(URANIUM PLUS THORIUM) HELIUM DATING OF APATITE - EXPERIENCE WITH SAMPLES F ROM DIFFERENT GEOCHEMICAL ENVIRONMENTS

U-He dating, which during its history has been very problematic, might find special application in the field of hydrothermal mineralizations where other methods generally fail. The mineral apatite is a potential candidate for this method. In order to shed light on various aspects of apatite dating by the (U+Th)/He method, two samples each from different geochemical environments (plutonic, pneumatolytic and hydrothermal) were probed. Isotope dilution techniques were used for concentration analyses from which He-4 model dates, also called ''helium indices'' after N.B. Keevil, were calculated. He-4 diffusion measurements were carried out for comparison of the He-4 retentivities between the samples. Comparisons of the apatite He indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the (U+Th)/He dating method applied to apatite yields an age of cooling rather than an emplacement age. The calculated closure temperatures are similar to those obtained by the fission-track method (approximately 100-degrees-C). Our results suggest that the high-temperature apatites (plutonic and pneumatolytic) may be more suitable for He dating than hydrothermal apatite. Important criteria for the dating of apatite by (U+Th)/He are the presence of moderate radionuclide concentrations (tens of ppm) and a minimum abundance of inclusions.