Ar-40/Ar-39 ages in deformed potassium feldspar: evidence of microstructural control on Ar isotope systematics

Detailed field and microstructural studies have been combined with high spa tial resolution ultraviolet laser Ar-40/Ar-39 dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-relat ed microstructure and Ar isotope systematics. The sample studied is a simil ar to1,000 Ma Torridonian arkose from Skye, Scotland, that contains detrita l feldspars previously metamorphosed at amphibolite-facies conditions simil ar to1,700 Ma. The sample was subsequently deformed similar to 430 Ma ago d uring Caledonian orogenesis. The form and distribution of deformation-induc ed microstructures within three different feldspar clasts has been mapped u sing atomic number contrast and orientation contrast imaging, at a range of scales, to identify intragrain variations in composition and lattice orien tation. These variations have been related to thin section and regional str uctural data to provide a well-constrained deformation history for the feld spar clasts. One hundred and forty-three in-situ Ar-40/Ar-39 analyses measu red using ultraviolet laser ablation record a range of apparent ages (317-1 030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420-1,030 Ma, with the oldest apparent ages being fo und close to the centre of the feldspar away from fractures and the detrita l grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent ag e and the detrital grain boundary. Within this feldspar, the oldest apparen t ages are recorded from orientation domain boundaries and fracture surface s where an excess or trapped Ar-40 component resides, Orientation contrast images at a similar scale to the Ar analyses illustrate a significant defor mation-related microstructural difference between the feldspars and we conc lude that deformation plays a significant role in controlling Ar systematic s of feldspars at both the inter- and intragrain scales even at relatively low 'bulk' strains. The data show that Ar loss and trapping within the defo rmed K-feldspars reflects the presence of a deformation-induced population of small diffusion domains in combination with 'short-circuit' diffusion al ong deformation-induced defects. The complex history of microstructures ind uced in the K-feldspars during their cooling, alteration, erosion and sedim entation do not appear to be as significant as deformation-induced microstr uctures in controlling the distribution of apparent ages at the grain scale .