ABSTRACT

The Rb, Sr contents and ⁸⁷Sr/⁸⁶Sr ratios were studied for mafic and ultramafic inclusions (four lherzolites, six gabbros and amphibolites) and their host basaltic rocks from Itinome-gata, Northeast Japan. These lherzolites are relatively depleted in both Rb and Sr as compared with similar inclusions from other regions in the world. It is noteworthy that three of four lherzolites show significantly higher ⁸⁷Sr/⁸⁶Sr ratios (0.7044-0.7053) than those of host basaltic rocks (0.7030-0.7033), suggesting an accidental origin for these inclusions. However, one lherzolite is isotopically similar to the host rocks and may be related to the recent magma genesis in this region. The mafic inclusions are characterized by relatively low Rb contents (0.3 ∼ 1.8ppm), and amphibolites show ⁸⁷Sr/⁸⁶Sr ratios (0.7030-0.7032) similar to that of host basaltic rocks, but other gabbroic rocks show higher ⁸⁷Sr/⁸⁶Sr ratios (0.7035∼0.7048). These results suggest vertical inhomogeneities of the uppermost part of the mantle and lower crust in the Itinome-gata region with respect to Sr isotopes.