Effects Of Etching On Zircon Grains And Its Implications For The Fission Track Method

Studies of zircon grains using optical microscopy, micro-Raman spectroscopy, and scanning electron microscopy (SEM) have been carried out to characterize the surface of natural zircon as a function of etching time. According to the surface characteristics observed using an optical microscope after etching, the zircon grains were classified as: (i) homogeneous; (ii) anomalous, and (iii) hybrid. Micro-Raman results showed that, as etching time increases, the crystal lattice is slightly altered for homogeneous grains, it is completely damaged for anomalous grains, and it is altered in some areas for hybrid grains. The SEM (energy dispersive X-ray spectroscopy, EDS) results indicated that, independent of the grain types, where the crystallinity remains after etching, the chemical composition of zircon is approximately 33% SiO 2:65% ZrO 2 (standard natural zircon), and for areas where the grain does not have a crystalline structure, there are variations of ZrO 2 and, mainly, SiO 2. In addition, it is possible to observe a uniform surface density of fission tracks in grain areas where the determined crystal lattice and chemical composition are those of zircon. Regarding hybrid grains, we discuss whether the areas slightly altered by the chemical etching can be analyzed by the fission track method (FTM) or not. Results of zircon fission track and U-Pb dating show that hybrid and homogeneous grains can be used for dating, and not only homogeneous grains. 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Projected length annealing of etched 152Sm^{152}Sm ion tracks in apatite

Slices of apatite (cut similar to 45 degrees apart from c-axis) were irradiated with Sm-152 ions and heated at different steps in order to investigate the thermal annealing property of tracks generated by these ions. The ions were impinged with 45 degrees and similar to 150 MeV at apatite surface. Samples were etched with diluted nitric acid. Results of annealed projected lengths are presented for isochronal 10, 100 and 1000 h thermal treatments (runs) for samples with and without pre-annealing preparation. For low annealing temperatures, a distinct behavior of these samples was observed: pre-annealed samples presented a faster annealing rate. At elevated temperatures, the behavior seems to be equal. A single activation energy model was fitted to data and the energy obtained is in agreement with literature. Finally, despite the different trend in comparison with annealing rates of confined fission tracks, extrapolation to geological timescales presents reasonable estimates, indicating small influence of surface effects and, in principle, the possibility to employ ion tracks as proxies for annealing kinetics. (C) 2012 Elsevier B.V. All rights reserved.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Zircon fission track and U–Pb dating methods applied to São Paulo and Taubaté Basins located in the southeast Brazil

Zircon samples from the Cenozoic São Paulo and Taubaté Basins and Mantiqueira Mountain Range (southeast Brazil) were concomitantly dated by zircon Fission Track Method (FTM) and in situ U–Pb dating method. While FTM detrital-zircon data are ideally used to provide low-temperature information, U–Pb single detrital grain ages record the time of zircon formation in igneous or high grade metamorphic environments. This methodology may be used to study the possible sources of the basins sediments. The results suggest that the São Paulo Basin is composed of sediments from just one source, the Mantiqueira Mountain Range. On the other hand, the Taubaté Basin presents further sediment sources besides the Mantiqueira Mountain Range

Projected Length Annealing Of Etched 152sm Ion Tracks In Apatite

Slices of apatite (cut ∼45° apart from c-axis) were irradiated with 152Sm ions and heated at different steps in order to investigate the thermal annealing property of tracks generated by these ions. The ions were impinged with 45° and ∼150 MeV at apatite surface. Samples were etched with diluted nitric acid. Results of annealed projected lengths are presented for isochronal 10, 100 and 1000 h thermal treatments (runs) for samples with and without pre-annealing preparation. For low annealing temperatures, a distinct behavior of these samples was observed: pre-annealed samples presented a faster annealing rate. At elevated temperatures, the behavior seems to be equal. A single activation energy model was fitted to data and the energy obtained is in agreement with literature. 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