Nitrogen and noble gas isotopic compositions of mantle xenoliths from Far Eastern Russia: Implications for nitrogen isotopic characteristics of mantle wedge fluid

摘要

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 delta N-15 values are -13.26 to +0.19 parts per thousand, some of which are lower than those of air (0 parts per thousand) and upper mantle (-5 +/- 2 parts per thousand). The He-3/He-4 and Ar-40/Ar-36 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 N-2/Ar-36 are 1.5 x 10(4) - 4.5 x 10(4), which are much lower than the mantle value (6.7 x 10(6)) and which are rather more similar to atmospheric (2.5 x 10(4)) and deep seawater (1.1 x 10(4)) values. Kinetic fractionation can lower the delta N-15 value to -15 parts per thousand without marked change of the N-2/Ar-36 ratio. Therefore, the low delta N-15 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. (C) 2020 Elsevier B.V. All rights reserved.