Compositional effects on etching of fossil confined fission tracks in apatite
Compositional effects on etching of fossil confined fission tracks in apatite
American Mineralogist (May 2024) 109 (12): 2026-2036
Fission-track analysis is a thermochronologic method for dating rocks and reconstructing their low-temperature thermal histories. We investigate the influence of the apatite composition on the etching of fossil confined fission tracks, and its consequences for the fission-track method. We conducted step-etch experiments with 5.5 M HNO (sub 3) at 21 degrees C on samples with etch pit diameters (Dpar) spanning most of the range for natural apatites (Panasqueira: 1.60 mu m, Slyudyanka: 2.44 mu m, Brazil: 3.92 mu m, and Bamble: 4.60 mu m) to determine their apatite etch rates v (sub R) (the rate at which each lattice plane is displaced parallel to itself) as a function of crystallographic orientation (Phi '). Our measurements revealed significant differences between the four samples. We fitted three-parameter functions, v (sub R) = a(Dpar)Phi ' e (super b(Dpar)Phi ') + c, describing v (sub R) as a function of the angle to the apatite c-axis for our hexagonal samples (excluding Bamble) and Durango apatite. The parameters a and b both exhibit a linear correlation with Dpar, whereas the constant c is small ( approximately 0.1 mu m min (super -1) ) and its between-sample variation negligible at the resolution of our measurements. Bamble exhibits a different, bimodal relationship between v (sub R) and Phi ', which we fitted with a sum of two sine functions. In all cases, including Bamble, there is a striking correlation between the angular frequencies of horizontal confined tracks and the magnitude of the apatite etch rate v (sub R) perpendicular to the track axes. This shows that the sample of confined tracks selected for measurement and modeling is to a much greater degree determined by the etching properties of the apatite sample than by geometric or subjective biases. The track etch rate v (sub T) is constant along most of the track length but varies from track to track. The mean v (sub T) correlates with Dpar, so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, v (sub L) , also correlates with Dpar. The length increments of individual tracks are however irregular. This points to an intermittent structure at the ends of the tracks.