Advancing the fission-track method: a practical approach to transparent and reliable fission-track ages and thermal histories

• Applicant: Privatdozent Dr. Raymond Jonckheere
• Subject Area: Geology ; Mineralogy, Petrology and Geochemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550001175

This proposal deals with fundamental aspects of the fission-track method for dating rocks and reconstructing their thermal histories. It is based on counting and measuring the trails of lattice damage from uranium fission in minerals. Apatite is the most used mineral and the focus of our research. Apatite grains mounted in resin are polished and etched in nitric acid for observation with a microscope. Etching creates hollow channels where the damage trails used to be. Etched fission tracks have different shapes and dimensions depending on their orientation and that of the etched surface. It is however not clear how their numbers and lengths are related to those of the unetched tracks. Instead, protocols and a calibration against age standards must guarantee accurate results. We outlined an etch model and measured the corresponding etch rates for apatite. From the track shape we can infer its orientation, effective etch time, and etch rate (and so if it is under- or over-etched) in addition to its length. It is also possible to determine the etch rate and crystallographic orientation of the surface. We propose to use these advances to date grains that are undatable with the present protocols. We further aim to refine thermal histories by raising the number of confined tracks tenfold and by evaluating each measured length. These advances are possible because our etch model frees us from the opaque and rigid protocols that impeded progress to this date. The validation of fission-track results rests on geological benchmarks. Our earlier investigation showed that the Kontinentale Tiefbohrung on the western border of the Bohemian Massif is the most suited for this purpose. To make it accessible, we plan to create a virtual Kontinentale Tiefbohrung online, comprising images of each counted grain and measured confined track, together with the data obtained with our techniques. This should result in better agreement between scientists and the instatement of the KTB as the definitive benchmark.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Thorsten Nagel