U. Mass Lowell |
Prof. Nelson Eby | Department of Environmental, Earth, & Atmospheric Sciences |
| Fission-Track Geochronology |
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238U and 235U usually decay by the emission of alpha and beta particles. 238U, however, occasionally decays via spontaneous fission. During this process fission fragments are ejected which damage the crystal structure in the immediate vicinity of the original U atom. By using proper etchants, these tracks can be enlarged until they are visible using an optical microscope. The track density (fossil tracks) gives a measure of the number of 238U atoms that have decayed since the formation of the mineral (or the time when its temperature fell below the annealing temperature). The amount of U present in the mineral can be determined by subjecting the mineral to thermal neutrons thus fissioning 235U atoms and producing fission tracks. The density of these tracks (induced tracks), and a measure of the total neutron fluence, can be used to determine the amount of 235U in the mineral and, using the fixed relationship between 235U and 238U, the amount of 238U in the mineral. The age can now be determined. Three minerals, apatite, titanite, and zircon, are routinely dated using the fission-track method. Apatite and titanite ages are determined in this laboratory. Apatite and titanite are separated from crushed rock samples using standard heavy liquid and electromagnetic techniques (Figs. 1 and 2).
Apatite. Apatite ages are determined using the population method. Apatite grains are separated into two populations. One population is annealed at 600oC for 6 hours removing all the fossil fission tracks in the apatite grains (the geological annealing temperature for apatite is ~120oC). This population is subjected to a known neutron fluence and induced tracks are formed. Both populations are mounted in epoxy, the grains are polished, and then etched for 20-30 s in 10% HNO3 producing optically visible fission tracks (Fig. 3). Fifty or more grains are counted from each population giving the density of fossil tracks (unannealed population) and induced tracks (annealed population) from which the age can be calculated.
Titanite. Titanite ages are determined using the external detector method. Titanite grains are mounted in epoxy and etched for several hours at ~120oC in a concentrated NaOH solution producing optically visible fission tracks (Fig. 4). U-free muscovite detectors are then placed on each grain mount and the package is subjected to a known neutron fluence. Induced fission tracks are formed in the muscovite detectors. After irradiation the muscovite detectors are removed and etched in HF. The same areas are counted on the titanite grain and the muscovite detector giving, respectively, the fossil and induced track density. From these measurements an age can be determined.
Determination of fluence. Glass standards are included in each irradiation package along with mineral standards of known age. Muscovite detectors placed over the glass standards are etched to reveal the number of induced tracks (Fig. 5). Final ages are calculated using the zeta calibration which is based on the relationship between the known age of the mineral standards and the track density recorded in the muscovite detectors.
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