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06/11/91

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Geologists find gases that blew away from ancient volcano

STANFORD -- Two Stanford University geologists have found a way to estimate what was released into the atmosphere when a volcano erupted 45,000 years ago. By looking at gases trapped in tiny glass beads within volcanic rock, they have found that a single moderate-sized eruption can add 10 times as much copper to the atmosphere as the amount added by a year of modern industry and other human activities. This is far more than scientists had previously thought a moderate eruption could contribute. Their findings give new evidence about how copper ore is formed, and the new technique they used will help atmospheric scientists compare natural versus human sources of pollution. The research, reported June 7 in Science Magazine, was conducted by Jacob Lowenstern, a graduate student, and Gail Mahood, associate professor of geology at Stanford. They worked with Mark Rivers and Stephen Sutton, scientists at the Brookhaven National Laboratory in Upton, N.Y. Lowenstern and Mahood studied pinpoint-sized beads, called glass inclusions, that had been trapped inside grains of quartz from Pantelleria, an island volcano in the Strait of Sicily. As it crystallized deep within the magma chamber, the quartz trapped drops of molten glass, riddled with strange bubbles. With Rivers and Sutton, they focused Brookhaven's high-powered X- ray microprobe into the interior of these bubbles, each of which is about 1/250th of an inch in diameter. The microprobe measures trace elements in very small samples, and its results were surprising: Copper was a thousand times more concentrated inside the trapped bubbles than in the surrounding rocks. After a variety of tests, the group concluded that the bubbles contained remnants of copper-rich volcanic gas formed 45,000 years ago. Their findings give some of the first direct evidence that mineral- rich gas forms in magma, or molten rock, early in the formation of a volcano. They concluded that copper -- plus other substances such as carbon dioxide and chlorine -- had separated out of the molten magma into this hot gas. The gas spewed into the air when the volcano erupted. When the magma cooled, the only remaining evidence of the gaseous phase was hidden inside the glass inclusions. Lowenstern plans to study glass inclusions for information on other elements sent into the atmosphere by volcanoes, including such major pollutants as chlorine and sulfur. Atmospheric scientists will be able to add this to data from the Greenland ice cores -- to establish the natural baseline of pollutants that the atmosphere carried at any given time. This baseline is used to measure the effects of human-caused pollution. The research also offers ideas about a topic that mining geologists have debated for decades: the relationship between magma and certain kinds of ore deposits. Minerals like copper are normally scattered in trace amounts in the earth. Lowenstern and Mahood's study shows that copper can be boiled out of molten earth into a gas; future research may show that when mineral-rich gases do not escape in eruptions, they eventually cool into ore deposits. Until now, scientists have had few ways to calculate the trace elements given off by dormant volcanoes. Vapors of active volcanoes are measured by flying an airplane into the plume of smoke and ash. However, airplane samples are normally taken at less explosive periods in the eruption, after much of the vapor has already escaped. They provide a minimum estimate of a volcano's metal output. By studying glass inclusions, Lowenstern and Mahood are measuring the much higher concentration of metal in a vapor that formed in the magma chamber and that would have escaped in the first few moments of the volcano's eruption.

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