Shock-effects on the K-Ar system of plagioclase feldspar and the age of anorthosite inclusions from North-Eastern Minnesota
Artificial shock pressures up to 52.5 GPa have no influence on the K-Ar system of plagioclase feldspar. The 40Ar- 39Ar analysis of feldspar (labradorite An 67 from anorthosite of North-Eastern Minnesota) shocked up to 45 GPa - in vacuo, to prevent massive entrapment of atmospheric argon-shows that the age spectra and the argon diffusion properties remain unaltered. Similar feldspar samples (labradorite An 51 from Nain, Labrador), shocked in air up to 52.5 GPa and dated by the conventional K-Ar method, also yield the same age as unshocked samples but with a higher atmospheric argon contribution. The Minnesota anorthosite has an 40Ar- 39Ar age of 1075 ± 10 m.y. No information on a possible previous history of the anorthosite became apparent.