Iron oxyhydroxide as water carrier to the deep mantle

Carbonates play a key role in the chemistry and dynamics of our planet. They are directly connected to the CO2 budget of our atmosphere and have a great impact on the deep carbon cycle. Moreover, recent studies have shown that carbonates are stable along the geothermal gradient down to Earth's lower mantle conditions, changing their crystal structure and related properties. Subducted carbonates may also react with silicates to form new phases. These reactions will redistribute elements, such as calcium (Ca), magnesium (Mg), iron (Fe) and carbon in the form of carbon dioxide (CO2), but also trace elements, that are carried by the carbonates. The trace elements of most interest are strontium (Sr) and rare earth elements (REE) which have been found to be important constituents in the composition of the primitive lower mantle and in mineral inclusions found in super-deep diamonds. However, the stability of carbonates in presence of mantle silicates at relevant temperatures is far from being well understood. Related to this, very little is known about distribution processes of trace elements between carbonates and mantle silicates. To shed light on these processes, we studied reactions between Sr- and REE-containing CaCO3 and Mg/Fe-bearing silicates of the system (Mg,Fe)2SiO4 - (Mg,Fe)SiO3 at high pressure and high temperature using synchrotron radiation based μ-X-ray diffraction (μ-XRD) and μ-X-ray fluorescence (μ-XRF) with μm-resolution in a laser-heated diamond anvil cell. X-ray diffraction is used to derive the structural changes of the phase reactions whereas X-ray fluorescence gives information on the chemical changes in the sample. In-situ experiments at high pressure and high temperature were performed at beamline P02.2 at PETRA III (Hamburg, Germany) and at beamline ID27 at ESRF (Grenoble, France). In addition to μ-XRD and μ-XRF, ex-situ measurements were made on the recovered sample material using transmission electron microscopy (TEM) and provided further insights into the reaction kinetics of carbonate-silicate reactions. Our investigations show that CaCO3 is unstable in presence of mantle silicates above 1700 K and a reaction takes place in which magnesite plus CaSiO3-perovskite are formed. In addition, we observed that a high content of iron in the carbonate-silicate system favours dolomite formation during the reaction. The subduction of natural carbonates with significant amounts of Sr leads to a comprehensive investigation of the stability not only of CaCO3 phases in contact with mantle silicates but also of SrCO3 (and of Sr-bearing CaCO3). We found that SrCO3 reacts with (Mg,Fe)SiO3-perovskite to form magnesite and gained evidence for the formation of SrSiO3-perovskite. To complement our study on the stability of SrCO3 at conditions of the Earth's lower mantle, we performed powder X-ray diffraction and single crystal X-ray diffraction experiments at ambient temperature and up to 49 GPa. We observed a transformation from SrCO3-I into a new high-pressure phase SrCO3-II at around 26 GPa with Pmmn crystal structure and a bulk modulus of 103(10) GPa. This information is essential to fully understand the phase behaviour and stability of carbonates in the Earth's lower mantle and to elucidate the possibility of introducing Sr into mantle silicates by carbonate-silicate reactions. Simultaneous recording of μ-XRD and μ-XRF in the μm-range over the heated areas provides spatial information not only about phase reactions but also on the elemental redistribution during the reactions. A comparison of the spatial intensity distribution of the XRF signal before and after heating indicates a change in the elemental distribution of Sr and an increase in Sr-concentration was found around the newly formed SrSiO3-perovskite. With the help of additional TEM analyses on the quenched sample material the elemental redistribution was studied at a sub-micrometer scale. Contrary to expectations from combined μ-XRD and μ-XRF measurements, we found that La and Eu were not incorporated into the silicate phases, instead they tend to form either isolated oxide phases (e.g. Eu2O3, La2O3) or hydroxyl-bastnasite (La(CO3)(OH)). In addition, we observed the transformation from (Mg,Fe)SiO3-perovskite to low-pressure clinoenstatite during pressure release. The monoclinic structure (P21/c) of this phase allows the incorporation of Ca as shown by additional EDX analyses and, to a minor extent, Sr too. Based on our experiments, we can conclude that a detection of the trace elements in-situ at high pressure and high temperature remains challenging. However, our first findings imply that silicates may incorporate the trace elements provided by the carbonates and indicate that carbonates may have a major effect on the trace element contents of mantle phases.

Superdeep diamonds and their syngenetic inclusions are crucial for understanding Earth’s deep carbon cycle and slab–mantle redox dynamics. The origins of these diamonds, especially their links to iron (Fe) carbides and ferropericlase with varying Mg# [=Mg/(Mg+Fe)at], however, remain elusive. In this study, we performed high pressure–temperature (P-T) experiments (10–16 GPa and 1200–1700 K) across cold-to-warm subduction zones using a multi-anvil press. The results reveal a stepwise Fe-mediated carbonate reduction process for the formation of superdeep diamonds: MgCO3 → Fe-carbides (Fe3C/Fe7C3) → graphite/diamond. This mechanism explains two phenomena regarding superdeep diamonds: (1) anomalous 13C depletion results from kinetic isotope fractionation during 12C enrichment into the intermediate Fe-carbides; (2) nitrogen scarcity is due to Fe-carbides acting as nitrogen sinks. Ferropericlase [(Mg,Fe)O] formed during the reactions in our experiments shows Mg# variations (0.2–0.9), similar to those found in natural samples. High Mg# (>0.7) variants from lower temperature experiments indicate diamond crystallization from carbonatitic melts in the shallow lower mantle, while the broad Mg# range (0.2–0.9) from experiments at higher temperatures suggests multi-depth formation processes as found in Brazilian diamonds. These findings suggest that slab–mantle interactions produce superdeep diamonds with distinctive Fe-carbides and ferropericlase assemblages as inclusions, coupled with their 13C- and nitrogen-depleted signatures, which underscore thermochemical carbon cycling as a key factor in deep carbon storage and mantle mineralogy.

A simple four‐parameter model for calculating thermoelastic properties of MgSiO3 perovskite is presented based on the Vinet model for static lattice and the Debye approximation for lattice vibration. The input parameters are the volume of the unit cell, V0, the bulk modulus, K0, its pressure derivative, K0′, and the Debye temperature, Θ0, in the static lattice at zero pressure. For V0, K0 and K0′ the theoretical values by Stixrude and Cohen [1993] are used and Θ0 is determined to reproduce the experimental value at ambient conditions, 980 K, by Akaogi and Ito [1993]. The resulting isobars are in good agreement with experimental data to 1300 K and 11 GPa by Wang et al. [1994], with those to 1200 K at 20 GPa by Utsumi et al. [1995], and with those to 1500 K and to 2000 K, respectively, by Kato et al. [1995] and Funawori et al. [1996] both at 25 GPa. Using the present equation of state together with the method for calculating adiabatic Lamé constants λS and μS for isotropic medium given in the present paper, density ρ, and sound velocities νp and νs of MgSiO3 perovskite under lower mantle conditions have been calculated where the constant‐entropy model is assumed with the temperature at the core‐mantle boundary being taken to be 3000 K. The results for ρ, νp, and νs are in agreement with the preliminary reference Earth model (PREM) within −2.4%∼ −3.7%, +3.3%∼+1.1%, and +0.8%∼−6.8%, respectively, over the lower mantle from 670 to 2891 km in depth. The calculated thermal expansivity under lower mantle conditions is in good agreement with that of the lower mantle estimated by Anderson [1982]. Using the present model with the parameters determined from experimental data at room temperature by Knittle and Jeanloz [1987], assuming Θ0 to be the same as that of MgSiO3 perovskite, thermoelastic properties of (Mg0.9, Fe0.1)SiO3 perovskite under lower mantle conditions have been calculated. The density becomes in much better agreement (+0.4%∼−0.8 %) with PREM and νp and νs remain almost unchanged from those of MgSiO3 perovskite.

Water, in trace amounts, can greatly alter chemical and physical properties of mantle minerals and exert primary control on Earth’s dynamics. Quantifying how water is retained and distributed in Earth’s deep interior is essential to our understanding of Earth’s origin and evolution. While directly sampling Earth’s deep interior remains challenging, the experimental technique using laser-heated diamond anvil cell (LH-DAC) is likely the only method available to synthesize and recover analog specimens throughout Earth’s lower mantle conditions. The recovered samples, however, are typically of micron sizes and require high spatial resolution to analyze their water abundance. Here we use nano-scale secondary ion mass spectrometry (NanoSIMS) to characterize water content in bridgmanite, the most abundant mineral in Earth’s lower mantle. We have established two working standards of natural orthopyroxene that are likely suitable for calibrating water concentration in bridgmanite, i.e., A119(H2O) = 99 ± 13 μg/g (1SD) and A158(H2O) = 293 ± 23 μg/g (1SD). We find that matrix effect among orthopyroxene, olivine, and glass is less than 10%, while that between orthopyroxene and clinopyroxene can be up to 20%. Using our calibration, a bridgmanite synthesized by LH-DAC at 33 ± 1 GPa and 3,690 ± 120 K is measured to contain 1,099 ± 14 μg/g water, with partition coefficient of water between bridgmanite and silicate melt ∼0.025, providing the first measurement at such condition. Applying the unique analytical capability of NanoSIMS to minute samples recovered from LH-DAC opens a new window to probe water and other volatiles in Earth’s deep mantle.

The regularities of the joint genesis of diamonds and their paragenetic inclusions under lower mantle conditions are controlled by the melting relations at the liquidus of the multicomponent diamond-forming system. The boundary compositions of this system are evident from the generalized data on the analytical mineralogy of paragenetic inclusions in lower-mantle diamonds. The structure of the liquidus of the diamond-forming system was studied in a physicochemical experiment for P–T parameters typical of the depths of 670–800 km. The compositions of parental melts/solutions for diamonds and paragenetic inclusions correspond to the multicomponent system MgO–FeO–СаО–SiO2–MgCO3–FeCO3–CaCO3–Na2CO3–C. Its primary melting is controlled by the peritectic phase relations at the solidus, which was shown as a result of experimental studies of polythermal sections of the system during a study of their phase diagrams. Of key importance is the effect of the “stishovite paradox,” i.e., the peritectic reaction between the ultrabasic bridgmanite phase and melt with the formation of basic oxide associations of periclase–wustite solid solutions and stishovite. The peritectic reaction of bridgmanite is a fundamental feature of the diamond-forming system and determines the major peculiarity in its liquidus structure. Structure of the peritectic liquidus provides the physicochemical basis for the evolution of growth melts of diamonds and their paragenetic minerals. Based on the experimental data, we have constructed a fractional diagram of syngenesis of diamonds and inclusions clearly illustrating the solution–melt mechanism of diamond genesis and sequence of growth trapping of primary inclusions by diamonds under the lower-mantle conditions. The physicochemical factors of the genesis of diamonds and primary inclusions are agreed and generalized in a compositional diagram of lower-mantle diamond-forming media, and they provide a natural basis for the genetic classification of inclusions of rock-forming and accessory minerals in diamonds

High-temperature equation of state of CaSiO 3 perovskite and its implications for the lower mantle

The deep carbon cycle and carbon reservoir in the Earth’s lower mantle are of great interest in high-pressure mineral physics since they are thought to have important contributions to the dynamical and chemical evolution of the solid Earth. A high-pressure silicate-carbonate, CaSiC2O7, has recently been discovered, but its stability in reactions with surrounding mantle materials remains unclear. To investigate this, we perform ab initio density functional calculations assuming several reactions with deep mantle mineral components. The calculated results show that this new class of carbon-bearing silicate compound is stabilized by the reactions of CaCO3 and MgCO3 with SiO2, CaSiO3 perovskite, and MgSiO3 bridgmanite at different pressures and inversely inhibits the stabilization of CaCO3, MgCO3, and CaC2O5 above 60 GPa. These results suggest that CaSiC2O7 represents an important host for carbon in the Earth’s deep mantle and influences the form in which carbon is stored and transported under lower mantle conditions.

The presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the Earth's lower mantle in what is generally known as “hydrolytic weakening”. This weakening will have profound effects on global convection, but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative. In order to constrain the likelihood of hydrolytic weakening being important in the lower mantle, we use first principles methods to calculate the partitioning of water (strictly protons) between mineral phases of the lower mantle under lower mantle conditions. We show that throughout the lower mantle water is primarily found either in the minor Ca-perovskite phase or in bridgmanite as an Al3+–H+ pair. Ferropericlase remains dry. However, neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small. We find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents (>1000 ppm). We conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle.

Our understanding of the deep carbon cycle has witnessed amazing advances in the last decade, including the discovery of tetrahedrally coordinated high pressure (P) carbonate phases. However, little is known on the properties of molten carbonates at depth, while their properties at lower mantle conditions are unknown. Here, we report the structure and density of FeCO3 melts and glasses from 44 GPa to 110 GPa by means of in situ x-ray synchrotron diffraction, and ex situ Raman and x-ray Raman spectroscopies. Carbon is fully transformed to 4-fold coordination, a bond change recoverable at ambient P. While low P melts react with silica, resulting in the formation of silico-carbonate glasses, high P melts are not contaminated but still quench as glasses. Carbonate melts are therefore polymerized, highly viscous and poorly reacting with silicates in the lower mantle, in stark opposition with their low P properties.

The Earth's deep interior is a hidden water reservoir on a par with the hydrosphere that is crucial for keeping the Earth as a habitable planet. In particular, nominally anhydrous minerals (NAMs) in the silicate Earth host a significant amount of water by accommodating H point defects in their crystal lattices. Water distribution in the silicate Earth is highly heterogeneous, and the mantle transition zone may contain more water than the upper and lower mantles. Plate subduction transports surface water to various depths, with a series of hydrous minerals and NAMs serving as water carriers. Dehydration of the subducting slab produces liquid phases such as aqueous solutions and hydrous melts as a metasomatic agent of the mantle. Partial melting of the metasomatic mantle domains sparks off arc volcanism, which, along with the volcanism at mid-ocean ridges and hotspots, returns water to the surface and completes the deep water cycle. There appears to have been a steady balance between hydration and dehydration of the mantle at least since the Phanerozoic. Earth's water probably originates from a primordial portion that survived the Moon-forming giant impact, with later delivery by asteroids and comets. Water could play a critical role in initiating plate tectonics. In the modern Earth, the storage and cycling of water profoundly modulates a variety of properties and processes of the Earth's interior, with impacts on surface environments. Notable examples include the hydrolytic weakening effect on mantle convection and plate motion, influences on phase transitions (on the solidus of mantle peridotite in particular) and dehydration embrittlement triggering intermediate- to deep-focus earthquakes. Water can reduce seismic velocity and enhance electrical conductivity, providing remote sensing methods for water distribution in the Earth's interior. Many unresolved issues around the deep water cycle require an integrated approach and concerted efforts from multiple disciplines.

Al2O3 is estimated to total 4 to 5 mol% in all mantle compositional models, and is believed to be incorporated into (Mg,Fe)SiO3‐perovskite at lower mantle conditions. Using synchrotron X‐ray diffraction, we have measured the 300 K equation of state of a perovskite with XAl=Al/(Al+Mg+Si)=.077, up to 32 GPa. A least squares refinement of two independent data sets yields the following equation of state parameters V0=163.52(3) ų, K0=229(4) GPa, and K0=2.5(4). These values allow us to assess the most probable substitution mechanism for aluminium into perovskite at pressure conditions of the shallow lower mantle. Al3+ is likely to be incorporated in place of Si4+ in the octahedral site of perovskite, hence requiring the creation of oxygen vacancies for charge balance. As a consequence, aluminous perovskites may have a high affinity for water. The enhanced compressibility of aluminous perovskite certainly has also strong geophysical and geochemical implications, as it may revive perovskite‐rich lower mantle models.

δs, a measure of the sensitivity of the adiabatic bulk‐modulus to isobaric density changes, controls lateral variations of P‐wave velocities. To estimate the effect of pressure on δs, we have performed lattice dynamic calculations of the cubic alkaline earth oxides. We used a many‐body parametric Hamiltonian that reproduces the available laboratory data on thermoelastic properties. The calculations were carried out for P‐T conditions that correspond to those of the lower mantle. We find that δs decreases from 4±1 at standard conditions to ≤ 2 at lower mantle conditions. The latter value is consistent with D.L. Anderson's estimates based on geophysical data.With δs ≤ 2, and a corresponding parameter for shear modulus not decreasing with compression, no partial melting is required to account for the observation that ΔlnVs/ΔlnVp ≥ 2 in the lower mantle. Although more data and calculations on additional minerals are needed to draw any firm conclusions, our results indicate that the relatively small lateral variations of lnVp, when compared to lnVs, are due entirely to compressional effects.

Lower mantle electrical conductivity based on measurements of Al, Fe-bearing perovskite under lower mantle conditions

The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO2 and periclase–wustite solid solutions, (MgO · FeO)ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO2 react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO2–CaSiO3 system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO3 (bridgmanite) + L = SiO2 + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO3 (bridgmanite) + (MgO · FeO)ss (ferropericlase), and basic minerals, (FeO · MgO)ss (magnesiowustite) + SiO2 (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.

We report cloudy micro- and nano-inclusions in a superdeep diamond from Sao-Luiz, Brazil which contains inclusions of ferropericlase (Mg, Fe)O and former bridgmanite (Mg, Fe)SiO3 and ringwoodite (Mg, Fe)2SiO4. Field emission-SEM and TEM observations showed that the cloudy inclusions were composed of euhedral micro-inclusions with grain sizes ranging from tens nanometers to submicrometers. Infrared absorption spectra of the cloudy inclusions showed that water, carbonate, and silicates were not major components of these micro- and nano-inclusions and suggested that the main constituent of the inclusions was infrared-inactive. Some inclusions were suggested to contain material with lower atomic numbers than that of carbon. Mineral phase of nano- and micro-inclusions is unclear at present. Microbeam X-ray fluorescence analysis clarified that the micro-inclusions contained transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) possibly as metallic or sulfide phases. The cloudy inclusions provide an important information on the growth environment of superdeep diamonds in the transition zone or the lower mantle.