Fission-track analysis: principles, methodology and implications for tectono-thermal histories of sedimentary basins, orogenic belts, and continental margins

Abstract

Fission tracks, formed by natural fission of 232Th,235U and 238U, are damage zones in the crystal lattice. The decay constants of the first two isotopes are so small that, for all practical purposes, all fission tracks are derived from fission of 238U. The spontaneous fission-track (FT) density is proportial to the elapsed time and the uranium content. The latter parameter is determined by irradiation of the sample with thermal neutrons, causing the 235U-isotope to fission. A new set of induced fission tracks is made and the induced FT density is proportional to the amount of uranium, because the 235U/238U ratio is constant. FT dating is commonly performed on volcanic glass and accessory minerals such as apatite, zircon and sphene. Compared to other radiogenic age determinations, FT apparent ages are systematically younger, 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 important 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 timescale of the heating event and the chemical composition of the mineral. Data 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 progresses the length of the fission track shortens. This results in a reduction of the spontaneous track density and hence in a decrease of the FT age. The apparent age, single-age grain distribution, FT mean length and length distribution are diagnostic of the temperature histories of rocks. Recent advances in understanding annealing kinetics of fission tracks in apatite permit computer modelling of age and length parameters for given T-t pathways. FT analysis thus constitutes a powerful and unique tool for the reconstruction of thermal, uplift and subsidence histories, and also for provenance studies of sediments. Particularly in hydrocarbon exploration, the applicationof fission tracks to the study of thermal and burial histories has proven the unique ability of the method in understanding the formation and evolution of sedimentary basins. FT analyses are also used for studying uplift, exhumation, unroofing, denudation and erosion histories of basement rocks. These parameters are important for our understanding of tectonic processes and for numerical modelling studies, because they constrain temperature histories in diverse geological settings like subduction and collision zones, extensional areas of continental breakup during rifting, and intra-plate settings.