Carbon and Nitrogen Speciation in N-poor C-O-H-N Fluids at 6.3 GPa and 1100-1400 °C

Deep carbon and nitrogen cycles played a critical role in the evolution of the Earth. Here we report on successful studying of speciation in C-O-H-N systems with low nitrogen contents at 6.3 GPa and 1100 to 1400 °C. At fO₂ near Fe-FeO (IW) equilibrium, the synthesised fluids contain more than thirty species. Among them, CH₄, C₂H₆, C₃H₈ and C₄H₁₀ are main carbon species. All carbon species, except for C₁-C₄ alkanes and alcohols, occur in negligible amounts in the fluids generated in systems with low H₂O, but С₁₅-С₁₈ alkanes are slightly higher and oxygenated hydrocarbons are more diverse at higher temperatures and H₂O concentrations. At a higher oxygen fugacity of +2.5 Δlog fO₂ (IW), the fluids almost lack methane and contain about 1 rel.% C₂-C₄ alkanes, as well as fractions of percent of C_15-18 alkanes and notable contents of alcohols and carboxylic acids. Methanimine (CH₃N) is inferred to be the main nitrogen species in N-poor reduced fluids. Therefore, the behaviour of CH₃N may control the nitrogen cycle in N-poor peridotitic mantle. Oxidation of fluids strongly reduces the concentration of CH₄ and bulk carbon. However, higher alkanes, alcohols, and carboxylic acids can resist oxidation and should remain stable in mantle hydrous magmas.