The geochemists believe that Dixie Valley, which lies in an isolated island of elevated helium-3, shows that fracturing of the normally impermeable barrier between the earth's mantle and crust can set up a natural geothermal system. They speculate that, once sheared open, the fractures fill with high-pressure fluids that transport helium to the crust along with lots of heat. The heat sets up a convection cycle in the crust in which heated groundwater rises to the surface, dumps its heat, and then circulates back down for more.

"Dixie Valley is a very productive geothermal field," says Kennedy. "The question now is, are all [such helium] anomalies potential geothermal resources? That would take somebody to go and do some more exploration work, but I would point them to those areas of anomalies first."

MIT chemical engineer Jefferson Tester, who directed the geothermal study for the DOE, says that the helium-3-to-helium-4 ratio "seems to have potential" as a geothermal prospecting tool. But he notes that validating a site for a geothermal project "may always require drilling and hydraulic pressurization in the field."

Other researchers are equally cautious. Albert Genter, scientific coordinator for an enhanced geothermal project in Alsace, France, that's managed by a consortium of European energy companies, points out that helium isotopes might not be predictive in less "geodynamic" regions such as Alsace. (See "Tapping Rocks for Power.") "The transposition of the results is not obvious," says Genter.

Kennedy agrees and says that the next step is to expand the survey of helium isotopes. .That may be difficult in the short term. For one thing, measuring helium isotopes is costly. Kennedy and van Soest accurately measured the faint helium-3 signals in their samples by cleaning out their equipment with a vacuum system one thousand times stronger than those normally used with mass spectrometers. "You have to have a very tight, clean system," says Kennedy. That and other steps to eliminate background noise cost $2,000 to $3,000 per sample, although Kennedy says that automation could bring the cost well below $1,000 per sample.

Even that looks expensive in light of the DOE's current budget for geothermal research: $0.