Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmdi51a..01b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #DI51A-01
Physics
1025 Composition Of The Mantle, 5430 Interiors (8147), 5455 Origin And Evolution, 5480 Volcanism (6063, 8148, 8450), 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105)
Scientific paper
Terrestrial planets are expected to melt to some extent during their accretion. The melting extent depends upon planetary mass and composition and accretionary history, and the planet is likely to experience serial magma oceans of partial or even whole mantle depth. Magma ocean solidification, even in the presence of a thick atmosphere, is likely to be on the order of or shorter than the timescale of giant accretionary impacts. When the majority of accretion is complete the planet's mantle is therefore likely to consist of regions that have been melted and solidified more than once, possibly in combination with some remaining undifferentiated material. Fractional solidification of magma bodies proceeds from the bottom up and produces compositional and density stratification among the resulting silicate cumulates. The densest material is likely to be the last solidifying, as iron has been progressively enriched during solidification, and therefore it will also contain an excess of incompatible and radiogenic elements. Initially, this latest-solidifying, most iron-rich materials will form nearest the surface. This unstable stratigraphy will overturn. This densest material will likely sink deep in the mantle, displacing upward less dense differentiated and even undifferentiated materials. Material that rises up in the mantle may melt adiabatically to produce the earliest crust, or that crust may have been produced by mineral flotation in a magma ocean, as on the Moon. The mantle before convective onset may then consist of density-stratified silicate differentiates, with or without intermixed undifferentiated material. Many models produce deep dense reservoirs unlikely to be remixed by later convection, particularly in larger planets, and all models produce mantles resistant to convective onset through density stratification. Initial mantle compositions in the range of chondritic meteorite compositions can produce magma oceans that float quartz in addition to plagioclase, and mantles that during overturn produce earliest andesitic rather than basaltic crusts. The more silica-rich early crusts are likely to be more stable because of their high buoyancy.
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