Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System
Gregory A. Brennecka and Meenakshi Wadhwa
NAS 2012 109 (24) 9299-9303; published ahead of print May 30, 2012, doi:10.1073/pnas.1114043109
Events occurring within the first 10 million years of the Solar System’s approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be “anchored” to absolute ages of appropriate meteoritic materials using the high-precision lead–lead (Pb–Pb) chronometer. Previously reported Pb–Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant 238U/235U ratio (= 137.88). In this work, we report measurements of 238U/235U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb–Pb ages of ≥1 million years. There is no resolvable variation in the 238U/235U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb–Pb age for the commonly used anchor, the D’Orbigny angrite, to be 4563.37 ± 0.25 Ma. An adjustment to the Pb–Pb age of a time anchor (such as D’Orbigny) requires a corresponding correction to the “model ages” of all materials dated using that anchor and a short-lived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.