Apatite LA-ICP-MS fission-track age determination using absolute ϕICP- and θ-ICP methods and the comparison with ζ-calibration

Fission-track thermochronology (FTT) is a technique used to determine the age and thermal history of minerals by counting and measuring the length of fission fragment tracks generated during the spontaneous fission of ²³⁸U. The minerals suitable for FTT are ionic crystals that contain trace amounts of uranium, which substitute other elements within the crystal structure.

For age determination, the number of tracks is counted on a polished surface after appropriate chemical etching. Until recently, the External Detector Method (EDM) was the preferred way of determining the uranium content in minerals. This method involves attaching an external detector, usually a muscovite, to the polished and etched mineral surface. The set is then irradiated with thermal neutrons in a nuclear reactor, and the neutron fluence is determined by counting tracks in a muscovite that is placed next to a co-irradiated U-doped standard glass.

EDM (External Detector Method) has been a widely used technique for many years. It has been tested extensively and has provided significant geological information. However, there is a major drawback to the method: the samples become activated during irradiation, and it takes a considerable amount of time to handle the samples after irradiation.

To avoid these complications, the fission-track community has been working on a new method to directly determine U (uranium) concentration using laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Using LA-ICP-MS, the waiting times for sample return from the reactor are avoided and there is no need for further sample preparation. The LA-ICP-MS machine is easily accessible and can be found in most geochemical laboratories. Additionally, a series of other isotopes can be determined simultaneously (e.g., Ansberque et al., 2020), which yields geochemical signatures. Also, the LA-ICP-MS makes it possible to apply other dating techniques such as U-Th/Pb to the same mineral grains. Such additional information can be valuable for geological interpretation, especially for basin samples, which can contain multiple age populations from various provenances.

We will present results for two absolute methods of calibration for apatite and compare with z-method using Durango as age standard sample (e.g., Donelick et al., 2005): Such methods are: 1) an absolute approach (ϕ_ICP-method) (modified from Soares et al., 2014b); 2) A new method is presented, based on the calibration of LA-ICP-MS efficiency parameters via EDM dating, through θ_ICP-method (using U-doped glass calibration against U thin film (Soares et al., 2014a)).

Reference

Ansberrque, C., Chew, D.M., Kerstin, D. (2020). Apatite Fission-track dating by LA-Q-ICP-MS imaging. Chemical Geology. 560. 1-13.

Soares, C.J., Guedes, S., Curvo, E.A.C., Hadler, J.C., Jonckheere, R., Tello, C.A., Lixandrão-Filho, A.L., Siqueira, P.T.D., Madi Filho, T. (2014a). Recalibration of U-doped standard glasses through uranium thin film for neutron-fluence measurements. Journal of Radioanalytical and Nuclear Chemistry 302, 17–26.

Soares C.J., Guedes, S., Hadler, J.C., Mertz-Kraus, R., Zack, T., Iunes, P.J. (2014b). Novel calibration for LA-ICP-MS-based fission-track thermochronology. Physics and Chemistry of Minerals, 41, 65-73.