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
Other:
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
Subsections:General
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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040 |a ViAlAGI  |c ViAlAGI 
072 7 |a 04  |2 georeft 
100 1 |a Rushmer, Tracy  |e analytic author  |u University of Vermont, Department of Geology, Burlington, VT 
245 1 0 |a Fe-liquid segregation in deforming planetesimals; coupling core-forming compositions with transport phenomena 
300 |a p. 185-202 
500 |a In English. 49 refs. 
500 |a Abstract number: 05M/4348 
500 |a Category Section: Experimental mineralogy 
500 |a Category Subsection: General 
500 |a Affiliation: University of Vermont, Department of Geology; Burlington, VT; USA; United States 
500 |a Affiliation: Kingston University; ; GBR; United Kingdom 
500 |a Affiliation: Florida State University; ; USA; United States 
500 |a Affiliation: University of Maryland; ; USA; United States 
500 |a Key title: Earth and Planetary Science Letters 
500 |a Source note: Earth and Planetary Science Letters, 239(3-4), p.185-202. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X 
500 |a Publication type: journal article 
504 |b 49 refs. 
510 3 |a GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands 
520 |a 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 D<SM/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. 
650 7 |a Core  |2 georeft 
650 7 |a Coupling  |2 georeft 
650 7 |a Dilatancy  |2 georeft 
650 7 |a Experimental studies  |2 georeft 
650 7 |a ICP mass spectra  |2 georeft 
650 7 |a Iron  |2 georeft 
650 7 |a Laboratory studies  |2 georeft 
650 7 |a Liquid phase  |2 georeft 
650 7 |a Magmas  |2 georeft 
650 7 |a Mass spectra  |2 georeft 
650 7 |a Metals  |2 georeft 
650 7 |a Outer core  |2 georeft 
650 7 |a Planetary interiors  |2 georeft 
650 7 |a Planetesimals  |2 georeft 
650 7 |a Planetology  |2 georeft 
650 7 |a Segregation  |2 georeft 
650 7 |a Siderophile elements  |2 georeft 
650 7 |a Simulation  |2 georeft 
650 7 |a Spectra  |2 georeft 
650 7 |a Strain  |2 georeft 
650 7 |a Textures  |2 georeft 
650 7 |a Transport  |2 georeft 
700 1 |a Petford, Nick,  |e analytic author  |u Kingston University 
700 1 |a Humayun, Munir,  |e analytic author  |u Florida State University 
700 1 |a Campbell, Andrew J.,  |e analytic author  |u University of Maryland 
773 0 |t Earth and Planetary Science Letters  |d Amsterdam : Elsevier, Nov. , 15 2005  |x 0012-821X  |y EPSLA2  |n Earth and Planetary Science Letters, 239(3-4), p.185-202. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X Publication type: journal article  |g Vol. 239, no. 3-4  |h illus., incl. 2 tables 
856 |u urn:doi: 10.1016/j.epsl.2005.08.006