Hf–W chronometry of primitive achondrites T. Schulz a,b,c, * , C. Mu ¨ nker a,b,c , K. Mezger c , H. Palme a,d a Institut fu ¨ r Geologie und Mineralogie, Albertus Magnus Universita ¨t zu Ko ¨ln, Zu ¨ lpicherstr. 49b, D-50674 Ko ¨ ln, Germany b Steinmann Institut, Bereich Endogene Prozesse, Rheinische Friedrich-Wilhelms-Universita ¨ t Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany c Institut fu ¨ r Mineralogie, Westfa ¨ lische Wilhelms-Universita ¨t Mu ¨ nster, Corrensstrasse 24, D-48149 Mu ¨ nster, Germany d Sektion Meteoritenforschung, Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany Received 24 June 2009; accepted in revised form 14 December 2009; available online 24 December 2009 Abstract Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived 182 Hf– 182 W radionuclide system. Here we present new 182 Hf– 182 W data for major types of primitive achondrites including acapulcoites, winonaites and one lodra- nite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites. For acapulcoites we derived a 182 Hf– 182 W age of Dt CAI = 4.1 +1.2 / 1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of Dt CAI = 4.8 +3.1 / 2.6 Ma (assuming a bulk Hf/W ratio of 1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of Dt CAI = 14.3 +2.7 / 2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between 1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf–W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between 35 and 4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were 40 K/Ma to 720 K and then 3 K/Ma to 550 K. Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achon- drite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse correlation between accretion-age of asteroids and intensity of heating in their interiors as expected from heating by 26 Al and 60 Fe decay. How- ever, the early accretion of the parent asteroid of primitive IAB silicates (1.0 Ma after CAIs; Schulz et al., 2009) and the possibly impact-induced melting-history of winonaites show that this concept is too simplistic. Parent body size, impact-dri- ven melting as well as heat-insulating regolith cover also need to be considered in the early history of asteroid differentiation. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Primitive achondrites have approximately chondritic composition and generally show evidence for higher tem- peratures than even the most highly metamorphosed chon- drites. Meteorites that belong to this category include acapulcoites, lodranites, winonaites and silicates present in IAB iron meteorites (e.g., Prinz et al., 1983; Clayton 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.12.016 * Corresponding author. Address: Institut fu ¨ r Geologie und Mineralogie, Albertus Magnus Universita ¨t zu Ko ¨ln, Zu ¨ lpicherstr. 49b, D-50674 Ko ¨ln, Germany. Fax: +49 228 732766. E-mail address: toni.schulz@arcor.de (T. Schulz). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 1706–1718