Chemical etching effects in apatite and zircon minerals and their implications in the Fission Track Method: Canastra Mountain Range Geochronology.

Abstract

This study aims at analyzing the effects of chemical etching, required to reveal spontaneous fission tracks of 238U, in apatite and zircon minerals and their implications in the Fission-Track Method. Therefore, a characterization of the minerals mentioned before will be done using optical microscopy, micro-Raman spectroscopy and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), as well as an theoretical-experimental analysis of the different physical-chemical phenomena that occur in the natural crystallization of each mineral. The intention behind those procedures is determining the relations between the chemical etching and crystallographic characteristics of the minerals. The surface fission-track density observed on the mineral grains can vary due to either natural variations of the chemical composition or variations in the lattice. In the zircon case, it can be observed that some grains show anisotropic fission-track density, that is, in some areas of the same grain surface the fission-track density is different. It can happen due to the incorporation of other minerals during the mineral crystallization, whose identification and characterization are some goals of this work, and even due to natural conditions of crystallization by itself. The grains surface characterization will be done with the collaboration of Prof. Dr. Carlos José Leopoldo Constantino and the chemical analysis with the collaboration of Prof. Dr. Neri Alves. The focus of this work is to get improvements of FTM under way. The FTM will be used to determine both fission-track age and thermal history of Canastra Mountain Range crystalline. In addition, the U/Pb dating Method will be applied to the same grains. Therefore, simultaneously to FT ages a set of U/Pb ages will be obtained. This will make a more comprehensive geochronological study of the Bauru Group, by comparing the ages previously obtained from this geological unit with those to be obtained in the Canastra Mountain Range. In other words, it can be made a source-to-sink correlation, as the region of the Canastra Mountain Range may have been an important source of sediments that came to form the Bauru Group, located north of the Paraná Basin. Forty samples will be collected from profiles previously established, giving preference to sites located near fault lines.