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  1. Abstract

    Magma oceans were once ubiquitous in the early solar system, setting up the initial conditions for different evolutionary paths of planetary bodies. In particular, the redox conditions of magma oceans may have profound influence on the redox state of subsequently formed mantles and the overlying atmospheres. The relevant redox buffering reactions, however, remain poorly constrained. Using first-principles simulations combined with thermodynamic modeling, we show that magma oceans of Earth, Mars, and the Moon are likely characterized with a vertical gradient in oxygen fugacity with deeper magma oceans invoking more oxidizing surface conditions. This redox zonation may be the major cause for the Earth’s upper mantle being more oxidized than Mars’ and the Moon’s. These contrasting redox profiles also suggest that Earth’s early atmosphere was dominated by CO2and H2O, in contrast to those enriched in H2O and H2for Mars, and H2and CO for the Moon.

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  2. Earth’s core is likely the largest reservoir of carbon (C) in the planet, but its C abundance has been poorly constrained because measurements of carbon’s preference for core versus mantle materials at the pressures and temperatures of core formation are lacking. Using metal–silicate partitioning experiments in a laser-heated diamond anvil cell, we show that carbon becomes significantly less siderophile as pressures and temperatures increase to those expected in a deep magma ocean during formation of Earth’s core. Based on a multistage model of core formation, the core likely contains a maximum of 0.09(4) to 0.20(10) wt% C, making carbon a negligible contributor to the core’s composition and density. However, this accounts for ∼80 to 90% of Earth’s overall carbon inventory, which totals 370(150) to 740(370) ppm. The bulk Earth’s carbon/sulfur ratio is best explained by the delivery of most of Earth’s volatiles from carbonaceous chondrite-like precursors.

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  3. Abstract

    Astronomical surveys have discovered thousands of transiting exoplanets, revealing that rocky planets are common in the galaxy. A planet's interior chemistry is frequently inferred by average density, described by mass‐radius (M‐R) relationships. However, M‐R relationships give rise to non‐unique interpretations of a planet's interior composition, an issue that limits our ability to characterize far‐away worlds. We present experimental and density functional theoretical results addressing the influence of an ultra‐reducing (oxygen‐poor) interior chemistry on rocky mantle phases and discuss the possible implications for atmospheric observables. We show that silicon carbide (SiC) and molecular nitrogen (N2) react to form solid silicon nitride (γ‐Si3N4) at high pressures and high temperatures in a laser‐heated diamond‐anvil cell, consistent with ab initio computations. Si3N4remains stable under extreme conditions and when quenched to ambient conditions. As SiC is a common compound found under very reducing conditions, these results indicate that nitrogen may form solid phases in an oxygen‐poor rocky planet. If, by sequestering nitrogen in a planet's mantle, the distribution of nitrogen between a planet's interior and atmosphere is altered (i.e., a nitrogen‐rich mantle and nitrogen‐poor atmosphere), these results indicate that there may be atmospheric observables connected to the mantle‐redox state of a rocky planet besides the oxygen‐containing phases ubiquitous in exoplanet literature.

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  4. Abstract

    The amount of ferric iron Fe3+in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe2+into metallic Fe and Fe3+triggered by the formation of bridgmanite. Recent work has shown that Fe3+has a strong effect on the density and seismic wave speeds of bridgmanite and the incorporation of impurities such as aluminum. In order to further investigate the effects of ferric iron on mineral behavior at lower mantle conditions, we conducted laser‐heated diamond‐anvil cell (LHDAC) experiments on two sets of samples nearly identical in composition (an aluminum‐rich pyroxenite glass) except for the Fe3+content; with one sample with more Fe3+(“oxidized”: Fe3+/ΣFe ~ 55%) and the other with less Fe3+(“reduced”: Fe3+/ΣFe ~ 11%). We heated the samples to lower mantle conditions, and the resulting assemblages were drastically different between the two sets of samples. For the reduced composition, we observed a multiphase assemblage dominated by bridgmanite and calcium perovskite. In contrast, the oxidized material yielded a single phase of Ca‐bearing bridgmanite. These Al‐rich pyroxenite samples show a difference in density and seismic velocities for these two redox states, where the reduced assemblage is denser than the oxidized assemblage by ~1.5% at the bottom of the lower mantle and slower (bulk sound speed) by ~2%. Thus, heterogeneities of Fe3+content may lead to density and seismic wave speed heterogeneities in Earth's lower mantle.

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  5. Abstract

    At nearly 2,900‐km depth, the core‐mantle boundary (CMB) represents the largest density increase within the Earth going from a rocky mantle into an iron‐alloy core. This compositional change sets up steep temperature gradients, which in turn influences mantle flow, structure, and seismic velocities. Here we resolve the thermodynamic parameters of (Mg,Fe)O and compute the melting phase relations of the MgO‐FeO binary system at CMB conditions. Based on this phase diagram, we revisit iron infiltration into solid ferropericlase along the CMB by morphological instability and find that the length scale of infiltration is comparable with the high electrical conductivity layer inferred from core nutations. We also compute the (Mg,Fe)O‐SiO2pseudo‐binary system and find that the solidus melting temperatures near the CMB decrease with FeO and SiO2content, becoming potentially important for ultralow velocity zones. Therefore, an ultralow velocity zone composed of solid‐state bridgmanite and ferropericlase may be relatively enriched in MgO and depleted in SiO2and FeO along a hot CMB.

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  6. Abstract

    We present results from high‐pressure and high‐temperature experiments on mixtures of SiC and SiO2to explore the stability of SiC in the presence of oxygen‐rich silicates at planetary mantle conditions. We observe no evidence of the ambient pressure predicted oxidation products, CO or SiO, resulting from oxidation reactions between SiC and SiO2at pressures up to ~40 GPa and temperatures up to ~2500 K. We observe the decomposition of SiC through releasing C, resulting in vacancies in the SiC lattice and consequently the contracted SiC ambient volume V0observed in the heated regions of sample. The decomposition is further supported by the observations of diamond formation and the expanded SiO2V0in the heated regions of samples indicating the incorporation of C into SiO2stishovite. We provide a new interpretation of SiC decomposition on laboratory timescales, in which kinetics prevent the reaction from reaching equilibrium. We consider how the equilibrium decomposition reaction of SiC will influence the differentiation of a SiC‐containing body on planetary timescales and find that the decomposition products may become isolated during early planetary differentiation. The resulting presence of elemental Si and C within a planetary body may have important consequences for the compositions of the mantles and atmospheres of such planets.

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  7. Abstract

    Pyrite‐type FeO2Hx(P phase) has recently been suggested as a possible alternative to explain ultralow‐velocity zones due to its low seismic velocity and high density. Here we report the results on the congruent melting temperature and melt properties of P phase at high pressures from first‐principles molecular dynamics simulations. The results show that P phase would likely be melted near the core–mantle boundary. Liquid FeO2Hxhas smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO2Hxcould account for the observed seismic anomaly of ultralow‐velocity zones. However, to maintain the liquid FeO2Hxwithin the ultralow‐velocity zones against compaction requires special physical conditions, such as relatively high viscosity of the solid matrix and/or vigorous convection of the overlying mantle.

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  8. Abstract

    Density of silicate melt dictates melt migration and establishes the gross structure of Earth's interior. However, due to technical challenges, the melt density of relevant compositions is poorly known at deep mantle conditions. Particularly, water may be dissolved in such melts in large amounts and can potentially affect their density at extreme pressure and temperature conditions. Here we perform first‐principles molecular dynamics simulations to evaluate the density of Fe‐rich, eutectic‐like silicate melt (Emelt) with varying water content up to about 12 wt %. Our results show that water mixes nearly ideally with the nonvolatile component in silicate melt and can decrease the melt density significantly. They also suggest that hydrous melts can be gravitationally stable in the lowermost mantle given its likely high iron content, providing a mechanism to explain seismically slow and dense layers near the core‐mantle boundary.

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