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Exploration Technique: Thermochronometry

Exploration Technique Information
Exploration Group: Geochemical Techniques
Exploration Sub Group: Geochemical Data Analysis
Parent Exploration Technique: Geochemical Data Analysis
Information Provided by Technique
Thermal: Thermal history of area, rate of cooling, age that minerals reached closure temperature
The study of the thermal evolution of a mineral, rock, or geologic region using radiometric dating of two or more different minerals which have different closure temperatures
Other definitions:Wikipedia Reegle

Thermochronometry involves comparing the radiometric dates of two or more minerals with different closure temperatures. The closure temperature is when the crystal structure of a mineral has formed and cooled sufficiently to prevent diffusion of isotopes. At this point the mineral begins to display measurable radioactive decay. For radiometric dating when the mineral reaches the closure temperature that is when the clock starts. In other words, the age of the rock is calculated based on the point in time that it reached the closure temperature. Different minerals have different closure temperatures, so dating the different minerals within the same rock can give information about the thermal history of the rock (i.e., rate at which the rock cooled). This is known as thermochronometry.

There are many isotopes and minerals that can be used for radiometric dating and therefore many different ways to perform a thermochronology study. Some examples of thermochronology methods used in geothermal studies are: 40Ar/39Ar dating in the minerals hornblende, muscovite, biotite, and k-feldspar[1][2][3], (U-Th)/He dating of apatite, zircon, sphene, and fluorite.[4][3], and U-Pb dating of garnet, zircon, allanite, monzanite, and sphene.[5][3]

There are multiple techniques for capable of providing thermochronometry data in geothermal areas. The U-Pb methods in green determine the ages of high temperature minerals (500-950°C), the 40AR/39AR methods in red have application in medium temperature minerals (150-550°C), and the (U-Th)/He methods cover the low temperature range (40-220°C).[3]

Use in Geothermal Exploration
A study conducted by Gorynski et al. 2010[4] using (U-Th)/He dating of apatite and zircon from samples from the Wassuk Range has identified the presence of a known geothermal anomaly. By using (U-Th)/He dating and Thermochronometry methods it is claimed that complexities and unknowns in subsurface thermal evolutions can be resolved which can play an important role in lowering geothermal exploration uncertainties and risks. (U-Th)/He dating of zircon in conjunction with apatite has enabled researchers to deduce thermal histories of the upper ~8 km of the crust.[4] The youngest ages measured by this technique correspond to areas where temperatures were above the closure temperature most recently. The thermochronometry method provides information on the thermal history of samples, the magnitude of thermal perturbation, and the longevity of the thermal structure which may reveal a geothermal resource. Thermochronometry could become a useful tool in exploration for blind geothermal systems as the absence of hydrothermal activity at the surface would not affect the results of a thermochronometry study.

Field Procedures
Thermochronometry begins with developing strategic plan for rock sample collection locations within the study area. The location of the samples is very important when determining the thermal history of the geothermal area and tracing the data back to a potential location of a heat source. The samples are then brought back to a lab where radiometric dating can be conducted on minerals within the rocks. For a thermal history to be generated radiometric dating must be conducted using two or more minerals with different closing temperatures. Once a sufficient amount of radiometric dates have been determined the data can be analyzed and modeled to show a thermal history of the area. A location with anomalously young rocks could be an indicator of a geothermal anomaly which has reset the rocks closure temperature the most recently. In known geothermal areas It has been found that the youngest ages correspond to the samples closest in proximity to the geothermal anomaly.[4]

  1. Kurilovitch, L.; Norman, D.; Heizler, M.; Moore, J.; McCulloch, J.. 1/1/2003. 40AR/39AR THERMAL HISTORY OF THE COSO GEOTHERMAL FIELD. Proceedings of (!) ; (!) : PROCEEDINGS, Twenty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 27-29, 2003.
  2. Wells, M.; Sneem L.; Blythe, A.  . 7/10/2000. Dating of major normal fault systems using thermochronology- An example from the Raft River detachment, Basin and Range, western United States. J. Geophys. Res.. (!) .
  3. 3.0 3.1 3.2 3.3 Curtin University. John De Laeter Centre For Isotope Research [Internet]. 2013. Curtin University. Curtin University. [cited 2013/10/28]. Available from:
  4. 4.0 4.1 4.2 4.3 Kyle Gorynski,Danny Stockli,J. Douglas Walker,Andrew Sabin. 2010. Application of (U-Th)/He Thermochronometry as a Geothermal Exploration Tool in Extensional Tectonic Settings: The Wassuk Range, Hawthorne, Nevada. GRC Transactions. 34:685-688.
  5. Terrence Blackburn,Samuel A. Bowring,Blair Schoene,Kevin Mahan,Francis Dudas. 2011. U-Pb Thermochronology: Creating a Temporal Record of Lithosphere Thermal Evolution. Contributions to Mineralogy and Petrology. 162(3):479-500.

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