Earth's Lower Mantle Research Articles

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.

Core formation and the oxidation state of the Earth

We performed experiments to separate the effects of temperature, pressure and metal composition on the metal-silicate partitioning of V, Ni, Co, Mn and Si in order to refine the ‘deep magma ocean’ model of core formation. Interactions in the liquid metal were accounted for by using a well-known metallurgical model combined with data from the literature. Temperature effects were calculated using free energy data for liquid metals and liquid oxides. This approach enabled us to separate pressure effects from those of the other two variables. If we assume that the core formed by a single-stage process then we find that the mantle concentrations of the refractory elements can be matched at a temperature of 3750 K and a pressure of 40 GPa. These values are in good agreement with recent estimates based on other partitioning data. The calculated temperature is, however, 700 °C above the peridotite liquidus at 40 GPa, and is therefore physically implausible. The base of the magma ocean, at which metal would pond during accretion must be saturated in crystals and should, therefore lie at or below the silicate liquidus. We find that dynamic accretion models, in which pressure increased as the earth grew, do not improve the match to the peridotite liquidus so long as the oxygen fugacity is fixed by the current Fe content of the mantle. If, however, we force temperature to lie on the peridotite liquidus as the earth grew, the mantle concentrations of refractory elements can be matched provided oxygen fugacity increased during accretion. To illustrate this we use accretionary models in which, as the earth grew, the base of the magma ocean was at half the mantle depth and applied gradual or step-changes in oxygen fugacity. If we apply a 2 log unit increase in oxygen fugacity, the siderophile element contents of the mantle are matched, the Si content of the core is 5 to 7 wt.% and the average core segregation temperature is 2990 K, within the experimental range. Oxidation during accretion is an important component of heterogeneous accretion models, but the mechanisms proposed are generally untestable. Here we propose that an increase in oxidation state is a natural consequence of the size of the earth. Magnesium silicate perovskite is the principal phase in the earth's lower mantle. This phase forces the disproportionation of ferrous iron into ferric iron plus metal. When perovskite started to grow at the base of the magma ocean, dissolution and reprecipitation acted as an ‘oxygen pump’ injecting oxidized ferric iron into the upper mantle. As the earth continued to grow, infalling metal was oxidized by the ferric iron into ferrous iron, raising the iron content of the magma ocean and the oxygen fugacity of metal-silicate equilibration. This process did not occur to any great extent on Mars because of the limited stability of perovskite in the smaller planet

The spin state of iron in minerals of Earth's lower mantle

The spin state of Fe(II) and Fe(III) at temperatures and pressures typical for the Earth's lower mantle is discussed. We predict an extended high‐spin to low‐spin crossover region along the geotherm for Fe‐dilute systems depending on crystal‐field splitting, pairing energy, and cooperative interactions. In particular, spin transitions in ferromagnesium silicate perovskite and ferropericlase, the dominant lower mantle components, should occur in a wide temperature‐pressure range. We also derive a gradual volume change associated with such transitions in the lower mantle. The gradual density changes and the wide spin crossover regions seem incompatible with lower mantle stratification resulting from a spin transition.

Melting behavior of H2O at high pressures and temperatures

Water plays an important role in the physics and chemistry of planetary interiors. In situ high pressure‐temperature Raman spectroscopy and synchrotron x‐ray diffraction have been used to examine the phase diagram of H2O. A discontinuous change in the melting curve of H2O is observed at approximately 35 GPa and 1040 K, indicating a triple point on the melting line. The melting curve of H2O increases significantly above the triple point and may intersect the isentropes of Neptune and Uranus. Solid ice could therefore form in stratified layers at depth within these icy planets. The extrapolated melting curve may also intersect with the geotherm of Earth's lower mantle above 60 GPa. The presence of solid H2O would result in a jump in the viscosity of the mid‐lower mantle and provides an additional explanation for the observed higher viscosity of the mid‐lower mantle.

Thermoelastic properties of (Mg 0.64Fe 0.36)O ferropericlase based on in situ X-ray diffraction to 26.7 GPa and 2173 K

We present 53 in situ measurements of the unit-cell volume ( V) of polycrystalline (Mg 0.64Fe 0.36)O ferropericlase (FP) at simultaneous high pressures ( P) and temperatures ( T) up to P = 26.7 GPa and T = 2173 K, including pressure–temperature conditions at the top of Earth's lower mantle. FP volumes were determined through in situ energy-dispersive synchrotron X-ray diffraction in a multi-anvil press, using gold and MgO as pressure markers. Our data, combined with 112 previously published P– V– T measurements for the same FP sample, were fitted to high-temperature Birch-Murnaghan and Mie-Grüneisen equations of state (EOS). Experimental data are reproduced accurately, with a standard deviation lower than 0.31 GPa for both EOS. EOS calculations show that the thermal expansion and P– T derivatives of the bulk modulus of this iron-rich FP are virtually identical to those of pure MgO to pressures >55 GPa and temperatures of 3000 K. This result is confirmed by measurements of the normalised unit-cell volumes for FP and MgO at identical simultaneous high P– T conditions, which are identical to within 0.6 per cent relative to 26.7 GPa and 2173 K. The pressure and temperature derivatives of the bulk modulus, and thermal expansion are concluded to be independent of iron content across the range of plausible FP compositions in Earth's lower mantle.

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.

Electrical Conductivities and Conduction Mechanisms of Perovskite‐type Na1‐xKxMgF3 (x = 0, 0.1, 1) and KZnF3

AbstractElectrical conductivities of NaMgF3, Na0.9K0.1MgF3, KMgF3, KZnF3 and an aggregate of NaMgF3 + 2 mol% NaF were measured as functions of temperature and frequency by the complex impedance method. The conductivities of NaMgF3 and Na0.9K0.1MgF3 as a function of temperature show a little jump at 1040 K and 710 K with the phase transition from orthorhombic to cubic. Conductivities of NaMgF3 KMgF3 and KZnF3 increase at 1100 K, 1120 K and 980 K, respectively. These temperatures correspond to the reaction temperatures such as eutectic reaction caused by a slight compositional deviation to alkaline fluoride component side. The extremely high conductivity and the increment of the activation energy were observed just below the melting points of these compounds. The intrinsic conductivity of 0.1 S/m is estimated at most by extrapolation to the melting temperature. From the results on these fluoride perovskite as analog compounds of constituents of the Earth's lower mantle, it is suggested that intrinsic and extrinsic conduction mechanisms are not sufficient to explain high conductivity of uppermost‐part of lower mantle, estimated to be 1‐10 S/m, and an occurrence of low temperature eutectic reaction is necessary to attain such a high conductivity.

Variable azimuthal anisotropy in Earth's lowermost mantle.

A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the D'' layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20 degrees are consistent with the data. Small-scale convection cells within the mantle above the D'' layer may cause the observed variations by inducing laterally variable crystallographic or shape-preferred orientation in minerals in the D'' layer.

Phase transition in MgSiO 3 perovskite in the earth's lower mantle

A new polymorph of MgSiO 3 more stable than the Pbnm-perovskite phase has been identified by first-principles computations. It has the CaIrO 3 structure with Cmcm symmetry and consists of SiO 3 layers intercalated by eightfold-coordinated Mg ions. High-temperature calculations within the quasiharmonic approximation give a volume change of ∼1.5% and a Clapeyron slope of ∼7.5±0.3 MPa/K at ∼2750 K and ∼125 GPa. These pressure–temperature ( P– T) conditions are close to those in which a phase transition in MgSiO 3-perovskite has been observed by in situ angle dispersive X-ray diffraction measurements. This transition appears to be associated with the Dʺ discontinuity.

Equations of state of the high-pressure phases of a natural peridotite and implications for the Earth's lower mantle

To help determine the chemical composition of the Earth's mantle, we characterised the high-pressure mineral assemblage of an undepleted natural peridotite—thought to be representative of the Earth's upper-mantle—to 107 GPa using high-resolution X-ray diffraction. At lower-mantle conditions, the peridotite transforms to the assemblage 76 (±2)% (Mg 0.88Fe 0.05 2+Fe 0.01 3+Al 0.12Si 0.94)O 3 orthorhombic perovskite by volume (at zero pressure), 17 (±2)% (Mg 0.80Fe 0.20)O magnesiowüstite and 7 (±1)% CaSiO 3 perovskite. The measured room-temperature bulk properties of this high-pressure assemblage, together with a range of estimates of thermal properties of the constituent minerals, appear to be inconsistent with seismological constraints on the density and bulk modulus of the lower mantle. Our results suggest that the lower mantle differs in bulk composition (e.g., richer in iron, Mg#∼0.85) from current estimates for the upper mantle, requiring some amount of segregation between the upper and lower mantle over geological history.

Thermochemical structures within a spherical mantle: Superplumes or piles?

Heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology. In particular, two regions in the Earth's lower mantle, under the Pacific and Africa, are often interpreted as being either piles or superplumes of dense material. We perform numerical modeling of thermochemical convection in a three‐dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated, rounded piles or superplumes of material. We study the effect of temperature and compositionally dependent viscosity, and we find two different modes of dense layer deformation. Temperature‐dependent rheology leads to a low‐viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle. The addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material, forming large, isolated superplumes. Our results indicate that piles and superplumes are separate features which, in general, do not occur together and, in order for isolated, rounded superplumes to form, an intrinsic compositional viscosity increase is required for the dense material.

Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer.

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3 (ref. 1). But it has not been possible to explain many unusual properties of the lowermost approximately 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D" layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.

Shock‐induced melting of MgSiO3 perovskite and implications for melts in Earth's lowermost mantle

New shock wave equation of state (EOS) data for enstatite and MgSiO3 glass constrain the density change upon melting of Mg‐silicate perovskite up to 200 GPa. The melt becomes denser than perovskite near the base of Earth's lower mantle. This inference is confirmed by shock temperature data suggesting a negative pressure‐temperature slope along the melting curve at high pressure. Although melting of Earth's mantle involves multiple phases and chemical components, this implies that the partial melts invoked to explain anomalous seismic velocities in the lowermost mantle may be dynamically stable.

The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle.

MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200-300 km above the core-mantle boundary (the D" discontinuity) or the polarization anisotropy observed in the lowermost mantle. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase. However, recent in situ X-ray diffraction measurements have revealed a transition to a 'post-perovskite' phase above 125 GPa and 2,500 K--conditions close to those at the D" discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D" layer.

Sound velocities and elasticity of aluminous MgSiO3 perovskite: Implications for aluminum heterogeneity in Earth's lower mantle

Aluminum has been reported to have a remarkably strong effect on the thermoelastic properties of MgSiO3 perovskite. However, the sound velocities of aluminous MgSiO3 perovskite have not been previously measured, even though this phase likely dominates most of the chemistry in Earth's lower mantle. Here we report the first sound velocity measurements on aluminous MgSiO3 perovskite using Brillouin spectroscopy and obtain the following values for the room‐pressure room‐temperature adiabatic bulk and shear moduli: KS = 252 ± 5 GPa and μ = 165 ± 2 GPa, respectively. The presence of 5.1 ± 0.2 wt.% Al2O3 in MgSiO3 perovskite decreases the shear modulus by 5.6%. However, within experimental uncertainties, there is no discernable effect of aluminum on the bulk modulus. We find that variations in the aluminum content of MgSiO3 perovskite may provide an explanation for some observed lateral heterogeneity in Earth's lower mantle.

Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3–AlO1.5 join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30–100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+ causes a mild increase in compressibility, with the bulk modulus (K0) decreasing at a rate of −67±35 GPa/XAl. This decrease in K0 is consistent with recent theoretical calculations if essentially all Al3+ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+ and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3–AlO1.5–SiO2 indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3–AlO1.5–FeO1.5–SiO2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+ and Fe3+ substitutions. Primitive mantle with about 5 mol% AlO1.5 and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+ and Fe3+ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.

Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3–AlO1.5 join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30–100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+ causes a mild increase in compressibility, with the bulk modulus (K0) decreasing at a rate of −67±35 GPa/XAl. This decrease in K0 is consistent with recent theoretical calculations if essentially all Al3+ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+ and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3–AlO1.5–SiO2 indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3–AlO1.5–FeO1.5–SiO2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+ and Fe3+ substitutions. Primitive mantle with about 5 mol% AlO1.5 and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+ and Fe3+ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.

Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O.

The emerging picture of Earth's deep interior from seismic tomography indicates more complexity than previously thought. The presence of lateral anisotropy and heterogeneity in Earth's mantle highlights the need for fully anisotropic elasticity data from mineral physics. A breakthrough in high-frequency (gigahertz) ultrasound has resulted in transmission of pure-mode elastic shear waves into a high-pressure diamond-anvil cell using a P-to-S elastic-wave conversion. The full elastic tensor (c(ij)) of high-pressure minerals or metals can be measured at extreme conditions without optical constraints. Here we report the effects of pressure and composition on shear-wave velocities in the major lower-mantle oxide, magnesiowüstite-(Mg,Fe)O. Magnesiowüstite containing more than approximately 50% iron exhibits pressure-induced c(44) shear-mode softening, indicating an instability in the rocksalt structure. The oxide closer to expected lower-mantle compositions ( approximately 20% iron) shows increasing shear velocities more similar to MgO, indicating that it also should have a wide pressure-stability field. A complete sign reversal in the c(44) pressure derivative points to a change in the topology of the (Mg,Fe)O phase diagram at approximately 50-60% iron. The relative stability of Mg-rich (Mg,Fe)O and the strong compositional dependence of shear-wave velocities (and partial differential c(44)/ partial differential P) in (Mg,Fe)O implies that seismic heterogeneity in Earth's lower mantle may result from compositional variations rather than phase changes in (Mg,Fe)O.

Dislocation creep in MgSiO3 perovskite at conditions of the Earth's uppermost lower mantle.

Seismic anisotropy provides an important observational constraint on flow in the Earth's deep interior. The quantitative interpretation of anisotropy, however, requires knowledge of the slip geometry of the constitutive minerals that are responsible for producing rock fabrics. The Earth's lower mantle is mostly composed of (Mg, Fe)SiO3 perovskite, but as MgSiO3 perovskite is not stable at high temperature under ambient pressure, it has not been possible to investigate its mechanical behaviour with conventional laboratory deformation experiments. To overcome this limitation, several attempts were made to infer the mechanical properties of MgSiO3 perovskite on the basis of analogue materials. But perovskites do not constitute an analogue series for plastic deformation, and therefore the direct investigation of MgSiO3 perovskite is necessary. Here we have taken advantage of recent advances in experimental high-pressure rheology to perform deformation experiments on coarse-grained MgSiO3 polycrystals under pressure and temperature conditions of the uppermost lower mantle. We show that X-ray peak broadening measurements developed in metallurgy can be adapted to low-symmetry minerals to identify the elementary deformation mechanisms activated under these conditions. We conclude that, under uppermost lower-mantle conditions, MgSiO3 perovskite deforms by dislocation creep and may therefore contribute to producing seismic anisotropy in rocks at such depths.

Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle.

The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.