Phase Relations in the FeO-Fe3C-Fe3N System at 7.8 GPa and 1350 °C: Implications for Oxidation of Native Iron at 250 km

Abstract

Oxidation of native iron in the mantle at a depth about 250 km and its influence on the stability of main carbon and nitrogen hosts have been reconstructed from the isothermal section of the ternary phase diagram for the FeO-Fe3C-Fe3N system. The results of experiments at 7.8 GPa and 1350 °C show that oxygen increase in the system to > 0.5 wt % provides the stability of FeO and leads to changes in the phase diagram: the Fe3C, L, and Fe3N single-phase fields change to two-phase ones, while the Fe3C + L and Fe3N + L two-phase fields become three-phase. Сarbon in iron carbide (Fe3C, space group Pnma) is slightly below the ideal value and nitrogen is below the EMPA (Electron microprobe analysis) detection limit. Iron nitride (ε-Fe3N, space group P63/mmc) contains up to 2.7 wt % С and 4.4 wt % N in equilibrium with both melt and wüstite but 2.1 wt % С and 5.4 wt % N when equilibrated with wüstite alone. Impurities in wüstite (space group Fmm) are within the EMPA detection limit. The contents of oxygen, carbon, and nitrogen in the metal melt equilibrated with different iron compounds are within 0.5–0.8 wt % O even in FeO-rich samples; 3.8 wt % C and 1.2 wt % N for Fe3C + FeO; and 2.9 wt % C and 3.5 wt % N for Fe3N + FeO. Co-crystallization of Fe3C and Fe3N from the O-bearing metal melt is impossible because the fields of associated C- and N-rich compounds are separated by that of FeO + L. Additional experiments with excess oxygen added to the system show that metal melt, which is the main host of carbon and nitrogen in the metal-saturated (~0.1 wt %) mantle at a depth of ~250 km and a normal heat flux of 40 mW/m2, has the greatest oxygen affinity. Its partial oxidation produces FeO and causes crystallization of iron carbides (Fe3C and Fe7C3) and increases the nitrogen enrichment of the residual melt. Thus, the oxidation of metal melt in the mantle enriched in volatiles may lead to successive crystallization of iron carbides and nitrides. In these conditions, magnetite remains unstable till complete oxidation of iron carbide, iron nitride, and the melt. Iron carbides and nitrides discovered as inclusions in mantle diamonds may result from partial oxidation of metal melt which originally contained relatively low concentrations of carbon and nitrogen.