Disproportionation of Iron in Almandine-Pyrope-Grossular Garnet From 25 to 65 GPa

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 FeAlO₃ content, 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.