Graduation Year

2011

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Jeffrey G. Ryan

Keywords

achondrite, asteroids, meteorite, pallasite, taxonomic A-class, ureilite

Abstract

The apparent rarity of taxonomic A-class asteroids poses a significant paradox for understanding asteroid differentiation and the dynamical evolution of the early solar system. Based on results from asteroid taxonomic surveys, and on the abundances and mineralogy of different achondrite meteorites, it appears that olivine-dominated mantle remnants are missing from both the asteroid population and in meteorite collections. Several scenarios to explain this paradox have been proposed: (1) olivine mantle material has been stripped away by collisions and only remains as small fragments (< ~5 km), (2) A-class asteroids are abundant but have been altered in some way masking their presence, or (3) differentiated asteroids did not form thick olivine-rich mantles.

We have approached these questions through the collection of taxonomic and observational data on known A-class asteroids, and the geochemical characterization of olivine grains from pallasite and ureilite igneous meteorites. Examination of four taxonomic surveys reveals discrepancies in the classification of A-class objects. Recent data with spectral coverage to 2.45 μm have reclassified some asteroids previously thought to belong to the class. Data complied from these taxonomies reveal only 17 A-class asteroids out of ~2100 individual objects surveyed (<1%). Physical and orbital characteristics of A-class asteroids indicate that the majority are small (<13 km) collisional fragments that reside in orbits interior to, or within the inner main-belt.

Photometric observations of five A-class asteroids obtained during this study have constrained the rotational periods of, 246 Asporina, 289 Nenetta, 446 Aeternitas, 1600 Vyssotsky, and the Mars-crossing asteroid 1951 Lick. Robust photometric data for 446 Aeternitas collected over three apparitions yielded a precise rotation period (15.737496 ± 0.000005 h) and a pole orientation of Β = 49º, and λ = 342º. A shape model produced from these data revealed that 446 Aeternitas has a distinctly angular shape suggestive of a collisional fragment.

Olivine compositions between our pallasite meteorites span a narrow range (Fa10.5 - Fa13.4), while the ureilite olivine compositions, generally more fayalitic, display wide variations in the eight examined meteorites (Fa8.5 - Fa22.1). Major and trace element behavior in olivines from pallasite meteorites is consistent with a model of slow, in situ cooling and crystallization, allowing for near-equilibrium exchange between crystallizing olivines and coexisting silicate and FeNi melt, preserving near-uniform olivine major element compositions, and limited trace element variation. Trace element signatures of ureilite silicates (olivine and pigeonite) show large variations, consistent with residual solids from fractional melting processes. Ureilite olivines are uniformly more enriched in both compatible lithophile and siderophile elements (Ca, Li, Sc, V, Cr, Ni, and Mn) than pallasite olivines. corroborating models for ureilite petrogenesis as low-degree partial melting residues in the absence of an FeNi melt phase. Uniformity of elemental signatures among different pallasites point to a chemically homogeneous parent body.

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