High-pressure and high-temperature experiments were conducted to determine the stability field of phase Egg, AlSiO 3 OH, in the pressure range of 16.5-23.5 GPa and in the temperature range of 800-1500°C. We found that phase Egg decomposes to δ-AlOOH + stishovite below~1200°C (16.5 GPa and 800°C; 20.6 GPa and 1000°C) and to Al-phase D + corundum + stishovite above~1200°C (21.8 GPa and 1500°C) at P-T conditions corresponding to the mantle transition zone.This indicates that phase Egg is unstable at the top of the lower mantle and can be a water reservoir only in the mantle transition zone. In addition, the present results imply that the superdeep diamonds that include phase Egg do not originate from the lower mantle but from the wet mantle transition zone.

Nitrogen is a crucial volatile element in the early Earth’s evolution and the origin of life. Despite its importance, nitrogen’s behavior in the Earth's interior remains poorly understood. Compared to other volatile elements, nitrogen is depleted in the Earth’s atmosphere (the so-called “missing nitrogen”), calling for a hidden deep reservoir. To investigate nitrogen’s behavior in the deep Earth including how the reservoir formed, high-pressure and high-temperature experiments were conducted at 28 GPa and 1,400–1,700 °C. To reproduce the conditions in the lower mantle, the redox was controlled using a Fe–FeO buffer. We observed that depending on the temperature conditions, stishovite can incorporate up to 90–404 ppm nitrogen, experimentally demonstrating that stishovite has the highest nitrogen solubility among the deep mantle minerals. Stishovite is the main mineral component of subducted nitrogen-rich sedimentary rocks and eroded continental crust that are eventually transported down to the lower mantle. Our results suggest that nitrogen could have been continuously transported into the lower mantle via subduction, ever since plate tectonics began.

Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D2O) as Mg(OD)2 in the ideal composition (Fe–MgSiO3–D2O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)2, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeDx) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x <  ~ 0.4). Both H and S can be incorporated into solid Fe and other light elements could have dissolved into molten iron hydride and/or FeS during the later process of Earth’s evolution.

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