Sources and sinks of natural and anthropogenic iodine in the hyperarid Atacama Desert of northern Chile
Abstract
Iodine is a strongly biophilic element. Most of the global Iodine budget concentrates in marine sediments, but in continental settings, the occurrence of iodine minerals is rare and restricted to hyper-arid environments. Currently, the Atacama Desert hosts the world's largest iodine crustal anomaly, where the occurrence of iodine minerals is constrained to the nitrate-iodine deposits located along the eastern side of the Coastal Range, and the supergene zones of copper deposits in the Central Depression and Precordillera. Although iodine enrichment has been documented for decades in this region, little is known about its setting, source and mechanism(s) of enrichment. In this study, we present the first survey of iodine concentrations and isotopic ratios (129I/I) of the different geochemical reservoirs in Atacama, including nitrate deposits, supergene copper ores, marine sedimentary rocks, geothermal fluids, groundwater and meteoric water. Our results indicate that nitrate deposits contain the highest iodine concentrations of all reservoirs in Atacama, with a mean concentration of ~700 ppm. These anomalous values are followed by highly enriched soil samples above supergene copper deposits, and Mesozoic shales and limestones averaging ~50 ppm. On the other hand, the highest concentrations of iodine in fluids were measured in groundwater below nitrate ore fields (3.5-10 ppm) and in geothermal fluids (1-3 ppm). The calculated 129I/I ratios in nitrate ores range from ~300 to ~400 x 10-15. Supergene iodine minerals in copper deposits present values between 200 and 550 x 10-15 and ratios obtained from marine rocks vary from 300 to 400 x 10-15. Regarding isotopic ratios in fluids, seawater presents the highest 129I/I ratios (~11000 x 10-15). Isotopic composition of groundwater below nitrate deposits is ~10000 x 10-15, while 129I/I ratios in fluids from Western Cordillera are between 2000 and 5000 x 10-15. In most of the reservoirs it is possible to identify a strong iodine enrichment compared to average crustal values. To reach these high concentrations it is necessary to have: (1) the presence of an enriched iodine source, (2) the removal of iodine by fluids from the source to the surficial reservoirs, and (3) the accumulation and preservation of iodine in these reservoirs. Isotopic ratios in nitrates and supergene iodine minerals are in agreement with previously reported 129I/I ratios in crustal fluids derived from organic material (200-400 x 10-15). Mesozoic shales are the most probable source for iodine because of the similar 129I/I ratios with rock reservoirs, high organic content and regional occurence. Finally, with respect to 129I/I ratios in fluids, most of the samples present values above the marine pre-anthropogenic value (1500±150 x 10-15), and it is possible to identify a decreasing trend from sea to Western Cordillera. This indicates a more important marine influence in western samples and a meteoric component in estern samples.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSM.V53A..12A
- Keywords:
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- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry