Radiative heating of carbonaceous near-Earth objects as a cause of thermal metamorphism for CK chondrites Noël Chaumard a,b,c,⇑ , Bertrand Devouard a,b,c , Marco Delbo d , Ariel Provost a,b,c , Brigitte Zanda e a Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France b CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France c IRD, R 163, LMV, F-63038 Clermont-Ferrand, France d Laboratoire Cassiopée, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France e Laboratoire de Minéralogie et Cosmochimie du Muséum, MNHN & CNRS UMR 7202, 61 rue Buffon, 75005 Paris, France article info Article history: Received 7 December 2011 Revised 27 March 2012 Accepted 15 April 2012 Available online 24 April 2012 Keywords: Asteroids Near-Earth objects Meteorites Solar radiation Thermal histories abstract Metamorphic CK carbonaceous chondrites display matrix textures that are best explained by a transient thermal event with temperatures in the 550–950 K range and durations in the order of days to years, longer than what is commonly admitted for shock events but shorter than what is required for nuclide decay. We propose that radiative heating of small carbonaceous meteoroids with perihelia close to the Sun could account for the petrological features observed in CK chondrites. Numerical thermal modeling, using favorable known NEOs orbital parameters (perihelion distances between 0.07 and 0.15 AU) and physical properties of CV and CK chondrites (albedo in the range 0.01–0.1, 25% porosity, thermal diffu- sivity of 0.5–1.5 W m 1 K 1 ), shows that radiative heating can heat carbonaceous meteoroids in the meter size range to core temperatures up to 1050 K, consistent with the metamorphic temperatures esti- mated for CK chondrites. Sizes of known CV and CK chondrites indicate that all these objects were small meteoroids (radii from a few cm to 2.5 m) prior to their atmospheric entry. Simulations of dynamic orbits for NEO objects suggest that there are numerous such bodies with suitable orbits and properties, even if they are only a small percentage of all NEOs. Radiative heating would be a secondary process (superim- posed on parent-body processes) affecting meteoroids formed by the disruption of an initially homoge- neous CV3-type parent body. Different petrologic types can be accounted for depending on the sizes and heliocentric distances of the objects in such a swarm. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction CK chondrites are the only group of carbonaceous chondrites (CCs) forming a metamorphic series from petrologic type 3 to type 6(Kallemeyn et al., 1991). This metamorphic evolution is charac- terized by the chemical and textural equilibration of chondritic components: matrix, chondrules, and calcium–aluminum-rich inclusions (CAIs) (Geiger and Bischoff, 1991; Kallemeyn et al., 1991; Noguchi, 1993; Chaumard et al., 2009a). 92% (in number) of known CK samples are chemically equilibrated (types 4–6). Estimated metamorphic temperatures range from 550 to 1270 K (e.g. Geiger and Bischoff, 1991; Neff and Righter, 2006). Based on mineralogy, petrology, and oxygen isotope analyses, it has been proposed that CK and CV carbonaceous chondrites form a continuous metamorphic series from a same parent body, rather than distinct groups (Greenwood et al., 2003, 2004, 2010; Devouard et al., 2006; Chaumard et al., 2009a). The similar cos- mic-ray exposure age distributions for CVs and CKs (between 1 and 40 Myr) also support a common source for these groups (Scherer and Schultz, 2000). Recent paleomagnetic data on CVs give evidence that the Allende CV chondrite could be derived from the surface of a differentiated asteroid (Weiss et al., 2010; Elkins- Tanton et al., 2011; Humayun and Weiss, 2011). In this model, a continuous CV–CK series could imply that CKs come from the low- er part of the undifferentiated layer at the surface of the differen- tiated asteroid. A striking and ubiquitous feature of type 4–5 CK chondrites is the texture of their matrices which contain numerous micron- and nanometer-sized vesicles and inclusions and are much coarser than they are in ordinary chondrites (OCs) of the same petrological types (Kallemeyn et al., 1991; Rubin, 1992; Tomeoka et al., 2001, 2005; Ohnishi et al., 2007; Brearley, 2009). Heat sources commonly involved to explain parent-body metamorphism are the short and long-lived radionuclides decay, and the accretional or collisional residual heat (e.g. Ghosh et al., 2006; Huss et al., 2006). All these 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.04.016 ⇑ Corresponding author at: Clermont Université, Université Blaise Pascal, Labo- ratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France. Fax: +33 (0)4 73 34 67 44. E-mail address: n.chaumard@opgc.univ-bpclermont.fr (N. Chaumard). Icarus 220 (2012) 65–73 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus