Effect of sulfur on hydrogenation of metallic iron and evolution of the earth's core

The authors determined the amount of hydrogen incorporated into iron based on neutron diffraction experiments under high temperature and high pressure simulating the primitive earth. Regarding hydrogen and sulfur, which are light element candidates contained in the earth's core that composes of iron as the main component, they clarified the mutual effects when both elements dissolve into the earth's core. They showed the possibility that after hydrogen and sulfur preferentially reacted with solid iron in the early stage of earth formation, other light elements became more likely to dissolve in the iron. They improved high-pressure cells and conducted experiments in the pressure-temperature range of 6-7 GPa, 850-1000 K and 10-12 GPa, 750-1200 K. Samples had a composition resembling the primitive material of the chondrite composition, and were arranged so that the presence or absence of sulfur and water could be compared. The crystal structure and the amount of hydrogen were obtained by neutron diffraction. The following four points were clarified regarding the water-mediated hydrogenation reaction of iron that was found this time. (1) Iron sulfide itself is not hydrogenated, but it inhibits the hydrogenation of coexisting iron. (2) Hydrogenation of iron sulfide does not occur until the hydrogenation of iron is completed. (3) Hydrogenation of iron via water is an endothermic reaction, and the amount of dissolution increases with increasing temperature and pressure, but the reaction proceeds very slowly. (4) Under the experimental conditions this time, the amount of hydrogen in iron is lower than that in the previous report under different conditions, regardless of the presence or absence of sulfur. There remain issues such as further studies on higher temperature and higher pressure, and the contribution of other light elements. (A.O.)