Bridgmanite, with (Mg,Fe)SiO3 as the major component, is the most abundant mineral on Earth and is stable under lower mantle conditions. When incorporating trivalent cations (Al3+ and Fe3+), bridgmanite can form various components such as the oxygen vacancy ((Mg,Fe2+)(Al,Fe3+)O2.5) and A-site vacancy ((Al,Fe3+)2/3□1/3)SiO3, □ = vacancy) components in addition to the charge-coupled (Fe2O3, Al2O3, FeAlO3) components. These vacancy-bearing components potentially allow the storage of volatiles in the lower mantle and the modification of the viscosity and compressibility of the lower mantle. However, our understanding of these defect components has been limited due to the previous focus on relatively simple systems. This study determined the compositions of bridgmanite and its associated phases in the MgO–SiO2–Al2O3–Fe2O3–FeO system as a function of temperature (1700–2300 K) at a fixed pressure of 27 GPa, relevant for uppermost lower mantle conditions. Our results suggest the presence of oxygen vacancies in bridgmanite, implying that the Earth’s lower mantle may store water and noble gases. Furthermore, our results agree with geophysical observations indicating slab stagnation in the deep mantle.

Antimony is a priority pollutant, whose mobility in redox-dynamic environments may be controlled by interactions with Fe(III) hydroxide minerals that form via Fe(II) oxidation. In this study, we examined the Fe(III) hydroxide precipitates and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V) under neutral pH conditions. To achieve this aim, oxidation experiments were carried out in O2-saturated, Fe(II)-bearing solutions (buffered at pH 7) over a range of environmentally relevant Sb(V) concentrations (equivalent to Sb(V):Fe(II) molar ratios of 0, 0.01, 0.04, 0.1 and 0.25). Under these experimental conditions, Fe(II) oxidation occurred rapidly (within 20 minutes) causing associated removal of Sb(V) from solution via coprecipitation with the resulting Fe(III) hydroxides. At low Sb(V):Fe(II) ratios (< 0.1), lepidocrocite was the only Fe(III) mineral product of Fe(II) oxidation, whereas higher ratios resulted in formation of feroxyhyte. Both lepidocrocite and feroxyhyte retained Sb(V) within their crystal structure via Sb(V)-for-Fe(III) substitution. This mechanism of Sb(V) retention largely protected the solid-phase Sb(V) from release processes. Collectively, these results highlight the coupled role that interactions between Sb geochemistry and the Earth’s near-surface Fe cycle can play in controlling both Fe(III) hydroxide mineralogy and Sb mobility.

Grospydites are a rare variety of eclogites composed of grossular–pyroxene – disthene (kyanite) occurring only in a few kimberlite pipes worldwide. The pressures, temperatures and redox conditions of formation for grospydites have been poorly investigated, even though such parameters are needed to better understand their origin and the scarcity of elemental carbon (graphite/diamond), a common feature of these rocks. In this study, we determined the chemical composition and Fe3+/∑Fe of coexisting garnet and clinopyroxene from nine C-free kyanite-bearing eclogite xenoliths from Zagadochnaya pipe (Yakutia). Five samples show garnet with Ca# >45 mol

Bridgmanite, a magnesium-rich silicate perovskite, is the most prevalent mineral in Earth's lower mantle and contains substantial quantities of ferric (oxidized) iron, even in equilibrium with iron metal. Mixing of oxygen-rich material from the lower mantle could have raised the oxidation state of the upper mantle to its present level after the more reducing conditions during core formation. However, it remains unclear how the lower-mantle oxygen content was established to achieve this level. Here we use high-pressure and temperature multi-anvil experiments at known oxygen fugacities to show that the bridgmanite ferric iron content is independent of pressure but decreases with temperature. Using these data, we build a thermodynamic model to calculate the ferric iron content of the lower mantle as bridgmanite crystallized from a reduced magma ocean in the early Earth. We determine that this ferric iron content would have been sufficient to explain the current upper mantle's ferric iron content after whole mantle mixing.

The catalytic effect of aqueous Fe(II) (Fe2+aq) on the transformation of Fe(oxyhydr)oxides has been extensively studied in the laboratory. It involves the transfer of electrons between Fe2+aq and Fe-(oxyhydr)oxides, rapid atomic exchange of Fe between the two states, and recrystallization of the Fe-oxides into more stable Fe-(oxyhydr)oxides. The potential occurrence of these reactions in natural soils and sediments can have an important impact on biogeochemical cycling of iron, carbon, and phosphorus. We investigated the possible isotopic exchange between Fe2+aq and sedimentary Fe(III) in Fe-Si-C-rich lake sediments. 57Fe Mössbauer spectroscopy was used to evaluate Fe mineral speciation in unaltered lake sediments. Unaltered and oxidized sediment laboratory incubations were coupled with a classical kinetic approach that allows a quantitative description of the reactivity of assemblages of Fe-(oxyhydr)oxides found in sediments. Specifically, unaltered and oxidized sediment samples were separately incubated with an 55Fe2+aq-enriched solution and exchange was observed between 55Fe2+aq and sedimentary Fe(III), highest in the top of the sediment and decreasing with depth with the 55Fe2+aq tracer distributed within the bulk of the sedimentary Fe(III) phase. Our results indicate that atomic exchange between Fe2+aq and sedimentary Fe(III) occurs in natural sediments with electrons transferred from the Fe(III)-particle to Fe(III)-particle via Fe2+aq intermediates.

Two large low shear velocity provinces (LLSVPs) near the core-mantle boundary beneath the Pacific Ocean and Africa were discovered about 40 years ago and are characterized by tomographic images as reductions of shear and compression wave velocities by up to 3-4% and 1%, respectively, over an area of several thousand kilometers and a height of more than a thousand kilometers. It was recently proposed that the Fe3+ enrichment in bridgmanite could reduce its shear wave velocity and thus account for the formation of LLSVPs (Wang et al., 2021). However, the viscosity of the Fe3+-enriched bridgmanite is unknown, while it is critical for the long-term stability of LLSVPs at the base of the lower mantle against mantle convection. Considering the operation of diffusion creep in the lower mantle, viscosity will be positively correlated with grain size. Therefore, we measured the grain growth rate of bridgmanite under lower mantle conditions as a function of Fe3+ content. The experimental results show a significant enhancement of grain growth by Fe3+ incorporation. Thus, a larger grain size of bridgmanite is expected in the Fe3+-enriched LLSVPs than in the Fe3+-poor surrounding mantle, leading to highly viscous LLSVPs. As a result, they are stabilized at the base of the lower mantle over geological time.

The redox state of the Earth’s interior (i.e., the oxygen fugacity, fo2) is related to the Fe speciation (Fe2+, Fe3+) in mantle rock-forming minerals and controls the speciation of volatiles like carbon at depth. To date, the fo2 of the lower mantle has been mostly constrained by HP-T experiments, due to the extreme rarity of natural samples represented by mineral inclusions in sub-lithospheric diamonds. Experimental evidence suggests that the lower mantle is reduced and saturated in Fe(-Ni) metal (about 1 wt%). However, coexisting minerals like ferropericlase and bridgmanite are predicted to contain 0.02 and 0.6 of Fe3+ /∑Fe, respectively. A slight increase of Fe3+ /∑Fe (less than 1 wt%) is expected in the case of fo2 > iron-wüstite buffer. This would imply the complete oxidation of Fe(Ni) alloys promoted by reduction of carbonates (either fluids or melts). The finding of carbonates trapped in sub-lithospheric diamonds is natural evidence of the (local) oxidative redox state of the deep and inaccessible lower mantle and this is enhanced by the lack of metallic inclusions coexisting with Fe3+-poor ferropericlase in sublithospheric diamonds. Moreover, the variation of Fe3+ in bridgmanite appears, at least currently, to be better explained by its crystal chemistry while the effect of pressure and fo2 remains unclear, mainly due to the lack of oxybarometers applicable to lower mantle assemblages.In this study, we combined an experimental investigation of the Fe3+/∑Fe in ferropericlase and bridgmanite equilibrated at known high pressure, temperature and oxygen fugacity conditions with Fe3+ /∑Fe measurements conducted on bridgmanite(-like) and ferropericlase inclusions in sublithospheric diamonds from Rio Sorriso and São Luís (Brazil) and Kankan (Guinea). Some inclusions are composite for which the Fe3+/∑Fe was determined by in situ synchrotron Mössbauer source spectroscopy and the bulk Fe3+ /∑Fe determined.Our preliminary results show a discrepancy between natural inclusions and experimental products in terms of i) modal abundance of ferropericlase and bridgmanite, likely related to their diverse role in diamond formation (redox) processes; ii) chemical compositions expected for both peridotitic and metabasaltic parageneses; and iii) Fe3+ /∑Fe content.

The role that subducted carbonates play in sourcing and storing carbon in the deep Earth's interior is uncertain, primarily due to poor constraints on the stability of carbonate minerals when interacting with mantle phases. Magnesite (MgCO3) is the most prominent carbonate phase to be present at all mantle pressure-temperature conditions. In this study, we combined multi-anvil apparatus and laser-heated diamond anvil cell experiments to investigate the stability of magnesite in contact with iron-bearing bridgmanite. We examined the presence of melt, decarbonation, and diamond formation at shallow to mid-lower mantle conditions (25 to 68 GPa; 1350 to 2000 K). Our main observation indicates that magnesite is not stable at shallow lower mantle conditions. At 25 GPa and under oxidizing conditions, melting of magnesite is observed in multi-anvil experiments at temperatures corresponding to all geotherms except the coldest ones. Whereas, at higher pressures and under reducing conditions, in our laser-heated diamond-anvil cell experiments, diamond nucleation is observed as a sub-solidus process even at temperatures relevant to the coldest slab geotherms. Our results indicate that magnesite was reduced and formed diamonds when in contact with the ambient peridotite mantle at depths corresponding to the shallowest lower mantle (33 GPa). Thus, we establish that solid magnesite decomposes at depths of similar to 700 km as it contacts the ambient mantle. Consequently, the recycling of carbonates will hinder their transport deeper into the lower mantle.

The densification mechanisms of silicate melts under high pressure are of key interest in understanding the evolution of the early Earth and its present-day internal structure. Here, we report Brillouin spectroscopy-derived transverse acoustic wave velocities VS $\left({V}_{S}\right)$ from a basaltic glass at high pressures up to 163 GPa and ambient temperature to provide insight into pressure-induced changes in its elasticity and, by extension, its density. We find that the pressure dependence of VS ${V}_{S}$ below 110-140 GPa follows a trend nearly tantamount to those of pyrolite and Fe- and (Fe,Al)-bearing MgSiO3 glasses, indicating that the large compositional differences among these glasses do not exert variable acoustic wave velocity trends. However, at higher pressures we observe a small departure from the VS ${V}_{S}$ profiles of the Al-poor compositions toward higher acoustic wave velocities to eventually become stiffer. This pressure-induced steepening in VS ${V}_{S}$ is comparable to that of (Mg, Fe, Al)(Si, Al)O3 glass, and suggests a possible structural change toward a denser state caused by more rapidly changing Al-O coordination in network-forming Al. Coupled with the high Fe content in basalt, this may render basaltic melt denser than surrounding minerals in the deep lower mantle, and may provide an additional mechanism for the existence of ultralow-velocity zones. Silicate melts are subject to different densification mechanisms from the counterpart solids. Although less dense at ambient/low pressures and for most of the mantle, it has long been speculated that at high enough pressures a density inversion may take place, where melts may become denser than the corresponding solids. This would have profound implications for the present-day structure and dynamics of the Earth and its evolution through geological history. We have experimentally measured the sound wave velocities of a basaltic glass, which serves as a laboratory analogue to melt, up to 163 GPa. Through comparison of our sound wave velocity-pressure profile to other glass compositions we observe a slight deviation in steepness above 110-140 GPa to a potentially stiffer and denser state. This pressure-induced change may be the result of changes in the Al-O coordination environment. Extrapolated onto expected melt compositions in the lower mantle, which are enriched in both Al and heavy Fe, this may render dense pools of silicate melt gravitationally stable at the bottom of the mantle. We have carried out acoustic wave velocity measurements of a basaltic glass up to 163 GPa The VS-pressure profile of basalt shows close similarities to other, more depolymerized glasses up to 110-140 GPa, where it becomes steeper This anomaly is likely induced by changes in Al-O coordination

Abstract Fe1-x O, although chemically simple, possesses a complex structural and magnetic phase diagram. The crystal structures of Fe1-x O and its magnetic properties at extreme conditions are still a matter of debate. Here, we performed a systematic investigation on Fe0.94O up to 94 GPa and 1700 K using synchrotron X-ray diffraction and synchrotron Mössbauer source spectroscopy. We observe a transition of Fe0.94O to the monoclinic phases above 40 GPa and at high temperatures and use the group theory analysis of the observed phases to discuss their properties and their relation to the ambient pressure phases. The Mössbauer spectra of the rhombohedral and the room temperature monoclinic phase contain a component attributed to Fe2.5+, caused by the electron exchange between the Fe3+ defect and neighboring Fe2+ atoms. Our results present a structural and magnetic transitional pressure-temperature diagram of Fe1-x O and show the complex physicochemical properties of simple Fe1-x O binary oxide under extreme conditions.

Trivalent cations such as Al3+ and Fe3+ can be incorporated into the crystal structure of bridgmanite either by the charge-coupled mechanism forming the FeFeO3, AlAlO3, and FeAlO3 components or by the oxygen vacancy mechanism forming the MgAlO2.5 and MgFeO2.5 components. They may affect the physical properties of bridgmanite and thus affect lower mantle dynamics. In this study, we investigated the effects of Al and Fe3+ on the grain growth kinetics of bridgmanite at a pressure of 27 GPa and temperatures of 2000 - 2300 K by multi anvil experiments. The experimental results indicate that the FeFeO3, AlAlO3, and FeAlO3 components enhance the growth rate of bridgmanite, while the MgAlO2.5 and MgFeO2.5 components have negligible effects. However, due to the relatively low Fe3+ and Al3+ contents of bridgmanite in the lower mantle, none of them affect the lower mantle rheology significantly. In particular, the mid-mantle viscosity jump interpreted from geoid analysis is unlikely to be caused by the decreasing of MgAlO2.5 and MgFeO2.5 concentrations with increasing depth.

Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between -6 (Kasai, Congo and Udachnaya, Siberia) and -0.1 (Udachnaya, Siberia) log units (relative to the fayalite-magnetite-quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/& sum;Fe values range between 0.03 and 0.19 for garnet and 0.18-0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/& sum;Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of +/- 0.02 and +/- 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0-5.2 (+/- 0.4) GPa and 720-1050 (+/- 60) degrees C for both high-MgO and low-MgO eclogites, slightly lower than previous P-T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, triangle logfo(2) (FMQ) calculated using the available oxythermobarometric model varies from -1.7 to -0.6 log units for high-MgO eclogites and from -2.9 to 0.9 log units for low-MgO eclogites. Samples recording triangle logfo(2) (FMQ) <= -1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of -1.2 (+/- 0.4) log units for high-MgO and -0.6 (+/- 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/& sum;Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

Mossbauer spectra of polycrystalline e-57FeOOH were measured up to 14.6 GPa at room temperature using the energy-domain synchrotron 57Fe Mossbauer source at BL11XU in SPring-8. Based on measurements with=without an external magnetic field, e-FeOOH appears to be antiferromagnetic at ambient pressure and temperature. On compression, the quadrupole shift 2e shifted from negative to positive at -8 GPa, which is consistent with a magnetic spin rotation. Previous XRD studies did not detect any transitions at this pressure; thus we infer the change in 2e to be the third pressure-induced transition without major structural change in addition to the hydrogen-bond symmetrization and electronic spin transition.

The role of slab-derived hydrous silicate melts in the transfer of the oxidised signature in subduction zones remains poorly constrained. We have investigated the mobility and redox state of iron in hydrous silicate melts by carrying out solubility measurements of hematite-magnetite assemblages in a piston-cylinder apparatus combined with electron microprobe and Mossbauer analysis of the recovered glasses. The experiments were performed at subcritical conditions, i.e. two-fluid phases coexisting with the solid assemblage. We observe concentrations of total FeO as high as 1.85 +/- 0.18 wt. % (2.07 +/- 0.41 wt. % in saline systems) at 2 GPa and 900 degrees C, with Fe3+/Fe-tot ratios of 0.79 +/- 0.04 (0.45 +/- 0.07) that indicate the dominance of oxidised iron in the melt phase. Combined with thermodynamic modelling to reconstruct the composition and speciation of the coexisting fluid phase, we demonstrate that hydrous silicate melts can transport 20 times more dissolved iron, preferentially as oxidised iron, than aqueous fluids at sub-arc conditions. Our results support the efficient dissolution of 'fluid-insoluble' iron oxides in slab-melts, which are thus efficient agents for the transfer of oxidised iron to the mantle wedge, ultimately contributing to the oxidation of the arc magma source.

Grain-scale pore geometry primarily controls the fluid distribution in rocks, affecting material transport and geophysical response. The dihedral angle (theta) in the olivine-fluid system is a key parameter determining pore fluid geometry in mantle wedges. In the system, curved and faceted olivine-fluid interfaces define theta, resulting in faceted-faceted (FF), faceted-curved (FC), and curved-curved (CC) angles. The efect of faceting on theta under various pressure and temperature (P-T) conditions and fluid compositions, however, has not been constrained, and mineralogical understanding remains unresolved. This study evaluated facet-bearing theta and their proportions in olivine-multicomponent aqueous fluid systems. Our results show that 1/3 of olivine-fluid theta are facet-bearing angles, regardless of the P-T conditions and fluid composition. Faceting produces larger dihedral angles than CC angles. The grain boundary plane (GBP) distribution reveals that the GBPs of faceted interfaces at triple junctions have low Miller index faces ({100}, {010}, and {101}). The misorientation angle/axis distributions of adjacent grain pairs are in accord with a theoretical distribution of random olivine aggregate. Moreover, the calculation of the FF angles for adjacent grain pairs with low Miller index GBPs reproduces measured angle values based on the olivine crystal habit. Therefore, our study suggests that the FF angle is strongly afected by olivine crystallography. The presence of faceting increases theta and a critical fluid fraction (phi(c)) for percolation, lowering permeability. In the mantle wedge, where olivine crystallographic preferred orientation (CPO) is expected owing to corner flow, increasing the FF angle proportion with associated changes in fluid pore morphology will lead to permeability anisotropy, and controlling the direction of the fluid flow, and it will result in geophysical anomalies such as seismic wave attenuation and high electrical conductivity.

The melt viscosity (η) of anhydrous and hydrous peridotite was investigated using a multipronged approach combining micropenetration viscometry, conventional DSC, flash DSC and Brillouin spectroscopy. Raman spectroscopy measurements were used to verify the absence of crystallization and/or degassing during high-temperature measurements of these extremely reactive glasses and melts, ensuring that the data corresponded to the crystal-free melt viscosity. Based on the experimental data, an accurate description of the viscosity of peridotite melts over thirteen orders of magnitude (from Tg to η ≈ 10−1 Pa s) is provided in a broad range of compositions and oxidation states. Since empirical models predict viscosities that can significantly deviate from measured data to varying degrees, a new model was developed for the temperature- and H2O-dependent viscosity of peridotite melts (up to 12 mol% H2O content).

We have carried out in situ high‐pressure acoustic velocity measurements of (Fe 2+ , Al)‐bearing MgSiO 3 glass up to pressures of 155 GPa, which confirmed a distinct pressure‐induced trend change in the transverse acoustic velocity ( V S ) profile around 98 GPa, likely caused by the Si‐O coordination number (CN) change from 6 to 6 + . Although it has been reported that the substitution of Fe 2+ in MgSiO 3 glass induces almost linear velocity reduction up to ∼160 GPa, we revealed that the V S profile of (Fe 2+ , Al)‐bearing MgSiO 3 becomes anomalously steeper above ∼100 GPa and eventually came to be equivalent to MgSiO 3 glass above ∼125 GPa. This implies the incorporation of Al into Fe‐bearing MgSiO 3 glass significantly facilitates making it far elastically stiffer and thus the densification under pressures well within the Earth's lower mantle. Our results indicate the possible presence of stiff and highly dense silicate melts in deep MOs in the rocky terrestrial planets.