Abstract
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 mid‐ocean ridge basalts and pristine diamonds, a δ15N value of about ‐ 5±2‰ (where δ15N = [( 15N/ 14)sample/ ( 15N/ 14N)air − 1] × 1000) is assigned to the upper mantle. In contrast, the origin of nitrogen has not been well documented in subduction zones. We report here δ15N values, N2/36Ar, and 40Ar/ 36Ar ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a newly developed static mass spectrometry method. The δ15N values and the N2/ 36Ar ratios vary significantly from −2.7‰ to 5.0‰ and from 2.28×104 to 7.15×l05, respectively. Taking a typical δ 15N value for sediments, assuming that the N2/ 36Ar 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 back‐arc basin basalts originated from the upper mantle, whereas island arc samples contain only about 15% of the mantle‐derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.
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