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

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 4 GPa, 1550 degrees C, and oxygen fugacity (fO(2)) from 2.1 to 3.3 log units below IW buffer. The technique of hydrogen fugacity (fH(2)) buffering via the dissociation of H2O employed here relies upon the diffusion of H-2 through Pt to achieve equal chemical potentials of H-2 in the Pt capsule and outer assemblage elements. The nitrogen source was Si3N4. The fO(2) imposed on the charge was controlled by redox reactions between H-2 buffered externally, Si3N4 and components of the Fe-bearing melt that was reduced with O-2 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) + 3O(2) -> 3SiO(2) (melt) + 2N(2) (melt) with a subsequent participation of nitrogen in the reactions with H-2, the components of silicate and metallic melts. The nitrogen and hydrogen solubility, calculated as N and H, ranges from 0.4 to 1.9 wt.% and from 0.2 to 0.3 wt.%, accordingly. The nitrogen content in iron globules at Delta logfO(2)(IW) = -3.3 was measured as 4.4 wt.%. Characterization by Raman and IR spectroscopy indicates that at fO(2), where a metallic Fe phase is stable, the silicate melt would contain species with N-H bonds (NH3, NH4+, NH2-, NH2+) as well as N-2, oxidized H species (OH- and H2O) and H-2. Experimental studies have shown that the fO(2) evolution during metal segregation would have strongly influenced the nature of nitrogen and hydrogen species in reduced magmas of the early Earth. (C) 2012 Elsevier B.V. All rights reserved.