太陽系物質中における酸素-16の異常濃縮  [in Japanese] Anomalous Enrichment of Oxygen-16 in Materials within the Solar System  [in Japanese]

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Author(s)

    • Kuroda P. K.
    • Meteorological Research Institute; Institute for Nuclear Study, University of Tokyo

Abstract

Correlated variations in <sup>18</sup>O/<sup>16</sup>O and <sup>17</sup>O/<sup>16</sup>O ratios in Al-rich inclusions present in carbonaceous chondrites Allende, Murchison, Murray etc. (Clayton et al, Science 182, 485, 1973) may be explained as due to a combined effect of the processes of mass-fractionation and cosmic-ray irradiation. Consider the relationship<br> (<sup>17</sup>O/<sup>16</sup>)<sub>Q</sub>=(<sup>17</sup>O/<sup>16</sup>)<sub>P</sub>(1+μ+C<sub>17</sub>)<br> and<br>(<sup>18</sup>O/<sup>16</sup>)<sub>Q</sub>=(<sup>18</sup>O/<sup>16</sup>)<sub>P</sub>(1+2μ+C<sub>18</sub>)<br> where the subscripts P and Q refer to isotopic ratios in a reference standard and in the sample, μ is a term which may be defined as mass-fractionation factor, and C<sub>17</sub> and C<sub>18</sub> are the contributions from the cosmic-ray-produced <sup>17</sup>O and<sup>18</sup>O. The above equations and the condition that δ<sup>17</sup>O=δ<sup>18</sup>O=-50 (permil), where<br>δ<sup>17</sup>O=[{(<sup>17</sup>O/<sup>16</sup>O)<sub>Q</sub>/(<sup>17</sup>O/<sup>16</sup>O)<sub>P</sub>}-1]・1000(permil)<br> and<br> δ<sup>18</sup>O = [{(<sup>18</sup>O/<sup>16</sup>O)<sub>Q</sub>/(<sup>18</sup>O/<sup>16</sup>O)<sub>P</sub>}-1]・1000(permil), <br>yield the relationships μ=-0.050-C<sub>17</sub> and C<sub>18</sub>=2C<sub>17</sub>+0.050. The condition δ<sup>17</sup>O=δ<sup>18</sup>O=-50 is met if, for example, the cosmic-ray production ratio of <sup>17</sup>O and <sup>18</sup>O is near unity and μ=-0.020, which corresponds to a value of δ<sup>18</sup>O=-40 (permil). Under these conditions, one finds C<sub>17</sub>= -0.0275 and C<sub>18</sub>=-0.005. The present-day cosmic-ray flux is not sufficient to produce such large variations in the oxygen isotope ratios, but the so-called early irradiation of matter must have taken place during the early history of the solar system. The puzzles of the oxygen, neon, magnesium and xenon isotope anomalies can be explained in terms of a model such as described in this paper.

Journal

  • Chikyukagaku

    Chikyukagaku 9(1), 5-15, 1975

    The Geochemical Society of Japan

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