東北日本，三ノ目潟火山からの初生玄武岩の発見とその組成化

Abstract

Much more primitive basalt (SiO2∼48.5%, FeO*/MgO∼0.90, Ni∼180 ppm, Cr∼490 ppm) than previously reported ones (SiO2>51.2%) of Sannome-gata volcano, northern Japan, is found from a new proximal outcrop. The basalt is nearly aphyric and contains olivine (Fo∼89) and clinopyroxene phenocrysts. Judging from Fe-Mg partitioning, these magnesian olivine phenocrysts can coexist with the basaltic magmas. Forsterite-NiO contents of olivine, and Fo (olivine)-Cr value (spinel) diagrams suggest that the basalts are nearly primary equilibriating with a mantle peridotite. On the other hand, the previously reported basatic rocks contain resorbed quartz and plagioclase phenocrysts with magnesian olivine phenocryst indicating that these rocks were formed by either assimilation or magma mixing with silicic materials (or magmas). Ratios of incompatible trace elements of these silicic materials are largely different from those of the primary basalt. Therefore, it should be noted that previously reported geochemical data of Sannome-gata basalt do not represent chemical characteristics of the primary magma.