Early Disk dynamics Inferred from Isotope Systematics of Individual Chondrules: Absolute Chronology of the Solar Protoplanetary Disk Revisited

Research output: Book/ReportPh.D. thesisResearch

A full understanding of the formation and earliest evolution of our Solar System is one of the most fundamental goals of the natural sciences. Chondritic meteorites contain pristine relict material from the birth of our Solar System, ~4.56 billion years ago, such that studying their constituents can provide direct insights into the environment and dynamics that prevailed at that time. In particular, chondrules are mm-sized igneous silicate spherules that formed throughout the protoplanetary disk by transient heating events in the first few million years. Their sheer abundance in chondrites imply that they must be the product of one of the most energetic processes that operated in the early Solar System. Chondrules are the only samples available to investigate the early Solar System and the chemical and thermal evolution of the Solar protoplanetary disk.

Although relative 26Al-26Mg dating of chondrules has suggested that their formation began approximately 1-2 Myr after the condensation of Calcium-Aluminum-rich Inclusions (CAIs), the oldest known solids, recent absolute dating of individual chondrules by the 207Pb-206Pb chronometer has refuted this supposed age gap. Instead, this chronometer indicates that chondrule formation started contemporaneously with CAIs and lasted ~3 Myr. However, this result is based on the analyses of only five individual chondrules and, thus, does not statistically evaluate the transient heating events.

The principal goal of this project is to define a set of reference inclusions with U-corrected 207Pb-206Pb absolute ages for combined multi-isotopic analysis (i.e. 54Cr, 26Al-26Mg). This third chapter of this dissertation presents four identical Pb-Pb ages for chondrules from the CBa-type chondrite. This single chondrule formation age confirms the viability of the impact plume model of chondrule formation for this meteorite and validates the accuracy and robustness of the Pb-Pb dating method by stepwise dissolution method. The fourth chapter presents a suite of absolute U-corrected Pb-Pb ages of individual chondrules from various chondrite types. They confirm the contemporaneous formation of chondrules with CAIs and extends the chondrule forming period to ~4 Myr, with most of the chondrules produced during the first million years. Similar age ranges of chondrules from different chondrite types, argues against the existence of distinct chondrule formation regions and/or reservoirs over time. Finally, the fifth chapter explores the Solar System 26Al homogeneity paradigm by comparing of Al-Mg systematics in chondrules with their respective Pb-Pb ages. We report a consistent age offset between the two chronometers that we infer reflects a reduced abundance of 26Al relative to 27Al in chondrule precursors and, therefore, that 26Al/27Al heterogeneity existed in the Solar protoplanetary disk. Regardless of the mechanism(s) that produced this 26Al heterogeneity in the early Solar System, it rules out any chronological significance of the Al-Mg systematics when applied to different reservoirs. Lastly, a reduced abundance of 26Al relative to 27Al in inner Solar System objects implies that asteroid accretion must have occurred very early in the Solar System’s formation, to allow melting and differentiation driven by 26Al decay. With respect to our Pb-Pb chondrule chronology, further accretion after ~1-2 Myr will lead to the formation of undifferentiated outer shells of planetesimals that serve as the source of chondrite meteorites.

Original languageEnglish
PublisherNatural History Museum of Denmark, Faculty of Science, University of Copenhagen
Number of pages195
Publication statusPublished - 2016

ID: 164301685