Evidence for the Disproportionation of Iron in a Eucrite Meteorite: Implications for Impact Processes on Vesta.

Pure metallic iron is a common component in extraterrestrial samples (e.g., ordinary chondrite, returned lunar samples), and is thought to be an indicator of extremely reducing conditions. However, tiny pure metallic iron (<1 mm) particles are rarely found in Howardite–Eucrite–Diogenite meteorites, and their formation mechanism on Vesta is poorly understood. Sub‐micron sized pure metallic iron particles were identified by transmission electron microscope studies of a melt pocket in the basaltic eucrite meteorite Northwest Africa 11592. Our results demonstrate that the pure metallic iron likely formed through a dissociation reaction of ferrous iron (i.e., 3Fe2+ = Fe0 + 2Fe3+) in pyroxene. Another disproportionation reaction product, ferric iron, was confirmed to be incorporated into Al‐rich clinopyroxene in the melt pocket by electron energy loss spectra analyses. The temperature conditions required for the formation of the melt pocket are estimated to be above 1,310°C, as indicated by the crystallization of nano‐sized hercynite within acicular plagioclase. Such high temperatures on Vesta could only have been reached if an impact occurred during peak thermal metamorphism, early in the asteroid's evolution. This formation mechanism of these pure metallic iron particles could play a role in redox environment of the Vesta. Plain Language Summary: Vesta, the second‐largest asteroid in the main belt and one of the most primitive solar system bodies, is among the few extraterrestrial objects visited by an orbital spacecraft. The Howardite–Eucrite–Diogenite (HED) clan of meteorites has been associated with Vesta for decades, and the Dawn mission provided strong evidence for their genetic relationship. Pure metallic iron (Fe0) particles, an indicator of reducing conditions, are rarely found in HED meteorites, and their origins are not clear. In basaltic eucrite Northwest Africa 11592, abundant sub‐micron sized Fe0 particles embedded in a shock‐induced melt pocket provide new insight into their formation. The presence of Fe3+‐bearing Al‐rich clinopyroxene and nano‐sized hercynite minerals in the melt pocket was also confirmed, which indicates that the Fe0 formed through the dissociation reaction of Fe2+ (i.e., 3Fe2+ = Fe0 + 2Fe3+) present in pyroxene at a temperature above 1,310°C. This formation mechanism provides a possibility that Fe0 particles may have formed as a result of cratering processes, which would have affected the redox conditions on Vesta. Key Points: Evidence for the dissociation reaction of Fe2+, which formed pure metallic iron particles in eucrite meteorite Northwest Africa 11592, was foundAl‐rich clinopyroxene is another carrier of Fe3+ in shocked meteorites, in addition to the high‐pressure phases ringwoodite and bridgmaniteThe disproportionation reaction would occur during cratering on Vesta and may have influenced the redox environment [ABSTRACT FROM AUTHOR]