Nitrogen in the Earth: abundance and transport

摘要

The terrestrial nitrogen budget, distribution, and evolution are governed by biological and geological recycling. The biological cycle provides the nitrogen input for the geological cycle, which, in turn, feeds some of the nitrogen into the Earth’s interior. A portion of the nitrogen also is released back to the oceans and the atmosphere via N~(2)degassing. Nitrogen in silicate minerals (clay minerals, mica, feldspar, garnet, wadsleyite, and bridgmanite) exists predominantly as NH~(4)_(+). Nitrogen also is found in graphite and diamond where it occurs in elemental form. Nitrides are stable under extremely reducing conditions such as those that existed during early planetary formation processes and may still persist in the lower mantle. From experimentally determined nitrogen solubility in such materials, the silicate Earth is nitrogen undersaturated. The situation for the core is more uncertain, but reasonable Fe metal/silicate nitrogen partition coefficients (>?10) would yield nitrogen contents sufficient to account for the apparent nitrogen deficiency in the silicate Earth compared with other volatiles. Transport of nitrogen takes place in silicate melt (magma), water-rich fluids, and as a minor component in silicate minerals. In melts, the N solubility is greater for reduced nitrogen, whereas the opposite appears to be the case for N solubility in fluids. Reduced nitrogen species (NH~(3), NH~(2)_(?), and NH~(2)_(+)) dominate in most environments of the modern Earth’s interior except the upper ~?100?km of subduction zones where N~(2)is the most important species. Nitrogen in magmatic liquids in the early Earth probably was dominated by NH~(3)and NH~(2)_(?), whereas in the modern Earth, the less reduced, NH~(2)_(+)functional group is more common. N~(2)is common in magmatic liquids in subduction zones. Given the much lower solubility of N~(2)in magmatic liquids compared with other nitrogen species, nitrogen dissolved as N~(2)in subduction zone magmas is expected to be recycled and returned to the oceans and the atmosphere, whereas nitrogen in reduced form(s) likely would be transported to greater depths. This solubility difference, controlled primarily by variations in redox conditions, may be a factor resulting in increased nitrogen in the Earth’s mantle and decreasing abundance in its oceans and atmosphere during the Earth’s evolution. Such an abundance evolution has resulted in the decoupling of nitrogen distribution in the solid Earth and the hydrosphere and atmosphere.