火山岩の同位体地質学

抄録

Volcanic rocks have been studied with respect to oxygen and sulfur as light stable isotopes and to the isotope variations of strontium and lead due to radioactive decay, and variations of rare-earth elements (REE). This paper deals with studies on oxygen, strontium and lead isotopes and rare-earth elements which have provide their usefulness in the field of isotope geology. The ¹⁸O/¹⁶O ratios of a magma would vary during crystallization differentiation if the weighted average of the isotope ratio of the crystallizing minerals differs from the isotopic ratio of the remaining liquid phase. The oxpgen isotope data on volcanic rocks are in agreement with Sr isotope study in that they are compatible with the rocks having derived by fractional crystallization of magma with some contamination by sialic crust, or the initial enrichment of ¹⁸O in the source material in those samples from Kiso-ontake Volcano and Oki-Dogo Island in Japan. The ¹⁸O-enrichment in magma during crystallization differentiation is recognized in every rock series in Japanese Islands. Isotopic composition of strontium is not only a useful indicator of the ages of rocks and minerals, but it also contains information about the origin of igneous rocks and about the geologic processes that have affected their chemical compositions. No evidence has ever been presented to suggest that strontium isotopes are fractionated in nature, and it is therefore widely believed that fractionation effects are negligible. Rocks formed by melting, metasomatism, or assimilation of crustal materials will be labeled by having higher initial ⁸⁷Sr/⁸⁶Sr ratios than the ratios of uncontaminated rocks derived from the mantle. There is a positive correlation between Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios in some island arc tholeiitic suites and a decrease of ⁸⁷Sr/⁸⁶Sr ratios with increasing distance from the trench in some island arc. The ⁸⁷Sr/⁸⁶Sr ratios in volcanic rocks from Kurile and East Japan arcs in Japan, decrease from 0.7040-0.7055 closest to the trench to 0.7026-0.7034 furthest from it. Note that Kurasawa (1970b) resports the opposite trend in south-west Honshu, but this is due to high ratios in the volcanic rocks of West Japan. These volcanic rocks differ in such distinctive feature as K/Na ratios and TiO₂ content from other island arc rocks and may only indirectly reflect the typical island arc phenomenon. In either case, it is important to note that West Japan is situated in a region of a continental margin that do not correspond to typical island arc. These results are in agreement with Pb isotope studies on the volcanic rocks form Japanese Islands. In this report in situ magma differentiation of basaltic magmas are discussed by using Sr isotopes and variation of REE concentrations in the rocks.