Graphite Capsules Research Articles

Deciphering the corrosion puzzle: Nano-iron-biochar composite – Not a quick fix for metal immobilization in peat soils

The use of zero-valent iron nanoparticles in the remediation of metal-contaminated soils has received considerable attention. Upon introduction into the soil, zero-valent iron particles corrode into iron oxides, known for their high adsorption capacity for potentially toxic metals. While limited research has directly compared zero-valent iron micro- and nanoparticles, it is important to investigate whether particle size contributes to effective soil remediation. This study focuses on elucidating the comparative kinetics of iron corrosion using iron powder and a nano-iron-biochar composite. In a model experiment, these materials were exposed to a cellulose/biochar mixture and peat for five months. Mössbauer spectroscopy, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and transmission electron microscopy with electron diffraction were used to analyze the corrosion products. The results show a slow corrosion rate in the nano-iron-biochar composite in cellulose due to the protective effect of biochar on the embedded iron nanoparticles. In addition, iron corrosion in peat was inhibited, likely due to the presence of humic substances. Transmission electron microscopy after five months of corrosion revealed round metal particles encased in a graphite capsule with visible channels. Large dissolved organic matter molecules in the peat likely block these channels, inhibiting metallic iron corrosion. Consequently, the nano-iron-biochar composite emerges as a slow-reacting option for immobilizing soil metals in peat. This study highlights the need for further research involving long-term field experiments.

Experimental study of the interaction between garnets of eclogitic and lherzolitic parageneses and H2O-CO2 fluid under the P-T parameters of the lithospheric mantle

Experimental modeling of the interaction between garnets of mantle parageneses with H2O–CO2 fluid was performed in order to determine the characteristic features and trends in garnet compositions due to metasomatic alterations, and to study the patterns of formation of carbon and carbon-bearing phases in these processes. Experiments were carried out in the lherzolitic garnet–CO2–H2O–C and eclogitic garnet–CO2–H2O–C systems at 6.3 GPa in the temperature range of 950–1550 °C using double graphite+platinum ampoule on a multi-anvil high-pressure apparatus of a split-sphere type (BARS). The interaction of mantle garnets with H2O–CO2 fluid was found to cause garnet recrystallization and formation of new phases, namely magnesian carbonate, coesite, kyanite, corundum–eskolaite solid solution and silicate–carbonate melt, as well as metastable graphite (950–1550 °C) and diamond growth on seeds (1250–1550 °C). It was established that the garnet-fluid interaction was accompanied by the bivalent cations' redistribution between recrystallized garnet, carbonate and carbonate–silicate melt. In these processes calcium was preferentially partitioned in the melt, while magnesium was partitioned in garnet and carbonate. Recrystallization of garnets of lherzolitic and eclogitic parageneses under effect of H2O–CO2 fluid resulted not only in compositional changes, but in the formation of carbonate, kyanite, coesite, graphite, corundum–eskolaite solid solution and carbonate-silicate melt inclusions in these garnets. The revealed patterns of variations in garnet compositions, as well as the typical inclusions in them, can be regarded as indicators of metasomatic alteration of garnets of mantle parageneses involving H2O–CO2 fluid. The obtained results allow one to infer that H2O–CO2 fluids play a crucial role in redistribution of divalent cations under the P-T parameters of the lithospheric mantle and control over the composition of carbonate–silicate melts coexisting with the eclogitic and lherzolitic assemblages. Another important result of this study is the experimental demonstration that graphite capsules on their own fail to ensure reliable hermeticity with respect to the fluid and melt phases in the high-pressure, high-temperature experiments. It was substantiated that the procedure involving a hermetically sealed platinum ampoule with inner graphite capsule prevents uncontrollable loss of matter in long-term petrological experiments and is optimal for modeling systems containing melts and fluids.

A novel method for experiments in a one-atmosphere box furnace

Abstract We present a conceptually simple method to perform high-temperature experiments in a one-atmosphere box furnace that has a negligible cost of materials. The experimental setup consists of two commercially available materials and can be customized to sample or furnace size with few limitations. Furthermore, the design allows easy extraction of samples in one piece, making them eligible for textural analysis. The setup comprises a graphite capsule and a fireclay shell, the latter of which acts as a heat-resistant protective shield. Containers must be individually hand-crafted, but each can hold multiple samples. The setup can be reliably used in temperature conditions below the heat tolerance limit of the commercial fireclay, commonly ~1400 °C. Moreover, the graphite capsule buffers the oxygen fugacity to strongly reducing conditions during the experiment. The main advantage of our method lies in the utilization of easily accessible and low-cost materials that provides a widely applicable experimental setup easily used at larger scales. The method was developed during an experimental study of magmatic crystal-liquid suspensions and was reliable for experiments lasting for up to 36 h.

Experimental Modeling of the Interaction between Garnets of Mantle Parageneses and CO2 Fluid at 6.3 GPa and 950–1550 °C

Abstract —Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

An experimental investigation of factors controlling the oxygen content of sulphide melts in the Earth’s upper mantle

To determine the factors that control the oxygen content of sulphide melts in the upper mantle, mono-sulphides were equilibrated with mantle peridotite assemblages, with varying FeO and NiO contents, between 3 and 13 GPa and 1300 to 1819 °C. Carbonates were added as a flux and the use of graphite capsules allowed oxygen fugacities (fo2) to be estimated for most of the experiments. Sulphide melt O concentrations were in the range 0.2–3.7 wt. %, coexisting with silicate assemblages with olivine Fe/(Fe + Mg) ratios between 0.04 and 0.25. Except in Ni-bearing experiments, variations in the sulphur/metal cation ratio did not affect the sulphide melt O contents, which also appeared to be independent of fo2. The silicate FeO contents, temperature and pressure were found to be the main controls on O contents. Nickel lowers the O content, although all Ni-bearing experiments produced sulphides with lower sulphur/metal ratios, making it difficult to categorically separate the effect of Ni alone. A preliminary geothermometer expression was developed based on the O content of the sulphide melts and the iron oxide concentrations of coexisting olivine and orthopyroxene. With this expression, the experimental temperatures are reproduced to within 74 K for Ni-free experiments and within 135 K for Ni-bearing samples. Using measurements of the O contents of sulphide inclusions of peridotite affinity in diamonds from the Lac de Gras kimberlite field, a plausible average entrapment temperature of 1318 ± 48 °C is calculated. Mantle peridotite assemblages would be expected to contain sulphide melts with approximately 0.4–0.6 wt. % O along a typical mantle adiabat down to 200 km depth.

Tin disulfide with bright near-IR luminescence centers obtained at high pressures

In this work, the high-pressure and high-temperature (HPHT) method was applied to obtain crystalline tin disulfide (${\mathrm{SnS}}_{2}$). The synthesis was performed in a sulfur-tin-carbon growth system by placing the reaction mixture of sulfur and tin in a graphite capsule. The synthesized ${\mathrm{SnS}}_{2}$ demonstrated to be a highly crystalline $2H$-polytypic form suitable for mechanical exfoliation of atomically thin films. The crystals reveal strong near-IR photoluminescence attributed to a new luminescence center in ${\mathrm{SnS}}_{2}$. At 5 K, this center is characterized by a narrow zero-phonon line at 885 nm and phonon sideband whose structure is governed by ${\mathrm{SnS}}_{2}$ phonon density of states. Based on $\mathit{ab}$ initio calculations, the center is attributed to carbon in the tin sublattice. The side product of the HPHT synthesis is the diamond phase containing optically active tin-vacancy centers (${\mathrm{SnV}}^{\ensuremath{-}}$).

The fate of nitrogen during parent body partial melting and accretion of the inner solar system bodies at reducing conditions

Evolution of nitrogen (N), a life-essential volatile element, in highly reduced magmatic systems is a key for the origin of N on rocky planets formed via accretion of reduced chondritic parent body materials, planetesimals, and embryos that underwent partial or complete differentiation. However, the storage capacity of N in phases relevant for reduced silicate systems undergoing thermal processing is poorly known. To investigate the stability of N-bearing phases in partially molten silicate-rich systems as well as solubility of nitrogen in silicate melts and minerals, we performed laboratory experiments on a 80:20 synthetic basalt-Si3N4 mixture at 1.5–3.0 GPa and 1300–1600 °C in graphite capsules, yielding oxygen fugacity ranging from ∼ IW– 3.0 to ∼ IW – 4.0. All experiments produced silicate melt + nierite + Fe-rich alloy melt + N-rich vapor ± sinoite ± cpx. Sinoite was restricted to above while cpx was restricted below 1400–1500 °C. Nitrogen solubility and Nitrogen Concentration at Silicon-Nitride Saturation (NCNS) in silicate melts increase with increasing pressure and temperature and range between 3.6 and 9.5 wt%. Using our high pressure N solubility data and similar data at ambient and lower pressures, we derived a new N solubility model in silicate melts. Solubility of nitrogen in cpx was between 1.51 and 2.05 wt% and resulted in cpx/silicate melt partition coefficients for nitrogen, DNcpx/silicatemelt of ∼ 0.4 to ∼ 0.2. These DNcpx/silicatemelt are distinctly higher than those previously estimated at more oxidizing conditions, suggesting N maybe much less incompatible during thermal processing of rocky reservoirs at highly reducing conditions. Partition coefficient of N between Fe-rich alloy melt and cpx, DNFe-richalloymelt/cpx was found to be between 1.6 and 2.1. The application of our N solubility data and model suggests that mobilization of N from the deeper, partially molten reservoirs to shallower reservoirs is possible in reduced planetesimals and internally differentiated meteorite parent bodies – leading to net loss of N via melt degassing or reprecipitation of N-bearing solid phases, depending on whether the surficial shell is oxidized or reduced, respectively. Similarly, comparison of the first measured DNcpx/silicatemelt values from our highly reducing experiments with those estimated at more oxidizing conditions suggest that N would be much less incompatible during internal and external magma ocean processing of rocky bodies under highly reducing conditions. Therefore, enrichment of N in the atmospheres of Earth and Venus is likely a result of more oxidizing penultimate phase of accretion, which would lead to N being more readily partitioned to residual liquid, which would also more readily degas at oxidizing conditions.

Metal Saturated Cumulates from Siberia — Lunar Basalt Analogues?

Abstract It is not well known which chemical differentiation pathways basaltic melts will take when they are iron metal saturated. Thermodynamically, the pathway seems predictable. So long as Fe metal is a stable liquidus phase and relative oxygen fugacity (fO2) is not subject to major fluctuations, the activity of FeO (aFeOmelt) is buffered by the iron–wüstite (IW) equilibrium 2Femetal + O2 → 2FeOmelt. Metallic Fe also stabilizes olivine through the equilibrium 2Femetal + O2 + SiO2 melt → Fe2SiO4 olivine. That equilibrium tends to suppress the enrichment in bulk SiO2 when Fe saturated basaltic melts differentiate. We document the differentiation history of tholeiitic cumulates from the Siberian craton that carry up to 30 modal % metallic Fe. Our study is complemented by differentiation experiments at two redox states, one set in Fe metal capsules at 1.6 log units below IW (IW-1.6) and a second set in graphite capsules at IW + 1.5. Iron saturated differentiation pathways do not show enrichments in FeO nor in bulk SiO2 because olivine remains stable along the entire liquid line of descent. By contrast, melts equilibrated at IW + 1.5, that is, outside metallic Fe saturation, crystallize pigeonite as first silicate and follow a normal (terrestrial) differentiation pathway involving marked SiO2 enrichment. The Fe-saturated path duplicates in detail the liquid line of descent we derive for the cumulates. Iron-saturated experiments have limited applicability to the Earth because there are so few terrestrial basalts saturated with metallic Fe; however, they might apply to the Moon. Many lunar basalts appear to have been saturated with an Fe-Ni phase during their emplacement on the lunar surface, and potentially during generation within the lunar mantle.

The influence of variable oxygen fugacity on the source depths of lunar high-titanium ultramafic glasses

We present the results of high pressure, high temperature multiple saturation experiments at variable oxygen fugacity (fO2) conditions (IW+1.5 and IW−2.1) on three lunar high titanium ultramafic glasses: the Apollo 17 Orange glass (A17O, 9.1 wt% TiO2), the Apollo 15 Red glass (A15R, 13.8 wt% TiO2), and the Apollo 14 Black glass (A14B, 16.4 wt% TiO2). We performed experiments in graphite (fO2 = IW+1.5) and iron (fO2 = IW−2.1) capsules. The experimentally determined multiple saturation points (MSPs) in graphite capsules are 2.5 GPa and ∼1530 °C (A17O), 1.3 GPa and ∼1350 °C (A15R), and 1.55 GPa and ∼1425 °C (A14B). In iron, we found MSPs of 3.3 GPa and ∼1565 °C (A17O), 2.8 GPa and ∼1490 °C (A15R), and 4.0 GPa and ∼1540 °C (A14B). These results, when combined with previous experiments on the lunar ultramafic glasses, indicate that the increase in the pressure of multiple saturation is linearly proportional to the TiO2 content of the melt (ΔMSPGPa=0.14×TiO2(wt%)-0.15, R2 = 0.93, RMSE = 0.2 GPa). The high depths of melting correlated with the lowest fO2 conditions are hard to reconcile with buoyancy constraints on these iron and titanium rich magmas. In addition, measurements of fO2 on the orange glass as well as the presence of iron blebs in the glasses suggest that the glasses were reduced during eruption. To reconcile buoyancy constraints with fO2 estimates, we present a model in which the high titanium magmas experienced higher fO2 conditions at their source, but underwent subsequent reduction at shallow depths (4–52 km) just prior to their eruption. In this model, we can then further bracket the depth of melting to be from the minimum multiple saturation pressure in graphite to the deepest depth at which the magmas are buoyant: assuming the Hess and Parmentier (1995) post overturn cumulate mantle, the depths of melting range from ∼550–770 km for the A17O glass, ∼260–490 km for the A15R glass, and ∼320–350 km for the A14B glass.

The effect of composition on chlorine solubility and behavior in silicate melts

Abstract We have performed experiments at 1.5 GPa and 1400 °C on 25 different bulk compositions to determine the effects of major element compositions on the Cl contents of silicate melts at known fugacities of Cl2 and O2. The experimental method involved mixing a “sliding” Cl buffer, a mixture of AgCl, AgI, and Ag with the silicate bulk composition and performing the experiment in a graphite capsule together with a source of CO2 (AgCO3). The graphite capsules were sealed inside welded Pt tubes to maintain a CO2-CO atmosphere with oxygen fugacity fixed at the C-CO-CO2 (CCO) buffer. During the experiment, the Cl buffer segregates leaving a Cl-bearing melt, which quenches to a glass. We used the results to define chloride capacity CCl for each melt at the pressure and temperature of the experiment: C Cl = Cl ( wt% ) f ( Cl 2 ) + f O 2 4 Chloride capacity was found to correlate positively with optical basicity and NBO/T and negatively with ionic porosity and the Larsen index. We combined our new data with the results of Thomas and Wood (2021) to derive an equation describing the composition, pressure and temperature dependence of the chloride capacity: log ⁡ C C l = 1.604 + ( 4470 X C a − 3430 X S i + 2592 X F e − 4092 X K − 894 P ) / T . In this equation, XCa, XSi, and so on refer to the oxide mole fractions on a single-cation basis, P is in GPa and T in K. The equation reproduces 58 data points with an r2 of 0.96 and a standard error of 0.089. The addition of literature data on hydrous experiments indicates that the effects of <4.3 wt% H2O are small enough to be ignored. We also performed experiments aimed at determining the conditions of NaCl saturation in melts. When combined with literature data we obtained: log ⁡ ( C l − ) = log ⁡ ( a N a C l ) + 0.06 − ( 2431 X C a + 3430 X S i − 2592 X F e + 3484 X N a + 4092 X K − 2417 ) / T where (Cl–) is the Cl content of the melt in wt% aNaCl is the activity of NaCl (liquid) and the other symbols are the same as before. The results indicate that basalt dissolves ~8 times more Cl than rhyolite at a given NaCl activity i.e., Cl is ~8 times more soluble in basalt than in rhyolite.

Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ({D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}}) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} = 0.63 ± 0.10 and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{cpx}/mathrm{melt}} = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}} function alone allows ~ 40% of the variation in MORB compositions. If we allow {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

Defect-Enhanced Electromagnetic Wave Absorption Property of Hierarchical Graphite Capsules@Helical Carbon Nanotube Hybrid Nanocomposites.

Development of high-performance materials for electromagnetic wave absorption has attracted extensive interest, but it still remains a huge challenge especially in reducing density and lowering filler loading. Herein, a hierarchical all-carbon nanostructure is rationally designed as follows: the defect-rich hollow graphite capsules (GCs) controlled by the size/density of ZnO templates are synthesized on the surface of helical carbon nanotubes (HCNTs) to form a hybrid nanocomposite, denoted as GCs@HCNTs. As a result, the GCs@HCNTs demonstrate a strong and wide absorption performance with a very low filler loading of 10 wt %. The minimum reflection loss reaches -51.7 dB at 7.6 GHz, and the effective bandwidth (below -10 dB) ranges from 8 to 14 GHz, covering the whole X or Ku bands. The hierarchical nanostructure and homoatomic heterogeneous interface are beneficial to impedance matching and bring additional dipole polarization enhanced by the structural defects, which may enlighten the design of ultralight and broadband high-performance electromagnetic wave absorption materials.

Partitioning of chalcophile and highly siderophile elements (HSEs) between sulfide and carbonated melts – Implications for HSE systematics of kimberlites, carbonatites, and melt metasomatized mantle domains

Highly Siderophile Elements (HSEs; Os, Ru, Ir, Rh, Pt, Pd, Au and Re) combined with their isotopic systematics (Re-Os and Pt-Os) are powerful tools for tracking evolution and genesis of mantle derived magmas. Given sulfides (accessory sulfide minerals and/or molten sulfides) are the primary hosts of HSEs in the mantle and low-degree carbonated melts are extracted from large portions of mantle volume, partitioning of HSEs between sulfide and carbonated melt might play a critical role in distributing HSEs between the mantle and crustal reservoirs. Although, partitioning of HSEs and chalcophile elements between sulfide melt and silicate melt has been previously studied, partitioning of these elements between sulfide melt and carbonated melts has not received much attention. Here we use high P-T experiments to determine the partitioning of HSEs and chalcophile elements (Ni, Co, Mo, Os, Ru, Pd, Pt and Re) between (i) sulfide melt and carbonated silicate melt (CO2 ~ 17 wt.%) and (ii) sulfide melt and carbonatitic melt (CO2 ~ >30 wt.%) at a pressure (P) of 3 GPa and temperatures (T) of 1300–1600 °C in graphite capsules. All experiments produced quenched Fe-sulfide melt blobs + carbonated silicate melt matrix. Concentrations of major elements were measured using electron microprobe, and HSEs and chalcophile elements were measured using LA-ICP-MS. We find that all the elements measured are compatible in the sulfide melt to varying degrees and their Dsulfide/carb. melt sequence is Mo < Co < Ni < Re < Pt ≤ Pd < Ru ≤ Os varying from around 10 for Mo to 105 for Os. Comparing the Dsulfide-carb. melt with Dsulfide-silicate from previous studies, we show that the partition coefficients of HSEs between sulfide and carbonated melts are lower than the partition coefficients of these elements between sulfide and silicate melts, indicating greater mobilization of these elements in carbonatites and carbonated silicate melts. Calculating bulk D (D¯) for carbonated peridotite using our experimentally measured D values, we model the HSE contents of mantle derived low-degree partial melts using an aggregate fractional melting equation and compare the primitive mantle normalized HSE patterns of our model with natural kimberlites, carbonatites, ocean island basalts, and alkaline basalts. We also calculate proportions of sub-lithospheric continental mantle (SCLM) xenolith detritus in the natural kimberlite and carbonatite samples from Karelian, Kaapvaal, Canadian shield and North China craton by using mass balance calculations based on Ru concentration in the primary carbonated melt and the SCLM xenoliths. Our calculations show that detritus proportion in natural kimberlites are 2–28% for Karelian, 7–28% for Kaapvaal, and 6–16% for Canadian shield, which are in agreement with previous studies using various other proxies. We also show that the extent of Re/Os fractionation is less for events of carbonate melt metasomatism as compared to similar events of basaltic melt metasomatism.

Rapid reduction of basaltic glasses in piston-cylinder experiments: a XANES study

Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (µ-XANES) to obtain their Fe3+/ΣFe profiles near the graphite–melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3–1.9 mm thickness. Even for a “time-zero” experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 µm in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 °C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/ΣFe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/ΣFe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at ~ 1300 °C and ~ 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO −0.9, which agrees well with the measured Fe3+/ΣFe ratios in the basaltic glasses and the estimated oxygen fugacity for graphite and Pt–graphite capsule experiments from a previous study. Future studies are needed to better constrain the kinetics of this process at different temperature, pressure, and in silicate melts of different compositions. The dynamic process of H2 diffusing and reducing Fe3+ to Fe2+ shown in our experiments also provides a potential way to determine the diffusivity and solubility of molecular hydrogen in silicate melts, which are crucial in understanding volatile behaviors on reducing planetary bodies, such as the Moon and Mercury.

Carbon partitioning between metal and silicate melts during Earth accretion

In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5–6.0 GPa, 1300–2000°C at oxygen fugacities of −0.9 to −1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt.Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain ∼7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients DCmetal/silicate at 1.5 GPa, 1300–1750°C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, DCmetal/silicate is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4–2.6 wt% carbon, resulting in 0.14–0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer.

Metasomatic interaction of the eutectic Na- and K-bearing carbonate melts with natural garnet lherzolite at 6 GPa and 1100–1200°C: Toward carbonatite melt composition in SCLM

The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed.We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K‑carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2–0.3 wt%), and higher K2O (0.5–1.0 wt%), whereas the Na‑carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3∙60Ca0.5Mg0.4Fe0.1CO2 + 0.6–1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100∙Ca/(Ca + Mg + Fe). We have also found that alkali-poor (≤ 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures.Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100–1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3∙70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40–45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ≤ 30–40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM.Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30–40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40–60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.

Sulfur extraction via carbonated melts from sulfide-bearing mantle lithologies – Implications for deep sulfur cycle and mantle redox

Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep global sulfur cycle. Given that sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur concentration at sulfide saturation (SCSS) of mantle-derived magmas. CO2 ± H2O-induced melting beneath oceanic and continental mantle produces incipient CO2-rich melts. Although, SCSS of silicate melts of a variety of compositions is extensively studied, the SCSS of carbonatitic and carbonated silicate melts have not received much attention. Here we present experiments in graphite capsules at pressures (P) of 2.5–6.0 GPa and temperatures (T) of 1350–1650 °C investigating the SCSS of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe ± Ni-sulfide melt blobs + carbonated melt matrix ± ol ± cpx ± opx ± gt, with melt composition on a CO2-free basis varying from 7 to 40 wt.% SiO2, 0.5 to 7 wt.% Al2O3, and 9 to 17 wt.% FeO* (total FeO). SCSS measured using EPMA increases with SiO2 and T but is not affected by P; the effect of composition being more pronounced than P-T. The composition of sulfide melt phase also affects SCSS. With increasing Ni in the molten sulfide phase, the SCSS changes from 2000 to 4000 ppm (Ni-free) to is 800–3000 ppm (33 wt.% Ni). Comparison of our measured SCSS with the existing SCSS models for nominally CO2-free silicate melts and with one study for carbonated melts show that these parameterizations fail to capture the sulfide saturation values in CO2-rich melts from our study. Using our new SCSS data and previous SCSS data for melt compositions that span the range from carbonatite to basalts via carbonated silicate melts, we develop a new empirical SCSS parameterization. Unlike a previous model, which suggested SCSS of carbonated melt is only affected by melt FeO* (other than P-T) and did not constrain how SCSS evolves from low-silica carbonatitic melt to low-CO2 basaltic melt, our new parameterization captured complex effects of many melt compositional parameters, including silica on SCSS. Using our new SCSS model, we constrained the efficiency of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated melts beneath ridges are expected to mobilize 5–15% of the initial sulfur before nominally-volatile-free peridotite melting begins. In continental mantle, deep kimberlitic melt can act as an agent to mildly enrich the shallow mantle in sulfide as it evolves to a carbonatitic melt upon reactive cooling. Application of our data to subduction zones suggests that low degree carbonatitic melt is not an efficient agent to extract residual sulfide from the subducting oceanic crust.

Synthesis and Raman spectra of K-Ca double carbonates: K2Ca(CO3)2 bütschliite, fairchildite, and K2Ca2(CO3)3 at 1 ATM

Here we present results on synthesis of double K-Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K2CO3 and CaCO3 corresponding to stoichiometry of K2Ca(CO3)2 and K2Ca2(CO3)3 were used as starting materials. The low-temperature modification of K2Ca(CO3)2 was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K2Ca(CO3)2 as well as the K2Ca2(CO3)3 compound were synthesized both by a solid-state reaction at 600°C during 72 h and during cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the presence of an intense band at 1093 cm-1 and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm-1. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm-1, and several bands at 1760, 1739, 719, 704, 167, 100 сm-1. In the Raman spectrum of K2Ca2(СO3)3 intense bands at 1078 and 1076 cm-1 and several bands at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm-1 were found. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks.

Trace element partitioning between CAI-type melts and grossite, melilite, hibonite, and olivine

We determined the mineral-melt partition coefficients (Di’s) and the compositional and/or temperature dependency between grossite, melilite, hibonite, olivine and Ca-, Al-inclusion (CAI)-type liquids for a number of light (LE), high field strength (HFSE), large ion lithophile (LILE), and rare earth (REE) elements including Li, Be, B, Sr, Zr, Nb, Ba, La, Ce, Eu, Dy, Ho, Yb, Hf, Ta, Th. A series of isothermal crystallization experiments was conducted at 5 kbar pressure and IW + 1 in graphite capsules. The starting compositions were selected based on the calculated and experimentally confirmed phase relations during condensation in CI dust-enriched systems (Ebel and Grossman, 2000; Ebel, 2006; Ustunisik et al., 2014).The partition coefficients between melt and gehlenite, hibonite, and grossite show that the trace element budget of igneous CAIs is controlled by these three major Al-bearing phases in addition to pyroxene. In general, LE, LILE, REE, and HFSE partition coefficients (by mass) decrease in the order of Gehlenite-MeltDi > Hibonite-MeltDi > Grossite-MeltDi. The partition coefficients between gehlenitic melilite and CAI melt are approximately Be:0.14, B:0.07, Sr:0.79, Zr:0.02, Nb:0.01–0.02, Ba:0.05, La:1.03–3.44, Ce:1.2–3.86, Eu:1.19–2.88, Dy:1.14–3.23, Ho:1.04–2.91, Yb:0.7–1.70, Hf:0.02, Ta:0.01–0.02, Th:0.31–1.71. These results suggest that Gehlenite-MeltDi vary by a factor of 2–3 in different melt compositions at the same T (∼1500 °C). Hibonite-MeltDi exhibit a range as Be:0.02–0.04, B:0.01, Sr:0.21–0.66, Zr:0.02–0.18, Nb:0.03–0.05, Ba:0.02–0.06, La:0.56–4.38, Ce:0.52–3.54, Eu: 0.33–0.84, Dy: 0.25–0.32, Ho:0.17–0.29, Yb:0.05–0.19, Hf:0.05–0.38, Ta:0.02, Th:0.31–1.71. Increased Al and Ca, relative to earlier work, increases the compatibility of Gehlenite-MeltDi, and also the compatibility of Hibonite-MeltDi, especially for La and Ce. Grossite-MeltDi of individual mineral-melt pairs are Be:0.43, Sr:0.31, Zr:0.09, Nb:0.01, Ba:0.03, La:0.06, Ce:0.07, Eu:0.13, Dy:0.04, Ho:0.04, Yb:0.03, Hf:0.01, Ta:0.01, Th:0.01 for #18 at 1550 °C and as Sr:0.29, Nb:0.03, La:0.07, Ce:0.09, Eu:0.10, Dy:0.05, Ho:0.04, Yb:0.02, Hf:0.003, Ta:0.02, Th:0.02 for #7 at 1490 °C.Olivine partitioning experiments confirm that olivine contribution to the trace element budget of CAIs is small due to the low Olivine-MeltDi at a range of temperatures while Olivine-MeltDEu, Yb are sensitive to changes in T and fO2. The development of a predictive model for partitioning in CAI-type systems would require more experimental data and use of analytical instruments capable of obtaining single phase analyses for crystals <5 µm.

Crystallization of the lunar magma ocean and the primordial mantle-crust differentiation of the Moon

We present crystallization experiments on silicate melt compositions related to the lunar magma ocean (LMO) and its evolution with cooling. Our approach aims at constraining the primordial internal differentiation of the Moon into mantle and crust. We used graphite capsules in piston cylinder (1.35–0.80 GPa) and internally-heated pressure vessels (<0.50 GPa), over 1580–1020 °C, and produced melt compositions using a stepwise approach that reproduces fractional crystallization. Using our new experimental dataset, we define phase equilibria and equations predicting the saturation of liquidus phases, magma temperature, and crystal/melt partitioning for major elements relevant for the crystallization of the LMO. These empirical expressions are then used in a forward model that predicts the liquid line of descent and crystallization products of a 600 km-thick magma ocean. Our results show that the effects of changes in the bulk composition on the sequence of crystallization are minor. Our experiments also show the crystallization of a silica phase at ca. 1080 °C and we suggest that this phase might have contributed to the building of the lower anorthositic crust. Calculation of crustal thickness clearly shows that a thin crust similar to that revealed by GRAIL cannot have been generated through solidification of whole Moon magma ocean. We discuss the role of magma ocean depth, trapped liquid fraction (with implication for the alumina budget in the mantle and the crust), and the efficiency of plagioclase flotation in producing the thin crust. We also constrain the potential range of pyroxene compositions that could be incorporated into the crust and show that delayed crustal building during ca. 4% LMO crystallization on the nearside of the Moon may explain the dichotomy for Mg-number. Finally, we show that the LMO can produce magnesian anorthosites during the first stages of plagioclase crystallization.