40Ar/39Ar Dating of Feldspars from the KTB Deep Drill Hole: Insight into Post-Orogenic Thermal Relaxation and AR Diffusion Systematics During Closure

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9219595 Zeitler The 40Ar/39Ar dating of feldspar has enormous potential for helping geologists determine quantitative temperature-time paths over the range of 150oC to 350oC. Such time-temperature information over such a range of low temperatures is of great importance to scientists evaluating such issues as the architecture and evolution of the crust or the origin and evolution of sedimentary basins and their attendant water and hydrocarbon resources. However, to this point the technique has only been tested in short-term laboratory heating experiments and in presumably well-characterized geological contexts. Now that sophisticated interpretative models have been developed for use in quantitative thermochronometry, it is important that the 40Ar/39Ar dating of feldspar be evaluated in a slow-cooling geological environment. The KTB deep drill hole in Germany offers an ideal natural laboratory with which to examine the closure of feldspar to diffusive Ar loss during slow cooling, as this hole already has reached temperatures which overlap the closure interval for feldspars and is projected to reach ~300oC. In this project, a suite of samples from deep in the KTB hole will be examined which will permit the principal investigator to view the process of Ar closure as it is happening, and would be an invaluable benchmark for virtually all applied studies which use feldspar thermochronometry. At the same time, the understanding of 40Ar/39Ar systematics in feldspar is sufficiently good that study of shallow samples from the KTB hole will permit the principal investigator to monitor in detail the post-orogenic cooling of the local crust; he will assess the relative roles that thermal relaxation and denudation have played, and will look for evidence of even a very mild Tertiary thermal event that might be responsible for the high gradients seen in the KTB hole. Such information will be highly useful for models of thermal evolution of the lithosphere du ring and after mountain building. ***