Major element versus MgO variations in higher-K subgroup liquids of Izu-Oshima volcano. (a) SiO2. (b) Al2O3. (c) Al2O3/SiO2 weight ratio. (d) FeO* (wt.%). (e) CaO (wt.%). Fe content is given as FeO* (total iron), and the composition is normalized to 100 wt.%. All of the geochemical data are listed in Additional file 1: Table S1.