APATITE FROM AN OLIVINE PYROXENITE OF THE CANAÃ VALLEY, ESPÍRITO SANTO, BRAZIL: FISSION-TRACK AGE DETERMINATION AND CONFINED TRACK LENGTH MEASUREMENTS IN ORDER TO ESTABLISH AN INTERNAL STANDARD

Abstract

We present the first results of fission-track age determinations and natural confined track length measurements in apatite crystals of an olivine pyroxenite from the Canaã Valley, Espírito Santo, Brazil. This apatite was chosen for its high abundance in the rock and also for its high uranium content. Confined track length measurements were performed by three different analysts, whereas fission-track ages corresponding to two different irradiations were determined by two analysts. Confined track length distributions as well as ages showed good reproducibility. The characteristics of this apatite (its abundance in the rock and the relatively large number of confined tracks) make it a very promising internal standard which experienced a more complex thermal history in comparison with the volcanic apatite international age standards. The fission-track age of around 95 Ma of the apatite sample from the olivine pyroxenite is consistent with the age interval 65-110 Ma previously measured by Fonseca and Poupeau (1984), Fonseca (1989) and Fonseca et al. (1992) in the Ribeira Belt. This age interval corresponds to the basaltic and alkaline volcanic activities related to the opening of the Atlantic Ocean.