doi:10.1016/j.gca.2004.05.047 Unraveling the evolution of chondrite parent asteroids by precise U-Pb dating and thermal modeling YURI AMELIN, 1, *AMITABHA GHOSH, 2 and ETHAN ROTENBERG 3 1 Jack Satterly Geochronology Laboratory, Geology Department, University of Toronto, Toronto, Ontario, Canada, M5S 3B1 2 Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA 3 Geology Department, University of Toronto, 22 Russell Street, Toronto, Ontario, Canada, M5S 3B1 (Received November 26, 2003; accepted in revised form May 21, 2004) Abstract—U-Th-Pb isotopic data are reported for mineral fractions, individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite Richardton (H5). Chondrules and milligram- sized fractions of pyroxene-rich chondrule fragments contain highly radiogenic Pb and concordant or nearly concordant U-Th-Pb isotopic systems, and are suitable for precise Pb-Pb age determinations. Olivine and sulfide have low U concentrations and contain less radiogenic Pb. The ages of individual chondrules, pyroxene-rich and phosphate fractions are determined using U-Pb and Pb-Pb isochron and model date calculations. The Pb-Pb isochron date of 4562.7  1.7 Ma of the Richardton chondrules and chondrule fragments is resolved from the Pb-Pb isochron date of 4550.7  2.6 Ma obtained from multiple phosphate fractions. Possible biases of the isochron dates due to single-stage approximation of multi-stage evolution, contamination with modern common Pb, and disturbance to the system by reheating, are examined and are found to be insignificant. The chondrule and phosphate dates are interpreted as the timing of cessation of Pb diffusion during cooling following metamorphism in chondrite parent bodies. The difference in estimated closure temperatures, 950 –1150 K for pyroxenes, and 700 – 800 K for phosphates (temperature estimates are based on published diffusion rates for Pb in pyroxenes and apatite), allows evaluation of the average cooling rate at 26  13 K/million years for the Richardton parent body over the period of 4563– 4551 my. Thermal modeling of the H-chondrite parent body (which is assumed to be asteroid 6 Hebe, heated by decay of 26 Al) suggests a scenario in which accretion initiated at 1.7 m.y. after formation of calcium-aluminum–rich inclusions and continued for 3.5 m.y. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Chondrites are primitive meteorites from asteroids that never melted. These meteorites are complex mixtures of mineral grains and aggregates that formed by various processes (Kerridge and Matthews, 1988; Brearley and Jones, 1998): condensation from protosolar dust-gas disk and melting of the early condensates (refractory inclusions, metal grains, chon- drules) and metamorphism (metamorphic minerals, e.g., phos- phates). Parent asteroids of ordinary chondrites underwent ther- mal metamorphism at 600 –1200 K (Dodd, 1981; McSween et al., 1988) and preserve a record of the early thermal history of the solar system. Cooling paths for meteorite parent asteroids can be estab- lished by isotopic dating of two or more minerals from the same meteorite with different closure temperatures for diffu- sion of the element of interest. If these minerals are dated using the same isotopic system, the determined age interval is free of uncertainties related to poorly known decay constants (Begemann et al., 2001) and can be used to determine precise cooling rates. The cooling path deduced this way could be used to constrain the timing and duration of formation of the mete- orite parent body, and to interpret its structure. The U-Pb dating method, based on the decay of 235 U and 238 U, is particularly useful for dating chondritic minerals be- cause it can produce precise absolute ages, and contains an internal check for concordance. In addition, use of the U-Pb method as a thermochronometer is facilitated by a recently established extensive database of Pb diffusion rates in various minerals (Cherniak and Watson, 2000; Cherniak, 2001, and references therein). Calcium phosphates are the most uranium- rich minerals in ordinary chondrites, with uranium concentra- tions 10 –300 times higher than the whole rock (Göpel et al., 1994). The studies of U distribution between minerals in ordi- nary chondrites (Jones and Burnett, 1979; Ebihara and Honda, 1984; Crozaz et al., 1989; Hagee et al., 1990) show, however, that the fraction of U that resides in phosphate minerals— apatite and merrillite—varies greatly between meteorites, from less than 10% to 100% (Göpel et al., 1994). Much of the uranium contained in many chondrites therefore resides in silicate minerals. If these minerals contain sufficiently low common Pb, they also may be used as U-Pb chronometers. The most abundant silicate minerals in ordinary chondrites, both in chondrules and in the matrix, are olivine and low-Ca pyroxene. Plagioclase and high-Ca pyroxene are also found but are less abundant. Chondrules, making up to 50% or more of the chondrite volume, are composed of pyroxene, olivine, and recrystallized glass (mesostasis) with lesser amounts of metal, sulfide, and other minerals. The abundance of phosphates does not exceed 1% and is much lower in most chondrites. Here we report U-Th-Pb data, along with supporting structural and compositional data, for individual chondrules and fractions of pyroxene-rich chondrule fragments as well as phosphates, olivine, sulfide, and low-density plagioclase-rich fractions from Richard- * Author to whom correspondence should be addressed. (yamelin@NRCan.gc.ca). Present address: Geological Survey of Canada, 601 Booth St., Ottawa, ON, Canada, K1A 0E8. Geochimica et Cosmochimica Acta, Vol. 69, No. 2, pp. 505–518, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 505