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1. Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

   https://doi.org/10.3390/min11050512  

   Dorfman, Susannah M.; Nabiei, Farhang; Boukaré, Charles-Edouard; Prakapenka, Vitali B.; Cantoni, Marco; Badro, James; Gillet, Philippe (May 2021, Minerals)

   Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures. 

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2. Evolution of Mercury’s Earliest Atmosphere

   https://doi.org/10.3847/PSJ/ac2dfb  

   Jäggi, Noah; Gamborino, Diana; Bower, Dan J.; Sossi, Paolo A.; Wolf, Aaron S.; Oza, Apurva V.; Vorburger, Audrey; Galli, André; Wurz, Peter (November 2021, The Planetary Science Journal)

   Abstract MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing of C- and H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, and photoionization. To quantify atmospheric loss, we combine constraints on the lifetime of surficial melt, melt composition, and atmospheric composition. Consideration of two initial Mercury sizes and four magma ocean compositions determines the atmospheric speciation at a given surface temperature. A coupled interior–atmosphere model determines the cooling rate and therefore the lifetime of surficial melt. Combining the melt lifetime and escape flux calculations provides estimates for the total mass loss from early Mercury. Loss rates by Jeans escape are negligible. Plasma heating and photoionization are limited by homopause diffusion rates of ∼10⁶kg s⁻¹. Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from ∼10^6.6to ∼10^9.6kg s⁻¹. The material for photoevaporation is sourced from below the homopause and is therefore energy limited rather than diffusion limited. The timescale for efficient interior–atmosphere chemical exchange is less than 10,000 yr. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust (0.3% of Mercury’s mass). Accordingly, ≤0.02% of the total mass of H₂O and Na is lost. Therefore, cumulative loss cannot significantly modify Mercury’s bulk mantle composition during the magma ocean stage. Mercury’s high core:mantle ratio and volatile-rich surface may instead reflect chemical variations in its building blocks resulting from its solar-proximal accretion environment. 

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3. The Lithophile Element Budget of Earth's Core

   https://doi.org/10.1029/2021GC009986  

   Chidester, B. A.; Lock, S. J.; Swadba, K. E.; Rahman, Z.; Righter, K.; Campbell, A. J. (February 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The relative composition of Earth's core and mantle were set during core formation. By determining how elements partition between metal and silicate at high pressures and temperatures, measurements of the mantle composition and geophysical observations of the core can be used to understand the mechanisms by which Earth formed. Here we present the results of metal‐silicate partitioning experiments for a range of nominally lithophile elements (Al, Ca, K, Mg, O, Si, Th, and U) and S to 85 GPa and up to 5400 K. With our results and a compilation of literature data, we developed a parameterization for partitioning that accounts for compositional dependencies in both the metal and silicate phases. Using this parameterization in a range of planetary growth models, we find that, in general, lithophile element partitioning into the metallic phase is enhanced at high temperatures. The relative abundances of FeO, SiO₂, and MgO in the mantle vary significantly between planetary growth models, and the mantle abundances of these elements can be used to provide important constraints on Earth's accretion. To match Earth's core mass and mantle composition, Earth's building blocks must have been enriched in Fe and depleted in Si compared with CI chondrites. Finally, too little Mg, Si, and O are partitioned into the core for precipitation of oxides to be a major source of energy for the geodynamo. In contrast, several ppb of U can be partitioned into the core at high temperatures, and this energy source must be accounted for in thermal evolution models. 

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4. Solubility of Rock in Steam Atmospheres of Planets

   https://doi.org/10.3847/0004-637X/824/2/103  

   Fegley, Bruce; Jacobson, Nathan S.; Williams, K. B.; Plane, J. M.; Schaefer, L.; Lodders, Katharina (June 2016, Astrophysical journal)

   Extensive experimental studies show that all major rock-forming elements (e.g., Si, Mg, Fe, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky-element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases (e.g., Si(OH)4, Mg(OH)2, Fe(OH)2, Ni(OH)2, Al(OH)3, Ca(OH)2, NaOH, KOH) and via reaction with HF and HCl as volatile halide gases (e.g., NaCl, KCl, CaFOH, CaClOH, FAl(OH)2) in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at high temperatures expected for steam atmospheres on the early Earth and hot rocky exoplanets. We quantitatively compute the extent of fractional vaporization by defining gas/magma distribution coefficients and show that Earth's subsolar Si/Mg ratio may be due to loss of a primordial steam atmosphere. We conclude that hot rocky exoplanets that are undergoing or have undergone escape of steam-bearing atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Al, Ca, Na, and K. This loss can modify their bulk composition, density, heat balance, and interior structure. 

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5. Investigating the Effects of Extended Equilibrium Crystallization on a Cooling Lunar Magma Ocean

   Cone, Kim A; Elardo, Stephen M; Spera, Frank J; Bohrson, Wendy A; Trail, Dustin; Nakajima, Miki; Palin, Richard (December 2024, American Geophysical Union)

   We investigate the implications of prolonging the equilibrium crystallization (EQX) stage of lunar magma ocean (LMO) solidification beyond the oft-modeled 50% volume solids, to 60%. Most models of two-stage LMO crystallization halt the EQX phase once 50% of a molten Moon (post-core formation) solidifies, after which the remaining 50% of the LMO solidifies via fractional crystallization (FRX). We quantitatively show through a simple scaling analysis that compares crystal settling velocity to vertical convective velocity that the early EQX regime can operate up to (and possible even slightly beyond) 60% volume solids. Phases that stabilize during the EQX and FRX regimes are then computed using Perple_X (thermodynamic calculator) along with the hp633ver database and associated activity-composition relations for solid solutions, and consider an adiabat that remains between the liquidus and solidus. Early results show two key findings: 1) only low volumes (~2%) of ilmenite form over ~50-km thick upper mantle layers for both 50% and 60% EQX regimes, suggesting that a mantle overturn may have been sluggish and/or limited in depth (dense ilmenite is thought to have been a critical driver of late-stage mantle mixing); and 2) contrary to most published two-stage LMO models, a refractory-enriched (i.e. high Al2O3) bulk silicate Moon is not required to produce garnet in the lunar mantle, assuming an Earth-like bulk silicate Moon composition with an alumina content of ~4 wt.%. To complement and test these numerical phase equilibria model results, a series of piston-cylinder experiments is underway that simulate the pressures and temperatures experienced by an FeO+TiO2-rich residual LMO in order to assess the volume and distribution of ilmenite produced during LMO solidification. These results are compared to those of the numerical phase equilibria models. Despite the model-dependent nature of these results, they provide a unique insight into potential LMO crystallization that has not been previously considered in the literature. 

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