Excess radiogenic argon in pyroxenes

Abstract

In a locality whose age is known from U--Pb dating to be 1050 to 1150 million years (m.y.), K-Ar dating of two pyroxene samples yields apparent ages of 1500 to 10,400 ' m.y. Concordant K--Ar dating of two hornblende samples yields ages that agree more closely with the U--Pb results, namely 900 and 1000 m.y. The high apparent ages of the pyroxene samples are surmised to result from the inclusion of excess Ar/sup 40/ in the pyroxenes during their initial crystallization or during a later recrystallization. It is noted that loss of K at some time during the history of the pyroxenes, without simultaneous loss of Ar, could also raise the apparent age; the probability of such an occurrence is considered to be very slight. (T.F.H.)