Defect-controlled 40Ar diffusion-domain structure of white micas from high-resolution 40Ar/39Ar crystal-mapping in slowly-cooled muscovite

Using target-matching techniques combining 40Ar/39Ar crystal-mapping with elemental mapping and high-resolution electron microscopy, this study investigates the 40Ar behavior in very-slowly cooled mus-covite from the Harney Peak Granite (HPG, South Dakota, USA). Detailed age mapping along (001) in sin-gle crystals from different localities of the HPG documents age gradients in excess of ti 300-400 m.y., with conspicuous internal 40Ar/39Ar zoning. This suggests (001) layer-parallel 40Ar transport driven by diffusion, consistent with previous 40Ar/39Ar crystal-mapping studies. The age distribution pattern is complex, however, and defines a mosaic of sub-grain domains with more retentive core zones, broadly ti 250-300 lm across, separated by zones of high diffusivity varying in shape and extent. The maximum ages preserved in the core domains are independent of their size but vary linearly with the bulk areal extent of the peripheral (or surrounding) high-diffusivity zones. Spatial 40Ar/39Ar relationships inside each grain point to a mechanism of multipath continuum-diffusion interaction between subdomains across the whole crystal, rather than via discrete non-interracting domains such as in K-feldspars. A close spatial correlation exists between younger ages, Na-depleted (K-enriched) zones, and density of microstructural defects. These defects, identified as lenticular voids and basal partings (< 100 nm-long), developed in response to inward K $ Na interdiffusion during late-magmatic stages, in the absence of deformation. Coupled variations in density of microstructural defects and Na-K interchange are inferred to control the bulk diffusion-domain structure of HPG muscovite. Quantitative diffusion modeling of cou-pled compositional-defect-isotopic variations indicates that 40Ar diffusivity may be enhanced by up to six orders of magnitude in defect-controlled high-diffusivity zones relative to less defective (pristine) domains. On the other hand, empirical diffusivity estimates required to preserve the core ages are com-mensurate with diffusion estimates independently derived from recent atomistic simulations. (c) 2022 Elsevier Ltd. All rights reserved.