Argon isotopic composition of some Hawaiian historical lavas

A total of 28 historically erupted lava samples were collected from three active volcanoes (Hualalai, Mauna Loa and Kīlauea) on the island of Hawaiʽi, and their argon isotopic compositions were determined in order to clarify whether mass-dependent fractionation occurs in hotspot volcanoes and to find out how the effect of extraneous 40Ar can be avoided in K-Ar or 40Ar/39Ar dating. Comparison between groundmass and whole rock samples revealed that the latter has higher 40Ar/36Ar ratios than the former, indicating extraneous 40Ar contamination. The amount of extraneous 40Ar contained in whole rock samples corresponds to an apparent age increase of up to 0.5 my. Groundmass samples of Mauna Loa and Kīlauea agreed well with the mass fractionation line through the atmospheric composition. Many of the Hualalai groundmass samples, however, showed evidence of extraneous 40Ar contamination. Comparison between a 180-250 μm and 250-500 μm groundmass fraction revealed that the argon isotopic composition of a 180-250 μm fraction agreed with the mass fractionation line, while the 250-500 μm fraction still contained extraneous 40Ar, suggesting that the 180-250 μm fraction is more suitable for the accurate dating of young and/or low-K volcanics.