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Geochemistry of Impactites

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Geochemistry of Impactites Geochemistry of Impactites Suevitic polymict breccia from the 1 2 3 Bosumtwi impact crater Christian Koeberl , Philippe Claeys , Lutz Hecht , in Ghana, showing a and Iain McDonald4 variety of rock fragments; the foamy 1811-5209/12/0008-0037$2.50 DOI: 10.2113/gselements.8.1.37 inclusions are impact glass that may carry the geochemical signature eochemical analysis is an essential tool for the confirmation and study of the impactor. Sample is ca 10 cm wide. of impact structures and the characterization of the various rock types Ginvolved (target rocks, impact breccias, melt rocks, etc.). Concentrations and interelement ratios of the platinum-group elements, as well as the proximal ejecta, melt rocks, and osmium and chromium isotope systems, allow quantification of extraterres- pseudotachylitic breccias, the trial components and the identification of impactor types in impact deposits. latter being dikes of melt rock at In addition, chemolithostratigraphy can reveal the possible role of impacts the bottom of an impact structure; we also consider a few examples of in environmental change throughout the geologic record. This article deals distal ejecta. Impact processes predominantly with terrestrial impact structures. produce brecciation, shock meta- morphism, and melting and Keywords: impacts, ejecta, geochemistry, platinum-group elements, vaporization of the target rocks. chromium isotopes The chemical composition of impactites provides important INTRODUCTION information that supplements petrological data. It depends The geochemistry and cosmochemistry of impact craters on (1) the composition and spatial distribution of the target and impact processes constitute a rapidly developing lithologies; (2) impact energy, which affects the size of the research area encompassing such wide-ranging topics as crater, the depth of material involved, and the volume of the chemical characterization of rock types; the formation, rocks vaporized or melted; (3) the emplacement and emplacement, and differentiation of impactites as revealed cooling history of impactites; (4) the admixture of projec- by petrologic studies; the identification of extraterrestrial tile material; and (5) post-impact modifications by meta- components in impact ejecta and crater fills; the derivation morphism and/or hydrous alteration (including of the impactor (projectile) composition; and the determi- weathering). nation of the causes of environmental change from anal- yses of samples in the stratigraphic record. One of the most The chemical compositions of rock types at a crater or important roles of geochemistry in impact studies is the distal sites can also be used to determine whether any confirmation of the impact origin for terrestrial structures unusual or extraneous (noncrustal) components are present (see French and Koeberl 2010). If an impact structure is and to determine the origin of impact glasses. Once all buried, drill core samples are essential. Breccias and melt rock types present at a particular impact site are analyzed, rocks often carry unambiguous evidence for the impact mixing calculations allow the reconstruction of the propor- origin of a structure, such as the presence of shocked tions of the different rock types that combined to form mineral and lithic clasts or contamination from the extra- breccias or melt rocks. Such data have been used to estab- terrestrial projectile (for more details see Koeberl 2007). lish scaling relationships for the impact process (for example, the geometry of melt zones, the volume of melt produced, and the sizes of craters). Also, these calculations FROM CHEMISTRY TO PETROLOGY are important for defining the indigenous contents of General Chemistry of Impactites siderophile elements in breccias and melt rocks, which are essential for establishing the extent of contamination by The term impactite comprises a large variety of rocks formed extraterrestrial components. by the modification of crustal rocks due to impact processes. Here we focus on the chemistry of impactites that have In geochemical work, proper sampling and sample prepara- been formed or deposited at or close to the crater (i.e. tion are crucial and depend on the analyses to be done. Each preparation and treatment step increases the chance of contamination or loss, and a compromise must be 1 Department of Lithospheric Research, University of Vienna reached between available sample mass and what consti- 1090 Vienna, Austria, and Natural History Museum tutes a representative sample (details in chapter 6 of 1010 Vienna, Austria Montanari and Koeberl 2000). In the study of impacto- E-mail: [email protected] clastic layers (distal ejecta), this problem is even more 2 Earth Systems Science, Vrije Universiteit Brussel severe, because of the low abundance of impact-derived Pleinlaan 2, 1050 Brussels, Belgium debris within a large amount of local matrix. 3 Natural History Museum Berlin Invalidenstrasse 43, 10115 Berlin, Germany 4 School of Earth & Ocean Sciences, Cardiff University Cardiff, CF10 3AT, UK ElEmEnts, Vol. 8, pp. 37–42 37 February 2012 Differentiation and Emplacement obvious mantle component, as was later confirmed by Os of Impact Melt isotope studies. Early, rapidly cooled dikes of impact melt that were emplaced into the crater floor may preserve the The composition of the resulting bulk impact melt depends composition of the initial impact melt at Sudbury (Hecht on the efficiency of mixing between the individual coex- et al. 2008a) and at the Vredefort impact structure isting melts. In general, the resulting impact melt is homo- (South Africa). geneous at hand-specimen scale (Dressler and Reimold 2001), indicating that melt mixing during impact dynamics The heterogeneity of impact glasses is increased by incom- or during post-impact convection within thicker melt plete melting, incomplete assimilation of rock or mineral sheets is very efficient. In the case of the 200 km wide, fragments, and rapid cooling. These factors are mostly 1.86-billion-year-old Sudbury impact structure in Canada, relevant for melt clasts in suevite and for impact melt however, Zieg and Marsh (2005) proposed that superheated bodies in small craters where a melt pool large enough to melts derived from siliceous and mafic target rocks were allow homogenization has not developed. Rapid disequi- so different in density and viscosity that they did not mix librium crystallization in quenched impact melt may also but remained separate, forming a layered melt sheet (sili- induce small-scale heterogeneity (Hecht et al. 2008b). ceous top layer and mafic bottom layer). The Sudbury impact melt sheet was differentiated, resulting in the Post-impact Modifications of Impactites formation of a crudely layered complex comprising grano- Metamorphism and/or hydrothermal alteration are facili- phyre and norite layers (Fig. 1), along with major Ni–Cu– tated by the porosity of impact breccias and the sensitivity PGE deposits. Fractional crystallization and host-rock of impact glass to alteration. Post-impact metamorphism assimilation were involved in the differentiation of the causes recrystallization but does not necessarily change impact melt sheet and were probably enhanced by its the composition of impactites. Impact-induced heating can cooling history, leading to significant post-impact hetero- produce hydrothermal systems that may significantly geneity of the melt body. modify the composition of impactites (Naumov 2002). The isotope systems Rb–Sr and Sm–Nd can be used to date impact events, and also to confirm that impact melt rocks Need for Future Work were derived from near-surface crustal rocks and not from Our understanding of processes such as the formation and the deep crust or mantle. For example, the Nd isotope emplacement of impact melt and the mechanisms of composition of Sudbury melt rocks shows that the target mixing between impactites and meteoritic material is still rocks were predominantly crustal rocks without any incomplete. Therefore, petrological and geochemical studies of impact structures, combined with numerical K O modeling and laboratory experiments, are very important. 2 Hypervelocity impact experiments may also help to better understand the extreme, short-term dynamics of far-from- > 10 km ∅ equilibrium impact processes. < 10 km ∅ UCC PROJECTILE IDENTIFICATION II CC Siderophile Element Studies Although projectile fragments rarely survive an impact event, detectable amounts of melted and recondensed projectile are often incorporated into impact-produced BR EG breccias and melt rocks during crater formation. This SIC-G dispersed projectile (meteoritic) material can be conclu- HB RK sively identified by distinct chemical and isotopic signa- tures in the host rocks, thus providing reliable evidence WP RS for a meteorite impact event. IM-avg During impact, original projectile material is diluted by SIC PG MS mixing with a volume of vaporized, melted, and frag- CX MK mented target rock that may be orders of magnitude larger VF MC than the volume of the projectile. As a result, the actual SIC-QD amount of projectile material incorporated into impact- SIC-N crater rocks is generally small, typically <1 wt%. Siderophile elements, such as Ni, Co, and the platinum-group elements (PGEs, i.e. Pt, Pd, Os, Ru, Rh, Ir), occur at significantly MgO CaO higher concentrations in meteorites than in average crust. They also show interelement ratios distinct from those of K O–MgO–CaO plot showing average compositions of crustal rocks and mantle melts. With target that
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