环境地球化学国家重点实验室知识库

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 Soufrie`re, 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 (d66Zn values from +0.21& to +0.35&) similar to or slightly higher than fresh andesite (+0.21&). 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 d66Zn values of most of the spring waters are relatively constant (approximately 0.70&), indicating that the thermal springs from La Soufrie`re are enriched in heavy isotopes (i.e., 66Zn) compared to the host rocks (from 0.14& to +0.42&). Only three thermal springs have lower d66Zn 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 66Zn-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 d66Zn 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., 66Zn). 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.