Single crystals of dispersed trace pyrite from two igneous rocks were dated by using the 40Ar-39Ar laser probe method, and the resulting ages were compared to step-heating ages of associated amphibole and phlogopite. The isochron ages for 0.1–0.3 mm cubes of pyrite agree well with the respective amphibole ages of these igneous samples. Preliminary step-heating analyses of single pyrite crystals yield internal isochrons with indistinguishable, although less precise, ages. Both pyrite and amphibole are significantly older than associated phlogopite in one sample, which has undergone postcrystallization alteration. These results suggest that pyrite can give reliable and precise 40Ar-39Ar ages even in the presence of subsequent alteration. Given the ubiquity of pyrite in many geologic environments, this technique has great promise for application to the dating of ores, sediments, and hydrocarbon migration, all of which have been very difficult to date directly.

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