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Turbulent mixing of metal and silicate during planet accretion — And interpretation of the Hf–W chronometer

Dahl, Tais W. and Stevenson, David J. (2010) Turbulent mixing of metal and silicate during planet accretion — And interpretation of the Hf–W chronometer. Earth and Planetary Science Letters, 295 (1-2). pp. 177-186. ISSN 0012-821X. https://resolver.caltech.edu/CaltechAUTHORS:20100709-105723910

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Abstract

In the current view of planet formation, the final assembly of the Earth involved giant collisions between proto-planets (> 1000 km radius), with the Moon 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. We investigate to what extent large self-gravitating iron cores can mix with surrounding silicate and how this 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 (> 10 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 core would 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–W chronometer. The time scale of core formation derived from the Hf–W chronometer 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 system in the silicate Earth. Excess ^(182)W in bulk silicate Earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.epsl.2010.03.038 DOIArticle
ORCID:
AuthorORCID
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2010 Elsevier B.V. Received 26 October 2009; revised 16 February 2010; accepted 24 March 2010. Editor: T. Spohn. Available online 10 May 2010. We thank Robin Canup for supporting data and helpful guidance through the interpretation of her SPH results. H. Schmeling and an anonymous reviewer greatly improved the manuscript. TWD thanks Peter D. Ditlevsen, Kaveh Pahlevan, Andy Knoll, andMinik T. Rosing for their comments,motivation, and fruitful discussions. We are thankful to our sponsors Danmarks Grundforskningsfond (Nordic Center for Earth Evolution), and Caltech SURF fellowship (TWD).
Funders:
Funding AgencyGrant Number
Danmarks GrundforskningsfondUNSPECIFIED
Caltech Summer Undergraduate Research Fellowship (SURF)UNSPECIFIED
Subject Keywords:core formation; tungsten isotopes; siderophile elements; turbulence; Earth; Moon
Issue or Number:1-2
Record Number:CaltechAUTHORS:20100709-105723910
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20100709-105723910
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18959
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:09 Jul 2010 18:12
Last Modified:03 Oct 2019 01:50

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