The subaerial tholeiitic to alkalic basalts of Kohala, the oldest volcano on the Big Island of Hawaii, have been difficult to date accurately using K/Ar and 40Ar/39Ar dating techniques due to low K content and young ages (Lipman & Calvert, 2013). To address these issues, nine whole rock samples were each divided into three subsamples of varying magnetic susceptibility for 40Ar/39Ar dating. Subsample 1 was a highly magnetic split composed of magnetite bearing interstitial feldspar and glassy matrix. Subsample 2 was a low magnetic group of mainly plagioclase, olivine, and pyroxene. Subsample 3 was an intermediate group that is a mixture of the phases in the high and low magnetic groups. In addition to providing three independent age determinations for each whole rock sample, the method evaluates the initial 40Ar/36Ar ratio and the internal Ar isotopic distribution within the whole rock sample.Results indicate the initial 40Ar/36Ar ratio is not always equal to the modern atmospheric value nor is it always the same in different phases of a whole rock. The method utilized this conclusion to recalculate three independent isochron intercept plateau ages using the respective subsample initial 40Ar/36Ar ratio. Splitting the whole rock into subsamples to emulate the argon partitioning of mineral phases while using the determined 40Ar/36Ar ratio from each subsample’s inverse isochron in place of atmospheric argon (Schaen et al., 2020) yields more accurate age calculations. Compared to the reported ages of the oldest subaerial tholeiites on Kohala, “greater than 450 ka” (Lipman & Calvert, 2013) and 403 ± 141 (± 35%) ka (McDougall & Swanson, 1972), this study’s oldest exposed tholeiite, age 565.3 ± 5.9 (±1.0%) ka, is within two standard deviations of the previously reported ages. Thus, the method brings more precision and accuracy to 40Ar/39Ar chronology.
Zakiya A. Chikwendu (Thu,) studied this question.