Origin of iodine in volcanic fluids I-129 results from the Central American Volcanic Arc

• Published in: Geochimica et cosmochimica acta Vol. 66; no. 21; pp. 3827 - 3838
• Main Authors: Snyder, G T, Fehn, U
• Format: Journal Article
• Language: English
• Published: 01.11.2002
• ISSN: 0016-7037
• DOI: 10.1016/S0016-7037(02)00825-6

Results of I/I-129 ratios in volcanic fluids, collected during a comprehensive study of fluids and gases in the Central American Volcanic Arc, are reported. I/I-129 ratios, together with I, Br, and Cl concentrations, were determined in 79 samples from four geothermal centers and a number of crater lakes, fumaroles, hot springs, and surface waters in Costa Rica, Nicaragua, and El Salvador. Geothermal and volcanic fluids were found to have iodine concentrations substantially higher than values in seawater or meteoric waters. I/I-129 ratios in most of the geothermal fluids are below the preanthropogenic input ratio of 1500 x 10 exp -15, demonstrating that recent anthropogenic additions are largely absent from the volcanic systems. The majority of the I/I-129 ratios are between 500 and 800 x 10 exp -15. These ratios indicate minimum iodine ages between 25 and 15 Ma, in good agreement with the age of subducted sediments in this region. In all four geothermal systems, however, a few samples were found with iodine ages older than 40 Ma, that is, considerably below the expected age range for subducted sediments from the Cocos Plate. These samples probably reflect the presence of iodine derived from sediments in older accreted oceanic terraines. The iodine ages indicate that the magmatic end member for the volcanic fluids originates in the deeper parts of the subducted sediment column, with small additions from older iodine mobilized from the overlying crust. The high concentrations of iodine in geothermal fluids, combined with the observed iodine ages, demonstrate that remobilization in the main volcanic zone (and probably also in the forearc area) is an important part in the overall marine cycle of iodine and similar elements. (Author)