title: Evidence for the late formation of hydrous asteroids from young meteoritic carbonates.
abstract: The accretion of small bodies in the Solar System is a fundamental process that was followed by planet formation. Chronological information of meteorites can constrain when asteroids formed. Secondary carbonates show extremely old (53)Mn-(53)Cr radiometric ages, indicating that some hydrous asteroids accreted rapidly. However, previous studies have failed to define accurate Mn/Cr ratios; hence, these old ages could be artefacts. Here we develop a new method for accurate Mn/Cr determination, and report a reliable age of 4,563.4+0.4/-0.5 million years ago for carbonates in carbonaceous chondrites. We find that these carbonates have identical ages, which are younger than those previously estimated. This result suggests the late onset of aqueous activities in the Solar System. The young carbonate age cannot be explained if the parent asteroid accreted within 3 million years after the birth of the Solar System. Thus, we conclude that hydrous asteroids accreted later than differentiated and metamorphosed asteroids.
authors.affiliation: Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. fujiya@eps.s.u-tokyo.ac.jp
authors.forename: Wataru
authors.initials: W
authors.lastname: Fujiya
authors.forename: Naoji
authors.initials: N
authors.lastname: Sugiura
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Hotta
authors.forename: Koji
authors.initials: K
authors.lastname: Ichimura
authors.forename: Yuji
authors.initials: Y
authors.lastname: Sano