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).