Evidence for the charge disproportionation of iron in extraterrestrial bridgmanite

Extensive iron-water exchange at Earth's core-mantle boundary can explain seismic anomalies

Seismological observations indicate the presence of chemical heterogeneities at the lowermost mantle, just above the core-mantle boundary (CMB), sparking debate over their origins. A plausible explanation for the enigmatic seismic wave velocities observed in ultra-low-velocity zones (ULVZs) is the process of iron enrichment from the core to the silicate mantle. However, traditional models based on diffusion of atoms and penetration of molten iron fail to account for the significant iron enrichment observed in ULVZs. Here, we show that the chemical reaction between silicate bridgmanite and iron under hydrous conditions leads to profound iron enrichment within silicate, a process not seen in anhydrous conditions. Our findings suggest that the interaction between the core and mantle facilitates deep iron enrichment over a few kilometres at the bottom of the mantle when water is present. We propose that the seismic signatures observed in ULVZs indicate whole mantle convection, accompanied by deep water cycles from the crust to the core through Earth's history.

Perovskite-bearing crystal-controlled oxide-silicate mantle xenoliths: Resolution to controversial origins?

Classic lamellar clinopyroxene-ilmenite intergrowths (type 1) are extended to include discovery of olivine-ilmenite-perovskite-wüstite (type 2) and olivine-spinel-perovskite (type 3) xenoliths in kimberlites from Liberia. Low titanium solubilities in olivine, garnet, and pyroxene cannot account for exsolution-like relations. Because the oxides coexist with high-pressure perovskite-structured silicate minerals in diamond, a permissive conclusion is that type 1 to type 3 xenoliths are of super-deep origin. Phase equilibria and thermodynamic studies show that type 1 xenoliths are stable at P > 80 GPa, with type 2 and type 3 at 35 to 50 GPa consistent with an origin in anomalous large low shear velocity province bodies anchored at the core-mantle boundary. Dissociated precursor perovskite-structured Ca-Fe-Ti bridgmanite is proposed and is indirectly supported by the copresence of type II diamonds with a sublithospheric lower mantle origin.

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5 under Pressure

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5.

Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite

Bridgmanite, the most abundant mineral of the Earth's lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide intergrowths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe³⁺/ΣFe ratio (0.69 ± 0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth's lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of ∼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth's formation.

Formation of large low shear velocity provinces through the decomposition of oxidized mantle

Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO₃-10 mol% Fe₂O₃ system, rather than forming a homogeneous single phase. The Fe³⁺-rich bridgmanite has substantially lower velocities and a higher V_P/V_S ratio than MgSiO₃ bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe³⁺-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe³⁺-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.

Six 'Must-Have' Minerals for Life's Emergence: Olivine, Pyrrhotite, Bridgmanite, Serpentine, Fougerite and Mackinawite

Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]2SiO4), bridgmanite ([Mg,Fe]SiO3), serpentine ([Mg,Fe,]2-3Si2O5[OH)]4), and pyrrhotite (Fe(1-x)S), are an essential requirement in planetary bodies to produce such disequilibria and, thereby, life. Yet only two minerals, fougerite ([Fe2+6xFe3+6(x-1)O12H2(7-3x)]2+·[(CO2-)·3H2O]2-) and mackinawite (Fe[Ni]S), are vital-comprising precipitate membranes-as initial "free energy" conductors and converters of such disequilibria, i.e., as the initiators of a CO2-reducing metabolism. The fact that wet and rocky bodies in the solar system much smaller than Earth or Venus do not reach the internal pressure (≥23 GPa) requirements in their mantles sufficient for producing bridgmanite and, therefore, are too reduced to stabilize and emit CO2-the staple of life-may explain the apparent absence or negligible concentrations of that gas on these bodies, and thereby serves as a constraint in the search for extraterrestrial life. The astrobiological challenge then is to search for worlds that (i) are large enough to generate internal pressures such as to produce bridgmanite or (ii) boast electron acceptors, including imported CO2, from extraterrestrial sources in their hydrospheres.

Dehydration of δ-AlOOH in Earth’s Deep Lower Mantle

δ -AlOOH has been shown to be stable at the pressure–temperature conditions of the lower mantle. However, its stability remains uncertain at the conditions expected for the lowermost mantle where temperature is expected to rise quickly with increasing depth. Our laser-heated diamond-anvil cell experiments show that δ -AlOOH undergoes dehydration at ∼2000 K above 90 GPa. This dehydration temperature is lower than geotherm temperatures expected at the bottom ∼700 km of the mantle and suggests that δ -AlOOH in warm slabs would dehydrate in this region. Our experiments also show that the released H 2 O from dehydration of δ -AlOOH can react with metallic iron, forming iron oxide, iron hydroxide, and possibly iron hydride. Our observations suggest that H 2 O from the dehydration of subducting slabs, if it occurs, could alter the chemical composition of the surrounding mantle and core regions.