Nitrogen Speciation in Silicate Melts at Mantle Conditions From Ab Initio Simulations

Abstract

Nitrogen (N) is a major ingredient of the atmosphere, but a trace component in the silicate Earth. Its initial inventory in these reservoirs during Earth's early differentiation requires knowledge of N speciation in magmas, for example, whether it outgasses as N2 or is sequestered in silicate melts as N3−, which remains largely unconstrained over the entire mantle regime. Here we examine N species in anhydrous and hydrous pyrolitic melts at varying P-T-redox conditions by ab-initio calculations, and find N-N bonding under oxidizing conditions from ambient to lower mantle pressures. Under reducing conditions, N interacts with the silicate network or forms N-H bonds, depending on the availability of hydrogen. Redox control of N speciation is demonstrated valid over a P-T space encompassing probable magma ocean depths. Finally, if the Earth accreted from increasingly oxidized materials toward the end of its accretion, an N-enriched secondary atmosphere might be produced and persist until later impacts.