Melting of the Fe-C-H System and Earth's Deep Carbon-Hydrogen Cycle

The occurrences and cycling of slab-originated carbon and hydrogen are considered to be controlled by their reactions with metallic iron from mantle disproportionation and slab serpentinization, to form Fe alloys containing carbon and hydrogen. Here we show experimental results on the phase relations and melting of the Fe-C-H system using laser-heated diamond anvil cell and X-ray diffraction techniques up to 72 GPa. The incorporation of hydrogen was found to lower the eutectic melting temperatures of Fe-C alloy by similar to 50-178 K at 20-70 GPa, facilitating the formation of metallic liquids in the deep mantle and thus enhancing the mobility and deep cycling of subducted carbon and hydrogen. Hydrogen also substitutes with carbon in Fe-C metal to form hydride and diamond at relatively high-temperature conditions (e.g., 42.6 GPa, >1885 K and 71.8 GPa, >1798 K). The hydrogen-carbon-enriched metallic liquids provide the necessary fluid environment for superdeep diamond growth.

Automated summary

This study examines the phase relations and melting processes of the Fe-C-H system under high pressure. The results show that the addition of hydrogen can lower the melting point of Fe-C alloy, facilitating the formation of metallic liquids and enhancing the mobility and deep cycling of subducted carbon and hydrogen. Additionally, hydrogen can form hydrides and diamonds with carbon.