酸消解法从碳酸盐中提取CO2，并测定CO2中的O-17含量，获得了碳酸盐矿物中O-17的精确同位素组成。我们的三途径机制模型也可以很好地预测磷酸消化过程中的三氧同位素关系。
The kinetic isotope effect (KIE) produced by the phosphoric acid digestion reaction of carbonates has been well studied since the beginning of stable isotope geochemistry, but its molecular-level mechanism remains elusive. Importantly, the validity of carbonate C-13-O-18 clumped isotope thermometry, which is heavily based on the phosphoric acid digestion treatment, needs further study. The existing Delta(47)-T relationships calibrated by different groups are incompatible and create substantial confusion in the community. Here, we propose a new model of the molecular-level mechanism of the phosphoric acid digestion reaction of carbonates. This new model, which points out that there are three parallel pathways undergoing for the phosphoric acid digestion process, can explain a large part of discrepancies of the Delta(47)-T relationships obtained by different groups in slightly different experimental conditions. Specifically, it provides different clumped isotope enrichment factors for the reaction at 25, 70, and 90 degrees C than previous theoretical studies. Under the absolute reference frame (ARF) treatment, these factors are important to obtain comparable Delta(47)-T relationships. Combined with the equilibrium clumped isotope fractionation factors, which are recalculated using higher-order theoretical treatments, e.g., the anharmonic corrections, a new theoretical Delta(47)-T relationship is built. This new relationship is no longer a single line but a range that varies to a small extent due to the changes of individual contributions of the three parallel pathways and reflects slight differences in experimental procedures and conditions. These calibration lines have a constant slope close to those of digested at 90 degrees C but significantly smaller than those at 25 degrees C. Based on linear mathematical analysis, an unknown isotope effect causing heavy O isotope enrichment during the experiments at 25 degrees C can be clearly identified. We speculate that this hidden factor was the cause of the rise of slopes. Using a strictly controlled experiment, the distribution of those calibration lines at 90 degrees C can be largely narrowed down, providing a good base for constructing an ideal clumped isotope thermometer. Extraction of CO2 from carbonate by phosphoric acid digestion and measurement of O-17 in CO2 have been used to obtain accurate O-17 isotope compositions in carbonate minerals. Our three-pathway mechanism model also provides good predictions of triple oxygen isotope relationships during the phosphoric acid digestion process.