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 δ15N 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 δ15N value to −15‰ without marked change of the N2/36Ar ratio. Therefore, the low δ15N 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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science