Quaternary basaltic and iron-rich flows from the Snake River Plain, Idaho, have chemical characteristics of both tholeiitic and alkali olivine basalt suites. The minerals present are olivine (Fo80–Fo44), Ca-rich augite (CaO 18 wt.%), plagioclase (An71–An40), and oxides. The spinel phases include a Mg–Al rich variety as inclusions in olivine phenocrysts. Both magnetite and ilmenite are present and temperatures estimated from the compositions of coexisting groundmass Fe–Ti oxides range from 915 to 1095 °C. Temperatures estimated from the compositions of coexisting groundmass olivine and pyroxene range from 928 to 1030 °C. Oxygen fugacities are in the expected basalt range (log values from −9.6 to −12.8). Silica activities calculated for these rocks fall in the alkali olivine basalt field but are above the Ab–Ne silica buffer.The aphyric basaltic rocks could be representative of liquids generated by partial melting (15–20%) of pyrolite. The generation of these rocks would leave a residuum of lherzolite composition. Equilibrium of the basalts with the residual composition recalculated to spinel lherzolite could occur between 15 and 25 kbar (15 × 105 and 25 × 105 kPa) at approximately 1300 °C. This is in agreement with the seismically determined depth of the low velocity zone (of partial melting) at approximately 60 km.A crystal fractionation model suggests that the porphyritic basalts are products of minor fractionation and accumulation from parental magma represented by the aphyric basalts. The iron-rich lavas (total FeO 17–18 wt.%) appear to be later stage fractionation products of the porphyritic basalts. However, basalts with approximately 14 wt.% FeO can be generated from partial melting of a model mantle.