A seismologically consistent compositional model of Earth’s core

Badro, James; Côté, Alexander S.; Brodholt, John P.

doi:10.1073/pnas.1316708111

Cited by 293 publications

(321 citation statements)

References 39 publications

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“…however, recent experiments found that adding up to 10% of Ni does not change the hexagonal close-packed crystal structure of the solid 42 , while ab initio calculations suggest that at high T the seismic properties of Fe-Ni alloys are almost indistinguishable from those of pure iron 43 . Recent studies of core composition [44][45][46] given in section 1 of Table 1; each model is named after the corresponding molar concentration.…”

Section: Materials Properties For Earth's Corementioning

confidence: 99%

Constraints from material properties on the dynamics and evolution of Earth’s core

et al. 2015

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The Earth's magnetic field is powered from energy supplied by slow cooling and freezing of the liquid iron core. Core thermal history calculations have been hindered in the past by poor knowledge of the properties of iron alloys at the extreme pressures and temperatures pertaining in the core. This obstacle is now being overcome by developments in high pressure experiments and computational mineral physics. Here we review the relevant properties of iron alloys at core conditions and discuss their uncertainty and geophysical implications.Powerful constraints on core evolution are now possible, due partly to recent factor 2-3 upward revisions of the all-important electrical and thermal conductivities. This has dramatic implications for the thermal history of the entire Earth, not just the core: the inner core is very young, the core is cooling quickly, and was so hot in the past that the lowermost mantle

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Section: Materials Properties For Earth's Corementioning

confidence: 99%

Constraints from material properties on the dynamics and evolution of Earth’s core

et al. 2015

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“…(10) and (19)) is depicted in Fig. 3 (d); the convergence of this contribution with respect to the k-mesh density is presented in Fig.…”

Section: Anharmonic Fementioning

confidence: 99%

“…A prominent example is behavior at conditions found at the Earth's inner core. The Earth's core is composed primarily of iron, with about 10% nickel and a number of light impurities, such as sulfur, oxygen, silicon, carbon, and hydrogen that are likely to be present [10][11][12] (it has even been proposed that the "missing xenon paradox" can be explained via reactions of Xe with Fe and Ni at inner-core conditions 13 ). An outstanding problem is determination of the structure of the core materialamong other things, this knowledge can inform theories of the Earth's evolution, aid understanding of its physical features (e.g., magnetic, gravitational fields), and guide interpretation of seismological measurements.…”

Section: Introductionmentioning

confidence: 99%

Accurate and precise ab initio anharmonic free-energy calculations for metallic crystals: Application to hcp Fe at high temperature and pressure

et al. 2017

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A framework for computing the anharmonic free energy (FE) of metallic crystals using BornOppenheimer ab initio molecular dynamics (AIMD) simulation, with unprecedented efficiency, is introduced and demonstrated for the hcp phase of iron at extreme conditions (up to ≈ 290 GPa and 5000 K). The advances underlying this work are: (1) A recently introduced harmonically-mapped averaging temperature integration (HMA-TI) method reduces the computational cost by order(s) of magnitude compared to the conventional TI approach. The TI path starts from zero Kelvin, where it assumes the behavior is given exactly by a harmonic treatment; this feature restricts application to systems that have no imaginary phonons in this limit. (2) A Langevin thermostat with the HMA-TI method allows use of a relatively large MD time step (4 fs, which is about eight times larger than the size needed for the Andersen thermostat) without loss of accuracy. (3) AIMD sampling is accelerated by using density functional theory (DFT) with a low-level parameter set, then the measured quantities of selected configurations are robustly reweighted to a higher level of DFT. This introduces a speedup of about 20-30× compared to directly simulating the accurate system. (4a) The temperature (T ) dependence of the hcp equilibrium shape (i.e. c/a axial ratio) is determined (including anharmonicity), with uncertainty less than ±0.001. (4b) Electronic excitation is included through Mermin's finite-temperature extension of the T = 0 K DFT. A simple FE perturbation method is introduced to handle the difficulty associated with applying the TI method with a Tdependent geometry and (due to electronic excitation) potential-energy surface. (5) The FE in the thermodynamic limit is attained through extrapolation of only the (computationally inexpensive) quasiharmonic FE, because the anharmonic FE contribution has negligible finite-size effects. All methods introduced here do not affect the AIMD sampling -results are obtained through postprocessing -so established AIMD codes can be employed without modification. Analytical formulas fitted to the results for the variation of the equilibrium c/a ratio and FE components with T are provided. Notably, effects of magnetic excitations are not included, and may yet prove important to the overall FE; if so, it is plausible that such contributions can be added perturbatively to the FE values reported here. Notwithstanding these considerations, FE values are obtained with an estimated accuracy and precision of 2 meV/atom, suggesting that the capability to compute the phase diagram of iron at Earth's inner core conditions is within reach.

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“…Even for the Earth's core, the core composition is not well-known and compositional models for the core (e.g., a silicon-versus an oxygen-bearing core) are offered as competing hypotheses (e.g., McDonough 2014). In contrast to other terrestrial bodies, compositional models for the Earth's core can be constrained by seismological data (e.g., Badro et al 2014).…”

Section: Core Compositionmentioning

confidence: 99%

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Breuer

Rueckriemen

Spohn

2015

Prog. in Earth and Planet. Sci.

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Recent planetary space missions, new experimental data, and advanced numerical techniques have helped to improve our understanding of the deep interiors of the terrestrial planets and moons. In the present review, we summarize recent insights into the state and composition of their iron (Fe)-rich cores, as well as recent findings about the magnetic field evolution of Mercury, the Moon, Mars, and Ganymede. Crystallizing processes in iron-rich cores that differ from the classical Earth case (i.e., Fe snow and iron sulfide (FeS) crystallization) have been identified and found to be important in the cores of terrestrial bodies. The Fe snow regime occurs at pressures lower than that in the Earth's core on the iron-rich side of the eutectic, where iron freezes first close to the core-mantle boundary rather than in the center. FeS crystallization, instead, occurs on the sulfur-rich side of the eutectic. Depending on the core temperature profile and the pressure range considered, FeS crystallizes either in the core center or close to the core-mantle boundary. The consequences of the various crystallizing mechanisms for core dynamics and magnetic field generation are discussed. For the Moon, revised paleomagnetic data obtained with advanced techniques suggest a peculiar history of its internal dynamo, with an early strong field persisting between 4.25 and 3.5 Ga, and subsequently a much weaker field. In addition, the long-lasting dynamo and the possible presence of an inner core, as inferred from a revised interpretation of Apollo seismic data, suggest core crystallization as a viable process of magnetic field generation for a substantial period during lunar evolution. The present-day magnetic fields of Mercury and Ganymede (if they occur on the iron-rich side of the Fe-FeS eutectic) and the related dynamo action are likely generated in the Fe snow regime and seem to be recent features. An earlier dynamo in Mercury would have been powered differently. For Mercury, MESSENGER data further suggest core formation under reducing conditions that may have resulted in an Fe-S-Si composition, further complicating the core crystallization process. Mars, with its early and strong paleo-field, likely has not yet started to freeze out an inner iron core.

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A seismologically consistent compositional model of Earth’s core

Cited by 293 publications

References 39 publications

Constraints from material properties on the dynamics and evolution of Earth’s core

Constraints from material properties on the dynamics and evolution of Earth’s core

Accurate and precise ab initio anharmonic free-energy calculations for metallic crystals: Application to hcp Fe at high temperature and pressure

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

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