We conducted high-pressure experiments on hexagonal close packed iron (hcp-Fe) in MgO, NaCl, and Ne pressure-transmitting media and found general agreement among the experimental data at 300K that yield the best fitted values of the bulk modulus K-0=172.7(1.4)GPa and its pressure derivative K-0=4.79(0.05) for hcp-Fe, using the third-order Birch-Murnaghan equation of state. Using the derived thermal pressures for hcp-Fe up to 100GPa and 1800K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp-Fe. The thermal equation of state of hcp-Fe is further used to calculate the densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earth's inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000K.

In this chapter, we present a review of experimental data in geochemistry used to place constraints on Earth's accretion and core formation. Siderophile element abundances combined with partitioning experiments potentially give insights about pressure, temperature, and oxygen fugacity conditions during core formation. The interplay between siderophile partitioning and light elements in the core can help depict accurate models of accretion and core formation eventually. Additionally, resolvable metal-silicate isotopic fractionations in core formation experiments have been evidenced over the past few years. This new experimental tool merging the fields of experimental petrology and isotope geochemistry represents a promising approach, providing new independent constraints on the nature of light elements in the core and the conditions of Earth's core formation.

Information about the materials and conditions involved in planetary formation and differentiation in the early Solar System is recorded in iron isotope ratios. Samples from Earth, the Moon, Mars and the asteroid Vesta reveal significant variations in iron isotope ratios, but the sources of these variations remain uncertain. Here we present experiments that demonstrate that under the conditions of planetary core formation expected for the Moon, Mars and Vesta, iron isotopes fractionate between metal and silicate due to the presence of nickel, and enrich the bodies' mantles in isotopically light iron. However, the effect of nickel diminishes at higher temperatures: under conditions expected for Earth's core formation, we infer little fractionation of iron isotopes. From our experimental results and existing conceptual models of magma ocean crystallization and mantle partial melting, we find that nickel-induced fractionation can explain iron isotope variability found in planetary samples without invoking nebular or accretionary processes. We suggest that near-chondritic iron isotope ratios of basalts from Mars and Vesta, as well as the most primitive lunar basalts, were achieved by melting of isotopically light mantles, whereas the heavy iron isotope ratios of terrestrial ocean floor basalts are the result of melting of near-chondritic Earth mantle.

We apply an experimentally based thermodynamic model of Si+O saturation for the core to determine the saturation level of these elements under the conditions when the core formed. The model limits the bulk Si content of the core to between 0.4 and 3.1 wt% depending on the pressure, temperature, and oxygen content of the metal when it segregated from silicate. With knowledge of the core's Si content, the measured Si-30 content of the silicate Earth, and the experimentally determined metal-silicate fractionation factor, we can calculate the core's delta Si-30, which is between -0.92 to -1.36%. SiO2 cycled through the core and then released into the mantle might be trapped in inclusions in diamond formed in the lower mantle. These would be characterized by significantly lighter delta Si-30 values of -1.12 +/- 0.13 parts per thousand (1 sigma), compared to bulk silicate earth values of -0.29% and a potentially key indicator of mass transfer from the core to the mantle.

Iron isotopic compositions are demonstrably powerful tracers of foundational planetary processes, including crust and core formation. In many volcanic environments, however, geochemical vestiges of these processes are obscured by the effects of magmatic differentiation on Fe isotopic compositions. Recent decades have witnessed continued refinement of observational and experimental approaches to Fe isotope fractionation during silicate differentiation. In contrast, the influence of sulfide fractionation on Fe isotopic compositions in terrestrial environments is known only from theoretical approaches and limited experimental data for relatively siliceous magmatic systems. One reason for this may be that sulfide fractionation is difficult to definitively trace using traditional major and minor element variation patterns. We utilize well-characterized lavas and cumulate xenoliths from Piton de la Fournaise and Piton des Neiges, Reunion Island, that have previously been examined for their highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents to investigate the effect of sulfide fractionation on Fe isotopes. The Fe isotopic compositions of the basalts range from delta Fe-56 values of 0.04 to 0.15 parts per thousand (average: 0.10 parts per thousand) and the compositions of the cumulate xenoliths range from delta Fe-56 values of -0.07 to 0.08 parts per thousand (average: 0 parts per thousand). In the absence of metal, HSE preferentially partition into sulfide phases, making them important tracers of sulfide segregation during magmatic differentiation. We find that commonly-observed co-variations between Fe isotopic compositions and major element oxide abundances are relatively underdeveloped for Reunion lavas. The correlation between Fe isotopic composition and MgO, for example, has a similar statistical significance to the correlation between Fe isotopic composition and Pd/Ir ratios, suggesting an important role of sulfides during Fe isotopic fractionation. After accounting for sulfide segregation, we determine that the parental magma Fe isotopic composition calculated for Piton de la Fournaise would be overestimated by 0.04 parts per thousand (within propagated error, 0.01-0.06 parts per thousand) when considering silicate differentiation alone. An analogous calculation for Kilauea Iki basalts, for which there is available Fe isotopic and HSE data, yields a somewhat smaller difference of 0.02 parts per thousand (0-0.03 parts per thousand). These differences may partially explain Fe isotopic compositions in other settings that could not previously be reconciled with a dominantly peridotitic and/or chondritic mantle source. This discovery may warrant discussion of the apparent decoupling between Fe and radiogenic isotopes in ocean island basalts, where the latter shows significant global variations and the former may show little or none. Our work highlights the need for additional constraints on the behavior of Fe isotopes during crustal recycling processes and reinforces the notion that consideration must be given to the effect of magmatic differentiation on Fe isotopic compositions. (C) 2018 Elsevier Ltd. All rights reserved.

Isotopic fractionation associated with diffusion in crystals is the most reliable means of understanding the origin of mineral zoning in igneous and metamorphic rocks. We have experimentally determined the relative diffusivities of iron isotopes in olivine as a function of crystallographic orientation, composition, and temperature. For two isotopes i and j of an element, the isotope effect for diffusion is parameterized as D-i/D-j = (m(j)/m(i))(beta), where beta is a dimensionless parameter, and D and m stand for diffusivity and mass, respectively. A series of single crystal diffusion couple experiments were conducted at an oxygen fugacity of QFM - 1.5 at temperatures of 1200, 1300, and 1400 degrees C. For the Fo(83.4)-Fo(88.8) composition pair, beta(Fe) is isotropic and a value of 0.16 +/- 0.09 can be used to describe diffusion along all major crystallographic axes in olivine. Based on our experiments and previously reported coupled Mg-Fe isotopic data, we also estimate beta(Mg) = 0.09 +/- 0.05 for this range of olivine composition. For the Fo(88.8)-Fo(100) composition pair, beta(Fe) becomes anisotropic with beta(Fe [100]) = 0.11 +/- 0.03, beta(Fe [010]) = 0.14 +/- 0.03 (both within error of the value measured for the Fo(83.4)-Fo(88.8) pair), and beta(Fe [001]) = 0.03 +/- 0.03. For Fo# between 83.4 and 100, beta(Fe [100]) and beta(Fe [010]) are thus independent of composition. The reason why beta(Fe) ([001]) transitions from similar to 0.16 to similar to 0.03 close to the Mg-endmember is unclear. Over the temperature range studied, a dependence of beta(Fe) on temperature was not resolved. General analytical expressions are introduced to calculate isotopic fractionation as a function of distance, time, beta, and the concentration contrast between the diffusing media for spherical, cylindrical, and planar geometries. (C) 2018 Elsevier Ltd. All rights reserved.