Direct comparison of astronomical and40Ar/39Ar ages of ash beds: Potential implications for the age of mineral dating standards

Abstract

We present first results of 40Ar/39Ar multiple single fusion datings on biotites and feldspars from two volcanic ash beds found on Crete, Gavdos and Koufonisi (Greece). Preferred 40Ar/39Ar ages - calculated against TCR sanidine with an age of 27.92 Ma, intercalibrated to an age of 28.09 ± 0.10 Ma (1σ) for FCT-3 biotite and an age of 24.99 ± 0.07 Ma for DRA sanidine - are slightly but consistently younger than astronomical ages obtained independently for the same ash beds. The best fit to the astronomical ages is obtained when the age of TCR sanidine is increased slightly to 27.98 ± 0.19 Ma, the age of FCT-3 biotite to 28.15 ± 0.19 Ma and the age of DRA sanidine to 25.05 ± 0.17 Ma. The ages for the standards arrive slightly younger if the 40Ar/39Ar age - of 6.936 ± 0.006 Ma (1σ) - for a single pure sanidine separate of the lower ash - dated astronomically at 6.941 Ma - is considered most reliable. Copyright 1997 by the American Geophysical Union.