A History of Geothermal Energy Research and Development in the ...

GEOLOGICAL TECHNIQUE DEVELOPMENT / 5
5.6 Duration and Age of Hydrothermal Activity
The presence of young volcanic rocks (< 1 my in age) was found to be among the
best diagnostic criteria for locating potential geothermal resources. From this,
researchers inferred that geothermal activity typically spanned long time periods. 167
More recent studies, however, suggested that geothermal activity was frequently
episodic and that individual pulses could be relatively short-lived.
Sinter deposits at Roosevelt Hot Springs and Steamboat Hot Springs were
dated. 168-169 Carbon-14 ages of organic material encapsulated in the deposits were
much younger than volcanic rocks at the same locations. Dates obtained on the
basal portions of two sinters at the Opal Mound at Roosevelt Hot Springs yielded
ages of 1,630 ± 90 and 1,920 ± 160 y BP. Dating of a 15-meters (49-feet) thick
sequence of sinter deposits at Steamboat Hot Springs suggested that activity
began at approximately 11,493 ± 70 y BP. An important observation was that
the transformation of amorphous silica to quartz may have occurred very rapidly,
within a few thousand years, and that this transformation could not be used to
estimate the age of hot spring activity.
Other studies constrained the ages of geothermal activity in two volcanic systems—
Tiwi, Philippines and Karaha-Telaga Bodas, Indonesia. 163/170 Argon 40/39
( 40 Ar/ 39 Ar) spectrum dating of vein adularia from Tiwi, when combined with fluidinclusion
temperatures, documented a complex thermal history that began with
adularia deposition between approximately 314 and 279 ka. 170 Between 279 and
200 ka, temperatures of at least 300°C were reached intermittently in response to a
short-term (e.g. 20,000 y) thermal pulse(s) or were maintained by slow cooling. A
long period of quiescence lasting from 200 to 50 ka followed. The present system is
no older than 10 to 50 ka. It is significant that fracture zones, which are permeable
during the early phase of geothermal activity, also control fluid flow today.
14
C ages of altered lake beds in the volcanic-hosted system at Karaha-Telaga Bodas
also documented recent thermal activity. In this case, hydrothermal and magmatic
activity was found to be no older than approximately 5,910 ± 76 y. 163 Similarly, recent
heating events were suggested by preliminary 40 Ar/ 39 Ar spectrum dating of rocks from
Coso, indicating that the present temperatures may not have persisted for more than
10 ka. 171 In another instance, young thermal pulses at the Salton Sea were implied
by zircons with uranium-thorium (U-Th) ages between 30 ± 13 and 9 ± 7 ka. 172
DOE scientists used geochemistry in the exploration stages of a geothermal
program to estimate reservoir temperatures, locate permeable fault zones, trace fluid
sources, and evaluate past fluid temperatures and salinities. Toward these goals, the
trace-element, stable-isotope and noble-gas compositions of soils, rocks and fluids
were analyzed. Changes in the thermal and geochemical conditions within reservoirs
were monitored by integrating mineral distributions with microthermometric
measurements on the fluid inclusions trapped within these minerals.
A History of Geothermal Energy Research and Development in the United States | Exploration 61