The determination of a series of ages of a Hawaiian volcano by the potassium-argon method

Abstract

A mass spectrometer, gas extraction system, and associated apparatus has been assembled in order to perform potassium-argon age determinations on young, low-potassium rocks typically found in the Hawaiian Is lands. The Waianae Volcano Range of Oahu, Hawaii, was systematically dated; the results generally corroborating earlier work by other investigators. Discordantly greater ages were found for certain biotite and hornblende fractions obtained from a 2.3 my rhyodacite. It is suggested that the discordant ages result from excess radiogenic argon held in fluid inclusions in the minerals. This belief is substantiated by visual evidence, vacuum crushing experiments, and the synthesis of a f1uoromica that contained inclusions. Apparent ages of 160 to over 3000 my were obtained from olivines, pyroxenes, and a feldspar separated from a number of Hawaiian xenoliths and inclusions. Again, it is believed that these abnormally great ages can be attributed to excess radiogenic argon contained in the many fluid inclusions observed in these rocks. The results of this research emphasize the need of multiple, yet selective sampling for potassium-argon dating of young volcanic environments in order to preclude any possibility of excess radiogenic argon cognately or accidentally incorporated into random samples.