Search for: All records

The production of metal via the iron disproportionation reaction in the deep Earth has been a long debated topic with important implications for the geochemistry of the lower mantle. To explore the occurrence of the iron disproportionation reaction from 25 to 65 GPa, a natural almandine‐pyrope‐grossular garnet was studied with in situ X‐ray diffraction (XRD) in the laser‐heated diamond anvil cell and ex situ scanning electron microscopy (SEM) techniques. Upon heating the natural almandine‐pyrope‐grossular garnet up to 3000 K up to 65 GPa, the formation of phase assemblage consisting of bridgmanite, stishovite, and davemaoite was confirmed by XRD, but because of the low abundance of Fe metal and small grain size, XRD was determined not to be effective in detecting the disproportionation reaction. Examination of the samples recovered between 39 and 64 GPa by SEM analysis revealed the presence of nm‐scale disproportionated iron metal grains as an additional product of this reaction that was not detectable in the XRD patterns. Volume compression data of bridgmanite synthesized in the experiments were fit to the Birch‐Murnaghan equation of state and compared to similar compositions. Bridgmanite was found to decompress to the LiNbO₃‐type structure, indicating a high FeAlO₃content, in accordance with the occurrence of a disproportionation reaction. The experimental confirmation of disproportionated metallic Fe has significant implications for the distribution of siderophile and volatile elements in the lower mantle.

Ab initio molecular dynamics calculations on a carbonate‐silicate‐metal melt were performed to study speciation and coordination changes as a function of pressure and temperature. We examine in detail the bond abundances of specific element pairs and the distribution of coordination environments over conditions spanning Earth’s present‐day mantle. Average coordination numbers increase continuously from 4 to 8 for Fe and Mg, from 4 to 6 for Si, and from 2 to 4 for C from 1 to 148 GPa (4,000 K). Speciation across all pressure and temperature conditions is complex due to the unusual bonding of carbon. With the increasing pressure, C‐C and C‐Fe bonding increase significantly, resulting in the formation of carbon polymers, C‐Fe clusters, and the loss of carbonate groups. The increased bonding of carbon with elements other than oxygen indicates that carbon begins to replace oxygen as an anion in the melt network. We evaluate our results in the context of diamond formation and of metal‐silicate partitioning behavior of carbon. Our work has implications for properties of carbon and metal‐bearing silicate melts, such as viscosity, electrical conductivity, and reactivity with surrounding phases.

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.