Late accretion as a natural consequence of planetary growth

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Late accretion as a natural consequence of planetary growth

Postby David Entwistle » Sun Jul 22, 2012 11:09 pm

:ugeek: from Nature Geoscience

Late accretion as a natural consequence of planetary growth by James M. D. Day, Richard J. Walker, Liping Qin & Douglas Rumble III.

Core formation should strip highly siderophile elements (HSEs) from planetary mantles according to the expected metal–silicate partition coefficients. However, studies of Earth, the Moon and Mars indicate mantles with HSE abundances in chondrite-relative proportions that exceed the values expected from metal–silicate partitioning. Competing hypotheses have been proposed to account for these observations, including metal–silicate partitioning at higher pressures and temperatures and late accretion. Here we present petrological and geochemical analyses of diogenite meteorites that represent mantle and crustal materials from two or more differentiated asteroids. We find that diogenites show HSE abundances that are consistent with metal–silicate equilibration, followed by minor continued accretion. Isotope chronometry supports diogenite crystallization ages within 2–3 million years of Solar System formation, indicating that late accretion occurred earlier than postulated for Earth, the Moon and Mars. The early timing and occurrence on differentiated asteroids, as well as on the larger terrestrial planets, therefore ties late accretion to planetary growth. On asteroidal bodies, such as the diogenite parent bodies, variations in HSE compositions may reflect regional rather than global effects. In contrast, for Earth, the Moon and Mars, compositional variations in mantle materials seem to be consistent with more homogeneous distributions through prolonged melting and/or solid-state convection.


:? Yeah, what they said!....

Possibly explained in a more digestible way by e! Science News....
David Entwistle
 
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