岩石形成過程における^<18>Oの分配

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Oxygen isotope chemistry of various rock-forming minerals is discussed from theoretical and geological points of view. Semi-quantitative calculation based on the vibrational spectra predicts the isotopic sequence of rock-forming minerals, or the order of enrichment of ^<18>O, which is in good agreement with that empirically obtained by Taylor and Epstein (1962). It is pointed out, however, that the abnormal depletion of ^<18>O in hydrous minerals, such as biotite and hornblende, is not due to the depletion of the isotope in OH ion in them as stated by these authors, but due to the fact that the aluminosilicate sheets or double chains have, by some unknown reason, a weaker affinity toward the heavy oxygen isotope. Granodiorite and granite are, in average, enriched by 2‰ or so in the heavy isotope as compared with basaltic and ultrabasic rocks according to the data by Taylor and Epstein (1962) and Silverman (1951). Theoretical consideration on the isotope chemistry of magma, however, reveals that it is difficult to obtain such a large fractionation of the isotopes during the magmatic differentiation of basaltic chemistry. This prediction is also supportcd by the isotopic data on the Skaergaard intrusion, East Greenland, Koyama gabbros, Japan and other differentiates of similar chemistry. It may be stated that "isotopic eutectic" could be maintained during the most part or the chemical differentiation of magma. Water is shown to be the heaviest constituent of magma system at elevated temperatures. Therefore, when magmatic water is transfered from the upper mantle to the crust, the transfer of the heavy oxygen would also take place. The process is anticipated to be very effective in making the crustal materials heavier than original mantle rocks.
日本地質学会の論文
1969-03-25

岩石形成過程における^<18>Oの分配

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