| Summary: | 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/R[a]) 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.
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