Credit: © NOAA

The pathway of the fluids from black smokers near slow-spreading ocean ridges is channelled by faults in the oceanic crust, suggests a new geochemical analysis of a massif 25 km west of the mid-Atlantic ridge. This is contrary to the standard model of hydrothermal systems in which recharge of the hydrothermal fluids from seawater is slow and diffuse, and a vertical outlet pipe leads into the vents.

Andrew McCaig of Leeds University in the UK and colleagues1 analysed the oxygen and strontium isotope compositions of the rocks from a mid-Atlantic detachment fault — a shallow-angle fault in the ocean crust — at about 15°N. From the combination of these isotopic compositions, the researchers concluded that the rocks were geochemically altered by hot fast-flowing hydrothermal fluids that were geochemically similar to seawater, suggesting that the fault provides a fast pathway for fluid flow through a hydrothermal system.

Because of the shallow angles of fluid flow associated with detachment faults, hydrothermal vents can be located several kilometres away from the volcanic zone or magma chamber whose heat fuels the circulation. Different stages in the evolution of the detachment faults explain distinct types of hydrothermal circulation systems, such as the Transatlantic Geotraverse, the Rainbow and the Lost City vent fields.