Date of Degree

2-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Harold C. Connolly, Jr.

Committee Members

Denton Ebel

Michael Weisberg

Subject Categories

Cosmochemistry

Keywords

Cosmochemistry, chondrules, 26-Aluminum, chondrites, meteorites, Calcium-Aluminum Inclusions

Abstract

This study focuses on the challenges associated with the historical development of the interpretative role of 26Al as both a thermodynamic contributor to planetesimal formation processes and an acutely precise chronometer of protoplanetary disk events. Conventional thought asserts that 26Mg concentrations can occur as isotopic excesses in meteorites and represent the daughter product of 26Al, a radioactive form of aluminum (t½ = ~0.73 ka). The greatest 26Mg concentrations generally occur in calcium aluminum inclusions (CAIs), meteoric constituents considered among the oldest known materials in the Solar System whose formation appear to predate the less refractory protoplanetary building blocks known as “chondrules.” The tendency for CAIs to exhibit higher 26Mg concentrations than chondrules is considered support for assigning a later formation age (e.g., 1-2 Ma younger) to the latter, a notion bolstered by some literature supporting this perspective based on Pb-age dating (e.g., Amelin et al., 2002; Bouvier and Wadhwa, 2010; Zinner E. and C. Göpel. 2002).

Given that 26Al is the parent material of meteoric 26Mg excesses, then certain minimum criteria must be met before 26Al can be accepted as a fine-tuned chronometer or the proposed chief source of internal heat for planetesimals. First, 26Mg excesses must be genuine and solely a result of a specific decay pathway associated with 26Al decay rather than an artifact of extraneous factors, including analytical method biasing or data misinterpretation. Secondly, 26Al was uniformly distributed throughout the protoplanetary disk (at least within the orbits that terrestrial bodies reside). Last, the original 26Al concentration in the protoplanetary disk must be known before this isotope can be considered a chronometer. When 26Mg excesses are compared to the ratio of stable isotopic concentrations (27Al/24Mg), the resulting calculated value of the initial 26Al/27Al ratio consistently appears to be approximately 5x10-5. This special value is considered ‘canonical’ and referred to as such. It is also worth mentioning that if any of the aforementioned criteria are not satisfied, then the case for pointing to 26Al as the chief internal heat source for planetesimals or as a viable chronometer fails.

The principal goal of this thesis is to test the validity of the results published by MacPherson et al. (1995). Their work is arguably one of the most comprehensive canonical model studies to date. This thesis also evaluated the strength of the canonical model via new perspectives and the results thereof are discussed herein and pose serious considerations regarding the nature and meaning of the ‘canonical’ value as well as the cogency of utilizing 26Al as a relative dating tool.

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