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

摘要

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., 3Fe(2+) = Fe-0 + 2Fe(3+)) 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 degrees 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.