Magma ocean

Our planet was probably transformed to a ball of magma 4.45 billion years ago by an impact with a small planet-like body the resulting debris formed the Moon. Yet uranium-lead dating shows us how quickly this magma ocean must have cooled, since it reveals that the oldest zircons, found in Western Australia, crystallized about 4.4 billion years ago. What is more, these ancient zircons show signs of having been formed in contact with water, implying that even in that distant era the world had... 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

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... 

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). 

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. 

The compositions of the crusts of the Moon and Mars are distinct - one is dominated by feldspathic cumulates from an early magma ocean, and the other by basaltic lavas. Regional patterns reflect differences in subjacent mantle compositions. The compositions of the mantles and cores of these bodies can be constrained by chemical analyses of mantle-derived basalts. The interiors of both bodies have remained geochemically isolated, because of the absence of plate tectonics. 

Righter, K. and Drake, M. J. (1997) A magma ocean on Vesta core formation and petro-genesis of eucrites and diogenites. Meteoritics and Planetary Science, 32, 929-944. 

Crystallization of the magma ocean would have allowed fractionation of high-pressure silicate and oxide minerals that dominate the Earth s deep interior. The olivine-rich... 

However, the strong and uniform fractionation of some element ratios that would be expected from crystallization of a magma ocean is not apparent on Earth. Perhaps our planet s magma ocean was vigorously stirred by convection, obliterating such geochemical effects. 

Taylor G. J., Keil K., McCoy T., Haack H., and Scott E. R. D. (1993) Asteroid differentiation—pyroclastic volcanism to Magma Oceans. Meteoritics 28, 34-52. 

Righter K. and Drake M. J. (1999) Effect of water on metal-silicate partitioning of siderophile elements a higji pressure and temperature magma ocean and core formation. Earth Planet Sci. Lett. 171, 383—399. 

The above models differ with respect to timing and therefore can be tested with isotopic techniques. However, not only are the models very different in terms of timescales, they also differ with respect to the environment that would be created on Earth. In the first two cases the Earth would form with a hot dense atmosphere of nebular gas that would provide a ready source of solar noble gases in the Earth. This atmosphere would have blanketed the Earth and could have caused a dramatic buildup of heat leading to magma oceans (Sasaki, 1990). Therefore, the evidence from dynamic models can also be tested with compositional data for the Earth, which provide information on the nature of early atmospheres and melting. 

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