Iron-wustite Buffer Research Articles

Potential of Earth’s core as a reservoir for noble gases: Case for helium and neon

doi: 10.7185/geochemlet.2028 lower mantle upper mantle OIB MORB outer core Low 3 He/ 22 Ne High 3 He/ 4 He Co re C on tr ib ut io n This study investigates metal-silicate partitioning of neon (D Ne) under the likely conditions of early Earth's core formation: up to 16 GPa, ∼ 3000 K and an oxygen fugacity near IW-2 (2 log units below the Iron-Wustite buffer). We find that the D Ne coefficients range between 10 −2 and 10 −1. These partition coefficients are only one of the controlling factors of noble gas distributions within the early Earth: because, even if D He and D Ne are low (∼10 −4), there may have been sufficient noble gases present in the mantle to supply a significant quantity of He and Ne to the core. Assuming gas-melt equilibrium of the molten proto-Earth with a nebular gas composition and con-comitant metal-silicate differentiation, the core would have inherited and maintained throughout Earth's history high 3 He/ 4 He ratios and low 3 He/ 22 Ne ratios (<0.6), making the core a potential source of primordial light noble gases in mantle plumes.

Formation of primitive achondrites by partial melting of alkali-undepleted planetesimals in the inner solar system

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW −0.5/−1.0 for brachinites, ΔIW −1.3/−2.5 for ureilites, ΔIW −1.6/−2.7 for acapulcoites/lodranites and ΔIW −2.5/−3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete condensation of the solar nebula.

Widespread production of silica- and alkali-rich melts at the onset of planetesimal melting

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1–13.1 MPa) and over a range of oxygen fugacity (log fO2 − (log fO2)IW = −2.5 to −1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5–25 wt.%) of chondritic planetesimals. Most experiments (IW −2.5 to −1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun’s photosphere (e.g. CI, H and LL compositions) produced low-degree melts (<15 wt.%) rich in SiO2, Al2O3 and alkali elements, regardless of the fO2. Despite their high apparent viscosities (104-5 Pa.s), such low-density partial melts (2400–2500 kg/m3) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase An10-30 (i.e. oligoclase) such as the trachyandesite achondrites Graves Nunataks 06128/9, Northwest Africa 6698 and 11575, the Almahata Sitta clast ALM-A, as well as smaller “albitic clasts” in polymict ureilites and “alkali-silica-rich” inclusions in non-magmatic iron meteorites. We suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation.

Extreme reduction: Mantle-derived oxide xenoliths from a hydrogen-rich environment

Coarse-grained xenoliths of hibonite + grossite + Mg-Al-V spinel from Cretaceous pyroclastic rocks on Mt. Carmel, N. Israel, and from Sierra de Comechingones, Argentina, include spherules, rods and dense branching structures of native vanadium and VAl alloys. Microstructures suggest that vanadium melts became immiscible with the host Ca-Al-Mg-Si-O melt, and nucleated as droplets on the surfaces of the oxide phases, principally hibonite. Many extended outward as rods or branching structures as the host oxide crystal grew. The stability of V0 implies oxygen fugacities ≥9 log units below the Iron-Wustite buffer, suggesting a hydrogen-dominated atmosphere. This is supported by wt%-levels of hydrogen in gasses released by crushing, by Raman spectroscopy, and by the presence of VH2 among the vanadium balls. The oxide assemblage formed at 1400–1200 °C; the solution of hydrogen in the metal could lower the melting point of vanadium to these temperatures. These assemblages probably resulted from reaction between differentiated mafic melts and mantle-derived CH4 + H2 fluids near the crust-mantle boundary, and they record the most reducing magmatic conditions yet documented on Earth.

Raman spectroscopy study of C-O-H-N speciation in reduced basaltic glasses: Implications for reduced planetary mantles

Abstract To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒO2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒO2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H2, NH2–, NH3, CH4, CO, CN–, N2, and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N2, CH4, NH3 and H2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒO2, CO band areas increase while those of N2 decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN–, NH2– and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H2, NH2–, NH3, CH4, CO, CN–, N2, and OH– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒO2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation.

Effects of pH2O, pH2 and fO2 on the diffusion of H-bearing species in lunar basaltic liquid and an iron-free basaltic analog at 1 atm

Abstract We have experimentally determined the diffusivity of water in a representative lunar basaltic liquid composition (LG) and in an iron-free analog of a basaltic liquid (AD) at the low water concentrations and low oxygen fugacities (fO2) relevant to the eruption of lunar basalts. Experiments were conducted at 1 atm and 1350 °C over a range of pH2/pH2O from near zero to ∼10 and a range in fO2 spanning ∼9 orders of magnitude (from 2.2 log units below the iron-wustite buffer, IW−2.2, to IW+6.7). The water concentrations measured in our quenched experimental glasses by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) vary from a few ppm to ∼430 ppm. Water concentration gradients in the majority of our AD experiments are well described by models in which the diffusivity of water ( D w a t e r ∗ ) has a constant value of ∼2 × 10−10 m2/s, while our LG results indicate that D w a t e r ∗ in LG melt has a constant value of ∼6 × 10−10 m2/s under the conditions of our experiments. Water concentration gradients in hydration and dehydration experiments that were run simultaneously in H2/CO2 gas mixtures are well described by the same D w a t e r ∗ , and water concentrations measured near the melt-vapor interfaces of these experiment pairs are approximately the same. These observations strongly support an equilibrium boundary condition for our experiments containing >70 ppm H2O. However, dehydration experiments into nominally anhydrous CO2, N2, and CO/CO2 gas mixtures leave some scope for the importance of kinetics during dehydration of melts containing less than a few 10′s of ppm H2O. Comparison of our results with the modified speciation model (Ni et al., 2013) in which both molecular water and hydroxyl are allowed to diffuse suggests that we have resolved the diffusivity of hydroxyl ( D O H ) in AD and LG melts. Our results support a positive correlation between D O H and melt depolymerization. Best-fit values of D w a t e r ∗ for our LG experiments vary within a factor of ∼2 over a range of pH2/pH2O from 0.007 to 9.7 and a range of logfO2 from IW−2.2 to IW+4.9. The relative insensitivity of our best-fit values of D w a t e r ∗ to variations in pH2 suggests that H2 diffusion did not control the rate of degassing of H-bearing species from the lunar glasses of Saal et al. (2008); however, we cannot rule out a role for molecular H2 diffusion under lower-temperature and/or higher-pressure conditions than explored in our experiments. The value of D w a t e r ∗ chosen by Saal et al. (2008) for modeling the diffusive degassing of the lunar volcanic glasses is within a factor of ∼2 of our measured value in LG melt at 1350 °C. By coupling our LG results at 1350 °C with an activation energy of 220 kJ/mol (Zhang et al., 2017), we obtain the following Arrhenius relationship, which can be used to model syneruptive diffusive water loss from lunar melt beads: D w a t e r ∗ ( m 2 / s ) = 7.2 × 1 0 - 3 e x p - 2.6 × 1 0 4 T ( K ) .

Not all moissanites are created equal: New constraints on moissanite from metamorphic rocks of Bulgaria

Terrestrial moissanite (SiC) is widely reported as an ultra-high pressure mineral occurring in kimberlites, diamonds and ultramafic/mafic rocks of mantle origin. However, the conditions of crystallization remain largely unknown. Moreover, dozens of SiC occurrences have been reported from continental crust sources such as granitoids, andesite–dacite volcanic rocks and their breccia, metasomatic and metamorphic rocks, and even limestones. The validity of many of these reports is still debated primarily due to possible contaminations from the widespread use of synthetic SiC abrasives in samples preparation. Indeed, reports of well-documented in-situ occurrences of moissanite in association with co-existing minerals are still scarce. The only condition of moissanite formation that is agreed upon is that extremely reducing media are required (e.g. 4.5–6 log units below the iron-wustite buffer). Here, we report the new occurrence of moissanite that was found in-situ within the garnet–staurolite–mica schists of Topolovgrad metamorphic group of Triassic age in Southern Bulgaria. The 10–300 μm moissanite crystals are situated within 0.1–1.2 mm isolated clusters, filled with amorphous carbon and nanocrystalline graphite. Most of moissanite crystals are 15R (rhombohedral) and 6H (hexagonal) polytypes, and one prismatic crystal, found within them, exhibits unusual concentric polytypical zoning with core (15R), intermediate zone (6H) and rim (3C-cubic). Experimental data show that this type of polytypical zonation is likely due to a decrease in temperature (or/and pressure?) and changes in Si/C ratio. Indeed, amphibolite facies metamorphism (500–580 °C – garnet–staurolite zone) followed by a subsequent cooling during the retrograde stage of green schist facies metamorphism (∼400–500 °C) could have provided a change in temperature. The SiC containing clusters exhibit evidence that they are pre-metamorphic, and we hypothesize that their protolith was a “black shale” material likely rich in carbon, hydrocarbon and terrigenous silica. The latter served as a source of isolated chemically-reduced media, which is required for SiC formation. Other concepts to explain moissanite occurrences in metasedimentary rocks are also discussed. Importantly, our findings show that the formation conditions of moissanite are likely more variable than previously recognized.

Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle

Oxygen fugacity (ƒO2) is a key parameter of Earth’s mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping ƒO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low ƒO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of δ13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing ƒO2 with depth (e.g. Yaxley et al. in Lithos 140:142–151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.

Formation of the Fe,Mg-Silicates, Fe0, and Graphite (Diamond) Assemblage as a Result of Cohenite Oxidation under Lithospheric Mantle Conditions

Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.

Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of N–С–О–Н molecules and complexes in the iron-bearing silicate melts at high pressures

Large-scale melting of the Earth’s early mantle under the effect of global impact processes was accompanied by the generation of volatiles, which concentration was mainly controlled by the interaction of main N, C, O, and H gas-forming elements with silicate and metallic melts at low oxygen fugacity (fO2), which predominated during metallic segregation and self-oxidation of magma ocean. The paper considers the application of Raman and IR (infrared) Fourier spectroscopy for revealing the mechanisms of simultaneous dissolution and relative contents of N, C, O, and H in glasses, which represent the quench products of reduced model FeO–Na2O–Al2O3–SiO2 melts after experiments at 4 GPa, 1550°C, and fO2 1.5–3 orders of magnitude below the oxygen fugacity of the iron—wustite buffer equilibrium (fO2(IW)). Such fO2 values correspond to those inferred for the origin and evolution of magma ocean. It was established that the silicate melt contains complexes with N–H bonds (NH3, NH2+, NH2-), N2, H2, and CH4 molecules, as well as oxidized hydrogen species (OH– hydroxyl and molecular water H2O). Spectral characteristics of the glasses indicate significant influence of fO2 on the N–C–O–H proportion in the melt. They are expressed in a sharp decrease of NH2+, NH2-(O–NH2), OH–, H2O, and CH4 and simultaneous increase of NH2-(≡Si–NH2) and NH3 with decreasing fO2. As a result, NH3 molecules become the dominant nitrogen compounds among N–C–H components in the melt at fO2 two orders of magnitude below fO2(IW), whereas molecular СН4 prevails at higher fO2. The noteworthy feature of the redox reactions in the melt is stability of the ОН– groups and molecular water, in spite of the sufficiently low fO2. Our study shows that the composition of reduced magmatic gases transferred to the planet surface has been significantly modified under conditions of self-oxidation of mantle and magma ocean.

Effect of redox conditions on iron metal phase segregation during experimental high-temperature centrifuge modeling of the origin of the Moon’s core

The possible origin of the Moon’s metallic core at the precipitation of iron–sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %) + ferrobasalt (10 wt %) + metallic phase Fe95S5 (wt %) in a high-temperature centrifuge at 1430–1450°C. The oxygen fugacity fO2 was determined from the composition of the quenched experimental silicate melts (glasses). A decrease in fO2 is proved to be favorable for the segregation of iron–sulfide melt from the silicate matrix. The metallic phase is most effectively segregated in the form of melt droplets, and these droplets are accumulated in the lower portions of the samples under strongly reduced conditions, at fO2 ∼ 4.5–5.5 orders of magnitude lower than the iron–wustite buffer.

Discovery of in situ super-reducing, ultrahigh-pressure phases in the Luobusa ophiolitic chromitites, Tibet: New insights into the deep upper mantle and mantle transition zone

![][1] Previous research on super-reducing ultrahigh-pressure (SuR UHP) phases from the Tibetan ophiolitic chromitites were mainly conducted on isolated grains extracted from extremely large samples. This approach has been questioned because of possible contamination. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitites from the Yarlung Zangbo suture zone. Here we report and analyze unambiguously in situ SuR UHP assemblages from the ophiolitic chromitites by electron probe micro-analyzer, scanning microscope and Laser Raman spectroscope. The SuR UHP and associated phases include: (1) blue moissanite as inclusions in olivine (Fo96–98), and in olivine domains between disseminated chromite grains; (2) multiple inclusions of moissanite + wustite + native Fe in olivine; (3) FeNi and FeCr alloys in olivine and chromite; and (4) native Fe and Si in chromite. Crustal asphaltum and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases, combined with the previously inferred existence of ringwoodite + stishovite, all indicate that these assemblages formed under a highly reducing environment (oxygen fugacities several orders of magnitude lower than that of the iron-wustite buffer) in the mantle transition zone (MTZ) and in the deep upper mantle. Diamond + moissanite with distinct 13C-depleted compositions from chromitites have a metasedimentary carbon source. Associations with existing crustal minerals in chromitites demonstrate that carbon-bearing metasedimentary rocks were recycled into the mantle through subduction, and locally modified its composition. Finally we propose a three-stage model to explain the formation of SuR UHP phase-bearing chromitite. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Ural Mountains and from Myanmar imply existence of a new type of ophiolitic chromitite. Such occurrences provide an additional window to explore the physical-chemical conditions of the MTZ, mantle dynamics, and the profound recycling of crustal materials. [1]: /embed/graphic-1.gif

Ultra-reducing conditions in average mantle peridotites and in podiform chromitites: a thermodynamic model for moissanite (SiC) formation

Natural moissanite (SiC) is reported from mantle-derived samples ranging from lithospheric mantle keel diamonds to serpentinites to podiform chromitites in ophiolites related to suprasubduction zone settings (Luobusa, Dongqiao, Semail, and Ray-Iz). To simulate ultra-reducing conditions and the formation of moissanite, we compiled thermodynamic data for alloys (Fe–Si–C and Fe–Cr), carbides (Fe3C, Fe7C3, SiC), and Fe-silicides; these data were augmented by commonly used thermodynamic data for silicates and oxides. Computed phase diagram sections then constrain the P–T–fO2 conditions of SiC stability in the upper mantle. Our results demonstrate that: Moissanite only occurs at oxygen fugacities 6.5–7.5 log units below the iron–wustite buffer; moissanite and chromite cannot stably coexist; increasing pressure does not lead to the stability of this mineral pair; and silicates that coexist with moissanite have X Mg > 0.99. At upper mantle conditions, chromite reduces to Fe–Cr alloy at fO2 values 3.7–5.3 log units above the moissanite-olivine-(ortho)pyroxene-carbon (graphite or diamond) buffer (MOOC). The occurrence of SiC in chromitites and the absence of domains with almost Fe-free silicates suggest that ultra-reducing conditions allowing for SiC are confined to grain scale microenvironments. In contrast to previous ultra-high-pressure and/or temperature hypotheses for SiC origin, we postulate a low to moderate temperature mechanism, which operates via ultra-reducing fluids. In this model, graphite-/diamond-saturated moderately reducing fluids evolve in chemical isolation from the bulk rock to ultra-reducing methane-dominated fluids by sequestering H2O into hydrous phases (serpentine, brucite, phase A). Carbon isotope compositions of moissanite are consistent with an origin of such fluids from sediments originally rich in organic compounds. Findings of SiC within rocks mostly comprised by hydrous phases (serpentine + brucite) support this model. Both the hydrous phases and the limited diffusive equilibration of SiC with most minerals in the rocks indicate temperatures below 700–800 °C. Moissanite from mantle environments is hence a mineral that does not inform on pressure but on a low to moderate temperature environment involving ultra-reduced fluids. Any mineral in equilibrium with SiC could only contain traces of Fe2+ or Cr3+.

The W-WO2oxygen fugacity buffer (WWO) at high pressure and temperature: Implications forfO2buffering and metal-silicate partitioning

Synchrotron X-ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO2 at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO2 unit-cell volume data were fit to Mie-Gruneisen equations of state; parameters for W are K T = 307 (±0.4) GPa, K ′T = 4.05 (±0.04), γ = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO2 with structures in the P 21/ c , Pnma , and C 2/ c space groups. The transition pressures are 4 and 32 GPa for the P 21/ c-Pnma and Pnma-C 2/ c phase changes, respectively. The P 21/ c and Pnma phases have previously been described, whereas the C 2/ c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters K T = 238 (±7), 230 (±5), 304 (±3) GPa, K ′T = 4 (fixed), 4 (fixed), 4 (fixed) GPa, γ = 1.45 (±0.18), 1.22 (±0.07), 1.21 (±0.12), and q = 1 (fixed), 2.90 (±1.5), 1 (fixed) for the P 21/ c , Pnma , and C 2/ c phases, respectively. The W-WO2 buffer (WWO) was extended to high pressure using these W and WO2 equations of state. The T - f O2 slope of the WWO buffer along isobars is positive from 1000 to 2500 K with increasing pressure up to at least 60 GPa. The WWO buffer is at a higher f O2 than the iron-wustite (IW) buffer at pressures lower than 40 GPa, and the magnitude of this difference decreases at higher pressures. This implies an increasingly lithophile character for W at higher pressures. The WWO buffer was quantitatively applied to W metal-silicate partitioning by using the WWO-IW buffer difference in combination with literature data on W metal-silicate partitioning to model the exchange coefficient ( K D) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the WWO-IW buffer difference. Calculation of K D along a peridotite liquidus predicts a decrease in W siderophility at higher pressures that supports the qualitative behavior predicted by the WWO-IW buffer difference, and agrees with findings of others. Comparing the competing effects of temperature and pressure the results here indicate that pressure exerts a greater effect on W metal-silicate partitioning.

Indigenous nitrogen in the Moon: Constraints from coupled nitrogen–noble gas analyses of mare basalts

Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by step-wise CO2 laser-extraction, static-mass spectrometry analysis, are reported for bulk fragments and mineral separates of ten lunar mare basalts (10020, 10057, 12008, 14053, 15555, 70255, 71557, 71576, 74255, 74275), one highland breccia (14321), and one ferroan anorthosite (15414). The mare basalt sub-samples 10057,183 and 71576,12 contain a large amount of solar noble gases, whereas neon and argon in all other samples are purely cosmogenic, as shown by their 21Ne/22Ne ratios of ≈0.85 and 36Ar/38Ar ratios of ≈0.65. The solar-gas-free basalts contain a two-component mixture of cosmogenic 15N and indigenous nitrogen (<0.5 ppm). Mare basalt 74255 and the olivine fraction of 15555,876 record the smallest proportion of 15Ncosm; therefore, their δ15N values of −0.2 to +26.7‰ (observed at the low-temperature steps) are thought to well represent the isotopic composition of indigenous lunar nitrogen. However, δ15N values ≤−30‰ are found in several basalts, overlapping with the isotopic signature of Earth's primordial mantle or an enstatite chondrite-like impactor. While the lowest δ15N values allow for nitrogen trapped in the Moon's interior to be inherited from the proto-Earth and/or the impactor, the more 15N-enriched compositions require that carbonaceous chondrites provided nitrogen to the lunar magma ocean prior to the solidification of the crust. Since nitrogen can efficiently be incorporated into mafic minerals (olivine, pyroxene) under oxygen fugacities close to or below the iron-wustite buffer (Li et al., 2013), the mare basalt source region is likely characterized by a high nitrogen storage capacity. In contrast, anorthosite 15414 shows no traces of indigenous nitrogen, suggesting that nitrogen was not efficiently incorporated into the lunar crust during magma ocean differentiation.

Natural moissanite (SiC) – a low temperature mineral formed from highly fractionated ultra-reducing COH-fluids

Natural moissanite (SiC) is reported from dozens of localities, most commonly from ultramafic rocks where it may be associated with diamond and iron silicides. Yet, formation conditions of moissanite remain in the realm of speculation. The key property of SiC is its extremely reduced nature. We have experimentally equilibrated SiC with olivine and orthopyroxene at 1300-1700°C, 2 and 10 GPa, to determine the oxygen fugacity of the C + orthopyroxene = SiC + olivine + O2 buffer (MOOC) and the equilibrium XMg of coexisting mantle silicates. The experiments resulted in olivine and orthopyroxene with XMg of 0.993-0.998 in equilibrium with SiC and iron silicides. Calculated oxygen fugacities are 5-6.5 log units below the iron-wustite (IW) buffer at 2-10 GPa. The experimental results concur with calculated phase relations for harzburgitic mantle under reducing conditions that include metal alloys, carbides and silicides. The extremely reducing character of SiC precludes coexistence with silicates with appreciable Fe 2+ , and hence excludes equilibrium with mantle phases with typical XMg’s of ~0.9. Calculated Fe-Mg diffusion lengths reveal that SiC grains of 1 mm would react with the Fe-component of olivine to iron carbide or metal and orthopyroxene within <1 Ma at te mperatures above 800°C. We thus conclude that SiC forms through a low-temperature process (<700-800°C) where equilibrium is only reached at the grain scale. The most plausible formation mechanism is a strong fractionation of a C-O-H fluid from metamorphosed sediments originally rich in organic material. Such a fluid is initially saturated with graphite or diamond and is slightly more reduced than the H2O-C join. Fluid percolation in the mantle leads to H2O-sequestration by crystallizing hydrous phases (most likely serpentine, brucite or phase A), and hence O2-removal from the fluid causing its reduction and continuous graphite or diamond precipitation. A small, highly fractionated fluid fraction may then reach CH4 and H2 concentrations that allow SiC formation on grain boundaries without equilibration with the bulk rock on a larger scale. Such a mechanism is corroborated by the strongly negative δ 13 C of moissanites (-20 to -37), consistent with reduced fluids originating from

Solution behavior of C-O-H volatiles in FeO-Na2O-Al2O3-SiO2 melts in equilibrium with liquid iron alloy and graphite at 4 GPa and 1550°C

In order to elucidate the solution behavior of carbon and hydrogen in iron-bearing magmatic melts in equilibrium with a metallic iron phase and graphite at oxygen fugacity (fO2) values 2–5 orders of magnitude below the iron-wustite buffer equilibrium, fO2 (IW), experiments were carried out at 4 GPa and 1550°C with melts of FeO-Na2O-SiO2-Al2O3 compositions. Melt reduction in response to an fO2 decrease was accompanied by a decrease in FeO content. The values of fO2 in the experiments were determined on the basis of equilibrium between Fe-C-Si alloy and silicate liquid. Infrared and Raman spectroscopy showed that carbon compounds are formed in FeO-Na2O-SiO2-Al2O3 melts: CH4 molecules, CH3 complexes (Si-O-CH3), and complexes with double C=O bonds. The content of CO2 molecules and carbonate ions (CO 3 2− ) is very low. In addition to carbon-bearing compounds, dissolved hydrogen occurs in melt as H2 and H2O molecules and OH− groups. The spectral characteristics of FeO-Na2O-SiO2-Al2O3 glasses indicate the occurrence of redox reactions in the melt, which are accompanied at decreasing fO2 by a significant decrease in H2O and OH−, a slight decrease in H2, and a significant concomitant increase in CH4 content. The content of species with the double C=O bond increases considerably at decreasing fO2 and reaches a maximum at ΔlogfO2(IW) = −3. According to the obtained IR spectra, the total water content (OH− + H2O) in the glasses is 1.2–5.8 wt % and decreases with decreasing fO2. The high H2O contents are due largely to oxygen release related to FeO reduction in the melt. The total carbon content at high H2O (4.9–5.8 wt %) is approximately 0.4 wt %. The carbon content in liquid iron alloys depends on silicon content and, probably, oxygen solubility and ranges from 0.3 to 3.65 wt %. Low carbon contents were observed at a significant increase in Si content in liquid iron alloy, which may be as high as ∼13 wt % at fO2 values 4–5 orders of magnitude below fO2(IW).

Phase relations in the carbon-saturated C–Mg–Fe–Si–O system and C and Si solubility in liquid Fe at high pressure and temperature: implications for planetary interiors

The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = −1.7 to −1.2, where ΔIW is the deviation of the oxygen fugacity (fO2) from an iron-wustite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < −3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO2, whereas the solubility of Si decreased with increasing fO2. The carbon-bearing phases were graphite, Fe3C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < −6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core.

Oxygen potential of diamond formation in the lower mantle

Thermodynamic calculations have shown that when a metallic phase arising due to ferroan ion disproportionation is contained in lower-mantle rocks, carbon occurs as iron carbide and the oxygen fugacity corresponds to the equilibrium of ferropericlase with Fe-Ni alloy. The typical values of oxygen fugacity in zones of diamond formation in the lower mantle lie between the iron-wustite buffer and six logarithmic units above this level. The processes that proceed in the lower mantle give rise to variation of \(f_{O_2 }\) within several orders of magnitude above the elevated \(f_{O_2 }\) values, which are necessary for the formation of diamond, as compared with a common level typical of the lower mantle. The mechanisms responsible for redox differentiation in the lower mantle comprise the subduction of oxidized crustal material, mechanical separation of metallic phase and silicate-oxide mineral assemblage enriched in ferric ions, as well as transfer of fused silicate material presumably enriched in Fe3+ through the mantle.

CO2 solubility in primitive martian basalts similar to Yamato 980459, the effect of composition on CO2 solubility of basalts, and the evolution of the martian atmosphere

To determine the influence of basalt composition on the CO2 solubility in martian lavas, we investigated experimentally a synthetic melt based on the martian meteorite Yamato 980459 (Y 980459), an olivine-phyric shergottite and a picritic rock (19 wt% MgO) thought to be a near-primary liquid derived from high-temperature (>1540 °C) partial melting of the martian mantle. Experiments were performed in a piston-cylinder apparatus at 1–2 GPa and 1600–1650 °C. CO2 contents in quenched glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.45–1.26 wt%. Despite large differences in FeO* and MgO contents, the CO2 solubilities in Y 980459 are similar to that in a less primitive synthetic martian basalt based on the Humphrey rock and to a Hawaiian tholeiite. The lack of enhanced solubility in Fe2+- and Mg2+-rich melts is likely owing to the complex structural role of these cations in silicate melts, acting partly as network formers, rather than network modifiers. The small sensitivity of CO2 solubility to compositional variations among martian and tholeiitic basalts means that the experimentally determined solubilities may be applicable to a wide spectrum of martian magmatic products. Using experimentally determined CO2 solubilities of Y 980459 and Humphrey allows the calibration of the thermodynamic parameters governing dissolution of CO2 vapor as carbonate in martian basalts. This relation facilitates calculation of the CO2 dissolved in magmas derived from graphite-saturated martian basalt source regions as a function of P , T , and f O2. The hot conditions in the source of Y 980459, 1540 ± 10 °C, and 1.2 ± 0.5 GPa, are plausible for plume-related magmas forming the giant Tharsis volcanic complex, which accounts for 50% of martian igneous activity since stabilization of the primordial crust. If oxygen fugacity in the sources of hot Tharsis magmatism were equivalent to that at the iron-wustite buffer (IW) or 1 log unit above (IW+1), respectively, then the entire Tharsis event would outgas 30–300 mbars of CO2 to the martian atmosphere, which is far from the 2 bars required to stabilize an equable climate in the late Noachian and early Hesperian epochs. This mismatch could be reconciled if significant martian igneous activity derived from comparatively oxidized mantle sources (i.e., IW+2) similar to those responsible for the nakhlite meteorites.