(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 potentia l 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 diff erent geochemical environments (plutonic, pneumatolytic and hydrotherm al) were probed. Isotope dilution techniques were used for concentrati on 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 samp les. Comparisons of the apatite He indices with plausible reference ag es of the sampling locations in combination with results obtained from diffusion experiments show that the (U+Th)/He dating method applied t o 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 sugges t that the high-temperature apatites (plutonic and pneumatolytic) may be more suitable for He dating than hydrothermal apatite. Important cr iteria for the dating of apatite by (U+Th)/He are the presence of mode rate radionuclide concentrations (tens of ppm) and a minimum abundance of inclusions.