Nitrogen recycling in subduction zones

The isotopic composition of nitrogen bears important information concerning the fate of this volatile in the mantle-crust-atmosphere system. Based on the isotopic compositions of midocean ridge basalts and pristine diamonds, a delta(15)N value of about 5+/-2 parts per thousand (where delta(15)N = [((15)N/(14)N)(sample)/((15)N/(14)N)(air) - 1] x 1000) is assigned to the upper mantle. In contrast, the origin of nitrogen has not been well documented in subduction zones. We report here delta(15)N values, N(2)/(36)Ar, and (40)Ar/(36)Ar ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a newly developed static mass spectrometry method. The delta(15)N values and the N(2)/(36)Ar ratios vary significantly from -2.7 parts per thousand to 5.0 parts per thousand and from 2.28x10(4) to 7.15x10(5), respectively. Taking a typical delta(15)N value for sediments, assuming that the N(2)/(36)hAr ratio of mid-ocean ridge basalts is representative of the upper mantle, and that later isotopic and elemental fractionation are not significant, it is possible to deconvolve each fraction of nitrogen on the basis of simple mixing equations. About 50% of nitrogen in Sack-are basin basalts originated from the upper mantle, whereas island are samples contain only about 15% of the mantle-derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.