Model ages in fission-track dating

Abstract

v reproduced from the unrefereed volume entitled Fission-Track IMting Workshop Al~tracls: Pisa, 10-13 September 1980,* MODEL AGES IN FISSION-TRACK DATING J. CARPENA, D. MAILHE, G. POUPEAU + and D. VINCENT Centre des Faibles Radioactivit6s, Domaine du C.N.R.S., Laboratoire Mixte C.N.R.S.-C.E.A., 91 190 Gif-sur-Yvette, France SINCE it was realised, more than 10 years ago (Bigazzi, 1967), that incomplete fossil track retention was the cause of anomalously young fissiontrack (FT) ages, much attention has been paid to the thermal stability of uranium fission tracks in natural minerals. This has led to the establishment of various methods of FT age correction procedures. We report here and in a companion paper in this Workshop the results of a continuous investigation initiated in 1978 (Poupeau et al., 1978a, b) of the stepwise plateau heating methods earlier proposed by Storzer and Poupeau (1973) and Burchart et al. (1975). 1. MODEL AGES An incomplete track retention in minerals is revealed by a shortening of the etchable track length. In uranium FT dating, this corresponds to a lowering of the etchable fossil track density, which leads to a lowered FT age, as the usual age equation does not take into account the difference of etching efficiency of fossil and induced tracks (Poupeau, 1981a; Poupeau et al., 1980b). In the mean time, as the central part of the tracks is more stable, the volumic density of recorded fossil fission events is kept constant. This is precisely which permits correction procedures to be applied. One way to overcome the above mentioned effects of partial geological track annealing on FT ages is to reduce both the fossil and induced track populations to the same degree of fading by an appropriate thermal treatment (i.e., to equalize again their etching efficiency), before FT measurements, as in the plateau methods depicted in the figure on the next page: 1.1 Isochronal Plateau Ages (ICPA) Figure la Storzer and Poupeau (1973); Poupeau et al. (1980a, 1981a). In this procedure, FT ages are successively measured after thermal annealing steps (one hour duration each in our standard routine) of fossil and induced fission tracks at increasing temperatures. In the case of total fossil track retention (left), both populations of tracks behave similarly, and the resulting age distribution with temperature is fiat; in the case of partial geologic track annealing, the fossil track component reacts only progressively to the thermal treatment and the FT age evolves from an age to an older plateau-age value. 1.2 IsoThermal Plateau-Age (ITPA) This is the Isothermal Cumulative Heating Method of Burchart et al. (1975), into which FT ages are measured after a series of cumulative steps of thermal heating at a constant temperature. The FT ages and densities behave as described above, now in function of the heating time. 1.3. lsochrone FT age This formalism, first proposed by Naeser and co-workers (1978) to calculate apparent ages as given by a population of zircons treated with the external detector method, can also be used to calculate FT-isochron ages for materials submitted to a plateau mode of dating (Poupeau, 1981). *Abstracts volume edited by G. Bigazzi, CNR, lnstituto di Geocronologia e Geochimica lsotopica, Via Cardinale Maffi 36, Pisa. Italy. tAuthor to whom correspondence may be addressed.