The initial 129I/I ratio and the presence of 'old' iodine in continental margins

The natural radioisotope I-129 covers ail important age range for applications in geological systems, particularly for fluids derived from or associated with organic material. Crucial for the application of this isotopic system is the initial ratio used for the calculation of ages, the marine input ratio of I-129/I. Determinations of this ratio in recent marine sediments led to a value R-i = (1500 +/- 150) x 10(-15), which has been used in the determination of source ages in hydrocarbon systems, in gas hydrate occurrences and the tracing of fluids in subduction related volcanoes. We investigated the validity of this ratio in view of a number of determinations which resulted in ratios considerably below this value. Investigations of fluids along continental margins demonstrate that the likely cause for these low values is the release of methane-rich fluids with high iodine concentrations derived from old organic sources. Although these fluids locally influence the ratio in some areas along continental margins, they do not disturb the isotopic balance of the open ocean, justifying the use of Ri for the application of the I-129/I System. (c) 2007 Elsevier B.V. All rights reserved.