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Topic Name: Researcher discover Helium Isotopes Point to New Sources of Geothermal Energy
Category: BioFuels
Research persons: Mack Kennedy, Matthijs van Soest
Location: Lawrence Berkeley National Laboratory, DOE, United States
Details
In a survey of the northern Basin and Range province of the western United
States, geochemists Mack Kennedy of the Department
of Energy's Lawrence Berkeley National
Laboratory and Matthijs van Soest of Arizona
State University have discovered a new tool for identifying potential
geothermal energy resources.
Currently, most developed geothermal energy comes from regions of volcanic
activity, such as The Geysers in Northern California. The potential resources
identified by Kennedy and van Soest arise not from volcanism but from the flow
of surface fluids through deep fractures that penetrate the earth's lower crust,
in regions far from current or recent volcanic activity. The researchers report
their findings in the November 30, 2007 issue of Science.
"A good geothermal energy source has three basic requirements: a high
thermal gradient — which means accessible hot rock — plus a rechargeable
reservoir fluid, usually water, and finally, deep permeable pathways for the
fluid to circulate through the hot rock," says Kennedy, a staff scientist
in Berkeley Lab's Earth Sciences Division. "We believe we have found a way
to map and quantify zones of permeability deep in the lower crust that result
not from volcanic activity but from tectonic activity, the movement of pieces of
the Earth's crust."
Kennedy and van Soest made their discovery by comparing the ratios of helium
isotopes in samples gathered from wells, surface springs, and vents across the
northern Basin and Range. Helium-three, whose nucleus has just one neutron, is
made only in stars, and Earth's mantle retains a high proportion of primordial
helium-three (compared to the minuscule amount found in air) left over from the
formation of the solar system. Earth's crust, on the other hand, is rich in
radioactive elements like uranium and thorium that decay by emitting alpha
particles, which are helium-four nuclei. Thus a high ratio of helium-three to
helium-four in a fluid sample indicates that much of the fluid came from the
mantle.
High helium ratios are common in active volcanic regions, where mantle fluids
intrude through the ductile boundary of the lower crust. But when Kennedy and
van Soest found high ratios in places far from volcanism, they knew that mantle
fluids must be penetrating the ductile boundary by other means.
The geology of the region was the clue. The Basin and Range is characterized
by mountain ranges that mostly run north and south, separated by broad,
relatively flat-floored valleys (basins), which are blocks of crust that have
sunk and become filled with sediment eroded from the uplifted mountains. The
alternating basin and range topography is the result of crustal spreading by
east to west extension, which has occurred over the past approximately 30
million years. The Earth's crust in the Basin and Range is some of the thinnest
in the world, resulting in unusually high thermal gradients.
The faces of mountain blocks in the Basin and Range clearly exhibit the
normal faults that result as the blocks are pulled apart by the extension of the
crust. Normal faults form high-angle pathways deep down into the brittle upper
crust. But as the fault plane approaches the ductile lower crust, changes in the
density and viscosity of the rock refract the principle stress acting on the
fault, deflecting the fault plane, which becomes more horizontal. It is from
these deep, horizontally-trending faults that Kennedy thinks permeable
passageways may emanate, penetrating the ductile boundary into the mantle.
One of the most seismically active areas in the Basin and Range occurs in
what is called the central Nevada seismic belt. The researchers' detailed
studies in this area, notably at the Dixie Valley thermal system next to the
Stillwater range, established that the highest helium ratios were restricted to
fluids emerging from the Stillwater range-front fault system.
The northern Basin and Range, which Kennedy and van Soest surveyed on behalf
of DOE's Office of Basic Energy Sciences and Office of Geothermal Technologies,
includes parts of California, Nevada, Oregon, Idaho, and Utah. In their survey
the researchers mapped the steady progression from low helium ratios in the east
to high ratios in the west. The distribution of the increasing ratios
corresponds remarkably with an increase in the rate and a change in the
direction of crustal extension, which shifts from an east to west trend across
the Basin and Range to a northwest trend.
This change in rate and direction reflects the added shear strain induced by
the northward movement of the Pacific Plate past the North American Plate.
Kennedy and van Soest believe that the added component of shear strain and
increasing extension rate tear open fluid pathways through the ductile lower
crust, into the mantle. The high helium isotope ratios they found, indicating
potential new sources of geothermal energy, were superimposed upon the general
background trend: anomalously high ratios map zones of higher than average
permeability.
"We have never seen such a clear correlation of surface geochemical
signals with tectonic activity, nor have we ever been able to quantify deep
permeability from surface measurements of any kind," says Kennedy. The
samples they collected on the surface gave the researchers a window into the
structure of the rocks far below, with no need to drill.
With the urgent need to find energy sources that are renewable and don't emit
greenhouse gases, geothermal energy is ideal — "the best renewable energy
source besides the sun," Kennedy says. Accessible geothermal energy in the
United States, excluding Alaska and Hawaii, has been estimated at 9 x 1016
(90 quadrillion) kilowatt-hours, 3,000 times more than the country's total
annual energy consumption. Determining helium ratios from surface measurements
is a practical way to locate some of the most promising new resources.
Pictures overview
In figure 1, Berkeley Lab geochemist B. Mack Kennedy used this mass
spectrometer (foreground) to determine helium isotope ratios in samples of
surface fluids from the northern Basin and Range.
In figure 2, Arizona State geochemist Matthijs van Soest samples surface
water in the northern Basin and Range. The sample is collected without direct
exposure to air and stored in the copper tube, foreground left, which will be
sealed by crimping.
About Researchers
B. Mack Kennnedy
Scientist
Geochemistry Department
Phone: 510-486-6451
Fax: 510-486-5686
Email: bmkennedy@lbl.gov
Research Interests:
Application of isotopic systematics to terrestrial problems related to our understanding of the origin, history, and current processes of the earth and solar system. Specifically, noble gas isotope geochemistry as applied to sources, geochemical evolution, transport processes, and flow rates of fluids in the crust, cosmogenic and radiometric dating techniques, tectano-magmatic processes, and paleoclimatology. Research areas of application include geothermal energy, fossil energy, and basic energy sciences. Specific titles are: (1) Integrated isotopic studies of geochemical processes; (2) Isotopic and chemical composition of fault zone fluids; (3) Geochemical and isotopic constraints on processes in oil hydrogeology; and (4) Isotope geochemistry in geothermal research.
Education:
Ph.D., Earth and Planetary Sciences, Washington Univ., St. Louis, Missouri, 1981.
B.A., Earth and Planetary Sciences, Washington Univ., St. Louis, Missouri, 1974.
Matthijs van Soest
Research Associate
Geochemistry Department
Phone: 510-486-5659
Fax: 510-486-5496
email: mcvansoest@lbl.gov
Research Interests
The application of geochemical tracer and isotope techniques to problems in natural systems as a means to furthering our understanding of the EarthÕs geological system and processes. With the main focus on noble gas isotope and abundance systematics of magmatic, geothermal, and hydrocarbon systems. Of particular importance are the characterization of: (a) the specific sources of noble gases in these systems, (b) the transport mechanisms of noble gases in these systems, and (c) the times scales on which processes in these different systems take place.
Education
Ph.D. Sediment subduction and crustal contamination in the Lesser Antilles island arc. Isotope Geochemistry, Vrije Universiteit, Amsterdam, The Netherlands, 2000.
B.A. (Doctoral Degree) Petrology and Isotope Geochemistry, Vrije Universiteit, Amsterdam, The Netherlands, 1994.
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