Nitrogen in the deep mantle
Final Report Abstract
More than 20 successful high-pressure experiments were carried out in multi anvil presses at 20 – 33 GPa and 1600 – 1800 ˚C in order to quantify the solubility of nitrogen in the minerals of the deep transition zone and the lower mantle. Oxygen fugacity in the experiments was buffered to be close to the Fe-FeO equilibrium. 15N-doped NH4NO3 was used as a source of nitrogen and 15N concentrations in the samples were measured by secondary ion mass spectrometry (SIMS). The two most important experimental observations of this study are: (i) The presence of Al enhances N solubility in bridgmanite and the effect is particularly strong at Al2O3 concentrations of a few wt.%; (ii) nitrogen solubility in ferropericlase (Fe,Mg)O may reach several wt. %, likely due to the coupled substitution of Fe3+ + NH4+ for 2 Mg2+. The data suggest that the nitrogen storage capacity of the lower mantle is far larger than previously thought. After solidification of the magma ocean, the lower mantle may have been the largest nitrogen reservoir on the planet. The often discussed “subchondritic N/C ratio” of the Earth may be an artifact due to the poor sampling of the lower mantle reservoir.
Publications
- (2018) Nitrogen solubility in the deep mantle and the origin of Earth’s primordial nitrogen budget. Earth Planet Sci Lett 488: 134–143
Yoshioka T, Wiedenbeck M, Shcheka S, Keppler H
(See online at https://doi.org/10.1016/j.epsl.2018.02.021)
