ArAr

ArAr dating, short for argon-argon dating, is a radiometric dating method used in geochronology to determine the ages of potassium-bearing minerals and rocks. It is a refinement of the traditional K-Ar method, eliminating the need to introduce external argon and improving precision and internal checks.

Principle and measurement: In the ArAr technique, a mineral sample is irradiated in a nuclear reactor to convert a known portion of its 39K to 39Ar. The amount of radiogenic 40Ar produced since formation (40Ar*) is inferred from the measured ratio of 40Ar to 39Ar. Because 39Ar serves as a proxy for the sample’s potassium content and for the neutron fluence during irradiation, the age is calculated from the relation t = (1/λ) ln(1 + 40Ar*/39Ar), where λ is the decay constant of 40K. The sample is then heated, often stepwise or by laser, to release argon in steps. Isotopic ratios are determined by mass spectrometry, producing an age spectrum from which a plateau age (consistent ages over several consecutive steps) is interpreted as the true formation age.

Procedure and standards: The irradiation batch includes a monitor mineral with a known age to calibrate neutron flux. A standard of known age is measured alongside the sample to ensure inter-laboratory comparability. Potassium-bearing minerals suitable for ArAr dating include sanidine, orthoclase, biotite, and feldspars in volcanic rocks.

Applications and limitations: ArAr dating is widely used to date volcanic sequences, eruption ages, and tectonic events, often with high precision for the Quaternary and Cenozoic. Limitations include the need for a neutron source, potential argon loss or excess argon, and the requirement that the sample contains sufficient potassium and remains closed to Ar diffusion since formation.