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Helium isotopes point to the best sources of geothermal energy

December 03, 2007

TEMPE, Ariz. - With fossil fuel sources depleting and global warming on the rise, exploring alternative means of power for humans is a necessary reality. Now, looking to the sky, relying on the wind or harnessing water power are not the only remaining options. Deep within Earth is an untapped source of energy: geothermal energy.

It has been estimated that within the continental United States, there is a sizable resource of accessible geothermal energy - about 3,000 times the current annual U.S. consumption.

Two important reasons this storehouse of energy has not been tapped is that locating the specific energy hot spots is difficult and expensive.

"Since many geothermal resources are hidden, that is, they do not show any clear indications of their presence at the surface, locating them by just using observations made at the surface is difficult," explains Matthijs van Soest, associate research professional at the Noble Gas Geochemistry and Geochronology Laboratory within the School of Earth and Space Exploration at Arizona State University.

"Often when people thought there might be a geothermal resource below the surface the only way to determine if their assumption was correct was drilling and drilling is extremely expensive," he says.

Now, research by van Soest and B. Mack Kennedy at Lawrence Berkeley National Laboratory reveals that geothermal exploration doesn't have to be high-priced.

And it doesn't even have to require drilling.

"We wanted to show that certain surface indicators, specifically the ratio of helium isotopes, can be used to identify areas with high resource potential for geothermal energy," says van Soest, co-author of a research report that appears in the Nov. 30 issue of the scientific journal Science.

Different parts of the Earth are composed of a variety of elements in varying amounts. Earth's crust contains a variety of noble gases, one of those being helium. Natural helium occurs as two isotopes, helium-4 (4He) and helium-3 (3He.) Typically, helium-4 is more abundant in Earth's crust, whereas helium-3 is more abundant in the mantle below. Thus, the helium-3/helium-4 ratio of the gas found in groundwater can provide an indication of the extent to which the water has interacted with volcanic rocks derived from the mantle.

Waters that have equilibrated only with crustal rocks typically have low helium-3/helium-4 ratios, but Kennedy and van Soest found that some waters from hot springs near the Dixie Valley geothermal power plant in Nevada contained anomalously high ratios.

"When we found the elevated ratios, we knew that the only way these waters could be enriched with helium-3 was if they had interacted with fluids from the Earth's mantle," explains van Soest. "The area directly surrounding the power plant has about two to three times the values found elsewhere in the region."

The analysis of samples taken from more than 60 features (mostly from hot springs and shallow wells) in the northern Basin and Range showed that other areas with characteristics similar to those of Dixie Valley - higher 3He/4He ratios - could be very favorable for geothermal development.

That wasn't the only trend the researchers discovered. They uncovered a correlation between the helium ratios and deformation.

The area they studied is within the Basin and Range physiographic province of the western United States, where the crust has a history (over the last few tens of millions of years) of extending. The distribution of anomalously high helium-3/helium-4 ratios correlates well with areas of higher extensional deformation rate.

"This suggests that, as deformation increases, fluids circulate more deeply into the Earth, thus scavenging up more of the mantle helium," van Soest says. "Areas where we can sample fluids near the surface provide a way of getting a relatively cheap and easy indication of what's happening deep down. Applying what we know about the helium ratios makes the exploration for geothermal resources cheaper and faster."

Arizona State University