Hydrogen isotopic exchange kinetics between organic matter and water: Implications for chemical evolution during meteorite parent body processing.
Hydrogen isotopic exchange kinetics between organic matter and water: Implications for chemical evolution during meteorite parent body processing.
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有机物和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。
DOI：
10.1111/maps.13629
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发表时间：
2021
影响因子：
2.2
通讯作者：
G. D.
中科院分区：
文献类型：
--
作者：
Kebukawa;Y.;Kobayashi;S.;Kawasaki;N.;Wang;Y.;Yurimoto;H. and Cody;G. D.
不溶性有机质（IOM）中氢同位素比值的变化很大，这可能是由于水蚀变过程中IOM与水之间的氢同位素交换。我们进行了D-H交换实验（1）在由甲醛、乙醇醛和氨与水合成IOM模拟物（以下称为有机类似物，COA）期间，以及（2）用合成的COA与二级水储存器。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来自水。我们进一步研究了富D COA和贫D水之间的D-H交换动力学，以及相反的情况，贫D COA和富D水。为了帮助评估交换动力学，使用同位素显微镜获得的二维同位素成像显示，COA颗粒中不存在梯度D-H交换曲线，表明D-H交换的限速步骤不是扩散。因此，D/（D + H）比的变化通过一级反应速率定律拟合。表观动力学参数-速率常数，活化能，和频率因子-得到的Arrhenius方程。使用这些动力学表达式，氢同位素交换配置文件估计的时间和温度的行为。有机质和水之间的D-H交换显然相对较快，这意味着水蚀变温度应该非常低，可能接近0 °C，以维持有机质和液态水之间的氢同位素不平衡数百万年。
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.