40AR/39AR AGE GRADIENTS AMONG SINGLE CRYSTALS OF COEXISTING ALKALI FELDSPARS: THE BENSON MINES ORTHOCLASE REVISITED

⁴⁰Ar/³⁹Ar dating of potassium feldspars provides powerful means for evaluating low-temperature thermochronologic histories. In typical practice, for a vast number of studies, ⁴⁰Ar/³⁹Ar ages are determined for bulk alkali feldspar separates prepared from a grain size fraction following crushing and concentrating a pure separate. Early studies of orthoclase from the Benson Mines in the northwestern Adirondack highlands (Foland, 1974) emphasized argon diffusion over the physical scale of orthoclase crystals, a result reaffirmed by more recent studies of Foland & Xu (1990) and also Cassata & Renne (2013). If the diffusion length scale approaches the physical grain size, crushing and grain size reduction for the analysis of a bulk sample can sensibly be expected to alter and misrepresent the actual ⁴⁰Ar distribution in feldspars. In order to assess the role of grain size and other physical characteristics of feldspars upon resulting ⁴⁰Ar/³⁹Ar ages, we have undertaken a single crystal study of lithologies from the Benson Mines in the ANIMAL facility. The mine interiors are presently inaccessible, and samples of sillimanite-orthoclase-biotite-magnetite gneiss, granite, amphibolite, and pegmatite were selected from waste piles. K-feldspar crystals from the sillimanite gneisses yield the oldest ⁴⁰Ar/³⁹Ar ages observed that begin with ages of ca. 840 Ma and increase to a maximum of ca. 875 Ma. In contrast, alkali feldspars from pegmatites yield very discordant spectra with ages beginning as young as ca. 460 Ma and increasing to ca. 850 Ma. The younger and more discordant results for the feldspars from pegmatites may reflect the fact that they are post-metamorphic intrusions with a higher fluid activity than that which characterized the high-grade gneiss. Differences in the age distributions we observe for feldspars of a given rock (more or less discordance, or differences in absolute age) are consistent with our analysis of fragments that differ in size and shape and present variable representations of their natural grain-scale diffusion geometries. Results of the present study expand the earlier work of Foland and others for Benson Mines, and emphasize the importance of analytical strategies to recover natural diffusion gradients in radiogenic ⁴⁰Ar concentrations at the individual feldspar crystal scale.