Catherine McCammon

Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle

The oxidation state recorded by rocks from the Earths upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantles oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.

Perovskite as a possible sink for ferric iron in the lower mantle

The lower mantle constitutes more than half the Earths interior by volume, and is believed to consist predominantly of (Mg,Fe)SiO3 perovskite with up to approximately 20% (Mg,Fe)O. In the system FeO–MgO–SiO2, iron partitions preferentially into (Mg,Fe)O relative to the perovskite phase and has been believed to be nearly all in the form of Fe2+on the basis of experiments in the MgO–FeO–SiO2 system. Here, however, we present a Mössbauer study of (Mg,Fe)SiO3 perovskite containing 3.3 mol% Al2O3, which shows that approximately 50% of the iron is Fe3+. These results, combined with evidence from other experiments, suggest that the proportion of iron present as Fe3+in the lower-mantle perovskite phase is probably much higher than is currently believed. Because the oxidation state of iron in the perovskite phase affects the electrostatic charge balance and equilibrium defect concentration, the presence of Fe3+is likely to significantly affect physical and chemical properties of the lower mantle such as sub- and super-solidus phase relations, transport properties, mechanical behaviour, trace-element partitioning and the concentration of species such as hydroxide ions.

The oxidation state of the mantle and the extraction of carbon from Earth's interior.

Determining the oxygen fugacity of Earth’s silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet’s formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe3+ in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe3+ to total iron in the bulk rock. This ‘redox melting’ relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.

Iron Partitioning and Density Changes of Pyrolite in Earth’s Lower Mantle

Spin Transitions and Mantle Phases The extreme temperatures and pressures at depth can cause mantle minerals to undergo phase transformations. For example, it is thought that an electronic spin transition that occurs at high pressures in the lower mantle leads to the partitioning of various iron mineral phases. Using a multianvil apparatus to study a synthetic material similar in composition to the lower mantle, Irifune et al. (p. 193, see the Perspective by Hirose; published online 3 December) demonstrate that the spin transition occurs at lower pressures, and thus shallower depth, than suggested by earlier studies based on simpler compositions. As verification of the materials applicability in lower mantle studies, its density profile with pressure overlaps very well with those predicted by seismological models. Increasing the compositional complexity of mantle samples causes an electronic spin transition to occur at lower pressures. Phase transitions and the chemical composition of minerals in Earth’s interior influence geophysical interpretations of its deep structure and dynamics. A pressure-induced spin transition in olivine has been suggested to influence iron partitioning and depletion, resulting in a distinct layered structure in Earth’s lower mantle. For a more realistic mantle composition (pyrolite), we observed a considerable change in the iron-magnesium partition coefficient at about 40 gigapascals that is explained by a spin transition at much lower pressures. However, only a small depletion of iron is observed in the major high-pressure phase (magnesium silicate perovskite), which may be explained by preferential retention of the iron ion Fe3+. Changes in mineral proportions or density are not associated with the change in partition coefficient. The observed density profile agrees well with seismological models, which suggests that pyrolite is a good model composition for the upper to middle parts of the lower mantle.

Structure and elasticity of single‐crystal (Mg,Fe)O and a new method of generating shear waves for gigahertz ultrasonic interferometry

investigated Fe 3+ -bearing (Mg,Fe)O single crystals prepared by interdiffusion having Fe/(Fe + Mg) = 0.06, 015, 0.24, 0.27, 0.37, 0.53, 0.56, 0.75, and 0.79, with ferric iron contents ranging from � 1 to 12% of the total Fe. The elastic constants (c11, c12, c44) are determined from compressional and shear wave velocities in the (100) and (111) propagation directions in the range of 0.5-1.2 GHz. The c11 and c44 elastic constants soften from periclase to wustite, whereas the c12 elastic constant increases. The rate of change in the elastic constants with composition (@cij/@x) is greatest between MgO and (Mg,Fe)O with � 25 mol % FeO implying that substitution of Fe into periclase has a greater effect on the elastic properties than adding Mg to wustite. The elastic anisotropy of (Mg,Fe)O has rather unusual behavior, being essentially constant for the range 0-25 mol % FeO but then decreases linearly with Fe content such that wustite is elastically isotropic. The elastic properties of (Mg,Fe)O having similar total Fe but varying Fe 3+ contents are identical within uncertainty. The isothermal compressibility of samples with Fe/(Fe + Mg) = 0.27, 0.56, and 0.75 is determined by single-crystal X-ray diffraction in a diamond anvil cell to � 9 GPa. For these samples, K0T = 158.4(4), 155.8(9), and 151.3(6) GPa with @KT/@P = 5.5(1), 5.5(1), and 5.6(2), respectively (where values in parentheses indicate standard deviations). The deviation of @KT/@P from 4.0 corresponds to a difference in calculated density of about one percent for ferropericlase (Mg0.8Fe0.2)O at 30 GPa from the value predicted by second-order truncation of the Birch- Murnaghan equation of state. INDEX TERMS: 3620 Mineralogy and Petrology: Crystal chemistry; 3909 Mineral Physics: Elasticity and anelasticity; 3919 Mineral Physics: Equations of state; 3924 Mineral Physics: High-pressure behavior; KEYWORDS: magnesiowustite, elastic constants, ultrasonics, crystal chemistry, bulk moduli

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle

Abstract (Mg,Fe)(Si,Al)O3 perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe3+/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80 K, and shows a nearly linear dependence of Fe3+/ΣFe with Al composition of (Mg,Fe)(Si,Al)O3 perovskite. The Fe3+/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O3 perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe3+/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe3+/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O3 perovskite.

Oxygen fugacity, temperature reproducibility, and H2O contents of nominally anhydrous piston-cylinder experiments using graphite capsules

Abstract The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe3+/Fe2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5-3.0 GPa, 1100-1500 °C). Under those conditions, the amount of Fe3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe3+/ ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO₂ cannot be achieved using Pt-graphite or graphite only capsules, but fO₂ can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO₂ that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H2O are always present in nominally dry experiments. These small amounts of H2O should not, however, significantly change phase relations.

Hydrogen solubility and speciation in natural, gem-quality chromian diopside

Abstract A new technique for performing long duration (up to 300 hours) high-pressure annealing experiments under water-saturated conditions has been developed. This technique has been used to investigate watersolubility and speciation in natural, gem-quality chromian diopside. Capsule design for the technique is a variant of the double-capsule technique, and relies on the use of a semi-permeable Pt membrane, which permits free hydrogen diffusion into samples, but protects samples from reacting with buffer mixtures. The investigation of a natural single crystal of chromian diopside revealed a very unusual annealing behavior: water contents increase sharply after a short annealing period and then decrease slowly to some metastable equilibrium value. The main process that takes place during the annealing experiments is hydrogen diffusion coupled with Fe3+ reduction. This essentially reverses the main mechanism for hydrogen loss from mantle samples during exhumation, and the technique therefore provides sample-specific information on original water contents. Absorption bands at 3646 and 3434 cm-1 in IR spectra from annealed samples suggest two main mechanisms for hydrogen incorporation in the diopside sample: (1) incorporation of hydrogen onto the O2 site, with vibration of the OH dipole in the direction of a nearby O3 site (along the edge of an M2 site), and (2) incorporation of hydrogen onto the O2 site with vibration of the OH dipole toward a nearby O1 site (along a shared M1-M2 edge) or O2 site (along the edge of an M1 site). The ratio of peak heights between the absorption bands at 3646 and 3434 cm-1 is independent of water fugacity but dependent on oxygen fugacity, and appears to provide a measure of the redox state “frozen” into the sample. This ratio could be used to determine whether pyroxenes from upper-mantle xenoliths had experienced concurrent hydrogen-loss and oxidation during exhumation.

Structural systematics of hydrous ringwoodite and water in Earth’s interior

Abstract Seven separate samples of hydrous ringwoodite with compositions ranging from Fo100 to Fo89 and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 °C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mössbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mössbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 cm-1 with maxima ranging from 3105 for the pure magnesian samples to 3150 cm-1 for the Fo89 samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

MossA: a program for analyzing energy-domain Mössbauer spectra from conventional and synchrotron sources

The program MossA provides a straightforward approach to the fitting of 57Fe conventional and synchrotron energy-domain Mossbauer spectra. Sites can be defined simply by mouse clicks and hyperfine parameters can be constrained to constant values, within specific ranges, and can be coupled linearly between different subspectra. The program includes a full transmission integral fit with Lorentzian line shape (conventional source) or Lorentzian-squared line shape (synchrotron source). The fitting process is graphically displayed in real time while fitting and can be interrupted at any time. Gaussian-shaped quadrupole splitting distributions for analyzing nonmagnetic amorphous materials are included. MossA is designed especially for the rapid and comprehensive analysis of complex Mossbauer spectra, made possible by its native graphical user input.