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

Abstract

The Champagne Hot Springs (CHS) shallow submarine hydrothermal system is located along the submerged flank of the Plat Pays volcanic system on the southwest section of the island of Dominica, Lesser Antilles. We have conducted a detailed geochemical study of the hydrothermal system, with the objectives to investigate the source of the hydrothermal fluids and gases, their effect on sediment and precipitate chemistry, as well as comparing the submarine vent chemistry with nearby on-land hydrothermal vents. Finally, we compare our findings to previous preliminary data from CHS, and compare sediment chemistry to that of average Caribbean sediments. We also report on a newly discovered area of submarine hydrothermal venting, located approximately 40 m to the north of CHS. This area consists of hydrothermally altered areas of sand that contain abundant coatings of hydrous ferric oxides (HFO) on sediment grains.

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.