ARGON GEOCHRONOLOGY OF SMALL SAMPLES USING THE VULKAAN ARGON LASERPROBE

Argon geochronology is a versatile dating tool based on the accumulati on of radiogenic Ar-40 as it decays from K-40 over time. The K/Ar tech nique can be used with success from a younger limit of several 10's of ka to the oldest rocks available. In this paper, the argon laserprobe facility (VULKAAN) at the Vrije Universiteit in Amsterdam is describe d. We demonstrate the performance of the facility in terms of operatin g characteristics and intercalibration using mineral standards of well known age. There are no systematic biases in our operating technique when comparing our results to those of other laboratories. We demonstr ate that our analytical uncertainty is in the order of 0.2%, which mea ns that we can measure age differences ca. 10 times better than absolu te ages where we are limited by external uncertainties such as the unc ertainty of the decay constant of K-40 and the certainty in the absolu te argon content of our standards. Results are presented on a new set of in-house mineral standards, biotite standard IGO2 (10.60 +/- 0.42 M a) suited for monitoring conventional K/Ar experiments, and sanidine s tandard DRA1 (24.99 +/- 0.07 Ma) for laser fusion experiments. We have used 3 different facilities for irradiation with fast neutrons: the O regon State University TRIGA reactor (both the standard facility B3 an d the Cd-shielded CLICIT facility), and the HFPIF facility in the high flux research reactor at Petten, The Netherlands. For each of these f acilities the correction factors for undesired argon isotopes produced by neutron irradiation from isotopes of calcium and potassium was det ermined. We have demonstrated that the Cd shielded facility at OSU TRI GA has a very low production of Ar-40, which makes it very well suited for dating young minerals, whereas the Petten facility has the higher neutron fluxes that make that reactor more appropriate for irradiatin g very old samples.