Fission track geochronology

Fission track geochronology is a method for dating minerals containing uranium, particularly apatites and zircon. Apatite is a calcium phosphate that is common in granites and metamorphic rocks. Zircon is a zirconium silicate that is also common in similar rocks. The sample age is determined by counting fission tracks in the material from spontaneous fission of These tracks are a function of the U content and the age since closure when the fission track clock started. A research reactor is then used to irradiate the samples and induce fission in the present in the sample. By comparison of the before and after track count, the U content in the sample is determined. 

Since tracks are annealed at temperatures above about 120°C for apatites and 200 to 300°C for zircons, the method is also useful for the determination of the thermal history of a sample. 

Because of the specialized nature of the analysis, most reactor facilities will only be performing the irradiations and the samples will be sent off-site for analysis. 

In addition to the uranium, all samples will contain low concentrations of thorium. Both the thorium and will fission when irradiated with fast neutrons. Therefore, it is important that the samples are irradiated with well thermalized neutrons. This allows for the assumption that all of the fission events produced during irradiation are from the fission of 

It is important that the flux is stable during the irradiation. 

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Carpena J (1998) Uranium-235 fission track aimealing in minerals of the apatite group. In Van den haute P, De Corte F (eds) Advances in Fission Track Geochronology. Kluwer Academic, Dordrecht, p 81-92 Carter A (1999) Present status and future avenues of source region discrimination and characterization using fission track analysis. Sed Geol 124 31-45... 

Dumitra TA (2000) Fission-Track Geochronology. In Noller JS, Sowers JM, Lettis, WR (eds) (Jrratemary Geochronology Methods and Applications. Am Geophys Union Ref Shelf 4, Washington, D(3, American Geophysical Union, p 131-155... 

Except for the fission-track and chemical lead methods, all geochronologic techniques involve... 

Dahl P. S. (1997) A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology. Earth Planet. Sci. Lett. 150(3-4), 277-290. 

DeWolf C. P., Zeissler C. J., Halliday A. N., Mezger K., and Essene E. J. (1996) The role of inclusions in U-Pb and Sm-Nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis. Geochim. Cosmochim. Acta 60, 121-134. 

The Geochronology section focuses principally on apatite and monazite for U-Th-Pb, (U-Th)/He, and fission-track age determinations it covers both classical geochron-ologic techniques as well as recent developments. 

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