Abstract
High magnesian olivines of Quaternary basalts and Ichinomegata Iherzolites were analyzed to estimate the compositions of primary magmas and residual mantles under Northeast (NE) Japan arc. The olivine phenocrysts from the back-arc side basalts have a trend with higher Fo content (lOO × Mg/Mg + Fe) in Fo-NiO wt% relationship than those from the volcanic front side basalts whose trends appear to be derived from a field of the olivines of Ichinomegata Iherzolites. The residual mantle olivine of the back-arc side primary magma (Fo = 94) is higher in Fo than that of the volcanic front side one (Fo≦90). Initial mantle before melt extraction would also have different bulk compositions in the sources for both magmas. The initial composition for the back-arc side primary magma may be higher in Xmg value (100 × Mg/Mg + Fe) and lower in FeO content than that for the volcanic front side one. The latter, which is similar to the Ichinomegata Iherzolite, may be shallower-seated than the former. It is postulated that the across-arc variations of Xmg and FeO in basalts results from the compositionally-layered mantle wedge under NE Japan arc.
