ANOTHER WRINKLE ON THE PYROXENE LIQUIDUS SURFACE

Complimentary to the maximum on the pyroxene liquidus surface along the En-Di join near the Di composition is a minimum with higher En content. This minimum is metastable at low pressures, but it has been inferred to emerge from the clinopyroxene solvus as pressure increases. Because the composition of the liquid coexisting with pigeonite and diopside has higher Di content that the critical cpx, the minimum emerges on the high-Di side of the solidus and changes the solidus reaction from pgt + di=liq to pgt + liq=di. To date no one has described analogous behavior in the natural system. In a series of experiments on compositions designed to produce assemblages analogous to those present in lunar ferroan anorthosites (intermediate Mg’, anorthitic plag, pigeonite ± olivine) low-pressure experiments produced expected results, whereas runs at 0.6 GPa on the same starting materials did not. At low pressure the ol+plag-liquidus surface expanded across the pgt and opx liquidus fields and all pgt projected with distinctly lower Wo content than coexisting liq, whereas at 0.6 GPa the pgt + plag liquidus surface expanded to cover the trace of the Opx-Wo join and most of the pgt projected with higher Wo than coexisting liq — the one exception being a run in which a low-Wo pgt projected with slightly lower Wo than its coexisting liq. Also, the arrangement of pgt, di, and liq required a Di peritectic: di + liq=pgt. These phase arrangements require a minimum on the pgt + plag liquidus surface on the low-Di side of the cpx-solvus and suggest a way to generate plag-saturated liquids with relatively low Wo contents. The latter stages of crystallization of a lunar magma ocean are expected to produce cumulates of plag, pgt, and aug. Lunar anorthosites, however, are mostly either troctolitic or noritic. Overturn, heating, and partial melting of the mafic portion of the cumulate pile would produce ol-saturated liquids with lower Wo than the liquids that generated the cumulates. In this way melting of mafic sources may generate lunar anorthosites — a mode similar to that propsed for Proterozoic anorthosites.