Abstract
This is a report on the first comparative study of the effects of experimental and natural shock on the K-Ar system. First, mineral separates from a 451 ± 1 Ma old gneiss, experimentally shocked up to 60 GPa, were investigated by stepwise heating 40Ar- 39Ar technique. Compared with an unshocked sample, the shocked samples have lower 40Ar- 39Ar ages, and the diffusion properties have changed depending on peak shock pressure and mineralogic composition. Total resetting of the K-Ar clock did not occur. Secondly, to compare these results with the effects of natural shock, samples from the ~23 Ma old Haughton impact crater, Devon Island, Canada, with different shock stages were investigated. Up to shock stage III, no total resetting of the K-Ar clock was observed, but severe argon losses, depending on shock pressure, result in 40Ar- 39Ar plateau ages which are incorrect in terms of the true crater age, yet they provide an upper limit to the time of the cratering event.
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