Metal-silicate partitioning systematics of siderophile elements at reducing conditions: A new experimental database

E. S. Steenstra, A. X. Seegers, R. Putter, J. Berndt, S. Klemme, S. Matveev, E. S. Bullock, W. van Westrenen

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO2) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO2 and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO2 and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB). To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO2 based on the Si-SiO2 buffer (ΔSi-SiO2) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (εM Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, εM Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

Original languageEnglish
Article number113391
Pages (from-to)1-28
Number of pages28
JournalIcarus
Volume335
Early online date5 Aug 2019
DOIs
Publication statusPublished - 1 Jan 2020

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siderophile elements
siderophile element
silicon
silicates
partitioning
silicate
metal
metals
buffers
iron
silicate melt
fugacity
chladnite
valence
oxygen
terrestrial planets
activity coefficient
light elements
parent body
coefficients

Keywords

  • Accretion
  • Aubrites
  • Core
  • Mercury
  • Metal-silicate
  • Siderophile

Cite this

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title = "Metal-silicate partitioning systematics of siderophile elements at reducing conditions: A new experimental database",
abstract = "The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO2) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO2 and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO2 and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB). To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-w{\"u}stite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO2 based on the Si-SiO2 buffer (ΔSi-SiO2) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (εM Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, εM Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.",
keywords = "Accretion, Aubrites, Core, Mercury, Metal-silicate, Siderophile",
author = "Steenstra, {E. S.} and Seegers, {A. X.} and R. Putter and J. Berndt and S. Klemme and S. Matveev and Bullock, {E. S.} and {van Westrenen}, W.",
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Metal-silicate partitioning systematics of siderophile elements at reducing conditions : A new experimental database. / Steenstra, E. S.; Seegers, A. X.; Putter, R.; Berndt, J.; Klemme, S.; Matveev, S.; Bullock, E. S.; van Westrenen, W.

In: Icarus, Vol. 335, 113391, 01.01.2020, p. 1-28.

Research output: Contribution to JournalArticleAcademicpeer-review

TY - JOUR

T1 - Metal-silicate partitioning systematics of siderophile elements at reducing conditions

T2 - A new experimental database

AU - Steenstra, E. S.

AU - Seegers, A. X.

AU - Putter, R.

AU - Berndt, J.

AU - Klemme, S.

AU - Matveev, S.

AU - Bullock, E. S.

AU - van Westrenen, W.

PY - 2020/1/1

Y1 - 2020/1/1

N2 - The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO2) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO2 and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO2 and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB). To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO2 based on the Si-SiO2 buffer (ΔSi-SiO2) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (εM Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, εM Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

AB - The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO2) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO2 and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO2 and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB). To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO2 based on the Si-SiO2 buffer (ΔSi-SiO2) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (εM Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, εM Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

KW - Accretion

KW - Aubrites

KW - Core

KW - Mercury

KW - Metal-silicate

KW - Siderophile

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