TY - JOUR
T1 - A helium isotope perspective on the Dixie Valley, Nevada, hydrothermal system
AU - Kennedy, B. Mac
AU - van Soest, Matthijs C.
N1 - Funding Information:
The authors would like to thank Stu Johnson and Dick Benoit for their assistance and support, particularly with field logistics and access to relevant field data. Thanks are also due to Dave Shuster for assistance with sample collection and analysis, and to Dave Hilton and Dave Blackwell for reviews. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences and Office of Geothermal Technologies, under contract DE-AC03-76SF00098.
PY - 2006/2
Y1 - 2006/2
N2 - Fluids from springs, fumaroles, and wells throughout Dixie Valley, NV were analyzed for noble gas abundances and isotopic compositions. The helium isotopic compositions of fluids produced from the Dixie Valley geothermal field range from 0.70 to 0.76 Ra, are among the highest values in the valley, and indicate that ∼7.5% of the total helium is derived from the mantle. A lack of recent volcanics or other potential sources requires flow of mantle-derived helium up along the valley bounding Stillwater Range Front Fault, from which the geothermal fluids are produced. Using a one-dimensional flow model, a lower limit fluid flow rate up through the fault of 7 mm/yr is estimated, corresponding to a mantle 3He flux of ∼104 atoms m-2 s-1. A comparison between the fluids from Dixie Valley springs, fumaroles, and wells and the fluids produced from the geothermal field reveals a mixing trend between the geothermal fluid and younger, cooler groundwaters. The exceptions are those features that either emanate directly from the Stillwater fault or wells that penetrate and extract fluids from the fault zone, all of which have helium isotopic compositions that are indistinguishable from the geothermal production fluids. The results of our study indicate that the Stillwater Range Front Fault system must act as a permeable conduit that can sustain high vertical fluid flow rates from deep within the crust and crust-mantle boundary and that high permeability may exist along most of its length. This suggests that the geothermal potential of the Stillwater fault may be significantly greater than the 6-8 km long system presently under production. Since all the numerous springs, wells, and fumaroles in the valley also contain a fluid component that is indistinguishable from the geothermal/Stillwater fault fluid, the potential for an additional deeper and more pervasive geothermal system also exists and should be further evaluated. Furthermore, we suggest that elevated helium isotope compositions in regions with little or no recent magmatism are an indicator of the deep crustal permeability that is required to drive and sustain extensional geothermal systems.
AB - Fluids from springs, fumaroles, and wells throughout Dixie Valley, NV were analyzed for noble gas abundances and isotopic compositions. The helium isotopic compositions of fluids produced from the Dixie Valley geothermal field range from 0.70 to 0.76 Ra, are among the highest values in the valley, and indicate that ∼7.5% of the total helium is derived from the mantle. A lack of recent volcanics or other potential sources requires flow of mantle-derived helium up along the valley bounding Stillwater Range Front Fault, from which the geothermal fluids are produced. Using a one-dimensional flow model, a lower limit fluid flow rate up through the fault of 7 mm/yr is estimated, corresponding to a mantle 3He flux of ∼104 atoms m-2 s-1. A comparison between the fluids from Dixie Valley springs, fumaroles, and wells and the fluids produced from the geothermal field reveals a mixing trend between the geothermal fluid and younger, cooler groundwaters. The exceptions are those features that either emanate directly from the Stillwater fault or wells that penetrate and extract fluids from the fault zone, all of which have helium isotopic compositions that are indistinguishable from the geothermal production fluids. The results of our study indicate that the Stillwater Range Front Fault system must act as a permeable conduit that can sustain high vertical fluid flow rates from deep within the crust and crust-mantle boundary and that high permeability may exist along most of its length. This suggests that the geothermal potential of the Stillwater fault may be significantly greater than the 6-8 km long system presently under production. Since all the numerous springs, wells, and fumaroles in the valley also contain a fluid component that is indistinguishable from the geothermal/Stillwater fault fluid, the potential for an additional deeper and more pervasive geothermal system also exists and should be further evaluated. Furthermore, we suggest that elevated helium isotope compositions in regions with little or no recent magmatism are an indicator of the deep crustal permeability that is required to drive and sustain extensional geothermal systems.
KW - Basin and Range
KW - Dixie Valley
KW - Geothermal
KW - Helium isotopes
KW - Magmatic
KW - Noble gases
KW - Permeability
UR - http://www.scopus.com/inward/record.url?scp=32644432825&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=32644432825&partnerID=8YFLogxK
U2 - 10.1016/j.geothermics.2005.09.004
DO - 10.1016/j.geothermics.2005.09.004
M3 - Article
AN - SCOPUS:32644432825
SN - 0375-6505
VL - 35
SP - 26
EP - 43
JO - Geothermics
JF - Geothermics
IS - 1
ER -