The Effect of Nitrogen on the Dihedral Angle Between Fe-Ni Melt and Ringwoodite: Implications for the Nitrogen Deficit in the Bulk Silicate Earth

Nitrogen (N) is extremely depleted in the bulk silicate Earth (BSE). However, whether the silicate magma ocean was as N-poor as the present-day BSE is unknown. We performed multi-anvil experiments at 20 GPa and 1,673-2,073 K to determine the dihedral angle of Fe-Ni-N alloy melt in ringwoodite matrix to investigate whether percolation of Fe-rich alloy melt in the solid mantle can explain N depletion in the BSE. The dihedral angles ranged from 112 degrees to 137 degrees, surpassing the wetting boundary. Our experiments suggest that N removal from the mantle by percolation of Fe-rich alloy melt to the Earth's core is unlikely. Therefore, besides N loss to space during planetesimal and planetary differentiation, as well as its segregation into the Earth core, the stranded Fe-rich metal in the deep mantle could be a hidden N reservoir, contributing to the anomalous depletion of N in the observable BSE. Understanding how and when the present-day inventory of nitrogen (N) in the bulk silicate Earth (BSE) was established is important to gain insights into Earth's habitability. A key question remains as to why N is strongly depleted in the BSE than carbon and hydrogen. Efficient segregation of N into the metallic core in the final stage of Earth's formation is postulated to be one of the primary causes behind this depletion. However, it is not clear whether the silicate magma ocean (MO) was as depleted in N as the present-day BSE due to the uncertainties in the degree of metal-silicate equilibration during the final stages of Earth's formation. Post-MO crystallization, Fe-Ni alloy precipitated in the reduced mantle owing to the disproportionation of ferrous iron. We used high-pressure experiments to examine whether this Fe-Ni alloy melt can trap the excess N and percolate through the solid mantle to the core. Our experiments show that the percolation of Fe-Ni-N melt is unlikely owing to its dihedral angle in silicate phases being larger than the wetting boundary. Instead, the Fe-Ni-N alloy stranded in the mantle can be a hidden N reservoir and the present-day BSE may not be as N-depleted as predicted. Dihedral angles of Fe-Ni-N melt in ringwoodite matrix ranged from 112 degrees to 137 degrees, surpassing the wetting boundary Percolation of Fe-Ni-N melt through the solid mantle cannot explain N depletion in the bulk silicate Earth Fe-Ni-N alloy can be a hidden N reservoir in the mantle if excess N was present in the Earth's mantle post core-mantle differentiation