Sm. Reddy et al., The effects of deformation-induced microstructures on intragrain Ar-40/Ar-39 ages in potassium feldspar, GEOLOGY, 27(4), 1999, pp. 363-366
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.