Seawater interaction with the oceanic lithosphere crucially impacts on global geochemical cycles, controls ocean chemistry over geologic time, changes the petrophysical properties of the oceanic lithosphere, and regulates the global heat budget. Extensive seawater circulation is expressed near oceanic ridges by the venting of hydrothermal fluids through chimney structures. These vent fluids vary greatly in chemistry, from the metal-rich, acidic fluids that emanate from “black smokers” at temperatures up to 400 °C to the metal-poor, highly alkaline and reducing fluids that issue from the carbonate–brucite chimneys of ultramafic-hosted systems at temperatures below 110 °C. Mid-ocean ridge hydrothermal systems not only generate signifi-cant metal resources but also host unique life forms that may be similar to those of early Earth.

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