A Reconnaisance Study of Phase Boundaries in Low-Alkali Basaltic Liquids

Abstract

New data, derived from microprobe analyses of melting experiments, are presented in the form of sub-projections in the model basalt system, Ol-Pl-Wo-SiO2, to illustrate the shifts in the liquidus fields of olivine, plagioclase, spinel, pyroxene, and silica in low-alkali liquids as functions of variable Wo component and Mg. The projections are calculated in terms of oxygen units to avoid the ‘non conservative’ properties of mole units and to provide a better representation of the volume proportions of minerals. These projections show that over a wide range of composition the complex, natural basalt system has liquidus boundaries that resemble many of those in the simpler CaO-MgO-Al2O3-SiO2 system. One important feature of the natural system is the progressive change in the dominant low-Ca pyroxene along the olivine liquidus surface from protoenstatite to orthoenstatite to pigeonite as Mg decreases. Another notable feature is the expansion of the plagioclase liquid field at the expense of the olivine and low-Ca pyroxene fields as Mg decreases in low-alkali liquids. Increasing alkali concentration has almost exactly the opposite effect so that evolved terrestrial basalts have higher proportions of feldspar components than their lunar and eucritic counterparts. The new phase diagrams indicate that low-alkali liquids residual to those that produced the ancient, ferroan lunar anorthosites should have crystallized an amount of ferro-quartz-gabbro equivalent to 10 per cent of the volumes of their parents. The scarcity of such rocks among the returned lunar samples poses a problem for simple models of lunar petrogenesis.