The age of the Mistastin Lake crater, Labrador, Canada

Abstract

40Ar/ 39Ar age determinations have been carried out on eight samples of melt rocks and one of the maskelynite from Mistastin Lake impact crater, Labrador. The observed 40Ar * evolution spectra of the impact melts fall into distinct groups which correlate with petrographic variations. The release patterns of six of the melt rock samples define an age plateau in the range 34–41 m.y.; the other two have complex spectra which indicate incomplete equilibration of inclusions. Four of the samples with well-defined plateaux exhibit a high-temperature sag in their 40Ar/ 39Ar ratio similar to that observed in some lunar samples. Maskelynite gives a partially overprinted spectrum which rises monotonically to a final age near 700 m.y., approximately half the age of the country rocks. The data from the melt samples are interpreted as indicating an age of 38 ± 4 m.y. for the Mistastin Lake impact event. Previously, it had been considered that this crater was 202 ± 25 m.y. old.