Nitrogen and carbon fractionation in planetary magma oceans and origin of the superchondritic C/N ratio in the bulk silicate Earth

Magnesiowüstite as a major nitrogen reservoir in Earth’s lowermost mantle

“…While many ocean island basalts do indeed have N/ 40 Ar ratios similar to MORB, there are also some samples with much higher values (Johnson and Goldblatt, 2015) that may hint at a deep hidden reservoir. Based on our current state of knowledge, the bulk N/C ratio of the Earth appears to be rather uncertain and this ratio should not be used as a parameter for constraining planetary accretion models (e.g., Hirschmann, 2016;Li et al, 2023).…”

“…If one considers the upper mantle to be representative of the entire bulk silicate Earth, these data would suggest that nitrogen on Earth is selectively depleted relative to carbon (Marty, 2012). This "subchondritic N/C ratio" has been extensively discussed and has often been used to constrain details of models for the early evolution of Earth (e.g., Hirschmann, 2016;Li et al, 2023).…”

The origin and evolution of Earth's nitrogen

“…It was found that nitrogen solubility in liquid metal obeys Sievert's law [ 106 ], which states that nitrogen solubility in liquid metal is proportional to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $\surd {{P}_{{{N}_2}}}$\end{document} , where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} ${{P}_{{{N}_2}}}$\end{document} is the partial pressure of N 2 . Recent experimental studies, which were more relevant to the Earth's core and mantle [ 80 , 83 , 107 , 108 ], showed that nitrogen solubility in Fe-rich liquid metal, as high as 12 wt%, increased strongly with increasing pressure but decreased moderately with increasing Ni content at P – T conditions up to 18 GPa and 2580°C. The presence of sulfur, carbon and/or silicon may also decrease the nitrogen solubility in Fe-rich liquid metal [ 80 , 109 ].…”

“…3 ) [ 78 , 80 , 82–86 , 107 , 149 ]. The nitrogen solubility in silicate melts at the saturation of N 2 -rich gas ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $S_N^{{silicate}}$\end{document} ) and at f O 2 < IW was determined at pressures up to 3 GPa (see below), and also shows a strong dependence on f O 2 [ 67 , 72 , 83 , 150 ]. The f O 2 -dependence of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $D_N^{{metal}/silicate}$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $S_N^{{silicate}}$\end{document} is primarily attributable to the transformation of nitrogen speciation in silicate melt from N 2 dominance at oxidizing conditions to N–H and N 3− dominance at reducing conditions.…”

“…The f O 2 -dependence of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $D_N^{{metal}/silicate}$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $S_N^{{silicate}}$\end{document} is primarily attributable to the transformation of nitrogen speciation in silicate melt from N 2 dominance at oxidizing conditions to N–H and N 3− dominance at reducing conditions. Previous studies have also determined \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $D_C^{{metal}/silicate}$\end{document} , which are ∼10–10 5 and largely higher than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} $D_N^{{metal}/silicate}$\end{document} (see a most recent study [ 83 ] for a review). The metal/silicate nitrogen isotope fractionation was determined at 1–8 GPa and 1400–2200°C.…”