A new approach for electron microprobe zircon fission track thermochronology

Author(s)

Dias, A. N. C. ; Chemale, Jr., F. ; Soares, C. J. ; Guedes, S.

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 459, p. 129-136, MAY 25 2017.
• Web of Science Citations: 5

Abstract

Fission track thermochronology (FTT) has been applied for decades to quantify rates and timing of processes in the shallow crust. The most widely used approach is the external detector method (EDM). In this conventional approach, an age is obtained by counting both the fossil (U-238 spontaneous fission) and the induced (U-235 induced fission) tracks using an optical microscope. The induced tracks are obtained through the irradiation of the sample with thermal neutrons in the nuclear reactor, which causes fission of U-235. Based on the studies carried out by Gombosi et al. (2014), we present an alternative method of dating zircons using electron probe microprobe analysis (EPMA) to measure uranium concentration {[}U]. The electron microprobe analysis fission track (EPMA-FT) method was applied to three samples of rapidly cooled zircons: the Fish Canyon Tuff, Pocos de Caldas (syenite) and Serra Geral zircons. The analyses were performed using two approaches: 1) using the age equation described in Gombosi et al. (2014) and 2) using a new age equation calibration developed for this work. The results using the Gombosi et al. (2014) age equation were 26.7 +/- 4.1 Ma, 80.6 +/- 12.8 Ma and 130.9 +/- 20.1 Ma, respectively, and the results using the age equation from this work were 27.8 +/- 1.9 Ma, 83.8 +/- 7.7 Ma and 136 +/- 12 Ma, respectively. The uncertainty of the age is affected mainly by U-238 concentration and ps (the spontaneous fission track density) determinations. Other factors can affect the uncertainty of the age, but their contributions are smaller. For all samples, the yield ages found by the two methods are consistent and overlap within two standard deviations of published reference ages determined from other radiometric techniques (i.e., K/Ar, Ar-40/Ar-39, and/or U/Pb) and traditional FIT by the EDM. (AU)

• FAPESP's process: 14/13792-5 - Evolution study of Brazilian sedimentary basins by zircon fission track thermochronology
• Grantee: Airton Natanael Coelho Dias
• Support Opportunities: Regular Research Grants
• FAPESP's process: 12/23508-7 - Zircon fission-track annealing data set applied to sudies of fission-track thermochronology: case study of a Brazilian sample
• Grantee: Airton Natanael Coelho Dias
• Support Opportunities: Scholarships in Brazil - Post-Doctoral