A Petrologic Model for the Moon Based on Petrogenesis, Experimental Petrology, and Physical Properties

Abstract

Crystallization experiments under highly reducing conditions of synthetic material of mean Apollo 11 rock composition yielded: ilmenite in at 1,150° C; clinopyroxene in at 1,130° C, plagioclase in at 1,120° C. The same sequence of crystallization for the A pollo 11 vesicular ferrobasalts was interpreted from the mineral textures. The low liquidus temperature, narrow crystal-liquid interval, and delayed appearance of plagioclase are interpreted by crystallization from a magma formed by remelting of a mixture of accumulated pyroxene and ilmenite crystals in a liquid which has undergone advanced fractional crystallization. Under the Sea of Tranquillity, a residual ferrobasaltic liquid was formed by fractional crystallization of ultrabasic magma deeper in the moon. Heavy ilmenite and pyroxene crystals sank in the liquid, while light plagioclase floated to augment a crust. Meteorite impact blasted away the plagioclase-rich crust and released a ferro-basaltic magma formed by melting of ilmenite and pyroxene into the differentiated liquid. Fractional crystallization of a molten moon of modified chondritic composition yielded a small metallic core surrounded by pressure-stable, Mg-rich olivine and pyroxene. The fractionated liquid became Fe-and Ti-rich. Primitive ultrabasic crust in the highlands was augmented by dominant plagioclase-rich cumulates. The preferential occurrence of large irregular seas on the earth side of the moon is explained by differential tidal attraction of the late liquid fraction, and release by meteorite impact. The model can satisfy the density and moment of inertia of the moon if the olivine is Mg-rich. Early removal of radioactive material from the center by fractional crystallization, and enhancement of radiative heat transfer in volatile-free silicates would ease the problem of cooling the center of the moon. The low volatile content of A pollo 11 rocks may be a feature of the entire moon, permitting high rigidity and refractoriness. Incorporation of metal-seeking elements into a metal core, loss of volatiles from a hot surface, and crystal-liquid fractionation can explain some element concentrations of A pollo 11 rocks. Nevertheless the low density of the moon indicates a low Fe content of the primary material of the moon. Although there are no'meteorites which match the Apollo 11 rocks in all chemical and textural respects, several types have one or more features in common.