Comparison of iodine dates from mud volcanoes and gas hydrate occurrences: Relevance for the movement of fluids and methane in active margins

We compare here results of iodine dating in fluids collected from mud volcanoes and gas hydrate occurrences associated with active margins. This is a compilation of previously reported data for 7 subduction zones around the Pacific Rim with slab ages ranging from 6 Ma to 130 Ma, where we determined iodine concentrations and 129I/I ratios in more than two hundred pore water samples. Iodine ages consistently are older than the host sediments and show an age distribution independent of the slab ages associated with the subduction zones. The results suggest that iodine in gas hydrates and mud volcanoes is predominantly derived from organic matter in the upper plates of subduction zones and that iodine derived from host sediments or subducting marine sediments has only a minor presence in these fluids. Because potential source sediments typically are found at lateral distances of 20 km or more, our results also suggest that fluid movement is possible over considerable distances in fractures present in the upper plate sediments. The association between iodine and methane suggests that our results can be extrapolated to the origin and transport of methane to mud volcanoes and gas hydrates. Throughout all studied sites around the Pacific margins, ages of source sediments for iodine were found to fall into the same range, which starts at the early Eocene (̃50 Ma) and has a broad peak around 30 Ma. Our results indicate that the occurrence of iodine and methane in gas hydrates and mud volcanoes is the result of transport in aqueous fluids and long-term remobilization of carbon in the upper plates of subduction zones.