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

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

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

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

Geochimica et Cosmochimica Acta, 2020, 284, 120-133

Abstract :

Knowledge of N solubility in silicate melts is key for understanding the origin of terrestrial N and the distribution andexchanges of N between the atmosphere, the silicate magma ocean, and the core forming metal. To place constraints onthe incorporation mechanism(s) of N in silicate melts, we investigated the effect of the oxygen fugacity (fO2) and melt com-position 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-wu ̈stite buffer, IW) was varied from IW –8 to IW +4.1, and meltcompositions covered a wide range of polymerization degrees, defined by the NBO/T ratio (the number of non-bridging oxy-gen atoms per tetrahedrally coordinated cations). The N contents of the quenched run products (silicate glasses) were ana-lyzed byin-situsecondary ion mass spectrometry and bulk CO2laser extraction static mass spectrometry, yielding resultsthat are in excellent agreement even for N concentrations at the (sub-)ppm level. The data obtained here highlight the fun-damental control offO2and 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-1/2in highly polymerized melts(NBO/T = 0) to 6710 ± 102 ppm.atm-1/2in 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 constraintson N solubility in enstatite chondrite melts and in the shallow part of a planetary magma ocean. The nitrogen storage capacityof an enstatite chondrite melt, which may approximate that of planetesimals that accreted and melted early in the inner SolarSystem, varies between ˜60 and ˜6000 ppm at IW –5.1 and IW –8, respectively. In contrast, a mafic to ultra-mafic magmaocean could have incorporated ˜0.3 ppm to ˜35 ppm N under thefO2conditions inferred for the young Earth (i.e., IW –5 toIW). 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 toIW –4), the N storage capacity is significantly higher (˜35 ppm) ; in this case, Earth would have lost N to the atmosphereand/or N would have been transported into and stored within its deep interior (i.e., deep mantle, core).

Voir en ligne : https://doi.org/10.1016/j.gca.2020....




publié mercredi 15 juillet 2020