Ultralow Melting of Iron Nitrides and the Fe-N-C System at High Pressure

Iron nitrides and carbonitrides are the two primary hosts of deep nitrogen in the Earth's interior; hence, the storage and circulation of deep nitrogen can be largely controlled by the melting behavior of iron nitrides and the Fe-N-C system. In this study, the melting temperatures of these minerals were determined individually and jointly using the new diagnostics of electrical resistance jump up to 80 GPa in laser-heated diamond-anvil cells. The solidus curves of these samples are extremely low at mantle pressures, and are marginally affected by the iron spin transition, nitrogen concentration, and carbon incorporation, suggesting that nitrogen-rich metallic liquids likely exist at depths of <1,500 km in the present-day mantle. Meanwhile, our Raman spectra are indicative of N-2 degassing out of iron nitride liquids at <12 GPa. These results suggest that low-pressure N-2 degassing in the proto-Earth coupled with nitrogen-rich liquids hidden in present-day lower mantle play important roles in the redistribution of deep nitrogen through geologic time and space.

Automated summary

This study determined the melting temperatures of iron nitrides and carbonitrides, and found that nitrogen-rich metallic liquids likely exist at depths of <1,500 km in the mantle. Raman spectra results indicate N-2 degassing from iron nitride liquids. These findings suggest that low-pressure N-2 degassing in the proto-Earth and nitrogen-rich liquids in the present-day lower mantle play important roles in the redistribution of deep nitrogen through geologic time and space.