The composition of Earth's atmosphere depends on the redox state of the mantle, which became more oxidizing at some stage after Earth's core started to form. Through high-pressure experiments, we ...found that Fe
in a deep magma ocean would disproportionate to Fe
plus metallic iron at high pressures. The separation of this metallic iron to the core raised the oxidation state of the upper mantle, changing the chemistry of degassing volatiles that formed the atmosphere to more oxidized species. Additionally, the resulting gradient in redox state of the magma ocean allowed dissolved CO
from the atmosphere to precipitate as diamond at depth. This explains Earth's carbon-rich interior and suggests that redox evolution during accretion was an important variable in determining the composition of the terrestrial atmosphere.
Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character ...of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe(3+) in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe(3+) to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.
The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as
Kd
Fe
T
-
Mg
= (Fe
ol
/Fe
m
)·(Mg
m
/Mg
ol
), with all Fe
T
expressed as Fe
2+
, is one of the most widely used ...parameters in petrology. We explore the effect of redox conditions on
Kd
Fe
T
-
Mg
using experimental, olivine-saturated basaltic glasses with variable H
2
O (≤ 7 wt%) over a wide range of
f
O
2
(iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe
3+
to total Fe (Fe
3+
/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe
3+
/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H
2
O. Combining our new data with published experimental data having measured glass Fe
3+
/∑Fe, we show that for Fo
65–98
olivine in equilibrium with basaltic and basaltic andesite melts,
Kd
Fe
T
-
Mg
decreases linearly with Fe
3+
/∑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 Fo
50
olivine; 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 Fe
3+
/∑Fe in olivine-bearing experimental melts, from which
f
O
2
can be calculated. In contrast to
Kd
Fe
T
-
Mg
, the Mn–Mg exchange coefficient,
Kd
Mn
-
Mg
, is relatively constant over a wide range of P–T–
f
O
2
conditions. We present an expression for
Kd
Mn
-
Mg
that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for
Kd
Fe
T
-
Mg
and
Kd
Mn
-
Mg
to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe
3+
/∑Fe to a precision of ≤ 0.04.
The program MossA provides a straightforward approach to the fitting of 57Fe conventional and synchrotron energy‐domain Mössbauer spectra. Sites can be defined simply by mouse clicks and hyperfine ...parameters can be constrained to constant values, within specific ranges, and can be coupled linearly between different subspectra. The program includes a full transmission integral fit with Lorentzian line shape (conventional source) or Lorentzian‐squared line shape (synchrotron source). The fitting process is graphically displayed in real time while fitting and can be interrupted at any time. Gaussian‐shaped quadrupole splitting distributions for analyzing nonmagnetic amorphous materials are included. MossA is designed especially for the rapid and comprehensive analysis of complex Mössbauer spectra, made possible by its native graphical user input.
Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the ...speciation and distribution of sulfur on Mercury, we performed high temperature (1200–1750 °C), low- to high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S2− and forms complexes with Fe2+, Mg2+ and Ca2+. The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from <1 wt.% S at IW-2 to >10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity (fO2), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4±0.4. We also calculate that the mantle of Mercury contains 7–11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.
•We performed S solubility experiments on Mercurian lavas and enstatite chondrite.•The solubility of sulfur (S2−) increases with temperature and reducing conditions.•Oxygen fugacity in Mercurian lavas is IW-5.4±0.4.•7–11 wt.% S in the mantle of Mercury and <1.5 wt.% S in the metallic core.•The external core of Mercury is made up of a thin FeS layer (<90 km).
Abstract 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+/∑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+/∑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) °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, ∆logfo2 (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 ∆logfo2 (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+/∑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.
The dihedral angle (θ) between olivine and aqueous fluid is a critical parameter in identifying the grain-scale fluid connectivity which controls the distribution and migration of aqueous fluids in ...subduction zones besides physical properties of mantle wedges. Recent magnetotelluric observations have suggested the occurrence of significant fluid circulation in deep fore-arc regions, which can be explained by infiltration of saline fluid with a low θ in the mantle wedge (Huang et al., 2019). Along with the salt component, non-polarized gas such as CO2, is a crucial constituent of subduction zone fluids. CO2 is known to increase the olivine–fluid θ under conditions in which the olivine does not react with CO2, which is in contrast to the effect of NaCl on θ. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained θ in olivine + H2O–CO2 fluid and olivine + H2O–CO2–NaCl (multicomponent) fluid systems at 1–4 GPa and 800–1100°C. For the H2O–CO2 system, we found that CO2 tends to increase θ at 1 GPa and 800–1100°C and at 2 GPa and 1100°C. In contrast, CO2 reduced θ even below 60° at relatively high-pressure (P) and low-temperature (T) conditions, in which the olivine partly reacts with CO2 to form magnesite and orthopyroxene (opx). The consumption of non-wettable CO2 components in aqueous fluid alone cannot explain θ lower than those in a pure H2O system. Additional experiments on olivine–magnesite + H2O and olivine–opx + H2O systems showed that the presence of magnesite or opx decreased the olivine–fluid θ, which implies that coexisting minerals affect the olivine–fluid interfacial energy by changing the fluid chemistry. The results of the multicomponent system showed that the effect of NaCl on θ is much more significant than that of CO2. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems. Our results suggest that slab-derived fluid can infiltrate into the deep fore-arc mantle wedge through an interconnected network even in a CO2-bearing multicomponent system at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation. The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity in a wide condition may allow for the possibility of mapping partial melt in the mantle wedge.
•Effects of H2O–CO2, H2O–CO2–NaCl fluid on olivine wetting properties are studied.•CO2 decreases the dihedral angle once it reacts with olivine.•The effect of NaCl on the dihedral angle is more significant than that of CO2.•H2O–CO2–NaCl (multicomponent) fluid can effectively wet olivine.•Significant fore-arc circulation of multicomponent fluid can occur.
Recently, high electrical conductors have been detected beneath some fore-arcs and are believed to store voluminous slab-derived fluids. This implies that the for-arc mantle wedge is permeable for ...aqueous fluids. Here, we precisely determine the dihedral (wetting) angle in an olivine-NaCl-H
O system at fore-arc mantle conditions to assess the effect of salinity of subduction-zone fluids on the fluid connectivity. We find that NaCl significantly decreases the dihedral angle to below 60° in all investigated conditions at concentrations above 5 wt% and, importantly, even at 1 wt% at 2 GPa. Our results show that slab-released fluid forms an interconnected network at relatively shallow depths of ~80 km and can partly reach the fore-arc crust without causing wet-melting and serpentinization of the mantle. Fluid transport through this permeable window of mantle wedge accounts for the location of the high electrical conductivity anomalies detected in fore-arc regions.
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.
Phase transitions and the chemical composition of minerals in Earth's interior influence geophysical interpretations of its deep structure and dynamics. A pressure-induced spin transition in olivine ...has been suggested to influence iron partitioning and depletion, resulting in a distinct layered structure in Earth's lower mantle. For a more realistic mantle composition (pyrolite), we observed a considerable change in the iron-magnesium partition coefficient at about 40 gigapascals that is explained by a spin transition at much lower pressures. However, only a small depletion of iron is observed in the major high-pressure phase (magnesium silicate perovskite), which may be explained by preferential retention of the iron ion Fe³⁺. Changes in mineral proportions or density are not associated with the change in partition coefficient. The observed density profile agrees well with seismological models, which suggests that pyrolite is a good model composition for the upper to middle parts of the lower mantle.
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