Raman Spectroscopy and Scanning Electron Microscopy Characterizations of Fission Track Method Datable Zircon Grains

Abstract

Spectroscopic and morphological studies, designed to improve our understanding of the physicochemical phenomena that occur during zircon crystallization, are presented. The zircon fission track method (ZFTM) is used routinely in various laboratories around the world; however, there are some methodological difficulties needing attention. Depending on the surface fission track density observed under an optical microscope, the zircon grain surfaces are classified as homogeneous, heterogeneous, hybrid, or anomalous. In this study, zircon grain surfaces are characterized using complementary techniques such as optical microscopy (OM), micro-Raman spectroscopy, and scanning electron microscopy (SEM), both before and after chemical etching. Our results suggest that anomalous grains have subfamilies and that etching anisotropy related to heterogeneous grains is due to different crystallographic faces within the same polished surface that cannot be observed under an optical microscope. The improved methodology was used to determine the zircon fission track ages of samples collected from the Bauru Group located in the north of Paraná Basin, Brazil. A total of 514 zircon grains were analyzed, consisting of 10% homogeneous, about 10% heterogeneous, about 20% hybrid, and 60% anomalous grains. These results show that the age distributions obtained for homogeneous, heterogeneous, and hybrid grains are both statistically and geologically compatible.

AVANÇOS METODOLÓGICOS NA DATAÇÃO POR TRAÇOS DE FISSÃO E U-Pb EM ZIRCÃO

“…The fission-track age distributions for the three grain types described above are shown in Figure 1 (Resende et al, 2014). The age distribution for homogeneous grains has a mean value of (271 ± 22) Ma, while for heterogeneous and hybrid grains it is about (270 ± 18) Ma and (251 ± 13) Ma, respectively.…”

“…Therefore, the results in Figure 1 indicate a major advance in the dating method via fission tracks, especially in the case of detrital rocks, where the concentration of homogeneous grains is usually low. Thus, excluding the heterogeneous and hybrid grains from the FTM analysis make the dating of many samples unfeasible (Resende et al, 2014). (1996) and Williams (1998).…”

“…According to the characterization studies performed on the zircon surface as a function of the etching time, this behavior may occur in different micro-areas of a grain (Dias et al, 2009;Tello et al, 2012). After etching, depending on the fission track distribution on the zircon prismatic surface (parallel to the c-axis), the grains, previously sanded and polished, can be optically classified into: homogeneous, heterogeneous, hybrid and anomalous (Dias et al, 2009;Tello et al, 2012;Resende et al, 2014). Homogeneous grains have a uniform surface-track distribution over the surface, while heterogeneous grains present some irregularities in the track distribution throughout the surface, i.e., some micro-areas of the grain have a higher fission-track density than others.…”

“…According to Resende et al (2014), a total of 514 zircon grains collected from the Bauru Group, located north of the Paraná Basin, Brazil, were analyzed to determine the percentage of each type of grain. They were classified as being 10% heterogeneous, 20% hybrid and 60% anomalous.…”