Solution behavior of reduced N–H–O volatiles in FeO–Na2O–SiO2–Al2O3 melt equilibrated with molten Fe alloy at high pressure and temperature

Abstract

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.