Geochemistry of Champagne Hot Springs shallow hydrothermal vent field and associated sediments, Dominica, Lesser Antilles

Geochemical and mineralogical analyses of vent waters, pore waters, gases, sediments and precipitates reveal that the vent fluids consist of a mixture of entrained seawater and meteorically derived hydrothermal fluid in varying proportions. Vent fluids are depleted in Br−, SO42−, Cl−, Na+, K+, and Sr2+ relative to ambient seawater. These species are all positively correlated with Mg2+, which is also depleted relative to seawater. Boron, Fe, As, Sb, Mn, Si and Li are all enriched relative to ambient seawater. Pore waters in the hydrothermally altered sand patches have essentially the same chemistry. Mixing between Fe2+ rich vent fluids and seawater causes rapid oxidation of Fe2+ to insoluble Fe3+ and leads to precipitation of HFO at the vent site and subsequent formation of hydrothermally altered sand patches. The elevated concentrations of As and Sb in the precipitates and sediments relative to average Caribbean seafloor sediments reflect adsorption by HFO. Gas samples from the vent site are typical arc-type gases and have both meteoric and magmatic signatures.