日本における同位体岩石学の進歩

NDLデジタルコレクション 被引用文献1件

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  • Progress of isotope petrology in Japan

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The study on the source materials of granitic rocks based on their strontium and lead isotope compositions was first made by GoRAI at the beginning of l960's in Japan. Thereafter, during the last twenty years, a large number of available data concerning the strontium isotope ratios of the Cenozoic volcanic rocks, late Mesozoic granitic rocks and other igneous rocks in the Japanese Islands have been presented by the Japanese petrologists and geochemists. From some of the results significant hypotheses were proposed for the origin of granitic and volcanic rocks. Lead isotope data of volcanic and granitic rocks from the Japanese Islands, obtained by the beginning of 1970's also contributed to the investigation of the origin of these igneous rocks and geochemical character of the upper mantle. On the other hand, Nd isotope study of the igneous rocks, which has shown remarkable progress in other countries during the last decade is not yet conducted sufficiently in Japan. The study of this field is expected to make rapid progress in Japan because the combined application of both Nd and Sr isotope data obtained so far in various igneous rocks from the continental and oceanic circumstances and island arcs has played important roles in considering the origin of the igneous rocks and evolutional history of the earth's crust and mantle. Since 1970's, many available oxygen isotope data have been measured for mainly Quaternary volcanic rocks and late Mesozoic granitic rocks in the Japanese Islands. These results indicate that oxygen isotope ratios are very usefull as a tracer in studying of magmatic differentiation processes. Recently some Japanese petrologists and geochemists measured the hydrogen isotope ratios of granitic rocks in Japan. Based on these data they estimated the hydrogen isotope ratios of the granitic magma and the upper mantle. Sulfur isotope ratios also are measured on granitic and gabbroic rocks of the Creta ceous and Tertiary ages and Quatemary volcanic rocks in Japan. These data show that the two series of granitic rocks, the magnetite-series and ilmenite-series, have different sulfur isotope characteristics and that regional variations of sulfur isotope ratios exist in the Quaternary volcanic rocks in the Japanese Islands. In summary, isotope petrology up to this time in Japan contributed mainly in investigating the chemical characteristics of the source materials of granitic rocks and the problems on solidification processes of magmas, such as fractional crystallization of magma and effect of crustal contamination on the chemistry of magma and to recognize the regional variations of isotope ratios of some elements in mainly Mesozoic plutonic rocks and Cenozoic volcanic rocks in the Japanesie Islands. However, we have not yet the detailed models for the evolution of the earth's crust and mantle except for one or more models based on the strontium isotope ratios of granitic rocks and have not confirmed the concrete evidences on the chemical heterogeneity of the upper mantle and lower crust from which are probably generated the various types of magmas. Through the progress of isotope petrology in Japan, it is expected that the Japanese petrologists and geochemists will make great progress in solving these problems.

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