The association of natural hydrogen and nitrogen: The ammonium clue?

Abstract

It is generally accepted that hydrogen is generated by ferrous iron oxidation and/or water radiolysis but no attempt has been done so far to explain the origin of the nitrogen that is associated to ‘hydrogen systems’ such as those located in Brazil and Oman and New Caledonian ophiolites.We carried out 25–300 °C geochemical equilibrium calculations on model water and water-mineral systems including ammonium (NH4+) to see if this ion could be a ‘precursor’ in the generation of hydrogen and/or nitrogen. The considered minerals of the water-mineral systems were the magnetite-hematite or fayalite-magnetite-quartz redox-buffering assemblages.The water systems including NH4+ only or NH4+ and ferrous iron co-generate hydrogen and nitrogen through the ammonium decomposition reaction:NH4+ = 1.5 H2 + 0.5 N2 + H+In the water-mineral systems, hydrogen is generated by the magnetite and fayalite ferrous iron oxidation while nitrogen is generated independently by the hematite and magnetite ferric iron reduction by NH4+:3 Fe3+ + NH4+ = 0.5 N2 + 3 Fe2+ + 4 H+With magnetite-hematite, the produced gas is mainly composed of nitrogen. At low temperature with fayalite-magnetite-quartz, the produced gas is mainly composed of hydrogen; approaching 300 °C, and with a sufficient initial ammonium stock, a significant amount of nitrogen is produced to form a mixed gas.If we consider low (magnetite-hematite) or high (fayalite-magnetite-quartz) reducing conditions occurring respectively within the central or peripheral zones of the hydrogen systems, NH4+ passing through and reacting with these two kinds of redox environments could explain why the central zones release mainly hydrogen (with some amounts of nitrogen) and why mainly nitrogen is emitted at the peripheries.