The 40Ar/39Ar geochronology method is capable of high precision (<0.05%), but remains limited by relatively large uncertainties in 40K decay constants and the ages natural reference mineral standards. most widely used well-known ca. 28 Ma Fish Canyon Tuff sanidine (FCTs). Several studies have attempted to calibrate FCTs against astronomically tuned tephra Crete (Faneromeni A1 tephra) Morocco (M...