Hafnium–tungsten chronometry of angrites and the earliest evolution of planetary objects Agnès Markowski a, ⁎ , Ghylaine Quitté a,1 , Thorsten Kleine a , Alex N. Halliday b , Martin Bizzarro c , Anthony J. Irving d a Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH NW C81.1, Clausiusstrasse 25, CH-8092 Zürich, Switzerland b Department of Earth Sciences, Oxford University, Parks Road, OX1 3PR, United Kingdom c Geological Institute, University of Copenhagen, Copenhagen, Denmark d Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Received 20 March 2007; received in revised form 17 July 2007; accepted 17 July 2007 Editor: R.W. Carlson Available online 25 July 2007 Abstract Angrites are amongst the oldest basalts in the solar system and their origins are enigmatic, some even proposing the planet Mercury as the parent body (APB). Whatever their exact provenance their chronometry provides insights into early stages of planetary melting and differentiation. We present the first high-precision internal 182 Hf– 182 W isochrons for such early differentiated objects. Angrites Sahara 99555, D'Orbigny, and Northwest Africa 2999 define ages of 5.1 ± 1.3 Ma, 4.7 ± 1.3 Ma and 9.5 ± 3.3 Ma respectively after formation of calcium–aluminum-rich refractory inclusions (CAIs). These data are in good agreement with 26 Al– 26 Mg, 53 Mn– 53 Cr and most 207 Pb– 206 Pb ages for other angrites and provide evidence for two texturally and temporally well-resolved groups. The quenched angrites (SAH 99555, D'Orbigny and five others) have a weighted mean age of 4562.1 ± 0.4 Ma and are the products of igneous crystallization on the APB ∼ 5 Ma after the formation of CAIs, whereas the more slowly cooled angrites (NWA 2999, Angra dos Reis, LEW 86010, average age: 4557.7± 0.2 Ma) reflect metamorphic closure ∼5 Ma later following second reheating process or a complex cooling history. The concordance obtained between various short-lived chronometers provides evidence that 26 Al, 53 Mn and 182 Hf were homogeneously distributed in the solar nebula, although we cannot rule out the possibility of local small heterogeneities. Contrary to recent proposals, the data are also consistent with the previously determined age of the solar system based on 207 Pb– 206 Pb systematics of CAIs. The Hf–W data are discussed in the context of two endmember models for the early differentiation of the angrite parent body. In the first model, core formation occurred at 3–4 Ma after CAIs and both groups of angrites formed by two distinct partial melting events from the bulk mantle of the angrite parent body. In the second model, the angrite parent body underwent progressive core formation with an increasing degree of W-depletion over time. In this model, the two groups of angrites derive from distinct reservoirs. The heat sources responsible for such late melting and core formation are unclear. Quenched angrites are coeval with non-magmatic IAB iron meteorites and CB chondrules at ∼4562 Ma. However, demonstration of a genetic link between angrite melting and impact events must await the acquisition of still higher resolution chronometry. © 2007 Elsevier B.V. All rights reserved. Keywords: tungsten isotopes; angrites; planetesimals; chronology; isotopic homogeneity; intercalibration; Sahara 99555; Northwest Africa 2999; D'Orbigny; short-lived radionuclides Earth and Planetary Science Letters 262 (2007) 214 – 229 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: markowski@erdw.ethz.ch (A. Markowski). 1 Present address: Laboratoire des Sciences de la Terre, CNRS/ENS Lyon/Univeristé de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 7, France. 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.07.035