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Title: Metal-silicate Partitioning and Its Role in Core Formation and Composition on Super-Earths

Abstract

We use a thermodynamic framework for silicate-metal partitioning to determine the possible compositions of metallic cores on super-Earths. We compare results using literature values of the partition coefficients of Si and Ni, as well as new partition coefficients calculated using results from laser shock-induced melting of powdered metal-dunite targets at pressures up to 276 GPa, which approaches those found within the deep mantles of super-Earths. We find that larger planets may have little to no light elements in their cores because the Si partition coefficient decreases at high pressures. The planet mass at which this occurs will depend on the metal-silicate equilibration depth. We also extrapolate the equations of state (EOS) of FeO and FeSi alloys to high pressures, and present mass–radius diagrams using self-consistent planet compositions assuming equilibrated mantles and cores. We confirm the results of previous studies that the distribution of elements between mantle and core will not be detectable from mass and radius measurements alone. While observations may be insensitive to interior structure, further modeling is sensitive to compositionally dependent properties, such as mantle viscosity and core freeze-out properties. We therefore emphasize the need for additional high pressure measurements of partitioning as well as EOSs, and highlightmore » the utility of the Sandia Z-facilities for this type of work.« less

Authors:
; ;  [1]; ;  [2]
  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)
  2. Harvard University, Department of Earth and Planetary Sciences, 20 Oxford St., Cambridge, MA 02138 (United States)
Publication Date:
OSTI Identifier:
22663909
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; DISTRIBUTION; EQUATIONS OF STATE; FREEZING OUT; IRON OXIDES; IRON SILICIDES; MASS; MELTING; METALS; PARTITION; PLANETS; POWDERS; PRESSURE MEASUREMENT; PRESSURE RANGE GIGA PA; SATELLITES; SILICATES; SIMULATION; THERMODYNAMICS; VISCOSITY; VISIBLE RADIATION

Citation Formats

Schaefer, Laura, Petaev, M. I., Sasselov, Dimitar D., Jacobsen, Stein B., and Remo, John L., E-mail: lschaefer@asu.edu. Metal-silicate Partitioning and Its Role in Core Formation and Composition on Super-Earths. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/2/234.
Schaefer, Laura, Petaev, M. I., Sasselov, Dimitar D., Jacobsen, Stein B., & Remo, John L., E-mail: lschaefer@asu.edu. Metal-silicate Partitioning and Its Role in Core Formation and Composition on Super-Earths. United States. doi:10.3847/1538-4357/835/2/234.
Schaefer, Laura, Petaev, M. I., Sasselov, Dimitar D., Jacobsen, Stein B., and Remo, John L., E-mail: lschaefer@asu.edu. Wed . "Metal-silicate Partitioning and Its Role in Core Formation and Composition on Super-Earths". United States. doi:10.3847/1538-4357/835/2/234.
@article{osti_22663909,
title = {Metal-silicate Partitioning and Its Role in Core Formation and Composition on Super-Earths},
author = {Schaefer, Laura and Petaev, M. I. and Sasselov, Dimitar D. and Jacobsen, Stein B. and Remo, John L., E-mail: lschaefer@asu.edu},
abstractNote = {We use a thermodynamic framework for silicate-metal partitioning to determine the possible compositions of metallic cores on super-Earths. We compare results using literature values of the partition coefficients of Si and Ni, as well as new partition coefficients calculated using results from laser shock-induced melting of powdered metal-dunite targets at pressures up to 276 GPa, which approaches those found within the deep mantles of super-Earths. We find that larger planets may have little to no light elements in their cores because the Si partition coefficient decreases at high pressures. The planet mass at which this occurs will depend on the metal-silicate equilibration depth. We also extrapolate the equations of state (EOS) of FeO and FeSi alloys to high pressures, and present mass–radius diagrams using self-consistent planet compositions assuming equilibrated mantles and cores. We confirm the results of previous studies that the distribution of elements between mantle and core will not be detectable from mass and radius measurements alone. While observations may be insensitive to interior structure, further modeling is sensitive to compositionally dependent properties, such as mantle viscosity and core freeze-out properties. We therefore emphasize the need for additional high pressure measurements of partitioning as well as EOSs, and highlight the utility of the Sandia Z-facilities for this type of work.},
doi = {10.3847/1538-4357/835/2/234},
journal = {Astrophysical Journal},
number = 2,
volume = 835,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2017},
month = {Wed Feb 01 00:00:00 EST 2017}
}
  • We present gallium concentration (normalized to CI chondrites) in the mantle is at the same level as that of lithophile elements with similar volatility, implying that there must be little to no gallium in Earth's core. Metal-silicate partitioning experiments, however, have shown that gallium is a moderately siderophile element and should be therefore depleted in the mantle by core formation. Moreover, gallium concentrations in the mantle (4 ppm) are too high to be only brought by the late veneer; and neither pressure, nor temperature, nor silicate composition has a large enough effect on gallium partitioning to make it lithophile. Wemore » therefore systematically investigated the effect of core composition (light element content) on the partitioning of gallium by carrying out metal–silicate partitioning experiments in a piston–cylinder press at 2 GPa between 1673 K and 2073 K. Four light elements (Si, O, S, C) were considered, and their effect was found to be sufficiently strong to make gallium lithophile. The partitioning of gallium was then modeled and parameterized as a function of pressure, temperature, redox and core composition. A continuous core formation model was used to track the evolution of gallium partitioning during core formation, for various magma ocean depths, geotherms, core light element contents, and magma ocean composition (redox) during accretion. The only model for which the final gallium concentration in the silicate Earth matched the observed value is the one involving a light-element rich core equilibrating in a FeO-rich deep magma ocean (>1300 km) with a final pressure of at least 50 GPa. More specifically, the incorporation of S and C in the core provided successful models only for concentrations that lie far beyond their allowable cosmochemical or geophysical limits, whereas realistic O and Si amounts (less than 5 wt.%) in the core provided successful models for magma oceans deeper that 1300 km. In conclusion, these results offer a strong argument for an O- and Si-rich core, formed in a deep terrestrial magma ocean, along with oxidizing conditions.« less
  • Cited by 6
  • The partitioning of Fe, Ni and Co between Mg-rich olivine, lunar basaltic liquid and metal has been measured over a range of temperatures and pressures. The results of the olivine/metal partitioning may be compared to those predicted from thermodynamic calculation; in general, the agreement is good, although the calculated distribution coefficients are slightly greater than those experimentally determined. This discrepancy increases with increasing temperature and pressure, but it is not possible to ascertain unambiguously which thermodynamic data might be responsible. At lower temperatures, inversion of the equilibria to yield cosmothermometers for measuring the temperature of equilibration between olivine and metalmore » gives results for the pallasite meteorites in excellent agreement with independent estimates. The results have been applied to Apollo 15 Green Glass, presumed to approximate a primitive melt from the lunar mantle, to deduce the composition of Fe-Ni-Co metal in equilibrium with the lunar mantle. This composition is approximately (by weight) 54-60% Fe, 38-45% Ni and 1% Co; consequently, if metal has separated from the lunar mantle under equilibrium conditions to form a small lunar core, this core will be nickel rich, with a Ni/Fe ratio of {approximately}0.7 {plus minus} 0.15.« less
  • The abundances of siderophile elements in the Earth's mantle are the result of core formation in the early Earth. Many variables are involved in the prediction of metal/silicate siderophile partition coefficients during core segregation: pressure, temperature, oxygen fugacity, silicate and metal compositions. Despite publications of numerous results of metal-silicate experiments, the experimental database and predictive expressions for elements partitioning are hampered by a lack of systematic study to separate and evaluate the effects of each variable. Only a relatively complete experimental database that describes Ni and Co partitioning now exists but is not sufficient to unambiguously decide between the mostmore » popular model for core formation with a single stage core-mantle equilibration at the bottom of a deep magma ocean (e.g. Li and Agee, 2001) and more recent alternative models (e.g. Wade and Wood, 2005; Rubie et al., 2007). In this experimental work, systematic study of metal silicate partitioning is presented for elements normally regarded as moderately siderophile (Mo, As, Ge, W, P, Ni, Co), slightly siderophile (Zn, Ga, Mn, V, Cr) and refractory lithophile (Nb, Ta). Using a new piston-cylinder design assembly allows us to present a suite of isobaric partitioning experiments at 3 GPa within a temperature range from 1600 to 2600 C and over a range of relative oxygen fugacity from IW-1.5 to IW-3.5. Silicate melts range from basaltic to peridotite in composition. The individual effect of pressure is also investigated through a combination of piston cylinder and multi anvil isothermal experiments from 0.5 to 18 GPa at 1900 C. Absolute measurements of partitioning coefficients combining EMP and LA-ICPMS analytical methods are provided. New results are obtained for elements whose partitioning behavior is usually poorly constrained and not integrated into any accretion or core formation models. We find notably that Ge, As, Mo become less siderophile with increasing temperature. In contrast Zn, Nb and Ta become more siderophile while Ga, W and P show negligible dependence with increasing temperature. Moreover, As, Mo, W, Ga and Nb become less siderophile with increasing pressure while a small influence of pressure is observed for Ta, Ge and Zn. At 3 GPa, regressions of the partitioning data show a 5+ valence state for As, Mo, W and Ta, 3+ for Ga, and 2+ for Ge and Zn. Finally regressions show that highly charged cations (Nb, P, W, Mo and As) are, as expected, the most sensitive to variations in silicate melt composition with the exception of Ta that shows a surprisingly small dependence. Generally, models of partitioning behaviors during core segregation are obtained for each element and seem to exclude the possibility of a single stage equilibrium scenario for the earth's core formation.« less