Heterogeneous accretion, composition and core–mantle differentiation of the Earth

The authors note the following: "The calculations that we presented for high-MgO concentration silicate melts are unduly sensitive to small variations in the K 2 O content of the liquid, stemming from the equation of state (EOS) [(1), hereafter referred to as Part IV EOS] that we utilized to calculate thermodynamic properties of the silicate liquid at high pressure. Although we attributed the strong decrease in ΔIW values for peridotite seen in Figs. 4 and 5 to the possibility of Fe 2 O 3 being stabilized in the presence of alkalis in the basalt composition (as well as to the effect of extremely high temperature conditions on the melt volume), the Part IV EOS does not include treatment of Fe 2 O 3 , so this argument may only be relevant to the natural system; in the Part IV EOS all oxidized iron is treated as FeO. Instead, the strong decrease can be attributed to K 2 O contents; above 20 GPa, the Part IV EOS calibration pinned the komatiite fit to a K 2 O content of 0.25 wt%; i.e., K 2 O is not an independent variable for high-MgO liquid above 20 GPa. A similar conclusion can be reached for the K 2 O of MORB liquid above 20 GPa, and to a lesser extent about the other alkali, Na 2 O. Given that there are more data constraints in the Part IV EOS to calibrate variation of Na 2 O content, we expect the sensitivity of derived thermodynamic properties on Na 2 O variation to be muted. However, due caution should be exercised and Na 2 O contents should not stray far from the komatiite and MORB calibrants used to derive the Part IV EOS model. We now conclude that the only consistent way to deal with high-magnesia liquids in the context of the Part IV EOS model is to use a K 2 O ∼0.25 wt%, instead of the 0.03 wt% K 2 O in natural peridotite. As a result, Figs. 4, 5, and 6 and their respective legends appeared incorrectly."Any use of the online fO 2 calculator above 20-25 GPa for peridotite, komatiite or similar high-MgO bulk composition liquids must fix K 2 O content of silicate melt to 0.25 wt% K 2 O, otherwise erroneous fO 2 values may result."The corrected figures and their corrected legends appear below. These errors do not affect the conclusions of the article.

Section: Resultscontrasting

confidence: 58%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Redox systematics of a magma ocean with variable pressure-temperature gradients and composition

Righter

Ghiorso²

2012

“…% 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: 50%

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

Mao

Lin

Liu

et al. 2012

103

138

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...

“…The temperature and pressure of the simulation are fixed at 3200 K and 40 GPa, which are chosen based on the single-stage magma ocean depth inferred from siderophile element distribution (17,18). In general, the Earth's accretion history is expected to be more complicated (19), but the single-stage magma ocean model can be viewed as an effective model reflected by the siderophile element distribution in the mantle (20).…”

Section: Resultsmentioning

confidence: 99%

Carbon and other light element contents in the Earth’s core based on first-principles molecular dynamics

Zhang

Yin

2012

Carbon (C) is one of the candidate light elements proposed to account for the density deficit of the Earth's core. In addition, C significantly affects siderophile and chalcophile element partitioning between metal and silicate and thus the distribution of these elements in the Earth's core and mantle. Derivation of the accretion and core-mantle segregation history of the Earth requires, therefore, an accurate knowledge of the C abundance in the Earth's core. Previous estimates of the C content of the core differ by a factor of ∼20 due to differences in assumptions and methods, and because the metal-silicate partition coefficient of C was previously unknown. Here we use two-phase first-principles molecular dynamics to derive this partition coefficient of C between liquid iron and silicate melt. We calculate a value of 9 ± 3 at 3,200 K and 40 GPa. Using this partition coefficient and the most recent estimates of bulk Earth or mantle C contents, we infer that the Earth's core contains 0.1-0.7 wt% of C. Carbon thus plays a moderate role in the density deficit of the core and in the distribution of siderophile and chalcophile elements during core-mantle segregation processes. The partition coefficients of nitrogen (N), hydrogen, helium, phosphorus, magnesium, oxygen, and silicon are also inferred and found to be in close agreement with experiments and other geochemical constraints. Contents of these elements in the core derived from applying these partition coefficients match those derived by using the cosmochemical volatility curve and geochemical mass balance arguments. N is an exception, indicating its retention in a mantle phase instead of in the core.