Publications

2018
Fischer R.A., Nimmo F., and O'Brien D.P. 2018. “Radial mixing and Ru–Mo isotope systematics under different accretion scenarios.” Earth and Planetary Science Letters, 482, Pp. 105–114. Publisher's VersionAbstract
The Ru–Mo isotopic compositions of inner Solar System bodies may reflect the provenance of accreted material and how it evolved with time, both of which are controlled by the accretion scenario these bodies experienced. Here we use a total of 116 N-body simulations of terrestrial planet accretion, run in the Eccentric Jupiter and Saturn (EJS), Circular Jupiter and Saturn (CJS), and Grand Tack scenarios, to model the Ru–Mo anomalies of Earth, Mars, and Theia analogues. This model starts by applying an initial step function in Ru–Mo isotopic composition, with compositions reflecting those in meteorites, and traces compositional evolution as planets accrete. The mass-weighted provenance of the resulting planets reveals more radial mixing in Grand Tack simulations than in EJS/CJS simulations, and more efficient mixing among late-accreted material than during the main phase of accretion in EJS/CJS simulations. We find that an extensive homogeneous inner disk region is required to reproduce Earth's observed Ru–Mo composition. EJS/CJS simulations require a homogeneous reservoir in the inner disk extending to ≥3–4 AU (≥74–98% of initial mass) to reproduce Earth's composition, while Grand Tack simulations require a homogeneous reservoir extending to ≥3–10 AU (≥97–99% of initial mass), and likely to ≥6–10 AU. In the Grand Tack model, Jupiter's initial location (the most likely location for a discontinuity in isotopic composition) is ∼3.5 AU; however, this step location has only a 33% likelihood of producing an Earth with the correct Ru–Mo isotopic signature for the most plausible model conditions. Our results give the testable predictions that Mars has zero Ru anomaly and small or zero Mo anomaly, and the Moon has zero Mo anomaly. These predictions are insensitive to wide variations in parameter choices.
Wordsworth R.D., Schaefer L., and Fischer R.A. 2018. “Redox evolution via gravitational differentiation on low mass planets: Implications for biosignatures, water loss, and habitability.” The Astronomical Journal, 155, Pp. 195. Publisher's VersionAbstract

The oxidation of rocky planet surfaces and atmospheres, which arises from the twin forces of stellar nucleosynthesis and gravitational differentiation, is a universal process of key importance to habitability and exoplanet biosignature detection. Here we take a generalized approach to this phenomenon. Using a single parameter to describe redox state, we model the evolution of terrestrial planets around nearby M-stars and the Sun. Our model includes atmospheric photochemistry, diffusion and escape, line-by-line climate calculations and interior thermodynamics and chemistry. In most cases we find abiotic atmospheric O2 buildup around M-stars during the pre-main sequence phase to be much less than calculated previously, because the planet’s magma ocean absorbs most oxygen liberated from H2O photolysis. However, loss of non-condensing atmospheric gases after the mantle solidifies remains a significant potential route to abiotic atmospheric O2 subsequently. In all cases, we predict that exoplanets that receive lower stellar fluxes, such as LHS1140b and TRAPPIST- 1f and g, have the lowest probability of abiotic O2 buildup and hence are the best targets for future biosignature searches. Key remaining uncertainties can be minimized in future by comparing our predictions for the atmospheres of hot, sterile exoplanets such as GJ1132b and TRAPPIST-1b and -c with observations.

2017
Fischer R.A., Campbell A.J., and Ciesla F.J. 2017. “Sensitivities of Earth’s core and mantle compositions to accretion and differentiation processes.” Earth and Planetary Science Letters, 458, Pp. 252–262. Publisher's VersionAbstract
The Earth and other terrestrial planets formed through the accretion of smaller bodies, with their core and mantle compositions primarily set by metal–silicate interactions during accretion. The conditions of these interactions are poorly understood, but could provide insight into the mechanisms of planetary core formation and the composition of Earth's core. Here we present modeling of Earth's core formation, combining results of 100 N-body accretion simulations with high pressure–temperature metal–silicate partitioning experiments. We explored how various aspects of accretion and core formation influence the resulting core and mantle chemistry: depth of equilibration, amounts of metal and silicate that equilibrate, initial distribution of oxidation states in the disk, temperature distribution in the planet, and target:impactor ratio of equilibrating silicate. Virtually all sets of model parameters that are able to reproduce the Earth's mantle composition result in at least several weight percent of both silicon and oxygen in the core, with more silicon than oxygen. This implies that the core's light element budget may be dominated by these elements, and is consistent with ≤1–2 wt% of other light elements. Reproducing geochemical and geophysical constraints requires that Earth formed from reduced materials that equilibrated at temperatures near or slightly above the mantle liquidus during accretion. The results indicate a strong tradeoff between the compositional effects of the depth of equilibration and the amounts of metal and silicate that equilibrate, so these aspects should be targeted in future studies aiming to better understand core formation conditions. Over the range of allowed parameter space, core and mantle compositions are most sensitive to these factors as well as stochastic variations in what the planet accreted as a function of time, so tighter constraints on these parameters will lead to an improved understanding of Earth's core composition.
2016
Deep Earth: Physics and Chemistry of the Lower Mantle and Core
2016. Deep Earth: Physics and Chemistry of the Lower Mantle and Core. American Geophysical Union / John Wiley and Sons. Publisher's VersionAbstract

Deep Earth: Physics and Chemistry of the Lower Mantle and Core highlights recent advances and the latest views of the deep Earth from theoretical, experimental, and observational approaches and offers insight into future research directions on the deep Earth. In recent years, we have just reached a stage where we can perform measurements at the conditions of the center part of the Earth using state-of-the-art techniques, and many reports on the physical and chemical properties of the deep Earth have come out very recently. Novel theoretical models have been complementary to this breakthrough. These new inputs enable us to compare directly with results of precise geophysical and geochemical observations. This volume highlights the recent significant advancements in our understanding of the deep Earth that have occurred as a result, including contributions from mineral/rock physics, geophysics, and geochemistry that relate to the topics of: 

I. Thermal structure of the lower mantle and core

II. Structure, anisotropy, and plasticity of deep Earth materials

III. Physical properties of the deep interior

IV. Chemistry and phase relations in the lower mantle and core

V. Volatiles in the deep Earth 

The volume will be a valuable resource for researchers and students who study the Earth's interior. The topics of this volume are multidisciplinary, and therefore will be useful to students from a wide variety of fields in the Earth Sciences. 

Fischer R.A. 2016. “Melting of Fe-alloys and the thermal structure of the core.” In Deep Earth: Physics and Chemistry of the Lower Mantle and Core. American Geophysical Union / John Wiley and Sons. Publisher's VersionAbstract

The temperature of the Earth’s core has significant implications in many areas of geophysics, including applications to Earth’s heat flow, core composition, age of the inner core, and energetics of the geodynamo. The temperature of the core at the inner core boundary is equal to the melting temperature of the core’s Fe-rich alloy at the inner core boundary pressure. This chapter is a review of experimental results on melting temperatures of iron and Fe-rich alloys at core conditions that can thus be used to infer core temperatures. Large discrepancies exist between published melting curves for pure iron at high pressures, with better agreement on the melting behavior of Fe–light element alloys. The addition of silicon causes a small melting point depression in iron, while oxygen and especially sulfur cause larger melting point depressions. The inner core boundary temperature likely falls in the range 5150–6200 K, depending on the identity of the light element(s) in the core, which leads to a core–mantle boundary temperature of 3850–4600 K for an adiabatic outer core. The most significant sources of uncertainties in the core’s thermal structure include the core’s composition, phase diagram, and Grüneisen parameter.

Thompson E.C., Chidester B.A., Fischer R.A., Myers G.I., Heinz D.L., Prakapenka V.B., and Campbell A.J. 2016. “Equation of state of pyrite to 85 GPa and 2400 K.” American Mineralogist, 101, Pp. 1046–1051. Publisher's VersionAbstract
The high-cosmic abundance of sulfur is not reflected in the terrestrial crust, implying it is either sequestered in the Earth’s interior or was volatilized during accretion. As it has widely been suggested that sulfur could be one of the contributing light elements leading to the density deficit of Earth’s core, a robust thermal equation of state of iron sulfide is useful for understanding the evolution and properties of Earth’s interior. We performed X-ray diffraction measurements on FeS2 achieving pressures from 15 to 80 GPa and temperatures up to 2400 K using laser-heated diamond-anvil cells. No phase transitions were observed in the pyrite structure over the pressure and temperature ranges investigated. Combining our new P-V-T data with previously published room-temperature compression and thermochemical data, we fit a Debye temperature of 624(14) K and determined a Mie-Grüneisen equation of state for pyrite having bulk modulus KT = 141.2(18) GPa, pressure derivative KT = 5.56(24), Grüneisen parameter γ0 = 1.41, anharmonic coefficient A2 = 2.53(27) × 10−3 J/(K2·mol), and q = 2.06(27). These findings are compared to previously published equation of state parameters for pyrite from static compression, shock compression, and ab initio studies. This revised equation of state for pyrite is consistent with an outer core density deficit satisfied by 11.4(10) wt% sulfur, yet matching the bulk sound speed of PREM requires an outer core composition of 4.8(19) wt% S. This discrepancy suggests that sulfur alone cannot satisfy both seismological constraints simultaneously and cannot be the only light element within Earth’s core, and so the sulfur content needed to satisfy density constraints using our FeS2 equation of state should be considered an upper bound for sulfur in the Earth’s core.
Shofner G.A., Campbell A.J., Danielson L.R., Righter K., Fischer R.A., Wang Y., and Prakapenka V.B. 2016. “The W–WO2 oxygen fugacity buffer (WWO) at high pressure and temperature: Implications for fO2 buffering and metal–silicate partitioning.” American Mineralogist, 101, Pp. 211–221. Publisher's VersionAbstract
Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO2 at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO2 unit-cell volume data were fit to Mie-Grüneisen equations of state; parameters for W are KT = 307 (±0.4) GPa, KT = 4.05 (±0.04), γ0 = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO2 with structures in the P21/c, Pnma, and C2/c space groups. The transition pressures are 4 and 32 GPa for the P21/c-Pnma and Pnma-C2/c phase changes, respectively. The P21/c and Pnma phases have previously been described, whereas the C2/c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters KT = 238 (±7), 230 (±5), 304 (±3) GPa, KT = 4 (fixed), 4 (fixed), 4 (fixed) GPa, γ0 = 1.45 (±0.18), 1.22 (±0.07), 1.21 (±0.12), and q = 1 (fixed), 2.90 (±1.5), 1 (fixed) for the P21/c, Pnma, and C2/c phases, respectively. The W-WO2 buffer (WWO) was extended to high pressure using these W and WO2 equations of state. The T-fO2 slope of the WWO buffer along isobars is positive from 1000 to 2500 K with increasing pressure up to at least 60 GPa. The WWO buffer is at a higher fO2 than the iron-wüstite (IW) buffer at pressures lower than 40 GPa, and the magnitude of this difference decreases at higher pressures. This implies an increasingly lithophile character for W at higher pressures. The WWO buffer was quantitatively applied to W metal-silicate partitioning by using the WWO-IW buffer difference in combination with literature data on W metal-silicate partitioning to model the exchange coefficient (KD) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the WWO-IW buffer difference. Calculation of KD along a peridotite liquidus predicts a decrease in W siderophility at higher pressures that supports the qualitative behavior predicted by the WWO-IW buffer difference, and agrees with findings of others. Comparing the competing effects of temperature and pressure the results here indicate that pressure exerts a greater effect on W metal-silicate partitioning.
2015
Fischer R.A., Nakajima Y., Campbell A.J., Frost D.J., Harries D., Langenhorst F., Miyajima N., Pollok K., and Rubie D.C. 2015. “High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O.” Geochimica et Cosmochimica Acta, 167, Pp. 177–194. Publisher's VersionAbstract
The distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100 GPa and 5700 K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45 GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54 ± 5 GPa and 3300–3400 K. These conditions would result in 8.5 ± 1.4 wt% Si and 1.6 ± 0.3 wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature–oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density.
Pigott J.S., Ditmer D.A., Fischer R.A., Reaman D.M., Hrubiak R., Meng Y., Davis R.J., and Panero W.R. 2015. “High-pressure, high-temperature equations of state using nanofabricated controlled-geometry Ni/SiO2/Ni double hot-plate samples.” Geophysical Research Letters, 42, Pp. 10239–10247. Publisher's VersionAbstract
We have fabricated novel controlled-geometry samples for the laser-heated diamond-anvil cell (LHDAC) in which a transparent oxide layer (SiO2) is sandwiched between two laser-absorbing layers (Ni) in a single, cohesive sample. The samples were mass manufactured (>104 samples) using a combination of physical vapor deposition, photolithography, and wet and plasma etching. The double hot-plate arrangement of the samples, coupled with the chemical and spatial homogeneity of the laser-absorbing layers, addresses problems of spatial temperature heterogeneities encountered in previous studies where simple mechanical mixtures of transparent and opaque materials were used. Here we report thermal equations of state (EOS) for nickel to 100 GPa and 3000 K and stishovite to 50 GPa and 2400 K obtained using the LHDAC and in situ synchrotron X-ray microdiffraction. We discuss the inner core composition and the stagnation of subducted slabs in the mantle based on our refined thermal EOS.
Fischer R.A. and Campbell A.J. 2015. “The axial ratio of hcp Fe and Fe–Ni–Si alloys to the conditions of Earth's inner core.” American Mineralogist, 100, Pp. 2718–2724. Publisher's VersionAbstract
The Earth’s iron-rich inner core is seismically anisotropic, which may be due to the preferred orientation of Fe-rich hexagonal close packed (hcp) alloy crystals. Elastic anisotropy in a hexagonal crystal is related to its c/a axial ratio; therefore, it is important to know how this ratio depends on volume (or pressure), temperature, and composition. Experimental data on the axial ratio of iron and alloys in the Fe–Ni–Si system from 15 previous studies are combined here to parameterize the effects of these variables. The axial ratio increases with increasing volume, temperature, silicon content, and nickel content. When an hcp phase coexists with another structure, sample recovery and chemical analysis from each pressure-temperature point is one method for determining the phase’s composition and thus the position of the phase boundary. An alternate method is demonstrated here, using this parameterization to calculate the composition of an hcp phase whose volume, temperature, and axial ratio are measured. The hcp to hcp+B2 phase boundary in the Fe–FeSi system is parameterized as a function of pressure, temperature, and composition, showing that a silicon-rich inner core may be an hcp+B2 mixture. These findings could help explain observations of a layered seismic anisotropy structure in the Earth’s inner core.
2014
Fischer R.A. and Ciesla F.J. 2014. “Dynamics of the terrestrial planets from a large number of N-body simulations.” Earth and Planetary Science Letters, 392, Pp. 28–38. Publisher's VersionAbstract
The agglomeration of planetary embryos and planetesimals was the final stage of terrestrial planet formation. This process is modeled using N-body accretion simulations, whose outcomes are tested by comparing to observed physical and chemical Solar System properties. The outcomes of these simulations are stochastic, leading to a wide range of results, which makes it difficult at times to identify the full range of possible outcomes for a given dynamic environment. We ran fifty high-resolution simulations each with Jupiter and Saturn on circular or eccentric orbits, whereas most previous studies ran an order of magnitude fewer. This allows us to better quantify the probabilities of matching various observables, including low probability events such as Mars formation, and to search for correlations between properties. We produce many good Earth analogues, which provide information about the mass evolution and provenance of the building blocks of the Earth. Most observables are weakly correlated or uncorrelated, implying that individual evolutionary stages may reflect how the system evolved even if models do not reproduce all of the Solar System's properties at the end. Thus individual N-body simulations may be used to study the chemistry of planetary accretion as particular accretion pathways may be representative of a given dynamic scenario even if that simulation fails to reproduce many of the other observed traits of the Solar System.
Fischer R.A., Campbell A.J., Caracas R., Reaman D.M., Heinz D.L., Dera P., and Prakapenka V.B. 2014. “Equations of state in the Fe–FeSi system at high pressures and temperatures.” Journal of Geophysical Research: Solid Earth, 119, Pp. 2810–2827. Publisher's VersionAbstract
Earth's core is an iron-rich alloy containing several weight percent of light element(s), possibly including silicon. Therefore, the high pressure-temperature equations of state of iron-silicon alloys can provide understanding of the properties of Earth's core. We performed X-ray diffraction experiments using laser-heated diamond anvil cells to achieve simultaneous high pressures and temperatures, up to ~200 GPa for Fe–9 wt % Si alloy and ~145 GPa for stoichiometric FeSi. We determined equations of state of the D03, hcp + B2, and hcp phases of Fe–9Si, and the B20 and B2 phases of FeSi. We also calculated equations of state of Fe, Fe11Si, Fe5Si, Fe3Si, and FeSi using ab initio methods, finding that iron and silicon atoms have similar volumes at high pressures. By comparing our experimentally determined equations of state to the observed core density deficit, we find that the maximum amount of silicon in the outer core is ~11 wt %, while the maximum amount in the inner core is 6–8 wt %, for a purely Fe-Si-Ni core. Bulk sound speeds predicted from our equations of state also match those of the inner and outer core for similar ranges of compositions. We find a compositional contrast between the inner and outer core of 3.5–5.6 wt % silicon, depending on the seismological model used. Theoretical and experimental equations of state agree at high pressures. We find a good match to the observed density, density profile, and sound speed of the Earth's core, suggesting that silicon is a viable candidate for the dominant light element.
Salamat A., Fischer R.A., Briggs R., McMahon M., and Petitgirard S. 2014. “In situ synchrotron X-ray diffraction in the laser-heated diamond anvil cell: melting phenomena and synthesis of new materials.” Coordination Chemistry Reviews, 277–278, Pp. 15–30. Publisher's VersionAbstract
The ability to produce high pressures and temperatures (PT), and study the physical and chemical properties of solids and melts at these conditions, is possible through the use of the laser-heated diamond anvil cell (LH-DAC). High PT experiments are commonly performed at synchrotron radiation facilities, where in situ X-ray diffraction (XRD) permits the detection of melting, crystallographic structural analysis, and density measurements at extreme conditions. Here we present an overview of recent experimental advances in the use of high pressure techniques in combination with laser heating and in situ X-ray diffraction. We summarize state-of-the-art capabilities, including recent advancements, in sample preparation, pressure and temperature measurements, and other technical aspects. Two main applications of the LH-DAC are discussed: the study of the melting curves of elements and compounds at high pressures, and materials synthesis at extreme conditions. The melting curves of Fe, Ta, and MgO are used as examples in a discussion of experimental techniques, technical developments, and sources of discrepancies in melting data. High PT syntheses of light molecular systems, superhard materials, nitrides, oxides, carbon compounds, and geologically important materials are reviewed.
2013
Fischer R.A., Campbell A.J., Reaman D.M., Miller N.A., Heinz D.L., Dera P., and Prakapenka V.B. 2013. “Phase relations in the Fe–FeSi system at high pressures and temperatures.” Earth and Planetary Science Letters, 373, Pp. 54–64. Publisher's VersionAbstract
The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9 wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200 GPa and ~145 GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42 GPa. The minimum temperature of an Fe–Si outer core is 4380 K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, only the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8 wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases.
2012
Fischer R.A., Campbell A.J., Caracas R., Reaman D.M., Dera P., and Prakapenka V.B. 2012. “Equation of state and phase diagram of Fe–16Si alloy as a candidate component of Earth's core.” Earth and Planetary Science Letters, 357–358, Pp. 268–276. Publisher's VersionAbstract
The outer core of the Earth contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is critical to understand the high pressure–temperature properties and behavior of an iron–silicon alloy with a geophysically relevant composition (16 wt% silicon). We experimentally determined the melting curve, subsolidus phase diagram, and equations of state of all phases of Fe–16 wt%Si to 140 GPa, finding a conversion from the D03 crystal structure to a B2+hcp mixture at high pressures. The melting curve implies that 3520 K is a minimum temperature for the Earth's outer core, if it consists solely of Fe–Si alloy, and that the eutectic composition in the Fe–Si system is less than 16 wt% silicon at core–mantle boundary conditions. Comparing our new equation of state to that of iron and the density of the core, we find that for an Fe–Ni–Si outer core, 11.3 ± 1.5 wt% silicon would be required to match the core's observed density at the core–mantle boundary. We have also performed first-principles calculations of the equations of state of Fe3Si with the D03 structure, hcp iron, and FeSi with the B2 structure using density-functional theory.
2011
Fischer R.A., Campbell A.J., Shofner G.A., Lord O.T., Dera P., and Prakapenka V.B. 2011. “Equation of state and phase diagram of FeO.” Earth and Planetary Science Letters, 304, Pp. 496–502. Publisher's VersionAbstract
Wüstite, Fe1 − xO, is an important component in the mineralogy of Earth's lower mantle and may also be a component in the core. Therefore the high pressure, high temperature behavior of FeO, including its phase diagram and equation of state, is essential knowledge for understanding the properties and evolution of Earth's deep interior. We performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures. Wüstite was mixed with iron metal, which served as our pressure standard, under the assumption that negligible oxygen dissolved into the iron. Our data show a positive slope for the subsolidus phase boundary between the B1 and B8 structures, indicating that the B1 phase is stable at the P–T conditions of the lower mantle and core. We have determined the thermal equation of state of B1 FeO to 156 GPa and 3100 K, finding an isothermal bulk modulus K0 = 149.4 ± 1.0 GPa and its pressure derivative K0′ = 3.60 ± 0.4. This implies that 7.7 ± 1.1 wt.% oxygen is required in the outer core to match the seismologically-determined density, under the simplifying assumption of a purely Fe–O outer core.
Fischer R.A., Campbell A.J., Lord O.T., Shofner G.A., Dera P., and Prakapenka V.B. 2011. “Phase transition and metallization of FeO at high pressures and temperatures.” Geophysical Research Letters, 38, Pp. L24301. Publisher's VersionAbstract
Wüstite, Fe1-xO, is an important component in the mineralogy of Earth's lower mantle and may also be a component of the core. Therefore its high pressure-temperature behavior, including its electronic structure, is essential to understanding the nature and evolution of Earth's deep interior. We performed X-ray diffraction and radiometric measurements on wüstite in a laser-heated diamond anvil cell, finding an insulator-metal transition at high pressures and temperatures. Our data show a negative slope for this apparently isostructural phase boundary, which is characterized by a volume decrease and emissivity increase. The metallic phase of FeO is stable at conditions of the lower mantle and core, which has implications for the high P-T character of Fe-O bonds, magnetic field propagation, and lower mantle conductivity
2010
Fischer R.A. and Campbell A.J. 2010. “High pressure melting of wüstite.” American Mineralogist, 95, Pp. 1473–1477. Publisher's VersionAbstract
Iron oxide (FeO) is an important component in the mineralogy of Earth’s lower mantle and possibly its core, so its phase diagram is essential to models of the planet’s interior. The melting curve of wüstite, Fe0.94O, was determined up to 77 GPa and 3100 K in a laser-heated diamond anvil cell. Melting transition temperatures were identified from discontinuities in the emissivity vs. temperature relationship within the laser-heated spot. The melting curve exhibits no obvious kinks that could be related to a subsolidus transition in wüstite, but there is evidence for a two-phase loop at pressures below 30 GPa. Comparison of these results to previous studies on Fe, Fe-O, and Fe-S confirms that the melting point depression in the Fe-O system remains significantly less, by a factor of 2 or more, than that in the Fe-S system up to pressures exceeding 80 GPa.
2009
Cottrell E., Kelley K.A., Lanzirotti A.T., and Fischer R.A. 2009. “High-precision determination of iron oxidation state in silicate glasses using XANES.” Chemical Geology, 268, Pp. 167–179. Publisher's VersionAbstract
Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectra were collected on natural basaltic glasses equilibrated over a range of oxygen fugacity (QFM − 3.5 to QFM + 4.5). The basalt compositions and fO2 conditions were chosen to bracket the natural range of redox conditions expected for basalts from mid-ocean ridge, ocean island, back-arc basin, and arc settings, in order to develop a high-precision calibration for the determination of Fe3+/∑Fe in natural basalts. The pre-edge centroid energy, corresponding to the 1s → 3d transition, was determined to be the most robust proxy for Fe oxidation state, affording significant advantages compared to the use of other spectral features. A second-order polynomial models the correlation between the centroid and Fe3+/∑Fe, yielding a precision of ± 0.0045 in Fe3+/∑Fe for glasses with Fe3+/∑Fe > 8%, which is comparable to the precision of wet chemistry. This high precision relies on a Si (311) monochromator to better define the Fe2+ and Fe3+ transitions, accurate and robust modeling of the pre-edge feature, dense fO2-coverage and compositional appropriateness of reference glasses, and application of a non-linear drift correction. Through re-analysis of the reference glasses across three synchrotron beam sessions, we show that the quoted precision can be achieved (i.e., analyses are reproducible) across multiple synchrotron beam sessions, even when spectral collection conditions (detector parameters or sample geometry) change. Rhyolitic glasses were also analyzed and yield a higher centroid energy at a given Fe3+/∑Fe than basalts, implying that major variations in melt structure affect the relationship between centroid position and Fe3+/∑Fe, and that separate calibrations are needed for the determination of oxidation state in basalts and rhyolites.
Lin J.F., Scott H.P., Fischer R.A., Chang Y.Y., Kantor I., and Prakapenka V.B. 2009. “Phase relations of Fe–Si alloy in Earth's core.” Geophysical Research Letters, 36, Pp. L06306. Publisher's VersionAbstract
Phase relations of an Fe0.85Si0.15 alloy were investigated up to 240 GPa and 3000 K using in situ X-ray diffraction in a laser-heated diamond anvil cell. An alloy of this composition as starting material is found to result in a stabilized mixture of Si-rich bcc and Si-poor hcp Fe-Si phases up to at least 150 GPa and 3000 K, whereas only hcp-Fe0.85Si0.15 is found to be stable between approximately 170 GPa and 240 GPa at high temperatures. Our extended results indicate that Fe0.85Si0.15 alloy is likely to have the hcp structure in the inner core, instead of the previously proposed mixture of hcp and bcc phases. Due to the volumetric dominance of the hcp phase in the hcp + bcc coexistence region close to the outer-core conditions, the dense closest-packed Fe-Si liquid is more relevant to understanding the properties of the outer core.

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