The Morphology of Etched and Unetched Ion Tracks in Apatite as a Function of Orientation and Thermal Annealling

Abstract

Apatite samples from Durango, Mexico, and Otter Lake, Canada, have been irradiated in different orientations with 185 MeV Xe, 284 MeV Au, and 2.2 GeV Au ions in order to simulate spontaneous fission track formation as a function of annealing temperature, etching time, apatite chemistry, and the orientation of the track relative to the apatite structure. We have characterized the unetched tracks using small angle x-ray scattering and atomic force microscopy and the etched tracks using optical microscopy. Apatite is commonly used in fission track dating, and the data presented here have implications for the use the dating parameter Dpar, used to assess kinetic annealing rates in apatite. Dpar is based on the diameter of an “etch figure,” or an etched fission track at its intersection with the (10-10) face, measured parallel to the c-axis. The Dpar values are proportional to the annealing rate. Larger Dpar values indicate slower annealing kinetics. Dpar measurements of natural apatite have been used to calibrate track-length reduction rates, against which observed track lengths can be evaluated to extract information regarding the thermal history of apatite, such as residence time below 100 °C, as well as the cooling rate. We observed that etch figures show systematic reductions in diameter with increasing annealing temperature for isochronal heating experiments. This decrease is accompanied by increased variation in diameter, with standard deviations as high as 40% of the mean. This reduction in mean diameter and increase in variability occurs gradually as a function of increasing temperature in the 320-360 °C range, with an accelerated rate in the range 360-400 °C. Extrapolated to geological time and temperature scales, this variation explains the variability reported in the literature for Dpar measurements on natural apatite, because fission tracks in natural apatite are of different ages and have experienced different amounts of annealing. The decrease in etch figure diameter, however, is proportional to track length decrease during annealing. While the relation between track length and etch figure diameter presented here is simplified, it suggests that the nature of the relation between etch figure diameter and track length is straightforward and that more accurate models of track annealing could be developed. Such models could then be used to correct anomalously low Dpar values measured in natural apatite by comparison with track length. Also incorporated into this model is the formation of local zones of complete annealing along the track length. These zones have been described as crystalline“gaps” by Green et al. (1986). Since gap formation can occur at random points along the track length, increased variability in track lengths is expected, explaining the increased variation in etch figure diameters at higher annealing temperatures. The relative sizes of etch figures between Otter Lake and Durango apatite have been compared by calculation of percent differences (difference in diameter/average of both diameters) for different orientations. A two-fold greater percent difference was observed between (10-11) etch figures than between (10-10) etch figures measured for Dpar determination. Since kinetic proxy measurements compare relative lengths of etch figures among different apatite grains, the comparison of etch figures on the (10-11) growth face in addition to the (10-10) growth face may significantly increase the resolution of kinetic proxies used in fission track dating. Latent (i.e., unetched) tracks were observed using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). “Hillocks” resulting from irradiation are reported on the surface of unetched apatite. Hillocks have diameters in the range 15-30 nm and heights in the range 3-7 nm, and size dimensions show a systematic dependence on apatite composition and orientation. This dependence is confirmed by SAXS measurements. In addition, etch figure sizes show a similar trend as latent tracks, suggesting that damage production and etching are controlled by similar crystallographic parameters. Lastly, development of microscopic streaks was observed on surfaces of apatite heated to temperatures above the track-annealing threshold and subsequently etched. These features are approximately 2-20 µm in length, and their origin is unknown.