TY - JOUR

T1 - Turbulent Mixing of Metal and Silicate during Planet Accretion – and interpretation of the Hf-W chronometer

AU - Dahl, Tais Wittchen

AU - Stevenson, David

PY - 2010

Y1 - 2010

N2 - 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 bodieshad 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 mixwith surrounding silicate and howthis influences the short-lived chronometer, Hf–W, used to infer the age of theMoon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to placeconstraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh–Taylor instability does lead tointimate mixing and equilibration, but large blobs (N10 km diameter) do not emulsify entirely. Emulsification isenhanced if most of the accreting metal cores deform into thin structures during descent through the Earth'smantle. Yet, only 1–20% of Earth's corewould equilibrate with silicate during Earth's accretion. The initial speed ofthe impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities wheresilicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain atthe 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 andthe 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 ofthe Moon-forming impact. However, we show that large cores have limited ability to reset the Hf–W systeminthe silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than toradiogenic ingrowth after the last giant impact.

AB - 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 bodieshad 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 mixwith surrounding silicate and howthis influences the short-lived chronometer, Hf–W, used to infer the age of theMoon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to placeconstraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh–Taylor instability does lead tointimate mixing and equilibration, but large blobs (N10 km diameter) do not emulsify entirely. Emulsification isenhanced if most of the accreting metal cores deform into thin structures during descent through the Earth'smantle. Yet, only 1–20% of Earth's corewould equilibrate with silicate during Earth's accretion. The initial speed ofthe impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities wheresilicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain atthe 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 andthe 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 ofthe Moon-forming impact. However, we show that large cores have limited ability to reset the Hf–W systeminthe silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than toradiogenic ingrowth after the last giant impact.

M3 - Journal article

VL - 295

SP - 177

EP - 186

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

ER -