Multivariable dependence of Fe-Mg partitioning in the lower mantle

Mineralogy of the deep lower mantle in the presence of H2O

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800-2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2H x (x ≤ 1) phase. The (Mg, Fe)O2H x has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2H x is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

Synthesis, characterization, and growth mechanism of motifs of ultrathin cobalt-substituted NaFeSi2O6 nanowires

We report on the synthesis and mechanistic study of Co-substituted pyroxene nanowires of controllable dimensions and their subsequent correlation with magnetic properties.

Spin and valence dependence of iron partitioning in Earth's deep mantle

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.

Stability of Fe,Al-bearing bridgmanite in the lower mantle and synthesis of pure Fe-bridgmanite

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.

Natural dissociation of olivine to (Mg,Fe)SiO3 perovskite and magnesiowustite in a shocked Martian meteorite

We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine (Fa(34-41)) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO(3) perovskite (Pv)+magnesiowüstite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular (Mg,Fe)SiO(3) Pv and Mw have 50-100 nm in diameter, and lamellar (Mg,Fe)SiO(3) Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, K(Pv/Mw) = [FeO/MgO]/[FeO/MgO](Mw), between (Mg,Fe)SiO(3) Pv and Mw in equigranular and lamellar textures are approximately 0.15 and approximately 0.78, respectively. The dissociation of olivine implies that the pressure and temperature conditions recorded in the shock melt vein and melt pockets during the dynamic event were approximately 25 GPa but 700 °C at least.

Ab initio thermodynamics of MgSiO3 perovskite at high pressures and temperatures

Using quantum-mechanical simulations based on density-functional perturbation theory, we address the problem of stability of MgSiO3 perovskite to decomposition into MgO and SiO2 at pressures and temperatures of the Earth's lower mantle. We show that MgSiO3 perovskite (and its post-perovskite phase) is more stable than the mixture of oxides throughout the pressure-temperature regime of the Earth's mantle. Structural stability and lattice dynamics of phases in the system MgO-SiO2 are discussed.

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.

Iron spin transition in Earth's mantle

High-pressure Mössbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowüstite solid solution confirms that ferrous iron (Fe(2+)) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4-5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowüstite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.

Iron-rich silicates in the Earth's D'' layer

High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the core-mantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.

Spin transition of iron in magnesiowüstite in the Earth's lower mantle

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by approximately 35 percent and bulk sound velocity increases by approximately 15 percent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

Ferromagnesian postperovskite silicates in the D'' layer of the Earth

Natural olivine with 12 mol % Fe(2)SiO(4) and synthetic orthopyroxenes with 20% and 40% FeSiO(3) were studied beyond the pressure-temperature conditions of the core-mantle boundary. All samples were found to convert entirely or partially into the CaIrO(3) postperovskite structure, which was recently reported for pure MgSiO(3). The incorporation of Fe greatly reduces the pressure needed for the transition and establishes the new phase as the major component of the D'' layer. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D'' layer.

Post-Perovskite Phase Transition in MgSiO 3

In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO 6 -octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

Post-Perovskite Phase Transition in MgSiO3

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D" seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D" discontinuity.

Stability of magnesiowustite in Earth's lower mantle

Magnesiowüstite [(Mg,Fe)O] is the second most abundant mineral of Earth's lower mantle. Understanding its stability under lower mantle conditions is crucial for interpreting the physical and chemical properties of the whole Earth. Previous studies in an externally heated diamond anvil cell suggested that magnesiowüstites decompose into two components, Fe-rich and Mg-rich magnesiowüstites at 86 GPa and 1,000 K. Here we report an in situ study of two magnesiowüstites [(Mg(0.39),Fe(0.61))O and (Mg(0.25),Fe(0.75))O] at pressures and temperatures that overlap with mantle conditions, using a laser-heated diamond anvil cell combined with synchrotron x-ray diffraction. Our results show that addition of Mg in wüstite (FeO) can stabilize the rock-salt structure to much higher pressures and temperatures. In contrast to the previous studies, our results indicate that Mg-rich magnesiowüstite is stable in the rock-salt structure in the lower mantle. The physical and chemical properties of magnesiowüstite should change gradually and continuously in the lower mantle, suggesting that it does not make a significant contribution to seismic-wave heterogeneity of the lower mantle. Stable Mg-rich magnesiowüstite in lowermost mantle can destabilize FeO in the core-mantle boundary region and remove FeO from the outer core.

Stability of Ferropericlase in the Lower Mantle

We have heated ferropericlases (Mg(0.60)Fe(0.40))O and (Mg(0.50)Fe(0.50))O to temperatures of 1000 kelvin at pressures of 86 gigapascals, simulating the stability of the solid solution at physical conditions relevant to Earth's lower mantle. The in situ x-ray study of the externally heated samples in a Mao-Bell-type diamond anvil cell shows that ferropericlase may dissociate into magnesium-rich and iron-rich oxide components. The result is important because the decomposition of ferropericlase into lighter and heavier phases will cause dynamic effects that could lead to mantle heterogeneity.

Compositional heterogeneity in the bottom 1000 kilometers of Earth's mantle: toward a hybrid convection model

Tomographic imaging indicates that slabs of subducted lithosphere can sink deep into Earth's lower mantle. The view that convective flow is stratified at 660-kilometer depth and preserves a relatively pristine lower mantle is therefore not tenable. However, a range of geophysical evidence indicates that compositionally distinct, hence convectively isolated, mantle domains may exist in the bottom 1000 kilometers of the mantle. Survival of these domains, which are perhaps related to local iron enrichment and silicate-to-oxide transformations, implies that mantle convection is more complex than envisaged by conventional end-member flow models.

(Mg,Fe)SiO3-perovskite stability under lower mantle conditions

In three different experiments up to 100 gigapascals and 3000 kelvin, (Mg,Fe)SiO3-perovskite, the major component of the lower mantle, remained stable and did not decompose to its component oxides (Mg, Fe)O and SiO2. Perovskite was formed from these oxides when heated in a diamond anvil cell at pressures up to 100 gigapascals. Both MgSiO3 crystals and glasses heated to 3000 kelvin at 75 gigapascals also formed perovskite as a single phase, as evident from Raman spectra. Moreover, fluorescence measurements on chromium-doped samples synthesized at these conditions gave no indication of the presence of MgO.