Discordant Potassium-Argon Model and Isochron “Ages” for Cardenas Basalt.indd


For more than twenty years it has been known that the Rb-Sr and K-Ar systems give discordant “ages” for Cardenas Basalt and associated Proterozoic diabase sills and dikes of Grand Canyon. Thirteen new K-Ar analyses of Proterozoic mafic rocks of Grand Canyon are added to nine published K-Ar analyses. We report a new fourteen-point K-Ar isochron “age” of 516 ± 30 Ma which is strongly discordant with the published Rb-Sr isochron “age” of 1.07 ± 0.07 Ga for Cardenas Basalt. By more than doubling the K-Ar data set we can test explanations for why the discordance exists. Advocates of the Rb-Sr isochron, recognizing the strong geochemical similarity of rubidium and potassium, have not argued for significant potassium addition to these rocks. Addition of potassium during alteration of these rocks would explain the anomously young K-Ar age, but it would also add rubidium and invalidate the Rb-Sr isochron age. Instead, advocates of the Rb-Sr isochron have argued only for significant argon loss. Two argon loss models (episodic loss and continuous loss) are tested in an attempt to explain why these altered rocks have about half the 40Ar required by the conventional Rb-Sr interpretation. Both argon loss models, although attempting to maintain the assumptions of conventional geochronology, fail to explain the data, especially the new data we offer. Three models are proposed as alternatives to argon loss models, but these invalidate using the K-Ar system as conventional geochronology would assume.


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