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).

We present the results of electrochemical measurements of the intrinsic oxygen fugacity of olivine separates representing seven rock types from the central part and southwestern termination of the Yoko–Dovyren mafic—ultramafic massif. The \({f_{{O_2}}}\) values were determined using a high-temperature solid-electrolyte double-cell assembly developed at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. A total of 59 experiments were performed (from 7 to 16 for each sample) at the atmospheric pressure and within the temperature range of 800–1050°C at the 30–50°C increment. The results were interpreted using the calculated log \({f_{{O_2}}}\) – 1/T(K) and log \({f_{{O_2}}}\) – T(°C) dependences. It was shown that the subsolidus temperature range of the rocks (below ~1050°C) is characterized by lowest intrinsic oxygen fugacity of olivine, which is 1–4 log units below the QFM buffer. For the solidus temperatures of ~1100°C, the more oxidized conditions ranging approximately from QFM to ~QFM-3 were measured. Extrapolating the log \({f_{{O_2}}}\) – T°C dependences to the temperatures of the original magmas (~1200–1300°C) produces the maximum scatter in oxygen fugacities from ~QFM+2.5 to QFM-1. The estimated range of redox conditions for the Dovyren magma crystallization lies between the QFM and ~QFM-2.5 buffer equilibria. This is consistent with the complete absence of primary magmatic titanomagnetite and the presence of ilmenite in the Dovyren rocks.

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, NH 2 + , NH 2 - ), 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 NH 2 + , NH 2 - (O–NH2), OH–, H2O, and CH4 and simultaneous increase of NH 2 - (≡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.

Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al2O3–Cr2O3–SiO2 system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg3Cr2Si3O12 included chromiumbearing enstatite MgSiO3 (up to 3.2 wt % Cr2O3) and eskolaite Cr2O3. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.

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 NH 2 – , NH 2 + , NH 4 + and CH 3 – 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 NH 4 + , NH 2 + (O–NH2), OH, H2O, and CH4 is accompanied by enrichment in NH 2 – (=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.

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).

Solubility and speciation of NOH volatiles in a model silicate melt (FeO–Na2O–Al2O3–SiO2) equilibrated with molten Fe alloy have been examined via nitrogen and hydrogen analyses and vibrational spectroscopy (Raman and FTIR). Experiments were performed in an anvil-with-hole apparatus conducted at 4GPa, 1550°C, and oxygen fugacity (fO2) from 2.1 to 3.3log units below IW buffer. The technique of hydrogen fugacity (fH2) buffering via the dissociation of H2O employed here relies upon the diffusion of H2 through Pt to achieve equal chemical potentials of H2 in the Pt capsule and outer assemblage elements. The nitrogen source was Si3N4. The fO2 imposed on the charge was controlled by redox reactions between H2 buffered externally, Si3N4 and components of the Fe-bearing melt that was reduced with O2 liberation and metallic Fe formation. The initial Si3N4 was unstable under the experimental conditions and completely consumed according to the reaction of oxidation: Si3N4 (initial)+3O2→3SiO2 (melt)+2N2 (melt) with a subsequent participation of nitrogen in the reactions with H2, the components of silicate and metallic melts.The nitrogen and hydrogen solubility, calculated as N and H, ranges from 0.4 to 1.9wt.% and from 0.2 to 0.3wt.%, accordingly. The nitrogen content in iron globules at ΔlogfO2(IW)=−3.3 was measured as 4.4wt.%. Characterization by Raman and IR spectroscopy indicates that at fO2, where a metallic Fe phase is stable, the silicate melt would contain species with N–H bonds (NH3, NH4+, NH2−, NH2+) as well as N2, oxidized H species (OH− and H2O) and H2.Experimental studies have shown that the fO2 evolution during metal segregation would have strongly influenced the nature of nitrogen and hydrogen species in reduced magmas of the early Earth.

Citation: Kadik, A. A., V. V. Koltashev, E. B. Kryukova, V. G. Plotnichenko (2011), Studying the forms of dissolution of hydrogen, carbon, nitrogen and oxygen volatiles in magmatic melts of the early earth's mantle by the methods of IR and Raman spectroscopy, Vestn. Otd. nauk Zemle, 3, NZ6031, doi:10.2205/2011NZ000161. Experimental studies of the features of dissolution of Н, C, N, O volatiles in the melting products of the early Earth’s mantle are continued. As a model system we have chosen the aluminosilicate melt (albite NaAlSi3O8 80 wt. %) + metallic Fe phase (FeO 20 wt. %) + H+C+N with nitrogen addition (Si3N4 1, 3, 5, 7 wt. %) which at quenching the products of experiments provides НС-N-O-containing silicate glasses. The quenching was carried out at a high pressure 4 GPa, high temperature 1550 о С, and low values of the chemical oxygen potential (fugacity fO2) from -2 to -4 below the iron-wustite buffer equilibrium lgfO2 (IW). The investigation techniques were IR (infrared) micro spectroscopy and micro Raman. The IR absorption spectra (Fig. 1) were measured on a vacuum Fourier-transform FTIR spectrometer “Bruker IFS-113v” with an optical microscope “IR Microscope A590”, focusing the radiation passing through the investigated samples into a spot from 15 to 400 m in diameter. The Raman spectra measurements (Fig. 2) were provided on a Triple Raman spectrograph Т-64000 (Jobin Yvon) in which the exciting radiation from Ar + -laser was focused on a sample surface into a 2 m spot. The application of these techniques has allowed to establish the influence oxygen fugacity (fO2) on the ratio between the "oxidized" (H2O, OH - CO2, C=O, CO3 2- ) and "reduced" (H2, CH4, SiC, C-C) Н- and C-species in the melts, as well as O2, N2, N-O, C-N and others N-H complexes (NH3, NH4 + , NH2 - (≡Si-NH2), NH2 + (≡Si-O-NH2)) in a rather wide region of the redox state

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.