Northwest Africa 6232: Visible-near infrared reflectance spectra variability of an olivine diogenite

Visible and near-infrared (VNIR) reflectance is an important spectroscopic technique to identify minerals, and their associations, on planetary body surfaces. Howardites, eucrites, and diogenites (HED) are a class of igneous-like meteorites whose genetic connection with asteroid 4 Vesta has since long been inferred and recently confirmed by Dawn mission results. Pyroxene and olivine are the two major mafic minerals present in HED which can be identified with VNIR reflectance measurements. Thus, studying the compositional variability of those phases and their mixtures by means of laboratory spectroscopic measurements on different diogenitic or eucritic samples is one of the prime methods to better understand the evolution of 4 Vesta's crust. Here, we report the VNIR reflectance spectral analysis of a harzburgitic olivine diogenite, Northwest Africa 6232 (probably paired with Northwest Africa 5480), containing variable amounts of olivine as small grains or aggregates. We found that the olivine diogenite spectral parameters (e.g., band position) of powdered samples and polished slabs are in agreement. Moreover, the olivine diogenite band position shifts from synthetic orthopyroxene in accordance with the presence of olivine and chromite. In particular, the presence of a large olivine clast permits us to determine a linear variation of the band position from synthetic orthopyroxene and olivine, but underestimates the presence of olivine in the olivine diogenite spot

Investigating the Petrogenesis of the Basaltic Crust of Asteroid 4 Vesta: A Combined Petrologic-Spectral Study of the Unbrecciated Eucrites

The Dawn mission, which launched in September 2007, will orbit and analyze two of the largest asteroids, 4 Vesta and 1 Ceres. These small, proto-planetary bodies retain a record of the conditions and processes that occurred in the early history of the Solar System, and, as such, provide us with a tool to understand its formation and evolution. Additional information can also be gleaned from the meteorite samples derived from such objects. This dissertation aims to increase understanding of the formation of the basaltic crust of the asteroid Vesta, and the lithologic variety that exists within it, by studying the eucrite meteorites, believed to originate from it. In order for us to be able to identify different units on the surface of Vesta, we must first understand the relationship that exists between the petrology of the samples and their spectral characteristics, which will be measured by the VIR (Visible Infrared) instrument aboard Dawn. The first part of this dissertation details an in-depth study into the petrology of the unbrecciated eucrites, focusing on those characteristics that impact the spectra, e.g. mineral chemistries and modal abundances. Select well-characterized unbrecciated eucrites from the petrologic study were then chosen for spectral work. Visible near-infrared (0.3 – 2.5 μm) spectra were collected and the amount of petrologic information that could be extracted from the spectra alone was assessed. We were able to distinguish the spectra of basaltic versus cumulate eucrites, as well as those samples that underwent slow- versus fast cooling and early- versus late-crystallization. This indicates that we may be able to map petrologic units using Dawn and determine the processes that occurred to form them. Dawn is scheduled to reach Vesta in 2011; however, the Vestoids, a group of asteroids believed to be from Vesta, offer us the opportunity to utilize the combined petrologic-spectral data set now. The spectra of 15 Vestoids were examined and compared to spectra for the howardite, eucrite, and diogenite meteorites, of which Vesta is believed to be the parent body. The Vestoids indicate that both large-scale (\u3e7 km) homogeneous and small-scale (\u3c1 \u3ekm) heterogeneous units exist on the surface of Vesta