Zircon fission-track annealing data set applied to sudies of fission-track thermochronology: case study of a Brazilian sample

Abstract

One of the most important phenomena of the Fission-Track Thermochronology, FTT is the shortening of fission tracks (spontaneous fission of 238U), mainly due to the heating experienced by the host rock in geological (millions years) or laboratory (hours or days) time. This phenomenon of shortening is known as annealing. The quantification of annealing kinetics is fundamental for the FTT because it serves as basis for obtaining the thermal histories of geological areas of interest. Thermal histories are obtained from annealing kinetic models, whose parameters are found by fitting to FT annealing datasets. A priori zircon datasets found in the literature are restricted to the analysis of the spontaneous fission-track lengths. Moreover, these sets were obtained by the same group of researchers, 80% of which are of the same sample (Nisatai Dacite - NST) and 95% of samples are from Japan. On this basis, the proposal of this project is to use a Brazilian sample (Poços de Caldas, Minas Gerais, Brazil) and obtain a more comprehensive dataset for the evaluation of the effect of anisotropy and composition of the selected sample on the annealing kinetics. This Brazilian sample (here called ZPC) has been dated by fission track (76.8 ± 10.4 Ma, unpublished original data) and the mineral characterization will be performed during this project (61.60% of ZrO2, 36,40% of SiO2, and ~ 2% HfO2 and other trace elements - previous analysis). ZPC aliquots will be submitted to thermal treatments at different temperatures in times of 1, 10, 100 and 1000 hours. Is worth mentioning that the behavior of spontaneous and induced fission tracks will be evaluated simultaneously, in order to investigate the possible difference between the annealing kinetics of the spontaneous and induced fission tracks. This difference is attributed to possible damage caused by alpha recoil during the mineral history. Such damage would change the tracks response to heat treatments. This simultaneous analysis is important for the FTT in zircon, because, if the annealing kinetics of these tracks depends on the amount of alpha recoil damage in the sample, the thermal history modeling in zircon FTT depends on the age and content uranium of samples, which would make necessary to adapt the method presently used.