[S-IT15] Study of the Earth's Deep Interior - Interaction and Coevolution of the Core and Mantle

[SIT15-02] Increase of nitrogen solubility in ferropericlase and bridgmanite by iron incorporation under lower-mantle conditions

★Invited Papers

*Ko Fukuyama^1,2, Hiroyuki Kagi², Toru Inoue³, Sho Kakizawa³, Toru Shinmei¹, Yuji Sano^4,5, Naoto Takahata⁴, Shunichi Hishita⁶, Cécile Deligny⁷, Evelyn Füri⁷ (1.Geodynamics Research Center, Ehime University, 2.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 3.Graduate School of Advanced Science and Engineering, Hiroshima University, 4.Atmosphere and Ocean Research Institute, The University of Tokyo, 5.Institute of Surface-Earth System Science, Tianjin University, 6.National Institute for Materials Science, 7.Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine)

Keywords:Lower mantle, "Missing" nitrogen, High-pressure and high-temperature experiment, Secondary ion mass spectrometry, Atmosphere-Mantle coevolution

Nitrogen occupies approximately 78% of the Earth's atmosphere and is one of the essential elements of life. Despite its importance, nitrogen behavior in the Earth's interior remains poorly understood. When the mass of volatile elements in the bulk silicate Earth is normalized to the chondrite composition, nitrogen in the bulk Earth is depleted compared to other volatile elements (Marty et al., 2012). Experimental studies suggested that this "missing" nitrogen was caused by nitrogen stored in the deep mantle through the solidification of the magma ocean (e.g., Li et al., 2013; Yoshioka et al., 2018). Yoshioka et al. (2018) are the only authors reporting the nitrogen solubility in bridgmanite, which occupies approximately 80 vol% of the lower mantle. Nitrogen solubility in ferropericlase, which occupies approximately 15 vol% of the lower mantle, has not been investigated.

In this research, we investigated the effect of iron incorporation on nitrogen solubility in periclase (MgO) and bridgmanite. High-pressure and high-temperature experiments were conducted using the multi-anvil apparatus installed at Geodynamics Research Center, Ehime University. The experimental pressure and temperatures were 28 GPa and 1400–1700 °C. Fe-FeO buffer was used to control the redox state corresponding to the lower-mantle condition. Nitrogen in recovered samples were analyzed using the NanoSIMS installed at Atmosphere and Ocean Research Institute, The University of Tokyo and the high-resolution secondary ion mass spectrometer (1280 HR2, CAMECA) installed at Centre de Recherches Pétrographiques et Géochimiques. Nitrogen-implanted standard samples for the NanoSIMS were prepared at National Institute for Materials Science.

Our results showed that the nitrogen solubility in periclase (MgO) increased from 1.1 ppm to 132 ppm with increasing FeO content and the nitrogen solubility in bridgmanite increased from 5 ppm to 14 ppm with increasing FeO content. These results implied that, assuming no pressure dependence on nitrogen solubility in minerals, bridgmanite ((Mg_0.9, Fe_0.1)SiO_3; nitrogen solubility of 8.7 ppm) and ferropericlase ((Mg_0.9, Fe_0.1)O; nitrogen solubility of 132 ppm) can store 5.2 PAN and 15.7 PAN (PAN: Mass of Present Atmospheric Nitrogen, 3.92 × 10¹⁸ kg (e.g., Johnson and Goldblatt, 2015)) in the lower mantle, respectively. We suggest that the lower mantle can be the largest nitrogen reservoir in the bulk silicate Earth.