KAr age determination of late Quaternary volcanic rocks using the “mass fractionation correction procedure”: application to the Younger Ontake Volcano, central Japan

We propose an age derivation procedure for the KAr dating method to be applied to late Quaternary volcanic rocks of low to medium K2O content, whose KAr ages calculated by the conventional derivation procedure provide insufficient age information. In this procedure, which we call the “mass fractionation correction procedure” (MFCP), the initial 40Ar/36Ar ratios of the samples are estimated from the stable 38Ar/36Ar ratios determined by the unspiked technique, assuming that the initial argon isotope ratios of all the samples should lie on a theoretical mass fractionation line of argon in the atmosphere. To examine the usefulness of the MFCP, systematic KAr age determinations using this procedure were made on volcanic rocks from the stratigraphically well-known Younger Ontake Volcano in central Japan, which has erupted repeatedly over the past 100 ka. The KAr ages determined for these volcanics by this procedure generally have analytical uncertainties within ± 25% (1σ level) and are consistent with the volcanostratigraphy. Exceptions were the large phenocrysts in pumice fall deposits: these may contain significant amounts of excess 40Ar and we considered them to be unsuitable for accurate KAr age determinations of young volcanic rocks. In the cases of lava flows which underlie a widespread fallout tephra, the KAr ages determined by the MFCP were consistent with the 14C age of the tephra (21–22 ka).