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 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/Ar-36 = 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 (similar to 1100 degreesC), 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. (C) 2001 Elsevier Science B.V. All rights reserved.