講演情報
[PPS04-P25]Earth's Nitrogen Deficit and δ15N Offset from Heterogeneous Accretion via Repeated Giant Impacts
*花沢 泰光1、佐々木 貴教1 (1.京都大学)
キーワード：
窒素欠乏、窒素同位体、ジャイアントインパクト、マグマオーシャン、大気散逸、不均質集積
Earth's bulk silicate Earth (BSE) is strongly depleted in nitrogen relative to chondritic expectations, while the atmosphere and mantle preserve distinct δ15N signatures. Explaining both the N deficit and the atmosphere–mantle isotopic offset requires a unified growth narrative that couples (i) heterogeneous volatile delivery, (ii) impact-driven atmospheric erosion and incomplete retention of impactor volatiles, and (iii) metal–silicate partitioning and isotope fractionation during core formation.
We develop a coupled accretion–geochemistry framework in which Earth grows through (a) early planetesimal accretion up to embryo scale and (b) a dominant final-assembly phase characterized by repeated embryo–embryo giant impacts (GIs), consistent with standard N-body terrestrial planet formation scenarios. The model tracks N mass and δ15N among atmosphere, silicate reservoir (melt/solid), and metal via stepwise mass balance across a sequence of impacts. Each GI can reset melt fraction and equilibration conditions (magma-ocean state), while inter-impact intervals allow progressive solidification, smaller impacts, and continued volatile loss.
At every impact, volatile supply competes directly with loss through atmospheric erosion and partial retention of impactor volatiles. Metal–silicate exchange and isotope fractionation are computed under parameterized equilibration pressure–temperature–redox conditions, and a variable equilibration fraction (Φ) controls the extent of atmosphere–silicate exchange under magma-ocean conditions. Heterogeneous accretion is represented by an evolving impactor population transitioning from reduced, volatile-poor inner-disk material toward more oxidized, volatile-richer sources as growth proceeds; optional outer-disk (CI-like) inputs are treated as endmember tests (e.g., late-veneer-like delivery).
We will present initial simulations and sensitivity analyses focusing on how (1) the number and timing of GIs, (2) evolving equilibration extent (Φ) and solidification between impacts, and (3) erosion/retention efficiencies jointly determine the final BSE N abundance and the atmosphere–mantle δ15N contrast. This framework provides a transparent bridge from dynamical giant-impact sequences to geochemical observables and identifies which processes are required—and which are dispensable—to reproduce Earth’s nitrogen inventory.
We develop a coupled accretion–geochemistry framework in which Earth grows through (a) early planetesimal accretion up to embryo scale and (b) a dominant final-assembly phase characterized by repeated embryo–embryo giant impacts (GIs), consistent with standard N-body terrestrial planet formation scenarios. The model tracks N mass and δ15N among atmosphere, silicate reservoir (melt/solid), and metal via stepwise mass balance across a sequence of impacts. Each GI can reset melt fraction and equilibration conditions (magma-ocean state), while inter-impact intervals allow progressive solidification, smaller impacts, and continued volatile loss.
At every impact, volatile supply competes directly with loss through atmospheric erosion and partial retention of impactor volatiles. Metal–silicate exchange and isotope fractionation are computed under parameterized equilibration pressure–temperature–redox conditions, and a variable equilibration fraction (Φ) controls the extent of atmosphere–silicate exchange under magma-ocean conditions. Heterogeneous accretion is represented by an evolving impactor population transitioning from reduced, volatile-poor inner-disk material toward more oxidized, volatile-richer sources as growth proceeds; optional outer-disk (CI-like) inputs are treated as endmember tests (e.g., late-veneer-like delivery).
We will present initial simulations and sensitivity analyses focusing on how (1) the number and timing of GIs, (2) evolving equilibration extent (Φ) and solidification between impacts, and (3) erosion/retention efficiencies jointly determine the final BSE N abundance and the atmosphere–mantle δ15N contrast. This framework provides a transparent bridge from dynamical giant-impact sequences to geochemical observables and identifies which processes are required—and which are dispensable—to reproduce Earth’s nitrogen inventory.