Iodine dating of pore waters associated with gas hydrates in the Nankai area, Japan

The Nankai Hydrate Field, Japan, is an example of gas hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, 129I/I ratios together with halogen concentrations were measured in a set of pore water samples collected from two bore holes in the Nankai Hydrate Field, Japan. Iodine concentrations are between 100 and 230 E M, i.e. strongly enriched compared to seawater, while Cl concentrations were found to be close to that of seawater. Except for one sample, 129I/I ratios are between 180 and 520x10-15, giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments (