Geothermal Resources:
Isotopes in Geothermal Research

Collaborators

(1) Prof. Donald J. DePaolo, Univesity of California, Berkeley and Lawrence Berkeley National Laboratory.

(2) D. Shuster, Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory.

(3) Alfred H. Truedsell, Consultant, Menlo Park, CA.

(4) Dr. J. N. Moore, Energy and Geoscience Institute, Salt Lake City, UT.

(5) Kathy Janik, U.S. Geological Survey, Menlo Park, CA.

(6) Dick Benoit, Oxbow Power Servies Inc., Reno, NV.

(7) Prof. Zhao Ping, Chinese Academy of Sciences, Beijing, P.R. China

(8) Dr. Mayflor Ramos-Candelaria, PNOC Energy Development Corporation, Philippines

An integrated isotopic approach can provide needed information to geothermal industry regarding:

• The sources of geothermal fluids and heat
• The spatial distribution of fluid types
• Subsurface fluid low directions and paths
• The physio-chemical processes effecting fluid composition, e.g. water-rock reaction paths and rates and
• The temporal evolution of geothermal systems.

The isotopic compositions of elements in geothermal fluids provide a quantitative measure of material balance and can be applied to fluid samples from production wells, hydrothermal and non-thermal springs, fumaroles, etc., fluid inclusions in rocks and minerals, as well as the host rocks and minerals themselves.

The isotopic compositions of elements in a fluid moving through the crust will be modified in space and time in response to varying chemical and physical parameters and/or by mixing with other fluids. During this process, elements will either be conserved, thus preserving isotopic information related to initial conditions and sources, or modified in a fashion that is diagnostic of chemical reactions along a flow path.

A. Conservative Elements:

• Noble gases (He, Ne, Ar, Kr, and Xe) are definitive tracers for heat and fluid sources. For instance, the isotopic composition of helium in the Earth's mantle is ~1000 times greater than that in the Earth's crust.

(³He/⁴He)Mantle = ~10^-5
(³He/⁴He)Cruat = ~ 10^-8

B. Non-Conservative or Reactive Elements:

• Fractionating elements (H, C, O, and N) are elements whose isotopic composition can be modified by chemical reactions involving the breakdown of chemical bonds, electron transfer, etc. or by phase change (e.g. boiling, condensation). These elements can provide useful information regarding the source of recharge (meteoric) fluids, water/rock ratios, chemical equilibration temperatures, etc.
• Non-fractionating elements (e.g. Sr, Nd, Pb, U, etc) are too heavy for chemical reactions or phase changes to have a significant impact on their isotopic compositions (also any natural fractionation that has occurred will be lost to normalization procedures used in the isotopic measurement). However, their isotopic compositions in fluids can be altered by fluid-rock exchange via dissolution/precipitation. The evolution of their isotopic compositions in fluids can provide insight into chemical reaction rates and paths and fluid flow directions and velocities.

I. Using Isotopes to Determine Fluid Velocities and Flow Pathways From Water-Rock Interactions

Isotopic measurements are increasingly used for hydrologic characterization in groundwater systems. Values of d¹⁸O and dD vary with time and space in the precipitation at recharge areas, and the isotope ratios of dissolved elements (e.g. C, S, Sr, Nd, Pb, U) are influenced by the rock the waters pass through. The resulting isotopic contrasts give rise to spatial and/or temporal patterns in isotope ratios that contain information about fluid flow paths, water-rock interaction, and mixing relationships. This approach has been used in hydrologic studies from the catchment to the regional scale.

The isotope ratios of "non-fractionating" elements (e.g. Sr, Nd, Pb) in solution deposited minerals are always equal to those of the parent water; thus directly recording water conditions at the time of precipitation. On the other hand, isotope ratios of "fractionating" elements (e.g. H, C, O) record the isotope ratios of the parent fluid, modified by temperature-dependent fractionation.

The isotopic ratios of both non-fractionating and fractionating elements in groundwater at any point along a fluid flow path will be the product of the isotopic composition of the original fluid and that of the solute acquired from the host rock. Therefore, isotope ratio measurements, either on sampled groundwater or water-deposited minerals, yield spatial or temporal patterns that contain information concerning groundwater flow and transport conditions.

Figure 1: The figure on the left (1a) is a schematic representation of a "fast path" or preferential flow zone as revealed by an isotope exchange front propagating through an aquifer. The exchange front propogates down-gradient and moves faster in zones of higher conductivity (faster fluid flow). In the figure on the left (1b), fluid conductivity contours for the Snake River Plain Aquifer are mapped by strontium isotopic composition contours. The aquifer (low ⁸⁷Sr/⁸⁶Sr) is recharged by groundwater coming of the mountains (high ⁸⁷Sr/⁸⁶Sr) in the northwest.

Radiogenic helium (⁴He) in groundwater is a special case for the reaction-transport models. Unlike other element systems, the radiogenic ⁴He accumulates in the fluid phase because the flux of helium from the fluid back to the host rock is neglible (helium is chemically inert). And in steady-state, the ⁴He accumulation rate does not depend on water-rock reaction rates, but is equivalent to the whole rock production rate which is readily calculated from the host rock U and Th concentrations. Therefore, along a fluid flow path a helium concentration gradient will be established which provides an estimate of fluid age and velocity.

As fluid containing mantle helium flows through the crust, the elevated ³He/⁴He ratio (elevated because of the mantle ³He contribution) will become diluted with the accumulating radiogenic ⁴He produced locally in the host rocks. The resulting gradient in helium isotopic composition along the fluid flow path will be a function of the fluid velocity and ⁴He-production rate.

Helium isotopic compositions in fluids associated with the San Andreas Fault in central California vary from ~0.1 Ra to 4.0 Ra (Figure 2). Assuming the elevated ³He/⁴He ratios reflect a flux of mantle volatile through the fault zone and scaling to the thickness of the brittle crust, fluid flow rates through the fautl of 1-10 mm/yr have been estimated (provide link to Link #2).

Figure 2: Helium isotopic composition (R) normalized to the ratio in air (Ra) plotted as a function of approximate distance from the main strike of the SAF. Associated fluid flow rates (q) and mantle contributions are also indicated.

II. Noble Gas Concentrations and Isotopic Abundances as Natural Tracers for Fluid Origins and Reservoir Processes

The noble gases are excellent natural tracers for fluid origins and reservoir processes:

1. They are chemically inert and, therefore, conserved in water-rock systems.

2. There are five stable noble gas elements (He, Ne, Ar, Kr, and Xe) providing 23 different isotopes spanning a mass range from 3 to 136 amu.

3. They are moderately soluble in water and their solubility in water as a function of temperature is known up to the critical point for water.

4. There are three natural terrestrial "reservoirs" (mantle, crust, atmo-hydrosphere) each characterized by a distinct noble gas isotopic and elemental composition.

Natural Terrestrial Reservoirs and Important Isotopic Distinctions

A. The Earth's Mantle: The Earth's mantle is believed to be partially or non-degassed and therefore is enriched in an indigenous or primordial noble gas component that was inherited from meteoritic material through accretion during Earth formation. The noble gas composition of the present day mantle reservoir is defined by the compositions of Mid-Ocean Ridge (MORB) and Ocean Island Basalts (OIB):

• ³He/⁴He = 1.2 Ð 4.8 x 10^-5 [9 (MORB)-32 (OIB) times the ratio in air]
• ⁴He/³⁶Ar >>> the ratio in air
• ⁴⁰Ar/³⁶Ar > 40,000 (>135 times the ratio in air)
• Radiogenic ⁴⁰Ar/⁴He ~ 0.27 (function of the mantle K/(U+Th) ratio)
• Solar neon + Nucleogenic ²¹Ne [from ¹⁸O(a,n)²¹Ne nuclear reactions]
• ³He/CO₂ ~ 5 x 10-10

B. The Earth's Crust: The processes involved in the formation of Earth's continental and oceanic crusts result in a general enrichment of incompatible lithophile elements such as K, U and Th, a depletion of noble gases, and therefore a concomitant enrichment in the K/Ar, and (U+Th)/He ratios. Therefore old crustal materials tend to be enriched in radiogenic (produced by decay of naturally occurring radioactive isotopes, e.g. ⁴⁰K, and the isotopes of U and Th) and nucleogenic (produced by nuclear interactions induced by natural radioactivity) noble gases. A theoretical composition of crustal noble gases can be calculated from nuclear theory using an average crustal elemental composition. The theoretical calculations have been confirmed by noble gas isotope and abundance measurements of hydrocarbon fluids from crustal regions void of any recent magmatic/volcanic/mantle involvement.

• ³He/⁴He ~ 3 x 10^-8 (~0.02 times the ratio in air)
• ⁴He/³⁶Ar = depends on the age of the crustal source
• ⁴⁰Ar/³⁶Ar = depends on the age of the crustal source
• Radiogenic ⁴⁰Ar/⁴He
• Nucleogenic ²¹Ne [produced from ¹⁸O(a,n)²¹Ne nuclear reactions]
• Nucleogenic ²¹Ne/⁴He ~ 5 Ð10 x 10^-8

C. The Atmo-Hydrosphere: To first order the composition of the atmo-hydrosphere represents an integrated mixture of noble gases degassed from the mantle and crust over 4.5 billion years of Earth history. The one exception is the atmospheric abundance of helium. Because helium is very light, it's isotopes are not bound by the Earth's gravitational field. Therefore, the atmosphere is strongly depleted with respect to helium relative to the other noble gases. By convention the elemental and isotopic composition of noble gases in the atmopshere are used as a standard for normalizing noble gas data. The concentrations of noble gases dissolved in water in equilibrium with air are determined by their temperature dependent solubility. Temperature dependent noble gas solubilities in water have been experimentally determined up to the critical point.

• ³He/⁴He = 1.4 x 10^-6
• ⁴He/³⁶Ar = 0.167 (<< crust or mantle)
• ⁴⁰Ar/³⁶Ar = 296
• Noble gas concentrations in water determined by temperature dependent solubility (Xe>Kr>Ar>Ne~He)

The influence of these three distinct reservoirs on the composition of noble gases in geothermal reservoirs is summarized in the figure below. Helium isotopic compositions (R) are given in units of Ra, the ³He/⁴He ratio in air. Helium relative abundances are given as F(⁴He) values, which are fractionation factors relative to atmospheric composition: F(⁴He) = (⁴He/³⁶Ar)_reservoir/(⁴He/³⁶Ar)_air.

Figure 3: A geothermal reservoir fluid contains noble gases from a variety of sources. Each source is characterized by a unique noble gas composition and contributions to a geothermal reservoir can be easily identified. For instance, magmatic fluids have large ⁴He excesses [F(⁴He) >>> ASW (air saturated water)], with a ⁴He/⁴He ratio (R) up to 9 times the ratio in air (Ra), and Heat to He ratios (Q/³He) of ~ 2 Joules/cc. Whereas, the helium in crustal fluids will have a ³He/⁴He ratio of ~0.02 Ra and Q/³He ~ 1000 Joules/cc.

Geothermal reservoirs recharged by natural meteoric waters will have noble gas concentrations and relative abundances equivalent to air-saturated water (ASW). Flashed brines used for re-injection into the system can be easily identified by noble gas concentrations up to ~100 times lower than the natural recharge waters.

Heat and Helium

Geothermal systems hosted in continental crust exhibit a wide range in ³He/⁴He ratios. There is evidence that the range may reflect variations in the contributions from different heat sources.