The abundance and isotopic composition of nitrogen in mantle xenoliths: Implications for mantle-atmosphere evolution.

摘要

Nitrogen is an interesting element as it functions as a bridge between lithospheric and atmospheric processes. It has several valence states in natural systems. Isotopic fractionation of nitrogen between different chemical forms is expected. Thus, nitrogen has great potential as a geochemical tracer for study of the Earth's evolution, especially for mantle degassing and formation of the atmosphere. Unfortunately, the state of nitrogen in the mantle, including concentration, isotopic composition, and its geochemical behavior, is not well understood.; This study analyzed the concentration and isotopic composition of nitrogen in mantle xenoliths (including peridotites, pyroxenites, eclogites and megacrysts) to constrain nitrogen abundance and {dollar}deltasp{lcub}15{rcub}{dollar}N value of the mantle, investigated the chemical form of nitrogen and explored the relationship between nitrogen and noble gases, potassium, REE and oxygen to understand the geochemical behavior of nitrogen in mantle.; The results show that the likely forms of nitrogen in the mantle are N{dollar}sp{lcub}3-{rcub}{dollar} and N{dollar}sb2{dollar}, the interconversion between these forms may strongly depend on ambient redox conditions; the large variation of {dollar}deltasp{lcub}15{rcub}{dollar}N ({dollar}-{dollar}8 to +30%) in mantle xenoliths has resulted from isotopic fractionation during various processes such as degassing, metasomatism, and subduction. Combined with oxygen, helium, and REE data, nitrogen concentration and {dollar}deltasp{lcub}15{rcub}{dollar}N can be used to constrain the compositions of mantle reservoirs. The inventory of nitrogen in the mantle has been constructed and the implications for the formation of the atmosphere have been obtained. Atmospheric nitrogen comes mainly from mantle degassing and is also modified by loss through hydrodynamic escape or by gain of icy planetesimals.