The use of hornblendes and pyroxenes for K-Ar dating

Abstract

The K-Ar ages of 12 hornblendes, one actinolite, and two pyroxenes were determined. When these ages are compared with ages of associated biotite, feldspar, or zircon, good agreement is found in most cases. No evidence is found for the existence of ‘excess’ radiogenic argon in these hornblendes. A maximum limit of 5×10−7 cc STP/g can be placed on possible ‘excess’ radiogenic argon in one sample. The potassium content of the amphiboles and pyroxenes is high enough so that Paleozoic or older samples can be easily dated, using present techniques. The rubidium content of the hornblendes is too low to be generally utilized for Rb-Sr dating.