Grewal Damanveer S, Dasgupta Rajdeep, Holmes Alexandra K, Costin Gelu, Li Yuan, Tsuno Kyusei
Department of Earth, Environmental, and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA.
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510460, China.
Geochim Cosmochim Acta. 2019 Apr 15;251:87-115. doi: 10.1016/j.gca.2019.02.009. Epub 2019 Feb 19.
Nitrogen, the most dominant constituent of Earth's atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth's major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is still poorly known. Here we present equilibrium partitioning data of N between alloy and silicate melt ( ) from 67 new high pressure ( = 1-6 GPa)-temperature ( = 1500-2200 °C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logO ΔIW - 4.2 to - 0.8), mafic to ultramafic silicate melt compositions (NBO/T = 0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1 wt.% and 0-3.1 wt.%, respectively). Under relatively oxidizing conditions (ΔIW - 2.2 to - 0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in O and increase in , and increases with increase in and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ~17 wt.% S content in the alloy melt, and then drops off at > ~20 wt.% S content in the alloy melt. Under increasingly reduced conditions (< ~ ΔIW - 2.2), N becomes increasingly lithophile ( between 0.003 and 0.5) with decreasing with decrease in O and increase in . At these conditions O, along with Si content of the alloy under the most reduced conditions (< ~ΔIW - 3.0), is the controlling parameter with playing a secondary role, while, , NBO/T, and S content of the alloy have minimal effects. A multiple linear least-squares regression parametrization for based on the results of this study and previous studies suggests, in agreement with the experimental data, that O (represented by Si content of the alloy melt and FeO content of the silicate melt), followed by , has the strongest control on . Based on our modeling, to match the present-day BSE N content, impactors that brought N must have been moderately to highly oxidized. If N bearing impactors were reduced, and/or there was significant disequilibrium core formation, then the BSE would be too N-rich and another mechanism for N loss, such as atmospheric loss, would be required.
氮气是地球大气中最主要的成分，对我们星球上生命的宜居性和存在至关重要。然而，其在地球主要储库之间的分布情况却鲜为人知，而这在很大程度上必定受其形成时期吸积和分异过程的影响。封存于金属核中，以及吸积前后与挥发性相关的损失，可能是一个关键过程，能够解释相对于原始球粒陨石而言，整体硅酸盐地球（BSE）中氮的消耗情况。然而，不同热力学参数对氮在合金 - 硅酸盐之间分配行为的相对影响仍知之甚少。在此，我们展示了在石墨饱和条件下，于广泛的氧逸度（logO₂ΔIW - 4.2至 - 0.8）、镁铁质到超镁铁质硅酸盐熔体成分（NBO/T = 0.4至2.2）以及合金熔体化学成分各异（S和Si含量分别为0 - 32.1 wt.%和0 - 3.1 wt.%）的情况下，通过67个新的高压（P = 1 - 6 GPa） - 温度（T = 1500 - 220 C）实验得出的氮在合金与硅酸盐熔体之间的平衡分配数据。在相对氧化的条件下（ΔIW - 2.2至 - 0.8），氮作为亲铁元素（KD在1.1至52之间），其中KD随O₂降低和S增加而减小，随Si和NBO/T增加而增大。在这些条件下，在无S条件和合金熔体中S含量高达~17 wt.%时，KD基本不受影响，而在合金熔体中S含量> ~20 wt.%时KD下降。在越来越还原的条件下（< ~ΔIW - 2.2），N变得越来越亲石（KD在0.003至0.5之间），KD随O₂降低和S增加而减小。在这些条件下，O₂以及在最还原条件下（< ~ΔIW - 3.0）合金的Si含量是控制参数，S起次要作用，而合金的Si、NBO/T和S含量影响极小。基于本研究及先前研究结果的KD多元线性最小二乘回归参数化表明，与实验数据一致，O₂（由合金熔体的Si含量和硅酸盐熔体的FeO含量表示），其次是S，对KD的控制作用最强。基于我们的模型，为了匹配现今BSE的N含量，带来N的撞击体必定是中度至高度氧化的。如果携带N的撞击体是还原的，和/或存在显著的不平衡核形成，那么BSE的N含量将会过高，就需要另一种N损失机制，比如大气损失。
