Semi-quantitative determination of apatite anion composition for fission-track analysis using infra-red microspectroscopy

Significant differences in fission-track annealing kinetics indicate that chlorapatites may give older fission-track ages and record different thermal histories to those derived from co-existing fluorapatites which have experienced the same geological history. Fourier-transform infra-red (FTIR) reflected light microspectroscopy offers a rapid, routine non-destructive technique to analyse those crystals upon which fission-track ages and lengths have been measured. IR analysis of a series of apatites of known F–Cl composition has shown an inverse relationship between chlorine content and wavenumber of peaks in the IR spectrum. Resultant calibration curves provide an estimation of F–Cl content of any IR-analysed crystal. FTIR analyses of single apatite grains upon which ages have been calculated show an increased resistance to fission-track annealing in apatite related to a decrease in IR wavenumber (corresponding to Cl-contents greater than a threshold of ∼1 wt.%) rather than a linear increase in annealing resistance corresponding to increase in chlorine content. Such behaviour suggests annealing is affected by modifications to crystal structure to accommodate the substitution of large ions such as, but not necessarily only, chlorine. The possible substitution of other large ions may offer hitherto unrecognised complications in deriving thermal history from apatite fission-track data.