Bridgmanite Research Articles

Elasticity of MgO to 130 GPa: Implications for lower mantle mineralogy

The aggregate shear wave velocities of MgO (periclase) have been determined throughout Earth's lower mantle pressure regime approaching 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. We found that the extrapolations of the high-pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies. However, the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus (d G/d P = G 0′) of MgO is 1.92(2), which is distinctly lower than that of previous lower-pressure experiments. Compared with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G 0′. We calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower G 0′ value of periclase can remarkably well reproduce the global seismological 1-D velocity profile model with uniform composition model. The best-fitting result indicates the possibility of a lower mantle mineralogy with ~ 92 vol.% silicate perovskite phase, implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic. The present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the Earth's lower mantle.

Radiative conductivity in the Earth’s lower mantle

Iron in crustal and mantle minerals adopts several possible oxidation states: this has implications for biogeochemical processes, oxygenation of the atmosphere and the oxidation state of the mantle. In the deep Earth, iron in silicate perovskite, (Mg(0.9)Fe(0.1))SiO(3), and ferropericlase, (Mg(0.85)Fe(0.15))O, influences the thermal conductivity of the lower mantle and therefore heat flux from the core. Little is known, however, about the effect of iron oxidation states on transport properties. Here we show that the radiative component of thermal conductivity in the dominant silicate perovskite material of Earth's lower mantle is controlled by the amount of ferric iron, Fe(3+). We obtained the optical absorption spectra of silicate perovskite and ferropericlase at pressures up to 133 GPa, corresponding to pressures at the core-mantle boundary. Absorption spectra of ferropericlase up to 800 K and 60 GPa exhibit minimal temperature dependence. The results on silicate perovskite show that optical absorption in the visible and near-infrared spectral range is dominated by O-Fe(3+) charge transfer and Fe(3+)-Fe(2+) intervalence transitions, whereas a contribution from the Fe(2+) crystal-field transitions is substantially smaller. The estimated pressure-dependent radiative conductivity, k(rad), from these data is 2-5 times lower than previously inferred from model extrapolations, with implications for the evolution of the mantle, such as generation and stability of thermo-chemical plumes in the lower mantle.

No interconnection of ferro‐periclase in post‐spinel phase inferred from conductivity measurement

Electrical conductivities of post‐spinel, which are thought to be a dominant assembly in the lower mantle, were investigated at the conditions of the uppermost lower mantle (25 GPa and temperatures ranging from 300 to 2000K) in a Kawai‐type multi‐anvil apparatus. Post‐spinel phases with a bulk composition of ((Mg,Fe)2SiO4: XFe = 0.09) were measured to constrain the bulk conductivity of the lower mantle. To investigate interconnectivity of ferro‐periclase in silicate perovskite matrix, a single phase composed of magnesium silicate perovskite ((Mg,Fe)SiO3: XFe = 0.1) and ferro‐periclase ((Mg,Fe)O: XFe = 0.13) were also measured. The conductivity values of the silicate perovskite are distinctly lower than those for the coexisting ferro‐pericalse. Both absolute values and change in activation enthalpy for the conductivity of the post‐spinel phases are similar to those for the silicate perovskite. These observations suggest that ferro‐periclase in post‐spinel are isolated in silicate perovskite matrix.

Disproportionation of Fe2+ in Al-free silicate perovskite in the laser heated diamond anvil cell as recorded by electron probe microanalysis of oxygen

Experimental evidence is reported for Fe2+ disproportionation in Al-free perovskite (Pv), when submitted to large temperature gradients (i.e., under off-equilibrium conditions) in a laser heated diamond anvil cell (LHDAC). To enable this effect, the experimental procedure was designed to produce large radial and axial temperature gradients. In the Pv and ferropericlase (Fp) assemblage synthesized after dissociation of natural olivine, the three chemical states of iron (i.e., Fe0, Fe2+ and Fe3+) could be evidenced by electron probe microanalysis (EPMA), through variations of oxygen contents attached to the Fe cations. Despite inherent difficulties for determination of O-contents and Fe3+/ΣFe ratios using EPMA, we recorded significant changes in iron oxidation state across the laser-heated strip. These changes are correlated with variations in composition for the major elements (Fe, Mg, and Si), which evidences that the Pv/Fp assemblage experienced large segregation under the strong temperature gradients. Grains of metallic iron were detected in parts of the laser-heated strip coexisting with a Pv phase with Fe/(Mg + Fe) = 6 at% and most of its iron as Fe3+. This Fe2+-disproportionation reaction involves insertion of Fe3+-defects in the Pv lattice. This Fe3+-bearing Pv phase is presumably unstable and decomposes into a mineral assemblage including magnesioferrite, which is detected at the border of the laser-heated strip.

Direct shock wave loading of MgSiO 3 perovskite to lower mantle conditions and its equation of state

Large MgSiO 3-perovskite samples were synthesized for shock wave experiments. Shock wave data on the pre-synthesized perovskite samples up to 107 GPa yielded a linear relationship between the shock wave velocity U s and particle velocity u p described by U s = 6.47(±0.63) + 1.56(±0.31) u p. Fitting the experiment data to the Rankin–Hugoniot equation, we obtained the Grüneisen parameter γ 0 = 1.33 with q = 1. The best fitted values for the adiabatic bulk modulus K 0 S and its pressure derivative K ′ 0 S are 254(±10) GPa and 3.9(±0.17), respectively, which are in general agreement with values derived from static compression data. By direct comparison with dynamic compression data using enstatite as starting material, we observed that MgSiO 3 enstatite completely transformed into perovskite phase above 90 GPa shock pressure. Improvement of the precision in determining the Hugoniot relationship by additional shock wave data is needed to further constrain the thermoelastic properties of perovskite. However, this is the first demonstration that direct shock wave loading of the pre-synthesized perovskite samples can provide a new way to determining the thermal equation of state (EOS) of silicate perovskite without the complication of phase transformations along the Hugoniot path, ultimately leading to a better constrained thermoelastic parameters for this important mantle mineral.

Thermal conductivity of lower-mantle minerals

Geodynamic models of heat transport and the thermal evolution of Earth's interior require knowledge of thermal conductivity for high-pressure phases at relevant temperatures and pressures. Here we present new data on radiative and lattice heat transfer in mantle materials determined from optical spectroscopy and time-resolved optical radiometry. The pressure dependence of optical absorption in ferropericlase (Mg,Fe)O, and silicate perovskite (Mg,Fe)SiO 3, has been determined in the IR through UV regions up to 133 GPa. Whereas (Mg,Fe)O exhibits a strong pressure dependence of absorption and spectral changes associated with the high-spin (HS) to low-spin (LS) transition of Fe 2+ [Goncharov, A.F., Struzhkin, V.V., Jacobsen, S.D. 2006. Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle. Science 312, 1205–1208], the pressure dependence of optical absorption in (Mg,Fe)SiO 3 is relatively weak. We observe a moderate increase in absorption with pressure for (Mg,Fe)SiO 3 in the visible and infrared spectral range due to a red-shift of absorption in ultraviolet, however the crystal-field transitions of Fe 2+ become weaker with pressure and disappear above 50 GPa as a result of the HS–LS transition in (Mg,Fe)SiO 3. Intervalence charge-transfer transitions in silicate perovskite shift to higher energies with pressure. The temperature dependence of the optical absorption of (Mg,Fe)O measured up to 65 GPa and 800 K is moderate below 30 GPa and weak above 30 GPa. Thus, the temperature correction of the radiative conductivity is insignificant. The estimated total pressure-dependent radiative conductivity (in approximation of a large grain size) is lower than expected from the pressure extrapolation of the ambient and low-pressure data [Hofmeister, A.M., 1999. Mantle values of thermal conductivity and the geotherm from phonon lifetimes. Science 283, 1699–1706; Hofmeister, A.M., 2005. Dependence of diffusive radiative transfer on grain-size, temperature, and Fe-content: implications for mantle processes. J. Geodyn. 40, 51–72]. A new method has been developed to measure thermal diffusivity of mantle materials at high P– T using time-resolved radiometry combined with a pulsed-IR source. Here, the technique is tested on MgO to 32 GPa and used to obtain a functional pressure dependence of thermal diffusivity and calculated thermal conductivity of the lower mantle.

Phase transition zone width implications for convection structure

In the Ringwood pyrolitic (pyroxene‐olivine) model, the upper mantle consists of peridotite (90%) and eclogite (10%). Peridotite includes olivine (60%), orthopyroxene (25%), clinopyroxene, and garnet. Eclogite contains clinopyroxene (60%), garnet (30%), and other minor admixtures. The lower mantle consists of silicate perovskite (Mg 0.9 Fe 0.1 )SiO 3 (75%), magnesiowustite (Mg 0.8 Fe 0.2 )O (15%), and calcium perovskite (CaSi)O 3 (10%) [Davies, 1999]. With an increase in pressure and temperature, the mantle material experiences phase transitions. Since it has a multicomponent composition, the transition does not occur in a jumpwise manner but is distributed over an interval of pressures ∆ p or depths d = ∆ p /( ρ g ) . At a depth of 410 km, olivine is transformed into wadsleyite with a density jump of δρ / ρ 0 = 0.07 at a phase equilibrium curve slope of γ = dp / dT = 1.6‐3.0 MPa/K; the width of the phase transition zone is d = 13 km. At a depth of 520‐580 km, wadsleyite is transformed into

A general test of the hypothesis that transformation-induced faulting cannot occur in the lower mantle

Transformation-induced faulting, which has been proposed as a mechanism for deep-focus (>300 km) earthquakes, requires a thermal runaway associated with an exothermic chemical reaction. This shearing instability has been demonstrated for several polymorphic phase transformations including the olivine → wadsleyite and olivine → ringwoodite reactions. However, it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability, as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite + magnesiowüstite. Here we report experiments designed to test this possibility using the proxy reaction albite → jadeite + coesite, a highly exothermic reaction with a large ΔV. We find no mechanical or microstructural evidence for transformation-induced faulting during this reaction. Applying these results to the reaction olivine (ol) → perovskite (pv) + mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes. Thus, transformation-induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone, but only if subduction zones are colder than currently proposed. We discuss other reasons for termination of earthquakes at ∼700 km and find them also difficult to reconcile with experimental evidence or natural observations.

Effect of Fe on the equation of state of mantle silicate perovskite over 1 Mbar

In order to investigate the effect of Fe on the equation of state (EOS), the volume of the perovskite (Pv) phases, (Mg 1− x Fe x )SiO 3, with different Fe contents ( x = 0 , 0.09, 0.15), has been measured at high pressures up to 1 Mbar in an Ar or NaCl medium. A bulk modulus ( K 0 ) ranging between 255 and 261 GPa is obtained when the Au scales consistent with the Pt scale are used and the data are fit to the second order Birch–Murnaghan (BM) equation. When the third order BM equation is used, we obtain K 0 = 250–264 GPa and K 0 ′ = 3.7–4.5, which is consistent with previous low-pressure X-ray diffraction and Brillouin measurements. Within experimental uncertainties (0.5% in volume and 6% in bulk modulus), we do not resolve differences in the EOS between Fe-bearing (up to 15%) and Fe-free Pv to 1 Mbar (2600-km depth in the mantle). This is in contrast with the much larger effect of the Fe spin transition for the EOS of ferropericlase (Fp) reported recently. This is perhaps due to much more diverse states of Fe (e.g., oxidation, coordination, and spin states) in Pv than in Fp.

Thermoelastic behaviour of silicate perovskites: Insights from new high-temperature ultrasonic data for ScAlO 3

(Mg,Fe,Al)(Si,Al)O 3 perovskite, stable only at pressures >20 GPa, is the dominant mineral of the Earth's lower mantle and thus controls its physical properties including seismic wave speeds. However, the development of a thorough understanding of the thermoelastic behaviour of this silicate perovskite is compromised by its marginal metastability on recovery at ambient conditions. Study of the close structural analogue ScAlO 3, stable at much lower pressure, has the potential to provide insight into the behaviour of its silicate cousin. Here, previous exploratory ultrasonic measurements of the compressional and shear wave speeds on a fine-grained polycrystalline specimen have been extended to 1000 K under 300 MPa confining pressure within an internally heated gas-medium high-pressure apparatus. The wave speeds and derived bulk and shear moduli vary approximately linearly with temperature with derivatives ( ∂K S / ∂T) P = −21.3(3) MPa K −1 and ( ∂G/ ∂T) P = −19.0(1) MPa K −1, respectively. The new data, along with previous measurements of thermal expansion and the pressure dependence of the elastic moduli, have been assimilated into the comprehensive internally consistent finite-strain model of thermoelastic behaviour proposed by [Stixrude, L., Lithgow-Bertelloni, C., 2005. Thermodynamics of mantle minerals—I. Physical properties. Geophys. J. Int. 162, 610–632]. Comparison of the newly constrained model for ScAlO 3 with emerging data for MgSiO 3 perovskite provides guidance in the analysis of the chemical composition and temperature of the Earth's lower mantle.

Self-diffusion of oxygen and silicon in MgSiO3 perovskite

We present results of simultaneous measurements of self-diffusion of oxygen and silicon in magnesium silicate perovskite. Oxygen diffusion is ~2 orders of magnitude faster than Si self-diffusion and Fe–Mg interdiffusion in perovskite, with D0=9.16×10−4 (+0.16/−9.2×10−6) m2/s and Q=501±80 kJ/mol at 25 GPa. Despite this high O diffusivity, however, the electrical conductivity of the lower mantle will be dominated by electronic conduction and the present-day core is likely to be out of redox equilibrium with the mantle.

Os isotope geochemistry of mantle peridotites from Sal Island, Cape Verde Archipelago

Sulfide globules with Fe‐Ni alloys and separate grains of metallic phases were found in the intergranular glass from mantle nodules on Sal Island [1, 2]. The occurrence of native metals and the high temperature of mineral equilibria in lherzolites from Sal Island [2] allow us to suggest the contribution of the lower mantle material to the ascending mantle plume that controls magmatism of Cape Verde Archipelago. This suggestion is substantiated by experimental data, which indicate that disproportionation of the ferrous component with formation of the metallic phase and excess of Fe 3+ incorporated into Al-bearing silicate perovskite must occur under the lower mantle conditions [3‐5]. The equilibrium with participation of the metallic phase is clearly expressed in the behavior of strongly siderophile elements, in particular PGE, and exerts a substantial influence on the Re‐Os isotopic system that serves as a sensitive indicator of the processes with participation of sulfides and native metals [6]. We determined PGE contents and measured the Os isotopic composition for a number of mantle nodules from Sal Island in order to provide possible evidence for participation of the lower mantle processes in the formation of plume material pertaining to Cape Verde Archipelago. Since both Os and Re have prominent siderophile and chalcophile properties, sulfide and metallic phases must be their main species in the mantle rocks. The partition of these elements between immiscible silicate and sulfide melts in the course of partial melting of mantle peridotites and crystallization of sulfide‐metallic melts results in an appreciable shift of Re/Os ratios. In these processes, Os behaves as a compatible element; Re, as a moderately incompatible component. In other isotopic systems (Sm‐Nd, Rb‐Sr, Lu‐Hf, U‐Th‐Pb) used as petrogenetic indicators, both parental and daughter radioactive elements are incompatible. As a result, the difference in the Os isotopic composition in various geological objects becomes much greater with time than, for example, for Nd or Sr. The Nd isotopic composition deviates from the meteoritic standard by a fraction of a percent, whereas the Os deviation is measured by several percent. In particular, the Re/Os ratio in basalts turns out to be several times higher than in mantle restite. Thus, Os in the Earth’s crust acquires a substantially radiogenic character with time, while the 187 Os/ 188 Os ratio in the residual mantle becomes subchondritic with time. These features make the Re‐Os isotopic system much more sensitive to participation of the crustal and mantle sources in geological processes in comparison with other systems of radiogenic isotopes.

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO 3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.

Spin transition of iron in the Earth's lower mantle

Electronic spin-pairing transitions of iron and associated effects on the physical properties of host phases have been reported in lower-mantle minerals including ferropericlase, silicate perovskite, and possibly in post-perovskite at lower-mantle pressures. Here we evaluate current understanding of the spin and valence states of iron in the lower-mantle phases, emphasizing the effects of the spin transitions on the density, sound velocities, chemical behavior, and transport properties of the lower-mantle phases. The spin transition of iron in ferropericlase occurs at approximately 50 GPa and room temperature but turns into a wide spin crossover under lower-mantle temperatures. Current experimental results indicate a continuous nature of the spin crossover in silicate perovskite at high pressures, but which valence state of iron undergoes the spin crossover and what is its associated crystallographic site remain uncertain. The spin transition of iron results in enhanced density, incompressibility, and sound velocities, and reduced radiative thermal conductivity and electrical conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's lower mantle. In addition, a reduction in sound velocities within the spin transition is recently reported. Our evaluation of the experimental and theoretical pressure–volume results shows that the spin crossover of iron results in a density increase of 2–4% in ferropericlase containing 17–20% FeO. Here we have modeled the density and bulk modulus profiles of ferropericlase across the spin crossover under lower-mantle pressure–temperature conditions and shown how the ratio of the spin states of iron affects our understanding of the state of the Earth's lower mantle.

The wetting ability of Si-bearing liquid Fe-alloys in a solid silicate matrix—percolation during core formation under reducing conditions?

The wetting characteristics of liquid Fe–Si alloys in a matrix of the respective predominating stable silicate mantle mineral (forsterite or silicate perovskite) at pressures of 2–5 and 25 GPa and temperatures of 1600–2000 °C were studied by determining the liquid metal–solid silicate contact angles. The median angle values from texturally equilibrated samples were found to be independent of pressure, temperature, silicate mineralogy and the Si content in the metal fraction and range between 130° and 140° which is far above the critical wetting boundary of 60°. This shows that within the studied range of conditions dissolved Si does not lower the surface energies between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions the presence of Si in the metal phase of planetary bodies would not have enhanced percolative flow as an effective metal–silicate separation process.

Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle

Changes in the electronic configuration of iron at high pressures toward a spin-paired state within host minerals ferropericlase and silicate perovskite may directly influence the seismic velocity structure of Earth's lower mantle. We measured the complete elastic tensor of ferropericlase, (Mg(1-x),Fe(x))O (x = 0.06), through the spin transition of iron, whereupon the elastic moduli exhibited up to 25% softening over an extended pressure range from 40 to 60 gigapascals. These results are fully consistent with a simple thermodynamic description of the transition. Examination of previous compression data shows that the magnitude of softening increases with iron content up to at least x = 0.20. Although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most, if not all, of the lower mantle.

Grain-boundary enrichment of iron on magnesium silicate perovskite

We present a combined experimental and numerical simulations study of grain boundaries in (Fe, Mg)SiO3 perovskite. TEM observations of grain boundaries in a well equilibrated perovskite sample consistently show small but significant enrichments of Fe. This result is in good agreement with classical potentials simulations of grain boundary structures in MgSiO3 perovskite which predict substitution of iron onto grain boundaries to be energetically favourable over iron substitution in the bulk perovskite structure.

The role of Al-defects on the equation of state of Al–(Mg,Fe)SiO 3 perovskite

We performed compression curves of aluminous silicate perovskite (Al–Pv) synthesized under various conditions of pressure, temperature and MgO or SiO 2 activities, using laser-heated diamond anvil cell at the ESRF (Grenoble). We refined bulk moduli ( K 0) from 235 to 270 GPa, in agreement with the wide range of values reported in the literature. We observe that Al–Pv phase synthesized at high temperature, in the SiO 2-rich system, is more compressible than Al–Pv phase synthesized at high pressure, in the MgO-rich system. As suggested by various authors, the resolution of this controversy rests on a better understanding of the crystal chemistry of Al in perovskite, which involves at least two competitive mechanisms, substitution of Si in the octahedral site only, or a coupled substitution on both Mg and Si sites. The vacancy mechanism is expected to reduce the K 0 significantly, due to the presence of oxygen vacancies. All compression curves performed in this study can be explained by considering that the vacancy mechanism is favored at high temperatures and that the coupled mechanism is favored at high pressures. These trends agree well with previous reports. For (Mg,Fe)(Si,Al)O 3 perovskite compositions relevant to the lower mantle, the two previous reports and our new data set for a MORB-type perovskite phase agree well with each other with higher K 0 values between 260 and 270 GPa, compared with K 0 = 253 GPa for the pure MgSiO 3 phase suggested from previous studies. In these compounds, coupled substitution of Al 3+ and Fe 3+ cations leads to a well constrained crystal chemistry. Therefore, the low K 0 value observed in some of the previous studies for Fe-free Al–Pv is likely to be irrelevant for mantle perovskite. Some questions may remain only for the mantle region just below the 670 km discontinuity, where pressures remain moderate, thus potentially allowing for at most 2% of oxygen vacancies. However, it is clear that using low K 0 values to extrapolate to greater depths is unjustified.

Chemical imaging with NanoSIMS: A window into deep-Earth geochemistry

We use a combination of nanometer-resolution secondary ion mass spectrometry (NanoSIMS) and analytical transmission electron microscopy (ATEM) for chemical imaging of material transformed in a laser-heated diamond anvil cell (LH-DAC), in the pressure and temperature range of Earth's lower mantle. MORB (mid-ocean ridge basalt), one of the components of subducted oceanic lithosphere, was transformed to an assemblage of Mg-perovskite, Ca-perovskite, stishovite and a calcium ferrite-structure phase at 55 GPa and 2100 °C in an LH-DAC. Elemental imaging spanning the entire range of concentrations, from major elements such as silicon (49.5 wt.% SiO 2) to trace elements such as strontium (118 ppm), scandium, and yttrium (both at 40 ppm) was obtained with a Cameca NanoSIMS 50. We observe a preferential partitioning of scandium, yttrium and strontium in the calcium silicate perovskite phase, and we compare this to recently measured solid–liquid partition coefficients and fractionation at lower pressures. This type of measurement demonstrates that even the most complex mineral assemblages can be probed using this combination of techniques and opens new pathways towards the characterization and quantification of geochemical interactions and processes occurring in the deep Earth.

XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite

Cr K -edge X-ray absorption near-edge structure (XANES) spectra were recorded on Cr:MgO periclase and Cr:(Mg,Fe)O ferropericlase synthesized at different pressures (4 and 12 GPa) and temperatures (1200 to 1400 °C) at reducing oxygen fugacity conditions (~iron-wustite buffer IW to IW – 2), and on Cr:MgSiO3 perovskite with 0.5 wt% Cr2O3. 57Fe Mossbauer spectra were collected on the Fe-containing samples. The aim of the study was to determine the Cr oxidation state in phases found in the Earth’s lower mantle, and to examine the possible relationship with the Fe oxidation state in the same materials. To calculate the amount of Cr2+, the intensity of the shoulder at the low-energy side of the edge crest was quantified using the area of the corresponding peak in the derivative XANES spectra (Berry and O’Neill 2004). In Cr:(Mg,Fe)O the relative Cr2+ content reached at most 12.5% but results from Mossbauer spectroscopy combined with chemical composition data suggest that some Cr2+ oxidized during cooling through the reaction Cr2+ + Fe3+ → Cr3+ + Fe2+. In iron-free Cr:MgO, the Cr2+ content is much higher and reaches ~40%. In Cr:MgSiO3 perovskite with 0.006 Cr pfu (similar to estimated lower mantle abundance), chromium is mainly divalent.