[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-P06] Heterogeneous accretion of Earth’s volatiles as constrained by nitrogen isotopes

*Lanlan Shi^2,1,3, Takanori Kagoshima², Yuji Sano² (1.State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of 7 Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, 2.Division of Ocean-Earth System Science, Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, Japan, 3.University of Chinese Academy of Sciences, Beijing 100049, China)

Keywords:nitrogen, isotope fractionation, core formation, oxygen fugacity, volatiles

The origin of planetary volatiles such as nitrogen is critical for understanding planetary accretion, differentiation, and habitability. However, the detailed processes for the origin of Earth’s volatiles remain unresolved. Nitrogen shows large isotopic fractionations among geochemical and cosmochemical reservoirs, which places tight constraints on Earth’s volatile accretion process. Here we experimentally determine N-partitioning and -isotopic fractionations between planetary cores and silicate mantles at 1–8 GPa and 1700–2200 °C. We show that the core/mantle N-isotopic fractionations increase from −4‰ to +10‰, as oxygen fugacity decreases from 0.3 to 4.7 log units below the iron-wüstite buffer. The core/mantle nitrogen partition coefficients, ranging from 0.03 to 80, are a multifunction of oxygen fugacity, temperature, pressure, and the compositions of core and mantle. We apply N partition coefficients and isotopic fractionations in in a state-of-the-art model of planetary accretion and core-mantle differentiation. We find that the N-budget and -isotopic composition of Earth’s atmosphere plus crust, silicate mantle, and the source region of oceanic island basalts are best explained by Earth’s early accretions of enstatite chondrite-like reduced impactors, followed by late accretions of increasingly oxidized impactors and minimal CI chondrite-like material shortly before and during the Moon-forming giant impact. Earth may thus have acquired its nitrogen and other major volatiles heterogeneously, and its volatile budget may have been established during the main accretion stages.