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Fe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth’s lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe3+,☐)(Al,Fe3+)2O4 composition (☐ = vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A-site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO3 charge-coupled component is dominant.

The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation

To determine the factors that control the oxygen content of sulphide melts in the upper mantle, mono-sulphides were equilibrated with mantle peridotite assemblages, with varying FeO and NiO contents, between 3 and 13 GPa and 1300 to 1819 °C. Carbonates were added as a flux and the use of graphite capsules allowed oxygen fugacities (fo2) to be estimated for most of the experiments. Sulphide melt O concentrations were in the range 0.2–3.7 wt. %, coexisting with silicate assemblages with olivine Fe/(Fe + Mg) ratios between 0.04 and 0.25. Except in Ni-bearing experiments, variations in the sulphur/metal cation ratio did not affect the sulphide melt O contents, which also appeared to be independent of fo2. The silicate FeO contents, temperature and pressure were found to be the main controls on O contents. Nickel lowers the O content, although all Ni-bearing experiments produced sulphides with lower sulphur/metal ratios, making it difficult to categorically separate the effect of Ni alone. A preliminary geothermometer expression was developed based on the O content of the sulphide melts and the iron oxide concentrations of coexisting olivine and orthopyroxene. With this expression, the experimental temperatures are reproduced to within 74 K for Ni-free experiments and within 135 K for Ni-bearing samples. Using measurements of the O contents of sulphide inclusions of peridotite affinity in diamonds from the Lac de Gras kimberlite field, a plausible average entrapment temperature of 1318 ± 48 °C is calculated. Mantle peridotite assemblages would be expected to contain sulphide melts with approximately 0.4–0.6 wt. % O along a typical mantle adiabat down to 200 km depth.

ACS earth and space chemistry 7(2), 471 - 478 (2023). doi:10.1021/acsearthspacechem.2c00326, The chemistry of bridgmanite is a crucial parameter for understanding the structure and dynamics of the Earth’s lower mantle. Incorporating Al, Fe$^{2+}$, and Fe$^{3+}$ into MgSiO$_3$ bridgmanite can significantly affect its physical properties. We investigated the substitution mechanisms of Fe$^{3+}$ in bridgmanite under SiO$_2$- and Fe$_2$O$_3$-saturated conditions at 27 GPa and 1700–2000 K using a multianvil apparatus. The fraction of Fe$^{3+}$ in bridgmanite increases from 0.085 to 0.202 atoms pfu with increasing temperature. We observed that only the charge-coupled mechanism takes place in bridgmanite under the studied conditions by forming the FeFeO$_3$ component, which increases from 4.0 ± 0.6 mol % at 1700 K to 9.9 ± 0.7 mol % at 2000 K. The absence of vacancies in bridgmanite under SiO$_2$-saturated conditions implies that bridgmanite in mid-ocean ridge basalt layers of subducted slabs in the lower mantle should have higher viscosity and lower electrical conductivity than that in the surrounding mantle. These differences in bridgmanite properties should affect lower-mantle dynamics, for example, enhance slab penetration more deeply into the lower mantle due to viscosity difference between mid-ocean ridge basalt and surrounding bridgmanite., Published by ACS Publications, Washington, DC

The ferric iron content in hydrothermally altered ultrabasic rocks and their major minerals, serpentines and Mg-chlorites, is important for establishing the oxidation state budget from oceanic ridges to subduction zones, in carbonaceous chondrites, and for modeling phase equilibria. A compilation of literature Mössbauer spectroscopic data on serpentines and magnesian chlorites from high-pressure ophiolites yields much lower ferric-to-total-iron ratios (Fe3+ / Fetotal) than those obtained on similar samples by X-ray absorption near-edge spectroscopy (XANES), leading to contradictory estimates of the ferric iron budget of subduction zones. New Mössbauer analysis of antigorite and Mg-chlorite samples from suites of high-pressure ophiolitic terrains of various Phanerozoic ages confirms the low and homogeneous values previously obtained by this technique, while lizardite inherited from oceanic hydrothermal alteration is ferric iron rich. We argue that XANES values may be biased by photo-oxidation when samples have a high Mg content, which is the case for serpentines and chlorites from subduction zones. Photo-oxidation is less important in Fe-poor phyllosilicates of the mica and talc families and does not affect the Fe-rich serpentines (greenalite, cronstedtite) of meteorites or Fe-rich terrestrial phyllosilicates. Mössbauer Fe3+ / Fetotal ratios of serpentine confirm the occurrence of a major redox change at the lizardite–antigorite transition near 300–400 ∘C rather than at the dehydration of antigorite at 500–650 ∘C in serpentinites from high-pressure ophiolites.

The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth’s lower mantle. To investigate the controls on the chemistry of Brg, the Fe3+ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo2) at 25 GPa. The Fe3+/∑Fe ratio of Brg increases with Brg Al content and fo2 and decreases with increasing total Fe content and with temperature. The dependence of the Fe3+/∑Fe ratio on fo2 becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al–Mg–Si–O, Fe–Mg–Si–O and Fe–Al–Mg–Si–O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of fo2 with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of fo2 and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly fo2 dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the fo2 at the top of the lower mantle is −0.86 log units below the iron–wustite buffer (IW) with a Brg Fe3+/∑Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7 wt.% Fe–Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe3+/∑Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the fo2 recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle.

Redox-driven biogeochemical element cycles play a central role in converting organic matter in aquatic ecosystems. They also perform key functions such as removing nitrate, mitigating the formation of greenhouse gases and weakening the effects of contaminants. Recent research has revealed the presence of redox-active compounds in these ecosystems with hitherto unknown redox properties. These substances are metastable (that is, non-equilibrium solid phases), which can both donate and accept electrons. They are highly redox reactive and recyclable and may act as biogeobatteries by temporarily storing electrons. Their lifetime, however, is limited, and with time they become more crystalline and less reactive. In this Review, we argue that these redox-active metastable phases require activation by fluctuating redox conditions to maintain their high reactivity. In aquatic ecosystems, switching between oxidizing and reducing conditions can be achieved only through hydrological perturbations at hydrological interfaces (for example, water level fluctuations). We present a novel framework that links microscale biogeochemical processes to large-scale hydrological processes, and discuss implications and future research directions for biogeochemical element cycles in aquatic systems exposed to frequent hydrological disturbances. Highly redox-active compounds play an important role in biogeochemical element cycles in aquatic systems that are exposed to frequent hydrological disturbances.

A versatile electrochemical corrosion process was used to generate green rust (GR) on a steel substrate and to transform it into the trivalent iron-only oxygen evolution catalyst mossbauerite (GR*) upon a subsequent oxidation process. Besides being scalable, cheap, and time-efficient, the demonstrated procedure is based on earth-abundant and non-hazardous materials only. Oxidation of mixed-valence Fe2+/Fe3+ GR to GR* was achieved by chemical and electrochemical processes. The latter induces less grafting of interlayer CO32−, leading to less contraction of the interlayer space and a reduction in the observable stacking disorder. The direct electrodeposition of the catalyst on a conductive substrate enabled the first systematic study of the impact of grafting on the performance of GR* in the oxygen evolution reaction (OER). The materials subjected to chemical (GR*ChOx) and electrochemical oxidation (GR*ElOx) allowed obtaining a current density of 100 mA cm−2 at 1.82 V and 1.84 V, respectively, thus improving the electrocatalytic capabilities and outperforming the bare steel substrate operated in 1 M KOH solutions.

As the most abundant material of rocky planets, high-pressure polymorphs of iron- and aluminum-bearing magnesium silicates have long been sought by both observations and experiments. Meanwhile, it was recently revealed that iron oxides form (FeO)m(Fe2O3)n homologous series above ∼10 GPa according to laboratory high-pressure experiments. Here, we report a new high-pressure iron-magnesium silicate, recently approved by the International Mineralogical Association as a new mineral (No. 2020-086) and named elgoresyite, in a shock-induced melt vein of the Suizhou L6 chondrite with a chemistry of (Mg,Fe)5Si2O9. The crystal structure of this new silicate is the same as the iron oxide Fe7O9, strongly suggesting that silicates also form ((Mg,Fe)O)m + n(SiO2)n series that are isostructural to iron oxides via (Mg2+,Fe2+) + Si4+ = 2Fe3+ substitution. To test this hypothesis, the phase relationships of the silicates and iron oxides should be further investigated at higher temperature conditions. Newly found iron-magnesium silicate is a potential constituent mineral in rocky planets with relatively high MgO + FeO content.

Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.

Acta crystallographica / Section E 76(5), 715 - 719 (2020). doi:10.1107/S2056989020005411, The crystal structure of MgCO$_3$-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp$^2$ to the sp$^3$ type of bonding. We have now determined the crystal structure of iron-bearing MgCO$_3$-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg$_{2.53}$Fe$_{0.47}$)C$_3$O$_9$ in the space group C2/m that contains tetra­hedrally coordinated carbon, where CO$_4$$^{4-}$ tetra­hedra are linked by corner-sharing oxygen atoms to form three-membered C$_3$O$_3$$^{6-}$ ring anions. The crystal structure of (Mg$_{0.85}$Fe$_{0.15}$)CO$_3$ (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles., Published by National Center for Biotechnology Information, U.S. National Library of Medicine15046, Bethesda, MD

Subduction of carbon-bearing phases throughout Earth’s history may be an important mechanism of sourcing carbon to the Earth’s lower mantle. As carbon has very low solubility in mantle silicates, it is primarily present in accessory phases such as carbonates, diamond, or metal carbides. Previous studies indicate that more than half of the carbonate contained in the oceanic crust may survive metamorphism and dehydration in the sub-arc and reach the lower mantle, even though the oxygen fugacity in the deep mantle may not favour their stability [1]. Indeed, the presence of carbonate in ultra-deep diamond inclusions provides evidence for carbonate subduction at least down to the transition zone [2].The carbonate phases present in the lower mantle depend on their bulk composition, the oxygen fugacity, and on their stability at a given pressure and temperature. Results from high-pressure experiments show that magnesite (MgCO3) can be stable up to deep lower mantle conditions (∼80 GPa and 2500 K) [3]. Accordingly, magnesite may be considered the most probable carbonate phase present in the deep Earth. Experimental studies on magnesite decarbonation in presence of SiO2 at lower mantle conditions suggest that magnesite is stable along a cold subducted slab geotherm [4, 5]. However, our understanding of magnesite’s stability in contact with bridgmanite [(Mg,Fe)SiO3], the most abundant mineral in the lower mantle, remains incomplete.Hence, to investigate sub-solidus reactions, melting, decarbonation, and diamond formation in the system MgCO3-(Mg,Fe)SiO3, we conducted a combination of high-pressure experiments using multi-anvil press and laser-heated diamond anvil cells (LH-DAC) at conditions ranging from 25 to 70 GPa and 1300 to 2100 K.Multi-anvil experiments at 25 GPa and temperatures below the mantle geotherm (1700 K) show the formation of carbonate-silicate melt associated with stishovite crystallization, indicating incongruent melting of bridgmanite to stishovite, in accordance with the recent finding of Litasov and Shatskiy [4]. LH-DAC data from in situ X-ray diffraction show crystallization of bridgmanite and stishovite. Diamond crystallization is detected using Raman spectroscopy. A melt phase could not be detected in situ at high temperatures.Our results suggest a two-step process that starts with melting at temperatures below the mantle geotherm, followed by crystallization of diamond from the melt produced. Therefore, we propose that subducted carbonate-bearing silicate rocks will not remain stable in the lower mantle and will instead melt at upper-most lower mantle conditions, fostering diamond formation. Our study also provides additional evidence that diamond production is related to carbonated melt. Consequently, the melting of recycled crust and chemical transfer to the surrounding mantle will hinder the transport of carbon deeper into the lower mantle.[1] Stagno et al. (2015) Contrib. Mineral. Petrol. 169(2), 16.[2] Brenker et al. (2007) EPSL 260(1-2), 1-9.[3] Binck, et al. (2020) Physical Review Materials, 4(5),1-9.[4] Litasov & Shatskiy (2019) Geochemistry International, 57(9), 1024-1033.[5] Drewitt, et al. (2019). EPSL, 511, 213-222.

Structure and properties of terrestrial magma oceans control the co-evolution of the core, mantle and atmosphere of the early Earth, but are poorly understood because discrepancies remain between experiments and theoretical calculations. Here we combine acoustic velocity measurements and ab initio simulations on pyrolite glass/melt with a silicate Earth-like composition. In the complex system, we find a gradual increase of sound velocity with increasing pressure. Through ab initio simulations, this is explicable by the transition from four- to six-fold coordinated Si occurring over the entire mantle regime. These results are at odds with recent X-ray diffraction measurements, which show an abrupt change in Si-O coordination at 35 GPa. It is however consistent with recent high-pressure data, where Ni partitioning between molten metal and silicate exhibits a similar gradual change with pressure. Unlike amorphous silica, smooth structural evolution in a multicomponent system implies progressive changes in magma ocean properties with depth, such as density, element partitioning and transport properties, which, when incorporated into magma ocean models, may improve our understanding of early history of the Earth and other rocky planets., Earth and Planetary Science Letters, 584, ISSN:0012-821X, ISSN:1385-013X

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth's lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93. Here we use Synchrotron Mössbauer Source (SMS) spectroscopy and single-crystal X-ray diffraction to determine the iron oxidation state and structure of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. Inclusion Mg#s vary between 16.1 and 84.5. Ferropericlase inclusions contain no ferric iron within the detection limit of SMS, while both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite ((Mg,Fe)Fe

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Radiation induced disorder in gadolinite that led to metamictization with an upper degree of amorphization of 18% was thermally annealed between room temperature and 1273 K. The degree of annealing was calibrated using the anti-symmetric Si–O–Si Raman-active stretching mode near 902 cm−1. Annealing increased with increasing temperature with a rapid critical recrystallization at ca. 943 K. This annealing on a short length scale was then complemented by investigations of long-range ordering seen by X-ray diffraction. The same critical temperature was found, and in addition further increase of long-range order extended to 1073 K. Metamict gadolinite contains only Fe2+ within experimental uncertainty.

Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1–3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500 °C in synthetic CaO–MgO–SiO2–CO2–H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1–3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate–carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.

Molecular hydrogen (H2) released during serpentinization of oceanic mantle is one of the main fuels for chemosynthetic-based deep life. Hydrogen is produced during the oxidation of ferrous to ferric iron, and the amount of H2 generated strongly depends on rock type, fluid composition, alteration temperature, and water-to-rock ratio.Progress has been made in understanding serpentinization and related H2 production at slow-spreading mid-ocean ridges (MORs). Less attention has been paid to the hydration of mantle rocks at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (200°C). To close this knowledge gap we investigated serpentinization and H2 production using drill core samples from the West Iberia margin (Ocean Drilling Program Leg 103, Hole 637A).Lherzolitic compositions indicate that the exhumed peridotites represent sub-continental lithospheric mantle. The rocks are strongly serpentinized and mainly consist of serpentine with little magnetite and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg+Fe) < 0.8, and shows distinct compositional trends towards a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios range from 0.6–0.92 and 0.58–0.8, respectively. Our data show that more than 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the initial serpentinization of the samples at temperatures of 2 per kg rock, which is 2–3 times more than previously estimated.These results lead us to suggest that the generation potential of H2 evolves from continental break-up to ultraslow and eventually slow MOR spreading. The observed metamorphic phase assemblages systematically vary between these different settings, which has consequences for H2 yields during serpentinization. At passive margins and ultraslow-spreading MORs, the main phase hosting Fe(III) appears to be serpentine, and H2 yields of 100–250 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of 2 per kg most of which is related to the formation of magnetite at >200°C. Within the same (low) temperature range, larger volumes of serpentinite should form at passive margins than at slow-spreading MORs, owing to lower geothermal gradients. Relative to both slow- and ultraslow-spreading MORs, serpentinization at passive margins likely produces more H2 and under conditions closer to/within the habitable zone. These sites may hence be suitable environments for hydrogenotrophic microbial life.

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe2O3 and the appearance of FeO2. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites. © 2021 American Physical Society. We acknowledge the Deutsches Elektronen-Synchrotron (DESY, PETRA III), the European Synchrotron Radiation Facility (ESRF), and the Advance Photon Source (APS) for provision of beamtime. N. D. and L. D. thank the Federal Ministry of Education and Research, Germany (BMBF, Grant No. 05K19WC1) and the Deutsche Forschungsgemeinschaft (DFG Projects No. DU 954–11/1, No. DU 393–9/2, and No. DU 393–13/1) for financial support. N. D. and I. A. A. thank the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). Electronic structure calculations were supported by the Russian Science Foundation (Project No. 18-12-00492). Theoretical analysis of chemical bonding was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” (No. K2-2019-001) implemented by a governmental decree, No. 211. Support from the Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant No. KAW-2018.0194), the Swedish Government Strategic Research Areas and SeRC, and the Swedish Research Council (VR) Grant No. 2019-05600 is gratefully acknowledged.

Mossbauer spectroscopy is an important tool in geoscience. Samples are often small or heterogeneous, requiring measurements with small beam size. Such measurements can be carried out in the laboratory using a radioactive source, or at a synchrotron facility in either time or energy domain. This chapter covers practical aspects of carrying out Mossbauer measurements with high spatial resolution in a tutorial style that covers radioactive source properties, how to choose between time and energy domain, practical aspects associated with preparing the sample and setting up a measurement, useful free software for data analysis and analytical data mapping, and concludes with a showcase of research questions in geoscience that have been addressed using Mossbauer spectroscopy with high spatial resolution.

co-auteur étranger; International audience; Iron speciation is linked to oxygen fugacity; hence the Fe3+/ΣFe ratio of glasses can reveal the oxygen fugacity of the system where they last equilibrated. Previous studies using X-ray Absorption Near Edge Structure (XANES) spectroscopy in silicate glasses and olivine-hosted melt inclusions have shown that arc basalts are more oxidized than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). However, Cottrell et al. (2018) recently demonstrated that hydrous glasses can experience beam-induced oxidation during XANES analysis, leading to an overestimation of their Fe3+/ΣFe ratios. Here, we determined Fe3+/ΣFe ratios in olivine-hosted melt inclusions from various arc, MORB and OIB localities by Mössbauer and XANES spectroscopy. We carried out a careful evaluation of beam damage during XANES measurements that included detailed analysis of consecutive XANES spectra and collection of time series on hydrous basaltic and basanitic glasses using different radiation doses. Fe3+/ΣFe ratios obtained using both spectroscopy methods on the same sample set allowed us to confirm the oxidation state of those magmas and the reliability of our approach to avoid the effect of beam-induced oxidation during XANES analysis. Our results show that melt inclusions from MORBs, OIBs and arc basalts display mean Fe3+/ΣFe ratios of 0.10 ± 0.05 (2σ; n = 5), 0.13 ± 0.05 (2σ; n = 2) and 0.25 ± 0.15 (2σ; n = 19), respectively, while Mount Etna melt inclusions display a mean Fe3+/ΣFe ratio of 0.26 ± 0.05 (2σ; n = 7). These results confirm that arc magmas are more oxidized than those from hot spots and mid-ocean ridges.

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Taenite, an iron-nickel alloy with 20-40 at% nickel, is a major constituent of iron meteorites that have been used to infer planetary core compositions. Many aspects of its magnetic properties are controversial, particularly near the Invar (“invariable”) composition around 36 at% nickel. Open questions include the conditions under which magnetism is lost, so to address this particular controversy, we undertook a combined magnetic remanence and Mössbauer study of synthetic taenite at high pressure. We synthesised polycrystalline iron-nickel alloy with 38 at% nickel, loaded the sample into a diamond anvil cell and collected Mössbauer spectra during decompression from 20 GPa. Our results show a clear loss of magnetism, but at pressures that differ considerably depending on the fitting model. The pressure obtained using the traditional approach involving a magnetic field distribution conflicts with results obtained from other methods, while a simple model based on magnetic field fluctuations gives results that are consistent with other data. Comparison of data from all methods provides insight that can be applied to planetary magnetism.

Earth’s inner core likely consists of Fe-Ni alloy(s) plus a minor fraction of light element(s) to match the density and sound wave velocities of seismological models such as the preliminary reference Earth model (PREM). Among possible alloying light elements (e.g., Si, O, H, S, C), silicon is a popular candidate based on its cosmochemical abundance and potential involvement in chemical reactions at the core-mantle boundary. Previous work has shown that the solubility of Si in hcp-(Fe,Ni) alloy increases the stability field of the bcc-phase at high pressure. Comparison of sound velocity and density data of Fe-Ni-Si alloys with geophysical observations and theoretical predictions provide important constraints on the structure and dynamics of Earth’s inner core. However, knowledge of the high-pressure and high-temperature behaviour and properties of Fe-Ni alloys that contain light elements is limited. We therefore investigated bcc-Fe0.78Ni0.07Si0.15 alloy to compare its sound velocity and density with ab initio calculations and PREM in order to clarify the role of Si as a light element in Earth’s inner core.Compressional velocities and densities of bcc-Fe0.78Ni0.07Si0.15 alloy have been measured using inelastic X-ray scattering (IXS) and powder X-ray diffraction at the SPring-8 synchrotron facility (BL35XU beamline). High pressure was generated using a BX90-type diamond anvil cell. The metal alloy sample was loaded together with Ne (pressure medium) in a Re sample chamber and was mechanically compressed to 75 GPa through steps of 10 GPa at room temperature. IXS data were acquired at each pressure point in the range of momentum transfer of 4.24 to 7.63 nm-1. To determine density, we collected X-ray diffraction patterns of the sample before acquisition of each IXS spectrum using a flat panel detector installed in the optical system. All IXS spectra were fitted using Lorentzian functions. The dispersion relationship between energy (E) and momentum transfer (Q) was obtained by fitting all data with the following equation:E (meV) = 4.192 x 10-4 vp (m/s) x Qmax (nm-1) x sin (π/2 x Q (nm-1)/ Qmax (nm-1),where vp is the sound velocity of the sample.Preliminary results for bcc-Fe0.78Ni0.07Si0.15 show that the energy of the longitudinal acoustic phonon increases with increasing pressure. Additionally, we found that vp follows Birch’s Law, i.e., there is a linear relationship between density and sound velocity. Based on the comparison of our results and those for hcp-Fe and Fe-Si alloys reported previously with PREM, we propose that bcc-Fe0.78Ni0.07Si0.15 alloy is a viable candidate as a component of Earth’s inner core.

Reactions involving carbon in the deep Earth have limited manifestations 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 10-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 has helped to identify gaps in our understanding that motivate and give direction to future studies. © 2020 Walter de Gruyter GmbH, Berlin/Boston 2020.

The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mg = (Feol/Fem)·(Mgm/Mgol), with all FeTexpressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgusing experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range offO2(iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H2O. Combining our new data with published experimental data having measured glass Fe3+/∑Fe, we show that for Fo65–98olivine in equilibrium with basaltic and basaltic andesite melts,$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgdecreases linearly with Fe3+/∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo50olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/∑Fe in olivine-bearing experimental melts, from whichfO2can be calculated. In contrast to$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mg, the Mn–Mg exchange coefficient,$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mg, is relatively constant over a wide range of P–T–fO2conditions. We present an expression for$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mgthat incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for$${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$KdFeT-Mgand$${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$KdMn-Mgto olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/∑Fe to a precision of ≤ 0.04.

Carbonates play an important role in the transport and storage of carbon in the Earth’s mantle. However, the abundance of carbon and carbonates in subduction zones is still an unknown quantity. To determine the most abundant accessory phases and how they influence the dynamical processes that operate within the Earth, investigations on the vibrational, elastic, and thermodynamic properties of these phases are crucial for interpreting seismological observations. Recently, the nuclear inelastic scattering (NIS) method has proved to be a useful tool to access information on the lattice dynamics, as well as to determine Debye sound velocities of Fe-bearing materials. Here we derive the acoustic velocities from two carbonate compositions in the FeCO3-MgCO3 binary system up to ~70 GPa using the NIS method. We conclude that more Mg-rich samples, in this case (Fe0.26Mg0.74)CO3, have ~19% higher sound velocities than the pure end-member Fe composition. In addition, we observed a significant velocity increase after the Fe2+ spin transition was complete. After laser heating of FeCO3 at lower mantle conditions, we observed a dramatic velocity drop, which is probably associated with thermal decomposition to another phase. Parallel to our NIS experiments, we conducted a single-crystal X-ray diffraction (SCXRD) study to derive the equation of states of FeCO3 and (Fe0.26Mg0.74)CO3. The combined information from NIS (i.e., Debye velocities) and SCXRD (i.e., densities and bulk moduli) experiments enabled us to derive the primary and shear wave velocities of our samples. Our results are consistent with results obtained by other methods in previous studies, including Brillouin spectroscopy, inelastic X-ray scattering, and DFT calculations, supporting NIS as a reliable alternative method for studying the elastic properties of Fe-bearing systems at high pressures and temperatures. Finally, we discuss the seismic detectability of carbonates. We determine that nearly 22 wt% CO2 must be present in the subduction slab to detect a 1% shear wave velocity decrease compared to non-carbonated lithologies at the transition zone to lower mantle boundary depths.

We report the stability and solubility of the FeAlO3 component in bridgmanite based on phase relations in the system MgSiO3-FeAlO3 at 27 GPa and 2000 K using a multi-anvil apparatus combined with in situ synchrotron X-ray diffraction measurements. The results demonstrate that the FeAlO3 component dominates Fe3+ and Al3+ substitution in bridgmanite, although trace amounts of oxygen- and Mg-site vacancy components are also present. Bridgmanite with more than 40 mol% FeAlO3 transforms into the LiNbO3-type phase upon decompression. The FeAlO3 end-member decomposes into corundum and hematite and does not form single-phase bridgmanite. We determined the maximum solubility of the FeAlO3 component in bridgmanite at 27 GPa and 2000 K to be 67 mol%, which is significantly higher than previously reported values (25–36 mol%). We determined the partial molar volume (27.9 mol/cm3) and bulk modulus (197 GPa) of hypothetical FeAlO3 bridgmanite, which are significantly higher and lower than those of AlAlO3 and FeSiO3 bridgmanite, respectively. The non-ideality of MgSiO3-FeAlO3 solid solution (W = 13 kJ/mol, where W is the interaction parameter) is significantly larger than that for MgSiO3-AlAlO3 (5 kJ/mol) and MgSiO3-FeSiO3 (3 kJ/mol) solid solutions. The rapid decrease in abundance of the MgAlO2.5 component in bridgmanite with increasing pressure is enhanced by the presence of the FeAlO3 component. The FeAlO3 content in pyrolite and mid-ocean ridge basalt is far below its solubility limit in bridgmanite and provides new insight into the mineralogy of the lower mantle.

Minerals 10(6), 542 (2020). doi:10.3390/min10060542, It is widely accepted that the lower mantle consists of mainly three major minerals—ferropericlase, bridgmanite and calcium silicate perovskite. Ferropericlase ((Mg,Fe)O) is the second most abundant of the three, comprising approximately 16–20 wt% of the lower mantle. The stability of ferropericlase at conditions of the lowermost mantle has been highly investigated, with controversial results. Amongst other reasons, the experimental conditions during laser heating (such as duration and achieved temperature) have been suggested as a possible explanation for the discrepancy. In this study, we investigate the effect of pulsed laser heating on the stability of ferropericlase, with a geochemically relevant composition of Mg$_{0.76}$Fe$_{0.24}$O (Fp24) at pressure conditions corresponding to the upper part of the lower mantle and at a wide temperature range. We report on the decomposition of Fp24 with the formation of a high-pressure (Mg,Fe)$_3$O$_4$ phase with CaTi$_2$O$_4$-type structure, as well as the dissociation of Fp24 into Fe-rich and Mg-rich phases induced by pulsed laser heating. Our results provide further arguments that the chemical composition of the lower mantle is more complex than initially thought, and that the compositional inhomogeneity is not only a characteristic of the lowermost part, but includes depths as shallow as below the transition zone., Published by MDPI, Basel