James Badro

Geophysical observations from the InSight mission suggest anomalously low seismic velocities near the Martian core–mantle boundary, possibly indicating a molten silicate layer. Such a layer would have major implications for the thermal and chemical evolution of Mars, but its origin remains poorly constrained because experimental data on iron-rich Martian melts at deep-mantle conditions remain scarce. Here we use in situ ultrasonic interferometry combined with X-ray imaging and diffraction to determine the melting phase relations, ultrasonic response and chemical compositions of co-existing melts and solids under conditions relevant to the Martian core–mantle boundary. Our experiments constrain a Martian mantle solidus about 100 K lower than previous estimates. We show that incipient melting rapidly weakens the elastic response, reducing wave speeds and modifying the ultrasonic signal character. Melts generated slightly above the solidus at 18–19 GPa, equivalent to a depth of approximately 1,550 km, are strongly enriched in iron (Fe# of 0.5) and reach densities comparable to or greater than surrounding solids, implying neutral to negative buoyancy. The combination of high melt density and low wetting angles favours the formation of an interconnected dense melt layer near the core–mantle boundary from a moderately FeO-rich Martian mantle without necessarily requiring overturn of an extremely Fe-enriched primordial reservoir and remains compatible with proposed thermal models.

Constraining the complex dynamics of the inner Earth unites research efforts across several scientific disciplines, including geochemistry, geophysics and geodynamics. Seismological and geodynamic studies offer insights into the present state of the mantle structure, whereas geochemical approaches characterize its chemical and isotopic heterogeneities ¹ , shedding light on the complexity of its evolution. One key challenge is determining the age and origin of its chemical heterogeneities. Here we present new high-precision Nd isotopic measurements in present-day volcanism that identify heterogeneities dating back to the Earth’s earliest history. We report significantly positive ¹⁴² Nd anomalies in lavas from the submarine Fani Maoré volcano in the Comoros archipelago. These anomalies require the preservation, in the mantle, of material depleted in light rare-earth elements (REE) and formed within the first 100 million years (Myr) of Earth’s history. We suggest that this material is mainly composed of bridgmanite that crystallized from an early Earth magma ocean. This Hadean bridgmanite may be more widespread in the present-day mantle than previously expected, raising new questions about its survival over billions of years of plate tectonics and vigorous mantle convection.

Geophysical observations from the InSight mission support low seismic wave velocity at the Martian core–mantle boundary (CMB) likely caused by the presence of a molten (or partially molten) layer. Such a layer would have had a key influence on the planet’s thermal evolution. However the relationship between its physical state and its seismic signature remain poorly constrained because experimental data on Fe-rich Martian melts at deep-mantle conditions are lacking. Here we determined the melting phase relations, seismic properties and densities of Fe-rich Martian mantle analogues at CMB pressures and temperatures. Using in situ ultrasonic interferometry combined with X-ray imaging and diffraction, we track phase transitions during progressive melting and directly determine seismic velocities in partially molten assemblages, while tightly constraining a Martian mantle solidus hundred kelvin lower than previously estimated. We find that small melt fractions (3–10 vol%) produce substantial seismic velocity reductions, with decreases of up to ~30% in shear wave speeds and ~15% in compressional wave speeds. Melts generated at pressures ≥14–18 GPa become strongly enriched in iron (Fe# > 0.45), yielding melt densities comparable to or greater than those of surrounding solids and promoting neutral to negative buoyancy. These results link Martian seismic observations to experimentally determined melt properties and support the existence of a long-lived, compositionally dense magma layer at the base of the Martian mantle.

The most abundant type of planet discovered in the Galaxy has no analogue in our Solar System and is believed to consist of a rocky interior with an overlying thick H₂ dominated envelope. Models have predicted that the reaction between the atmospheric hydrogen and the underlying magma ocean can lead to the production of significant amounts of water. The models suffer however from the current lack of experimental data on the reaction between hydrogen and silicate melt at high pressures and temperatures. Here we present novel experimental results designed to investigate this interaction. Laser heating diamond anvil cell experiments were conducted between 16 and 60 GPa at temperatures above 4000 K. We find that copious amounts of hydrogen dissolve into the silicate melt with a large dependence on temperature rather than pressure. We also find that the reduction of iron oxide leads to the production of significant amounts of water along with the formation of iron-enriched blebs. Altogether, the results predict that the typical processes attending planet formation will result in significant water production with repercussions for the chemistry and structure of the planetary interior as well as the atmosphere.

One of the main interpretations of deep-rooted geophysical structures in the mantle¹ is that they stem from the top-down solidification of the primitive basal magma ocean of Earth above the core^2–6. However, it remains debated whether solids first formed at the bottom of the mantle, solidifying upward, or above the melts, solidifying downward. Here we show that gravitational segregation of dense, iron-rich melts from lighter, iron-poor solids drives mantle evolution, regardless of where melting curves and geotherms intersect. This process results in the accumulation of iron-oxide-rich melts above the core, forming a basal magma ocean. We numerically model mantle solidification using a new multiphase fluid dynamics approach that integrates melting phase relations and geochemical models. This enables estimating the compositional signature and spatial distribution of primordial geochemical reservoirs, which may be directly linked to the isotopic anomalies measured in Archean rocks^7–11. We find that a substantial amount of solids is produced at the surface of the planet, not at depth, injecting geochemical signatures of shallow silicate fractionation in the deep mantle. This work could serve as a foundation for re-examining the intricate interplay between mantle dynamics, petrology and geochemistry during the first thousand million years of the evolution of rocky planets.

(Mg,Fe)O ferropericlase‐magnesiowüstite has been proposed to host the majority of Earth's sodium, but the mechanism and capacity for incorporating the alkali cation remain unclear. In this work, experiments in the laser‐heated diamond anvil cell and first‐principles calculations determine the solubility of sodium and favorability of sodium incorporation in iron‐rich magnesiowüstite relative to (Mg,Fe)SiO₃ bridgmanite. Reaction of Mg/(Mg + Fe) (Mg#) 55 and 28 olivine with NaCl at 33–128 GPa and 1600–3000 K produces iron‐rich magnesiowüstite containing several percent sodium, while iron‐rich bridgmanite contains little to no detectable sodium. In sodium‐saturated magnesiowüstite, sodium number [Na/(Na + Mg + Fe)] is 2–5 atomic percent at pressures below 60 GPa and drastically increases to 10–20 atomic percent at deep lower mantle pressures. For these two compositions, there is no significant dependence of the results on Mg#. Our calculations not only show consistent results with experiments but further indicate that such an increase in solubility and partitioning of Na into magnesiowüstite is driven by the spin transition in iron. These results provide fundamental constraints on the crystal chemistry of sodium at lower‐mantle conditions. If the sodium capacity of (Mg,Fe)O is not strongly dependent on Mg#, (Mg,Fe)O in the lower mantle may have the capacity to store the entire sodium budget of the Earth.