Variability of apatite fission-track annealing kinetics: I. Experimental results

Annealing rates for fission tracks in apatite vary markedly as a complex fu nction of composition-based on an experimental study of 15 well-characteriz ed, compositionally diverse apatites, Extensive annealing data were obtaine d in 69 experiments (durations of 1, 10, 100, and 1000 h at temperatures fr om 75 to 100 degrees C) on each of four apatites, three with near end-membe r occupancy of the halogen site by F, Cl, and OH, plus the well-known apati te from Durango, Mexico. These results were supplemented by less-comprehens ive annealing data from 12 experiments over the same range of time and temp erature on each of the remaining 11 apatites. Measurements of initial fissi on-track length, a parameter of considerable importance to the derivation o f time-temperature paths from fission-track data, reveal substantial variat ions from one apatite to another; initial lengths are best predicted from e tch figures. Interlaboratory comparisons of data on annealing kinetics high light discrepancies that appear to result largely from differences in the p recision and accuracy of experimental temperatures. None of the factors pre viously proposed as the dominant compositional controls on annealing rates can account completely for annealing behavior over the full range of compos itions studied. Nevertheless, relative rates of annealing among all apatite s are highly systematic, which allows this data set to be used in its entir ety to constrain multikinetic annealing models that predict fission-track l engths as a function of time and temperature.