SPEED DATING!: ADVICE ON SAMPLING AND APPLICATIONS FOR 40AR/39AR GEOCHRONOLOGY

The ⁴⁰Ar/³⁹Ar geochronological method can provide precise and accurate age constraints on rocks and minerals containing potassium, with an application range extending from the Holocene to as old as the Earth. The method has been applied to questions including tectonics, stratigraphy, volcanism, planetary geology, archaeology, and paleoclimate. However, obtaining age control from the ⁴⁰Ar/³⁹Ar method requires careful attention to the entire process, from sampling through to data reduction. Although much of this is done by ⁴⁰Ar/³⁹Ar specialists, project conception and sampling is frequently conducted by other geoscientists. The purpose of this presentation is to introduce the method, sampling techniques, and questions that can be answered using ⁴⁰Ar/³⁹Ar geochronology.

⁴⁰Ar/³⁹Ar geochronology relies on the decay of ⁴⁰K to ⁴⁰Ar, which has a half-life of 1.25 Ga. For most igneous and metamorphic rocks, the ⁴⁰Ar is trapped in material once it has cooled to a temperature below which the diffusion of argon is negligible. This decay forms the basis of the K-Ar method. ⁴⁰Ar/³⁹Ar is a variant of that method, which uses the transmutation of ³⁹K into ³⁹Ar via a neutron-proton reaction using a nuclear reactor. Details of the ⁴⁰Ar/³⁹Ar method will be presented, highlighting how ages are calculated by measurements of argon isotopes.

It is highly recommended that scientists begin incorporating geochronology early in their projects, to consider bringing geochronology specialists on board when planning the work, and collecting samples with their advice and/or in their presence. A wide variety of potentially datable materials exist, and consulting a specialist early in the process can help identify the most appropriate path forward. Ultimately sample selection depends on the question, but the material must be K-bearing, and the context of the formation of the material must be included in the selection criteria.

Our presentation and presence at the booth will allow for scientists to obtain information about the applicability of the ⁴⁰Ar/³⁹Ar method to their problem, via access to leading ⁴⁰Ar/³⁹Ar geochronologists and displays and hand-outs that will be available.