Raman spectroscopy study of C-O-H-N speciation in reduced basaltic glasses: Implications for reduced planetary mantles

To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒ_O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒ_O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H₂, NH₂ ^–, NH₃, CH₄, CO, CN^–, N₂, and OH^– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N₂, CH₄, NH₃ and H₂ vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒ_O2, CO band areas increase while those of N₂ decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN^–, NH₂ ^– and OH^– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H₂, NH₂ ^–, NH₃, CH₄, CO, CN^–, N₂, and OH^– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒ_O2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation.