The viability of leucite for ⁴⁰Ar/³⁹Ar dating and as a quaternary standard

The use of leucite (KAISi₂O₆) as a ⁴⁰Ar/³⁹Ar dating standard has been avoided largely because of concerns over ⁴⁰Ar_excess. Laser-fusion and resistance furnace step-heating ⁴⁰Ar/³⁹Ar analyses on leucite from a pyroclastic flow deposit from the Alban Hills are concordant and have atmospheric (⁴⁰Ar/³⁶Ar ≈ 292.1 ± 2.1 and 299.7 ± 2.5, respectively) initial isotopic ratios. The error weighted mean ⁴⁰Ar/³⁹Ar ratio from step-heating and laser fusion data (N ≈ 44) yields an age of 408 ± 2 ka, including propagated errors in standards' isotopic ratios. The step-heating analysis indicates that leucite degasses at quite low temperatures ( ∼ 1100°C), making it more suitable for K/Ar age analysis than sanidine. The mean and standard deviation from 10 potassium measurements by flame photometry and the error-weighted mean from five argon extractions produce a K/Ar age of 405 ± 3 ka, in good agreement with the ⁴⁰Ar/³⁹Ar data and with precision comparable to several widely used primary K/Ar standards. This work suggests that leucite could become a very useful primary ⁴⁰Ar/³⁹Ar standard for dating Quaternary age rocks, thereby avoiding errors introduced by intercalibrating a Quaternary-age ⁴⁰Ar/³⁹Ar standard with a primary K/Ar standard.