Physics
Scientific paper
Jun 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980georl...7..461m&link_type=abstract
Geophysical Research Letters, vol. 7, June 1980, p. 461-464.
Physics
4
Argon Isotopes, Diamonds, Geochemistry, Mass Spectroscopy, Primitive Earth Atmosphere, Chemical Composition, Crystallization, Earth Mantle, Outgassing
Scientific paper
The isotopic composition of argon entrapped in an Arkansas diamond is determined and its implications for the composition of the primordial earth are discussed. A 6.3-carat well-crystallized diamond obtained from the Arkansas kimberlite eruption with a diatreme age of 106 + or - 3 million years was examined for Ar-36, Ar-38 and Ar-40 content by mass spectrometry of the gases released upon crushing. Results indicate an Ar-40/Ar-36 ratio of 189 compared to an atmospheric value of 294 and an Ar-38/Ar-36 value of 0.18, almost identical to the present atmospheric value of 0.19. Observations suggest that the isotopic abundance of argon in the mantle at the time and depth of diamond crystallization was essentially the same as that of the earth's atmosphere at that time, that the isotopes are of primordial origin, and that the outgassing of the earth was neither complete nor isotopically selective at the time and depth of diamond crystallization. Comparison of the argon abundances with a model of the outer region of the earth reveals a minimum age of 3.1 billion years for the crystal and a mass of 10 to the 10th tons or 0.00025 ppm by weight for Ar-36 at that time.
Giardini A. A.
Melton C. E.
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