Early development of Argon-39 as a new geochronometer for geologic processes on short time scales

Up until now, the cosmogenic radioisotope ³⁹Ar has not been used for surface exposure or burial dating of minerals due to its low concentration in rocks and the large sample size requirements for its detection by low-level counting. The novel analytical method Atom Trap Trace Analysis (ATTA) – already well established for gas samples from groundwater, ocean water or ice cores – can measure the isotopic ratio of ³⁹Ar to stable Ar in the range of 10^-16 on just a few ml STP of argon and therefore opens up new possibilities for applying ³⁹Ar.

This talk will report on the initial steps taken towards using ³⁹Ar as a geochronometer. Calculations of production rates of ³⁹Ar in typical continental rocks, exposed to cosmic radiation at the Earth surface, show that sample sizes of the order of 100 g of rock should yield a sufficient number of ³⁹Ar atoms (order 10³ to 10⁴) for detection by ATTA. However, the amount of ⁴⁰Ar in such samples – and therefore the total extractable Ar amount - is much lower than what is typically extracted from ice and water samples, which contain atmosphere-derived Ar. The ⁴⁰Ar content in rock stems from ⁴⁰K-decay and depends on the rock formation age and the potassium content. Dilution with ³⁹Ar-free Ar results in sufficient total argon volumes for the standard ATTA analysis.

Gas extractions from heated rock samples indeed show ³⁹Ar isotope abundances 2-3 orders of magnitude above the atmospheric ratio, well within the measurement range of ATTA after dilution. In order to check the feasibility of exposure dating of rocks, several samples were taken for comparison from boulders of glacier moraines, previously dated with ¹⁰Be, in the Jamtal valley in Austria. Additionally, ¹⁰Be-dated samples from other moraine sites are to be analysed for ³⁹Ar for further validation of ³⁹Ar as a tool for exposure dating. As of now it remains to be seen whether reliable agreement between the dating methods can be achieved. Due to its relatively short half-life of 268 years, ³⁹Ar would be a useful addition in multi-tracer studies on geologic processes within the last two millennia.