(Table 3) Noble gas composition of dissociated gas hydrate specimens from ODP Leg 164 sites

Fractionation of the noble gases should occur during formation of a Structure I gas hydrate from water and CH4 such that CH4 hydrate is greatly enriched in Xenon. Noble gas concentrations and fractionation factors (F[4He], F[22Ne], F[86Kr], and F[132Xe] as well as R/Ra) were determined for eight gas hydrate specimens collected on Leg 164 to evaluate this theoretical possibility and to assess whether sufficient quantities of Xe are hosted in oceanic CH4 hydrate to account for Xe "missing" from the atmosphere. The simplest explanation for our results is that samples contain mixtures of air and two end-member gases. One of the end-member gases is depleted in Ne, but significantly enriched in Kr and Xe, as anticipated if the source of this gas involves fractionation during Structure I gas hydrate formation. However, although oceanic CH4 hydrate may be greatly enriched in Xe, simple mass balance calculations indicate that oceanic CH4 hydrate probably represents only a minor reservoir of terrestrial Xe. Noble gas analyses may play an important role in understanding the dynamics of gas hydrate reservoirs, but significantly more work is needed than presented here.

在以水与甲烷（CH₄）形成I型天然气水合物（Structure I gas hydrate）的过程中，稀有气体（noble gases）理应发生分馏，使得甲烷水合物显著富集氙（Xenon）。为验证这一理论可能性，并评估海洋甲烷水合物中赋存的氙量是否足以解释大气中‘缺失’的氙，研究人员对164航次（Leg 164）采集的8件天然气水合物样品开展了稀有气体浓度与分馏因子（fractionation factors，包括F[4He]、F[22Ne]、F[86Kr]、F[132Xe]及R/Ra）的测定工作。对本研究结果的最简解释为：样品为空气与两种端元气体（end-member gases）的混合体系。其中一种端元气体的氖（Neon）含量亏损，但氪（Krypton）与氙显著富集——若该气体的来源涉及I型天然气水合物形成过程中的分馏作用，则这一结果与理论预期相符。然而，尽管海洋甲烷水合物可显著富集氙，但通过简单的质量平衡计算（mass balance calculations）可知，海洋甲烷水合物可能仅为地球氙的一个次要储库（reservoir）。稀有气体分析或许对阐明天然气水合物储层的动力学特征具有重要意义，但后续需开展的工作远多于本研究已涵盖的内容。