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

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

Mar 1974

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974e%26psl..21..431b&link_type=abstract

Earth and Planetary Science Letters, Volume 21, Issue 4, p. 431-438.

Computer Science

8

Scientific paper

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 the K-Ar method. The preparation of the samples for the argon extraction work strongly affects the ages obtained, due to the presence of very large quantities of atmospheric argon held ``loosely'' within most of the rocks. Use of single large pieces of rock results in incomplete removal of this gas, argon isotopic fractionation and grossly incorrect ages. When powdered samples are used, the increase in surface area per unit mass helps in the removal of the ``loosely'' held atmospheric argon. The Picture Gorge Basalts contain traces of clays of the montmorillonite group; on hydration these clays swell up and apparently shut off minute cracks in the rocks, thereby trapping substantial amounts of atmospheric argon. It is recommended that when K-Ar dating of whole-rock samples showing traces of alteration has to be attempted, the specimens be crushed to approximately 10-30 mesh size prior to the argon extraction work. Present mailing address: c/o Comptroller and Auditor-General of India, Bahadur Shah Zafar Marg, New Delhi-1, India.

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