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Turbulent Mixing of Metal and Silicate during Planet Accretion – and interpretation of the Hf-W chronometer

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In the current view of planet formation, the final assembly of the Earth involved giant collisions between protoplanets
(N1000 kmradius), with theMoon formed as a result of one such impact.At this stage the colliding bodies
had likely differentiated into a metallic core surrounded by a silicate mantle. During the Moon-forming impact,
nearly all metal sank into the Earth's core. Weinvestigate towhat extent large self-gravitating iron cores can mix
with surrounding silicate and howthis influences the short-lived chronometer, Hf–W, used to infer the age of the
Moon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to place
constraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh–Taylor instability does lead to
intimate mixing and equilibration, but large blobs (N10 km diameter) do not emulsify entirely. Emulsification is
enhanced if most of the accreting metal cores deform into thin structures during descent through the Earth's
mantle. Yet, only 1–20% of Earth's corewould equilibrate with silicate during Earth's accretion. The initial speed of
the impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities where
silicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain at
the largest scale and do not mix to centimeter length scale,where diffusion operates and isotopes can equilibrate.
Thus, incomplete emulsification and equilibration of accreting iron cores is likely to occur.
The extent of metal–silicate equilibration provides key information for interpretation of siderophile budgets and
the timing of core formation using the Hf–Wchronometer. The time scale of core formation derived fromthe Hf–
Wchronometer is usually tied to the last major metal–silicate re-equilibration, believed to coincide with time of
the Moon-forming impact. However, we show that large cores have limited ability to reset the Hf–W systemin
the silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than to
radiogenic ingrowth after the last giant impact.
OriginalsprogEngelsk
TidsskriftEarth and Planetary Science Letters
Vol/bind295
Sider (fra-til)177-186
ISSN0012-821X
StatusUdgivet - 2010
Eksternt udgivetJa

ID: 43717382