NITROGEN RECYCLING IN SUBDUCTION ZONES

The isotopic composition of nitrogen bears important information conce rning the fate of this volatile in the mantle-crust-atmosphere system. Based on the isotopic compositions of midocean ridge basalts and pris tine diamonds, a delta(15)N value of about 5+/-2 parts per thousand (w here delta(15)N = [(N-15/N-14)(sample)/(N-15/N-14)(air) - 1] x 1000) i s assigned to the upper mantle. In contrast, the origin of nitrogen ha s not been well documented in subduction zones. We report here delta(1 5)N values, N-2/Ar-36, and Ar-40/Ar-36 ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a ne wly developed static mass spectrometry method. The delta(15)N values a nd the N-2/Ar-36 ratios vary significantly from -2.7 parts per thousan d to 5.0 parts per thousand and from 2.28x10(4) to 7.15x10(5), respect ively. 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 nit rogen on the basis of simple mixing equations. About 50% of nitrogen i n Sack-are basin basalts originated from the upper mantle, whereas isl and are samples contain only about 15% of the mantle-derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.