Geochemical variation of liquids of the Izu-Oshima volcano. (a) K2O versus MgO (wt.%) variation diagram. (b) K/Zr versus MgO (wt.%) variation diagram for liquids of the Izu-Oshima volcano. Analytical errors are estimated to be within the symbols, approximately 0.5% relative standard deviation (RSD) for K2O (wt.%) and approximately 1% RSD for K/Zr (e.g., Tani et al. 2002).