Anorthosite assimilation and the origin of the Mg/Fe-related bimodality of pristine moon rocks - Support for the magmasphere hypothesis

The geochemical bimodality of pristine rocks led to proposals that a major fraction of the crust (the Mg rich suite) formed in cumulates in numerous intrusions slightly younger than the magmasphere. It is suggested that assimilation helped to engender the bimodal patterns. Mass/energy balance calculations indicate that large proportions of plagioclase were probably assimilated from the older (Magmasphere-generated) ferroan anorthosite crust by most of the Mg-rich intrusive melts. The magmasphere, in the absence of assimilation probably did not yield appreciable plagioclase until fractional crystallization of mafic silicates had diminished the melt mg ratio to about 0.42. However, assuming identical melt composition, an Mg-rich intrusion assimilating ferroan anorthosite would have reached plagioclase saturation at a much higher mg, about 0.66. It is suggested that the current version of the magmasphere hypothesis (ferroan anorthosites = magmasphere flotation cumulates; Mg-rich rocks = products of younger, localized intrusions) is the only plausable mechanism for engendering the Mg/Fe-relate bimodality.