VOLCANIC FLUX OF NITROGEN FROM THE EARTH

Abstract

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 Ratm to 7.5 Ratm, 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 6.4×108 mol/year from island arc is comparable with 5.6×108 mol/year from back-arc basin, but smaller than 2.2×109 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 4.1×106 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 2.8×109 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 1.3×1019 mol in total, which is one order of magnitude smaller than 1.8×1020 mol of the present inventory of nitrogen at the Earth's surface.