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Nitrogen diffusion in silicate melts under reducing conditions

Boulliung, J. ; Dalou, C. ; Tissandier, L. ; Füri, E. ; Marrocchi, Y., Am. Min.

Nitrogen diffusion in silicate melts under reducing conditions

Boulliung, J. ; Dalou, C. ; Tissandier, L. ; Füri, E. ; Marrocchi, Y.

American Mineralogist, 2021, 106, 662-666

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 N3– diffusion on melt composition is greater than that of Ar. Furthermore, N3– diffusion in basaltic-andesitic 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.

Voir en ligne : https://doi.org/10.2138/am-2021-779...




publié mardi 20 avril 2021