Isotopic fractionation of a loosely held atmospheric argon component in the Picture Gorge Basalts

摘要: Abstract Seven lava flows from the Picture Gorge “type section”, central Oregon, U.S.A., have been dated at 15.2 ± 0.6 m.y. by K-Ar method. The preparation of samples for argon extraction work strongly affects ages obtained, due to presence very large quantities atmospheric held “loosely” within most rocks. Use single pieces rock results in incomplete removal this gas, isotopic fractionation and grossly incorrect ages. When powdered are used, increase surface area per unit mass helps argon. Basalts contain traces clays montmorillonite group; on hydration these swell up apparently shut off minute cracks rocks, thereby trapping substantial amounts It is recommended that when dating whole-rock showing alteration has be attempted, specimens crushed approximately 10–30 mesh size prior work.