[S-IT20] 地球深部科学

[SIT20-17] Dependence of iron content on nitrogen solubility in bridgmanite and implications for the formation of super-deep nitrogen reservoir

*福山 鴻¹、鍵 裕之²、入舩 徹男³、新名 亨³、井上 裕貴⁴、山本 順司¹、Villeneuve Johan⁵、Piani Laurette⁵、Füri Evelyn⁵ (1.九州大学 大学院理学研究院 地球惑星科学部門、2.東京大学 理学系研究科 附属地殻化学実験施設、3.愛媛大学 地球深部ダイナミクス研究センター、4.九州大学 大学院理学府 地球惑星科学専攻、5.Université de Lorraine, CNRS, Centre de Recherches Pétrographiques et Géochimiques)

キーワード：窒素、下部マントル、マグマオーシャン、ブリッジマナイト、窒化物イオン (N3−)

Nitrogen, an essential element for life, is abundant on the Earth's surface and occupies 78 vol.% of the Earth's atmosphere. However, relative abundance of nitrogen normalized by carbonaceous chondrites in the bulk silicate Earth is depleted compared to other volatile elements (Marty et al., 2012). This is so-called “missing” nitrogen which is one of the important unsolved problems in geochemistry. The estimation of the nitrogen abundance in the bulk silicate Earth was based on N₂/⁴⁰Ar correlation (Marty et al., 1995), which may underestimate nitrogen abundance in the deep mantle because nitrogen will exist as NH₄⁺ or N³⁻ in the lower mantle from observations of natural samples (e.g., Rudloff-Grund et al., 2016; Kaminsky and Wirth, 2017). Therefore, nitrogen solubilities in bridgmanite, which occupies 75 wt.% of the lower mantle (Hirose, 2006), with Fe-poor and MgSiO₃-endmember composition were determined under high-pressure, high-temperature, and near Fe-FeO or controlled Fe-FeO (Yoshioka et al., 2018; Fukuyama et al., 2023). However, these nitrogen solubilities ranged from 1.8 ± 0.4 ppm to 53.9 ± 1.2 ppm and this difference with one order of magnitude raised a question of whether bridgmanite in the present lower mantle can compensate for the mass of nitrogen to solve “missing” nitrogen or not. The difference of nitrogen solubilities between previous studies may be caused by differences of iron content in bridgmanite (Fukuyama et al., 2023).
Here, we conducted high-pressure and high-temperature experiments, Raman spectrometry, and secondary ion mass spectrometry to determine dependence of iron content on nitrogen solubility in bridgmanite. For carrying out these experiments, we used multi-anvil apparatus installed at Geodynamics Research Center, a Raman spectrometer installed at Kyushu University, and a secondary ion mass spectrometer (1280 HR2) installed at Centre de Recherches Pétrographiques et Géochimiques.
As a result, we precisely clarified that nitrogen solubility in Fe-bearing bridgmanite increases from 2.1 ± 0.3 ppm to 6.9 ± 1.5 ppm with increasing its FeO content from 1.9 wt.% to 7.7 wt.% at 28 GPa and 1500 °C. Simultaneously, we determined nitrogen solubility in bridgmanite with pyrolitic composition for the first time. The nitrogen storage capacity of the pyrolitic bridgmanite under the present temperature conditions is estimated to be 3.6 ± 0.8 ppm in the bulk Earth, corresponding to 5.5 ± 1.2 PAN (PAN: mass of present atmospheric nitrogen). Thus, bridgmanite cannot solve “missing” nitrogen by storing nitrogen from 22.8 ± 11.5 ppm to 34.7 ± 17.5 ppm in the present bulk Earth. In contrast, Fe-poor bridgmanite with low nitrogen solubility may have played an important role in segregating nitrogen in the deep mantle during solidification of magma ocean because bridgmanite is expected to be crystallized at the depth of 1000 km in the magma ocean (e.g., Xie et al., 2020).