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During accretion, nitrogen was distributed between metal melt, silicate melt, and the atmosphere and today's N deficit of bulk silicate Earth with respect to chondrites may be due to segregation into the core and/or atmospheric losses. To examine the former, we experimentally determined N solubilities in Fe-dominated metal melts at 1200-1800 degrees C, 0.4-9.0 GPa. Results show that pressure has a strong positive influence on N solubility, increasing from 1.0 to 7.4 wt% at 1-9 GPa (1400 degrees C) while temperature has the inverse effect, N solubility decreasing from 1.3 to 0.6 wt% at 1200-1800 degrees C (1 GPa). The solubility data are parameterized as function of pressure and temperature. Our results suggest that core-forming metal melts can dissolve large quantities of N, and the potential N contribution to the Earth's core density deficit could hence be much larger than previously assumed. Most importantly, N in the deep reduced mantle should be stored in the small metal fractions and not in silicates.

Plain Language Summary On the early Earth nitrogen was redistributed between three prevailing reservoirs: the core forming metal, the silicate magma ocean, and the atmosphere. To shed light on the behavior of N during core segregation, we have experimentally determined N solubilities in Fe-dominated metal melts at high temperatures and pressures (1200-1800 degrees C, 0.4-9.0 GPa) using high-pressure devices. Based on our experimental results a model has been developed to describe N solubility into metal melts as a function of pressure and temperature. The model suggests that core-forming metal melts can dissolve N quantities that are as high as the Earth's core density deficit. However, the N concentrations in the core-forming metal are dependent on the accretionary scenario and its partitioning with silicate magma ocean; our solubilities provide an upper limit for possible N concentrations within the Earth's core. Nevertheless, this study shows that N in the modern mantle will largely dissolve in its small metal fraction and not in the dominating silicates.