Studies on the age of the iodine-rich brine from Chiba using 129I systematics

Iodine-129 is a cosmogenic isotope with a half-life of 15.7 Ma, which allows age determinations up to 80 Ma. We have applied this technique to determine the age of iodine-rich brine collected from Chiba Prefecture, the world's largest production of iodine from brines. We also carried out chemical analyses of brines by ICP-MS and ICP-AES. The chemical characteristics of the brines suggest derivation from organic rich marine sediments under reducing conditions. Iodine ages for the brine samples were around 50 Ma. Since these ages are not compatible with the age range of the current reservoir formations (ca. 1-2 Ma), the Kazusa formation, cannot be the source of iodine. Based on our results we suggest that the iodine enrichment in the brines was caused by remobilization from subducting marine sediments associated with the release of pore waters in the fore arc area. Similar enrichments of iodine have also been observed in other fore arc areas such as New Zealand, Alaska and Trinidad. Study on the brines from the Chiba area could provide useful information on the geochemical cycle of iodine and also similar elements. (author)