MINERALOGY AND PETROLOGY OF DARK CLASTS IN POLYMICT EUCRITES

Examining the properties of HED meteorites aids the study of geologic processes on the asteroid 4 Vesta. In particular, fifteen thin sections from the ALH 76005 polymict eucrite pairing group contain dark clasts that contextualize Vestan impact processes. This project characterizes the dark clasts’ representative textures, mineralogies, and chemistries in order to address their mechanism of formation and its relationship with Vestan lithologies and impact events. Observational methods include preliminary textural and mineral identification with a petrographic microscope, qualitative chemical identification with a conventional SEM and a FEG-SEM (field emission gun), and quantitative chemical analysis with an electron microprobe.

Plagioclase, clinopyroxene, and silica are the major mineral phases within the dark clasts, while ilmenite, chromite, and troilite are minor phases. Dark clast groundmass textures of quenched plagioclase and clinopyroxene indicate rapid melting and cooling upon formation, and the presence of large skeletal clinopyroxene crystals in some dark clasts illustrates textural similarity to the NWA 1240 eucrite impact melt (Barrat et al., 2003). Plots of weight percent oxides, such as TiO₂ and FeO, versus weight percent MgO show that the dark clasts are eucritic in composition, as they are quite chemically similar to the Main Group, Stannern trend, and Nuevo Laredo trend eucrites (Mittlefehldt et al., 2015). These textural, mineralogical, and chemical observations reveal the dark clasts as endogenous to Vesta as opposed to from an exogenous impactor. Our current hypothesis asserts the dark clasts are eucritic impact melts and were darkened by mobilization of troilite upon impact. Potential source regions for the dark clasts are the ejecta of the Oppia and Octavia craters, as these have been interpreted to be composed of impact melts and are in eucrite-rich regions of Vesta (Le Corre et al., 2013; Peplowski et al., 2013). In addition, the presence of diogenite-rich material in the south pole’s Rheasilvia basin implies the previous existence of overlying eucritic material that was ejected during the Rheasilvia impact event. Eucritic meteorites or Vestoids derived from this event may include impact melt lithologies and therefore source material for polymict eucrites and the dark clasts.