Figure F21. Major-oxide discriminant diagrams. A, C. Al₂O₃ vs. Fe₂O₃. B, D. SiO₂ vs. Al₂O₃. Symbols distinguish vitric tuffs (red left-pointing triangles) and siltstones (gray triangles). Arrows in A and B indicate the effects of subtraction of 13% olivine = Fo₈₅ from a representative tuff composition. Additional symbols in A and B are large open triangles = basaltic glasses from Kilauea and Puna Ridge (Clague et al., 1995); half-filled squares = Honolulu Volcanic Series (Jackson and Wright, 1970; Clague and Frey, 1982); downward-pointing triangles = North Arch Volcanic Series (Dixon et al., 1997); blue diamonds = Hana Volcanic Series, Haleakala Volcano, Maui (Chen et al., 1991). Additional symbols in C and D are dark green squares = high-MgO basalts from Koolau Volcano, Oahu (Frey et al., 1994); light green right-pointing triangles = enriched mid-ocean-ridge basalt (E-MORB) (data compilation of J. Natland, from several literature sources).