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

Abstract

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 (fO₂) and melt composition on the N solubility through N equilibration experiments at atmospheric pressure and high temperature (1425 °C). Oxygen fugacity (expressed in log units relative to the iron-wüstite 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 fO₂ and the degree of polymerization of the silicate melt on N solubility. Under highly reduced conditions (fO₂ = 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 ∼ 2.0). In contrast, under less reducing conditions (fO₂ > 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 ∼60 and ∼6000 ppm at IW –5.1 and IW –8, respectively. In contrast, a mafic to ultra-mafic magma ocean could have incorporated ∼0.3 ppm to ∼35 ppm N under the fO₂ 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 (∼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).