Problems in the dating of volcanic rocks by the potassium-argon method
The potassium-argon method is attractive for dating volcanics since it can be applied to rocks of Pleistocene age and older, thus encompassing important periods of general volcanic activity. However it has been found that dates obtained on whole rocks and on included minerals frequently show gross discordances. In order to establish this dating method in this application an attempt has been made to trace the sources of the anomalies.
To illustrate these efforts, dating results from a rhyodacite of Mauna Kuwale, Oahu, Hawaii, are reported. Determinations on several minerals and the whole rock of this ridge give a concordant age of 2.3 ± 0.3 million years, excluding some few results on minerals which show old age anomalies.
It has been noted that xenoliths in certain Hawaiian volcanics contain fluid inclusions which show evidence of formation at depth. We have found that gas released from such inclusions by crushing contains radiogenic argon, and that the constituent minerals give very old potassium-argon ages (circa 800 million years). Similar gaseous inclusions have been noted in a variety of other lava phenocrysts, and their presence in a dated sample may produce an anomalous old age.
In the minerals from Mauna Kuwale sporadic occurrences of inclusions have been noted in biotites and hornblendes, and crushing of the mineral releases the excess radiogenic argon. The determination of the age of such a material would give an old age, and thus account for the anomalies found.
For meaningful dating of volcanics by the potassium argon method it is concluded that phenocryst-containing materials should be examined for fluid inclusion content, and samples which contain these should be rejected.
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- Dalrymple, G. B. andHirooka, K., 1965 —Variation in potassium, argon and calculated age in a late Cenozoic basalt. J. Geophys. Res. 70, 5291–96.Google Scholar
- Macdonald, G. A., 1940 —Petrography of the Waianae Range, Oahu. Hawaii Div. Hydrog. Bull., 5, 61–91.Google Scholar
- Macdonald, G. A. andKatsura, T., 1964 —Chemical composition of Hawaiian lavas. J. Petrology, 5, 82–133.Google Scholar
- Richter, D. H. andMurata, K. J., 1960 —Xenolith nodules in the 1800-01 Kaupulehu flow of Hualalai Volcano and their petrologic implication. Proc. Hawaiian Acad. Sci., 35th Ann. Meeting.Google Scholar
- Roedder, E., 1965 —Liquid CO2inclusions in olivine-bearing nodules and phenocrysts from basalts. Amer. Mineral., 50, 1746–82.Google Scholar