GEOCHEMISTRY OF HYDROTHERMAL VENTS AT SUBMARINE VOLCANOES

Hydrothermal fields at submarine volcanoes are generally classified as either ‘water-rock’ or ‘magmatic hydrothermal’ systems. Often, classification is based on visual cues with fluid geochemistry serving to confirm these observations. Systems controlled by water-rock interactions reveal massive sulfide chimneys (black smokers), and they discharge highly-altered seawater devoid of magnesium and sulfate. Magmatic hydrothermal systems are dominated by magmatic degassing, and they feature white smokers with elevated concentrations of sulfate and elemental sulfur due to the disproportionation reaction of sulfur dioxide gas.

The discovery of Iou vent field on Hinepuia submarine volcano prompted further investigation of these hydrothermal fluids. Here, mineralogy and gas content are highly suggestive of a magmatic hydrothermal system, but the fluids reveal characteristics attributed to water-rock systems. Ternary diagrams have been developed to compare some of the main components of these fluids, and clear distinctions can be made between water-rock and magmatic hydrothermal systems. Hinepuia falls at the intersection of these fluid end-members.

Ultimately, the Iou vent field is classified as a magmatic hydrothermal system with a profound water-rock influence, which is likely due to lower SO₂ degassing and a long pathway to the sea floor. In addition to the differences in fluid chemistry, the integration of both end-member types has implications for the types of minerals formed at this vent site and similar sites. Along with the high elemental sulfur, alunite and silica content present in magmatic hydrothermal systems, magmatic vents also experiencing strong water-rock influences within the hydrothermal system have some metal sulfide mineralization on the sea floor. Future work will focus on the interplay of these systems and how they evolve.