Earth and Planetary Sciences ETDs

Publication Date

5-1-2012

Abstract

Understanding the constituents of fine-grained nebular dust prior to asteroidal accretion is a significant challenge in meteoritics. The key to understanding nebular dust lies in chondritic meteorites, which are composed of undifferentiated solar system materials. Chondritic matrices are a subnanometer mixture of amorphous material, silicate grains, sulfides, and other phases, that act as the "glue" for larger components. In the pristine chondrites (petrologic type 3.00) the matrix preserves some record of the primary characteristics of nebular dust. Two CR carbonaceous chondrites, MET 00426 (CR3.00) and QUE 99177 (CR3.00), are likely the most pristine meteorites ever described (Abreu and Brearley, 2010). The mineralogy of crystalline silicate grains in the matrices of these two chondrites has been studied here using focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. Dark-field scanning transmission electron microscopy (STEM) imaging, along with bright-field TEM and energy dispersive X-Ray spectroscopy (EDS) techniques were used to understand mineralogical and chemical characteristics. FIB sections consist of extremely fine-grained, amorphous material (<50 nm in size). Additionally, each section contains crystalline or nanocrystalline silicate grains, varying in size from <100 to 1000 nm in diameter. These grains are MgO-rich or FeO-bearing olivines and pyroxenes. These crystalline silicates exhibit unequilibrated major and minor element compositions, but distinct compositional groupings of olivines and pyroxenes can be identified. We infer that the crystalline silicate materials in the matrices of these chondrites formed by different mechanisms that include nebular condensation under equilibrium and disequilibrium conditions, followed in some cases by annealing prior to accretion. Our data discount chondrule fragmentation as a significant source of the fine-grained matrix olivines. Although these chondrites have similar matrix mineralogy and chemistry, FeO-bearing crystalline silicates are more common in MET 00426 matrix than in QUE 99177, indicating that the matrices of these chondrites did not sample the same reservoir or source of crystalline silicate nebular dust. Nevertheless, in many cases, these grains have are similar in composition to other primitive silicates, like interplanetary dust particles (IDPs) and amoeboid olivine aggregates (AOAs), and could share a common formational mechanism or represent the nanoscale equivalent of these objects.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Advisor

Brearley, Adrian

First Committee Member (Chair)

Jones, Rhian

Second Committee Member

Sharp, Zachary

Third Committee Member

Shearer, Charles

Language

English

Keywords

chondrites, matrix, solar nebula, solar system evolution

Document Type

Thesis

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