Volcanic flux of nitrogen from the Earth

The global flux of nitrogen from subduction zones is estimated by the eleme ntal and isotopic compositions of nitrogen, argon and helium observed in vo lcanic gases and hydrothermal fluids in island arcs and in back-are basin b asalt (BABB) glasses. The He-3/He-4 ratios of island are samples vary from 4.7 R-atm to 7.5 R-atm, indicating a typical subduction signature. The Ar-4 0/Ar-36 ratios are consistent with atmospheric values except for a few samp les. The delta N-15 values range from +0.1 parts per thousand to +4.6 parts per thousand, which is generally higher than those of BABE glasses. Taking into account data distribution in the delta N-15-N-2/Ar-36 diagram, we dis tinguish three nitrogen components (mantle-derived, sedimentary and atmosph eric nitrogen) for the island are samples. Contribution of mantle-derived n itrogen is 9-30% in the samples, which is consistent with that of mantle-de rived carbon. It is possible to calculate nitrogen flux based on the He-3 f lux in the literature and N-2/He-3 ratios corrected for elemental fractiona tion. The nitrogen flux of 6.4 X 10(8) mol/year from island are is comparab le with 5.6 X 10(8) mol/year from back-are basin, but smaller than 2.2 X 10 (9) mol/year from mid-ocean ridges. In detail, island arcs show a large flu x of subducted sedimentary nitrogen, while back-are basins have a relativel y small but measurable subduction component. The nitrogen flux of 4.1 X 10( 6) mol/year from hot spot region is significantly small, which is consisten t with the characteristic of global carbon flux from the Earth. Total volca nic flux of nitrogen amounts to 2.8 X 10(9) mol/year by taking mid-ocean ri dge, hot spot and subduction values. The global nitrogen flux, if it has be en constant for the 4.55 billion years of geological time, leads to an accu mulation of 1.3 X 10(19) mol in total, which is one order of magnitude smal ler than 1.8 x 10(20) mel of the present inventory of nitrogen at the Earth 's surface. (C) 2001 Elsevier Science B.V. All rights reserved.