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Basalt–basaltic andesite (<55 wt % SiO2) and dacite–rhyolite (66–74 wt % SiO2) are the predominant eruptive products in the Sumisu caldera volcano, Izu–Bonin arc, Japan. The most magnesian basalt (8·5% MgO), as well as some of the other basalts, has a low Zr content (20–25 ppm), and cannot yield basalts with higher Zr contents (29–40 ppm) through fractionation and/or assimilation. The high- and low-Zr basalts have different phenocryst assemblages, olivine, plagioclase and pyroxene phenocryst chemistries, REE (rare earth element) patterns, and fluid-mobile element/immobile element ratios. Estimated primary olivine compositions are more magnesian (>Fo91) in the low-Zr basalts compared with those in high-Zr basalts (