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

Abstract

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.