¹Matthias M. M. Meier,²Kees C. Welten,^1,3My E. I. Riebe,^4,5Marc W. Caffee,^6,7,8Maria Gritsevich,¹Colin Maden,¹Henner Busemann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12874]
¹ETH Zurich, Institute of Geochemistry and Petrology, Zurich, Switzerland
²Space Sciences Laboratory, University of California, Berkeley, California, USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, USA
⁴Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, USA
⁵Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA
⁶Department of Physics, University of Helsinki, Helsinki, Finland
⁷Finnish Geospatial Research Institute, Masala, Finland
⁸Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia
Published by arrangement with John Wiley & Sons

The Park Forest (L5) meteorite fell in a suburb of Chicago, Illinois (USA) on March 26, 2003. It is one of the currently 25 meteorites for which photographic documentation of the fireball enabled the reconstruction of the meteoroid orbit. The combination of orbits with pre-atmospheric sizes, cosmic-ray exposure (CRE), and radiogenic gas retention ages (“cosmic histories”) is significant because they can be used to constrain the meteoroid’s “birth region,” and test models of meteoroid delivery. Using He, Ne, Ar, 10Be, and 26Al, as well as a dynamical model, we show that the Park Forest meteoroid had a pre-atmospheric size close to 180 g cm−2, 0–40% porosity, and a pre-atmospheric mass range of ~2–6 tons. It has a CRE age of 14 ± 2 Ma, and (U, Th)-He and K-Ar ages of 430 ± 90 and 490 ± 70 Ma, respectively. Of the meteorites with photographic orbits, Park Forest is the second (after Novato) that was shocked during the L chondrite parent body (LCPB) break-up event approximately 470 Ma ago. The suggested association of this event with the formation of the Gefion family of asteroids has recently been challenged and we suggest the Ino family as a potential alternative source for the shocked L chondrites. The location of the LCPB break-up event close to the 5:2 resonance also allows us to put some constraints on the possible orbital migration paths of the Park Forest meteoroid.

¹Addi Bischoff, ²Gerhard Wurm, ³Marc Chaussidon, ¹Marian Horstmann, ¹Knut Metzler, ⁴Mona Weyrauch, ¹Julia Weinauer
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12833]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
²Fakultät für Physik, Universität Duisburg-Essen, Duisburg, Germany
³Institut de Physique du Globe, Sorbonne Paris Cité, UMR CNRS 7154, Université Paris Diderot, Paris Cedex 05, France
⁴Institut für Mineralogie, Leibniz-Universität Hannover, Hannover, Germany
Published by arrangement with John Wiley & Sons

In Allende, a very complex compound chondrule (Allende compound chondrule; ACC) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (BO). The BO texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine-rich units. Because of secondary alteration of pristine Allende components and the ACC in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the ACC subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more 16O-poor reservoir. We modeled constraints on the particle density required at the ACC formation location, using textural, mineral-chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of ACC calculations suggest the existence of very local, kilometer-sized, and super-dense chondrule-forming regions with extremely high solid-to-gas mass ratios of 1000 or more.