K-Ar-Altersbestimmungen an Eisenmeteoriten--II spallogenes Ar 40 und Ar 40 -Ar 38 -Bestrahlungsalter

Details K-Ar-Altersbestimmungen an Eisenmeteoriten--II spallogenes Ar 40 und Ar 40 -Ar 38 -Bestrahlungsalter K-Ar-Altersbestimmungen an Eisenmeteoriten--II spallogenes Ar 40 und Ar 40 -Ar 38 -Bestrahlungsalter

Oct 1966

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1966gecoa..30.1059l&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 30, Issue 10, pp.1059-1074

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Scientific paper

A first attempt to determine the spallation Ar 40 in iron meteorites is reported. A new extraction technique for metallic samples with atmospheric contamination as low as 10 -9 cm 3 S.T.P. Ar 40 was developed to determine the spallation-produced Ar 40 in iron meteorites. The samples were degassed in steps with increasing temperature. The atmospheric and radiogenic components were released at low temperatures. At higher temperature the Ar 40 / Ar 38 values remain constant and give an upper limit of the spallation ratio. The spallation-produced Ar 40 is mainly important as a correction in the determination of the radiogenic Ar 40 content. The precision achieved is, however, high enough to determine also cosmic ray exposure ages. The measured Ar 40 / Ar 38 spallation ratios range from 0.188 to 0.226 and depend on the time of the decay of the cosmic-ray-produced K 40 . By comparison with exposure ages deduced from other isotopes relative production rates of Ar 40 / Ar 38 = 0.186 ± 0.005 and of K 40 / Ar 38 = 0.97 ± 0.05 are derived. The production cross sections of K 39 , K 40 and K 41 in iron targets bombarded with 575 MeV protons were also measured. For K 40 9.2 ± 0.8 mbarn have been obtained. This value is in agreement with the K 40 spallation yield calculated from the cosmic-ray-produced K isotopes in iron meteorites. The Ar 40 / Ar 38 determinations of this work give cosmic ray exposure ages which are in good agreement with the ages obtained from the K 40 -K 41 method.

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