Nitrogen is Not Always Noble in the Mantle

Abstract:

Tracing volatiles through the subduction system is pivotal for understanding the long-term evolution of the Earth’s atmosphere. To this regard, the ratio of molecular nitrogen over one or more of the noble gases is a commonly employed method, but is based on the assumption that nitrogen is a highly incompatible element and behaves as a noble gas. Thus, to compare nitrogen/noble gas data from volcanic arcs on a global scale requires the behaviour of nitrogen relative to the noble gases to be constant. We have calculated the speciation of nitrogen in equilibrium with eclogite (jadeite + kyanite + coesite) and peridotite (enstatite + forsterite) under lower crustal and upper mantle conditions (500-1,000°C & 1-5 GPa) and over several redox states (QFM +3 to -6) and pH values (0-8).

Our data demonstrates the duality for the behaviour of nitrogen during subduction, where nitrogen can either behave either as a noble gas (N₂) or as a moderately compatible element in K-bearing phases (NH₄⁺). Estimations for volatile abundance and fluxes based on N₂/noble gas systematics are potentially flawed, and datasets for N₂/noble gas systematics between different arc systems are not intuitively comparable (on the global scale). These data are also used to discuss the partitioning behaviour of nitrogen relative to other volatiles, such as carbon, and the noble gases.