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Basic Principles

Fission track thermochronology and (U-Th)/He thermochronology are methods for reconstructing the time-temperature histories of rock samples in the shallow parts of the Earth’s crust. Although the two methods are quite different from each other, they yield highly complementary data and are best applied together for most application.

Uranium and thorium are naturally concentrated in certain minerals, notably apatite and zircon, that are the main targets for FT, He, and U-Th-Pb analyses. U and Th nuclei experience chain decay, forming Pb isotope daughter nuclei, as well as emitting alpha particles (4He nuclei). 238U nuclei alternatively very rarely experience decay by spontaneous fission, forming two high velocity fission fragments, which create a single heavy ion damage trail (fission track) in the crystal lattice.

U-Pb, He, and FT dating exploit these various decay products. Pb isotopes are relatively immobile in many minerals, such as zircon, and are powerful tools for dating rock crystallization and high-temperature cooling (e.g., SHRIMP-RG). In contrast, 4He is relatively mobile and fission tracks are relatively unstable at low temperatures and can be used to date cooling through their respective partial retention temperature windows.

Apatite is the most useful mineral for FT and He studies. The apatite He and FT sensitivity windows partly overlap at about 45-85°C and 60-125°C respectively.  Assuming a typical geothermal gradient of 25°C/km and an Earth surface temperature of 15°C, this corresponds to nominal depths of 1.2-2.8 and 1.8-4.4 km in the Earth’s crust. These methods may therefore be used to investigate a wide variety of geologic phenomena that affect the upper crust, including mountain building, large-scale fault offset, thermal histories of sedimentary basins, sediment provenance, ore deposition, etc.

Introductory Fission Track and (U-Th)/He References

Dumitru, T. A., 2000, Fission-track geochronology, in Quaternary Geochronology: Methods and Applications, edited by J.S. Noller, J.M., Sowers, and W.R. Lettis, American Geophysical Union Reference Shelf, v. 4, p. 131-156. http://www.agu.org/books/rf/v004/RF004p0131/RF004p0131.pdf

Reiners, P. W., and T. A. Ehlers, editors, 2005, Low-Temperature Thermochronology: Techniques, Interpretations, and Applications, Reviews in Mineralogy and Geochemistry, v. 58, no. 1, 622 p. The 22 chapter titles in this volume are listed at http://www.minsocam.org/MSA/RIM/Rim58.html The individual chapters can be located on the web by searching for their titles.