The fate of nitrogen in deep magma oceans

Abstract

Nitrogen is important in planetary evolution because it is essential to life and the most abundant element in Earth’s atmosphere. Here, we investigate how core formation affects the distribution of N within accreting terrestrial planets. We conducted laser-heated diamond anvil cell experiments (LH-DAC) over a wide range of high pressure–temperature-compositional (PTX) conditions (38–103 GPa, 2728–5609 K, −1.95 to −1.03 ΔIW, 0.5–3.7 NBO/T) to study nitrogen partitioning in metal-silicate systems. Combining our data with existing low and high PT results, we developed a nitrogen partitioning model applicable from early accretion to extreme PT stages associated with giant impacts. We test the robustness of our model by accurately predicting nitrogen partitioning in a multi-anvil experiment conducted independently at 15 GPa, 2573 K with oxygen fugacity of −2.5 ΔIW. Our model shows that increasing pressure, oxygen fugacity, and N concentration in the alloy make nitrogen more siderophile, while increasing temperature, oxygen and silicon contents in the alloy, and the SiO2 content of the silicate melt make nitrogen less siderophile. Application of our model to core formation conditions under oxidized and reduced scenarios suggest that nitrogen can be siderophile or lithophile under low PT conditions but exhibits a neutral partitioning at high PT conditions (> 100 GPa, 5000 K) over a wide range of bulk planet compositions. Using our model, along with partitioning models for S and C, we examine how core formation scenarios can fractionate C/N and S/N ratios in the BSE. Our model suggests that backreaction of volatile rich cores from reduced, smaller impactors (sub-Mars-sized) within deep magma oceans can impart a wide range of C/N and S/N ratios on the magma ocean. We find that the amount of silicate entrainment has a strong control on elemental fractionations imparted to the magma oceans. Elevated C/N and S/N ratios are associated with larger degrees of silicate entrainment, and vice versa. Thus, Earth’s apparent depletion of N may relate to its volatiles being reprocessed within deep magma oceans, possibly during the end stages of accretion.