Page 120 - Geothermal Energy Systems Exploration, Development, and Utilization

96 2 Exploration Methods
in the kinetics of the 18 O exchange between minerals and water. Therefore, 18 O
enrichments in Na–Cl geothermal liquids with respect to meteoric waters have
◦
been traditionally considered as indicators of high temperatures (>150 C) in the
geothermal reservoirs of provenance (Truesdell and Hulston, 1980). However, the
18 O shifts depend also on the initial isotope composition of the liquid and solid
phases involved in the exchange process as well as on the water/rock ratio, or on
system dynamics (Giggenbach, 1991b). A second type of processes affecting the δD
18
and δ O values of geothermal liquids is mixing between local meteoric waters and
waters of different origin, such as marine waters; high-salinity waters, or connate
waters, especially in sedimentary basins and magmatic waters (Horita, Cole, and
Wesolowski, 1995). Steam separation (boiling) brings about isotopic fractionation,
with heavy isotopes concentrating in the liquid (at any temperature for 18 Oand
◦
at temperatures below 229 C for deuterium in a pure water system), whereas the
vapor phase is obviously depleted in heavy isotopes.
Deuterium, oxygen 18, and tritium contents of water are useful tracers of the
meteoric origin of the water and the marine origin of the salinity in the saline
thermal waters (Arnorsson, 2000). Carbon 14 is a good dating tool for geothermal
systems in the granite terrain if sufficient dissolved carbon is available for analysis.
The residence time of the geothermal fluid is defined as the time since it was last
isolated from atmosphere. The estimation of fluid ages permits to constrain flow
rates through the system. The presence of tritium in the thermal water is an
indicator of dilution by the young shallow groundwater. However, it seems not to
be a good dating method because the age of geothermal waters is usually far beyond
the limit (about 100 years) of the tritium method. Sulfur and oxygen isotopes of
dissolved sulfate are good tracers of the origin of the salinity in the case of marine
sulfate.
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 (Richet,
Bottinga, and Javoy, 1977). This approach has been used in hydrologic studies from
the catchments to the regional scale.
Nonfractionating elements (e.g., Sr, Nd, Pb, and U) are too heavy for chemical re-
actions or phase changes to have a significant impact on their isotopic compositions
(also any natural fractionation that has occurred will be lost to normalization pro-
cedures 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 chem-
ical reaction rates and paths and fluid flow directions and velocities. The isotope
ratios of ‘‘nonfractionating’’ elements (e.g., Sr, Nd, and 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, and O) record the isotope ratios of the parent
fluid, modified by temperature-dependent fractionation.