Nitrogen speciation in mantle aqueous fluids and the tectonic implications for the origin of Earth's unique atmosphere

A major process in the formation of planetary atmospheres is thought to be mantle degassing. Therefore, placing constraints on how this occurred is fundamental for understanding how the Earth became a habitable planet.

Earth's atmosphere is nitrogen rich relative to primordial noble gases when compared with the Venusian and Martian atmospheres. A primary control on the degassing of the atmophile elements is their compatibility in mantle minerals, which, in the case of N, is directly related to its speciation in mantle fluids. However, the speciation of nitrogen, particularly in supercritical aqueous fluids under mantle conditions is an enigma. Here we present the results of thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions.

Our results show that nitrogen in supercritical aqueous fluids under the redox conditions of Earth's mantle wedge at convergent plate margins should be present as N2, and therefore easily degassed. In contrast, in the rest of Earth's upper mantle, and those for Venus and Mars, nitrogen in aqueous fluids should be mostly present as NH4+, and therefore moderately compatible in upper mantle minerals, which would inhibit degassing. Earth's N-rich atmosphere, relative to Mars and Venus, can be explained as a direct result of the relatively oxidized nature of Earth's upper mantle wedge environments associated with plate tectonics.