The Absolute Pb-Pb Isotope Ages of Chondrules: Insights into the Dynamics of the Solar Protoplanetary Disk

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The parent nuclides 238U and 235U decay to 206Pb and 207Pb, respectively, with half-lives that makes this system uniquely suited to define the temporal framework of the solar protoplanetary disk, including the timing and duration of chondrule formation. Lead isotope data for 22 individual nebular chondrules indicate that the oldest chondrules formed contemporaneously with CAIs and that chondrules were recycled for ~4 Myr within the protoplanetary disk. Integrating the initial Pb isotopic compositions and ages of these individually-dated chondrules reveals that they appear to have formed in two distinct epochs. A primary phase of chondrule production occurred within 1 Myr of the formation of the Sun during the most energetic phase of the protoplanetary disk when mass accretion rates were highest. This epoch of primary chondrule production transitioned into a phase dominated by the reworking of existing chondrules, which lasted for the remainder of the protoplanetary disk’s lifetime. Such a model is consistent with a transition from heating by shock waves related to gravitational instabilities during the more energetic first 1 Myr to heating by bow shocks around early formed planetesimals and planetary embyros. The age of chondrules from the CB meteorite Gujba formed from a vapor-melt plume caused by impacting planetary embyros indicates that the solar protoplanetary disk had dissipated within 4.5 Myr. The Pb-Pb ages require that any appearance of chemical or isotopic complementarity between matrix and chondrules does not imply rapid chondrule formation and accretion or that matrix and chondrules in a single chondrite group have a strict cogenetic relationship. In this view, inferences about the range of ages for chondrule formation based on a 182Hf-182W decay method and the assumption of cogenetically-formed matrix and chondrules cannot be meaningful. Finally, the preponderance of chondrules (>50%) having formed in the first 1 Myr of the protoplanetary disk lifetime is consistent with models of early, efficient growth of planetary embryos by pebble accretion.

Original languageEnglish
Title of host publicationChondrules : Records of Protoplanetary Disk Processes
EditorsSara S. Russell, Harold C. Connolly, Alexander N. Krot
Number of pages24
PublisherCambridge University Press
Publication date2018
ISBN (Print)978-1-108-41801-0
ISBN (Electronic)9781108284073
Publication statusPublished - 2018
SeriesCambridge Planetary Science Series

Bibliographical note

Publisher Copyright:
© Harold Connolly Jr., Alexander Krot and The Trustees of the Natural History Museum, London 2018.

ID: 334860105