Fission-track annealing characteristics of meteoritic phosphates

Laboratory annealing experiments have been conducted on merrillite and chlorapatite crystals from the Estacado (H6) chondrite, and on merrillite from the Bondoc mesosiderite. The annealing behaviour of the two phosphate minerals in Estacado was found to differ in that the annealing curves obtained were distinct for each mineral and temperatures required for total annealing of fossil tracks were significantly higher for merrillite than for apatite. Nevertheless, estimates of effective closing temperatures for these minerals, on the basis of experimental results obtained to date, do not indicate a significant difference between the two (84 +- 10 deg C for Estacado merrillite; 66 +- 10 deg C for Estacado apatite); both estimates being based on the annealing of fossil tracks and assuming a cooling-rate of approx. 1 deg C/Myr. Annealing expriments were also conducted with merrillite from the Bondoc mesosiderite, and both fossil and freshly-induced tracks were found to be less easily annealed than in the case of Estacado merrillite. An estimated closing temperature of 134 +- 15 deg C was deduced from the results so far obtained, using freshly-induced tracks and assuming a cooling-rate of 0.2 deg C/Myr. (author)