High‐Pressure Phase Stability and Thermoelastic Properties of Iron Carbonitrides and Nitrogen in the Deep Earth. Issue 6 (24th June 2021)

• Record Type: Journal Article
• Authors: Huang, Shengxuan; Wu, Xiang; Zhu, Feng; Lai, Xiaojing; Li, Jie; Neill, Owen K.; Qin, Shan; Rapp, Robert; Zhang, Dongzhou; Dera, Przemyslaw; Chariton, Stella; Prakapenka, Vitali B.; Chen, Bin
• Is Part Of: Journal of geophysical research. Volume 126:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-24
• Subjects: Deep nitrogen host -- equation of state -- inner core -- iron carbonitride -- phase transition -- superdeep diamond; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB021934
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Iron‐dominant metallic phases are likely the primary hosts for nitrogen in the reduced deep Earth, hence the storage of nitrogen in the lower mantle and the core is governed by the behavior of the Fe‐N‐C system at high temperatures and pressures. In this study, phase transitions and thermoelastic properties of iron carbonitrides were investigated at high pressure‐temperature conditions by diamond anvil cell experiments and first‐principles calculations. Experimental data revealed no phase transition in ε ‐type Fe4 (N0.6 C0.4 ) or Fe7 (N0.75 C0.25 )3 up to 60 GPa at room temperature. At high temperature, Fe7 (N0.75 C0.25 )3 transforms into the Fe3 C‐type phase at ∼27 GPa, and then into the Fe7 C3 ‐type phase at ∼45 GPa, which is also corroborated by our theoretical calculations. We found that the phase stability of iron carbonitrides mainly depends on the N/C ratio, and the elastic properties of iron carbonitrides are dominantly affected by the Fe/(N+C) ratio. Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle. Some iron carbonitride inclusions in lower mantle diamonds could be the residue of the primordial mantle or originate from subducted nitrogen‐bearing materials, rather than iron‐enriched phases of the outer core. In addition, our experiments confirmed the existence of Fe7 C3 ‐type Fe7 C3 ‐Fe7 N3 solid solutions above 40 GPa. Fe7 C3 ‐type Fe7 (C, N)3 has comparable density and thermoelastic properties to its isostructural endmembers and may be a promising candidate constituent of the Earth's inner core. Plain Language Summary: Nitrogen is an essential element for the Earth's atmosphere and life. Most of the Earth's nitrogen may reside in Earth's interior, which may profoundly influence the partial pressure of atmospheric nitrogen. Knowledge on the storage and cycling of nitrogen in the deep Earth is crucial for our understanding of the Earth's evolution and dynamics. We combined high‐pressure and high‐temperature experiments and theoretical calculations to investigate the phase transitions and elasticity of iron carbonitrides. The results show that iron carbonitrides are stable upon compression at room temperature, but transformed into two high‐pressure structures at lower pressures at high temperatures than their iron carbide counterparts. N/C and Fe/(N+C) atomic ratios are two key factors affecting the phase stability and elasticity of iron carbonitrides. Nitrogen could be stored in the Earth's lower mantle in the form of iron carbonitrides. Iron carbonitrides may be the main nitrogen host and be trapped as inclusions in some superdeep diamonds and carried to the shallow regions through geodynamic processes, as we can find in some diamonds of sublithospheric origin. Key Points: ε‐type iron carbonitrides undergo two phase transitions at high pressure and high temperature N/C and Fe/(N+C) atomic ratios are two key factors affecting the phase stability and elasticity of iron carbonitrides Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle and encapsulated in superdeep diamonds