Effects of Melt Composition and Gas Speciation on Nitrogen Solubility in Basaltic Melt

To complement our survey of factors controlling nitrogen solubility in basaltic magmas, we present a new set of equilibration experiments aimed at documenting the specific effect of melt composition and gas speciation on nitrogen solubility in natural melts. Equilibration was performed at 1 atm and at 1425 °C over a large range of oxygen fugacity controlled by mixtures of gases belonging to either [C–N–O] or [C–N–O–H] systems. Nitrogen contents in equilibrated samples were measured by using a technique based on laser extraction under high vacuum and analysis by static vacuum mass spectrometry. This study shows, in agreement with previous studies, that the effect of the melt composition on nitrogen solubility depends mainly on the oxygen fugacity of the system. Between air and IW-1, we found that the solubility of nitrogen is low and increases with bulk polymerization of the melt, as noble gases do. This effect is consistent with a nitrogen solution controlled by the steric effect of the N₂ molecule. This differs from results obtained in more reducing conditions (<IW-1), in which high N solubility is anticorrelated with the bulk polymerization of the melt due to the substitution of nonbridging oxygen for nitrogen in the melt silicate network, very likely by forming Si(O₃N)^5– structural groups where nitrogen is 3-fold coordinated to silicon. Equilibration under H₂-bearing gaseous environments does not alter the physical solubility of nitrogen between the air and IW-1. In contrast, under highly reducing conditions (below IW-1), a significant decrease of approximately 33% in nitrogen solubility is observed compared to that of H₂-free gas mixtures. This reduction is attributed to the increased stability and abundance of reduced nitrogen species, including NH₂^–, NH^2–, and N^3–. As a direct consequence, the saturation of nitride phases, such as TiN and SiN, occurs at a lower oxygen fugacity.