The hydrogen isotopic composition of methane (CH₄) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH₄ and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH₄ and 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 CH₄ and supercritical water. Seven isothermal experiments were performed over a temperature range of 376 to 420°C in which deuterium-enriched water and CH₄ were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH₄ over the time course of an experiment. Regression of derived second order rate constants (k_r) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(k_r) = -17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of k_r of 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, CH₄ will 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 125-200°C, hydrogen isotope exchange between CH₄ and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH₄ may 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 CH₄ is 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 CH₄ and water to occur.

_{甲烷 (CH 4} )的氢同位素组成被用作气体来源的指纹。_{CH 4}和液态水之间的氢同位素交换已被提议在低温和高温环境中发生。_{然而，尽管环境证据表明 CH 4}和液态水之间有氢同位素交换，但对该过程的动力学几乎没有实验限制。_{我们展示了为限制 CH 4}和超临界水之间的氢同位素交换动力学而进行的水热实验的结果。在 376 至 420°C 的温度范围内进行了七次等温实验，其中富含氘的水和 CH ₄在灵活的金反应池系统中进行反应。_{通过测量 CH 4}的 δD在实验过程中的变化来确定交换率。导出的二阶速率常数 (k _r ) 与 1000/T（即 Arrhenius 图）的回归得出以下等式： ln(k _r ) = -17.32 (±4.08, 1 se) × 1000/T + 3.19 ( ±6.01, 1 se)（单位为 k _r of sec ^-1 [mol/L] ^-1）相当于 144.0 ± 33.9 kJ/mol (1 se) 的活化能。这些结果表明，在没有催化剂的情况下，CH ₄在低于 100-125°C 的温度下，不会在比地球年龄（即数十亿年）更短的时间尺度上与液态水交换氢同位素。在这些温度下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）和其他行星体上低温下的生物特征。在 125-200°C 的温度范围内，CH ₄和液态水之间的氢同位素交换可以在数百万年到数十万年的时间尺度上发生，这表明在热成因天然气系统中 CH ₄可以与水进行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，CH ₄的氢（以及因此聚集的）同位素组成可能由与活跃流动状态隔离的形成和/或储存条件决定。确定的动力学表明，一旦甲烷被夹带在循环流体中，预期的时间-温度路径不足以在 CH ₄和水之间发生可测量的氢同位素交换。