Spin Transitions and Compressibility of ε‐Fe7N3 and γ′‐Fe4N: Implications for Iron Alloys in Terrestrial Planet Cores

Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe7N3 and γ′‐Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe7N3 and γ′‐Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe7N3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe4N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yields V0 = 83.29 ± 0.03 (Å3), K0 = 232 ± 9 GPa, K0′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe7N3 above the spin transition completion pressure, and V0 = 54.82 ± 0.02 (Å3), K0 = 152 ± 2 GPa, K0′ = 4.0 ± 0.1 for γ′‐Fe4N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe3S, Fe3P, Fe7C3, and Fe3C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe3S, Fe3P, and Fe3C induces elastic stiffening, whereas that of Fe7C3 induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth’s and planetary cores.Key PointsSpin transition in ε‐Fe7N3 and γ′‐Fe4N at 300 K completes at 43 and 34 GPa, respectivelyThe completion of spin transition leads to stiffening in bulk modulus of ε‐Fe7N3, but not in γ′‐Fe4NEvidence for spin transitions in Fe‐light‐element alloys and their effects are reexaminedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163586/2/jgrb54505_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163586/1/jgrb54505.pd

Deep Earth carbon reactions through time and space

The authors acknowledge partial support from the Sloan Foundation grant G-2016-7157.Reactions involving carbon in the deep Earth have limited manifestation on Earth’s surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth’s accretion and may have sequestered substantial carbon in Earth’s core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth’s inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The ten-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and helped to identify gaps in our understanding that motivate and give direction to future studies.Publisher PDFPeer reviewe

Thermal cracking and crack healing of geomaterials

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2005.Cataloged from PDF version of thesis. Pages 29-37, 42 and 46 not in original thesis.Includes bibliographical references (pages 44-48).We explored two complementary mechanisms for change in porosity and permeability of geomaterials: thermal cracking and crack healing by diagenesis. A suite of thermal cracking experiments was performed on andesite plugs from the geothermal field in Awibengkok, Indonesia. Permeability (k) and specific storage capacity were measured by the oscillating flow method in a wide range permeameter, at room temperature, with effective pressures between 15 MPa and 95 MPa, before and after thermal cracking. The samples were cracked at 150 and 300 °C and ambient pressure. Andesite samples have low permeability, on the order of 10-²⁰ M² . With increased pressure, permeability is reduced by a factor of two. Contrary to expectations, thermal cracking reduced the permeability of this material by an order of magnitude. We also examined a set of samples from crack healing experiments performed on Sioux quartzite by M. Messar. In these experiments the quartzite permeability fell by three orders of magnitude within a few days when the samples were saturated with water and heated to temperatures from 300 to 500 °C and pressures from 25 to 200 MPa. In order to correlate Messar's permeability measurements and experimental conditions with visual observations of the pore structure (mainly consisting of grain boundary cracks), we took scanning electron microscope micrographs of the samples. We then counted the intersections of test lines with healed and unhealed cracks. This yielded a set of measurements of the crack area per volume of the quartzite. We found that the final permeability of the samples was related to the area per volume of unhealed cracks by a power relation. Combining the findings from the two sets of experiments, it seems that cracking and healing effects due to the ambient temperature and pressure in geothermal fields such as Awibengkok could eliminate any permeability through grain boundary scale cracks within a matter of days.by Susannah Dorfman.S.B

Strength and texture of Pt compressed to 63 GPa

Angle-and energy-dispersive X-ray diffraction experiments in a radial geometry were performed in the diamond anvil cell on polycrystalline platinum samples at pressures up to 63 GPa. Observed yield strength and texture depend on grain size. For samples with 70-300-nm particle size, the yield strength is 5-6 GPa at similar to 60 GPa. Coarse-grained (similar to 2-mu m particles) Pt has a much lower yield strength of 1-1.5 GPa at similar to 60 GPa. Face-centered cubic metals Pt and Au have lower strength to shear modulus ratio than body-centered cubic or hexagonal close-packed metals. While a 300-nm particle sample exhibits the texture expected of face-centered-cubic metals under compression, smaller and larger particles show a weak mixed and texture under compression. Differences in texture development may also occur due to deviations from uniaxial stress under compression in the diamond anvil cell. (C) 2015 AIP Publishing LLC

Effects of Fe-enrichment on the equation of state and stability of (Mg,Fe)SiO3 perovskite

Fe-rich natural orthopyroxenes with compositions of (Mg0.61Fe0.38Ca0.01)SiO3 (Fe#38) and (Mg0.25Fe0.74Ca0.01)SiO3 (Fe#74) were studied at pressures up to 155 GPa and temperatures up to 3000 K. Single-phase orthorhombic GdFeO3-type perovskite was synthesized by heating to similar to 2000 K at 63 GPa for the Fe#38 composition and at 72 GPa for the Fe#74 composition. At lower pressures, heating both compositions resulted in a mixture of perovskite, SiO2 and (Mg,Fe)O. These measurements provide new constraints on the dependence of (Mg,Fe)SiO3 perovskite stability on pressure and composition. Upon further compression and heating at 89 and 99 GPa, Fe#38 and Fe#74 perovskites transformed to two-phase mixtures of perovskite and post-perovskite, consistent with previous findings that increasing Fe content lowers the transition pressure. The volume of (Mg,Fe)SiO3 perovskites increases linearly with Fe-content. Volume data were fit to the Birch-Murnaghan equation of state. The bulk modulus at 80 GPa is 550-560 GPa for both Fe-rich perovskites, comparable to or slightly higher than the values measured for MgSiO3 perovskite at this pressure, indicating that the bulk modulus of perovskite is not strongly sensitive to Fe content. (C) 2012 Elsevier B.V. All rights reserved

The strength of ruby from X-ray diffraction under non-hydrostatic compression to 68 GPa

Polycrystalline ruby (alpha-Al2O3:Cr3+), a widely used pressure calibrant in high-pressure experiments, was compressed to 68.1 GPa at room temperature under non-hydrostatic conditions in a diamond anvil cell. Angle-dispersive X-ray diffraction experiments in a radial geometry were conducted at beamline X17C of the National Synchrotron Light Source. The stress state of ruby at high pressure and room temperature was analyzed based on the measured lattice strain. The differential stress of ruby increases with pressure from similar to 3.4 % of the shear modulus at 18.5 GPa to similar to 6.5 % at 68.1 GPa. The polycrystalline ruby sample can support a maximum differential stress of similar to 16 GPa at 68.1 GPa under non-hydrostatic compression. The results of this study provide a better understanding of the mechanical properties of this important material for high-pressure science. From a synthesis of existing data for strong ceramic materials, we find that the high-pressure yield strength correlates well with the ambient pressure Vickers hardness

Synthesis and equation of state of perovskites in the (Mg, Fe)(3)Al2Si3O12 system to 177 GPa

Natural and synthetic pyrope-almandine compositions from 38 to 100 mol% almandine (Alm38-Alm100) were studied by synchrotron X-ray diffraction in the laser-heated diamond anvil cell to 177 GPa. Single-phase orthorhombic GdFeO3-type perovskites were synthesized across the entire examined compositional range at deep lower mantle pressures, with higher Fe-contents requiring higher synthesis pressures. The formation of perovskite with Alm100 (Fe3Al2Si3O12) composition at 80 GPa marks the first observation of a silicate perovskite in a Fe end-member. Fe-enrichment broadens and lowers the pressure range of the post-perovskite transition for intermediate compositions such as Alm54, but the more Fe-rich Alm100-composition perovskite remains stable to pressures as high as 149 GPa. Volume compression data for the Alm54 and Alm100 compositions were fit to the Birch-Murnaghan equation of state. The compressibility of perovskites synthesized from compositions along the pyrope-almandine join is not strongly sensitive to Fe-content. The compression curves were smooth over the entire measured range, and no evidence for a volume anomaly associated with a spin transition was observed. (C) 2012 Elsevier B.V. All rights reserved

High Sodium Solubility in Magnesiowüstite in Iron-Rich Deep Lower Mantle

(Mg,Fe)O ferropericlase-magnesiowüstite has been proposed to host the majority of Earth's sodium, but the mechanism and capacity for incorporating the alkali cation remain unclear. In this work, experiments in the laser-heated diamond anvil cell and first-principles calculations determine the solubility of sodium and favorability of sodium incorporation in iron-rich magnesiowüstite relative to (Mg,Fe)SiO3 bridgmanite. Reaction of Mg/(Mg + Fe) (Mg#) 55 and 28 olivine with NaCl at 33–128 GPa and 1600–3000 K produces iron-rich magnesiowüstite containing several percent sodium, while iron-rich bridgmanite contains little to no detectable sodium. In sodium-saturated magnesiowüstite, sodium number [Na/(Na + Mg + Fe)] is 2–5 atomic percent at pressures below 60 GPa and drastically increases to 10–20 atomic percent at deep lower mantle pressures. For these two compositions, there is no significant dependence of the results on Mg#. Our calculations not only show consistent results with experiments but further indicate that such an increase in solubility and partitioning of Na into magnesiowüstite is driven by the spin transition in iron. These results provide fundamental constraints on the crystal chemistry of sodium at lower-mantle conditions. If the sodium capacity of (Mg,Fe)O is not strongly dependent on Mg#, (Mg,Fe)O in the lower mantle may have the capacity to store the entire sodium budget of the Earth

Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle

International audienceThe stable forms of carbon in Earth's deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth's mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth's surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO 3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO 3 reacts with silicates to form CaCO 3. In combination with previous work indicating that CaCO 3 is more stable than MgCO 3 under reducing conditions of Earth's lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth's lowermost mantle may include CaCO 3