Zn isotopes as a traccer of bedrock weathering in hydrothermal system of la Soufrière volcano, Guadeloupe (FWI)
Abstract
The active hydro-volcanic systems are characterized by intense hydrothermal activities associated with acidic fumaroles and hot springs and play an important role in global silicate weathering. As the ultimate weathering loads are mostly transported into ocean through water, studies of hydrothermal waters can give interesting clues about the complex interactions among magmatic fluids, bed-rock, and aquifers fed by meteoritic water or seawater. Zn is a volatile element during magma degassing. However, the behavior of Zn in hydrothermal water systems is still unclear. Recent studies have demonstrated the interest of Zn isotopes for investigating water-rock interactions. Speciation-related fractionation as well as source-related fractionation between its isotopes (about 3‰ in δ66Zn unit) make Zn isotopes a promising tracer for studying the mobility of metals during weathering, hydrothermalism, magma degassing and ore formation. Although previous studies have focused on the processes fractionating Zn isotopes in hydrothermal solid deposits, seafloor vents and fumarolic gas, Zn isotope composition of hydrothermal waters in continental arc setting has not been investigated so far. We developed a new one-step purification method for the separation of Zn from Fe- and SO4-enriched hydrothermal solutions using anion-exchange column. The protocol was validated by multiple tests on varying eluants and Zn concentrations, and by investigating the recovery and the reproducibility of measured isotopic ratios. Using this method, water samples from 8 hydrothermal springs and 6 gas samples from two fumaroles of la Soufrière active volcano on the Guadeloupe island (French West Indies, FWI) were analyzed for Zn isotope composition. Compared to the small δ66Zn range for the fumarolic gases (from 0.21‰ to 0.35‰) and local bedrocks (from -0.14‰ to 0.42‰), all water samples displayed a relative large δ66Zn variation of 1.44‰ (from -0.43‰ to 1.01‰). This is about 70% of the total variation reported to date for solid samples of hydrothermal systems. All water samples displayed an increase in δ66Zn with Zn concentration towards the dome summit. We show that water-rock interaction is probably the main contributing source of Zn in hydrothermal springs, while Zn of fumarolic gases is mainly derived from the magmatic gas. δ66Zn in redistributed solutions originated from the initial source can increase up to 1.01‰ due to precipitation effect of Fe(Mn) oxide-hydroxide and/or Zn sulfides. However, Zn adsorption onto solid surface may induce a decrease of δ66Zn even to -0.43‰ before leaking out of surface. Our study demonstrates that Zn isotope composition is a useful tool to monitor the flow path of trace metals during bedrock weathering and a powerful tracer of hydrothermalism in an active volcano system.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP13C0853C
- Keywords:
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- 1034 GEOCHEMISTRY / Hydrothermal systems;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8424 VOLCANOLOGY / Hydrothermal systems