The origin of Earth's mantle nitrogen: primordial or early biogeochemical cycling?

TL;DR

This study models Earth's mantle nitrogen origin, exploring whether it is primordial or derived from early biogeochemical cycling, using isotope tracers and partitioning models to evaluate different scenarios.

Contribution

It introduces a combined modeling approach using argon and nitrogen isotopes to differentiate between primordial and subduction-derived mantle nitrogen origins.

Findings

Only about 10% of N can be trapped in the mantle during magma solidification.

Excess N could be removed by impact erosion during late accretion.

Mantle N could originate from efficient subduction if early Earth's sedimentary N burial was high.

Abstract

Earth's mantle nitrogen (N) content is comparable to that found in its N-rich atmosphere. Mantle N has been proposed to be primordial or sourced by later subduction, yet its origin has not been elucidated. Here we model N partitioning during the magma ocean stage following planet formation and the subsequent cycling between the surface and mantle over Earth history using argon (Ar) and N isotopes as tracers. The partitioning model, constrained by Ar, shows that only about 10% of the total N content can be trapped in the solidified mantle due to N's low solubility in magma and low partitioning coefficients in minerals in oxidized conditions supported from geophysical and geochemical studies. A possible solution for the primordial origin is that Earth had about 10 times more N at the time of magma ocean solidification. We show that the excess N could be removed by impact erosion during late accretion. The cycling model, constrained by N isotopes, shows that mantle N can originate from efficient N subduction, if the sedimentary N burial rate on early Earth is comparable to that of modern Earth. Such a high N burial rate requires biotic processing. Finally, our model provides a methodology to distinguish the two possible origins with future analysis of the surface and mantle N isotope record.