火山と同位体学

抄録

Usefulness of isotope studies in volcanology is discussed by showing some examples. Multiple isotope approach gives us significant information on the characteristics of source materials of volcanic products. Based on the reported data, isotopic characteristics of volcanic materials from the Japanese Islands are summarized as follows. (1) In the εNd-εSr diagram, Quaternary volcanic rock samples nearly lie on the “mantle array” or show slightly higher ⁸⁷Sr/⁸⁶Sr ratios. (2) In the NE (northeast) Japan and the Izu-Bonin region, ⁸⁷Sr/⁸⁶Sr ratios decrease across-arc from the volcanic front to the back-arc. (3) Generally, lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr ratios are observed for volcanic rocks from the SW (southwest) Japan than those from the NE Japan. (4) In the NE Japan, Sr and Nd isotopic ratios seem to have changed around 15 Ma for volcanic rocks. Quaternary volcanic rocks show higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr ratios than those before 15 Ma. (5) In the NE Japan, Pb isotopes in Quaternary volcanic rocks also indicate more radiogenic components for those from the volcanic front area than those from the back-arc area. (6) The highest values for the ³He/⁴He ratio in each region in the Japanese Islands are close to those of MORBs or slightly lower and they are observed around the volcanic front in the back-arc side. No systematic difference is observed in the distribution of the ³He/⁴He ratio between the NE Japan and the SW Japan. (7) No systematic distribution is observed for δ¹⁸O in Quaternary volcanic rocks in the Japanese Islands. On the basis of these data, the source materials of volcanic rocks in the Japanese Islands are discussed. The incorporation of crustal materials including pelagic sediments to the MORB-like materials is suggested in the amount of around a few percent. This interpretation is compatible with a recent discovery of cosmogenic ¹⁰Be enrichment in some volcanic rocks from the Japanese Islands. This indicates that we have an evidence for the subduction of a slab together with some amounts of sediments on it. As a typical example for a hot-spot type volcanism, Hawaiian volcanism is examined on the basis of radiogenic isotope studies. Sr, Nd, Pb and Hf isotopic ratios suggest that the magma sources for Hawaiian volcanism are related to the MORB-type source and the other types by mixing. High ³He/⁴He ratios observed in Hawaiian volcanic samples such as those from Loihi Seamount and Kilauea volcano suggest the occurrence of relatively primordial volatile components derived from the deeper part of the mantle than that of MORB. Further, systematic variations in the ³He/⁴He ratios with the volume of individual volcanoes together with other isotopic signatures suggest a model for the evolution of Hawaiian volcanism, where the rise of volatile-enriched diapirs from the Earth’s deep interior is essential However, the amount of noble gas (volatile) components would vary at each stage of a volcano evolution. Voluminous outpourings during the Hawaiian volcanism do not always reflect the direct material supply from the hot-spot source, but would reflect the combined effects of material transfer and heat supply.