Cool waters.
Travertine deposits create marblelike terraces in CO2-rich springs near New Mexico's Rio Grande Rift.

Laura J. Crossey

Cold Springs Tell of Warm Mantle

Every hour or so, Old Faithful at Yellowstone National Park in the western United States sends up a spray of superheated water that signals the constant geological shifting deep underground. But dramatic geysers aren't the only places to see such signs. Scientists have now uncovered chemical traces of active tectonism in the more modest but much more numerous cool springs dotting the region--offering a new theory to explain widespread arsenic and other water quality problems in western U.S. aquifers.

Western North America is slowly pulling apart at a rate of several millimeters per year, creating a network of faults, hills, and valleys in the brittle crust. Deep under the landscape, cracks allow magma from the mantle to well up and interact with groundwater. The water, emerging as springs, contains chemical clues--isotopes of helium and carbon, as well as the concentration of carbon dioxide--that can help scientists track the activity of fluids and gases from deep within the mantle.

Such signs are common in springs hotter than 30 degrees Celsius, where such mixing is known to occur, says geology graduate student Dennis Newell of the University of New Mexico in Albuquerque. But now, Newell and a team of researchers working in New Mexico, Colorado, and Arizona have found geochemical signals of mantle gases mixing into quieter, often-ignored springs that are below 20 degrees Celsius. "We didn't expect to see that at all," Newell says.

In the cool waters, Newell's team observed telltale high ratios of helium-3, a primordial isotope of helium that can only come from deep inside the Earth, to helium-4, which is produced abundantly at the Earth's surface by the radioactive decay of uranium and thorium isotopes. The team also found high concentrations of carbon dioxide in the springs, another sign of mantle interaction, the team reports in the December issue of GSA Today.

The data suggest widespread mantle influence on groundwaters throughout the western U.S., Newell says--and that may help explain other unusual phenomena in groundwater chemistry, including high levels of arsenic, sulfate, and chloride. But hydrologist Fred Phillips of New Mexico Institute of Mining and Technology in Socorro says it may be tricky to prove conclusively that arsenic from mantle is lacing groundwater. "Some component may be mantle-derived," he adds, "but there's a lot of work to do."

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