The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH in active hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO (2.60–184 mmol/kg), CH (2.39·10–0.325 mmol/kg), dissolved organic carbon (4.78·10–0.112 mmol/kg), and CO (1.89·10–4.16·10 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO (δC-CO: −4.80 to −1.50 ‰, CO/He: 1.49·10–4.14·10C-CO: 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO, CH, CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH concentrations vary from lower, to significantly higher than those calculated using CO reduction kinetics. The isotope composition (δD-CH: −172 to −138 ‰, δC-CH: −32.0 to −24.6 ‰; C-CH: 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH originates from a radiocarbon inactive source, i.e. mantle CH, reduction of mantle CO and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH do not match those expected for mantle-derived CH as well as values generated from reduction of mantle-derived CO. Instead, differences in δD-CH and δC-CH values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH and δC-CH values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH concentrations, may contain a significant portion of CH that may originate from CO reduction.

中文翻译：

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冰岛热液中甲烷起源的同位素和动力学约束

热液中甲烷的起源长期以来一直是一个争论的话题——无论它是非生物的还是生物的。在这项研究中，我们的目标是通过采用整体方法分析冰岛特征良好的高温热液流体（∼230–310 °C），来阐明和量化活跃热液系统中 CH 的来源。我们采用多种地球化学和同位素指示剂，包括目标流体的化学和同位素组成。然后将这些特征与化学和同位素动力学模型的结果以及沉积物热液系统的数据进行比较。这些流体中的碳物质包括 CO (2.60–184 mmol/kg)、CH (2.39·10–0.325 mmol/kg)、溶解的有机碳 (4.78·10–0.112 mmol/kg) 和 CO (1.89·10–4.16) ·10毫摩尔/千克）。碳和氦同位素表明 CO 来源相对均匀（δ13C-CO：-4.80 至 -1.50 ‰，CO/He：1.49·10–4.14·10C-CO：0.11–2.42 pMC）。相比之下，甲烷有多种来源。总体而言，碳物种（CO、CH、CO）之间未达到化学平衡，表明存在动力学控制。氚含量（<0.8-1.42 TU）和水文限制表明热液停留时间相对较短（∼5-200年），并且偶尔有来自较旧水成分的输入。在这么短的时间内，CH 浓度从较低到显着高于使用 CO 还原动力学计算的浓度。同位素组成（δD-CH：-172 至 -138 ‰，δ13C-CH：-32.0 至 -24.6 ‰；C-CH：0.36–11.54 pMC）以及地球化学和同位素模型表明，大多数（>80–90 %） CH 源自放射性碳惰性源，即地幔 CH、地幔 CO 和/或旧有机质的还原，海洋 (<20%) 和陆地 (<10%) 溶解有机碳的贡献相对较小。测量的 CH 同位素组成与地幔来源的 CH 的预期值以及地幔来源的 CO 还原产生的值不匹配。相反，由大气水供给的系统和由大气水供给的系统之间存在 δD-CH 和 δC-CH 值的差异。通过海水，挑战了统一二氧化碳源和不变反应机制的假设。系统之间的差异最好通过热分解程度的不同以及海洋和陆地有机物同位素组成的主要变化来解释。此外，大气水供给系统中的 δD-CH 和 δ13C-CH 值与 Öxarfjördur 沉积系统中的值非常相似。总之，我们的数据支持冰岛海水和陆地热液中 CH 的主要热成因来源。有机质的来源似乎是现代溶解有机碳和古老沉积物的组合。此外，一些研究的热液系统（Krafla、Reykjanes、Theistareykir）的特点是 CH 浓度较低，可能含有很大一部分 CH，这些 CH 可能源自 CO 还原。