Nitrogen Retention in Feldspar: Implications for Nitrogen Transport in Subduction Zones.

Nitrogen recycling between Earth's interior and exterior determines the evolution of the atmosphere and habitability of the planet. However, the contribution of subduction to the deep nitrogen recycling is still highly debated. Ammonium‐bearing silicate minerals are the main nitrogen carriers in subduction zones. Nitrogen retention in subduction‐zone minerals determines the subducted nitrogen fluxes to the deep Earth. Here, we reveal ammonium behavior in a natural ammonium‐bearing feldspar at elevated temperatures to 1000°C and pressures to 19.99 GPa. With increasing temperature and pressure, ammonium interacts with the silicate framework and forms hydrogen bonding. The ammonium loss rates are on the order of 10−15, 10−14, and 10−13 at 800, 900, and 1000°C, respectively, with activation energy of 271 ± 13 kJ/mol. The activation energy of ammonium loss in feldspar is higher than that in phengite. Moreover, the ammonium diffusivities are similar to that of hydrogen in feldspar, but much slower than that of molecular water and faster than those of K and Ar. The results imply that most of nitrogen can be potentially retain in the feldspar structure before feldspar breakdown. Considering that feldspar could be stable to a great depth along cold subduction, it could be a potential carrier for nitrogen transport to the deep Earth. Plain Language Summary: Feldspar is an important constituent of Earth's crust and has a large affinity of nitrogen as ammonium in the structure. Here, we report nitrogen retention in feldspar at high temperature and high pressure. Our data show that ammonium interacts with the silicate framework and forms hydrogen bonding with increasing temperature and pressure. Ammonium is more stable in feldspar than in phengite, with similar diffusion kinetics to hydrogen in feldspar. Therefore, ammonium may be carried by feldspar to a great depth along cold subduction. Key Points: Ammonium interacts with the silicate framework and forms hydrogen bonding with increasing temperature and pressureAmmonium loss has the similar kinetics to hydrogen in feldspar, with larger activation energy in feldspar than that in phengiteFeldspar is a potential carrier for nitrogen transport in subduction zones [ABSTRACT FROM AUTHOR]