On the origin of metallic iron in eucrite breccias: Effects of impact shock and mixing on the surface of (4) Vesta

We performed scanning electron microscopy and electron microprobe analysis on the eucritic meteorites Errachidia 002 and Northwest Africa 11911, with a focus on the metallic iron disseminated in them, in order to constrain the processes involved in its formation, and discuss its implication for their parent body collisional history. Errachidia 002 is a polymict eucrite that consists of basaltic and noritic (diogenite) clasts set in a fragmental matrix. Fe-metals in this meteorite contain <0.04 wt% Ni, and 0.06 wt% Co, and they are mostly associated with silica in pyroxene. This suggests formation by decomposition of troilite, which generated S -vapors (S2) that subsequently reacted with FeO in pyroxene during an impact event, producing pure Fe-metal and silica, with escape of SO2 from the rock. The presence of late-stage phases in proximity of a few Fe-metal grains, as well as low Fe/Mn coupled with high Fe/Mg in pyroxene indicate a different scenario in which pure Fe-metal in this meteorite formed by reduction from a residual melt. NWA 11911 is also a polymict eucrite composed mainly of unequilibrated and equilibrated eucritic clasts set in a clastic groundmass. Here, metallic iron is either enclosed in the fragmental matrix, or within a vitrophyric impact melt clast. Its Ni and Co contents are similar to 2.8 and 0.6 wt%, implying an exogenous origin, namely contamination by an iron-bearing impactor. Our results point out the major role impacts played in controlling the surface features and mineral compositions of early formed bodies in the solar system such as (4) Vesta.