Fission-track dating applied to Peruvian volcanic glasses

Abstract

Fission-Track Dating (FTD) has been applied to six Peruvian volcanic glasses. These samples presented different ages and optical characteristics. One of them (Sara Sara) had inclusions, which could be mistaken for tracks, presenting also a poor surface quality after polishing. For that reasons, this sample could not be dated. For the others samples, two independent FTD determinations were performed. One of these determinations was carried out irradiating unknown age samples together with the age standard Jankov Moldavite. The apparent ages were corrected using the size-correction method. The other determination was based on the absolute dosimetry. This time, apparent ages were corrected using the plateau-age technique. In the first case the dated samples were Macusanite, Purupurini, Batan Orq'ó, Rajchi and Firura. In the second one they were Batan Orq'ós, Rajchi and Firura. The ages determined for Macusanite, Rajchi and Firura were: (4.98±0.57) Ma; (1.186±0.080) Ma; and (1.065±0.070) Ma, respectively. The values showed above for Rajchi and Firura are the mean values obtained from both determinations. In the case of the sample Purupurini only the apparent age, (0.0275±0.0094) Ma, was obtained due to the low density of fossil tracks. Concerning the sample Batan Orq'ó, a statistical procedure was developed in order to carry out FTD even if spontaneous tracks were missing. This statistical procedure was also compared with the method proposed by Galbraith [l]. Accordingly, an upper limit of 3430 years was found for this sample. This relatively small age's upper limit, added to the fact that the sample had been collected in an important archaeological site, suggests that the sample Batan Orq'ó could have been part of an artefact that experienced a heating during its use, which would have completely erased the pre-existing tracks. The comparison between the ages of the samples Macusanite, Purupurini, Rajchi and Firura, obtained both through the size-correction method plus the Jankov Moldavite and the plateau-age technique plus absolute dosimetry, shows that the responses provided by both methods are compatible.