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

The use of leucite (KAlSi2O6) as a 40Ar/39Ar dating standard has been avoided largely because of concerns over 40Arexcess. 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 standardsʹ 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.