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Asteroid (4) Vesta: I. the Howardite-Eucrite-Diogenite (HED) Clan of Meteorites

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Asteroid (4) Vesta: I. the Howardite-Eucrite-Diogenite (HED) Clan of Meteorites Chemie der Erde 75 (2015) 155–183 Contents lists available at ScienceDirect Chemie der Erde j ournal homepage: www.elsevier.de/chemer Invited Review Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED) clan of meteorites ∗ David W. Mittlefehldt XI3/Astromaterials Research Office, Astromaterials Research and Exploration Sciences Division, NASA/Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA a r a t b i c s t l e i n f o r a c t Article history: The howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocks Received 23 November 2013 and impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-based Accepted 21 August 2014 telescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compelling Editorial handling – K. Keil case that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted. Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addi- Keywords: tion to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites are Howardites gabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth within Eucrites Diogenites the crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills, Vesta and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites, Basaltic achondrites and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiation Differentiated asteroids occurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxy- gen isotope compositions and siderophile element contents favor a model of extensive melting of Vesta forming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield a metallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneous lithologies were subjected to post-crystallization thermal processing, and most eucrites show textural and mineral-compositional evidence for metamorphism. The cause of this common metamorphism is unclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusion rates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts, and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack of mixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymict breccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbers of meteorites, compositions of howardites, and models of magma ocean solidification, cumulate eucrites and their residual ferroan mafic melts are minor components of the vestan crust. © 2014 Published by Elsevier GmbH. Contents 1. Introduction . 156 2. Lithologies . 156 3. Mineralogy and petrology. 157 3.1. Diogenites. 157 3.2. Cumulate eucrites . 162 3.3. Basaltic eucrites. 164 3.4. Petrologically anomalous eucrites . 165 3.5. HED polymict breccias . 166 ∗ Tel.: +1 281 483 5043; fax: +1 281 483 1573. E-mail address: [email protected] http://dx.doi.org/10.1016/j.chemer.2014.08.002 0009-2819/© 2014 Published by Elsevier GmbH. 156 D.W. Mittlefehldt / Chemie der Erde 75 (2015) 155–183 4. HED compositions . 167 4.1. Oxygen isotopic composition. 167 4.2. Lithophile element compositions . 169 4.3. Siderophile element compositions . 171 4.4. Noble gas contents. 172 5. HED ages. 172 6. HED meteorite petrogenesis . 174 7. Thermal metamorphism of the HED parent body crust . 175 8. Fluid-mediated metasomatism? . 176 9. Mixing of the vestan crust . 176 10. Ungrouped basaltic achondrites . 178 11. The future . 178 Acknowledgments . 179 Appendix A. Supplementary data . 179 References . 179 1. Introduction geological, mineralogical, compositional and geophysical informa- tion on Vesta. The returned data are consistent with the identity Meteorites of the howardite, eucrite and diogenite (HED) clan of Vesta as the HED parent asteroid (McSween et al., 2013). Never- make up the largest suite of crustal igneous rocks available for study theless, there continues to be a minority view that Vesta is not the from any solar system body, baring the Earth and Moon. As of July HED parent. Wasson (2013) contends that the numerous V-type 2014, there were over 1450 named HED meteorites with 270 new asteroids in the inner main belt are debris from the disruption of HED meteorites announced during the previous year. Because most several differentiated asteroids, and that these are more probable are from collection fields in Antarctica and northern Africa, pairings as sources for HEDS than debris knocked off Vesta by the impact greatly reduces the number of individual fall events represented. that formed the Rheasilvia basin. He argues that the isotopic simi- HEDs include basalts, cumulate gabbros, norites, orthopyroxenites, larity of IIIAB irons and HEDs indicates that they were derived from harzburgites and rare dunites, plus brecciated mixtures of these the same asteroid, and that asteroid must have been completely igneous lithologies. The HED clan.
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