The large variations in hydrogen isotope ratios found in insoluble organic matter (IOM) in chondritic meteorites may be attributed to hydrogen isotopic exchange between IOM and water during aqueous alteration. We conducted D–H exchange experiments (1) during synthesis of IOM simulant (hereafter called chondritic organic analog, COA) from formaldehyde, glycolaldehyde, and ammonia with water, and (2) with the synthesized COA with a secondary reservoir of water. The changes in the D/H ratios obtained by infrared spectra of the COA suggest that most of the hydrogen in the COA is derived from water during synthesis. We further investigated the kinetics of D–H exchange between D‐rich COA and D‐poor water, as well as the opposite case, D‐poor COA and D‐rich water. To help assess understanding exchange kinetics, two‐dimensional isotope imaging obtained using isotope microscope revealed that no gradient D–H exchange profiles were present in the COA grains, indicating that the rate‐limiting step for D–H exchange is not diffusion. Thus, the changes in D/(D + H) ratios were fit by the first‐order reaction rate law. Apparent kinetic parameters—the rate constants, the activation energies, and the frequency factors—were obtained with the Arrhenius equation. Using these kinetic expressions, hydrogen isotopic exchange profiles were estimated for time and temperature behavior. The D–H exchange between organic matter and water is apparently relatively fast and this implies that the aqueous alteration temperatures should have been very low, likely close to 0 °C to maintain hydrogen isotopic disequilibrium between organic matter and liquid water for millions of years.

中文翻译：

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有机物与水之间的氢同位素交换动力学：对陨石母体加工过程中化学演化的影响

在粒状陨石的不溶性有机物（IOM）中发现的氢同位素比率的大变化可能归因于水蚀蚀变过程中IOM与水之间的氢同位素交换。我们进行了D–H交换实验（1）在甲醛，乙醇醛和氨与水的IOM模拟物（以下称为软骨状有机类似物，COA）的合成过程中，以及（2）在二级储水器中合成的COA。通过COA的红外光谱获得的D / H比的变化表明，COA中的大部分氢在合成过程中均来自水。我们进一步研究了富D的COA和贫D的水之间的D–H交换动力学，以及相反的情况，富D的COA和富D的水之间的交换。为了帮助评估对交换动力学的理解，使用同位素显微镜获得的二维同位素成像显示，COA颗粒中没有梯度D–H交换曲线，这表明D–H交换的限速步骤没有扩散。因此，D /（D + H）比的变化符合一阶反应速率定律。表观动力学参数-速率常数，活化能和频率因子-是通过Arrhenius方程获得的。使用这些动力学表达式，就时间和温度行为估算了氢同位素交换曲线。有机物与水之间的D–H交换显然相对较快，这意味着水相变化温度应非常低，可能接近0°C，以使有机物与液态水之间的氢同位素不平衡维持数百万年。