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Heterogeneous Growth of Meteorites and Planets, Especially the Earth and Moon

J. V. Smith
The Journal of Geology
Vol. 90, No. 1 (Jan., 1982), pp. 1-48
Stable URL: http://www.jstor.org/stable/30063988
Page Count: 48
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Heterogeneous Growth of Meteorites and Planets, Especially the Earth and Moon
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Abstract

Evidence concerning heterogeneous growth of meteorites and planets is summarized, and a generalized model for the growth of the Earth and Moon is presented. The solar nebula was chemically inhomogeneous and did not develop under physical and chemical equilibrium. A cloud containing interstellar dust and supernova debris yielded a range of planetesimals partly represented by meteorites. A range of oxidation state is needed to explain the components of the enstatite-, ordinary-, and carbonaceous chondrites, which may have formed at increasing distance from the Sun. Meteorites provide evidence on surviving planetesimals that range from cold mechanical aggregates to metamorphosed and melted varieties. Each terrestrial planet began to grow from nearby slow planetesimals, and the velocity of surviving planetesimals increased because of gravitational deflection. Growth was essentially completed within $10^{8}$ years. Deflections from Jupiter nearly emptied the asteroid zone, and the growth of Mars and Mercury was stunted by Jupiter and the Sun, respectively. Planetesimals rich in oxidized materials and volatiles (including $H_{2}O$ and $CO_{2}$) hit the inner planets, but the volatiles were retained only by the Earth, Venus, and partly by Mars. The inner planets began to melt early into a metal-sulfide core, a peridotite mantle, and a volcanic crust of ''basaltic" composition. Fronts of high-pressure minerals advanced outwards through the mantle, and several factors inhibited chemical equilibration. In the Earth, perovskite (mainly $MgSiO_{3}$) became a major component of the lower mantle, and a thermal boundary became established at ~700 km depth between convection systems in the upper and lower mantles. Heat was released by volcanic activity and foundering of cooled crust. Ultrabasic to basic rocks underwent remelting to intermediate and acidic rocks. The early crust was destroyed for the first 750 m.y. by a combination of igneous and sedimentary processes, coupled with the effects of impacts of planetesimals. The chemical distribution of the elements was determined by a complex sequence of physical processes involving recycling of crust and upper mantle. Polygonal tectonics developed stochastically into symmetrical linear tectonics and ultimately into asymmetrical linear tectonics. Crystallization of the inner core drove the magnetic dynamo. The Moon probably grew mainly by assembly of debris in orbit around the Earth and partly by direct capture of incoming material. One or more episodes of impact-induced fission of the Earth may have augmented the debris produced by disintegration of incoming planetesimals. The Moon may have melted during (not after) growth into a small metal-sulfide core (<700 km radius), an olivine-rich mantle, and a crust rich in Ca-plagioclase and pyroxene. Magma-loving elements became concentrated during the era of heavy bombardment (500 m.y.), and volcanism was concluded by eruption of basalts and pyroclastics generated by partial melting down to 500 km depth of the crystal-liquid residua from melting during growth. A magnetic dynamo died early as the core cooled.

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