Experimental Constraints on the Fate of Subducted Sedimentary Nitrogen in the Reduced Mantle

Nitrogen is considered to be transported from Earths surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth's interior during slab-mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser-heated diamond anvil cell experiments and first-principles calculations. Our results showed that subducted nitrogen-bearing silicates and fluids could not coexist with the metallic iron or iron-rich alloys, and reacted with them to form different products at high pressure-temperature conditions. Combining our results with previous data, we re-determined the relative stability of iron-light element binary compounds to 35 GPa and 1600 K to be Fe-O > Fe-N > Fe-S > Fe-C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur-depleted lower-mantle diamonds and are absent in sulfur-rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earths surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods. Plain Language Summary Nitrogen is an essential element for the Earths atmosphere and life. A large amount of nitrogen is transported to the Earth's interior through subduction and may finally reside in the deep Earth. The knowledge on the transportation and storage of nitrogen in the reduced mantle is crucial for our understanding of the Earth's nitrogen cycle. We combined high-pressure and high-temperature experiments and theoretical calculations to investigate the chemical reactions between subducted nitrogen carriers and metallic iron or iron-rich alloys. The results showed that nitrogen in the subducted silicates and fluids will be extracted to the iron-rich metallic phases when they are relatively excess. Iron nitrides are more stable than sulfides but less stable than oxides at pressure-temperature conditions relevant to the subduction down to 900 km. The incorporation of nitrogen in metallic iron could be an important mechanism to store surficial nitrogen in the reduced mantle. Iron nitrides could be trapped as inclusions in sulfur-depleted lower-mantle diamonds but nitrogen will reside in the metallic melt if superdeep diamonds are formed in a sulfur-rich environment.

Automated summary

This study investigates the stability and reaction processes of subducted nitrogen in the reduced mantle. The results show that nitrogen-bearing silicates and fluids cannot coexist with metallic iron or iron-rich alloys, and react to form different products. This contributes to understanding the presence of iron nitrides in sulfur-depleted lower-mantle diamonds.