Volcanic flux of nitrogen from the Earth

The global flux of nitrogen from subduction zones is estimated by the elemental and isotopic compositions of nitrogen, argon and helium observed in volcanic gases and hydrothermal fluids in island arcs and in back-arc basin basalt (BABB) glasses. The ³He/⁴He ratios of island arc samples vary from 4.7 R_atm to 7.5 R_atm, indicating a typical subduction signature. The ⁴⁰Ar/³⁶Ar ratios are consistent with atmospheric values except for a few samples. The δ¹⁵N values range from +0.1‰ to -4.6‰, which is generally higher than those of BABB glasses. Taking into account data distribution in the δ¹⁵N-N₂/³⁶Ar diagram, we distinguish three nitrogen components (mantle-derived, sedimentary and atmospheric nitrogen) for the island arc samples. Contribution of mantle-derived nitrogen is 9-30% in the samples, which is consistent with that of mantle-derived carbon. It is possible to calculate nitrogen flux based on the ³He flux in the literature and N₂/³He ratios corrected for elemental fractionation. The nitrogen flux of 6.4 x 10⁸ mol/year from island arc is comparable with 5.6 x 10⁸ mol/year from back-arc basin, but smaller than 2.2 x 10⁹ mol/year from mid-ocean ridges. In detail, island arcs show a large flux of subducted sedimentary nitrogen, while back-arc basins have a relatively small but measurable subduction component. The nitrogen flux of 4.1 x 10⁶ mol/year from hot spot region is significantly small, which is consistent with the characteristic of global carbon flux from the Earth. Total volcanic flux of nitrogen amounts to 2.8 x 10⁹ mol/year by taking mid-ocean ridge, hot spot and subduction values. The global nitrogen flux, if it has been constant for the 4.55 billion years of geological time, leads to an accumulation of 1.3 x 10¹⁹ mol in total, which is one order of magnitude smaller than 1.8 x 10²⁰ mol of the present inventory of nitrogen at the Earth's surface.