Using target-matching techniques combining Ar/Ar crystal-mapping with elemental mapping and high-resolution electron microscopy, this study investigates the Ar behavior in very-slowly cooled muscovite from the Harney Peak Granite (HPG, South Dakota, USA). Detailed age mapping along (001) in single crystals from different localities of the HPG documents age gradients in excess of 300400 m.y., with conspicuous internal Ar/Ar zoning. This suggests (001) layer-parallel Ar transport driven by diffusion, consistent with previous Ar/Ar crystal-mapping studies. The age distribution pattern is complex, however, and defines a mosaic of sub-grain domains with more retentive core zones, broadly 250300 m 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 Ar/Ar 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 nmlong), are 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 NaK interchange are inferred to control the bulk diffusion-domain structure of HPG muscovite. Quantitative diffusion modeling of coupled compositionaldefectisotopic variations indicates that Ar 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 commensurate with diffusion estimates independently derived from recent atomistic simulations.