PETROGRAPHIC AND SPECTROSCOPIC ANALYSES OF TEXTURES IN METEORITE NWA 5480 FROM 4 VESTA

Vesta, a differentiated asteroid, is the source of HED (howardite, eucrite, and diogenite) meteorites. NWA 5480 is a diogenite thought to originate from the 19 km-deep crater Rheasilvia and is unique because it lacks brecciated textures and has schlieren of olivine- and orthopyroxene-dominated zones. The olivine-dominated zone contains granular aggregates of larger olivine, whereas the orthopyroxene-dominated zone shows poikilitic orthopyroxene crystals containing small olivine grains. Previous studies hypothesize the schlieren texture could originate from deformation within Vesta’s mantle or from primary igneous or shock-induced melting processes. The lattice preferred orientations (LPO) of small poikilitic olivine as determined by EBSD analysis differs from the LPO of larger olivine crystals (Tkalcec 2014), but petrological studies concluded the schlieren texture are flow textures from impact melting (Yamaguchi 2015) or are igneous with re-equilibration textures (Peslier 2015). In this study, two samples of NWA 5480 were analyzed: a thin section with a cover slip from the JMU collection and a thick section created from a separate slab. Grain size distribution analysis was done on a Nikon Eclipse LV100 POL petrographic microscope. An Ocean Optics IDRaman with 532 nm laser was used for mineral identification and a Nicolet iS50 FTIR spectrometer was used to evaluate water in minerals. A Phenom XL desktop SEM was used to collect EDS data using a 15kV beam. NWA 5480 contains olivine and orthopyroxene with minor interstitial Fe metal, Fe sulfides, chromite and Fe oxide. Raman spectra confirm the presence of forsterite and enstatite. Transmission FTIR analysis indicates no significant water present in olivine or orthopyroxene. Rounded olivines reside within the poikilitic texture, a significant number of which are conjoined. Olivines range in grain size from 10-60μm with an average grain size of 40μm. The olivine grain size distribution suggests an estimated differential stress of 70-275 MPa. However, textures observed in our samples are consistent with melting or crystallization processes associated with impact. Previously reported olivine LPO could be explained by differential stress from rebound after impact.