LAUSR

Abstract To investigate nitrogen circulation via subduction, we measured isotopic compositions of nitrogen and noble gases of mantle-derived xenoliths from Far Eastern Russia, where the tectonic setting is a plate convergence margin in the Mesozoic to Cenozoic era. Kinetic fractionation accompanies the extraction of volatiles from a subducting plate, engendering the infiltration of fluids with light isotopic compositions into mantle wedge. The δ 15 N values are −13.26 to +0.19‰, some of which are lower than those of air (0‰) and upper mantle (−5 ± 2‰). The 3He/4He and 40Ar/36Ar ratios are 0.2–5.6 Ra and 304–1156, respectively, which are common for the mantle xenoliths in eastern Asia, a representative subduction setting. The N2/36Ar are 1.5 × 104 – 4.5 × 104, which are much lower than the mantle value (6.7 × 106) and which are rather more similar to atmospheric (2.5 × 104) and deep seawater (1.1 × 104) values. Kinetic fractionation can lower the δ 15 N value to −15‰ without marked change of the N2/36Ar ratio. Therefore, the low δ 15 N values of the present xenoliths might be regarded as values of the fluid experiencing fractionation during infiltration of the fluid from the subducting oceanic plate into the mantle wedge.