Hot, Deep Vents Without Fireworks

Deep sea chimney. A carbonate ledge juts out of a 49-meter chimney in a hydrothermal vent field.

At the bottom of the world's oceans, mountain ranges run along the boundary of tectonic plates, pushed up by rising molten rock that also heats water that spews out of vents found along the ridge tops. Oddly, some maverick hydrothermal vents exist far from the spine of the ridges and their reservoirs of magma. Now, a mathematical model shores up speculation that the furnace for these distant vents is a chemical reaction between seawater and rock exhumed from deep within the planet.

Marine geologists have known for years about the sizzling mix of seawater and peridotite--a greenish rock from Earth's mantle that is sometimes pushed up through the crust. The result is a soapy-textured rock called serpentine. But geophysicist Robert Lowell at the Georgia Institute of Technology in Atlanta said he never believed the reaction could produce enough heat to power a hydrothermal vent until a vent was discovered in 2000 in the Atlantic Ocean, atop a region of peridotite and away from the volcanically active backbone of the mid-ocean ridge. Waters from the vent site were a respectable 40° to 70°C, but hardly the scalding 200° to 300°C observed in vents near magma sources--which suggested that something other than magma might be at work.

So Lowell and Peter Rona of Rutgers University in New Brunswick, New Jersey, set out to determine the range of water temperatures that might issue from a hydrothermal vent on top of a mass of peridotite. They tallied the heat released for every kilogram of peridotite converted into serpentine and estimated how fast water moved through the vent system. They found that extra heat from magma may not be necessary; temperatures produced by a vent system like the one found in 2000 could be reached just by the seawater-rock interaction, the pair reports in the 14 June issue of Geophysical Research Letters.

This is the first demonstration that the peridotite conversion can create a "good-sized" temperature hike like that observed in the off-ridge vent, says chemical oceanographer David Butterfield of the Pacific Marine Environmental Laboratory in Seattle, Washington. But Butterfield questions whether the model can nail down any details of specific hydrothermal vents, because many of the model's variables, such as the amount of peridotite or flow rates in the crust, are very difficult to confirm.

Related sites
Robert Lowell's research page
Images of the unusual hydrothermal vent found in 2000

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