Initial Conditions? Nature of the Mantle after Earth Accretion and Core Separation

Details Initial Conditions? Nature of the Mantle after Earth Accretion and Core Separation Initial Conditions? Nature of the Mantle after Earth Accretion and Core Separation

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.t11g..01s&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #T11G-01

Other

1025 Composition Of The Mantle, 5410 Composition, 8124 Earth'S Interior: Composition And State (Old 8105)

Scientific paper

Earth formation was a high energy event, especially if the currently favored story of lunar formation is correct. Every part of the mantle underwent extensive melting (though not all at the same time!) Every part of the mantle had the opportunity to re-equilibrate with core-forming melt, probably at quite high pressures (10 -30 GPa) and at temperatures exceeding (perhaps greatly exceeding) 2000K. Contrary to most people's intuition, extensive melting does not necessarily lead to sustained differentiation (defined as a difference in deep mantle and shallow mantle intrinsic composition following Earth formation, e.g. 4.4 Ga). There are two reasons for this: One is extensive and vigorous convection which may prevent liquid-crystal separation, and the other (perhaps more important) is Rayleigh-Taylor instabilities of the mantle following solidification. The presence or absence of a sustained differentiation depends on aspects of the phase diagram that are not currently known. However, there are many plausible phase diagrams where a differentiated mantle structure will arise, with or without core interaction. The biggest effect of the core lies in the process of the last equilibration rather than interaction after core formation, even allowing for the possible percolating regime suggested by Groebner and Kohlstedt. However, the D" layer is thin enough that it might have arisen from core-mantle interaction when the lowermost mantle was still substantially partially molten. Geophysical evidence is equivocal on the question of a deep mantle layer ( thicker than D"). Geochemical evidence can be use to argue either for or against such a layer. The helium evidence and the Urey number are highly suggestive of such a layer , involving a large fraction of Earth mantle.

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