Argon geochronology of small samples using the Vulkaan argon laserprobe

Argon geochronology is a versatile dating tool based on the accumulation of radiogenic ⁴⁰Ar as it decays from ⁴⁰K over time. The K Ar technique 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 described. We demonstrate the performance of the facility in terms of operating 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 demonstrate that our analytical uncertainty is in the order of 0.2%, which means that we can measure age differences ca. 10 times better than absolute ages where we are limited by external uncertainties such as the uncertainty of the decay constant of ⁴⁰K and the certainty in the absolute argon content of our standards. Results are presented on a new set of in-house mineral standards, biotite standard IGO2 (10.60 ± 0.42 Ma) suited for monitoring conventional K Ar experiments, and sanidine standard DRA1 (24.99 ± 0.07 Ma) for laser fusion experiments. We have used 3 different facilities for irradiation with fast neutrons: the Oregon State University TRIGA reactor (both the standard facility B3 and the Cd-shielded CLICIT facility), and the HFPIF facility in the high flux research reactor at Petten, The Netherlands. For each of these facilities the correction factors for undesired argon isotopes produced by neutron irradiation from isotopes of calcium and potassium was determined. We have demonstrated that the Cd shielded facility at OSU TRIGA has a very low production of ⁴⁰Ar, 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 irradiating very old samples.