Earth and Planetary Sciences ETDs

Publication Date

Summer 7-30-2016

Abstract

The early solar nebula is thought to have been a turbulent disk of dust and gas. An unknown mechanism caused the disk to collapse, such as a nearby super nova. This collapse led to high temperatures and high particle densities at the mid-plane of the disk. An unknown mechanism caused the temperature in local area to become hot enough to cause some material to melt –creating the first igneous material of the solar system. These melt spherules, called chondrules, cooled and accreted together with dust and ice to form larger bodies called chondrites. Once the chondritic bodies were of sufficient size, and enough 26Al decayed which was the primary heat source for planetary interiors, these bodies began to melt and undergo differentiation (core, mantle and crust formation) and become planetesimals. These planetesimals were the building blocks for all of the planets we currently have today. Our window into understanding these early solar system processes come from our collection of meteorites. The study of meteorites has led to many challenges in fully understanding nebular processes. The identification of a mechanism for volatile element incorporation has remained a core problem. It is unlikely that many volatile elements were initially incorporated into chondrules because of the high temperature of formation. Further, there is little direct evidence for volatile incorporation, i.e. condensing phases identification. Therefore, it has been a long prevailing idea that many volatile elements were incorporated into the parent bodies post-accretion, mostly through fluids. These process are difficult to identify. However, understanding volatile element incorporation in chondritic bodies is paramount in the identification of volatile sources for the terrestrial planets. Of all the volatile elements, the halogen group is of particular interest. Halogens, such as Cl and F, are in relatively high concentration in many chondritic bodies, yet show high variability (between <100 ppm to 700ppm Cl across the chondrite groups). These differences may reflect either, or a combination of, different reservoir conditions, incorporation mechanisms, and/or post-accretion processing. Chlorine, however, is the focus of this study. Chlorine has many unique chemical characteristics. It is strongly hydrophilic. It does not exhibit appreciable isotopic fractionation at high temperatures, yet may fractionate significantly at low temperatures. And it is also strongly incompatible in magmatic systems. Thus, Cl is useful as a geochemical proxy for many systems. In the following chapters, this study will address the behavior of Cl on Mars and in ordinary chondrites. To do so, Cl isotopic measurements will be used in conjunction with in situ elemental mapping, and petrologic description of material. Chapter 1 will explore the Cl isotopic variation on Mars through Martian meteorites. Mars is a volatile rich body, and is enriched in chlorine relative to Earth by a factor of 3. The collection of known Martian meteorites spans a large range of geologic conditions. These samples include crustal regolith breccia, basaltic melts, and dunites. This work explores the differences in the Cl isotopic composition and the Cl concentration of the mantle and the crust. These differences will be used to assess Cl ability to trace crustal contamination. An unexpected outcome of this work is the isotopically light values measured for the Martian mantle-like samples; implying Mars sourced a different nebular reservoir than the Earth. By understanding Cl in the Martian mantle we can then reassess Cl in the early solar system. Chapter 2 looks at the Cl isotopic composition and host phases in the most pristine chondritic meteorites (3.00s). NWA 8276, L 3.00, has been shown to be isotopically light with respect to CL. This work finds that NWA 8276 is anomalous compared to other 3.00’s have near-Earth Cl isotopic compositions. A detailed study using in situ microbeam techniques of LL 3.00 Semarkona found some chondrules to have high Cl concentrations. This study will seek to identify the host phases of Cl in these enriched chondrules, and provide a mechanism for incorporation. NWA 8276 was also studied using in situ elemental mapping techniques in an effort to explain the difference in Cl isotopic composition. Understanding Cl in the 3.00 ordinary chondrites is important as it is most likely to represent the initial condition of Cl incorporation in the early solar nebula.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Sharp, Zachary

Second Committee Member

Shearer, Charles

Third Committee Member

McCubbin, Francis

Fourth Committee Member

Agee, Carl

Language

English

Keywords

Mars, Meteorites, Chondrites, Isotopes, Cl

Document Type

Thesis

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