In situ Raman spectroscopic study of nitrogen speciation in aqueous fluids under pressure

Nitrogen speciation in aqueous fluids is important for understanding the partitioning of N between fluid and coexisting melt or mineral phases and the storage and recycling of nitrogen in Earth's interior. Previously N speciation in aqueous fluids has only been investigated using the approach of quenched fluid inclusions or thermodynamic modeling. Here we present the results from in situ Raman spectroscopic measurements of N speciation in aqueous fluids held in hydrothermal diamond anvil cell, at P-T conditions up to above 800 degrees C and 2 GPa. We identify the presence of N in aqueous fluids as N-2 and NH3 (replaced by NH4+ at low pH) with N-2 being the favored species toward higher temperature. The fugacity equilibrium constant for reaction N-2 + 3 H2O = 2 NH3 + 3/2 O-2 is determined to be lnK(f) =-16.15-23,489/T with T being temperature in Kelvin. Our equilibrium constant is significantly lower than that from quenched fluid inclusions, but is in good agreement with that calculated from the Deep Earth Water (DEW) model. We suggest that the thermodynamic stability of N-2 relative to NH3 in aqueous fluids has been underestimated by quench experiments. Because N is stored in silicate minerals and melts mainly as NH4+, the bulk partition coefficient of N between fluid and mineral or melt should be greater than previously thought if one assumes a fixed partition coefficient for trivalent N. For subduction zones this means that a higher fraction of N is recycled by slab-derived fluids, and concomitantly less N is carried by subducting slabs to Earth's deeper interior.