Laser probe 40Ar [sbnd]39Ar measurements of loss profiles within individual hornblende grains from the Giants Range Granite, northern Minnesota, USA

Abstract

A Nd-glass laser has been used as a “probe”, mapping natural argon diffusion in partially outgassed hornblende grains at a resolution of around 50 μm. The hornblendes are from the Giants Range Granite (2700 Ma), Minnesota, taken approximately 2 km from the contact with the Duluth Gabbro Complex (1100 Ma). The full range of ages can be discerned within individual hornblende grains using the laser extraction technique, but more importantly, diffusion profiles up to 500 μm in length have been detected. The profiles reflect large variations in “domain” size within single grains. Diffusive argon loss was controlled not only by lattice-dominated volume diffusion, but a network of fast diffusion paths (now defined by biotite inclusions) and also indirectly by mineral reactions associated with the contact metamorphism. Three natural argon loss profiles from subgrains of different sizes yield good fits to theoretical volume diffusion loss curves, and a mean time-integrated diffusion parameter ( D(t)dt) of (7.1 ± 0.9) × 10 −5 cm 2. This parameter constrains the possible heat loss models for the Duluth Gabbro.