[S-IT20] Deep Earth Sciences

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

*Ko Fukuyama¹, Hiroyuki Kagi², Tetsuo Irifune³, Toru Shinmei³, Yuki Inoue⁴, Junji Yamamoto¹, Johan Villeneuve⁵, Laurette Piani⁵, Evelyn Füri⁵ (1.Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 2.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 3.Geodynamics Research Center, Ehime University , 4.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 5.Université de Lorraine, CNRS, Centre de Recherches Pétrographiques et Géochimiques)

Keywords:nitrogen, lower mantle, magma ocean, bridgmanite, nitride ion (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).