Geochronology GroupVAKGROEP GEOLOGIE EN BODEMKUNDE
Geochronology Group
Fission Track Dating
Research Team
Professor : Peter Van den haute
PhD Student : Johan De Grave
Technician : Nicole Seelen
History
Fission track research at our laboratory started in 1983 with a study by P. Van den haute on the track etching characteristics of fission tracks in glass and on the application of apatite fission track dating to the Precambrian basement rocks underlying the East-African rift shoulders in Rwanda and Burundi. Later in 1988, he worked alongside Prof. G. A. Wagner (one of the first scientists who introduced fission track dating in Europe) at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Since his return to the university, fission track research (in co-operation with Prof. Wagner) has been continued on a constant basis and PhD’s both in the areas of methodology and geological applications have been produced. Since 1987, there has also been a close co-operation between our FT research team and the group directed by Prof. Frans De Corte at the Institute of Nuclear Sciences. This co-operation resulted in the develoment of a procedure of thermal neutron-fluence determination that has internationally been accepted as a reference procedure.
The Fission Track Dating Method
Fission Tracks (FT) are micrometer-sized, linear damage tracks that occur in insulating minerals and that are caused by the spontaneous fission of heavy, unstable nuclides (mostly 238U in natural minerals). The spontaneous fission of 238U occurs at a specific rate, described by the decay constant (l f = 8.46 “ 10-17a-1). This implies that when the uranium concentration (CU) of a sample is known, the spontaneous FT density (r s = number of tracks/cm2) in that sample is an indication for the sample’s age. r s is determined by counting the tracks under an optical microscope (at 1250 “ magnification). Since FTs are features at an atomic scale, they require chemical techniques (etching) to make them observable under the optical microscope. CU is measured by irradiating the sample in a nuclear reactor with thermal neutrons. These neutrons induce fission of 235U isotopes in the sample. The induced FT density (r i) is obtained in an analogous manner as described above and provides a measure for the 235U concentration. Because in nature the 235U/238U ratio is a constant, the overall CU can be easily derived and all conditions are met to calculate the age of the sample. Several geological materials are dated with this method, in particular the minerals apatite, zircon and sphene and natural glasses (obsidian, pitchstone). Our laboratory is specialised in apatite fission track analysis and glass studies. Depending on age and CU, a vast time span is covered from archaeological and historical objects to e.g. several hundred million year old minerals from tectonically stable shields.
Fission tracks are thermally unstable. For a given time and temperature they will partially or even totally disappear (track fading, track annealing) due to restoration of the crystal lattice. A FT age is, in this respect, a cooling age. It yields the time when the mineral cooled down to a temperature at which the FTs became stable. Thermal annealing is reflected by a shortening of the track length. Combination of these track length measurements and age determination of a mineral sample will therefore yield information on its thermal history. For fission tracks in apatite this process is well understood and quantified. Computer models have been developed to reconstruct thermal histories from observed apatite FT data. In this way Aapatite FT analysis has become an important and successful tool in low-temperature thermochronology and during the last decade it became applied in numerical tectonic modeling, assessing tectonic hazards, landscape development, tectonic geomorphology, dating processes of mountain building, hydrocarbon exploration, sedimentary burial history, and much more.
Recent developments in FT reseach are announced in a six-monthly newsletter called On Track (latest FT newsletter On Track issue 24).
Equipment
Mineral separations are done using conventional magnetic and heavy liquid techniques. Mounting of the samples is done in epoxy resin, whereafter they are subjected to a sequence of grinding and polishing steps with diamond pastes down to 0.25 µm. Irradiation of the samples is carried out in the Tethis Research Reactor at the Institute of Nuclear Sciences (INW) of the University. For track counting and measuring, Olympus BH2 microscopes equipped with transmitted and reflected light, are used carrying a drawing tube attachment overlooking a high resolution digitising tablet. Self-developed software for counting and measuring the tracks and for data analysis is used. From 2003 on, track sizes will be measured from pictures taken with a digital camera and using an image analysis program. For apatite FT thermal history modelling the AFTSolve program is used (AFTSolve).
Research areas and perspectives
Methodological
Introducing the Q-factor, an absolute neutron fluence calibrated factor for use in the external detector method of FT dating.
Development of calibrated uranium-doped (15 and 50 ppm) glass monitors for use in the FT method in Cupertino with the EC-Institute for Reference Materials and Methods.
Geological
Low-temperature thermochronology and denudational history of the Altai Mountains, South Siberia, Russia.
Low-temperature thermochronology and denudational history of the Tien Shan Mountains, Kyrgyzstan, Central Asia.
Low-temperature thermochronology and cooling history of the Kazakhstan shield, Central Asia.
Assessing active tectonics and Cenozoic far-field effects of the India/Eurasia collision by apatite fission track analysis.
External services
Preparation of FT samples and irradiation with a calibrated neutron fluence in the thermal channels of the Thetis Reactor (in Cupertino with the INW).
Publications
Doctoral Dissertations
De Grave, J. (2003). Apatite fission track thermochronology of the Altai Mountains (South Siberia, Russia) and Tien Shan Mountains (Kyrgyzstan, Central Asia). Universiteit Gent. In prep.
Jonckheere, R. (1995). De absolute ouderdomsbepaling van apatiet gebaseerd op uranium-fissiesporen: een methodologisch onderzoek. Universiteit Gent, 504 p. (in Flemish).
Vercoutere, C. (1994). The thermotectonic history of the Brabant Massif (Belgium) and the Naab Basement (Germany): an apatite fission track analysis. Universiteit Gent, 191 p.
Van den haute, P. (1983). Bijdrage tot de studie van fissiesporen in glas en toepassing van de fissiesporendateringsmethode op apatieten uit precambrische gesteenten van Rwanda en Burundi. Universiteit Gent, 194 p. (in Flemish).
Books
Van den haute, P., De Corte, F. (1998) Advances in fission-track geochronology, Kluwer Academic Publishers, Dordrecht, 331 pp.
Wagner, G.A., Van den haute, P. (1992). Fission Track-Dating. Kluwer Academic Publishers, Dordrecht, 285 pp.
Selected articles (period 1997-2002)
De Grave, J., Van den haute, P. (2002). Denudation and cooling of the Lake Teletskoye region in the Altai Mountains (South Siberia) as revealed by apatite fission-track thermochronology. Tectonophysics, 349, 145-159.
Jonckheere, R., Van den haute, P. (2002). On the efficiency of fission-track counts in an internal and external apatite surface and in a muscovite external detector. Radiation Measurements, 35, 29-40.
Derbyshire, M., Ingelbrecht, C., De Corte, F., Van den haute, P. and Van Ham, J. (2001). Preparation of two uranium glass reference materials for fission-track dating of geological samples. Radiation Measurements, 30, 419-422.
De Corte, F., Bellemans, F., Van den haute P., Ingelbrecht C., Nicholl C. (1998). A new U doped glass certified by the European commission for the calibration of fission-track dating. In: Van den haute, P., De Corte, F. (Eds) Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Dordrecht, 67-78.
Van den haute, P., De Corte, F., Jonckheere, R., Bellemans, F. (1998). The parameters that govern the accuracy of fission-track age determinations: a re-appraisal. In: Van den haute, P., De Corte, F. (Eds) Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Dordrecht, 33-46.
Wagner, G.A., Coyle, D.A., Duyster, J., Hentjes-Kunst, F., Peterek, A., Schröder, B., Stökhert, B., Wemmer, K., Zulauf, G., Ahrendt, H., Bishoff, R., Hejl, E., Jacobs, J., Menzel, D., Lal, N., Van den haute, P., Vercoutere, C., Welzel B. (1997). Post-Variscan thermal and tectonic evolution of the KTB site and its surroundings. Journal of Geophysical Research,102, B8, 18221-18232.
