Uranium isotopic composition and absolute ages of Allende chondrules

Abstract

A handful of events, such as the condensation of refractory inclusions and the formation of chondrules, represent important stages in the formation and evolution of the early solar system and thus are critical to understanding its development. Compared to the refractory inclusions, chondrules appear to have a protracted period of formation that spans millions of years. As such, understanding chondrule formation requires a catalog of reliable ages, free from as many assumptions as possible. The Pb‐Pb chronometer has this potential; however, because common individual chondrules have extremely low uranium contents, obtaining U‐corrected Pb‐Pb ages of individual chondrules is unrealistic in the vast majority of cases at this time. Thus, in order to obtain the most accurate 238U/235U ratio possible for chondrules, we separated and pooled thousands of individual chondrules from the Allende meteorite. In this work, we demonstrate that no discernible differences exist in the 238U/235U compositions between chondrule groups when separated by size and magnetic susceptibility, suggesting that no systematic U‐isotope variation exists between groups of chondrules. Consequently, chondrules are likely to have a common 238U/235U ratio for any given meteorite. A weighted average of the six groups of chondrule separates from Allende results in a 238U/235U ratio of 137.786 ± 0.004 (±0.016 including propagated uncertainty on the U standard [Richter et al. 2010]). Although it is still possible that individual chondrules have significant U isotope variation within a given meteorite, this value represents our best estimate of the 238U/235U ratio for Allende chondrules and should be used for absolute dating of these objects, unless such chondrules can be measured individually.

Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules

“…The 238 U/ 235 U ratio measured for seven individual chondrules from NWA 5697 returns values identical within analytical uncertainty to the solar 238 U/ 235 U value of 137.786 ± 0.013 (Table 1 and fig. S24), confirming the lack of U isotope variability among individual chondrules (6,8,9) at the resolution of our analyses. This observation validates the approach of using the solar 238 U/ 235 U value to calculate Pb-Pb dates of individual chondrules, although for completeness, we used the measured 238 U/ 235 U values to calculate chondrule ages for which both U and Pb isotope compositions were determined.…”

“…The subset of chondrules we investigated comprises all major petrographic classes, including both porphyritic and non-porphyritic texture types as well as FeO-rich and FeO-poor varieties ( Fig. S24), confirming the lack of U-isotope variability amongst individual chondrules (6,8,9) at the resolution of our analyses. objects for which the solar 238 U/ 235 U value is used to calculate the Pb-Pb ages.…”

238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal

“…To confirm that our approach gives accurate results, we compare our d 238 U data from several replicate digests of the BCR-2 geostandard to all previously published values (Weyer et al, 2008;Brennecka and Wadhwa, 2012;Cheng et al, 2013;Murphy et al, 2014;Iizuka et al, 2014;Dahl et al, 2014Dahl et al, , 2017Dahl et al, , 2019Tissot and Dauphas, 2015;Holmden et al, 2015;Goldmann et al, 2015;Brennecka et al, 2015Brennecka et al, , 2018Andersen et al, 2015;Tissot et al, 2016Tissot et al, , 2018Tissot et al, , 2019Lau et al, 2016Lau et al, , 2017Wang et al, 2018;Avanzinelli et al, 2018;Dang et al, 2016Dang et al, , 2018Dang et al, , 2022Freymuth et al, 2019;Merle et al, 2020). We find that our data (d 238 U = -0.260‰ ± 0.016‰, ndigests = 11 separate digests, nanalyses = 142 replicate analyses) closely agree with prior work (d 238 U = -0.262‰ ± 0.005‰, ndigests = 33).…”

Chondrules: Ubiquitous Chondritic Solids Tracking the Evolution of the Solar Protoplanetary Disk

“…It is based on two isotopes of U that decay in a chain to (Connelly et al 2012;Tissot et al 2016). This view is supported by the homogeneous 238 U/ 235 U of chondrules and bulk planetary materials, which is used to define the solar 238 U/ 235 U value of 137.786 (Connelly et al 2012;Brennecka et al 2015).…”