Magnesiowüstite as a major nitrogen reservoir in Earth’s lowermost mantle

Abstract

Ferropericlase (Mg,Fe)O is after bridgmanite the most abundant phase in the lower mantle. The ultralow velocity zones above the core-mantle boundary may contain very Fe-rich magnesiowüstite (Fe,Mg)O, possibly as result of the fractional crystallisation of a basal magma ocean. We have experimentally studied the solubility of nitrogen in the ferropericlase-magnesiowüstite solid solution series as function of iron content. Multi-anvil experiments were performed at 20-33 GPa and 1600-1800 °C in equilibrium with Fe metal. Nitrogen solubility increases from a few tens ppm (μg/g) for Mg-rich ferropericlase to more than 10 wt. % for nearly pure wüstite. Such high solubilities appear to be due to solid solution with NiAs-type FeN. Our data suggest that during fractional crystallisation of a magma ocean, the core-mantle boundary would have become extremely enriched with nitrogen, such that the deep mantle today could be the largest nitrogen reservoir on Earth. The often discussed "subchondritic N/C" ratio of the bulk silicate Earth may be an artefact of insufficient sampling of this deep reservoir.

The origin and evolution of Earth's nitrogen

“…(2023) [ 74 ] experimentally determined 2–6 μg/g nitrogen in bridgmanite, which also increased with increasing temperature. More recently, it was shown that wüstite and Fe–N may form a solid solution in Earth's deep mantle [ 87 ]. All these experimental results demonstrate a significant nitrogen storage capacity of the Earth's mantle, indicating that nitrogen may not be anomalously depleted in the silicate Earth but reside in a reservoir in the deep Earth that is poorly sampled.…”

“…These experimental results, when considered alongside observations of natural samples, provide preliminarily constraints on the accretion processes of Earth's nitrogen [ 78 , 81 , 84 ]. They also shed light on the distribution of nitrogen between the proto-Earth's core, mantle and surface [ 78 , 80 , 85 , 86 ], the storage of nitrogen in Earth's mantle [ 68 , 70 , 74 , 87 ], and the long-term evolution of nitrogen in Earth's mantle and surface after the core formation [ 61 , 77 ].…”