Abstract
Recently, Mo isotope ratios in subduction zone volcanic rocks have been studied in a number of volcanic arcs to understand the recycling of subducted material in subduction zones. However, the processes that control the variations in Mo isotope ratios in rocks samples from the rear arc (RA) region remain unclear. The Northeast Japan (NE Japan) has many volcanoes from the volcanic front (VF) to the region of the RA, and is a suitable for comparing changes in geochemical composition along with the slab depth of each volcano. In this study, we measured Mo isotopic compositions of Kampu and Sannomegata in the RA volcanoes of NE Japan and compared them with previous studies. Samples from Kampu showed relatively low Mo isotope ratios similar to the other RA volcanos (Chokai, Iwaki) in the NE Japan (-1.20 to -0.29; N = 5), while those from Sannomegata showed Mo isotope ratios similar to those of VF in the NE Japan (+0.35 to +1.83; N = 3). The samples from Kampu have relatively high SiO2 abundances (53.0 to 63.8%), which may have been influenced by differentiation including hornblende. The low SiO2 abundance (48.1 to 51.6%) of the samples from the Sannnomegata, which are very close to those of primary magma, but with significantly larger Mo isotope ratios than DM and other magmas, require further investigation.
