Abstract
A new technique of Ar analysis by neutron activation utilizing the reactions Ar 40 (n,γ) Ar 41 (half-life 110 min) and Ar 36(n,γ) Ar 37 (half-life 35 days) has been perfected for use in potassium-argon dating. The gas is extracted from minerals by fusion, and a known fraction sealcd in an ultra-pure silica vial. Unknowns and standards are irradiated together and immediately after irradiation each sample is repurified in the presence of carrier argon and pumped into calibrated counter tubes, which are finally filled to 76 cm pressure with counting gas. Measurement of the Ar 41 and Ar 37 activities, which are distinguished by their differing half-lives, permits analysis for both Ar 40 and Ar 36.By comparison with air standards of known volume and fixed Ar 40 to Ar 36 ratio it is possible to obtain an analysis for argon and a correction for air contamination. The precision and accuracy of the analysis are ~5 per cent; due to Ar losses during sample preparation the confidence limits of the dates reported are +15 per cent, −5 per cent; counting statistics and uncertainity in the blank correction limit the precision and accuracy for very young samples with K contents greater than 5 per cent to ~±2 m.y. Thirty granitic plutons in eastern Nevada and western Utah have been dated. Plutons in the Southern Snake Range, Dolly Varden Mountains, Whitehorse Range, Cortez Range, Austin mining district, and at Mount Hamilton and Whistler Mountain are of Jurassic and early Cretaceous age. The Ruby Hill stock. Eureka mining district is latest Cretaceous, the Bingham Canyon stock early Eocene in age. All other dated plutons east of the Antler orogenic belt are of Oligocene and Miocene age, synchronous with major volcanism in the region. The older plutons and orogenic structures which they cross cut indicate that the Nevadan orogeny affected all of Nevada and not just the western half where eugeosynclinal rocks occur. There is no obvious association of age with chemical composition, association of ore deposits, petrography, or depth of emplacement; there appears to be a positive correlation between shallow depth of emplacement and association of major mineral deposits and occurrence of porphyritic texture. Most lead-alpha dates agree with the K-Ar results but a few are unrealistically old and must be explained by inherited radiogenic lead or contamination with common lead.
Published Version
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