A precise knowledge of the geological cycles of volatile elements, such as nitrogen (N) and carbon (C), is important to understand the distribution of these elements between the reservoirs of the solid Earth (core, mantle and crust), the ocean and the atmosphere. The exchange of nitrogen between these reservoirs during the geological evolution of our planet has had a decisive influence on the unique composition of the Earth’s atmosphere, which makes life on the Earth’s surface possible. Over the past two decades, fundamental aspects of nitrogen geochemistry have been investigated through studies of terrestrial samples as well as experimental and theoretical studies.
Subduction zones represent the most important exchange pathway for nitrogen between the Earth’s surface reservoirs and the deep Earth. A central question here is how large the proportions are that remain in the subducted plate and are fed into the Earth’s mantle, or return directly to the atmosphere through magmatism. The incorporation of nitrogen into minerals as an ammonium ion (NH4+) has an important influence here, as the stability of the minerals depends on the temperature. This means that in “cold” subduction zones with a low geothermal gradient, nitrogen can mainly be transported into the mantle, while in “warm” subduction zones with a high geothermal gradient, the nitrogen-containing minerals release nitrogen more easily, which is then immediately returned to the atmosphere.
While various studies show that there has been an increase in nitrogen in the Earth’s mantle over the course of geological development [2], this is a controversial topic, as other data indicate the opposite development. However, the fact that the majority of terrestrial nitrogen (approx. 57%) is bound in the mantle is undisputed. In addition to the Earth’s atmosphere (approx. 27%), the continental crust (approx. 14%) with an average content of 74±4 ppm is also a significant reservoir for nitrogen in the Earth [3].
