Page 117 - Geothermal Energy Renewable Energy and The Environment

Exploring for Geothermal Systems 103

Also shown in Figure 6.10 is the distribution of points from a few selected geothermal sites. The
geothermal sites delineated by open circles exhibit a trend that is commonly observed in geothermal
systems. For these sites, the local meteoric waters and the geothermal waters define a trend that
originates on the meteoric water line (MWL), with the geothermal waters displaced horizontally to
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higher δ O with little or no corresponding change in δD. Craig (1963) realized that the most likely
explanation for this behavior was that the geothermal waters must have originated as meteoric
water that had circulated to deeper, hotter regions where the oxygen in the H O molecule exchanged
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with oxygen in the minerals at elevated temperatures. Since the δ O of minerals is generally in the
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range of + 1 to + 10, δ O of meteoric water interacting with these minerals will increase. Since
the amount of oxygen in rocks far exceeds the hydrogen content, little shift in the δD values would
be expected. Giggenbach (1992b) recognized that not all geothermal waters follow this horizontal
trend toward higher δ O values with little or no change in δD. From an extensive suite of analyses,
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Giggenbach showed that geothermal waters associated with volcanic systems that occur at subduc-
tion zones commonly have a trend toward more positive δD as well as the commonly observed
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enrichment in O. He attributed that trend to mixing of water associated with the magmatic degas-
sing of subduction zone volcanic rocks and the resulting interaction with local meteoric fluids.
These results document that the variation in oxygen and hydrogen isotopes observed in geother-
mal systems can reflect competing processes. But, whichever process is dominant at any given site,
these stable isotopes can provide evidence of the presence of a geothermal reservoir at depth, if the
isotopic analyses show significant displacement away from the local meteoric water values. In the
absence of such a displacement, the presence of a significant geothermal resource connected to the
surface via circulating water is problematic.
Helium isotopic studies are another means for identifying potential geothermal resources. In the
case of this isotope system, however, the underlying processes controlling isotopic anomalies are
fundamentally different, and thus provide a different insight into the likely presence of a geothermal
resource.
The primary reservoirs for helium isotopes are the original He that the Earth inherited when it
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formed, and He that accumulates as U and Th decay via emission of alpha particles (which are He
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nuclei). Since the crust has the bulk of the Earth’s U and Th, He accumulates in the continental
crust and its abundance increases over time, progressively enriching the crust and the atmosphere in
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this isotope. The mantle, on the other hand, holds the Earth’s main store of He. Any volatile emis-
sions from the mantle that escape into the crust will increase the local ratio of He to He, and result
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4
in a helium isotope anomaly. Similarly, melts that are generated in the mantle will contain some
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small amount of He and virtually no He. When such magmas rise from the mantle and enter the
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crust, the magma will cool and slowly degas. As the degassed volatiles rise through the crust they
will interact with any local groundwaters, imposing on them a He signature. A survey of helium
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isotopes conducted on well or spring waters where such interactions have occurred will detect an
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anomalously high ratio of He to He if a crystallizing magma is degassing in the nearby subsur-
face or if mantle volatiles are escaping through the crust at that location. The current atmospheric
3 He/ He ratio (designated Ra) is 1.2 × 10 and crustal fluids lacking any mantle influence have
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values of approximately 0.02 Ra. Mantle derived-fluids have values of 6 to 35 Ra (Kennedy and van
Soest 2007), which are readily distinguished from crustal values.
Figure 6.11 depicts the results of a survey of helium isotopes across the western United States
(Kennedy and van Soest 2007), extending from the Cascade Mountains in Oregon to Colorado.
This region encompasses the volcanic mountain system of the Sierras and Cascades that are or were
part of the tectonic regime known as the eastern Pacific subduction zone, as well as the Basin and
Range extensional zone that includes Nevada, Arizona, New Mexico, Utah, and Colorado. The well-
defined peaks that fall at Rc/Ra values greater than 1.0 demark well-defined geographical zones
where mantle-derived helium is reaching the surface. This requires either that recent igneous intru-
sions reside below the surface, or that permeability exists in the deep crust sufficient for volatiles to
migrate in sufficient and persistent quantity to allow sampling at the surface. Such spikes in helium