A. A. Kadik

Influence of oxygen fugacity on the solubility of carbon and hydrogen in FeO-Na2O-SiO2-Al2O3 melts in equilibrium with liquid iron at 1.5 GPa and 1400°C

Equilibria in the model melt (NaAlSi3O8(80) + FeO(20))-C-H2 system were experimentally studied at ΔlogfO2(IW) from −2.2 to −5.6, a pressure of 1.5 GPa, and a temperature of 1400°C. The experiments were conducted in a piston-cylinder apparatus using Pt capsules. The low fO2 values were imposed during the experiments by adding 2, 5, and 7 wt % of finely dispersed SiC to NaAlSi3O8(80) + FeO(20) powder. The experimental products were investigated by electron microprobe analysis and Raman spectroscopy. The investigations showed that melting at 1.5 GPa and 1400°C in the stability field of a metallic iron phase produces silicate liquids containing both oxidized and reduced H and C species. Carbon and hydrogen are dissolved in the melt as C-H (CH4) complexes. In addition, OH− groups, molecular hydrogen H2, and molecular water H2O were observed in the melts. The proportions of dissolved C and H species strongly depend on oxygen fugacity. With decreasing fO2, the content of O-H species decreases and that of H-C species increases. The obtained data and previous results (Kadik et al., 2004, 2006) allow us to suppose a fundamental change in the character of magmatic transfer of C-O-H components during the evolution of the redox state of the Earth’s mantle in geologic time toward higher fO2 in its interiors.

Experimental characterization of diamond crystallization in melts of mantle silicate-carbonate-carbon systems at 7.0–8.5 GPa

Diamond crystallization from carbon solutions in compositionally variable melts of model eclogite with dolomite [CaMg(CO3)2], potassium carbonate (K2CO3), and multicomponent K-Na-Ca-Mg-Fe carbonates was studied at 7.0–8.5 GPa. Concentration barriers for the nucleation of the diamond were determined at a standard pressure of 8.5 GPa for variable proportions of silicate and carbonate components in the growth solutions. They correspond to 35, 65, and 40 wt % of silicate components for systems with dolomite, K2CO3, and carbonatites, respectively. At higher contents of silicates in silicate-carbonate melts, the nucleation of diamond phase ceases, but diamond crystallization on seed crystals continues and is accompanied by the spontaneous crystallization of thermodynamically unstable graphite. In melts of the albite (NaAlSi3O8)-K2CO3-C compositions, the concentration barrier of diamond nucleation at 8.5 GPa is up to 90–92 wt % of the albite component, and diamond growth on seeds was observed in albite-carbon melts. Using mineralogical and experimental data, we developed a model of mantle carbonate-silicate (carbonatite) melts as the main parental media for natural diamonds; it was shown that the composition of the silicate constituent of such parental melts is variable and corresponds to the mantle ultrabasic-basic series. With respect to concentration contributions and dominant role in the genesis of diamond in the Earth’s mantle, major (carbonate and silicate) and minor or admixture components were distinguished. The latter include both soluble in carbonate-silicate melts (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble components (sulfides, metals, and carbides). Both major and minor components may affect the position of the concentration barriers of diamond nucleation in natural parent media.

Solubility of hydrogen and carbon in reduced magmas of the early Earth’s mantle

The solubility of volatile compounds in magmas and the redox state of their mantle source are the main factors that control the transfer of volatile components from the planet’s interior to its surface. In theories of the formation of the Earth, the composition of gases extracted by primary planetary magmas is accounted for by the large-scale melting of the early mantle in the presence of the metallic Fe phase [1, 2]. The fused metallic Fe phase and the melted silicate material experienced gravitational migration that exerted influence upon the formation of the metallic core of the planet. The large-scale melting of the early Earth should have been accompanied by the formation of volatile compounds, whose composition was controlled by the interaction of H and C with silicate and metallic melts, a process that remains largely unknown as of yet.

Decompression mechanism of ferric iron reduction in tektite melts during their formation in the impact process

The analysis of available data on the Fe3+/Fe2+ ratio of impact-produced glasses showed that tektites and some other types of impact glasses are reduced compared with the precursor target material. Possible reasons for the change in the degree of iron oxidation in the impact process are still debatable. Based on the analysis of redox reactions in relatively simple systems with iron in different oxidation states (Fe-O and SiO2-FeO-Fe2O3) and the available data on the influence of temperature, oxygen partial pressure (pO2), and total pressure (Ptot) on the Fe3+/Fe2+ ratio of silicate melts, a model was proposed suggesting that the lower Fe3+/Fe2+ values of tektites formed in the impact process compared with the initial target material could be related to the characteristics of oxygen regime during the decompression stage following shock compression. One of the main prerequisites for the occurrence of reduction reactions involving iron and other elements is the attainment of high temperatures (>1800–2000°C) at a certain stage of decompression, providing the complete melting and partial evaporation of the material. When the vapor pressure in the system becomes equal to the total pressure during adiabatic decompression, a further decrease in Ptot will be inevitably accompanied by a decrease in pO2 and, correspondingly, partial reduction of Fe3+ to Fe2+ in the melt. The reactions of decompression reduction occur under closed-system conditions and do not require oxygen removal from the system. The higher the temperature and Fe3+/Fe2+ ratio of the melt, the more extensive iron reduction can be observed during the final stages of decompression. If the temperatures attained during decompression after an impact event are sufficient (>2500–3000°C) for the complete evaporation of the material, the melt produced during subsequent condensation must be significantly more reduced than the initial material. The final stage of the impact process is characterized by a catastrophic expansion of the explosion cloud, condensation, and rapid cooling. During this stage, the system is already not closed. The quenched glasses of this stage record the redox state of earlier melts. In addition, they can contain microinclusions of the products of nonequilibrium vapor condensation with iron compounds of different oxidation states, including metallic iron and iron oxides (wüstite, magnetite, and hematite).

Solubility of nitrogen, carbon, and hydrogen in FeO–Na2O–Al2O3–SiO2 melt and liquid iron alloy: Influence of oxygen fugacity

Abstract—Reactions of nitrogen, carbon, and hydrogen with FeO–Na2O–Al2O3–SiO2 melts, liquid iron alloys, and graphite were investigated at 4 GPa, 1550°C, and fO2 values 1.5–3.0 orders of magnitude below fO2(IW). A number of features important for the understanding of the formation conditions of volatile nitrogen compounds during melting of the Earth’s early reduced mantle were revealed. The nitrogen content of melt increases with decreasing fO2 from 0.96 wt % at ΔlogfO2(IW) =–1.4 to 4.1 wt % at ΔlogfO2(IW) =–3.0, whereas the hydrogen content of melt is weakly dependent on fO2 and lies within 0.40–0.47 wt %. The carbon content is approximately 0.3–0.5 wt %. The IR and Raman spectroscopy of the glasses indicated that the dissolution of nitrogen, carbon, and hydrogen in silicate liquids is accompanied by the formation of NH3, N2, and CH4 molecules, as well as NH2–, NH2+, NH4+ and CH3– complexes. Hydrogen is dissolved in melts as OH–, H2O, and H2. The experiments also demonstrated the presence of species with C=O double bonds in the melts. It was found that the solubility of nitrogen in FeO–Na2O–Al2O3–SiO2 melts increases in the presence of carbon owing to the formation of species with C–N bonds in the silicate liquid. One of the most remarkable features of nitrogen, carbon, and hydrogen interaction with FeO–Na2O–Al2O3–SiO2 melts is a significant change in the proportions of N–C–H–O species at fO2 2–3 orders of magnitude below fO2(IW). Under these conditions, a sharp decrease in the contents of NH4+, NH2+ (O–NH2), OH, H2O, and CH4 is accompanied by enrichment in NH2– (=Si–NH2) and NH3. As a result, NH3 becomes the dominant nitrogen species in the melt. The investigation revealed high nitrogen solubility in iron alloys at fO2 < fO2(IW). The nitrogen content increases from 2.47 wt % at ΔlogfO2(IW) =–1.4 to 3.63 wt % at ΔlogfO2(IW) =–3.0. The carbon content of N–C–Fe alloys ranges from 2.3 to 3.8 wt % and decreases with decreasing fO2. The siderophile behavior of nitrogen at fO2 < fO2(IW) suggests that part of nitrogen could be dissolved in iron alloys during large-scale melting of the early reduced mantle with subsequent nitrogen burial in the Earth’s metallic core. It was suggested that the self-oxidation of magmas in the Earth’s early mantle with the release of reduced N–C–H–O volatiles could be one of the reasons of extensive nitrogen degassing.

Solution behavior of C-O-H volatiles in FeO-Na 2 O-Al 2 O 3 -SiO 2 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 (CO32−) 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).

Use of platinum capsules in the study of the carbon and hydrogen solubility in silicate melts in equilibrium with liquid iron alloys at high pressures and temperatures

Formation of C–O–H volatile compounds in the melting products of early reduced mantle ay oxygen (fO2) and hydrogen (fH2) fugacities controlled by equi librium of silicate liquids with a metallic Fe phase is a major focus of recent experimental studies [1–6]. The solubility of C–O–H volatiles in magmatic melts depends on pressure, temperature, magma composi tion, and redox conditions of magma formation (fO2 and fH2). It is one of the most difficult problems of experimen tal geochemistry to provide controlled fO2 and fH2 regime in high pressure experiments. The values of fO2 could be given by solid phase buffer equilibria: Fe + 1/2O2 = FeO (IW), 2FeO + 1/2O2 = Fe2O3 (WM), Ni + 1/2O2 = NiO (NiNiO), Co + 1/2O2 = CoO (CoCoO), and other oxide equilibria [7], with loading of studied silicate melt, C–O–H fluid, graphite, and oxygen buffer into a single Pt capsule. For instance, Jacobsen and Holloway [8] applied this method for studying the interaction between carbon (graphite) sat urated C–H–H fluid and basic melts at 1–2 GPa and 1030–1140°C. Standard method of buffering hydrogen fugacity, so called double Pt capsule technique, is based on the reaction of thermal water decomposition Н2О = Н2 + 1/2O2 in the presence of oxygen buffer mixtures [9]. Traditionally, the double capsule assembly consists of the outer H2 generating capsule with buffer mixture + Н2О and the inner capsule with studied sample. The walls of double capsules made up of noble metals (Pt, AgPd alloy) are permeable for hydrogen. Buffering hydrogen is provided by fH2 dif fusion from the outer into the inner capsule with the achievement of an equal chemical hydrogen poten tial in both the capsules. According to equations for constants of thermal decomposition of water K1 = fH2 ⋅ (fO2) /fH2O, each value of fO2 determined by oxygen buffer should correspond to definite fH2 value. The use of such method (capsule in capsule) in experi ments is usually constrained by small amounts of stud ied material (20–30 mg) and buffer mixture.

Formation of N–С–О–Н molecules and complexes in the basalt–basaltic andesite melts at 1.5 Gpa and 1400°C in the presence of liquid iron alloys

The contents and speciation of nitrogen, carbon, and hydrogen were determined in basalt–basaltic andesite melts in equilibrium with liquid Fe alloys at 1.5 Gpa, 1400°C, and oxygen fugacity (fO2) 1.4–1.9 log units below that of the Fe–FeO buffer (ΔlogfO2(IW) =–1.4 …–1.9). Experiments were carried out on a piston- cylinder type apparatus using welded Pt capsules in the presence of excess С (graphite). Starting mixture consisted of natural ferrobasaltic glass and silicon nitride (Si3N4) as nitrogen source in the system. Experimental quench products representing glasses with spherical inclusions of iron alloy were analyzed using electron microprobe, Raman, and IR spectroscopy. With increase of Si3N4 in the starting mixture and, respectively, decrease of fO2, silicate melt forming during experiments became depleted in FeO and enriched in SiO2. It was established that the nitrogen content in the glasses increases from 0.13 to 0.44 wt % with decrease of ΔlogfO2(IW) from–1.4 to–1.9, whereas C content in the first approximation remains constant within 1.18–1.13 wt %, while the total water content (ОН– + Н2О) determined by IR spectroscopy decreases from 4.91 to 1.20 wt %. The N (0.13–0.48 wt %) and C (0.75–2.26 wt %) contents determined in the Fe alloy show no clear correlation with fO2. The IR and Raman spectroscopic study of the glasses indicates the formation of molecules and complexes with bonds N–H (NH3, NH2−, NH2+, NH4+), Н–О (Н2О, OH–), С–Н (СН4) as well as N2 and Н2 molecules in silicate melts. IR spectra also reveal the presence of complexes with С=О, С–N bonds and СО2 molecules. Obtained data are compared with results of previous studies on the solubility and speciation of N, С, and Н in the model FeO–Na2O–SiO2–Al2O3 melts in equilibrium with liquid iron alloys at 1.5 GPa (1400°C) and 4 GPa (1550°C) (Kadik et al., 2011, 2015).

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.