玄武岩の成因とSr・Pb同位体 : 火山岩類の成因を含めて

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The aspects of the development of isotopic methods of description that are pertinent to the petrology of basaltic rocks and associated differentiates are introduced. The amount of information available on isotopic compositions of lead and strontium in volcanic rocks is certainly small, neverthless it is sufficient to show that it will be an area rich in clues on questions of the origin and genesis of basalt-magma and associated volcanic-rocks. Almost all the measurment and systematic description of the isotopic properties of volcanic rocks which have in fact only begun to be made in the last 6 years are reviewed. Strontium-and lead-isotopes determinations may easily modify some of the interpretations which follow; however, the following significant observations may be drawn even from the relatively few data now available: (1) The Sr^<87>/Sr^<86> ratios in continental volanic rocks are more variable than are those in oceanic rocks. In particular they tend to be more radiogenic than oceanic volcanic rocks. (2) While much smaller differences in Sr^<87>/Sr^<86> ratios have been observed for basaltic rocks and differentiates from oceanic islands, relatively large systematic differences in the isotopic composition of lead are found among oceanic islands. (3) The relatively few comparisons available suggest that low Sr^<87>/Sr^<86> ratios are associated with high Pb^<206>/Pb^<204> ratios. (4) The variations in isotopic compositions may be generatcd in the source of the magma by the chemical and isotopic heterogeneities in the upper mantle. (5) In the continental interior or continental margin, it seems more probable that the large volumes of felsic differentiates were produced from abasaltic parent magma accompanied by contamination with crustal materials having higher silica and Sr^<87>/Sr^<86> ratios. (6) Assimilation of the lowest-melting-point components such as quartz, K-feldspar, and plagioclase in the ancient rocks of reasonable age would lower Pb^<206>/Pb^<204> and Pb^<208>/Pb^<204> ratios of a magma. (7) In Japanese islands, the mechanism of generating the three basalt magmas from different depths colud bring forth varieties in the isotopic composition of lead in Cenozoic basalts. (8) A wide difference is observed in lead isotopic compositions between the basalts from the Pacific side and the basalts from the continental side of the Japaneses islands. It is generally agreed that West Japan is situated in a region of a continental block structure.
日本地質学会の論文
1969-03-25

玄武岩の成因とSr・Pb同位体 : 火山岩類の成因を含めて

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