Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India.
Centre for Earth Sciences, Indian Institute of Science, Bangalore, India.
Rapid Commun Mass Spectrom. 2020 Dec 15;34(23):e8926. doi: 10.1002/rcm.8926.
In 'clumped isotope paleothermometry' carbonates are reacted with anhydrous phosphoric acid to extract CO that carries the isotopic signature of the reacting carbonates, and the amount of clumping in the product CO is measured. Previous theoretical models for determining clumped isotopic fractionation in product CO during acid digestion of carbonates are independent of the cations present in the carbonate lattice. Hence further study is required to understand the cationic effect.
We studied the acid reaction mechanism based on the protonation of carbonates, calculated the acid fractionation factor for dolomite using the partition functions and vibrational frequencies obtained for the transition state structure, and determined the effect of cations on the acid fractionation factor. Experimentally, carbonates are reacted using the modified sealed vessel method and analyzed in the dual inlet of a ThermoFinnigan MAT 253 isotope ratio mass spectrometer.
The oretically obtained acid fractionation factor can be expressed as Δ = -0.28563 + 0.49508 * (10 /T ) - 0.08231 * (10 /T ) for a temperature range between 278.15 and 383.15 K. The theoretical slope of the dolomite-acid digestion curve is lower than that of the calcite-acid digestion curve obtained using the identical reaction mechanism. Our theoretical slope is consistent with the result from the common acid bath experiments but higher than the slope obtained in our experimental study using the modified sealed vessel method and in a previous theoretical study using the H CO model.
The transition state structure, obtained in our study, includes the cations present in the carbonate minerals and provides distinct acid fractionation factors for calcite and dolomite. The observed gentler slope of the theoretically calculated dolomite-acid digestion curve than of the calcite curve is expected considering the stronger Mg-O bond. Our experimental approach invokes post-digestion isotopic exchange and agrees with the previous theoretical estimates where post-digestion isotopic fractionation was considered.
在“凝聚同位素古温度测定法”中，将碳酸盐与无水磷酸反应，以提取携带反应碳酸盐同位素特征的 CO，然后测量产物 CO 中的凝聚程度。以前用于确定碳酸盐酸消解过程中产物 CO 中凝聚同位素分馏的理论模型与碳酸盐晶格中存在的阳离子无关。因此，需要进一步研究以了解阳离子的影响。
我们基于碳酸盐的质子化研究了酸反应机制，使用过渡态结构获得的配分函数和振动频率计算了白云石的酸分馏因子，并确定了阳离子对酸分馏因子的影响。实验中，使用改进的密封容器法使碳酸盐反应，并在 ThermoFinnigan MAT 253 同位素质谱仪的双入口中进行分析。
理论上获得的酸分馏因子可以表示为 Δ = -0.28563 + 0.49508 * (10 /T ) - 0.08231 * (10 /T )，温度范围在 278.15 到 383.15 K 之间。与使用相同反应机制获得的方解石-酸消解曲线相比，白云石-酸消解曲线的理论斜率较低。我们的理论斜率与普通酸浴实验的结果一致，但高于我们使用改进的密封容器法进行的实验研究和以前使用 H CO 模型进行的理论研究中的斜率。
我们的研究中获得的过渡态结构包括碳酸盐矿物中存在的阳离子，并为方解石和白云石提供了独特的酸分馏因子。考虑到更强的 Mg-O 键，观察到理论计算的白云石-酸消解曲线比方解石曲线斜率更平缓是合理的。我们的实验方法涉及消解后的同位素交换，与考虑消解后同位素分馏的先前理论估计一致。
