Further investigation of the initial fission-track length and geometry factor in apatite fission-track thermochronology

Abstract

The external detector method (EDM) is a widely used technique in fission track thermochronology (FTT) in which two different minerals are concomitantly employed: spontaneous tracks are observed in apatite and induced ones in the muscovite external detector. They show intrinsic differences in detection and etching properties that should be taken into account. In this work, new geometry factor values, g, in apatite, were obtained by directly measuring the r ed /r is ratios and independently determined [GQR] ed/is values through the measurement of projected lengths. Five mounts, two of which were large area prismatic sections and three samples composed of random-orientation pieces have been used to determine the g-values. A side effect of applying EDM is that the value of the initial confined induced fission track, L 0 , is not measured in routine analyses. The L 0 -value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown-age samples, and is essential for accurate thermal history modeling. The impact of using arbitrary L 0 -values on the inference of sample thermal history is investigated and discussed. The measurement of the L 0 -value for each sample to be dated using an extra irradiated apatite mount is proposed. This extra mount can be also used for determining the g value as an extension of the r ed / r is ratio method. Eight apatite samples from crystalline basement, with grains at random orientation, were used to determine the g-values. The results found are statistically in agreement with the values found for apatite samples (from Durango, Mexico) measured in prismatic section and also measured at random orientation. There was no observable variation in efficiency regarding crystal orientation, showing that it is relatively safe using non-prismatic grains, especially in samples with paucity of grains, as it is the case of most basin samples. Implications for the z-calibration and for the calibration of the direct (spectrometer-based) fission-track dating are also discussed.

Standardless fission-track dating of the Durango apatite age standard

“…For all dating methods, the calibration factor can be determined using age standards, in which case the method ceases to be standardless but becomes a deconvoluted ζ-method (Hurford, 1998). On the other hand, it is also possible to construct the calibration factor from its components (G, Q, R, T; Jonckheere, 2003b;Enkelmann and Jonckheere, 2003;Enkelmann et al, 2005b;Soares et al, 2013) or to combine both these approaches in a single multiple calibration (ζ 0 -method: Jonckheere, 2003b). The ζ-method has the advantage of eliminating the need for neutron-fluence measurements but the disadvantage that the substituting track counts (ρ D ) in micas irradiated against standard uranium glasses lack the resolution for detecting all but large neutron-fluence gradients.…”

Recalibration of U-doped standard glasses through uranium thin film for neutron-fluence measurements

“…Besides, geometry factor was determined applying exactly the same experimental conditions in which unknown samples were dated [7][8][9]. As it will be seen in the next sections, this new calibration, together with a well determined geometry factor [9] resulted in much more consistent results when U-doped glasses and metal activation monitor are compared. An additional test for neutron dosimetry techniques is to date samples and compare the obtained ages with ages determined by independent radiometric ages.…”

“…This occurs because spontaneous fissiontrack density in apatite is determined under 4p-geometry, whereas induced fission-track density is determined under 2p-geometry. However, intrinsic differences between revelation by etching and observation under optical microscopy makes the g-value = 0.5, thus, this parameter should be measured in the same conditions samples are dated [7,9].…”

“…As the dispute on the value of k f has been solved [2][3][4][5][6], the neutron fluence determinations had been shown to be accurate [7,8] and the efficiency factors have been broadly studied [7][8][9], the absolute calibration yielded independent fission-track ages possessing geological meaning.…”

On the influence of etch pits in the overall dissolution rate of apatite basal sections

“…This method is named fission-track thermochronology (FTT). Regardless the model chosen for the annealing of fission tracks, their maximal etchable length is a key parameter for modelling thermal histories (Donelick et al 1990;Laslett and Galbraith 1996;Ketcham et al 1999;Barbarand et al 2003b;Soares et al 2013). It is also proposed that the dimension of an etch pit parallel to the crystallographic c-axis correlates with the chlorine content of apatite (Donelick 1993;Barbarand et al 2003a).…”