Ryosuke Sinmyo

The Electrical Conductivity of Post-Perovskite in Earth's D'' Layer

Recent discovery of a phase transition from perovskite to post-perovskite suggests that the physical properties of Earths lowermost mantle, called the D″ layer, may be different from those of the overlying mantle. We report that the electrical conductivity of (Mg0.9Fe0.1)SiO3 post-perovskite is >102 siemens per meter and does not vary greatly with temperature at the conditions of the D″ layer. A post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and the solid mantle, which can explain the observed changes in the length of a day on decadal time scales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.

Determination of post‐perovskite phase transition boundary in MgSiO3 using Au and MgO pressure standards

[1]xa0We have determined the post-perovskite phase transition boundary in MgSiO3 by in-situ X-ray diffraction measurements, using Au and MgO as internal pressure standards. Results demonstrate that phase transition occurs at 113 GPa and 2400 K with a positive Clapeyron slope of +4.7 MPa/K, based on the Au pressure scale. On the other hand, the simultaneous measurements of MgO standard show that the phase boundary is located at 119 GPa and 2400 K with a Clapeyron slope of +11.5 MPa/K, consistently with the previous experimental results based on the Pt scale and the theoretical calculations. Our experiments also indicate that the stabilities of perovskite and post-perovskite in all of (Mg0.89Fe0.11)2SiO4, natural pyrolitic mantle, and MORB compositions are consistent with the phase transition boundary in pure MgSiO3, when all the data are compared using the same Au scale. The compositional variations in natural systems have little effect on the post-perovskite phase transition.

Partitioning of iron between perovskite/postperovskite and ferropericlase in the lower mantle

[1]xa0The Fe2+-Mg distribution coefficients (KD) between perovskite/postperovskite and ferropericlase were determined in a (Mg0.9Fe0.1)2SiO4 bulk composition. High-pressure and high-temperature experiments were carried out between 36 and 128 GPa and 1760–2170 K in a laser-heated diamond anvil cell (LHDAC). Chemical analyses were made on recovered samples with transmission electron microscope (TEM) and field-emission-type scanning electron microscope (FE-SEM). All the samples exhibited a strong heterogeneity in iron content caused by relatively large temperature gradient during laser-heating, being depleted at the center of the hot spot and enriched at the margin. The KD values measured from coexisting perovskite/postperovskite and ferropericlase showed apparently very large variation in each sample, which could be the source of considerable discrepancy among previous measurements of KD using the LHDAC techniques. Nevertheless, the data obtained from a specific portion of the sample that retained the original bulk composition may represent the equilibrium partitioning. Present results demonstrate that the iron partitioning between perovskite/postperovskite and ferropericlase changes little at a given temperature throughout the lower mantle pressures. Both electronic transition of iron and postperovskite phase transition have small effects on iron partitioning.

Ferric iron in Al‐bearing post‐perovskite

[1]xa0The Fe3+/ΣFe ratios in both (Al, Fe)-bearing MgSiO3 post-perovskite phase and Ca-ferrite-type Al-phase, synthesized in a natural mid-oceanic ridge basalt (MORB) composition at 113 GPa and 2240 K, were determined by electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that post-perovskite has high proportions of Fe3+ with Fe3+/ΣFe ratio of 0.65 ± 0.04. The high Fe3+ concentration in post-perovskite may have significant effects on its physical properties, phase stability, and iron partitioning. In contrast, the Ca-ferrite-type Al-phase, which is the second Fe-bearing phase in a subducting MORB crust, is enriched in Fe2+ rather than Fe3+ with Fe3+/ΣFe ratio of 0.15 and 0.29.

The valence state and partitioning of iron in the Earth's lowermost mantle

[1]xa0While iron occurs predominantly in the form of Fe2+ in the Earths upper mantle and transition zone, Al-bearing (Mg,Fe)SiO3 perovskite (Pv), a primary mineral in the lower mantle, contains a large proportion of ferric iron (Fe3+). It has been demonstrated that such Fe3+ strongly affects physical and chemical properties of Pv. On the other hand, the iron substitution mechanism and valence state of iron in postperovskite (PPv) are still unclear. Here we determined the valence state of iron in PPv with changing Al3+ content, on the basis of electron energy-loss near-edge structure spectroscopy measurements. The results show that PPv includes a small amount of Fe3+, which is independent from the Al3+ content. This indicates that Fe3+ in Pv and coexisting metallic iron recombine to form Fe2+ upon phase transition from Pv to PPv in pyrolitic mantle. Such Fe2+ partitions preferentially into (Mg,Fe)O ferropericlase (Fp), and therefore PPv is depleted in iron compared to Pv. Such a marked change in the valence state of iron and resulting iron depletion in PPv have broad implications for seismic and transport properties in the lowermost mantle. Phase transition from Pv to Fe-poor PPv occurs in a much narrower pressure range than that for a fixed iron content.

Iron partitioning in pyrolitic lower mantle

The partitioning of iron between Mg-rich perovskite (Pv) and ferropericlase (Fp) was investigated for a pyrolitic bulk composition over a wide range of simulated lower-mantle pressures and temperatures from 28 to 114xa0GPa and from 1,900 to 2,300xa0K, in a laser-heated diamond anvil cell (DAC). The recovered DAC samples are chemically homogeneous, indicating a relatively small temperature gradient during laser heating. The chemical compositions of coexisting Pv, Fp, and Ca-rich perovskite (CaPv) were determined by energy-dispersive X-ray spectroscopy (EDS) using an EDS instrument attached to a transmission electron microscope. Our results demonstrate that at pressures above 90xa0GPa, Pv becomes more Fe-rich with increasing pressure, which is likely due to the effects of high-spin to low-spin crossover of Fe3+ in Pv. We highlight that such a change in Fe–Mg partitioning between Pv and Fp should have a strong influence on the physical properties of the deep lower mantle.

Crystallization of silicon dioxide and compositional evolution of the Earth’s core

The Earth’s core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40–60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core. However, only the binary systems Fe–Si and Fe–O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe–Si–O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe–Si–O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO2) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe–Si–O core crystallizes SiO2 as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity, from as early on as the Hadean eon. SiO2 saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.

The advanced ion-milling method for preparation of thin film using ion slicer: Application to a sample recovered from diamond-anvil cell

The advanced argon ion-milling technique using a new instrument called ion slicer was newly developed for preparation of thin foil. Compared to the conventional ion-milling methods, this technique facilitates very wide area to be homogeneously thinned by rocking the ion beam source with low angle and the specimen during milling. Here we applied this technique to a sample recovered from a laser-heated diamond-anvil cell (DAC). We obtained the thin film of almost entire cross section of the DAC sample along the compression axis, which possesses of approximately (10x50) microm(2). The laser-heated sample is often heterogeneous due to a large temperature gradient. However the chemical analyses are obtained from the whole hot spot under the transmission electron microscope by preparing the thin foil using the ion slicer.

Ferric iron content in (Mg,Fe)SiO3 perovskite and post-perovskite at deep lower mantle conditions

Abstract We have determined the Fe3+/ΣFe ratio of Al-free (Mg,Fe)SiO3 perovskite, post-perovskite, and (Mg,Fe)O ferropericlase synthesized at 99 to 187 GPa and 1830 to 3500 K based on the electron energyloss near-edge structure (ELNES) spectroscopy. The results demonstrate that post-perovskite includes minor amounts of ferric iron with Fe3+/ΣFe ratios of 0.11 to 0.21. These values are substantially lower than those of Al-rich post-perovskite (Fe3+/ΣFe = 0.59 to 0.69) reported in a previous study, suggesting that the Fe3+-Al3+ coupled substitution is important in post-perovskite, as in the case of perovskite. The Al-bearing post-perovskite in a pyrolitic mantle composition likely contains a considerable amount of ferric iron, which affects various physical properties in the lowermost mantle.

Electrical conductivity of NaCl-bearing aqueous fluids to 600 °C and 1 GPa

The electrical conductivity of aqueous fluids containing 0.01, 0.1, and 1xa0M NaCl was measured in an externally heated diamond cell to 600xa0°C and 1xa0GPa. These measurements therefore more than double the pressure range of previous data and extend it to higher NaCl concentrations relevant for crustal and mantle fluids. Electrical conductivity was generally found to increase with pressure and fluid salinity. The conductivity increase observed upon variation of NaCl concentration from 0.1 to 1xa0M was smaller than from 0.01 to 0.1xa0M, which reflects the reduced degree of dissociation at high NaCl concentration. Measured conductivities can be reproduced (R2xa0=xa00.96) by a numerical model with log