Zn isotope compositions of the thermal spring waters of La Soufriere volcano, Guadeloupe Island

To trace the sources and pathways of Zn in hydrothermal systems, the Zn isotope compositions of seventeen water samples from eight thermal springs and six gas samples from two fumaroles from La Soufriere, an active volcano on Guadeloupe Island (French West Indies, FWI), were analyzed using a method adapted for purifying Zn from Fe- and SO4-enriched thermal solutions. The fumaroles are enriched in Zn 100 to 8000 times compared to the local bedrock and have isotopic compositions (delta Zn-66 values from + 0.21 parts per thousand to + 0.35 parts per thousand) similar to or slightly higher than fresh andesite (+0.21 parts per thousand). The enrichment of Zn in the thermal springs compared with the surface waters shows that Zn behaves as a soluble element during hydrothermal alteration but is significantly less mobile than Na. The delta Zn-66 values of most of the spring waters are relatively constant (approximately 0.70 parts per thousand), indicating that the thermal springs from La Soufriere are enriched in heavy isotopes (i.e., Zn-66) compared to the host rocks (from 0.14 parts per thousand to + 0.42 parts per thousand). Only three thermal springs have lower delta Zn-66 values (as low as 0.43%). While the Zn in the fumaroles is essentially derived from magma degassing, which is consistent with a previous study on Merapi volcano (Toutain et al., 2008), we show that the Zn in the thermal springs is mainly derived from water-rock interactions. The Zn-66-enriched isotopic signature in most of the spring waters can be explained qualitatively by the precipitation at depth of sulfide minerals that preferentially incorporate the light isotopes. This agrees with the isotopic fractionation that was recently calculated for aqueous complexes of Zn. The few thermal springs with lower delta Zn-66 values also have low Zn concentrations, indicating the preferential scavenging of heavy Zn isotopes in the hydrothermal conduits. This study shows that unlike chemical weathering under surface conditions, hydrothermal alteration at high temperatures significantly fractionates Zn isotopes and enriches thermal waters in heavy Zn isotopes ( e. g., 66 Zn). Continental hydrothermal systems therefore constitute a source of heavy Zn isotopes to the oceans; this should be taken into account in the global oceanic budget of Zn. (C) 2013 Elsevier Ltd. All rights reserved.