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ORNL studies magmas formed during environmental cleanup

OAK RIDGE, Tenn., March 29, 1995 — What happens when molten volcanic rock cools in a lava lake or below the surface of the earth? How are large granite masses formed? Answers to these questions may be emerging from volcano-free Tennessee.

Geoscientists from around the nation have concluded that a "soil-to-glass" environmental cleanup process tested at the Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL) can mimic some of the processes of magma formation and cooling.

The process, in situ vitrification (ISV), developed for DOE by Battelle Pacific Northwest Laboratory in Richland, Wash., uses electrodes to heat soil contaminated with radioactive elements to temperatures up to 1600°C (2900°F). Upon cooling, the molten soil transforms to a mixture of glass and crystals in the ground, trapping the radioactive material.

ISV has been tested with and without radioactive materials at ORNL. In another test in June four megawatts of electrical power will heat and melt the soil at an ORNL waste pit and produce 600 tons of glass.

"ISV melts," says Mike Naney, a geochemist with ORNL's Environmental Sciences Division, "can provide artificial magmas in a controlled, monitored environment. The molten rock, crystals, and gases produced by the melting are formed at temperatures similar to those in crustal magma chambers that supply lava to volcanoes.

"But the ISV melts are not subjected to the explosively high pressures of volcanoes. By studying the cooling and crystallization in artificial magmas, we can learn more about what happens during the cooling of crustal magma chambers and lava lakes."

These magmatic processes include extraction of metals and their deposition as mineral ores, heating and circulation of water that can be tapped as geothermal energy, and venting of gases to the atmosphere that affect global climate.

A test of this technology for environmental remediation was sponsored by DOE's Office of Technology Development. Using thermocouples, the ORNL scientists measured the temperature profile of the ISV magma. Temperature ranged from 100°C several feet away from the melt to 1500°C in the molten soil. They observed vigorous bubbling and rapid convection of heat by circulation of heated liquid and gases at the melt surface. After the artificial magma cooled, they obtained samples by diamond-core-drilling the rock-like product. Then they analyzed the textures and chemistry of the minerals that formed during crystallization of the melt.

"Our studies of artificial magmas during and after cooling," Naney says, "help us understand how certain rocks form during cooling of pools of natural magma and lava lakes. The ORNL soils that we studied melt to form a basalt-like liquid, rich in calcium, magnesium, iron, and silicon. As the molten basalt cools, minerals containing calcium, magnesium, and iron precipitate out, leaving a liquid rich in silicon. As the last remaining liquid cools, it solidifies into a mixture of glass and crystals having the composition of granite.

"Our observation of small amounts of granite filling spaces between the larger feldspar and pyroxene crystals that form the predominantly basalt-like rock provides information on granite formation in the crust of the earth," Naney says. "The same phenomenon in natural magmas would trap granite liquid between interlocking crystals of a basalt, preventing migration and coalescence of the granite liquid to form large masses.

What conclusions have the ORNL researchers made from their observations? Says Naney: "The existence of some large bodies of granite, of the size exposed at Stone Mountain, Georgia, or in Yosemite National Park, may require multiple cycles of melting and squeezing of solidified basalts. These processes extracted the small amounts of granite liquid required to form granite masses as large as hundreds of cubic kilometers."

This idea is not new to geologists. But, Naney says, the ORNL studies provide a large-scale example of the 'fractionation' process that generates granite from a 'parent' basalt magma.

Although basalt is a common rock at volcanoes in the Cascades in the western United States and in Hawaii, it is not present in East Tennessee. The common rocks at the ORNL site are limestone and shales. "However," Naney notes, "the chemical compositions of basalt and the combination of shale and limestone are similar, so ISV melts in Tennessee can provide useful information on volcanic processes."

Naney and Gary Jacobs pursued the idea of using ISV melts as artificial magmas at a meeting of geologists in San Francisco and later in Boston. In the spring of 1994, he and Jacobs organized a workshop on this subject in Oak Ridge. It was attended by 23 geoscientists from universities, research institutions, and national laboratories across the country.

"The participants concluded that ISV technology can produce large silicate melts whose makeup and properties provide unique analogs for natural magmas," Naney says. "They agreed that ISV melts are useful for testing theories that cannot be investigated through direct observation using bench-top experiments or uncontrolled studies of natural systems such as lava lakes."

The workshop and other ISV magma research were supported by the Geosciences Research Program of DOE's Office of Basic Energy Sciences.

ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Martin Marietta Energy Systems, a Lockheed Martin company, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.