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Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere


Volatile elements stored in the mantles of terrestrial planets escape through volcanic degassing, and thereby influence planetary atmospheric evolution and habitability. Compared with the atmospheres of Venus and Mars, Earth's atmosphere is nitrogen-rich relative to primordial noble gas concentrations1,2,3. The compatibility of volatile elements in mantle minerals versus melts and fluids controls how readily these elements are degassed. However, the speciation of nitrogen in mantle fluids is not well constrained4,5,6. Here we present thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions. We find that, under the relatively oxidized conditions of Earth's mantle wedges at convergent plate margins7,8,9, nitrogen is expected to exist predominantly as N2 in fluids and, therefore, be degassed easily. In contrast, under more reducing conditions elsewhere in the Earth's upper mantle and in the mantles of Venus and Mars, nitrogen is expected predominantly in the form of ammonium (NH4+) in aqueous fluids. Ammonium is moderately compatible in upper mantle minerals10,11 and unconducive to nitrogen degassing. We conclude that Earth's oxidized mantle wedge conditions—a result of subduction and hence plate tectonics—favour the development of a nitrogen-enriched atmosphere, relative to the primordial noble gases, whereas the atmospheres of Venus and Mars have less nitrogen because they lack plate tectonics.

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Figure 1: Earth's atmosphere shows an enrichment of N2 relative to the primordial noble gases of the martian and venusian atmospheres.
Figure 2: Calculated log f O 2 –pH diagrams for nitrogen speciation in supercritical aqueous fluids at different temperatures and pressures using the Deep Earth Water (DEW) model20.

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This research was supported by a Carnegie Postdoctoral Fellowship, grants from the Sloan Foundation through the Deep Carbon Observatory (Reservoirs and Fluxes, and Extreme Physics and Chemistry programs) and grants DOE DE-FG-02-96ER-14616 and NSF EAR 1023865. We are also grateful for the help and support of the Johns Hopkins University and the Geophysical Laboratory of the Carnegie Institution of Washington. We wish to acknowledge reviews of the manuscript by R. W. Carlson, R. M. Hazen, J. Hopp and B. Marty, as well as helpful discussions with B. Mysen and C. Schiffries (CIW).

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S.M. conceived the idea and collated the planetary datasets, D.A.S. performed the thermodynamic calculations, and both authors contributed equally to the writing of the manuscript.

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Correspondence to Sami Mikhail.

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The authors declare no competing financial interests.

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Mikhail, S., Sverjensky, D. Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere. Nature Geosci 7, 816–819 (2014).

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