Precision, accuracy and meaning of fission track ages

Abstract

The precision, meaning, and accuracy of the fission track (Fr) dating method are reviewed from an examination of the recent literature as well as previously unpublished data from the author's laboratory. It is concluded that for apparent FT ages (i.e. ages derived from the canonical age equation) a precision (2~r level) of the order of 4-4 ~o to 4-5 Yo can be reached provided that (i) uranium is sufficiently homogeneously distributed in the dated samples, at least locally; and (ii) a large enough number of tracks can be counted. Model r'r ages, i.e. ages for which partial geological track annealing is taken into account, have variable degrees of precision. While model ages obtained with the track-size method seem, as evaluated from the literature, to have usually a limited precision of the order of 4-30yo (2~), plateau ages usually have a precision better than 4-5 ~0 at a 2~r confidence level. Because it provides an objective test on the accuracy of track identification, as well as some insight of the variability of closing temperatures between various samples of a given mineral phase, the Isochroua'l Plateau (icp) method, when applicable, will be preferred (Poupean et al 1980a). However, for phases which.could be damaged by heating at relatively high temperatures, as for example hydrated glass shards from tephra, an Isothermal Plateau (ITP) approach is to be preferred. Due to uncertainties about the value of the 28sU spontaneous fission decay constant ,~y, as well as difficulties inherent in the dosimetry of thermal neutrons in nuclear reactors, the rT method of dating is not an independent one. Presently, it relies on the existence of geological standards (volcanic rocks) of known age, allowing to determine an operational 'Ay' value (Naeser et al 1980). Accordingly, the accuracy of an FT age is limited by the accuracy on the age of the standard. It should be better than ,~ 5 Yo. For volcanic, hypovolcunic rocks, and shallow intrusives, the rr method dates the time of formation, provided they were not further reheated. More generally, the track method provides cooling ages. Closing temperatures calculated from laboratory experiments vary from ~ 300"C to 100~ according to minerals, for slow cooling rates (,~ l~ For apatites, recent geological calibrations (Naeser et al 1980; Gleadow and Duddy 1980) confirmed laboratory extrapolations. The association of the rT method with other geochronometers is therefore critical to the study of the cooling history of old cratons as well as to the evaluation of uplift/erosion rates in recent belts, Keywords. Fission tracks; fission track dating; track annealing; fission decay constant; neutron dosimetry; model ages; plateau ages; cooling and uplift rates.

Fission track geochronology of India

“…There are various dating methods employed by various workers in the field. These methods are summarised by Gleadow and Lovering (1975), and also discussed by Poupeau (1981). In our laboratory we have mostly used the subtraction method.…”

“…(6.9 q-0.2) x 10 -17 yr -1, Though we have been calculating the ages using the value of ?t F by Roberts et al (1968), yet this point is a matter of controversy and it should be resolved to assign a single value to AF" (See discussion in the article by Poupeau 1981).…”

“…Though the plateau method is more accurate, a large number of analysis is required to calculate the plateau age. Poupeau (1981) has discussed the age correction method in detail.…”

Datation par traces de fission et étude de provenance d'artefacts en obsidienne des sites archéologiques de la Tolita (Equateur) et Inguapi (Colombie)

“…3). Selon les échantillons, i l faut une ou plusieurs étapes de traitement thermique à des températures croissantes (plateau-isochrone, Storzer et Poupeau, 1973), ou à température constante mais pour des temps plus longs (plateau-isotherme, pour les verres hydratés, Poupeau, 1982), pour ramener le rapport Df/Dj à l'unité ( fig. 2).…”

Fission track geochronology of India

“…Though we have been calculating the ages using the value of A p by Roberts et al (1968), yet this point is a matter of controversy and it should be resolved to assign a single value to AF' (See discussion in the article by Poupeau 1981).…”