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

—Reactions of nitrogen, carbon, and hydrogen with FeO–Na 2 O–Al 2 O 3 –SiO 2 melts, liquid iron alloys, and graphite were investigated at 4 GPa, 1550°C, and f O 2 values 1.5–3.0 orders of magnitude below f O 2 ( 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 f O 2 from 0.96 wt % at Δlog f O 2 (IW) =–1.4 to 4.1 wt % at Δlog f O 2 (IW) =–3.0, whereas the hydrogen content of melt is weakly dependent on f O 2 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 NH 3 , N 2 , and CH 4 molecules, as well as NH 2 – , NH 2 + , NH 4 + and CH 3 – complexes. Hydrogen is dissolved in melts as OH – , H 2 O, and H 2 . 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–Na 2 O–Al 2 O 3 –SiO 2 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–Na 2 O–Al 2 O 3 –SiO 2 melts is a significant change in the proportions of N–C–H–O species at f O 2 2–3 orders of magnitude below f O 2 ( IW ). Under these conditions, a sharp decrease in the contents of NH 4 + , NH 2 + (O–NH 2 ), OH, H 2 O, and CH 4 is accompanied by enrichment in NH 2 – (=Si–NH 2 ) and NH 3 . As a result, NH 3 becomes the dominant nitrogen species in the melt. The investigation revealed high nitrogen solubility in iron alloys at f O 2 < f O 2 ( IW ). The nitrogen content increases from 2.47 wt % at Δlog f O 2 ( IW ) =–1.4 to 3.63 wt % at Δlog f O 2 ( IW ) =–3.0. The carbon content of N–C–Fe alloys ranges from 2.3 to 3.8 wt % and decreases with decreasing f O 2 . The siderophile behavior of nitrogen at f O 2 < f O 2 ( 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.