Models of formation of basaltic crust at mid-ocean ridges by adia-batic upwelling of fertile mantle lherzolite require knowledge of phase relations and phase compositions during melting at appropriate pressures. Spinel þ plagioclase lherzolites are found among peri-dotite samples from the ocean floor and ophiolitic exposures. At low pressure (5 1·2 GPa) the five-phase assemblage (olivine þ orthopyr-oxene þ clinopyroxene þ plagioclase þ spinel) is present at the anhydrous lherzolite solidus. New experimental data on mineral and melt compositions, at 0·75 GPa and 1140^12608C in the (Cr þ Na þ Fe þ Ca þ Mg þ Al þ Si) system, demonstrate smooth covariant relationships between oxides for melt compositions and in partition relationships both between mineral pairs and between minerals and melts. Molecular normative projections demonstrate that liquids on the five-phase þ liquid cotectic occupy a narrow compositional range. Of the mineral solid solutions that control the liquid composition, the [Ca/(Ca þ Na)] or anorthite/ albite content of the plagioclase is dominant and liquids vary from silica undersaturated and nepheline-normative at the sodic (oligo-clase) end to orthopyroxene and quartz-normative at the calcic (anorthite) end of the cotectic. Spinel (Cr^Al) solid solution has limited variation on the five-phase þ liquid cotectic. It is very Cr-rich at the sodic end and has limited compositional variation from $50 to 20 in Cr/(Cr þAl) at the anorthitic end. With fixed plagioclase composition on the cotectic, melt compositions show small compositional shifts with Fe^Mg (at Mg# between 85 and 95) and with Cr/(Cr þAl). The compositional vectors are consistent with effects observed in the end-member simple systems (FCMAS) and (CrCMAS). In comparing liquids at the anorthite end of the five-phase þ liquid cotectic with those on the Cr-free CMAS four-phase þ liquid cotectic at 0·75 GPa, it is evident that the presence of Al-rich Cr^Al spinel shifts liquid compositions to more silica-rich and silica-oversaturated composition. These experimentally defined melt compositions in equilibrium with plagio-clase þ spinel lherzolite are unlike quenched glasses from mid-ocean ridge settings. The data do not support models of mantle upwelling at low potential temperature (12808C) that produces low melt fractions at low pressures, leaving residual plagioclase þ spinel lherzo-lite. The detailed mineral compositional data at the solidus provide a template for comparison with natural plagioclase þ spinel lherzolites refertilized by porous reactive flow.