Experimental determination of hydrogen isotope exchange rates between methane and water under hydrothermal conditions
Experimental determination of hydrogen isotope exchange rates between methane and water under hydrothermal conditions
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DOI：
10.1016/j.gca.2022.04.029
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发表时间：
2022-05
影响因子：
5
通讯作者：
Andrew C. Turner;N. Pester;M. Bill;M. Conrad;K. Knauss;D. Stolper
中科院分区：
文献类型：
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作者：
Andrew C. Turner;N. Pester;M. Bill;M. Conrad;K. Knauss;D. Stolper
甲烷（CH4）的氢同位素组成被用作气体来源的指纹。ch4和液态水之间的氢同位素交换已被提出在低温和高温环境下都发生。然而，尽管环境证据表明ch4和液态水之间的氢同位素交换，但对这一过程的动力学几乎没有实验限制。我们提出了水热实验的结果，以约束ch4和超临界水之间氢同位素交换的动力学。在376-420 °C的温度范围内，在柔性金反应池体系中，富氘水和ch4进行了7次等温实验。交换速率是通过测量在实验过程中ch4的δD的变化来确定的。对导出的二阶速率常数（kr）与1000/T（即Arrhenius图）的回归得到以下等式：ln（kr） = −17.32（±4.08,1 s.e.） × 1000/T + 3.19（±6.01,1 s.e.）（单位为kof sec−1[mol/L]−1），相当于活化能为144.0 ± 33.9 kJ/mol（1 s.e.）。这些结果表明，如果没有催化剂，在低于100-125 °C的温度下，ch4不会在比地球年龄短的时间尺度（即数十亿年）内与液态水交换氢同位素。在这些温度或低于这些温度的交换被认为是由于生命活动而发生的，因此甲烷和水之间的氢同位素平衡可能是地球（现在或过去）和其他行星体在低温下的生物特征。在125 ~ 200 ℃的温度范围内，ch4与液态水之间的氢同位素交换可以发生在数百万~数十万年的时间尺度上，这表明在产热天然气系统中，ch4可能与水实现同位素平衡，并达到平衡同位素组成。最后，动力学表明，在深海热液系统中，ch4的氢（因此成团）同位素组成可能由与活跃流动状态分离的形成和/或储存条件决定。测定的动力学表明，一旦甲烷被带入循环流体，预期的时间-温度路径不足以使ch4与水之间发生可测量的氢同位素交换。
The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of krof sec−1[mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4and water to occur.