ABSTRACT

High-Mg andesites (HMAs) and basalts in the Setouchi volcanic belt exhibit large variations in both Os abundance and Os isotopic composition. For HMAs, ¹⁸⁷Os/¹⁸⁸Os ratios and Os concentrations range from 0.1718 to 0.2041 and from 8.1 to 11.5 ppt, respectively. For basalts, ¹⁸⁷Os/¹⁸⁸Os ratios and Os concentration range from 0.1556 to 0.1769 and from 31.0 to 53.7 ppt, respectively. The measured ¹⁸⁷Os/¹⁸⁸Os ratios are higher than that of the primitive upper mantle estimate. The petrological and geochemical characteristics of HMAs do not support a scenario whereby HMA magmas are derived from basalt magmas via assimilation-fractional crystallization processes within the crust. Instead, the observed elevated Os isotopic compositions may be explained by mixing between the wedge mantle and partial melts from subducted sediments, rather than slab-derived fluids.