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Although high pressure enables alloying between hydrogen and iron, hydrogen‐to‐iron molar ratio (H/Fe) so far found in experiments is mostly limited to 1 in the close‐packed iron metal under high pressure. We report a H/(Fe + Ni) ratio of 1.8 ± 0.1 from (Fe,Ni)H_x(orx ≥ 1.8) quenched from liquid, exceeding the amounts so far reported for densely packed Fe alloys. From the metastable behavior of the frozen (Fe,Ni)H_xliquid during decompression, we infer that the amount is a lower bound and therefore even a greater amount of H can be dissolved in the liquid part of Fe‐rich cores of planets. The significant H storage capacity of liquid Fe‐Ni alloy is important to consider for potential storage of H in the interiors of low‐density planets as well as rocky planets.

 

Nitrogen is considered to be transported from Earth′s surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth′s interior during slab‐mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser‐heated diamond anvil cell experiments and first‐principles calculations. Our results showed that subducted nitrogen‐bearing silicates and fluids could not coexist with the metallic iron or iron‐rich alloys, and reacted with them to form different products at high pressure‐temperature conditions. Combining our results with previous data, we re‐determined the relative stability of iron‐light element binary compounds to 35 GPa and 1600 K to be Fe‐O > Fe‐N > Fe‐S > Fe‐C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur‐depleted lower‐mantle diamonds and are absent in sulfur‐rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earth′s surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods.

 

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 ∼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.

 

The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe₉₀Ni₁₀‐3 wt.% C and Fe₉₀Ni₁₀‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe₉₀Ni₁₀liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (v_p) of Fe‐Ni‐C liquids are higher than that of pure Fe‐Ni alloy, promoting carbon as a possible candidate to explain the elevatedv_pin the Earth's outer core. However, the values and slopes of both density andv_pof the studied two Fe‐Ni‐C liquids do not match the outer core seismic models, suggesting that carbon may not be the sole principal light element in Earth's outer core. The highv_pof Fe‐Ni‐C liquids does not match the presumptivev_pof the lunar outer core well, indicating that carbon is less likely to be its dominant light element.

 

Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg_0.5Fe³⁺_0.5)(Al_0.5Si_0.5)O₃bridgmanite (FA50) with the highest Fe³⁺‐Al³⁺coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO₃structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yieldsV₀ = 172.1(4) ų,K₀ = 229(4) GPa withK₀′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO₃bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe³⁺‐Al³⁺substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe³⁺cations exchanged with Al³⁺and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe³⁺increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe³⁺‐Al³⁺‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe³⁺fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.

 

Making consistent and precise octahedral pressure media is crucial for reproducible high-pressure experiments in the multi-anvil press. Here we report a new approach of casting octahedra using 3D-printed molds, and pressure calibrations for octahedra both with and without pre-existing gaskets (“fins”). The 3D-printed molds for casting octahedra from either Ceramacast 584-OF or 646 cement improve the reproducibility of the octahedra and allow for a pre-existing central hole (for the high-pressure cell assembly) in the final cast product. Pressure and temperature calibrations of the octahedra have been performed based on phase transitions in bismuth (Bi) and silica (SiO2), respectively, in order to determine the efficiency and reproducibility of pressure generation and thermal insulation for cast octahedra designed for use with 18/12, 14/8, and 10/5 multi-anvil assemblies. The pressure-generating efficiency of the 14/8 and 10/5 octahedra with pre-existing gaskets, cast from the 584-OF cement, is similar to that of the corresponding COMPRES (Consortium for Materials Properties Research in Earth Sciences) octahedra, and more efficient than pre-cast octahedra made from the same material but lacking pre-existing gaskets. The efficiency of pre-gasketed 18/12 octahedra made of the 646 cement is markedly lower than those of the 584 cement. However, the 18/12 large-volume octahedra, cast (with fins) from the ZrO2-based 646 cement, also provides efficient thermal insulation. Casting octahedral solid pressure media for multi-anvil experiments using 3D-printed “injection” molds is a low-cost and low failure-rate alternative for conducting reproducible experiments at high pressure in the multi-anvil apparatus.