Doctor of Philosophy
Planetary Science and Exploration
Chondrules are ubiquitous igneous spherules, the main constituents of ordinary chondrites, and are considered to be critical building blocks of planetesimals, yet their age and formation mechanism(s) remain debated. Differences between ages determined from the long-lived Pb-Pb and the short-lived 26Al-26Mg chronometers have been attributed to the heterogeneous distribution of 26Al in the solar nebula, a radionuclide that was responsible for early differentiation of planetesimals.
To evaluate this hypothesis, the 26Al-26Mg and the 207Pb-206Pb isotopic compositions were both measured by multi-collector ICP-MS in individual chondrules and igneous clasts in unequilibrated ordinary chondrites, and three achondrites. A subset of the chondrules were also analyzed by in situ 26Al-26Mg secondary ionization mass spectrometry (SIMS), to compare with their bulk Mg model ages and Pb-Pb ages, and for 54Cr isotopic anomalies. A comparison between bulk Mg model ages and Pb-Pb ages in the same chondrules suggests that the absolute ages for CAIs may be too young, and that Pb-Pb ages date precursor formation rather than time of crystallization. Precursors may have been previous generations of chondrules that were recycled by transient shock waves in the protoplanetary disk.
Chronological results of EC 002 date its crystallization at 4566 Ma which is the oldest evidence of planetary crust formation. Analysis of two angrites NWA 10463 and NWA 8535 indicate a more diverse suite of processes that shaped the evolution of the angrite parent body and differentiation of early formed planetesimals.
All together, the presented results support a homogeneous distribution of 26Al in the solar nebula.
Summary for Lay Audience
This dissertation investigates the comparative chronology of the early Solar System materials using their 26Al-26Mg and 238,235U-206,207Pb isotopic compositions to evaluate the distribution of 26Al within the protoplanetary disk. Chondrules are crystallized spherules that formed from silicate melt droplets. They are a major component of ordinary chondritic meteorites. These meteorites are sourced from tens of kilometer-sized planetary bodies formed in the early Solar System, over 4560 million years ago. Achondrite meteorites come from planetesimals that experienced wide-scale melting, with the short-lived radionuclide 26Al providing an early source of radioactive heat. The distribution of 26Al is a critical constraint in understanding the nebular and planetary processes that shaped the early Solar System.
The introduction (Chapter 1) presents the current state of knowledge in cosmochemistry and present the objectives of this dissertation, while Chapter 2 details the methods used to achieve those objectives.
Chapter 3 deals with chondrules that were dated using the 26Al-26Mg and the Pb-Pb geochronometers and with their analyzed Cr isotope composition. The results show evidence that some chondrules formed within one million years of Solar System formation, and could be recycled by repeated heating events. Furthermore, based on 54Cr isotope anomalies, chondrules were not transported between the inner and outer Solar System.
Chapter 4 highlights the chronology of the unique achondrite Erg Chech 002, whose parent asteroid experienced wide-scale melting early in its lifetime. Using 26Al-26Mg and the Pb-Pb chronometry, we establish that this sample formed around 4566 million years ago, which makes it the oldest planetary crust found so far. The agreement between both chronometers is strong evidence for a homogeneous distribution of 26Al in the Solar nebula.
Chapter 5 details the chronology and trace element chemistry of two angrite meteorites, NWA 8535 and NWA 10463. The angrite parent asteroid experienced full-scale melting and produced a diverse suite of rocks. The petrology, chronology and geochemistry of the samples indicate they are unique from other angrite meteorites and that the angrite parent body may have a more complex history than previously thought.
Reger, Philip M., "Early Solar System chronology and isotopic linkage of non-carbonaceous chondrules, clasts and achondrites" (2022). Electronic Thesis and Dissertation Repository. 8525.
Available for download on Thursday, December 21, 2023