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The phase and spin transitions in single-crystal monoclinic ferrosilite, FeSiO3, were investigated using X-ray diffraction and Mössbauer spectroscopy up to lower-mantle pressures and room temperature in a helium pressure medium. Using single-crystal X-ray diffraction, we measured the equation of state of ferrosilite up to about 43 GPa. We observed a P21/c-to-C2/c phase transition between 1.5 and 1.7 GPa and a phase transition from C2/c to a distinct P21/c structure between 30 and 34 GPa. With time-domain Mössbauer spectroscopy, we determined the hyperfine parameters of ferrous iron up to 95 GPa. The phase transitions were correlated with discontinuities in Mössbauer spectral features. We observed the onset of high-spin-to-low-spin transitions in the M1 and M2 sites at ∼37 GPa and ∼74 GPa, respectively. Understanding the electronic structure of iron in a well-characterized single crystal of ferrosilite may help interpret the behavior of iron in complex dense silicate phases.

As an element ubiquitous in the Solar system, the isotopic composition of iron exhibits rich variations in different planetary reservoirs. Such variations reflect the diverse range of differentiation and evolution processes experienced by their parent bodies. A key in deciphering iron isotope variations among planetary samples is to understand how iron isotopes fractionate during core formation. Here we report new Nuclear Resonant Inelastic X‐ray Scattering experiments on silicate glasses of bulk silicate Earth compositions to measure their force constants at high pressures of up to 30 GPa. The force constant results are subsequently used to constrain iron isotope fractionation during core formation on terrestrial planets. Using a model that integrates temperature, pressure, core composition, and redox state of the silicate mantle, we show that core formation might lead to an isotopically light mantle for small planetary bodies but a heavy one for Earth‐sized terrestrial planets. Plain Language Summary: Planetary formation and differentiation are fundamental processes that physically and chemically shaped terrestrial planets. As a major element ubiquitous in terrestrial planets, the isotopic ratios of Fe are found to vary among different planetary reservoirs, reflecting the formation and differentiation processes that occurred on their parent bodies. Deciphering the information behind these variations require experimental and theoretical constraints on how the isotopic ratios of Fe evolve during different planetary processes. Here, we present a systematic model on how Fe isotopes behave during planetary core formation based on high pressure experiments. We show that Fe isotope fractionation between the mantle and core of terrestrial bodies depends strongly on the planet size. Key Points: We constrained the Fe isotope fractionation factor for the silicate mantle up to 30 GPa using a bulk silicate Earth compositionWe present a holistic Fe isotope fractionation model for core formation considering pressure, temperature, composition, and redox stateOur model shows that core formation can cause the mantle of small planetary bodies to be light in Fe isotopes, but heavy for that of Earth [ABSTRACT FROM AUTHOR]

A new synchrotron radiation experimental capability of coupling nuclear resonant inelastic X-ray scattering with the cryogenically cooled high-pressure diamond anvil cell technique is presented. The new technique permits measurements of phonon density of states at low temperature and high pressure simultaneously, and can be applied to studies of phonon contribution to pressure- and temperature-induced magnetic, superconducting and metal-insulator transitions in resonant isotope-bearing materials. In this report, a pnictide sample, EuFe2As2, is used as an example to demonstrate this new capability at beamline 3-ID of the Advanced Photon Source, Argonne National Laboratory. A detailed description of the technical development is given. The Fe-specific phonon density of states and magnetism from the Fe sublattice in Eu57Fe2As2 at high pressure and low temperature were derived by using this new capability. [ABSTRACT FROM AUTHOR]

The melting points of fcc - and hcp -structured Fe 0.9 Ni 0.1 and Fe are measured up to 125 GPa using laser heated diamond anvil cells, synchrotron Mössbauer spectroscopy, and a recently developed fast temperature readout spectrometer. The onset of melting is detected by a characteristic drop in the time-integrated synchrotron Mössbauer signal which is sensitive to atomic motion. The thermal pressure experienced by the samples is constrained by X-ray diffraction measurements under high pressures and temperatures. The obtained best-fit melting curves of fcc -structured Fe and Fe 0.9 Ni 0.1 fall within the wide region bounded by previous studies. We are able to derive the γ – ϵ –l triple point of Fe and the quasi triple point of Fe 0.9 Ni 0.1 to be 110 ± 5 GPa , 3345 ± 120 K and 116 ± 5 GPa , 3260 ± 120 K , respectively. The measured melting temperatures of Fe at similar pressure are slightly higher than those of Fe 0.9 Ni 0.1 while their one sigma uncertainties overlap. Using previously measured phonon density of states of hcp -Fe, we calculate melting curves of hcp -structured Fe and Fe 0.9 Ni 0.1 using our (quasi) triple points as anchors. The extrapolated Fe 0.9 Ni 0.1 melting curve provides an estimate for the upper bound of Earth's inner core–outer core boundary temperature of 5500 ± 200 K . The temperature within the liquid outer core is then approximated with an adiabatic model, which constrains the upper bound of the temperature at the core side of the core–mantle boundary to be 4000 ± 200 K . We discuss a potential melting point depression caused by light elements and the implications of the presented core–mantle boundary temperature bounds on phase relations in the lowermost part of the mantle. [ABSTRACT FROM AUTHOR]

Abstract: Many seismological studies have confirmed that Vp travels 3–4% faster along the rotation axis of the Earth than along the equatorial plane in the inner core, indicating that the inner core is elastically anisotropic. However, seismic and mineral physics observations of the polarized Vs are still emerging. Thus far, the Vs anisotropy of the constitute iron crystals at relevant pressures of the Earth''s core has remained mostly theoretical mainly because of the technical difficulties involved in measuring reliable Vs velocities of iron crystals. Here we have measured azimuthal Vs anisotropy of highly textured hcp-Fe at high pressures using nuclear resonant inelastic X-ray scattering, a technique sensitive to Vs, in a diamond anvil cell. Our results show that the azimuthal Vs is 2–4% faster along the crystallographic c axis than along the a axis at 158GPa and 172GPa. If one describes the Vp anisotropy of the inner core as a result of the textured hcp-Fe crystals, it is conceivable that azimuthal and polarized Vs anisotropies with a magnitude of a few percent also exist in the region. Since Vp and Vs of candidate iron phases behave quite differently in theoretical predictions, our results here indicate that future seismic observations of the Vs and Vp anisotropies of the inner core thus hold the key to deciphering the causes for the seismic and dynamic signatures as well as constitute iron phase(s) of the region. [Copyright &y& Elsevier]

The Earth's iron-dominant core is known to contain nickel from cosmochemical analysis and some amount of light elements from geophysical constraints on density and seismic wave velocities. Although there have been several studies to constrain thermoelastic properties of iron-alloys, there has been no systematic study on the effects of nickel and light elements on properties of iron using the same experimental methods and data analysis approach. We conducted nuclear resonant inelastic X-ray scattering and X-ray diffraction experiments on body-centered cubic and hexagonal close-packed (hcp) Fe 0.91 Ni 0.09 and Fe 0.8 Ni 0.1 Si 0.1 up to 104 GPa and 86 GPa, respectively, and compare to similar measurements conducted on hcp-Fe up to 171 GPa. Specifically, we determine the Debye sound velocity from the low-energy transfer region of the (partial) phonon density of states (DOS) using the equation of state determined for each material and a new approach which utilizes information criteria and probability distributions. Nickel decreases the shear velocity of iron, while 10 at% Si has little to no effect on the shear velocity of Fe 0.91 Ni 0.09. We observe that the shape of the phonon DOS of these alloys remains similar with increasing pressure. In the measured compression range, we therefore apply a generalized scaling law to describe the volume dependence of the phonon DOS and find that the vibrational Grüneisen parameters of hcp-Fe 0.91 Ni 0.09 are nearly indistinguishable from those hcp-Fe and those for Fe 0.8 Ni 0.1 Si 0.1 trend lower. From the vibrational free energy, we constrain the harmonic vibrational component of thermal pressure, which shows a significant positive deviation from theoretical calculations of hcp-Fe at pressures and temperatures of Earth's core. Collectively, our results demonstrate that the effects of nickel should be considered when modeling iron-rich planetary cores. • Nuclear resonant inelastic X-ray scattering on Fe-Ni and Fe-Ni-Si to 104 and 86 GPa • New Debye velocity determination method yields improved results and uncertainties. • Nickel decreases the shear velocity of iron, and 10 at% Si has little to no effect. • Vibrational Grüneisen parameters of hcp Fe-Ni-Si trend lower than Fe and Fe-Ni • hcp-Fe harmonic vibrational thermal pressure deviates from theoretical calculations. [ABSTRACT FROM AUTHOR]