The KTB apatite fission-track profiles: Building on a firm foundation?
Introduction
Apatite and other natural minerals incorporate trace amounts of uranium at crystallization. Over geological time, a fraction of the 238U undergoes spontaneous fission. The nucleus splits into two parts that move in opposite directions, creating a single long (∼20 μm), thin (∼10 nm) spontaneous fission track. Fission tracks are also created when the apatite is irradiated in a nuclear reactor. Thermal neutrons in the reactor spectrum cause a fraction of the 235U to fission, creating induced fission tracks. Fission tracks are enlarged by etching for observation with an optical microscope. Etching reveals the surface tracks intersecting plane sections created by grinding and polishing a collection of apatite grains fixed in resin. Cracks and surface tracks allow the etchant to access the grain interiors, where it reveals confined fission tracks. The lattice damage along an unetched track is subject to repair. Heating at first reduces etchable length of the track, followed by break-up, and its eventual erasure. The etchable length of a fossil fission track is thus an indicator of the temperature conditions to which it was exposed since it formed. Because each new track logs the temperature effects over a different time interval, the track-length distribution presents a record of temperature variations since the formation of the oldest surviving track.
Counting the number of spontaneous and induced surface tracks in a fixed area and measuring the lengths of spontaneous confined tracks permits calculation of the time elapsed since the formation of the oldest surviving track and reconstruction of subsequent temperature variations. The result is the temperature–time path (T,t-path) or thermal history of the sample. The foundation of T,t-path modelling was laid by Bertagnolli et al., 1983, Goswami et al., 1984 and Crowley (1985). The breakthrough came with the empirical approach published by Green, 1985, Green et al., 1986, Laslett et al., 1987, Duddy et al., 1988 and Green et al. (1989) and later refinements by Carlson et al., 1999, Donelick et al., 1999 and Ketcham et al. (1999), updated by Ketcham, 2003, Ketcham et al., 2007a, Ketcham et al., 2007b that include compositional and anisotropy corrections. Modelling algorithms of increasing sophistication were written by Corrigan, 1991, Lutz and Omar, 1991, Jonckheere, 1992, Crowley, 1993a, Gallagher, 1995, Issler, 1996a, Issler, 1996b, Willett, 1997, Ketcham et al., 2000, Ketcham, 2005, Hadler et al., 2001, Gallagher et al., 2005, Gallagher et al., 2009, Stephenson et al., 2006 and Sambridge et al. (2006). These work on a principle of search and selection: candidate T,t-paths are generated within defined constraints by random or guided-search algorithms. A forward calculation predicts the corresponding fission-track age and confined-track-length distribution. The programs use merit functions for evaluating their fit to the measured age and length distribution. Modern programs model related samples with fixed or variable offsets between their T,t-paths and partition sample sets with unknown relationships between their T,t-paths into subsets of correlated samples. Other software integrates the apatite fission-track data with thermochronological data obtained with other methods and on other minerals.
Our contribution is concerned with the question whether we can be confident that the results of apatite T,t-path modelling are consistent with the independent geological, thermochronological and methodological evidence. Apatite T,t-path modelling rests on three core elements: (1) the annealing model, (2) the Markov assumption (principle of equivalent time) and (3) the relationship between the apparent age and mean confined-track length. Except for the annealing model, these cornerstones of fission-track modelling have received scant attention in the more than two decades since T,t-modelling was established. Critical studies touching on the agreement of the modelling results with the independent evidence (Vrolijk et al., 1992, Holliday, 1993, Arne and Zentilli, 1994, House et al., 2002, Jonckheere, 2003a, Spiegel et al., 2007, Jonckheere et al., 2008) are not unanimous. This raises the question if geological constraints are ever tight enough or conflicts between geothermochronometers severe enough to validate or repudiate the modelling principles, rather than to resolve conflicts by considering less evident geological scenarios? What constitutes a validation of fission-track modelling? This paper attempts to address these questions based on new apatite fission-track age and length data for the Kontinentale Tiefbohrung, independent geological and thermochronological evidence, and methodological results.
A complication arises from the requirement that the measurements on geological samples are done in a manner that is consistent with those used to construct the calibrations and models. This applies to the etching and observation conditions and to the track counting and measurement criteria of the microscopist (Ketcham et al., 2009) and is more complex than it first appears. The choice of etching conditions rests on the assumption that fossil tracks in natural, radiation-damaged samples etch the same as induced tracks in annealed apatites, which is not certain (Donelick et al., 1999, Tamer, 2013). It is also unclear how it is to be established that a microscopist uses the same counting and measurement criteria as those used to construct the model equations.
Section snippets
Geological setting
The Kontinentale Tiefbohrung (KTB; continental deep drilling project) consist of two boreholes three km NW of Windischeschenbach in Bavaria (Fig. 1, Fig. 2). The pilot hole is 4000 m deep, of which 3276 m were cored. The main hole reaches 9101 m. The KTB has long been a test site for low-temperature geothermochronometers, e.g., Wagner et al., 1997, Hejl et al., 1997, Coyle et al., 1997, Warnock and Zeitler, 1998, Stockli and Farley, 2004 and Wolfe and Stockli, 2008, Wolfe and Stockli, 2010.
The KTB
Thermochronological data
Fig. 3 shows the low-temperature thermochronometric ages of dated KTB samples (apatite fission track (AFT): Hejl and Wagner, 1990, Wagner et al., 1994, Wagner et al., 1997, Coyle et al., 1997; this work; titanite (U–Th)/He (THe): Stockli and Farley, 2004; apatite (U–Th)/He (AHe): Warnock et al., 1997; zircon (U–Th)/He (ZHe): Wolfe and Stockli, 2010). Fig. 3 also shows a three-parameter sigmoidal fit to each dataset: , wherein: t [Ma] = fitted age, d [km] = sample depth, α [Ma] =
Sample preparation
Through the International Continental Drilling Program (ICDP) managed by the Geoforschungszentrum (GFZ) Potsdam, 67 ca. 2.5 kg cores from the KTB-Vorbohrung were made available for this research. Samples were selected at 50–100 m intervals, from 36 to 3889 m below the surface. Most samples were paragneisses except those from 1160 to 1610 m and 3574 to 3889 m that consisted of amphibolites and intercalated metagabbros. Most mineral separations were performed by Apatite to Zircon Inc. (R.A. and M.B.
Geological scenario
The geological evidence (Section 2) and the independent thermochronological data (Section 3) permit one to make the first-order assumption that the area of the KTB was exhumed around the end of the Cretaceous to the beginning of the Palaeocene and remained essentially undisturbed thereafter, at least as far as the effects on the apatite fission-track and (U–Th)/He low-temperature geochronometers are concerned. We make the further approximations that the exhumation and subsequent relaxation of
Model comparison
Fig. 11 plots the measured mean confined track lengths and published data (Hejl and Wagner, 1990, Wagner et al., 1994, Wagner et al., 1997, Coyle et al., 1997), together with their running mean and running median down to 4 km (10% sampling fraction), normalized to the same initial mean length. Fig. 11 also shows a compilation of the calculated mean length profiles for isothermal holding for 75 Ma. None of the calculated profiles provides a close fit to the data, although it is possible to
The apparent fission-track age profile
Fig. 20 plots the measured fission-track ages (ϕ-ages; Table 2), normalized to 75 Ma, against sample depth, together with age data from Coyle et al. (1997). Fig. 20, Fig. 21 show the calculated normalized age profiles for different annealing models and isothermal holding for 50, 75 and 100 Myr. In terms of normalized ages, the effect of the holding time is most pronounced in the lower borehole section but too small to affect the comparison with the measured age profile. A numerical comparison of
Summary and conclusions
Boreholes are unique laboratories for testing the kinetics of thermochronometers under geological conditions. The Kontinentale Tiefbohrung (KTB) is an outstanding case: (1) it is sunk into a single parametamorphic basement unit with a more or less uniform lithology; (2) the apatites down to 4 km depth have common near-end member fluorapatite compositions, with limited variation of their fission-track kinetic parameters within and between samples (Table 4); (3) the main borehole is deep enough to
Acknowledgement
This work was financed by grants RA 442/20, 26 and 27 from the German Research Council (DFG; Deutsche Forschungsgemeinschaft). We are indebted to T. Wöhrl (GFZ; Geoforschungzentrum Potsdam), the International Continental Drilling Program (ICDP) and the Bayrisches Landesamt für Umwelt (BLfU) for the KTB core material and to R.A. and M.B Donelick (Apatite to Zircon, Inc., Viola, Idaho, USA) for their part of the mineral separations. A. Renno (TU Bergakademie Freiberg) performed the electron
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2018, Chemical GeologyCitation Excerpt :Moreover, not only the etchable length but also the unetchable range varies among minerals (Jonckheere, 1995, 2003; Iwano and Danhara, 1998; Danhara and Iwano, 2013). By accounting for these factors, standardless fission-track dating has been re-established for apatite (Jonckheere, 2003; Enkelmann et al., 2005c; Jonckheere et al., 2015; Wauschkuhn et al., 2015) and zircon (Danhara et al., 2010; Danhara and Iwano, 2013). In this paper, we review a bias in fission-track counts based on the relationship between etchable and unetchable track ranges in the mineral and external detector, and demonstrate the feasibility of absolute fission-track dating of apatite, zircon and titanite by determining the standardless ages of the IUGS age standards.