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 VersionAbstractThe 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 K′T = 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 VersionAbstractSynchrotron 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, K′T = 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, K′T = 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.