The effects of deformation-induced microstructures on intragrain Ar-40/Ar-39 ages in potassium feldspar

A combination of forescattered scanning electron microscopy and both ultrav iolet (UV) laser and cycle stepped-heating Ar-40/Ar-39 dating techniques ha ve been applied to a single deformed grain of potassium feldspar to investi gate the relationship between microstructure and Ar-40/Ar-39 isotope system atics. The form and distribution of deformation-induced microstructures hav e been mapped by using intragrain variations in lattice orientation. These variations are related to thin section and regional structural data to prov ide a well-constrained deformation history for the feldspar grain. Thirty U V laser Ar-40/Ar-39 analyses show age variations that correlate with the in ternal microstructure. Stepped-heating data provide further evidence of thi s relationship. Our approach allows the recognition of different Ar reservo irs within the deformed feldspar and identifies microstructural controls on Ar migration. Such a relationship has important implications for the under standing and interpretation of Ar-40/Ar-39 geochronological data and the su bsequent use of such data to constrain the temperature vs. time evolution o f deformed rocks.