On the petrology and early development of the crust of a moon of fission origin

It is proposed that the primitive suite of upland rocks formed as a result of the cumulation of plagioclase which crystallized in disequilibrium from a convecting magma containing previously crystallized and co-crystallizing olivine and pyroxene. As the plagioclase was removed from this magma by flotation, it carried with it melt and mafic crystals in varying, but predictable proportions. This model successfully accounts for the major petrological characteristics of the upland suite of rocks, in particular, the reversed An vs Mg' trend, the quartz normative anorthosites and the olivine to pyroxene ratio variations vs plagioclase content of the rocks.It is shown that the crystallization sequence for the Moon is one where the pyroxenes of the peridotite upper mantle and crust were formed as a result of the reaction olivine + quartz (melt) pyroxene. This reaction occurred at depth (100–300 km) in the moon after the dunite lower mantle had formed, but while olivine was still crystallizing at the surface. As a result of this reaction, the crystallization of the last 20% of the Moon took place mainly along the olivine-plagioclase cotectic and not at the olivine-pyroxene-plagioclase peritectic as previously proposed. This crystallization sequence leads directly to an explanation of the fact that olivine rich rocks make up a significant fraction of the crust, despite the presence of a pyroxene dominated upper mantle directly below the crust. Also the reaction olivine + quartz (melt) pyroxene is exothermic and as such provided heat energy at the bottom of the magma system needed to set it into strong convective motion. As a result, the magma was kept stirred and the olivine and pyroxene in the cooling magma were kept in equilibrium with the melt, thus finally producing the relatively uniform peridotite of the upper mantle.A refined model for the distribution of U, Th and K in the crust of a pyroline moon is presented. It is demonstrated that the KREEP layer, which formed at the crust-upper mantle interface at the end of the crystallization of the Moon, was quickly destroyed by impact excavation and the upwards migration of the low melting KREEP materials. As a result of these processes the KREEP layer no longer exists in the Moon and all of its components are mixed in the crust. As a result, the crust contains about 80% of the heat producing U, Th and K of the Moon. The predicted values of the concentrations of U, Th and K in the crust based on this model are almost exactly those found for the average upland crust by the orbiting-ray experiment. This result not only strongly supports the models proposed in this paper but also supports the suggestion that the mean heat flow of the moon is 13–14 ergs/cm²/sec, i.e. that predicted for a Moon of fission origin in an earlier paper.The results and models presented in this paper further support the hypothesis that the Moon is a gravitationally differentiated body which originated by fission from a protoearth.Contribution No. 127, Institut für Geophysik, Kiel.