Lunar magma ocean

Two scenarios for the crystallization of the lunar magma ocean, involving different amounts of melt. In both models, olivine and orthopyroxene cumulates sink and plagiodase floats, but the thickness of the plagiodase-rich crust is greater for a completely melted Moon. Ilmenite crystallizes late in both models, but mixing of dense, ilmenite-rich rocks with early magnesium-rich cumulates could result from convection. After Ryder (1991). 

Neal C. R. and Taylor L. A. (1989) Metasomatic products of the lunar magma ocean the role of KREEP dissemination. Geochim. Cosmochim. Acta 53, 529-541. 

Snyder G. A., Taylor L. A., and Neal C. R. (1992) The sources of mare basalts a model involving lunar magma ocean... 

In this section, we discuss the question of the bulk planetary abundances of the rare earth elements. Central to the problem of planetary abundance determinations is the assumption that the composition of the original solar nebula, for the non-gaseous elements, is given by the composition of the Cl meteorites. It is accordingly of interest to see what evidence is available from the planets, and how it relates to the primordial nebula values. In the previous section, we have seen that although the moon is enriched in the lanthanides relative to those in the primordial solar nebula by about 2.5 times, the pattern is probably parallel to that of Cl. The evidence for an apparent depletion in the heavy lanthanides is readily explicable as a consequence of early lunar magma ocean crystallisation of phases such as olivine and orthopyroxene, which selectively accept Gd-Lu. 

Lunar surface materials (Apollo and Luna returned samples and lunar meteorites) are classified into three geochemical end members - anorthosite, mare basalt, and KREEP. These components are clearly associated with the various geochemically mapped terrains of different age on the lunar surface. The composition of the lunar interior is inferred from the geochemical characteristics of basalts that formed by mantle melting, and geochemistry provides constraints on the Moon s impact origin and differentiation via a magma ocean. 

The pronounced asymmetry of the lunar cmst, with PKT dominating the nearside and FHT dominating the farside, is apparently a result of magma ocean crystallization. Some... 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

It may also be that a hotter Earth had a surface that was inherently unstable. Some argued that the earliest crust was like the lunar highlands—made from a welded mush of crystals that had previously floated on the magma ocean. Others have suggested that it was made of denser rocks more like those of the Earth s present oceanfloor (Galer and Goldstein, 1991). But firm evidence has so far been sparse. 

Longhi J. and Ashwal L. D. (1985) Two-stage models for lunar and terrestrial anorthosites petrogenesis without a magma ocean. Proc. 15th Lunar Planet. Sci. Conf., C571-C584. 

Shirley D. N. (1983) A partially molten magma ocean model. Proc. 13th Lunar Planet. Sci. Conf. A519—A527. 

Warren P. H. (1990) Lunar anorthosites and the magma ocean hypothesis importance of FeO enrichment in the parent magma. Am. Mineral. 75, 46-58. 

Warren P. H. and Wasson J. T. (1979a) Effects of pressure on the crystallization of a chondritic magma ocean and implications for the bulk composition of the Moon. Proc. 10th Lunar Planet. Sci. Conf, 2, 2051-2083. 

Wood J. A. (1975) Lunar petrogenesis in a well-stirred magma ocean. Proc. 6th Lunar Sci. Conf, 1087-1102. 

Woolum D. S., Cassen P., PorceUi D., and Wasserburg G. J. (1999) Incorporation of solar noble gases from a nebula-derived atmosphere during magma ocean cooling. Lunar Planet. Sci. XXX, 1518 (CD-ROM). Lunar and Planetary Institute. 

Phinney, W. C. 1982. Petrogenesis of Archean anorthosites. In Walker, D. McCallum, I. S. (eds) Workshop on Magmatic Processes of Early Plcmetary Crusts Magma Oceans and Stratiform Layered Intrusions. Lunar and Planetary Institute Technical Report, 82-01, 121-124. 

The recognition of a 142Nd anomaly within the mantle implies that the Earth experienced a major, very early differentiation event. The study by Boyet and Carlson (2005) showed that lunar basalts also have elevated 142Nd/144Nd ratios relative to primitive chondrites, implying that the Moon was formed from an Earth that had already experienced major differentiation. This means that the early differentiation of the Earth took place within 30 Ma of the formation of the solar system. Whilst the precise nature of this differentiation event is not known, a favored model is the formation of an Fe- and trace element-enriched basaltic crust, perhaps as an initial crust to a magma ocean. It is postulated that this crust is now isolated from the convecting mantle and is located deep within the lower mantle. 

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