First Advisor

Alexander Ruzicka

Term of Graduation

Spring 2025

Date of Publication

1-22-2026

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Crystal Flow Alignment, Deformation, Lattice Preferred Orientation, Petrofabrics, Shape Preferred Orientation, Ureilites

Physical Description

1 online resource (xix, 170 pages)

Abstract

This thesis examines the origin and development of crystallographic fabrics in ureilites, which are primitive achondritic meteorites interpreted as residues of partial melting from a differentiated parent body. In this study, we evaluate three fabric-forming mechanisms: gravitational cumulate crystallization, shock-induced deformation, and flow alignment associated with melt extraction and convective stirring. A quantitative Electron Backscatter Diffraction (EBSD) analysis was conducted on six ureilitic meteorites to assess olivine Lattice Preferred Orientation (LPO), Shape Preferred Orientation (SPO), the connection between SPO and LPO, and other deformation metrics, including Grain Orientation Spread (GOS), and Crystal Rotation Axis (CRA). Our results suggest that most ureilites exhibit low to moderate deformation in olivine, equivalent to shock stages C-S2 to C-S3 for chondrites ("weakly deformed"), as indicated by low to moderate GOS values (mean GOSd50-100: 2.68, 2.66, 1.44, 2.46, 2.73, 2.24, 1.09, 0.9), where d is equivalent grain diameter in microns. These low to moderately shocked ureilites show weighted shock stages mostly within WSS 1-3, which is consistent with the observed C-S2 to C-S3 shock stages. However, one ureilite (Northwest Africa, NWA 7304) was largely recrystallized and fits the definition of shock stage S6, signifying a "strongly shocked ureilite." The weakly shocked ureilites display dominant < 001> LPO lineations and weak < 100> and < 010> LPO foliations, with long grain axes (SPO) subparallel to olivine < 001> directions. In contrast, the strongly deformed ureilite shows strong < 100> LPO lineations that are subparallel to textural banding, consisting of coarse and fine grain aggregates, with a-axis SPO of recrystallized grains roughly perpendicular to the textural banding and to < 010> LPO. The presence of subgrain boundaries and clustering in the Crystal Rotation Axis (CRA) diagrams indicates intracrystalline deformation via dislocation glide and climb, with no evidence of post-deformation annealing and no dynamic recrystallization, except in the strongly shocked ureilite (NWA 7304), which was extensively recrystallized into coarse and fine bands. This dislocation creep suggests both dislocation glide, implied by subgrain boundaries, and dislocation climb, involving movement to a different but parallel slip plane. The deformation signatures observed in the weakly shocked ureilites are not consistent with a cumulate origin, which would typically produce LPO lineation in olivine short axes (< 010>), a pattern not observed. Additionally, the fabrics cannot be attributed to shock-induced deformation in the weakly shocked samples, as their shock stages are low, and CRA diagrams for olivine indicate both a-type and c-type dislocation slip were active, pointing to high-temperature deformation, which typically would not result in a dominant < 001> LPO. However, the strongly shocked ureilite (NWA 7304) may have developed its LPO through intense shock deformation. Overall, the observed fabric data in weakly shocked ureilites are best explained by flow alignment of olivine crystals under magmatic conditions. This high-temperature flow process, associated with melt extraction and convective stirring, is strongly evidenced in the weakly shocked ureilites. These ureilites likely experienced a catastrophic collisional disruption near the solidus, followed by rapid cooling. Evidence for rapid cooling from near the solidus includes the presence of minor feldspathic glass and narrow FeO reduction rims, which are mainly observed in the weakly deformed ureilites. Additionally, the strongly shocked ureilite (NWA 7304) may have originated from a later or secondary shock event or from a location near the point of impact during the catastrophic disruption. This interpretation best explains the CRA data and the < 001> LPO, which aligns with predictions for a dominant a-type slip system.

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Persistent Identifier

https://archives.pdx.edu/ds/psu/44445

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