Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions

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 Fe²⁺ in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe³⁺/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe³⁺ in bridgmanite combined with the strong partitioning of Fe²⁺ 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.

Melting of subducted basalt at the core-mantle boundary

The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

Simultaneous determination of mean pressure and deviatoric stress based on numerical tensor analysis: a case study for polycrystalline x-ray diffraction of gold enclosed in a methanol-ethanol mixture

It is known that the {100} and {111} planes of cubic crystals subjected to uniaxial deviatoric stress conditions have strain responses that are free from the effect of lattice preferred orientation. By utilizing this special character, one can unambiguously and simultaneously determine the mean pressure and deviatoric stress from polycrystalline diffraction data of the cubic sample. Here we introduce a numerical tensor calculation method based on the generalized Hooke's law to simultaneously determine the hydrostatic component of the stress (mean pressure) and deviatoric stress in the sample. The feasibility of this method has been tested by examining the experimental data of the Au pressure marker enclosed in a diamond anvil cell using a pressure medium of methanol-ethanol mixture. The results demonstrated that the magnitude of the deviatoric stress is ∼0.07 GPa at the mean pressure of 10.5 GPa, which is consistent with previous results of Au strength under high pressure. Our results also showed that even a small deviatoric stress (∼0.07 GPa) could yield a ∼0.3 GPa mean pressure error at ∼10 GPa.

Quantitative Rietveld texture analysis of CaSiO(3) perovskite deformed in a diamond anvil cell

The Rietveld method is used to extract quantitative texture information from a single synchrotron diffraction image of a CaSiO(3) perovskite sample deformed in axial compression in a diamond anvil cell. The image used for analysis was taken in radial geometry at 49 GPa and room temperature. We obtain a preferred orientation of {100} lattice planes oriented perpendicular to the compression direction and this is compatible with [Formula: see text] slip.

The physical and chemical composition of the lower mantle

This article reviews some of the recent advances made within the field of mineral physics. In order to link the observed seismic and density structures of the lower mantle with a particular mineral composition, knowledge of the thermodynamic properties of the candidate materials is required. Determining which compositional model best matches the observed data is difficult because of the wide variety of possible mineral structures and compositions. State-of-the-art experimental and analytical techniques have pushed forward our knowledge of mineral physics, yet certain properties, such as the elastic properties of lower mantle minerals at high pressures and temperatures, are difficult to determine experimentally and remain elusive. Fortunately, computational techniques are now sufficiently advanced to enable the prediction of these properties in a self-consistent manner, but more results are required.A fundamental question is whether or not the upper and lower mantles are mixing. Traditional models that involve chemically separate upper and lower mantles cannot yet be ruled out despite recent conflicting seismological evidence showing that subducting slabs penetrate deep into the lower mantle and that chemically distinct layers are, therefore, unlikely.Recent seismic tomography studies giving three-dimensional models of the seismic wave velocities in the Earth also base their interpretations on the thermodynamic properties of minerals. These studies reveal heterogeneous velocity and density anomalies in the lower mantle, which are difficult to reconcile with mineral physics data.

Structure of liquid iron at pressures up to 58 GPa

We report structural data on liquid iron at pressures up to 58 GPa measured by x-ray scattering in a laser heated diamond anvil cell. The determined structure factor preserves essentially the same shape along the melting curve. Our data demonstrate that liquid iron at high pressures is a close-packed hard-sphere liquid. The results place important constraints on the thermodynamic and transport properties of liquid iron and the melting curve of iron.

Stability and structure of MgSiO3 perovskite to 2300-kilometer depth in Earth's mantle

Unexplained features have been observed seismically near the middle (approximately 1700-kilometer depth) and bottom of the Earth's lower mantle, and these could have important implications for the dynamics and evolution of the planet. (Mg,Fe)SiO3 perovskite is expected to be the dominant mineral in the deep mantle, but experimental results are discrepant regarding its stability and structure. Here we report in situ x-ray diffraction observations of (Mg,Fe)SiO3 perovskite at conditions (50 to 106 gigapascals, 1600 to 2400 kelvin) close to a mantle geotherm from three different starting materials, (Mg0.9Fe0.1)SiO enstatite, MgSiO3 glass, and an MgO+SiO2 mixture. Our results confirm the stability of (Mg,Fe)SiO3 perovskite to at least 2300-kilometer depth in the mantle. However, diffraction patterns above 83 gigapascals and 1700 kelvin (1900-kilometer depth) cannot presently rule out a possible transformation from Pbnm perovskite to one of three other possible perovskite structures with space group P2(1)/m, Pmmn, or P4(2)/nmc.

The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)2SiO4 from gamma-spinel (ringwoodite) to (Mg,Fe)SiO3-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures. But the first in situ examination of this phase transformation in Mg2SiO4 using a multi-anvil press indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to approximately 600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg2SiO4 at pressures of 20-36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from gamma-Mg2SiO4 to MgSiO3-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study, we find that the pressure and temperature of the post-spinel transformation in Mg2SiO4 is consistent with seismic observations for the 660-km discontinuity.