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.
Hawaii Institute of Geophysics Contribution No. 133.
Paper read at the IAV International Symposium on Volcanology (New Zealand), scientific session of Dec. 2, 1965.