FISSION-TRACK ANALYSIS - PRINCIPLES, METHODOLOGY AND IMPLICATIONS FORTECTONOTHERMAL HISTORIES OF SEDIMENTARY BASINS, OROGENIC BELTS, AND CONTINENTAL MARGINS

Fission tracks, formed by natural fission of Th-232, U-235 and U-238, are damage zones in the crystal lattice. The decay constants of the fi rst two isotopes are so small that, for all practical purposes, all fi ssion tracks are derived from fission of U-238. Th, spontaneous fissio n-track (FT) density is proportial to the elapsed time and the uranium content. The latter parameter is determined hv irradiation of the sam ple with thermal neutrons causing the U-235-isotope to fission. A new set of induced fission tracks is made and the induced FT density is pr oportional to the amount of uranium, because the U-235/U-238 ratio is constant. FT dating is commonly performed on volcanic glass and access ory minerals such as apatite, zircon and sphene. Compared to other rad iogenic age determinations, FT apparent ages are systematically younge r, except for rocks that cooled rapidly such as volcanics and shallow- depth intrusives. Laboratory experiments show that fission tracks are not stable at high temperatures. This provides an explanation for the comparatively young FT ages and at the same time, opens a new importan t field of application: FT analysis as a geochrono-thermometer. Within a mineral-specific temperature range, fission tracks begin to anneal until they are completely erased at the high temperature boundary. The temperature, at which total annealing occurs, depends on the timescal e of the heating event and the chemical composition of the mineral. Da ta from drill holes confirm the laboratory experiments over geological timescales. For apatite it is possible to establish an annealing zone for spontaneous fission tracks under geological time-temperature (T-t ) conditions. Annealing is temperature-dependent and as the process pr ogresses the length of the fission track shortens. This results in a r eduction of the spontaneous track density and hence in a decrease of t he FT age. The apparent age, single-age grain distribution, FT mean le ngth and length distribution are diagnostic of the temperature histori es of rocks. Recent advances in understanding annealing kinetics of fi ssion tracks in apatite permit computer modelling of age and length pa rameters for given T-t pathways. FT analysis thus constitutes a powerf ul and unique tool for the reconstruction of thermal, uplift and subsi dence histories, and also for provenance studies of sediments. Particu larly in hydrocarbon exploration, the application of fission tracks to the study of thermal and burial histories has proven the unique abili ty of the method in understanding the formation and evolution of sedim entary basins. FT analyses are also used for studying uplift, exhumati on, unroofing, denudation and erosion histories of basement rocks. The se parameters are important for our understanding of tectonic processe s and for numerical modelling studies, because they constrain temperat ure histories in diverse geological settings like subduction and colli sion zones, extensional areas of continental breakup during rifting, a nd intra-plate settings.