Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts

Knowledge of N solubility in silicate melts is key for understanding the origin of terrestrial N and the distribution and exchanges of N between the atmosphere, the silicate magma ocean, and the core forming metal. To place constraints on the incorporation mechanism(s) of N in silicate melts, we investigated the effect of the oxygen fugacity (fO(2)) and melt composition on the N solubility through N equilibration experiments at atmospheric pressure and high temperature (1425 degrees C). Oxygen fugacity (expressed in log units relative to the iron-wustite buffer, IW) was varied from IW -8 to IW +4.1, and melt compositions covered a wide range of polymerization degrees, defined by the NBO/T ratio (the number of non-bridging oxygen atoms per tetrahedrally coordinated cations). The N contents of the quenched run products (silicate glasses) were analyzed by in-situ secondary ion mass spectrometry and bulk CO2 laser extraction static mass spectrometry, yielding results that are in excellent agreement even for N concentrations at the (sub-)ppm level. The data obtained here highlight the fundamental control of f(O2) and the degree of polymerization of the silicate melt on N solubility. Under highly reduced conditions (fO(2) = IW -8), the N solubility increased with increasing NBO/T from 17.4 +/- 0.4 ppm.atm(-1/2) in highly polymerized melts (NBO/T = 0) to 6710 +/- 102 ppm.atm(-1/2) in depolymerized melts (NBO/T similar to 2.0). In contrast, under less reducing conditions (fO(2) > IW -3.4), N solubility is very low (<= 2 ppm.atm(-1/2)), irrespective of the NBO/T value. Our results provide constraints on N solubility in enstatite chondrite melts and in the shallow part of a planetary magma ocean. The nitrogen storage capacity of an enstatite chondrite melt, which may approximate that of planetesimals that accreted and melted early in the inner Solar System, varies between similar to 60 and similar to 6000 ppm at IW -5.1 and IW -8, respectively. In contrast, a mafic to ultra-mafic magma ocean could have incorporated similar to 0.3 ppm to similar to 35 ppm N under the fO(2) conditions inferred for the young Earth (i.e., IW -5 to IW). The N storage capacity of a reduced magma ocean (i.e., IW -3.4 to IW) in equilibrium with a N-rich atmosphere is <= 1 ppm, comparable to the N content of the present-day mantle. However under more reducing conditions (i.e., IW -5 to IW -4), the N storage capacity is significantly higher (similar to 35 ppm); in this case, Earth would have lost N to the atmosphere and/or N would have been transported into and stored within its deep interior (i.e., deep mantle, core). (C) 2020 The Author(s). Published by Elsevier Ltd.