13:45 〜 14:00
[SGC32-01] Wetting property of Fe-Ni-N alloy melt in the deep mantle and its application to depletion of nitrogen in the present-day bulk silicate Earth
★Invited Papers
キーワード：窒素、揮発性元素、ハビタビリティ、高温高圧実験、マルチアンビル
To understand the habitability of the Earth, it is crucial to constrain the process and timing by which the inventory of nitrogen (N) in the present-day bulk silicate Earth (BSE) was established. A key question remains as to why nitrogen is more strongly depleted than carbon (C) and hydrogen (H) in the present-day BSE [1, 2]. It has been suggested that one of the main reasons for this depletion is due to the efficient segregation of N into the core-forming alloy melt during core formation [3]. However, it is still unknown whether the silicate magma ocean (MO) was more N-depleted than the BSE today due to the uncertainty of the extent of metal-silicate equilibrium during the late stage of core formation [4, 5].
Fe-Ni alloy can precipitate deep in the mantle owing to the disproportionation reaction (3 Fe2+ (silicate) = 2 Fe3+ (silicate) + Fe (alloy)) [6]. Thus, it may incorporate N to form Fe-Ni-N melt if N in the post MO solid mantle is richer than in the BSE today [3, 7]. Here we examine the feasibility of percolation of Fe-Ni-N melt in the deep mantle to explain N depletion in the BSE. Multi-anvil experiments were performed at 20 GPa and 1673-2073 K to determine the dihedral angles of Fe-alloy melt containing 0-10 wt% Ni and 1-4 wt% N in ringwoodite matrix. The obtained dihedral angles ranged from 112° to 137°. The dihedral angles decreased with increasing temperature and N content in the alloy, but no discernible difference was observed with increasing Ni content in the alloy. The dihedral angles obtained in this study are much larger than the wetting boundary (60°), suggesting that N removal by percolation of Fe-Ni alloy melts deep in the mantle is an unlikely scenario to explain N depletion in the BSE today. If the metal-silicate equilibrium between the impactor's core and the proto-Earth's mantle was limited [8] and a significant portion of the atmosphere was retained [9, 10] during the late stage of Earth's growth, it is possible that the N content of the post MO solid mantle was not as depleted as the BSE today, in which case iron-nickel nitrides could be a hidden reservoir for N [11]. On the other hand, if the majority of nitrogen had already been lost to an amount of N equivalent to the BSE today, the building blocks of the Earth such as planetesimals had already lost most of N [12, 13] or an atmospheric loss had occurred during the main accretion stage [3, 14].
[1] Marty, B, (2012) EPSL. [2] Halliday, A.N. (2013) GCA. [3] Grewal, D.S. et al. (2021) Nat Geosci. [4] Deguen, R. et al, (2014) EPSL. [5] Landeau, M. et al. (2016) Nat Geosci. [6] Rohrbach, A. & Schmidt, M.W. (2011) Nature. [7] Grewal, D.S. et al. (2019) GCA. [8] Rudge, J.F. et al. (2010) Nature Geosci. [9] Genda, H., & Abe, Y. (2003) Icarus. [10] Kegerreis, J.A. et al. (2018) APJ. [11] Kaminsky, F., & Wirth, R. (2017) Am Mineral. [12] Grewal, D.S. & Asimow, P.D. (2023) GCA. [13] Grewal, D.S. et al. (2022) EPSL. [14] Kurokawa, H. et al. (2022) G-cubed.
This is a collaborative work with D.S. Grewal.
Fe-Ni alloy can precipitate deep in the mantle owing to the disproportionation reaction (3 Fe2+ (silicate) = 2 Fe3+ (silicate) + Fe (alloy)) [6]. Thus, it may incorporate N to form Fe-Ni-N melt if N in the post MO solid mantle is richer than in the BSE today [3, 7]. Here we examine the feasibility of percolation of Fe-Ni-N melt in the deep mantle to explain N depletion in the BSE. Multi-anvil experiments were performed at 20 GPa and 1673-2073 K to determine the dihedral angles of Fe-alloy melt containing 0-10 wt% Ni and 1-4 wt% N in ringwoodite matrix. The obtained dihedral angles ranged from 112° to 137°. The dihedral angles decreased with increasing temperature and N content in the alloy, but no discernible difference was observed with increasing Ni content in the alloy. The dihedral angles obtained in this study are much larger than the wetting boundary (60°), suggesting that N removal by percolation of Fe-Ni alloy melts deep in the mantle is an unlikely scenario to explain N depletion in the BSE today. If the metal-silicate equilibrium between the impactor's core and the proto-Earth's mantle was limited [8] and a significant portion of the atmosphere was retained [9, 10] during the late stage of Earth's growth, it is possible that the N content of the post MO solid mantle was not as depleted as the BSE today, in which case iron-nickel nitrides could be a hidden reservoir for N [11]. On the other hand, if the majority of nitrogen had already been lost to an amount of N equivalent to the BSE today, the building blocks of the Earth such as planetesimals had already lost most of N [12, 13] or an atmospheric loss had occurred during the main accretion stage [3, 14].
[1] Marty, B, (2012) EPSL. [2] Halliday, A.N. (2013) GCA. [3] Grewal, D.S. et al. (2021) Nat Geosci. [4] Deguen, R. et al, (2014) EPSL. [5] Landeau, M. et al. (2016) Nat Geosci. [6] Rohrbach, A. & Schmidt, M.W. (2011) Nature. [7] Grewal, D.S. et al. (2019) GCA. [8] Rudge, J.F. et al. (2010) Nature Geosci. [9] Genda, H., & Abe, Y. (2003) Icarus. [10] Kegerreis, J.A. et al. (2018) APJ. [11] Kaminsky, F., & Wirth, R. (2017) Am Mineral. [12] Grewal, D.S. & Asimow, P.D. (2023) GCA. [13] Grewal, D.S. et al. (2022) EPSL. [14] Kurokawa, H. et al. (2022) G-cubed.
This is a collaborative work with D.S. Grewal.