Ar-40-Ar-39 ages of rocks and glasses from the Noerdlinger Ries Crater and the temperature history of impact breccias

Abstract

We report on a ⁴⁰Ar–³⁹Ar study of hornblende, biotite, and glass samples from the Nordlinger Ries impact crater. The samples are derived from various depths (377–1,200 m) of the Forschungsbohrung 1973 drill core, from the ejecta blanket, and the crystalline crater rim. All mineral separates display ⁴⁰Ar–³⁹Ar plateau ages of 320 ± 3 m.y. The data represent the first direct age determination of the Ries bedrock. The plateau ages of suevite and moldavite, both generated in the Ries impact, date the cratering event which occurred 15 m.y. ago. The results from the mineral separates imply that shock pressure alone, even as high as 450 kbar, cannot reset K–Ar ages of hornblende and biotite. This result is significant for the interpretation of ages of impact breccias in general, and for the interpretation of lunar highland rock ages in particular. ⁴⁰Ar–³⁹Ar studies yield, as a side result, data on the natural loss of radiogenic ⁴⁰Ar. In this report we emphasize on this aspect to estimate the cooling history of the suevite layer. We compare the measured ⁴⁰Ar-loss to the loss calculated from a simple cooling model of the layer and from the diffusion properties of the minerals as obtained in the stepwise heating experiment. The resulting upper limit for the post-shock equilibrium temperature within the suevite layer is 450°C.

        ARK: https://n2t.net/ark:/88439/y028544

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