Melting Curve and Equation of State of β‐Fe₇N₃: Nitrogen in the Core?

Abstract

The chemical composition of carbonaceous chondrites permits up to 0.5 ± 0.4 wt% N in the Earth's core, which potentially affects the melting temperature, density profile, and phase relation of the core iron alloy. Here we have determined the melting curve and the high-temperature equation of state (EoS) of β-Fe 7 N 3 , which is stable above 40 GPa as the most Fe-rich iron-nitride. Experiments were performed in a laser-heated diamond-anvil cell up to 136 GPa, together with synchrotron X-ray diffraction measurements. We found that multiple melting criteria gave similar melting temperatures: (1) the appearance of diffuse X-ray scattering, (2) discontinuity in the laser power versus temperature relation, and (3) the reduction in diffraction peak intensity from solid. The validity of these melting criteria was confirmed by textural observation of recovered samples. We also observed rapid recrystallization at temperatures lower than the melting temperatures. The results demonstrate that β-Fe 7 N 3 melts congruently at about 3,100 K at 135 GPa, lower than the melting temperatures of FeSi and FeO and similar to that of FeS. The thermal EoS indicates that the density of Fe 7 (C,N) 3 matches the observed inner core density. Combining the melting curve and the EoS of β-Fe 7 N 3 , we also obtain the EoS of liquid Fe 7 N 3 . It shows that the density and compressibility of Fe + 10 wt% N is compatible with the outer core density profile. It supports the presence of some nitrogen in the liquid and solid parts of the core, although its concentration is difficult to constrain from the core density.

Equation of State of TiN at High Pressures and Temperatures: A Possible Host for Nitrogen in Planetary Mantles

“…The pressure dependence of the thermal expansion of TiN was previously investigated in a computational study (R. Yang et al, 2016), although we find a lower thermal expansion at high pressures. A previous study of the thermal equation of state of Fe 7 N 3, reported a higher K 0 (316 GPa), lower K 0 ′ (3.2), and higher  0 (2.1) compared to TiN (Kusakabe et al, 2019). DAVIAU ET AL.…”

“…The pressure dependence of the thermal expansion of TiN was previously investigated in a computational study (R. Yang et al, 2016), although we find a lower thermal expansion at high pressures. A previous study of the thermal equation of state of Fe 7 N 3, reported a higher K 0 (316 GPa), lower K 0 ′ (3.2), and higher  0 (2.1) compared to TiN (Kusakabe et al, 2019 Figure 3a displays the P-V-T data used in the equation of state fit, compared to isotherms calculated from our fitted equation of state. Also shown is a 300 K isotherm calculated from the equation of state of H. Chen et al (2010), which agrees well with our 300 K isotherm over the pressure range of their data (up to 45 GPa).…”

“…It is expected that TiN would be under subsolidus conditions in this temperature range, as its 1 bar melting point is ∼3,200 K (Haynes, 2014). In contrast, Fe 7 N 3 melts at temperatures below 2,000 K at pressures less than 60 GPa (Kusakabe et al., 2019). The melting curve of cubic Si 3 N 4 at high pressures is not well constrained, with a recent study to 1,300 K and 20 GPa reporting no indication of melting or decomposition (Nishiyama et al., 2019).…”

“…However, the oxygen and nitrogen fugacity conditions for their formation are more limited, according to thermodynamic calculations (Khodakovsky & Petaev, 1981), making them less likely candidates in the deep Earth. With these exotic nitrides being more difficult to form (Khodakovsky & Petaev, 1981), and Fe 7 N 3 melting at temperatures below the geotherm (Kusakabe et al., 2019), TiN's stability against melting and decomposition and its presence in mantle rocks (Dobrzhinetskaya et al., 2009) may make it one of the most plausible deep mantle nitrogen reservoirs.…”

Once Again on Preliminary Reference Earth Model

“…Detailed reviews of the experimental work on this topic (through 2016) have been presented in Anderson (2007), Litasov and Shatskiy (2016), Liu and Lin (2014), and Sumita and Bergman (2007), and the results of these studies are analyzed from the perspective of geomechanics in Guliyev (2017cGuliyev ( , 2018a. The results of new studies are reflected in numerous publications (Antonangeli et al, 2018;Birch, 1952;Chantel et al, 2016;Chen et al, 2018;Das et al, 2017;Fan, Xu, et al, 2019;Hakim et al, 2018;Huntington et al, 2017;Komabayashi, 2014;Komabayashi et al, 2019;Kusakabe et al, 2019;Mao et al, 2018;Qian et al, 2017;Sakairi et al, 2017;Sakamaki et al, 2016;Singh et al, 2017;Stekiel et al, 2017;Takahashi et al, 2019;Tanto et al, 2018;Tateno, 2018;Thompson et al, 2017;Wakamatsu et al, 2018;Xiong et al, 2018;Xu et al, 2019).…”