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doi: 10.7185/geochemlet.1919 Nitrogen and carbon are essential elements for life, and their relative abundances in planetary bodies are important for understanding planetary evolution and habitability. The high C/N ratio in the bulk silicate Earth (BSE) relative to chondrites has been difficult to explain through partitioning during core formation and outgassing from molten silicate. Here we propose a new model that may have released nitrogen from the metallic cores of accreting bodies during impacts with the early Earth. Experimental observations of melting in the Fe-N-C system via synchrotron X-ray radiography of samples in a Paris-Edinburgh press reveal that above the liquidus, iron-rich melt and nitrogen-rich liquid coexist at pressures up to at least 6 GPa. The combined effects of N-rich supercritical fluid lost to Earth’s atmosphere and/or space as well as N-depleted alloy equilibrating with the magma ocean on its way to the core would increase the BSE C/N ratio to match current estimates. Received 2 January 2019 | Accepted 8 June 2019 | Published 30 July 2019 1. Department of Earth and Environmental Sciences, Michigan State University, MI 48824, USA 2. Department of Earth and Environmental Sciences, University of Michigan, MI 48109, USA 3. HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, IL 60439, USA 4. Geodynamics Research Center, Ehime University, Ehime 790-8577, Japan * Corresponding author (email: jiacliu09@gmail.com, dorfman3@msu.edu) Introduction The habitability of Earth and other planetary bodies depends on the incorporation, distribution, and speciation of volatile elements including carbon and nitrogen. In Earth, the BSE C/N ratio is estimated to be higher than that of planetary building blocks including enstatite, carbonaceous chondrites, interstellar dust and gas (e.g., Marty, 2012; Bergin et al., 2015). Recent studies estimate the BSE C/N ratio to be superchondritic based on measurements of gas bubbles trapped in mid-ocean ridge and ocean island basalts (Marty, 2012; Halliday, 2013; Bergin et al., 2015), and recent estimates of the BSE C/N ratio converge to 49 ± 9 (Bergin et al., 2015). In contrast, C/N ratios measured in primitive CI chondrites (17 ± 3; Alexander et al., 2013) and enstatite chondrites (14 ± 12; Grady and Wright, 2003) are significantly lower. Explaining these observations thus requires one or more mechanisms that preferentially deplete N relative to C from the mantle source of basalt magmas. The key processes responsible for C and N redistribution among Earth reservoirs were equilibrium partitioning between the magma ocean and the growing metallic core and degassing of the magma ocean to the early atmosphere, but these processes have been insufficient to explain the estimated high BSE C/N ratio (Bergin et al., 2015; Hirschmann, 2016; Dalou et al., 2017). Core formation is expected to decrease the BSE C/N ratio because at relevant conditions C is a much stronger siderophile element than N (e.g., Dalou et al., 2017). Degassing has not been thought to significantly affect the C/N ratio, as chemical analysis of volatiles in silicate glasses quenched from high pressure-temperature (P-T) experiments indicates that the solubilities of C and N in the magma ocean are comparable (Hirschmann, 2016). The major host phase for both C and N in the solidifying planet is likely to be core-forming alloy, therefore understanding the Fe-N-C system under relevant conditions may hold the key to the paradox of the high BSE C/N ratio. While the Fe-C phase diagram has been well studied over a wide range of pressures (e.g., Fei and Brosh, 2014; Liu et al., 2016), very limited data are available on the phase relations of the Fe-N system (e.g., Guillermet and Du, 1994). In particular, melting behaviours are expected to be different in these two systems, as the C-rich end member, graphite/diamond, has a melting point of 4000-5000 K at relevant pressures (Grumbach and Martin, 1996), while the N-rich end member would be gas/ supercritical fluid in the BSE. Immiscible N-rich Fluid In order to test whether the mobility of N-rich gas/fluid could provide a possible mechanism to leak N during accretion and core formation, we investigated melting relations in the Fe-N-C system under high pressures by in situ X-ray radiography and

Loss of immiscible nitrogen from metallic melt explains Earth’s missing nitrogen