Geochemistry of

Suevitic polymict from the 1 2 3 Bosumtwi 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 , 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 ; 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 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 reached between available sample mass and what consti- 1090 Vienna, , 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 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 , 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 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 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 have low Figure 1 2 impact melt rocks from craters of different diameters siderophile element contents, it is possible to measure mete- in comparison with the compositions of bulk and upper continental oritic contributions down to 0.1% using the PGEs (Huber crust (data mostly from Dressler and Reimold 2001; Hecht et al. et al. 2001; Simonson et al. 2009). Distinctly higher sidero- 2008a, b; and references therein). With some exceptions there is a phile element contents in impact melts, compared to target- general trend from smaller towards larger craters with their impact melts changing from more upper crust (UCC) towards bulk crust rock abundances, can be indicative of the presence of either composition (CC). Il = Ilyinets, BR = Brent, HB = Henbury, RK = a chondritic or an iron projectile. Achondritic projectiles Roter Kamm, EG = El’gygytgyn, WP = Wanapitei, RS = Ries, PG = are much more difficult to discern because they have Popigai, MC = Manicouagan, MK = Morokweng, MS = Mistastin, significantly lower abundances of the key siderophile SIC = average of Sudbury Igneous Complex, SIC-G = average of SIC elements, and it is necessary to sample all possible target granophyre, SIC-N = average of SIC norite, SIC-QD=average of rocks to determine the so-called indigenous component quartz diorite dikes at Sudbury, VF = Vredefort, CX = Chicxulub, (i.e. the siderophile element content of the impact melt IM-avg = average of impact melt rocks rocks contributed by the target) and thus ascertain that no

Elements 38 February 2012 possibly siderophile element–rich mantle-derived target al. (2006), who discovered a 25 cm diameter fragment of rock has remained undetected. So far, meteoritic compo- an ordinary chondrite (LL type) in the Morokweng melt nents have been identified in about 45 out of the ca 180 sheet (Fig. 2a) that confirmed an earlier projectile charac- currently known impact structures on Earth (cf Koeberl terization by McDonald et al. (2001) based entirely on 2007). geochemistry. A detailed study of the abundances of siderophile elements The best-documented, impact-induced, geochemical signa- in the various target rocks is necessary so that mixing ture is the large positive iridium anomaly in the clay layer calculations can be used to constrain the relative propor- marking the Cretaceous–Tertiary (K–T, now called tions of the target rock types involved in the production Cretaceous–Paleogene, K–Pg) boundary. Today more than of a breccia or a melt–rock mixture. From this the indig- 120 K–Pg boundary locations worldwide displaying the enous concentrations can be determined and subtracted famous iridium anomaly have been found—from deep from the abundances found in the impact melt, thereby oceanic settings to continental lacustrine environments yielding “pure” meteoritic abundance ratios. In reality, it (Schulte et al. 2010). Iridium concentrations range from a is difficult to identify all target rocks involved in forming few hundred picograms/gram to almost 100 nanograms/ impact melt rocks or breccia (for example, because of the gram. There is no real concentration trend, except for a loss of some rock types due to erosion or because of lack dilution factor at very proximal K–Pg sites around the Gulf of exposure). Such identification can also be made difficult of Mexico. This dilution resulted from the shear volume by very low or highly variable indigenous siderophile of sediment and the high-energy deposition associated concentrations. with the formation of the . The reason why, in some cases, only the Ir concentrations were The PGEs are probably the most valuable elements for measured was that, until recently, the contents of this projectile characterization. Their variable abundances in element could be determined to lower concentrations and different meteorite groups allow the fingerprinting of with more ease than any of the other PGEs. Thus, Ir acts impactors (e.g. Palme 2008). When the contents of indi- as a marker for the other PGEs, which, if an Ir anomaly is vidual PGEs are plotted against each other (for example, found, may then also be determined in a small subset of as in figure 2a, using data from the Morokweng impact samples. In general, however, isolated Ir data, without structure, South Africa), the trend follows a mixing line complementary petrographic and geochemical informa- between the composition of the average target rocks (a few tion, are difficult to interpret and thus practically useless. parts per billion at Morokweng) and the impactor (whose composition is considered to be similar to that of a chon- At the K–Pg boundary, the concentrations of all the PGEs drite clast found in the Morokweng melt rock (Fig. 2a). The are significantly enriched compared to background. At slope of the mixing line, determined by regression analysis, most sites, the enrichment displays a sharp increase at the represents the PGE ratio (e.g. Ru/Ir, Pt/Ir, etc.) of the boundary, followed upwards by a progressive decrease in impactor, without the need to make a correction for indig- the lowermost Paleocene rocks. The shape of the peak is enous PGEs (McDonald et al. 2001; Tagle and Hecht 2006). conditioned by local sedimentological factors, such as the PGE ratios and associated uncertainties can then be intensity of bioturbation and diagenesis, the type of sedi- compared with those of different types of chondrites to ments, the chemistry of percolating fluids, and possible determine the most likely impactor. Figure 2b shows exam- reworking. In general, Ir, Ru, and Rh are the least remobi- ples of this for impact melt rocks at the Morokweng (80 km lized elements. The abundances of Ni, Cr, and Co also diameter, 145 Ma) and Popigai, Russia (100 km, 35 Ma) increase in the K–Pg boundary layer; however, their impact structures and early Archean impact spherules in concentrations are less diagnostic than those of the highly the Barberton greenstone belt of South Africa. The validity siderophile PGEs. of the regression technique was demonstrated by Maier et

A

490 B 480 Chondrite clastt 0.40 CI 470 CM 0.36 ≈ CV 70 H

60 0.32 LL L 50

Slope (Pt/Ir) = 2.08 ± 0.15 Rh/Ir 0.28 EH Pt (ppb) 40 EL 30 0.24 Morokweng Popigai 20 0.20 Barberton S3 10 3.6 4.0 4.4 4.8 5.2 5.6 6.0 Intercept = 2.4 ± 2.2 ppb Pt 0 ≈ Ru/Rh 0 10 20 30 225 230 235 Ir (ppb) (B) Rh/Ir versus Ru/Rh ratio plot of the Morokweng, rocks, and the Barberton S3 impact spherule layer, compared with (A) Regression between Pt and Ir contents at those of various chondrites. The PGE ratios were determined by Figure 2 Morokweng, South Africa (McDonald et al. 2001), regression (data from Reimold et al. 2000; McDonald et al. 2001; recalculated using ISOPLOT 3.0. A clast of LL ordinary chondrite Tagle and Berlin 2008; and references therein). Morokweng and discovered in the Morokweng M3 borehole (Maier et al. 2006) Popigai formed from the impact of ordinary chondrites, whereas plots close to the regression line projected through the impact the S3 layer involved a carbonaceous chondrite. Error bars are melt rocks. two sigma.

Elements 39 February 2012 Osmium Isotopes So far, the Cr isotope method has been used to confirm that several mid-size to large impact structures and Late The isotope 187Os (one of seven stable isotopes of Os) forms Archean impact spherule deposits were formed by ordinary by ß--decay of 187Re (half-life = 42.3 ± 1.3 billion years). chondrite projectiles (Koeberl et al. 2007; Simonson et al. Meteorites have Os concentrations several orders of magni- 2009). In addition, Cr isotope data from a globally distrib- tude higher than terrestrial crustal rocks and Re/Os ratios uted late Eocene impact layer (likely related to the Popigai less than or equal to 0.1 (e.g. Carlson et al. 2008). In impact structure, Russia) also indicate an ordinary chon- contrast, the Re/Os ratio of terrestrial crustal rocks (which drite projectile (Kyte et al. 2011). In contrast, for some have much lower Re and Os abundances) is usually greater much larger impact events, such as Chicxulub, Mexico (i.e. than 10. As is the practice for conventional isotope systems, the K–Pg boundary), and some early Archean spherule the abundance of the radiogenic isotope 187Os is normal- layers in South Africa and Australia, carbonaceous chon- ized to the abundance of a nonradiogenic isotope (188Os). drite impactors are indicated. The Cr isotope method has As a result of the high Re and low Os concentrations in revealed that about 80% of the Cr in samples from the old crustal rocks, their 187Os/188Os ratio increases rapidly K–Pg boundary originated from a CM2 carbonaceous chon- with time (average upper-crustal 187Os/188Os = 1–1.2). In drite projectile (Trinquier et al. 2006; Quitté et al. 2007). contrast, meteorites have low 187Os/188Os ratios of about 0.11 to 0.18, and, due to the low Re/Os ratio, only small Despite its selectivity, the Cr isotope method is complicated changes in the meteoritic 187Os/188Os ratio occur with time. and time-consuming, and a significant proportion of the Cr in an impactite, compared to the abundance in the Due to the relatively high meteoritic Os abundances, the target, has to be of extraterrestrial origin for it to be addition of even a small amount of meteoritic matter to detected. The detection limit is a function of the Cr content the crustal target rocks can lead to a significant change in in the target rocks involved in the formation of the impact the Os isotope signature of the resulting impactites. The breccias or melt rocks. For example, with a Cr concentra- addition of achondritic meteoritic matter requires a much tion in the target rocks of ~185 ppm (the average Cr concen- higher percentage of meteoritic contribution due to the tration in the bulk continental crust), an extraterrestrial much lower PGE abundances in achondrites compared to component in the impactite of more than 1.2 wt% can be chondritic and iron meteorites. A caveat is the present-day detected (Fig. 3). Nevertheless, the Cr isotope method holds 187Os/188Os ratio of mantle rocks of about 0.13, which is great potential for further projectile identifications at similar to meteoritic values; however, PGE abundances in impact structures where abundance data yield ambiguous typical mantle rocks are at least two orders of magnitude results. lower than those in chondritic and iron meteorites. Due to these differences in abundance, a mantle contribution Tungsten Isotopes needs to be about one hundred times larger than a mete- oritic component, and such a significant mantle compo- Another isotope recently proposed as a tracer for meteoritic 182 nent would be easily discernable petrographically and components in terrestrial material is W. This isotope was 182 geochemically. Compared to PGE elemental abundances produced by the decay of now-extinct Hf (half-life = 8.9 and ratios, the Os isotope method is superior with respect million years). Meteorites and the terrestrial crust have 182 to detection limits and selectivity. Several impact craters distinct W isotope compositions. W has been used to have been confirmed using the Os isotope method, which identify the impactor at the K–Pg boundary by analyzing also revealed a clear extraterrestrial signal at the K–Pg the sediments and Ni-rich spinels (cf Quitté et al. 2007). boundary (references in Koeberl 2007). Moynier et al. (2009) analyzed W isotopes in a variety of well-defined impactites and ejecta from four different Chromium Isotopes impact structures and two K–Pg boundary-layer locations, but in all these samples, the isotopic composition of W is Identifying an extraterrestrial component in impactites identical, within analytical error, to that of the Earth’s requires meticulous geochemical analyses; moreover, continental crust, and no 182W anomalies are present, even element abundances can, in some cases, yield ambiguous in the samples containing a significant (percent-level) results. The Os and Cr isotope methods potentially remove meteoritic component. Thus, earlier suggestions that W much of this ambiguity, and the Cr isotope method, in isotope analyses indicate a meteoritic component in early particular, can help identify the type of meteorite involved Archean (3.8 Ga) metasedimentary rocks are unfounded, in an impact event. The method is based on the determina- and W isotopes are not suitable for the identification of tion of the relative abundances of 53Cr, which is the meteoritic components in terrestrial rocks (Fig. 3). daughter product of the extinct radionuclide 53Mn (half-life = 3.7 million years). 53Cr relative abundances are measured as the deviations of the 53Cr/52Cr ratio in a sample relative STABLE AND OTHER ISOTOPES 53 52 to the standard terrestrial Cr/ Cr ratio. This is done using Stable Isotopes high-precision thermal ionization mass spectrometry, and the results are given in e units (1 e = 1 part in 104, or 0.01%). The most commonly measured stable isotopes in impact- Terrestrial rocks do not show any variation in the 53Cr/52Cr related materials are those of carbon, oxygen, and sulfur. ratio, because differentiation of the Earth was completed Such analyses have three main goals. (1) If the isotopic long after all 53Mn had decayed. In contrast, data for most compositions of impactites themselves are measured, meteorite groups, such as carbonaceous, ordinary, and source rocks can be determined and alteration processes enstatite chondrites, primitive achondrites, and other can be studied. For example, oxygen isotope measurements differentiated meteorites, show a variable excess of 53Cr have shed light on the sources of . (2) The geochem- relative to terrestrial samples. For the various meteorite istry of carbon in impactites can best be determined from types, the range is about +0.1 to +1.3 e. Only the carbona- isotopic studies of carbon components, such as impact- ceous chondrites show a deficit in 53Cr, of about –0.4 e. derived diamonds (e.g. Gilmour 1998). (3) Lithological or The presence of 54Cr excesses in bulk carbonaceous chon- paleoenvironmental effects of impact events in the geolog- drites distinguishes these meteorites clearly from the other ical record can be studied. In addition, some so-called meteorite classes. “nonconventional stable isotopes” have been analyzed in tektites, which are glasses thought to have formed very early in the impact process and been subjected to short-

Elements 40 February 2012 1.1 source rocks for tektites. 10Be has a short half-life (1.4 million 187 188 typical ε53Cr 2σ Os/ Os years) and thus can be studied only in correspondingly 53 typical Ir/Pd 2σ ε Cr young impact structures and glasses. The average content 182 Ir/Pd 0.9 typical ε W 2σ of 10Be in Australasian tektites is comparable to those ε182 W measured in near-surface source materials, such as soils (terrestrial) or sediments (marine and terrestrial), and 0.7 within the Australasian there is a correlation between type and 10Be concentration (Ma et al. 0.5 2004). Aerodynamically shaped tektites found distally from the presumed impact site in Southeast Asia have higher 10Be contents than more proximal, layered (Muong Nong– 0.3 type) tektites. This agrees with the Cu and Zn isotope frac-

Isotope Ratio tionation pattern for tektites, which shows that the greater the distance from the impact site, the greater is the frac- 0.1 tionation of the isotope systems (Moynier et al. 2010). Serefiddin et al. (2007) found 10Be in Ivory Coast tektites -0.1 and in the much older Central European tektites, confirming that tektites are early, high-temperature and high-velocity, distal impact ejecta that formed before the -0.3 main crater was excavated. 0 1 2 3 4 5 6 7 8 9 10 Meteoritic fraction (%) Other Geochemical Indicators Other geochemical indicators, which cannot be discussed Figure 3 Comparison of the detection limits of the Cr, Os, and here in detail, include the Ni and Cr spinels whose compo- W isotope and PGE (as Ir/Pd ratio) methods as tracers sitions are unique indicators of an extraterrestrial origin. of meteoritic components in terrestrial impact rocks for a carbona- ceous chondritic component. Each curve represents the evolution Studies of such indicators contributed significantly to the of either the element ratio (Ir/Pd) or the isotopic composition interpretation that during the Middle an (ε53Cr, ε182W, and 187Os/188Os) of the impactite as a function of the L-chondrite parent body was disrupted, leading to a fraction of meteoritic component. Typical error bars for the dramatically enhanced flux of extraterrestrial matter to different systems are plotted directly onto the graph, with the exception of the Os isotopes, where the error is within the thickness Earth (Schmitz et al. 2003). Extraterrestrial grains of the curve (after Moynier et al. 2009). dispersed in mid-Ordovician sediments contain abundant neon implanted by solar wind; this demonstrates that such chromite grains originated from micrometeorites that decomposed on the seafloor (Heck et al. 2008). lived, very-high-temperature conditions. Because volatiliza- Caveats tion can potentially fractionate isotopes, comparing the Geochemical data have at times been used in inappropriate isotopic compositions of volatile elements in tektites with ways, have not been properly verified, or have been over- those of their source rocks may improve understanding of interpreted. Claims of the presumed impact origin of some the physical conditions during tektite formation. spherule beds had to be rejected once appropriate chemical Interestingly, volatile chalcophile elements (e.g. Cu, Zn, data were obtained. The claimed presence of supposedly and Cd) are the only elements for which isotopic fraction- extraterrestrial fullerene molecules in impactites (e.g. at ation has been established in tektites (e.g. Moynier et al. Sudbury or at the Permo-Triassic boundary) has never been 2010). independently confirmed. Diamonds of supposed meteor- itic origin and allegedly related to the Younger Dryas event Helium-3 were found to be carbon forms of terrestrial origin. The helium-3 isotope is extremely rare in terrestrial crustal Suggestions that geochemical evidence (e.g. W isotopes, Ir rocks, but it is relatively abundant in interplanetary dust abundances) exists for the were particles because of reactions with cosmic rays. A signifi- based on incomplete data taken out of context. Similarly 3 cant enrichment of He was found in two Late Eocene to other aspects of impact studies, geochemistry is vulner- impactoclastic layers that are correlated with the Popigai able to overinterpretation and wishful thinking. It is and Chesapeake Bay impact events, which also had a imperative that data be carefully obtained and verified, 3 climatic influence. As He is a proxy for the influx of extra- using independent methods and multiple laboratories, and 3 terrestrial dust, the late Eocene He enrichment was inter- that they be calibrated with the appropriate methods and preted to indicate a time of enhanced dust activity in the standard reference materials (French and Koeberl 2010). inner Solar System; this dust was due to an increased flux of comets, which probably resulted in a higher impact rate CONCLUSIONS than usual. More recently a 3He signal in deep-sea sedi- ments was correlated with an asteroidal breakup event in Geochemistry is an extremely versatile tool for studying the mid-Miocene; this may resolve the discrepancy with impact events. Studies can range from simple major and other geochemical proxies (Cr isotopes, PGEs) that suggest trace element characterization of impact breccias, melt Eocene impactors were asteroids by indicating that particles rocks, glasses, and target rocks to elaborate isotope inves- derived from both cometary and asteroidal sources carry tigations and oxidation state determinations. Geochemistry a 3He signal (see Farley 2009 and references therein). can also be used to search for extraterrestrial components and to identify projectiles in impactites and ejecta, or to Beryllium-10 determine noble gas abundances in minute minerals and The cosmogenic radionuclide 10Be forms by the interaction the clay mineral composition of fracture fillings in impact of cosmic rays with nitrogen in the atmosphere and is breccias. Geochemical analyses are of crucial importance concentrated at the top of any sediment column. 10Be can for establishing the impact origin of suspicious geological be used to constrain the location and characteristics of the structures or stratigraphic units, and they contribute invaluable information about every part of the impact

Elements 41 February 2012 process. From nano- and microscale features to global ACKNOWLEDGMENTS processes, our understanding of impact as a geological We thank reviewers B. Schmitz and A. Wittmann for phenomenon would be incomplete and lack quantification helpful comments. without geochemical data.

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