Scientists examined chunks of basalt, a type of volcanic rock formed when lava cools, from 12,000 feet below the Pacific along a massive geographical formation called the midocean ridge. The scientists discovered in these basalts traces of carbon dioxide and other compounds that originated deep within the Earth's mantle, the source of most volcanic activity. Because compounds from this inaccessible region had never been found so well preserved, the rocks gave scientists a rare peek at what the mantle consists of – and what it might spew into the atmosphere through volcanoes.
"Most lava erupts at the surface and has lost its gases. From a geochemist's point of view, you need to know what the composition of the mantle really is," said Mike Perfit, a UF geology professor and co-author of the Nature paper. "This kind of data might be useful in talking about the contribution of the mantle to the atmosphere and hydrosphere and how those concentrations might affect the climate."
Carbon dioxide is the leading "greenhouse gas" that traps heat and contributes to warming of the Earth. Scientists have long speculated volcanic eruptions can spew enough of this and other gases into the atmosphere to cause significant warming trends – changes so massive they may even spur mass extinctions. By giving scientists an idea of how much carbon dioxide lies under the Earth, the basalt may help answer this question, Perfit said.
When magma rises to the Earth's surface and erupts as lava flows, Perfit said, it typically "de-gasses:" As the Earth's pressure on the lava declines, the amount of volatile compounds that become gases at the surface rapidly decrease. It's a bit like popping open a soft drink: The carbon dioxide bubbles off. As a result, carbon dioxide, water, sulfur dioxide, helium, chlorine and other "volatiles" are barely present in most basalt, making it difficult for geologists to figure out the amounts and proportions of these compounds in the mantle.
The deep ocean, however, is a unique environment. The water is so cold and the pressure so intense there it may keep the volatiles confined in the lava, known as magma when it first erupts and hardens. As a result, geologists have seen deep-sea geological formations such as so-called "pillow flows" as one of their best hopes for investigating the mantle question.
Perfit and colleague Dan Fornari, of the Woods Hole Oceanographic Institute, were among the scientists who dived in the manned deep-water submersible robot "Alvin" to probe a site a few hundred miles west of the Mexican coast. The area, known as the Siqueiros Transform fault, a deep part of the midocean ridge, was known to experience underwater eruptions, which is why the scientists chose it for their investigation. Perfit returned with a small load of golf ball- to basketball-sized pieces of basalt from the sea floor, where the water pressure was 350 times greater than at the surface.
This basalt not only had no bubbles, indicating that the volatile compounds remained in the lava, it also was very recently formed, making it an ideal study candidate. Scientists discovered that small crystals in the rock called olivines contained tiny bits of pure magma. Using newly developed technology that can analyze very small areas, researchers Alberto Saal and Eric Hauri, two of the other authors on the Nature paper from the Lamont-Doherty Earth Observatory of Columbia University, measured the volatiles in this magma.
Peter Michael, a professor of geosciences at The University of Tulsa familiar with the research, said other geologists have been able to measure some volatiles before, such as chlorine. But the Siqueiros Transform rocks provide a unique glimpse at carbon dioxide levels. This is important, because the midocean ridge is a mammoth, 40,000-mile long mountain where 85 percent of the world's volcanic activity occurs. As a result, if the basalts Alvin returned to the surface are typical of other midocean ridge basalts, it could help determine the rate at which Earth's below-ground carbon dioxide is supplied to the atmosphere through volcanoes, he said.
"What this work may allow us to do is actually compute the carbon dioxide not just for that magma, but for a lot of other midocean ridge basalt magmas," Michael said.
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Writer: Aaron Hoover
ahoover@ufl.edu
Source: Mike Perfit
352-392-2128
perfit@geology.ufl.edu
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