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

• Published in: Geochemistry international 2016-12, Vol.54 (13), p.1175
• Main Authors: Kadik, A A, Koltashev, V V, Kryukova, E B, Tsekhonya, T I, Plotnichenko, V G
• Format: Article
• Language: English
• Subjects: Magma; Molecules; Oxidation; Oxygen; Redox reactions; Silica; Spectroscopy; Spectrum analysis
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702916130073

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 (fO_2), 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-Na_2O-Al_2O_3-SiO_2 melts after experiments at 4 GPa, 1550°C, and fO_2 1.5-3 orders of magnitude below the oxygen fugacity of the iron--wustite buffer equilibrium (fO_2(IW)). Such fO_2 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 (NH_3, NH_2^+, NH_2^-), N_2, H_2, and CH_4 molecules, as well as oxidized hydrogen species (OH- hydroxyl and molecular water H_2O). Spectral characteristics of the glasses indicate significant influence of fO_2 on the N-C-O-H proportion in the melt. They are expressed in a sharp decrease of NH_2^+, NH_2^-O-NH_2), OH^-, H^2O, and CH_4 and simultaneous increase of NH_2^- (Si-NH_2) and NH_3 with decreasing fO_2. As a result, NH_3 molecules become the dominant nitrogen compounds among N-C-H components in the melt at fO_2 two orders of magnitude below fO_2(IW), whereas molecular СÐoe_4 prevails at higher fO_2. The noteworthy feature of the redox reactions in the melt is stability of the ÐzÐoe^- groups and molecular water, in spite of the sufficiently low fO_2. 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.