SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Planetary differentiation

39

The physical and chemical properties of Earth’s mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa. The compressibility of ferric iron-bearing bridgmanite is significantly different from any known bridgmanite, which has direct implications for the interpretation of seismic tomography data.

Concepts: Iron, Earth, Mineral, Mantle, Crust, Structure of the Earth, Olivine, Planetary differentiation

17

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3Bdg that Fe3+in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

Concepts: Water, Earth, Chemical element, Physical quantities, Terrestrial planet, Mantle, Crust, Planetary differentiation

6

The mineralogical constitution of the Earth’s mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases.

Concepts: Earth, Lithosphere, Terrestrial planet, Mantle, Crust, Asthenosphere, Outer core, Planetary differentiation

2

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe(2+) in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe(3+)/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe(3+) in bridgmanite combined with the strong partitioning of Fe(2+) into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.

Concepts: Iron, Earth, Redox, Aluminium, Mantle, Olivine, Post-perovskite, Planetary differentiation

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We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth’s lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth’s mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.

Concepts: Electron, Earth, Mantle, Crust, Structure of the Earth, Core–mantle boundary, Post-perovskite, Planetary differentiation

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Seismological observations show that, in some regions of the lower mantle, an increase in bulk sound velocity, interestingly, occurs in the same volume where there is a decrease in shear velocity. We show that this anti-correlated behavior occurs on cation substitution in bridgmanite by making single crystal elasticity measurements of MgSiO3 and (Mg,Fe,Al)(Si,Al)O3 using inelastic x-ray scattering in the ambient conditions. Cation substitution of ferrous iron and aluminum may explain large low shear velocity provinces in the lower mantle.

Concepts: Diffraction, Iron, Fundamental physics concepts, Earth, Solid, Resonant inelastic X-ray scattering, Planetary differentiation, Andrija Mohorovi ić

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The physical properties of iron (Fe) at high pressure and high temperature are crucial for understanding the chemical composition, evolution, and dynamics of planetary interiors. Indeed, the inner structures of the telluric planets all share a similar layered nature: a central metallic core composed mostly of iron, surrounded by a silicate mantle, and a thin, chemically differentiated crust. To date, most studies of iron have focused on the hexagonal closed packed (hcp, or ε) phase, as ε-Fe is likely stable across the pressure and temperature conditions of Earth’s core. However, at the more moderate pressures characteristic of the cores of smaller planetary bodies, such as the Moon, Mercury, or Mars, iron takes on a face-centered cubic (fcc, or γ) structure. Here we present compressional and shear wave sound velocity and density measurements of γ-Fe at high pressures and high temperatures, which are needed to develop accurate seismic models of planetary interiors. Our results indicate that the seismic velocities proposed for the Moon’s inner core by a recent reanalysis of Apollo seismic data are well below those of γ-Fe. Our dataset thus provides strong constraints to seismic models of the lunar core and cores of small telluric planets. This allows us to propose a direct compositional and velocity model for the Moon’s core.

Concepts: Earth, Sun, Planet, Mars, Thermodynamics, Venus, Planetary differentiation

0

Ever since the Apollo programme, isotopic abundances have been used as tracers to study lunar formation, in particular to study the sources of the lunar material. In the past decade, increasingly precise isotopic data have been reported that give strong indications that the Moon and the Earth’s mantle have a common heritage. To reconcile these observations with the origin of the Moon via the collision of two distinct planetary bodies, it has been proposed (i) that the Earth-Moon system underwent convective mixing into a single isotopic reservoir during the approximately 10(3) year molten disc epoch after the giant impact but before lunar accretion, or (ii) that a high angular momentum impact injected a silicate disc into orbit sourced directly from the mantle of the proto-Earth and the impacting planet in the right proportions to match the isotopic observations. Recently, it has also become recognized that liquid-vapour fractionation in the energetic aftermath of the giant impact is capable of generating measurable mass-dependent isotopic offsets between the silicate Earth and Moon, rendering isotopic measurements sensitive not only to the sources of the lunar material, but also to the processes accompanying lunar origin. Here, we review the isotopic evidence that the silicate-Earth-Moon system represents a single planetary reservoir. We then discuss the development of new isotopic tracers sensitive to processes in the melt-vapour lunar disc and how theoretical calculations of their behaviour and sample observations can constrain scenarios of post-impact evolution in the earliest history of the Earth-Moon system.

Concepts: Earth, Sun, Planet, Moon, Terrestrial planet, Crust, Natural satellite, Planetary differentiation