Problems with argon: K-Ar ages in Gulf Coast shales |
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Authors: | Mark Wilkinson |
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Affiliation: | Department of Geology and Geophysics, Grant Institute, West Mains Road, University of Edinburgh, Edinburgh, Scotland EH9 3JW, UK |
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Abstract: | K-Ar ages of Gulf Coast shales do not correspond to diagenetic ‘events’, but are a combination of inherited detrital ages and authigenic overprints. The key to understanding K-Ar ages of shales is the fate of the 40Ar when the host minerals dissolve or react during the illitisation reaction: is 40Ar incorporated within the illite, or is it lost to the porewaters? Ages are modelled as due to a single illitisation reaction, assuming a uniform precursor sediment and open-system import of K. Both whole-rock and clay mineral fraction data can be reproduced by modelling. The 40Ar incorporated into illite is a surprisingly high proportion (75-100%) of that produced within detrital phases (smectite, K-feldspar, mica), perhaps implying that the illitisation reaction is a solid-state transformation, or that 40Ar is imported into the shales along with incoming K. The model does not provide a good fit to the data below 4000 m, when the illitisation reaction is apparently complete, but shales lose 40Ar by solid-state diffusion or recrystallisation. |
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Keywords: | Diagenesis Shale Argon Gulf Coast Modelling |
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