Böttcher Michael Ernst, Vennemann Torsten W
Geochemistry & Isotope Biogeochemistry, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany.
Marine Geochemistry, University of Greifswald, Germany.
Rapid Commun Mass Spectrom. 2022 Dec 15;36(23):e9389. doi: 10.1002/rcm.9389.
The analytical method to determine the stable oxygen isotope ( O/ O) composition of carbonates via phosphoric acid digestion leads to temperature- and solid-dependent kinetic isotope fractionation. Values for the double carbonate norsethite (BaMg(CO ) ) have been unknown so far.
The temperature dependence of kinetic oxygen isotope fractionation during the reaction of synthetic and natural BaMg(CO ) with orthophosphoric acid (H PO ) according to the overall reaction BaMg(CO ) + 2H PO = Ba + Mg + 2HPO + 2CO + 2H O has been examined for the first time using separate carbonate decomposition via fluorination or phosphoric acid digestion, with the resulting gases analyzed by isotope ratio monitoring mass spectrometry.
In the temperature range between 25 and 70°C the kinetic fractionation factor between acid-generated CO and artificial and natural norsethite is described by (T in K): [Formula: see text] with A = 4.15 and B = 6.47 for natural norsethite, and A = 4.77 and B = 5.94 for synthetic norsethite. The fractionation factor measured for a poorly crystallized synthetic carbonate agrees with those for the other samples at 25°C, but is slightly lower at 50 and 70°C. No carbon isotope fractionation was found during the unidirectional acid dissolution.
The kinetic oxygen isotope fractionation during phosphoric acid liberation of CO from BaMg(CO ) is quantified. Based on published results for endmember carbonates, the results at 25°C for other double carbonates are estimated.
通过磷酸消解测定碳酸盐稳定氧同位素（¹⁸O/¹⁶O）组成的分析方法会导致温度和固体依赖性动力学同位素分馏。到目前为止，双碳酸盐碳镁钡矿（BaMg(CO₃)₂）的值尚不清楚。
首次通过氟化或磷酸消解分别进行碳酸盐分解，利用同位素比率监测质谱分析产生的气体，研究了合成和天然的BaMg(CO₃)₂与正磷酸（H₃PO₄）按照总反应BaMg(CO₃)₂ + 2H₃PO₄ = Ba²⁺ + Mg²⁺ + 2HPO₄²⁻ + 2CO₂ + 2H₂O反应时动力学氧同位素分馏的温度依赖性。
在25至70°C的温度范围内，酸生成的CO₂与人工和天然碳镁钡矿之间的动力学分馏因子由（T为开尔文温度）表示：[公式：见原文]，天然碳镁钡矿的A = 4.15，B = 6.47，合成碳镁钡矿的A = 4.77，B = 5.94。对于结晶不良的合成碳酸盐，在25°C下测得的分馏因子与其他样品的一致，但在50和70°C时略低。在单向酸溶解过程中未发现碳同位素分馏。
对从BaMg(CO₃)₂中磷酸释放CO₂过程中的动力学氧同位素分馏进行了量化。根据端元碳酸盐的已发表结果，估计了其他双碳酸盐在25°C时的结果。
