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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.

 

It is believed that the core formation processes sequestered a large majority of Earth’s carbon into its metallic core. Incorporation of carbon to liquid iron may significantly influence its properties under physicochemical conditions pertinent to the deep magma ocean and thus the chemical evolution of terrestrial planets and moons. Compared to available experimental data on the physical properties of crystalline iron alloys under pressure, there is a remarkable lack of data on the properties of liquid iron‐rich alloys, due to experimental challenges. Here we review experimental and computational results on the structure and properties of iron or iron‐nickel liquids alloyed with carbon upon compression. These laboratory data provide an important foundation on which the interpretation of ultrahigh pressure laboratory data and the verification of theoretical data will have to be based. The low‐pressure data can be used to validate results from theoretical calculations at the same conditions, and high‐pressure calculations can be used to estimate and predict liquid properties under core conditions. Availability of the liquid properties of Fe‐C liquids will provide essential data for stringent tests of carbon‐rich core composition models for the outer core.