Whole-rock dating of young extrusives by the potassium-argon method

Abstract

The feasibility of dating very young extrusives by the wholerock potassium-argon technique has been investigated. Rocks from the Columbia River Basalts, Oregon, Rio Grande de Santiago volcanics, Mexico, San Francisco Volcanic Field, Arizona, Hawaii and Australia have been studied. Some of the results point out the grave difficulties that can arise due to the presence of small quantities of excess argon in such rocks. ·Comparison on the ages of rocks in the age range 0 to 5 million years strongly suggest that inter-laboratory differences exist especially in the detection and measurement of small quantities of radiogenic argon. Study of a very thick extrusive of Recent age indicates that atmospheric argon may have been incorporated into the edges of the flow as it solidified. The quantity of argon present and its isotopic composition is not homogeneous throughout the lava flow. Wherever possible, the data are examined on potassium-argon isochrons. This yields information about the precision of the individual analyses. Lack of total control over the “blanks" during the argon analyses, does not allow unequivocal interpretation of the meaning of the "ages" obtained.