The effect of Si on metal–silicate partitioning of siderophile elements and implications for the conditions of core formation

Tuff, J.; Wood, Bernard J.; Wade, Jon

doi:10.1016/j.gca.2010.10.027

Cited by 67 publications

(72 citation statements)

References 51 publications

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“…% Si in the inner core. This amount of Si is much higher than the estimate in the power-law model and is inconsistent with recent cosmochemical and geochemical constraints (19,(59)(60)(61)(62)(63)(64)(65). Direct measurements of the V P − ρ relationship of Fe-light element alloys at relevant P-T conditions of the core now appear to be on the horizon, which in turn may eventually answer the longstanding question on the composition of the Earth's core.…”

Section: Resultsmentioning

confidence: 58%

Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Mao

Lin

Liu

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

103

138

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Compressional wave velocity-density (V P − ρ) relations of candidate Fe alloys at relevant pressure-temperature conditions of the Earth's core are critically needed to evaluate the composition, seismic signatures, and geodynamics of the planet's remotest region. Specifically, comparison between seismic V P − ρ profiles of the core and candidate Fe alloys provides first-order information on the amount and type of potential light elements-including H, C, O, Si, and/or S-needed to compensate the density deficit of the core. To address this issue, here we have surveyed and analyzed the literature results in conjunction with newly measured V P − ρ results of hexagonal closest-packed (hcp) Fe and hcp-Fe 0.85 Si 0.15 alloy using in situ highenergy resolution inelastic X-ray scattering and X-ray diffraction. The nature of the Fe-Si alloy where Si is readily soluble in Fe represents an ideal solid-solution case to better understand the lightelement alloying effects. Our results show that high temperature significantly decreases the V P of hcp-Fe at high pressures, and the Fe-Si alloy exhibits similar high-pressure V P − ρ behavior to hcp-Fe via a constant density offset. These V P − ρ data at a given temperature can be better described by an empirical power-law function with a concave behavior at higher densities than with a linear approximation. Our new datasets, together with literature results, allow us to build new V P − ρ models of Fe alloys in order to determine the chemical composition of the core. Our models show that the V P − ρ profile of Fe with 8 wt % Si at 6,000 K matches well with the Preliminary Reference Earth Model of the inner core.compressional-wave velocity | high pressure-temperature E nigmatic properties of the Earth's inner core have recently been discovered including differential super-rotation (1), seismic anisotropies (2-4), and fine-scale seismic heterogeneities (5, 6). Deciphering these observations requires solid knowledge about the composition of the Earth's inner core and, therefore, the elasticity of candidate Fe alloys (7-16). Since F. Birch pointed out in the 1950s that Earth's core is too dense if composed of Fe or Fe-Ni alloy alone (13), a number of candidate major light elements, including oxygen (O), silicon (Si), sulfur (S), carbon (C), and hydrogen (H), have been suggested via cosmochemical, geochemical, and geophysical evidence (17). To ascertain the identity and exact amount of light elements needed in the Earth's inner core, one key piece of information lies in the comparison of the seismic V P − ρ profiles with reliable laboratory measurements of these properties for candidate Fe alloys. Potential Fe-light element alloy must have V P − ρ profiles that match seismic models such as the Preliminary Reference Earth Model and AK135 (7,8). Thus, this requires precise experimental results describing the V P − ρ relationships of Fe alloys at pressure-temperature (P-T) conditions relevant to the Earth's core. To address this issue, here, we present new experimental measurements on the V...

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Section: Resultsmentioning

confidence: 58%

Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Mao

Lin

Liu

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

103

138

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“…S1 and S2) vary as a function of final magma ocean pressure for the whole range of model parameters (geotherm and redox). In addition, siderophile element activity coefficients in the metallic phase are also dependent upon the concentrations of Si and O in the metallic melt (16,18,27,28). Geochemically consistent models are those for which the mantle abundances for Ni, Co, Cr, and V are simultaneously satisfied, within uncertainties.…”

Section: Resultsmentioning

confidence: 99%

Core formation and core composition from coupled geochemical and geophysical constraints

Badro

Brodholt

Piet

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

166

178

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The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantlederived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal−silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.core formation | core composition | mineral physics | experimental petrology | earth's accretion E arth formed ∼4.56 billion years ago (1-3), over a period of several tens of millions of years, through the accretion of planetary embryos and planetesimals (4). The energy delivered by progressively larger impactors maintained Earth's outer layer as an extensively molten (4) magma ocean. Gravitational segregation of metal and silicate within the magma ocean resulted in the primary differentiation of the planet characterized by a metallic core and silicate mantle. Although numerous accretion/ core formation models have been proposed (5, 6), perhaps the simplest in terms of quantitative testability postulates that the molten metal and silicate maintained chemical equilibrium (7-11), allowing phase relations and partitioning constraints to be applied in modeling their chemical evolution. This scenario couples the chemical evolution of the mantle and core with the evolving conditions (depth, pressure, temperature, composition) in the magma ocean that directly influence partitioning behavior and the resulting composition of metal and silicate.The primary observations that constrain core formation models are (i) siderophile abundance patterns in the silicate mantle, (ii) the geophysically inferred requirement that the core contains elements lighter than iron (12), and (iii) the concentration of FeO (e.g., redox) in the primitive upper mantle. Assuming that core formation proceeds under con...

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“…The previous set of experiments (Kiseeva and Wood, 2013) demonstrated that 0.5 h were sufficient for the equilibrium to be achieved at 1400 • C and earlier work established metal-silicate equilibrium in ∼3 min at 1650 • C (Tuff et al, 2011). The duration of hightemperature runs (between 1500 and 1700 • C) was therefore between 1 h and 12 min with shortest run duration at 1700 • C. We did not observe any significant loss of Cu in our experiments, although up to 50% of Ni was lost when Ni was added as NiO or NiS.…”

Section: Experimental Methodsmentioning

confidence: 93%

The effects of composition and temperature on chalcophile and lithophile element partitioning into magmatic sulphides

Kiseeva

Wood

2015

Earth and Planetary Science Letters

Self Cite

128

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We develop a comprehensive model to describe trace and minor element partitioning between sulphide liquids and anhydrous silicate liquids of approximately basaltic composition. We are able thereby to account completely for the effects of temperature and sulphide composition on the partitioning of Ag, Cd, Co, Cr, Cu, Ga, Ge, In, Mn, Ni, Pb, Sb, Ti, Tl, V and Zn. The model was developed from partitioning experiments performed in a piston-cylinder apparatus at 1.5 GPa and 1300 to 1700 • C with sulphide compositions covering the quaternary FeS-NiS-CuS 0.5 -FeO. Partitioning of most elements is a strong function of the oxygen (or FeO) content of the sulphide. This increases linearly with the FeO content of the silicate melt and decreases with Ni content of the sulphide. As expected, lithophile elements partition more strongly into sulphide as its oxygen content increases, while chalcophile elements enter sulphide less readily with increasing oxygen. We parameterised the effects by using the ε-model of non-ideal interactions in metallic liquids. The resulting equation for partition coefficient of an element M between sulphide and silicate liquids can be expressed aswhere A is a constant related to the entropy change of the partitioning reaction, B is a constant related to its enthalpy and n is the valency of the element of interest. Interaction parameters ε FeO , ε NiS and ε CuS 0.5 refer to non-ideal interactions between trace element and matrix, x FeO , x NiS , x CuS 0.5 and x FeS are mole fractions of FeO, NiS, and CuS 0.5 in sulphide and FeO corr is FeO content of silicate liquid (wt%) corrected for the ideal activity of FeS in the sulphide as follows:We find, for most elements, that the effect of Ni and Cu on partitioning is significantly smaller than the effect of oxygen. The effects of temperature are greatest for Ni, Cu and Ag. We used our model to calculate the amount of sulphide liquid precipitated along the liquid line of descent of MORB melts and find that 70% of silicate crystallisation is accompanied by ∼0.23% of sulphide precipitation. The latter is sufficient to control the melt concentrations of chalcophile elements such as Cu, Ag and Pb. Our partition coefficients and observed chalcophile element concentrations in MORB glasses were used to estimate sulphur solubility in MORB liquids. We obtained between ∼800 ppm (for primitive MORB) and ∼2000 ppm (for evolved MORB), values in reasonable agreement with experimentally-derived models. The experimental data also enable us to reconsider Ce/Pb and Nd/Pb ratios in MORB. We find that constant Ce/Pb and Nd/Pb ratios of 25 and 20, respectively, can be achieved during fractional crystallisation of magmas generated by 10% melting of depleted mantle provided the latter contains >100 ppm S and about 650 ppm Ce, 550 ppm Nd and 27.5 ppb Pb.Finally, we investigated the hypothesis that the pattern of chalcophile element abundances in the mantle was established by segregation of a late sulphide matte. Taking the elements Cu, Ag, Pb and Zn as 281 examples we find that th...

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The effect of Si on metal–silicate partitioning of siderophile elements and implications for the conditions of core formation

Cited by 67 publications

References 51 publications

Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Sound velocities of Fe and Fe-Si alloy in the Earth’s core

Core formation and core composition from coupled geochemical and geophysical constraints

The effects of composition and temperature on chalcophile and lithophile element partitioning into magmatic sulphides

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