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

A combination of forescattered scanning electron microscopy and both ultraviolet (UV) laser and cycle stepped-heating ⁴⁰Ar/³⁹Ar dating techniques have been applied to a single deformed grain of potassium feldspar to investigate the relationship between microstructure and ⁴⁰Ar/³⁹Ar isotope systematics. The form and distribution of deformation-induced microstructures have been mapped by using intragrain variations in lattice orientation. These variations are related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar grain. Thirty UV laser ⁴⁰Ar/³⁹Ar analyses show age variations that correlate with the internal microstructure. Stepped-heating data provide further evidence of this relationship. Our approach allows the recognition of different Ar reservoirs within the deformed feldspar and identifies microstructural controls on Ar migration. Such a relationship has important implications for the understanding and interpretation of ⁴⁰Ar/³⁹Ar geochronological data and the subsequent use of such data to constrain the temperature vs. time evolution of deformed rocks.