Sang Heon Shim

The postspinel boundary in pyrolitic compositions determined in the laser-heated diamond anvil cell

In situ multianvil press (MAP) studies have reported that the depth and the Clapeyron slope of the postspinel boundary are significantly less than those of the 660 km discontinuity inferred from seismic studies. These results have raised questions about whether the postspinel transition is associated with the discontinuity. We determined the postspinel transition in pyrolitic compositions in the laser-heated diamond anvil cell (LHDAC) combined with in situ synchrotron X-ray diffraction. The Clapeyron slope was determined to be −2.5 ± 0.4MPa/K and did not vary significantly with compositions and used pressure scales. Using Pt scales, our data indicate that the postspinel transition occurs in pyrolitic compositions at 23.6–24.5GPa (1850K). The transition pressure and slope are consistent with the depth and topography of the 660 km discontinuity. Our data reveal that inaccuracy in pressure scales alone cannot explain the discrepancy and technical differences between MAP and LHDAC contribute significantly to the discrepancy.

Origin of Fe3+ in Fe-containing, Al-free mantle silicate perovskite

Abstract We have studied the ferrous (Fe 2+ ) and ferric (Fe 3+ ) iron concentrations in Al-free Fe containing Mg-silicate perovskite (Mg-Pv) at pressure ( P ), temperature ( T ), and oxygen fugacity ( f O 2 ) conditions related to the lower mantle using a thermodynamic model based on ab initio calculations. We consider the oxidation reaction and the charge disproportionation reaction, both of which can produce Fe 3+ in Mg-Pv. The model shows qualitatively good agreement with available experimental data on Fe 3+ / Σ Fe ( Σ Fe = total Fe in system), spin transitions, and equations of state. We predict that under lower-mantle conditions Fe 3+ / Σ Fe determined by the charge disproportionation is estimated to be 0.01–0.07 in Al-free Mg-Pv, suggesting that low Al Mg-Pv in the uppermost pyrolitic mantle (where majoritic garnet contains most of the Al) and in the harzburgitic heterogeneities throughout the lower mantle contains very little Fe 3+ . We find that the volume reduction by the spin transition of the B-site Fe 3+ leads to a minimum Fe 3+ / Σ Fe in Mg-Pv at mid-mantle pressures. The model shows that configurational entropy is a key driving force to create Fe 3+ and therefore Fe 3+ content is highly temperature sensitive. The temperature sensitivity may lead to a maximum Fe 3+ / Σ Fe in Mg-Pv in warm regions at the core–mantle boundary region, such as Large Low Shear Velocity Provinces (LLSVPs), potentially altering the physical (e.g., bulk modulus) and transport (e.g., thermal and electrical conductivities) properties of the heterogeneities.

Electronic structure of iron in magnesium silicate glasses at high pressure

[1]xa0A recent study attributed the source of an iron partitioning change between silicate melt and minerals at deep mantle conditions to a high-spin to low-spin change in iron, which was found in a Fe-diluted Mg-silicate glass at a similar pressure. We conducted X-ray emission spectroscopy and nuclear forward scattering on iron-rich Mg-silicate glasses at high pressure and 300xa0K in the diamond-anvil cell: Al-free glass up to 135xa0GPa and Al-bearing glass up to 93xa0GPa. In both glasses, the spin moment decreases gradually from 1xa0bar and does not reach a complete low-spin state even at the peak pressures of this study. The gradual change may be due to the existence of diverse coordination environments for iron in the glasses and continuous structural adjustment of the disordered system with pressure. If the result can be extrapolated to iron in mantle melts, the small, gradual changes in the spin state of iron may not be the dominant factor explaining the reported sudden change in the partitioning behavior of iron between silicate melt and minerals at lower-mantle pressures.

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

Significance This paper reports an unexpected change in the oxidation state of Fe in bridgmanite, the most dominant mineral in the lower mantle. The oxidation state change resolves the discrepancy between laboratory and seismic studies on the chemical composition of the lower mantle, showing that the lower mantle has major element chemistry similar to the upper mantle. The oxidation state change will also lead to a lower Fe content in bridgmanite in the midmantle, whereas the total Fe content remains the same. Such a change can lead to an increase in viscosity at 1,100- to 1,700-km depths, providing a viable mineralogical explanation on possible viscosity elevation suggested by geophysical studies at the same depth range. 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+/∑Feu2009=u200960%) 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.

Crystal structure and compressibility of lead dioxide up to 140 GPa

Abstract Lead dioxide is an important silica analog that has high-pressure behavior similar to what has been predicted for silica, only at lower pressures. We have measured the structural evolution and compressional behavior of different lead dioxide polymorphs up to 140 GPa in the laser-heated diamond-anvil cell using argon as a pressure medium. High-temperature heating prevents the formation of multi-phase mixtures found in a previous study conducted at room temperature using a silicone grease pressure medium. We find diffraction peaks consistent with a baddeleyite-type phase in our cold-compressed samples between 30 and 40 GPa, which was not observed in the previous measurements. Lead dioxide undergoes a phase transition to a cotunnite-type phase at 24 GPa. This phase remains stable to at least 140 GPa with a bulk modulus of 219(3) GPa for K′0 = 4. Decompression measurements show a pure cotunnite-type phase until 10.5 GPa, where the sample converts to a mixture of baddeleyite-type, pyrite-type, and OI-type (Pbca) phases. Pure α-structured lead dioxide (scrutinyite) is found after pressure release at room pressure even though our starting material was in the β-structure (plattnerite). Pressure quenching to the α-structure appears to be a common feature of all group IVa oxides that are compressed to structures with greater density than the rutile-type structure.

Raman spectroscopy of water-rich stishovite and dense high-pressure silica up to 55 GPa

Abstract Recent studies have shown that mineral end-member phases (δ-AlOOH phase, phase H, and stishovite) with rutile-type or modified rutile-type crystal structures and solid solutions between them in the MgO-Al2O3-SiO2 system can store large amounts of water and can be stable at high pressures and high temperatures relevant to the Earth’s lower mantle. The Al-H charge-coupled substitution (Si4+ → Al3+ + H+) has been proposed to explain the storage capacity found in some of these phases. However, the amount of H+ found in some recent examples does not match the expected value if such substitution is dominant, and it is difficult to explain the larger water storage in stishovite with such a mechanism alone. An octahedral version of the hydrogarnet-like substitution (Si4+ → 4H+) has been proposed to explain the incorporation of protons in Al-free, water-rich stishovite. Yet, the high-pressure structural behavior of OH in this phase has not yet been measured. In this study, we report high-pressure Raman spectroscopy measurements on Al-free hydrous stishovite with 3.2 ± 0.5 wt% water up to 55 GPa. At ambient pressure, we find that the OH stretching modes in this phase have frequencies lying in between those in low-water aluminous stishovite and those in δ-AlOOH, suggesting a strength of the hydrogen bonding intermediate between these two cases. After decompression to 1 bar, we observe modes that are similar to the IR-active modes of anhydrous and hydrous stishovite, suggesting that the existence of Si defects in the crystal structure can activate the inactive modes. For both lattice and OH-stretching modes, our data show a series of changes at pressures between 24 and 28 GPa suggesting a phase transition (likely to CaCl2-type). While some of the lattice mode behaviors are similar to what was predicted for the AlOOH polymorphs, the OH mode of our hydrous stishovite shows a positive frequency shift with pressure, which is different from δ-AlOOH. All our spectral observations suggest that water-rich pure dense silica has a distinct proton incorporation mechanism from aluminous low-water stishovite and δ-AlOOH, supporting the proposed direct substitution.

Crystal structure of CaSiO3 perovskite at 28–62 GPa and 300 K under quasi-hydrostatic stress conditions

Abstract To find the thermodynamically stable crystal structure of CaSiO3 perovskite (CaPv) at high pressure and 300 K, we have conducted synchrotron X-ray diffraction (XRD) on thermally stress-annealed samples in a Ne pressure medium in the diamond-anvil cell at 28–62 GPa. Rietveld refinements of the diffraction patterns are significantly improved in fitting the positions and intensities of the split lines of CaPv if the starting model is a tetragonal perovskite-type structure with the SiO6 octahedral rotation around the tetragonal c-axis. The result is in contrast with other previous experiments, but is consistent with first-principles calculations, reconciling the discrepancy between computations and experiments on the crystal structure of CaPv. We attribute the observed difference to the formation of the thermodynamically more stable phase under improved stress conditions in our experiments. Our fitting shows that the bulk modulus of CaPv is 223 ± 6 GPa when its pressure derivative fixed to 4, which is also consistent with first-principles calculations. The previous observations of the diffraction patterns of CaPv inconsistent with the first-principles studies could be due to the formation of a metastable crystal structure of CaPv under elevated deviatoric stresses.