Fe-liquid segregation in deforming planetesimals; coupling core-forming compositions with transport phenomena

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doi: 10.1016/j.epsl.2005.08.006
Authors:Rushmer, Tracy; Petford, Nick; Humayun, Munir; Campbell, Andrew J.
Author Affiliations:Primary:
University of Vermont, Department of Geology, Burlington, VT, United States
Kingston University, United Kingdom
Florida State University, United States
University of Maryland, United States
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 239(3-4), p.185-202. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:2005
Note:In English. 49 refs.; illus., incl. 2 tables
Summary:The segregation and macroscopic transport phenomena leading ultimately to the formation of metallic cores in planetary silicate mantles is a fundamental yet poorly understood process. Here we report the results of a series of deformation experiments on a sample of partially molten Kernouve H6 chondrite (T = 900-1050°C) aimed at determining the siderophile concentrations and associated partition coefficients in both Fe-S-Ni-O quench and Fe-Ni metal as a function of degree of melting, and to provide insight into the melt segregation mechanism(s). The geochemical results show the S content in the segregated Fe-rich liquid metal decreases with increasing degree of melting. As the S content of the liquid metal also strongly affects the partitioning of highly siderophile elements between solid and liquid metal, an increase in porosity (Fe liquid melt fraction) from c. 5% to 30% lowers DSM/LM for HSE by several orders of magnitude. The relationship between melt fraction and porosity is used to compare the migration rate of liquid metal driven by buoyancy pressure gradients with a new theoretical model of melt segregation in a deforming porous medium that takes into account the coupling between volume strain (dilatancy) and shear stress. For buoyancy driven porous flow, highest transport velocities occur at highest porosities, implying the fastest flow velocities will carry Fe-rich liquid metal with low sulfur contents, preferentially enriched in incompatible HSEs. Predicted characteristic timescales of liquid metal transport due to buoyancy effects (diapirism and porous flow) for a c. 100 km-sized planetesimal are contrasted with shear-induced segregation velocities set up in response to external perturbations via impacts, an important process during the final stages of planetary accretion. A novel feature of our analysis is that liquid metal segregated previously into a planetary core by buoyancy instabilities (e.g., porous flow or a raining mechanism), might be drawn locally back into the silicate lower mantle by pressure gradients linked to surface impacts providing a physical mechanism for return flow of siderophile elements across the CMB. Abstract Copyright (2005) Elsevier, B.V.
Sections:Experimental mineralogy
Subjects:Core; Coupling; Dilatancy; Experimental studies; ICP mass spectra; Iron; Laboratory studies; Liquid phase; Magmas; Mass spectra; Metals; Outer core; Planetary interiors; Planetesimals; Planetology; Segregation; Siderophile elements; Simulation; Spectra; Strain; Textures; Transport
Abstract Numbers:05M/4348
Record ID:2006082823
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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