Phase relations in the Fe-Fe3C-Fe3N system at 7.8 GPa and 1150°C: implications for C and N hosts in metal-saturated mantle

ABSTRACT The isothermal section of the Fe-Fe3C-Fe3N phase diagram at 7.8 GPa and 1150°С comprises a three-phase field of γ-Fe + Fe3C + ϵ-Fe3(С/N) in the central part and three two-phase fields (γ-Fe + Fe3C, γ-Fe + ϵ-Fe3 (С/N) and Fe3C + ϵ-Fe3(С/N)) bounded by three single-phase fields of γ-Fe, Fe3C and ϵ-Fe3(С/N) solid solutions along the triangle sides. Thus, native iron can host carbon and nitrogen in the γ-Fe + Fe3C or γ-Fe phases in the mantle depleted in volatiles (20 ppm C and 1 ppm N) at temperatures corresponding to the ∼35 mW/m2 heat flux, and in the Fe3C or ϵ-Fe3(С/N) phases if the contents of volatiles in the mantle reach 250 ppm C and 100 ppm N. Iron carbonitride has quite a large stability field even at low nitrogen concentrations in the system simulating native iron.