ストロンチウム同位体比からみた日本列島弧の火山岩類の成因

抄録

The aspects of the development of isotopic methods that are pertinent to the origin of volcanic rocks are introduced. This paper deals with studies on strontium isotopes and related elements which have provide their usefulness in the field of isotope geology.
In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. Decreasing ⁸⁷Sr/⁸⁶Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the southwest Japan, the ⁸⁷Sr/⁸⁶Sr ratios of Cenozoic basaltic rocks are clearly different between the San-in and the northwest Kyushu regions. The higher ⁸⁷Sr/⁸⁶Sr ratios of basalts from the San-in region than that of basalts from the northwest Kyushu region also reflect the different properties of the upper mantle, which means there is regional heterogeneity of Sr isotopic ratios under the southwest Japan arcs. Furthermore, the relatively high and variable ⁸⁷Sr/⁸⁶ ratios of volcanic rocks are partticularly concentrated in the southwestern Japan arcs which have probably more continental properties than the northeastern Japan arcs.
Arc volcanism is characterized by prevalence of calc-alkaline rock suites as well as island-arc tholeiites. Various models have been proposed to interpret its genesis of the calcalkaline rocks. ⁸⁷Sr/⁸⁶Sr ratios vary remarkably within individual rock sultes as well as volcano and volcano. In some volcanic suites, the date points in ⁸⁷Sr/⁸⁶Sr ratios vs. SO₂, SI and Rb/Sr plot accord with a model to produce more silisic differentiates by combined processes of crystal fractionation and wall rock assimilation in Mount St. Helens volcano. Adatara and Kirishima volcanoes in the Japanese island arcs, both the tholeiite series (pigeonitic rock series, P-series) and the calc-alkaline rock series (hypersthenic rock series, H-series) have been shown to have erupted from one volcano and difference in ⁸⁷Sr/⁸⁶Sr ratios shows that the H-series has lower ⁸⁷Sr/⁸⁶Sr rotios than the P-series, and also that each series had a separate crystal fractionation process. The results reveal that the nature of the primitive magmas of the H-series and the P-series seems to generate from individual source materials.
Many Tertiary volcanic rocks from the northeastern Japan were determined for strontium isotopic ratios for the purpose of examining the genesis among the volcanic rocks. Two distinct suites of volcanic rocks occur in the northeastern Japan : the rocks older than 16 Ma of predominantly intermediate composition volcanics and the rocks younger than 16 Ma with bimodal suite of mafic and felsic rocks. Initial values of ⁸⁷Sr/⁸⁶Sr in the Tertiary volcanic rocks from the northeastern Japan, lie in the range from 0.7033 to 0.7068. High (⁸⁷Sr/⁸⁶Sr) I ratios are observed for the rocks older than 16 Ma from the Japan Sea side (H zone). It is noteworthly lower (⁸⁷Sr/⁸⁶Sr) I ratios in the Dewa Hill, Japan Sea coast and North Akita areas (L zone). The rocks younger than 16 Ma from the L zone can also be interpreted as having been originated as a mantle-diapir associated with the spreading of the Japan Sea basin.