Spin transition zone in Earth's lower mantle

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.     

Density of phonon states in iron at high pressure

The lattice dynamics of the hexagonal close-packed (hcp) phase of iron was studied with nuclear inelastic absorption of synchrotron radiation at pressures from 20 to 42 gigapascals. A variety of thermodynamic parameters were derived from the measured density of phonon states for hcp iron, such as Debye temperatures, Gruneisen parameter, mean sound velocities, and the lattice contribution to entropy and specific heat. The results are of geophysical interest, because hcp iron is considered to be a major component of Earth's inner core.     

Body-centered cubic iron-nickel alloy in Earth's core

Cosmochemical, geochemical, and geophysical studies provide evidence that Earth's core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe(0.9)Ni(0.1) has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe(0.9)Ni(0.1) adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earth's inner core.     

The Melting Curve of Iron to 250 Gigapascals: A Constraint on the Temperature at Earth's Center

The melting curve of iron, the primary constituent of Earth's core, has been measured to pressures of 250 gigapascals with a combination of static and dynamic techniques. The melting temperature of iron at the pressure of the core-mantle boundary (136 gigapascals) is 4800 +/- 200 K. whereas at the inner core-outer core boundary (330 gigapascals), it is 7600 +/- 500 K. Corrected for melting point depression resulting from the presence of impurities, a melting temperature for iron-rich alloy of 6600 K at the inner core-outer core boundary and a maximum temperature of 6900 K at Earth's center are inferred. This latter value is the first experimental upper bound on the temperature at Earth's center, and these results imply that the temperature of the lower mantle is significantly less than that of the outer core.     

Earth's Core-Mantle Boundary: Results of Experiments at High Pressures and Temperatures

Laboratory experiments document that liquid iron reacts chemically with silicates at high pressures (>/=2.4 x 10(10) Pascals) and temperatures. In particular, (Mg,Fe)SiO(3) perovskite, the most abundant mineral of Earth's lower mantle, is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) and nonmetallic silicates (SiO(2) stishovite and MgSiO(3) perovskite) at the pressures of the core-mantle boundary, 14 x 10(10) Pascals. The experimental observations, in conjunction with seismological data, suggest that the lowermost 200 to 300 kilometers of Earth's mantle, the D" layer, may be an extremely heterogeneous region as a result of chemical reactions between the silicate mantle and the liquid iron alloy of Earth's core. The combined thermal-chemical-electrical boundary layer resulting from such reactions offers a plausible explanation for the complex behavior of seismic waves near the core-mantle boundary and could influence Earth's magnetic field observed at the surface.     

High-pressure and high-temperature experiments on core-mantle segregation in the accreting Earth

The abundances of siderophile elements in the Earth's silicate mantle are too high for the mantle to have been in equilibrium with iron in the core if equilibrium occurred at low pressures and temperatures. It has been proposed that this problem may be solved if equilibrium occurred at high pressures and temperatures. Experimental determination of the distribution of siderophile elements between liquid metal and liquid silicate at 100 kilobar and 2000 degrees C demonstrates that it is unlikely that siderophile element abundances were established by simple metal-silicate equilibrium, which indicates that the segregation of the core from the mantle was a complex process.     

Superheating effects on metal-silicate partitioning of siderophile elements

Uquid metal-liquid silicate partition coefficients for several elements at 100 kilobars and temperatures up to about 3000 kelvin in carbon capsules experimentally converge on unity with increasing temperature. The sense of change of the partition coefficients with temperature resembles the extrapolation of Murthy and may partially contribute to, but by no means provide a complete resolution of, the "excess" siderophile problem in the Earth's mantle. Sulfur and perhaps carbon successfully compete with oxygen for sites in the metallic liquid at these temperatures and pressures. This observation casts doubt upon the hypothesis that oxygen is the light element in the Earth's core.     

Phonon density of states of iron up to 153 gigapascals

We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.     

Lateral variations in Compressional/Shear velocities at the base of the mantle

Observations of core-diffracted P (Pdiff) and SH (SHdiff) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional (P) seismic velocities to horizontal shear (SH) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific (VP/VS = 1.88, also the value predicted by reference Earth models) to beneath Alaska (VP/VS = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.     

Sound velocities in olivine at Earth mantle pressures

The independent elastic constants of an upper mantle mineral, San Carlos olivine [(Mg(1.8)Fe(0.2))SiO(4)], were measured from 0 to 12.5 gigapascals. Evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400-kilometer discontinuity, and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers. In the absence of high-temperature data at high pressures, estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers. In contrast with low-pressure laboratory data, the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle.     

Origin of the low rigidity of the Earth's inner core

Earth's solid-iron inner core has a low rigidity that manifests itself in the anomalously low velocities of shear waves as compared to shear wave velocities measured in iron alloys. Normally, when estimating the elastic properties of a polycrystal, one calculates an average over different orientations of a single crystal. This approach does not take into account the grain boundaries and defects that are likely to be abundant at high temperatures relevant for the inner core conditions. By using molecular dynamics simulations, we show that, if defects are considered, the calculated shear modulus and shear wave velocity decrease dramatically as compared to those estimates obtained from the averaged single-crystal values. Thus, the low shear wave velocity in the inner core is explained.     

Elastic anisotropy of Earth's inner core

Earth's solid-iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body-centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body-centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.     

Sound velocities in iron to 110 gigapascals

The dispersion of longitudinal acoustic phonons was measured by inelastic x-ray scattering in the hexagonal closed-packed (hcp) structure of iron from 19 to 110 gigapascals. Phonon dispersion curves were recorded on polycrystalline iron compressed in a diamond anvil cell, revealing an increase of the longitudinal wave velocity (VP) from 7000 to 8800 meters per second. We show that hcp iron follows a Birch law for VP, which is used to extrapolate velocities to inner core conditions. Extrapolated longitudinal acoustic wave velocities compared with seismic data suggest an inner core that is 4 to 5% lighter than hcp iron.     

In Situ Determination of the NiAs Phase of FeO at High Pressure and Temperature

In situ synchrotron x-ray diffraction measurements of FeO at high pressures and high temperatures revealed that the high-pressure phase of FeO has the NiAs structure (B8). The lattice parameters of this NiAs phase at 96 gigapascals and 800 kelvin are a = 2.574(2) angstroms and c = 5.172(4) angstroms (the number in parentheses is the error in the last digit). Metallic behavior of the high-pressure phase is consistent with a covalently and metallically bonded NiAs structure of FeO. Transition to the NiAs structure of FeO would enhance oxygen solubility in molten iron. This transition thus provides a physiochemical basis for the incorporation of oxygen into the Earth's core.     

High-Pressure Brillouin Studies and Elastic Properties of Single-Crystal H2S Grown in a Diamond Cell

High-pressure Brillouin spectra of crystalline hydrogen sulfide (H(2)S) have been measured at up to 7 gigapascals at room temperature. The best fit of the angular dependence of Brillouin acoustic velocities between experimental values and calculations based on Every's expression for elastic waves of an arbitrary direction yielded the orientation of an H(2)S cubic crystal grown in the diamond-anvil high-pressure cell. In situ determinations of sound velocities, as a function of pressure, could be made for any direction, the refractive index, the density, and the elastic constants. This method provides a means for the systematic study of elastic properties and phase transitions of condensed gases under ultrahigh pressures.     

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earth's core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.     

Dislocation damping and anisotropic seismic wave attenuation in Earth's upper mantle

Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle.     

The structure of iron in Earth's inner core

Earth's solid inner core is mainly composed of iron (Fe). Because the relevant ultrahigh pressure and temperature conditions are difficult to produce experimentally, the preferred crystal structure of Fe at the inner core remains uncertain. Static compression experiments showed that the hexagonal close-packed (hcp) structure of Fe is stable up to 377 gigapascals and 5700 kelvin, corresponding to inner core conditions. The observed weak temperature dependence of the c/a axial ratio suggests that hcp Fe is elastically anisotropic at core temperatures. Preferred orientation of the hcp phase may explain previously observed inner core seismic anisotropy.     

The Phase Boundary Between agr- and beta-Mg2SiO4 Determined by in Situ X-ray Observation

The stability of Mg(2)SiO(4), a major constituent in the Earth's mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800 degrees to 1600 degrees C. The phase stability of alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) was determined by taking account of the kinetic behavior of transition. The phase boundary between alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) is approximated by the linear expression P = (9.3 +/- 0.1) + (0.0036 +/- 0.0002)T where P is pressure in gigapascals and T is temperature in degrees Celsius.