Experiments pertaining to the formation and equilibration of planetary cores

The phase diagram of FeO was experimentally determined to pressures of 155 GPa and temperatures of 4000 K using shock wave and diamond-cell techniques. Researchers discovered a metallic phase of FeO at pressures greater than 70 GPa and temperatures exceeding 1000 K. The metallization of FeO at high pressures implies that oxygen can be present as the light alloying element of the Earth's outer core, in accord with the geochemical predictions of Ringwood. The high pressures necessry for this metallization suggest that the core has acquired its composition well after the initial stages of the Earth's accretion. The core forming alloy can react chemically with oxides such as those forming the mantle. The core and mantle may never have reached complete chemical equilibrium, however. If this is the case, the core-mantle boundary is likely to be a zone of active chemical reactions

Equations of state of FeO and CaO

New shock-wave (Hugoniot) and release-adiabatic data for Fe_(0.94)O and CaO, to 230 and 175 GPa (2.3 and 1.75 Mbar) respectively, show that both oxides transform from their initial B1 (NaCl-type) structures at about 70 (±10) GPa. CaO transforms to the B2 (CsCl-type) structure and FeO is inferred to do the same. Alternatively, FeO may undergo an electronic transition, but it probably does not disproportionate under shock to Fe and Fe_2O_3 or Fe_3O_4. The Hugoniot data for the B1 phases of FeO and CaO agree with the ultrasonically-determined bulk moduli (K_0= 185, 112 GPa, respectively) and with the ultrasonically-determined pressure derivative for CaO (K′_0= 4.8); K′_0∼ 3.2 for FeO is determined from the present data. The Hugoniot data for both FeO and CaO are consistent with low- and high-pressure phases having identical K_0 and K′_0. Volume changes for B1/B2 transitions in oxides agree with theoretical expectations and with trends among the halides: -ΔV/V_1 ~ 4 per cent and 11 per cent for FeO and CaO respectively. Also, the transition pressures increase with decreasing cation/anion radius ratio for the oxides. The Hugoniot data show that the density of the outer core is equal to that of a 50–50 mix (by weight) of Fe and FeO (∼10 wt per cent oxygen), consistent with geochemical arguments for the presence of oxygen in the core. In terms of a mixture of simple oxides, the density of the lower mantle is satisfied by Fe/(Mg + Fe) ∼ 0.12, however, arbitrarily large amounts of CaO can be present; an enrichment of refractory components in the lower mantle is allowed by the shock-wave data. Because of the relatively low transition pressure in FeO, a B1/B2 transition in (Mg, Fe)O is likely to occur in the lower mantle even if MgO transforms at 150–170 GPa. Such a transition may contribute to the scattering of seismic waves and change in velocity gradient found near the base of the mantle

Release adiabat measurements on minerals: The effect of viscosity

The current inversion of pressure-particle velocity data for release from a high-pressure shock state to a pressure-density path usually depends critically upon the assumption that the release process is isentropic. It has been shown by Kieffer and Delaney that for geological materials below stresses of ∼150 GPa, the effective viscosity must be ≲10^3kg m^(−1) s^(−1) (10^4 P) in order that the viscous (irreversible) work carried out on the material in the shock state remains small in comparison to the mechanical work recovered upon adiabatic rarefaction. The available data pertaining to the offset of the Rayleigh line from the Hugoniot curve for minerals, the magnitude of the shear stress in the high-pressure shock state for minerals, and the direct measurements of the viscosities of several engineering materials shocked to pressures below 150 GPa yield effective viscosities of ∼10^3kg m^(−1) s^(−1) or less. We infer that this indicates that the conditions for isentropic release of minerals from shock states are achieved, at least approximately, and we conclude that the application of the Riemann integral to obtain pressure-density states along the release adiabats of minerals in shock experiments is valid

Anorthite: thermal equation of state to high pressures

We present shock-wave (Hugoniot) data on single-crystal and porous anorthite (CaAl_2Si_2O_8) to pressures of 120 GPa. These data are inverted to yield values of the Grüneisen parameter (γ), adiabatic bulk modulus (K_s) and coefficient of thermal expansion (α) over a broad range of pressures and temperatures which in turn are used to reduce the raw Hugoniot data and construct an experimentally-based, high-pressure thermal equation of state for anorthite. We find surprisingly high values of γ which decrease from about 2.2 to 1.2 over the density range 3.4 to 5.0 Mg m^(−3). Our data clearly indicate that whereas the zeroth order anharmonic (quasi-harmonic) properties such as γ and α decrease upon compression of a single phase, these properties apparently increase dramatically (200 per cent or more) in going from a low to a high pressure phase. The results for anorthite also support the hypothesis that higher-order anharmonic contributions to the thermal properties decrease more rapidly upon compression than the lowest order anharmonicities. We find an initial density p_0 ~ 3.4 Mg m^(−3) for the ‘high-pressure phase’ portion of the Hugoniot, with an initial value of Ks essentially identical to that of anorthite at zero pressure (90 GPa). This is surprising in light of recently documented candidate high-pressure assemblages for anorthite with significantly higher densities, and it raises the question of the non-equilibrium nature of Hugoniot data. By correcting the properties of anorthite to lower mantle conditions we find that although the density of anorthite is comparable to that of the lowermost mantle, its bulk modulus is considerably less, hence making enrichment in the mantle implausible except perhaps near its base

Pyrite: Shock compression, isentropic release, and composition of the Earth's core

New shock wave data (to 180 GPa) for pyrite (FeS_2) shocked along (001) demonstrate that this mineral, in contrast to other sulfides and oxides, does not undergo a major pressure-induced phase change over the entire pressure range (320 GPa) now explored. (This is probably so because of the initial, low-spin 3-d, orbital configuration of Fe^(+2)). The primary evidence which indicates that a large phase change does not occur is the approximate agreement of the shock velocity when extrapolated to zero particle velocity, 5.4 km/s, with the expected zero-pressure bulk sound speed of pyrite (5.36 to 5.43 km/s on the basis of previous ultrasonic data). Pyrite displays a prominent elastic shock (or Hugoniot elastic limit) of 8 ± 1 GPa. The velocity of the elastic shock approaches 8.72 km/s with decreasing shock pressure, the longitudinal elastic wave velocity. As shock pressure increases, the elastic shock velocity approaches 9.05 km/s and the elastic shock becomes overdriven for shock pressures greater than about 120 GPa. Analysis of release isentrope data obtained via the pressure-particle velocity buffer method indicates that buffer particle velocities in all experiments are from 1.7% to 20% greater than expected for a Grüneisen ratio given by 1.56 (V/V_o)^(1.0). This discrepancy appears to result from volume increases upon pressure release of 0.04% to 4.5% which may result from shock-induced partial melting. The normalized pressure, finite-strain formalism for reducing Hugoniot data is extended to take into account initial porosity and shock-induced phase transitions. A least squares fit to the present and previous shock data for pyrite yields an isentropic bulk modulus, K_s, of 162 ± 9 GPa and a value of dK_s/dP = 4.7 ± 0.3. This is close to the 145 ± 3 GPa bulk modulus observed ultrasonically. If the slight discrepancy in zero-pressure modulus is taken into account in the normalized pressure finite-strain formalism, a zero-pressure density of the shock-induced high-pressure phase having a density some 2% to 3% less than pyrite is inferred to occur in the high-pressure shocked state. We suggest from this result, the release isentrope results, and limited phase diagram data that the Hugoniot states probably correspond to material which is partially to completely melted. Using the above derived equation of state and previous shock wave data for iron, both the seismologically determined density and bulk modulus distribution in the outer core are fit to models with various temperature distributions and varying weight percent sulfur. Good agreement between the shock wave derived equation of state and the density/bulk modulus relations of the liquid outer core are obtained for temperatures of ∼3000 K at the core/mantle boundary extending to 4400 K at the outer core-inner core boundary. For this thermal model a calculated sulfur content of 11 ± 2% is obtained

Pressure-temperature stability studies of FeOOH using x-ray diffraction

The Mie-Gruneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (alpha-FeOOH) to combined conditions of T = 23-250o C and P = 0-29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be alpha0 = 2.3 (+-0.6) x 10-5 K-1 over this temperature range. Our data yield zero-pressure compressional parameters: V0 = 138.75 (+- 0.02) Angstrom3, bulk modulus K0 = 140.3 (+- 3.7) GPa, pressure derivative K0' = 4.6 (+- 0.4), Gruneisen parameter gamma0 = 0.91 (+- 0.07), and Debye temperature Theta0 = 740 (+- 5) K. We identify decomposition conditions for 2alpha-FeOOH --> alpha-Fe2O3 + H2O at 1 - 8 GPa and 100-400oC, and the polymorphic transition from alpha-FeOOH (Pbnm) to epsilon-FeOOH (P21mn). The non-quenchable, high-pressure epsilon-FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K0 = 158 (+- 5) GPa and V0 = 66.3 (+- 0.5) Angstrom3

High-Pressure Research Applications Seminar

The United States‐Japan seminar on “High‐Pressure Research Applications in Geophysics and Geochemistry” was held in Honolulu, Hawaii, January 13–16, 1986, under the auspices of the National Science Foundation (NSF) and the Japan Society for the Promotion of Science (JSPS). The seminar, the third in a series, was cocovened by Murli H. Manghnani (University of Hawaii, Honolulu) and Syun‐iti Akimoto (University of Tokyo). Coming together for this symposium were 25 researchers from Japan, 22 from the United States, and four others, from Australia, the People's Republic of China, the Netherlands, and the Federal Republic of Germany. Of the 52 papers presented, 38 were presented orally at seven scientific sessions, and the rest were displayed at a poster session