Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena

Tracy Rushmer, Nick Petford, Munir Humayun, Andrew J. Campbell

Research output: Contribution to Book/Report typesChapterResearchpeer-review

Abstract

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. © 2005 Elsevier B.V. All rights reserved.
Original languageEnglish
Title of host publicationEarth and Planetary Science Letters
Pages185-202
Number of pages18
DOIs
Publication statusPublished - 15 Nov 2005

Publication series

NameEarth and Planetary Science Letters
Volume239

Fingerprint

Metals
Porosity
Silicates
Freezing
Meteoroids
Pressure
Sulfur
Theoretical Models

Keywords

  • Core formation
  • Dilatancy
  • Planetesimals
  • Siderophile elements
  • Strain rate

Cite this

Rushmer, T., Petford, N., Humayun, M., & Campbell, A. J. (2005). Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena. In Earth and Planetary Science Letters (pp. 185-202). (Earth and Planetary Science Letters; Vol. 239). https://doi.org/10.1016/j.epsl.2005.08.006
Rushmer, Tracy ; Petford, Nick ; Humayun, Munir ; Campbell, Andrew J. / Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena. Earth and Planetary Science Letters. 2005. pp. 185-202 (Earth and Planetary Science Letters).
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Rushmer, T, Petford, N, Humayun, M & Campbell, AJ 2005, Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena. in Earth and Planetary Science Letters. Earth and Planetary Science Letters, vol. 239, pp. 185-202. https://doi.org/10.1016/j.epsl.2005.08.006

Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena. / Rushmer, Tracy; Petford, Nick; Humayun, Munir; Campbell, Andrew J.

Earth and Planetary Science Letters. 2005. p. 185-202 (Earth and Planetary Science Letters; Vol. 239).

Research output: Contribution to Book/Report typesChapterResearchpeer-review

TY - CHAP

T1 - Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena

AU - Rushmer, Tracy

AU - Petford, Nick

AU - Humayun, Munir

AU - Campbell, Andrew J.

PY - 2005/11/15

Y1 - 2005/11/15

N2 - 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. © 2005 Elsevier B.V. All rights reserved.

AB - 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. © 2005 Elsevier B.V. All rights reserved.

KW - Core formation

KW - Dilatancy

KW - Planetesimals

KW - Siderophile elements

KW - Strain rate

UR - http://www.mendeley.com/research/feliquid-segregation-deforming-planetesimals-coupling-coreforming-compositions-transport-phenomena

U2 - 10.1016/j.epsl.2005.08.006

DO - 10.1016/j.epsl.2005.08.006

M3 - Chapter

SN - 0012-821X

T3 - Earth and Planetary Science Letters

SP - 185

EP - 202

BT - Earth and Planetary Science Letters

ER -

Rushmer T, Petford N, Humayun M, Campbell AJ. Fe-liquid segregation in deforming planetesimals: Coupling Core-Forming compositions with transport phenomena. In Earth and Planetary Science Letters. 2005. p. 185-202. (Earth and Planetary Science Letters). https://doi.org/10.1016/j.epsl.2005.08.006