Bashar Baghdadi^1,*, Gaston Godard², Albert Jambon¹

¹UPMC Paris 6, Institut des Sciences de la Terre Paris, Paris Cedex 05, France
²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris-Diderot, Paris Cedex 05, France

Northwest Africa 3164 is a coarse-grained angrite that shows reaction coronas, a unique character among achondrites. Olivine (Fo₅₇; 1.2 wt% CaO), fassaitic clinopyroxene, anorthite, and spinel account for 46–47, 28–29, 8–13, and 4–8 vol%, respectively; kamacite is an accessory phase. The spinel grains in contact with clinopyroxene are bounded by discontinuous 20 μm thick coronas of anorthite and olivine, indicating the reaction Cpx + Spl → Ol + An (R1). In addition, irregular coronas of clinopyroxene and spinel developed around the primary anorthite in contact with primary olivine, during the reaction Ol + An → Cpx + Spl (R2). R2 also generated clinopyroxene and spinel films between the secondary olivine and anorthite coronas produced during R1, implying that R1 preceded R2. Both are metamorphic reactions that developed in the solid state. Finally, the coronas are cross cut by μm-thick veinlets due to a late shock. A mass-balance study shows that R2 is almost the reverse of R1. The P–T metamorphic evolution of the rock, modeled by calculating a P–T isochemical diagram, indicates an equilibrium T of 940 ± 120 °C at P < 0.9 GPa for the initial assemblage, followed by an increase of T up to approximately 1000–1200 °C during reaction R1 and a subsequent cooling during R2. Several causes are envisaged to account for this metamorphic evolution. Contact metamorphism due to a hot magmatic intrusion in the angrite parent body is favored, as similar metamorphic coronas are well known in metamorphic terrestrial rocks. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which would have been a large asteroid or a planetary-sized body.

Reference
Baghdadi B, Godard G and Jambon A (in press) Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164. Meteoritics & Planetary Science
[doi:10.1111/maps.12202]
Published by arrangement with John Wiley & Sons

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Zhiyong Xiao^a,b,*, Robert G. Strom^b, Clark R. Chapman^c, James W. Head^d, Christian Klimczak^e, Lillian R. Ostrach^f, Jörn Helbert^g, Piero D’Incecco^g

^aPlanetary Science Institute, Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan, Hubei, 430074, China
^bLunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85719, USA
^cDepartment of Space Studies, Southwest Research Institute, Boulder, Colorado, 80302, USA
^dDepartment of Geological Sciences, Brown University, Providence, Rhode Island, 02912, USA
^eDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., 20015, USA
^fSchool of Earth and Space Exploration, Arizona State University, Arizona, USA, 85281
^gInstitute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, 12489 Berlin, Germany

The impact cratering process is usually divided into the coupling, excavation, and modification stages, where each stage is controlled by a combination of different factors. Although recognized as the main factors governing impact processes on airless bodies, the relative importance of gravity, target and projectile properties, and impact velocity in each stage is not well understood. We focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater-exterior structures for similar-sized fresh complex craters on the Moon and Mercury. We find that the ratios of continuous ejecta deposits, continuous secondaries facies, and the largest secondary craters on the continuous secondaries facies between same-sized Mercurian and lunar craters are consistent with predictions from gravity-regime crater scaling laws. Our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the Moon and Mercury. On the other hand, similar-sized craters with identical background terrains on Mercury have different spatial densities of secondaries on the continuous secondaries facies, suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities. Moreover, some craters on Mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on Mercury or the Moon. This morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on Mercury. A possible interpretation is that at some places on Mercury, the target material might have unique properties causing larger ejection angles during the impact excavation stage. We conclude that gravity is the major controlling factor on the impact excavation stage of complex craters, impact velocity and target properties may also affect the excavation stage but their importance is to a less degree compared with gravity.

Reference
Xiao Z, Strom RG, Chapman CR, Head JW, Klimczak C, Ostrach LR, Helbert J, and D’Incecco P (in press) Comparisons of fresh complex impact craters on Mercury and the Moon: Implications of controlling factors in impact excavation processes. Icarus
[doi:10.1016/j.icarus.2013.10.002]
Copyright Elsevier

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A. J. Timothy Jull

Department of Geosciences, University of Arizona, Tucson Arizona, USA

No abstract is available for this article.

Reference
Jull AJT (in press) Book Review: Meteoriten—Meteorites: Zeitzeugen der Entstehung des Sonnensystems/Witnesses of the origin of the solar system. Meteoritics & Planetary Science
[doi:10.1111/maps.12210]
Published by arrangement with John Wiley & Sons

Link to Article