Nitrogen diffusion in silicate melts under reducing conditions

Abstract

Abstract The behavior of nitrogen during magmatic degassing and the potential kinetic fractionation between N and other volatile species (H, C, O, noble gases) are poorly known due to the paucity of N diffusion data in silicate melts. To better constrain N mobility during magmatic processes, we investigated N diffusion in silicate melts under reducing conditions. We developed uniaxial diffusion experiments at 1 atm, 1425 °C, and under nominally anhydrous reducing conditions (fO2 ≤ IW-5.1, where IW is oxygen fugacity, fO2, reported in log units relative to the iron-wüstite buffer), in which N was chemically dissolved in silicate melts as nitride (N3–). Although several experimental designs were tested (platinum, amorphous graphite, and compacted graphite crucibles), only N diffusion experiments at IW-8 in compacted graphite crucibles for simplified basaltic andesite melts were successful. Measured N diffusivity (DN) is on the order of 5.3 ± 1.5 × 10−12 m2 s−1, two orders of magnitude lower than N chemical diffusion in soda-lime silicate melts (Frischat et al. 1978). This difference suggests that nitride diffusivity increases with an increasing degree of melt depolymerization. The dependence of N 3– diffusion on melt composition is greater than that of Ar. Furthermore, N3– diffusion in basalticandesitic melts is significantly slower than that of Ar in similarly polymerized andesitic-tholeiitic melts at magmatic temperatures (1400–1450 °C; Nowak et al. 2004). This implies that N/Ar ratios can be fractionated during reducing magmatic processes, such as during early Earth's magma ocean stages.