Date of Degree
Earth & Environmental Sciences
Michael K. Weisberg
Denton S. Ebel
Harold C. Connolly, Jr.
Cosmochemistry | The Sun and the Solar System
chondrites, chondrules, meteorites, fine-grained rims, accretion, microchondrules
Fine-grained rims are ubiquitous, non-igneous, features that completely or partially envelope the majority of chondrules within the least equilibrated of the unequilibrated ordinary chondrites (UOCs). A detailed examination of such rims in 4 UOC samples less than petrologic type 3.2 was conducted in order to 1) characterize the relative distribution of rims within chondrite samples, 2) inspect differences between fine-grained rims and adjacent matrix material, 3) petrologically analyze the rims and their relationships with chondrule cores, 4) characterize an ungrouped UOC, Northwest Africa 5717, 5) conduct a microanalytical investigation of rim / matrix boundaries to discern relative chronologies of fine-grained material in UOCs, 6) provide context for the presence of microchondrules as ubiquitous inclusions within fine-grained rims, as well as provide constraints on the chondrule-forming environment of UOC chondrules, 7) compare fine-grained rims with well-characterized fine-grained rims from unbrecciated primary accretionary components of Mighei-like carbonaceous (CM) chondrites, so as to define different environments of formation for CM chondrules. The ultimate goal of this thesis is to provide the first comprehensive study in which multi-analytical techniques (basic petrology, electron microprobe, transmission electron microscopy, and stable isotope analyses) have been combined to characterize and understand fine-grained chondrule rims and the microchondrules within them.
Fine-grained rims record two distinct formation environments in the chondrules of UOCs and CM chondrites. Despite common mineralogical components among the different grouped chondrules, from my analysis I infer a single-stage reheating of chondrule surfaces to form fine-grained chondrule rims in UOCs. The dust that accreted onto reheated surfaces reacted with the chondrule core surface to produce predominantly FeO-rich sintered material that envelopes both type I and type II chondrules. However, the presence of 1) microchondrules preferentially within the rims of type I chondrules, coupling with their virtual absence from the rims of type II chondrules, 2) heavily irregular surfaces between type I chondrules and their rims, coupled with a sintered texture that reveals partial reaction of FeO-rich material with pyroxene-dominant outer chondrule shells, 3) the presence of disaggregated rim clasts within the matrix, and 4) the presence of chondrule-like fragments and refractory-like fragment within rims, all suggest that type I chondrules were reheated to higher temperatures than were the surfaces of type II chondrules, the latter of which accreted dust less efficiently than their type I counterparts, all within a pre-accretionary environment that predated final assemblages of the UOC parent body. CM chondrules are also pre-accretionary and experienced either single or multi-stage accretion or rim formation. Layering within rims, as the well as reheating products such as microchondrules, however, would have likely experienced deleterious effects of hydration. Since such features are not visible in all chondrule rims from CMs, I surmise that CM chondrules accreted dust in a lower temperature environment than UOC chondrules. Accretion of dust at farther distances from the Sun than the condensation region of water (the snowline) is the most probable environment for this process.
The presence of hydrated materials as secondary products in the rims of UOC chondrules leads to the hypothesis that they formed at near proximity to, though probably within, the snowline. Oxidation of dust-coated surfaces was facilitated by water vapor that would have elevated oxygen fugacities within microenvironments of the chondrule-forming region. Colder accretion of ices with dust around CM chondrules ultimately contributed to higher degrees of hydration of CM rims than UOC rims. In both cases, the parent body environment afforded more porous fine-grained material that evolved through fluid-assisted alteration into the opaque matrix in both chondrite groups. Such asteroidal alteration could also generate opaque nodules composed of Fe-Ni-metal, sulfides, and Fe-oxides by fluid mobilization of sulfur, oxygen, and metal.
Bigolski, John, "The Formation of Fine-Grained Chondrule Rims in Unequilibrated Ordinary Chondrites" (2017). CUNY Academic Works.