Surface Gas Sampling
Exploration Technique: Surface Gas Sampling
|Exploration Technique Information|
|Exploration Group:||Field Techniques|
|Exploration Sub Group:||Field Sampling|
|Parent Exploration Technique:||Gas Sampling|
|Information Provided by Technique|
|Hydrological:||Gas composition and source of fluids.|
|Thermal:||Distinguish magmatic/mantle heat inputs. Can be used to estimate reservoir fluid temperatures.|
A geochemical study of some 80 gas samples collected from fumaroles, springs, and wells confirmed that gases at the Valles Caldera, NM are chemically and isotopically similar to those in other volcanic-hosted geothermal systems, and that the gases are in apparent equilibrium at temperatures >200°C . Relative proportions of Ar, He, and N2 were similar to those measured at hot spot locations such as the Yellowstone and Kilauea calderas. He isotope R/Ra values of 4-6 within the caldera were suggestive of mantle/magmatic degassing, whereas R/Ra values of < 0.7 outside the caldera reflect an He input dominated by U/Th decay in crustal rocks. Major gas components of the caldera surface discharges have remained relatively constant over the course of the nearly 20 year sampling program, and generally resemble gas compositions of the geothermal wells. The study also revealed that Valles caldera gases contained relatively little CH4 and N2 compared to other geothermal systems hosted within sedimentary rocks, suggesting that organic carbon and nitrogen in Paleozoic and Miocene strata were depleted during 13 million years of magmatism in the Jemez volcanic field.
- Field Sampling
Fluid samples to be used for dissolved noble gas and tritium analyses can also be collected using a simple copper tube sampling apparatus attached to the tygon feed tube. The copper tube is suspended in an aluminum channel with steel pinch-off clamps. The liquid sample is fed through the copper tube, taking care to remove any bubbles that may have become entrained in the flow. The pinch-off clamps are then closed tightly to seal the copper tube with a cold weld at both ends.
More detailed descriptions of gas sampling procedures commonly used to evaluate geothermal systems are offered by Fahlquist & Janik (1999)  and online at the U.S. Geological Survey Reston Chlorofluorocarbon Laboratory website.
- Encyclopedia of Volcanoes
- Chapter 4: Geochemistry
- Geothermal Waters: A Source of Energy and Metals
- Fraser E. Goff,Cathy J. Janik. 2002. Gas Geochemistry Of The Valles Caldera Region, New Mexico And Comparisons With Gases At Yellowstone, Long Valley And Other Geothermal Systems. Journal of Volcanology and Geothermal Research. 116(3-4):299-323.
- Procedures for Collecting and Analyzing Gas Samples from Geothermal Systems
- Fraser E. Goff,Jamie N. Gardner. 1994. Evolution of a Mineralized Geothermal System, Valles Caldera, New Mexico. Economic Geology. 89(8):1803-1832.
- 3H/3He Sampling
- Sampling and Analysis of Geothermal Fluids
- Fenton Hill HDR Geothermal Area
- International Geothermal Area Mexico
- Jemez Springs Area
- Lassen Volcanic National Park Area
- Lightning Dock Area
- Valles Caldera - Redondo Area
- Valles Caldera - Sulphur Springs Area
- Yellowstone Caldera Geothermal Region