Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves^{1, 2}) in the protoplanetary disk around the early Sun. The ages of primitive chondrules^{3, 4, 5, 6} in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars⁷. Here we report the ²⁰⁷Pb–²⁰⁶Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 0.9 Myr), which formed during a single-stage, highly energetic event^{8, 9, 10, 11}. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave². We conclude that chondrules and metal grains in the CB chondrites formed from a vapour–melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System¹².

1. Hawai'i Institute of Geophysics & Planetology, School of Ocean & Earth Science & Technology, University of Hawai'i at Manoa, 2525 Correa Rd, Honolulu, Hawaii 96822, USA
2. Geology Department, University of Toronto, and Geological Survey of Canada, 601 Booth St, Ottawa, Ontario, K1A 0E8, Canada
3. SETI Institute, 515 N. Whisman Road, Mountain View, California 94043, USA
4. Museum National d'Histoire Naturelle, Laboratoire d'Etude de la Matière Extraterrestre, USM 0205, Case Postale 52, 57 rue Cuvier, 75005 Paris, France

Correspondence to: Alexander N. Krot¹ Correspondence and requests for materials should be addressed to A.N.K. (Email: sasha@higp.hawaii.edu).

Received 3 April 2005; Accepted 20 May 2005

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