¹Takashi Mikouchi et al. (>10)*
¹Department of Earth and Planetary Science, Graduate School of Science, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

*Find the extensive, full author and affiliation list on the publishers website.

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Reference
Mikouchi T et al. (2014) Mineralogy and crystallography of some Itokawa particles returned by the Hayabusa asteroidal sample return Mission.
Earth, Planets and Space , 66:82
Link to Article [doi:10.1186/1880-5981-66-82]

^1,2 Sebastian M. Stammler, ²Cornelis P. Dullemond
¹ Heidelberg University, Center for Astronomy, Institute of Theoretical Astrophysics, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
² Member of the International Max Planck Research School for Astronomy and Cosmic Physics at the Heidelberg University

In recent years many models of chondrule formation have been proposed. One of those models is the processing of dust in shock waves in protoplanetary disks. In this model, the dust and the chondrule precursors are overrun by shock waves, which heat them up by frictional heating and thermal exchange with the gas.
In this paper we reanalyze the nebular shock model of chondrule formation and focus on the downstream boundary condition. We show that for large-scale plane-parallel chondrule-melting shocks the postshock equilibrium temperature is too high to avoid volatile loss. Even if we include radiative cooling in lateral directions out of the disk plane into our model (thereby breaking strict plane-parallel geometry) we find that for a realistic vertical extent of the solar nebula disk the temperature decline is not fast enough. On the other hand, if we assume that the shock is entirely optically thin so that particles can radiate freely, the cooling rates are too high to produce the observed chondrules textures. Global nebular shocks are therefore problematic as the primary sources of chondrules.

Reference
Stammler SM, Dullemond CP (2014) A critical analysis of shock models for chondrule Formation. Icarus(in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.07.024]

Copyright Elsevier

^1,2Kun Wang, ^1,3,4Paul S. Savage, ^1,4Frédéric Moynier

¹ Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St Louis, One Brookings Drive, St. Louis, MO 63130, USA
² Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
³ Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom
⁴ Institut de Physique du Globe de Paris, Institut Universitaire de France, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238, Paris Cedex 05, France

Despite their unusual chemical composition, it is often proposed that the enstatite chondrites represent a significant component of Earth’s building materials, based on their terrestrial similarity for numerous isotope systems. In order to investigate a possible genetic relationship between the Fe isotope composition of enstatite chondrites and the Earth, we have analyzed 22 samples from different subgroups of the enstatite meteorites, including EH and EL chondrites, aubrites (main group and Shallowater) and the Happy Canyon impact melt. We have also analyzed the Fe isotopic compositions of separated (magnetic and non-magnetic) phases from both enstatite chondrites and achondrites.

On average, EH3-5 chondrites (δ56Fe = 0.003 ±0.042‰; 2 standard deviation; n=9; including previous literature data) as well as EL3 chondrites (δ56Fe = 0.030 ±0.038‰; 2SD; n=2) have identical and homogeneous Fe isotopic compositions, indistinguishable from those of the carbonaceous chondrites and average terrestrial peridotite. In contrast, EL6 chondrites display a larger range of isotopic compositions (−0.180‰ < δ56Fe < 0.181‰; n=11), a result of mixing between isotopically distinct mineral phases (metal, sulfide and silicate). The large Fe isotopic heterogeneity of EL6 is best explained by chemical/mineralogical fragmentation and brecciation during the complex impact history of the EL parent body.

Enstatite achondrites (aubrites) also exhibit a relatively large range of Fe isotope compositions: all main group aubrites are enriched in the light Fe isotopes (δ56Fe = −0.170 ±0.189‰; 2SD; n=6), while Shallowater is, isotopically, relatively heavy (δ56Fe = 0.045 ±0.101‰; 2SD; n=4; number of chips). We take this variation to suggest that the main group aubrite parent body formed a discreet heavy Fe isotope-enriched core, whilst the Shallowater meteorite is most likely from a different parent body where core and silicate material remixed. This could be due to intensive impact-induced shearing stress, or the ultimate destruction of the Shallowater parent body.

Analysis of separated enstatite meteorite mineral phases show that the magnetic phase (Fe metal) is systematically enriched in the heavier Fe isotopes when compared to non-magnetic phases (Fe hosted in troilite), which agrees with previous experimental observations and theoretical calculations. The difference between magnetic and non-magnetic phases from enstatite achondrites provides an equilibrium metal-sulfide Fe isotopic fractionation factor of Δ56Femetal-troilite = δ56Femetal − δ56Fetroilite of 0.129 ±0.060‰ (2SE) at 1060 ±80K, which confirms the predictions of previous theoretical calculations.

Reference
Wang K, Savage PS, Moynier F (2014) The iron isotope composition of enstatite meteorites: Implications for their origin and the metal/sulfide Fe isotopic fractionation factor. Geochimica et Cosmochimica Acta (In Press).

Link to Article [DOI: 10.1016/j.gca.2014.07.019]

Copyright Elsevier