An ordinary chondrite impactor for the Popigai crater, Siberia

Geochimica et Cosmochimica Acta, Vol. 69, No. 11, pp. 2877-2889, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 ⫹ .00

doi:10.1016/j.gca.2004.11.024

An ordinary chondrite impactor for the Popigai crater, Siberia ROALD TAGLE1,*,† and PHILIPPE CLAEYS2 1

2

Institut für Mineralogie, Museum für Naturkunde, D-10099 Berlin, Germany Dept. of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium (Received August 17, 2004; accepted in revised form November 30, 2004)

Abstract—With a diameter of ⬃100 km, Popigai in Northern Siberia is the largest crater known in the Cenozoic. The concentrations in platinum group elements (PGE) were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile. The method selected was preconcentration by NiS fire assay followed by inductively coupled plasma-mass spectrometry (ICP-MS). This technique measures all the PGE (except Os) and by using aliquots ⬎10g, the results are highly reproducible. The major and trace element composition of the impact melt resembles that of gneissic lithologies of the Anabar shield, which are representative of the target rock. The PGE are enriched in the melt by factors of 3 to 14 compared to the main target lithology, but the meteoritic contamination is only around 0.2 wt.%. Using plots of elemental ratios such as Ru/Rh vs. Pt/Pd or Ru/Rh vs. Pd/Ir, the Popigai impactor is clearly identified as an ordinary chondrite and most likely L-chondrite. This study indicates that PGE elemental ratios allow discrimination of the type of impactor, even in the case of low meteoritic contamination. This study confirms that a significant fraction of the crater-forming projectiles presently documented could have an ordinary chondrite composition. Their probable source, the S-type asteroids, appears to form the majority of the bodies in the main asteroid belt and among Near Earth Objects (NEOs). The ordinary chondrite origin of the Popigai projectile supports an asteroidal origin for the late Eocene impacts as a plausible alternative to the comet shower scenario proposed by Farley et al. (1998). Copyright © 2005 Elsevier Ltd 2000; Peucker-Ehrenbrink and Jahn, 2001; Horan et al., 2003). The PGE signature coupled with Ni, Co and Cr enrichments detected in impact melt formed within craters have often been used to document the type of impactor (e.g., Morgan et al., 1975, 1979; Palme et al., 1978, 1979; Palme, 1980; Schmidt et al., 1997). At most terrestrial craters, the impact melt rock contains a meteoritic contamination below 1 wt.% (e.g., Koeberl, 1998). This low contamination has frequently been an obstacle in the projectile characterization. In exceptional cases, the meteoritic enrichment reaches several wt.%. This is the situation for the Morokweng and Clearwater East craters, which were both formed by the fall of ordinary chondrites (McDonald et al., 2001; McDonald, 2002). A large number of craters show no detectable meteoritic contamination, e.g., Manicaougan, Ries, Clearwater West, Strangways (see Tagle et al., 2004, Table 5 and ref. within). Most of them are so far interpreted as produced by differentiated asteroid fragments that are poor in PGE (e.g., Morgan, 1979; Wolf et al., 1980; Morgan and Wandless, 1983). Numerical modeling of impact processes shows a relationship between the impact velocity, angle, and the amount of projectile material that remains in the crater (Pierazzo and Melosh, 1999, 2000). According to these results, a high angle impact at low speed is likely to produce greater contamination of the melt by meteoritic components. Not all types of meteorites are completely and precisely characterized in terms of their PGE compositions. This incomplete record also hampers the comparison with the PGE signature detected in impact melt rocks. In the last years, the situation has improved significantly because of new high precision analyses of all or most of the PGE in many meteorites from

1. INTRODUCTION

Approximately one hundred and sixty impact craters are recognized on Earth (Grieve, 2001). The projectiles that formed the majority of them are either unknown or their identification is problematic, except in the rare cases where meteorite fragments were found near the crater (Grieve and Shoemaker, 1994; Koeberl, 1998). Iron meteorites appear to be responsible for the majority of the craters with a diameter ⬍1.5 km. This is essentially due to their capacity to survive the passage through the atmosphere. In contrast, stony meteorites of the same sizerange are disrupted in the atmosphere and do not cause hypervelocity impacts (Palme et al., 1981; Melosh, 1989). For craters with a diameter ⬎1.5 km, the compositions of the projectiles involved are poorly constrained. Many are only identified as irons or chondrites. For several craters (e.g., Ternovka or Zhamanshin) both types of impactors are proposed (see Table 2 in Koeberl, 1998). The correct and precise identification of the crater-forming projectile is important to document the origin and frequency of the bodies that have impacted the Earth over geological time. The type of projectile can be recognized by the contamination in meteoritic elements imparted during the cratering event to the produced impact melt rock. Meteorites contain between 2 and 5 orders of magnitude higher concentrations of platinum group elements (PGE: Ir, Pd, Pt, Os, Rh, Ru) than crustal rocks (Barnes et al., 1985; Wasson and Kallemeyn, 1988; Wedepohl, 1995; Jochum, 1996; Koeberl et al., * Author to whom correspondence should be addressed. ([email protected]). † Present address: Dept. of Geology, Vrije Universiteit Brussel, Pleinlaan, 2 B-1050 Brussels, Belgium 2877

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Fig. 1. Geographical location and satellite image of the 100-km Popigai impact structure. Impact melt were collected along a large outcrop of impactites located west of the crater, GPS coordinates: 71° 45 530= N & 110° 15 171= E (see white dot on satellite image).

museum collections (McDonald et al., 2001; Friedrich et al., 2003; Horan et al., 2003; Tagle, 2004). 2. THE POPIGAI CRATER

2.1. Crater Structure and Petrography The late Eocene Popigai crater is located on the northeastern margin of the Anabar Shield in Northern Siberia (Fig. 1). With a diameter of 100 km, it is one of the largest craters on Earth (Grieve, 2001). The Popigai structure is well preserved; all impact lithologies are exposed within the crater. The Popigai impact structure has been described in detail by several authors (e.g., Masaitis, 1994; Vishnevsky and Montanari, 1999; Whitehead et al., 2002; Kettrup et al., 2003) and only a brief overview is given here. The target is mainly composed of Archean to early Proterozoic crystalline rocks covered by a northeast dipping sedimentary sequence up to 1.7 km thick. The crystalline rocks belong to the Khapshan terrain and have an estimated thickness of ⬃ 12 km (Rosen et al., 1994). The lithologies comprise different types of gneisses, occasionally intruded by lenticular bodies of mafic to ultramafic rocks up to 1 or 2 km in size. The sedimentary units are formed by Proterozoic to Cambrian conglomerates, quartzite, dolomites and limestones, Permian terrestrial sandstones and argillites, and a thin veneer of Cretaceous sandstones (Vishnevsky and Montanari, 1999). Popigai is a complex impact structure characterized by a central depression 2.0 –2.5 km deep, with no morphologic central uplift, surrounded by an annular ring and trough and an outer terrace zone (Masaitis, 1994). The crater is filled with impactites such as breccias, suevites and impact-melt rocks, reaching a thickness of ⬃ 1.5 km (Whitehead et al., 2002). Impact melt lithologies crop out predominantly to the west and southwest of the crater. The estimated total amount of preserved impact melt is ⬃1.750 km3 (Masaitis, 1994). The Popigai impact was dated at 35.7 ⫾ 0.2 Ma in the Late Eocene using the 40 Ar-39Ar step-heating technique (Bottomley et al., 1997).

2.2. Origin of the Studied Samples Eleven samples of impact melt were collected along a large outcrop of impactites located west of the crater along the Rossokhe river (GPS coordinates: 71° 45 530= N & 110° 15 171= E). The other nine impact-melt rocks and the suevite sample come from another set of outcrops 5 km further north along the river. Particular care was taken to sample fresh and unaltered material. One of the target-rock gneisses was recovered from a pile of drill cores gathered in the north of the crater. The second one was taken from an outcrop with abundant shatter cones some 5 km NNW of Popigai “village” on the bank of the Popigai river. The last one was collected along the Rassokhe river south of the large outcrop of impact melt mentioned above. 3. IMPACTOR IDENTIFICATION

3.1. Methodology Several authors have discussed the relevance of Ni, Co, Cr, and the PGE for the identification of an extraterrestrial component in impact-melt rocks (e.g., Morgan et al., 1975, 1979; Janssens et al., 1977; Palme et al., 1978; Palme et al., 1979; Palme, 1980, 1982; Koeberl, 1998). In general, the identification of the impactor was attempted by comparing the pattern and/or elemental ratios measured in impact-melt rock with similar data obtained from meteorites. In early studies, most authors made use of elements such as Ni, Cr, Co, Ir, and one or, rarely, two of the other PGE. Instrumental or radiochemical neutron activation was the most common analytical method, using small aliquots (⬃ 100 mg) of samples. The measurements of the entire group of PGE were rarely carried out, mainly because of the difficulties in detecting some of these elements at low concentrations due to short half-life, interferences, timeconsuming preparation etc. The reproducibility of the data were not always satisfactory, even between identical samples. In geological materials, micrometer-size nuggets of PGE can ac-

An ordinary chondrite impactor for the Popigai crater, Siberia

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Table 1. New PGE analyses of three ordinary chondrites (Acfer 132, Acfer 157 and Acfer 120), after Tagle (2004).

H-chondrite Acfer 132 L-chondrite Acfer 157 LL-chondrite Acfer 120

Ir [ng/g]

Ru [ng/g]

Rh [ng/g]

Pt [ng/g]

Pd [ng/g]

Au [ng/g]

418 385 202

618 563 266

125 122 61

864 820 398

476 514 193

91 84 33

count for a significant proportion of the PGE concentrations in the sample (Hall and Pelchat, 1994; Plessen and Erzinger, 1998). With small-size samples, the uneven distribution of these PGE-rich nuggets can in some cases lead to broad variation of PGE concentrations between aliquots of the same material. Under such conditions, chondrites can be distinguished from iron meteorites based on the Cr/Ir ratio, but the type of chondrite can rarely be determined. By combining INAA or ICP-MS analyses with NiS fire assay preconcentration, the measurement of all (or most of) the PGE, at once, becomes possible, providing a higher number of criteria with which to refine the impactor identification (Schmidt et al., 1997; McDonald et al., 2001; Tagle, 2004). More recently, analysis of Cr-isotope ratios (53Cr/52Cr) has contributed significantly to the recognition of impactors, in particular in the case of the Chicxulub and Morokweng craters (Shukolyukov and Lugmair, 1998; Shukolyukov et al., 1999). This method is discussed in detail by Lugmair and Shukolyukov (1998) and Shukolyukov and Lugmair (1998). Based on the different Cr isotope compositions of solar system bodies, it is possible to distinguish carbonaceous from ordinary or enstatite chondrites. This method works best in cases of high meteoritic contamination because of the relatively elevated background concentration of Cr in some terrestrial rocks (Koeberl et al., 2002). A database of PGE, Au, Cr, Co, and Ni concentrations in meteorites is being developed based on recent analytical results and an extensive compilation of the literature (Tagle, 2004). In the framework of this study, three ordinary chondrites, Acfer 132 (H-chondrite), Acfer 157 (L-chondrite) and Acfer 120 (LL-chondrite) (Grady, 2000) were analyzed to supplement the existing literature data (Table 1). Reduction and discrimination of the analyses included in the database was carried out to establish characteristic elemental ratios for the various groups of meteorites. The database revealed that most of the different groups of chondrites display distinctive PGE elemental ratios (Tagle, 2004). These ratios are in good agreement with those obtained on the 3 ordinary chondrites analyzed in this study. The combination and comparison of PGE elemental ratios precisely link a projectile with a specific chondrite group. This level of detail is more difficult to achieve in the case of differentiated meteorites. Meteorites originating from differentiated asteroids have compositions reflecting this differentiation processes. The disruption of a single differentiated asteroid can therefore produce both iron meteorites and achondrites (e.g., Palme, 1988; Burbine et al., 2003). 3.2. Analytical Techniques 3.2.1. PGE analyses The twenty Popigai impact melt fragments, three target rock gneisses and one suevite sample were analyzed for PGE, major,

and trace element concentrations. The samples were first cut into slices (⬃ 0.5 cm), then broken down by hand in a corundum mortar, and then finely powdered in a corundum ball mill. No detectable PGE contamination is added to the samples as a result of cutting, breaking down, or crushing the sample (Tagle, 2004). Several fractions of the same blank, processed through every step independently, do not show a detectable enrichment in PGE. The PGE and Au were analyzed using NiS fire assay combined with ICP-MS at the GeoForschungsZentrum (GFZ) in Potsdam, according to the method described by Plessen and Erzinger (1998). The quantitation limits for the PGE and Au determinations are 0.06 ng/g Ru, 0.01 ng/g Rh, 0.14 ng/g Pd, 0.06 ng/g Ir, 0.1 ng/g Pt, and 0.13 ng/g Au (Tagle, 2004). The meteoritic database indicates that Rh proves itself more diagnostic for distinguishing the different types of chondrites than Os. As a consequence, external calibration solutions, which allow to measure Rh, were used instead of isotope dilution that is required to detect Os. Since OsO4 volatilizes during sample treatment, it can only be determined quantitatively by adding an isotope spike, which was not done in this study. Previous work has showed that the PGE are often not uniformly distributed in geological materials (Hall and Pelchat, 1994; Plessen and Erzinger, 1998). The so-called “nugget effect” can account for a significant variation of PGE concentrations between several aliquots of the same sample. Representative and reproductive analyses can be obtained by taking large amounts of homogenized samples (10 –100 g) to average out this nugget effect (Hall and Bonham-Carter, 1988; McDonald 1998; Ely and Neal, 2002). The weight of the samples selected for this study varied between 10 and 50 g. The results of multiple analyses of the same sample of Popigai impact melt using aliquots of 2, 5 and 10 g revealed that the minimum amount required to obtain reproducible results was 10 g. With a lesser quantity, the values obtained by repetitive analyses are not reproducible. Using 10 g or more in identical analytical conditions, the results of several analyses of the same sample differed by less than 10%. To avoid the incorporation of such variation as an error, each aliquot was considered as a single “sample,” resulting in 32 measurements of the 20 independent impact melt samples. The accuracy of the PGE measurements was tested by recurring analyses of three certified reference materials: a diabase TDB-1 with low PGE, and two gabbros WGB-1 with low and WMG-1 with high PGE-concentrations (Govindaraju, 1994). The data showed good reproducibility. The results presented in Table 2 are consistent with the certified standard data and previous analyses by Plessen and Erzinger (1998). The PGE solutions (10 ml) obtained after the NiS fire-assay preconcentration were measured by ICP-MS independently on two separate days. The concentrations were averaged and the range of the two analyses was used to estimate the error according to the method of Doerffel (1990). The standard deviation of a number

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Table 2. PGE concentrations of the international reference standards from the Canadian Certified References Material Project CCRMP compared to the results of Plessen and Erzinger (1998) “§”, and this work. The “Ref.” values are certified values from Govindaraju (1994), concentrations presented without errors are not certified. Standard

Ru [ng/g]

Rh [ng/g]

Pd [ng/g]

Ir [ng/g]

Pt [ng/g]

Au [ng/g]

TDB-1 Ref. TDB-1§ (n ⫽ 25) TDB-1 (n ⫽ 3)

0.3 0.25 ⫾ 0.08 0.24 ⫾ 0.07

0.7 0.33 ⫾ 0.04 0.42 ⫾ 0.03

22.4 20 ⫾ 1.7 20 ⫾ 1.0

0.15 0.12 ⫾ 0.02 0.12 ⫾ 0.004

5.8 ⫾ 1.1 3.8 ⫾ 0.6 4.8 ⫾ 1.0

6.3 ⫾ 1.0 4.8 ⫾ 1 4.0 ⫾ 0.8

WGB-1 Ref. WGB-1 § (n ⫽ 30) WGB-1 (n ⫽ 8)

0.3 0.2 ⫾ 0.04 0.16 ⫾ 0.01

0.32 0.14 ⫾ 0.01 0.15 ⫾ 0.04

13.9 ⫾ 2.1 13 ⫾ 1.1 10 ⫾ 2.9

0.33 0.20 ⫾ 0.04 0.18 ⫾ 0.06

6.1 ⫾ 1.6 3.8 ⫾ 1.0 3.7 ⫾ 1.1

2.9 ⫾ 1.1 2.0 ⫾ 0.9 0.5 ⫾ 0.2

WMG-1 Ref. WMG-1 § (n ⫽ 6) WMG-1 (n ⫽ 3)

35 ⫾ 5 26 ⫾ 1.3 27 ⫾ 2.5

26 ⫾ 2 26 ⫾ 0.5 25 ⫾ 1

382 ⫾ 13 380 ⫾ 20 338 ⫾ 32

46 ⫾ 4 47 ⫾ 2.3 45 ⫾ 2.4

731 ⫾ 35 705 ⫾ 25 688 ⫾ 33

110 ⫾ 11 105 ⫾ 5 85 ⫾ 13

n ⫽ number of analyses.

(n) of related samples of similar composition is determined as the square root of the square sum of the range of duplicated analyses divided by 2n. 3.2.2. Major and trace elements Samples were also analyzed for major and trace element concentrations. The measurements were carried out by X-ray fluorescence spectroscopy (XRF) on glass beads with a SIEMENS SRS 3000 instrument, at the Museum of Natural History in Berlin. The glass beads were prepared using 0.6 g of dried sample powder (105°C for 4 hours) and 3.6 g of Di-lithiumtetraborate (BRA A10 Specflux). The aliquots were taken from the 20 to 150 g homogenized sample powder prepared for the analysis of PGE. For measurement and data analyses, a modified GEOQUANT program (SIEMENS) based on international rock standards and internal standards was used. Detection limits are 1.0 wt% for SiO2, 0.5 wt% for Al2O3, 0.1 wt% for SO3, 0.05 wt% for Fe2O3, 0.01 wt% for TiO2, MnO, MgO, CaO, Na2O, K2O, and P2O5, 30 ppm for Ba, and 15 ppm for Co, Cr, Ni, Rb, Sr, V, and Zr. The standard errors are 0.5 wt% for SiO2, 0.1 wt% for Al2O3, and SO3, 0.05 wt% for Fe2O3, MgO, CaO, Na2O, and K2O, 0.01 wt% for TiO2, MnO, and P2O5, 30 ppm for Ba, and 5 ppm for Co, Cr, Ni, Rb, Sr, V, and Zr (Schmitt et al., 2004). 3.2.3. Data processing Linear regression analysis was carried out on the results of the PGE analyses of the Popigai melt rock. Regression slopes, y-axis interceptions and correlation coefficients (r) were calculated using the “Origin 5.0” (Microcal Origin, Version 5.0) software program. By plotting the results for two PGE on a X-Y diagram, a mixing line is obtained between the meteorite and the target composition. Previously, this mixing line was used to subtract the indigenous PGE concentration of the target-rock from the values measured in the melt rock (Palme, 1980, 1982; Schmidt et al., 1997). Most commonly Ir was set as the X-axis and the concentration of another PGE, Ni or Cr was plotted on the Y-axis. The Ir concentration in the target was either assumed based on average crustal composition or assigned to be zero (Palme, 1982), more rarely it was measured directly. The intercept on the Y axis provided an estimation of

the amount of the element in the target. This target-rock concentration was then subtracted from the value measured in the melt rock to calculate the elemental ratios. Except for simple and homogeneous target lithologies, the precise estimation of the preimpact Ir concentration is difficult. However, this procedure is only necessary when comparing the element patterns in the melt rock with those in meteorites, as done for example by Palme (1982) and Schmidt et al. (1997). When this comparison is based on the elemental ratios of the PGE, the indigenous target component does not need to be subtracted as long as these ratios are obtained from the mixing line. Even in the case of high PGE concentration in the target rock and a low impactor contamination of the melt, the elemental ratios of the PGE can be determined efficiently. The PGE composition of the target rock has a rather marginal effect on the slope of the mixing line. Figure 2 illustrates the extreme and hypothetical case of the impact of a CI chondrite on a peridotite, an improbable but PGE-rich terrestrial target. The meteoritic contamination of the melt is assumed to be in the 0.1 to 0.2 wt.% range, compatible with the values typically reported for terrestrial craters (Koeberl, 1998). The PGE composition of the peridotite has little influence on the slope of the mixing line. Even in such an extreme case, the slope (1.547) of the melt mixing line diverges little from the elemental ratios of the impactor, in this case a CI-chondrite (Ru/Ir ⫽ 1.55). Because of the high PGE content of meteorites compared to terrestrial rocks, the slope of the linear regression is essentially controlled by the elemental ratio in the projectile. The difference is in most cases below the current analytical resolution. Consequently, the impactor’s elemental ratio can be deduced directly from the slope of the regression line without taking into consideration the indigenous PGE components. However, this is not valid when Ni is plotted vs. Cr. A stronger influence on the slope results for these elements, because the differences in concentration between crustal and meteoritic materials are not as large as those for the PGE. The direct comparison of elemental ratios between impact melt and meteorites eliminates the uncertainties induced by having to estimate the PGE content of the target rock. This method works best for well homogenized impact melt samples. The indigenous PGE, Ni, or Cr concentrations may differ significantly between the different lithologies comprising

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Fig. 2. Method for the determination of the elemental ratios in an impactor based on the composition of impact melts. The figure shows the result of a simulated impact of a CI-chondrite on a peridotite target (Tagle, 2004). Black squares represent the contamination of the impact melt with 0.1, 0.12, 0.15, 0.18 and 0.2 wt.% projectile.

the target. If the melt rock was heterogeneous, local variations in the proportion of the target components could disturb the linear regression of the mixing line. However, several studies indicate that impact crater melt sheets are usually homogeneous and relatively uniform in composition (Floran and Dence, 1976; Floran et al., 1978; Grieve et al., 1977; Reimold et al., 1984; Dressler and Reimold, 2001). Selecting melt rock samples from a specific region of the crater should therefore ensure a high degree of compositional homogeneity.

4. RESULTS

The major and the trace element compositions of the melt rock samples (Table 3) resemble those of the 3 gneisses analyzed and the average composition of the Anabar Shield (Masaitis, 1998; Vishnevsky and Montanari, 1999). This study confirms that the Popigai impact melt is homogeneous and that the Anabar Shield gneissic lithologies form its main component (Masaitis, 1994; Tagle and Claeys, 1999; Vishnevsky and Montanari, 1999, see in particular Tables 4 and 5 in Whitehead et al., 2002). The uniform composition of the melt indicates that it was produced by melting of a rather homogeneous target or that the different constituents of the melt were well homogenized by the cratering process (Whitehead et al., 2002). The PGE, Au, Ni, Co and Cr concentrations of the melt and target gneisses are presented in Table 4. All the samples of impact melt show a significant enrichment in PGE compared to the target lithology. Individual PGE concentrations in the melt are at least 3 times higher than the corresponding maximum concentration measured in the 3 target-rock gneisses. Several

samples are enriched by more than an order of magnitude compared to the target rocks. The 20 melt rock samples analyzed are relatively homogeneous in terms of PGE concentrations. Masaitis (1994) and Vishnevsky and Montanari (1999) had reported, based on Ir concentration, a significant variation in the distribution of the meteoritic component in the Popigai melt. This discrepancy can be explained in several ways. The samples from Vishnevsky and Montanari (1999) originated from outcrops of melt rock spread over the entire 100 km in crater diameter. It is thus possible that the PGE distribution is not homogeneous throughout the whole impact structure. However, these previous results were obtained by INAA on small aliquots (⬃ 50 mg). It must therefore be considered that this apparent inhomogeneity may simply be the result of nugget effects. Similar homogeneous distribution of the projectile material has been found in the impact melt of the Morokweng crater. Using NiS fire assay, McDonald et al. (2001) reported a rather homogeneous distribution of the PGE in 15 impact melt samples collected from 3 boreholes and spread over more than four hundred meters of impact melt recovered from cores in the center of the Morokweng crater. There is only a factor ⬃ 5 variation between the lowest and highest data, if the samples M3-356 and M3-357 (239 and 131 ppb Ir, respectively) containing abundant sulfidebearing nodules that may have concentrated the PGE from the melt, are excluded. The CI-normalized element patterns (Fig. 3) are relatively flat and nearly chondritic. Gold concentration in the target is similar or even slightly higher than in the impact melt (Table 4), resulting in a disturbance of the pattern observed in Figure

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Table 3. Major and trace element compositions of the Anabar Shield target rocks and the Popigai impact melt rocks from [1] Masaitis (1994) (wet chemistry); [2] Vishnevsky and Montanari (1999) XRF data average of 16 analyses, compared with the results for gneiss, suevite and the average of 50 analyses [3] from Tagle (2004), error 1 sigma. Target composition

Composition of the impact melt

Anabar-Shield wt. %

[1]

[2]

gneiss

garnet gneiss 1

garnet gneiss 2

suevite

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 LOI total

63.14 0.59 15.02 6.27 3.06 3.77 3.02 2.39

64.40 0.60 16.60 5.77 0.10 4.20 2.30 3.00 3.10

97.26

100.07

65.42 0.68 14.66 6.87 0.06 3.03 3.15 2.65 1.68 0.05 ⬍0.10 0.30 98.33

58.00 0.84 18.80 9.32 0.10 3.75 2.61 2.47 2.31 0.07 ⬍0.10 1.50 99.77

59.35 0.76 17.10 7.96 0.07 3.69 2.88 2.52 4.00 0.09 0.20 0.95 99.43

63.00 0.69 14.85 6.70 0.06 3.64 3.83 2.09 2.57 0.11 0.20 1.90 99.51

ppm Ba Ce Co Cr Mo Nb Ni Rb Sr Th V Y Zn Zr

166 86 24 109 ⬍10 16 22 44 152 ⬍10 133 46 79 263

833 72 33 96 ⬍10 27 64 62 348 ⬍10 135 46 95 188

1111 92 31 91 ⬍10 13 40 92 231 14 129 39 98 213

[1] 63.13 0.76 14.68 7.51 0.08 3.82 3.43 1.96 2.72

98.09

[2] 63.12 ⫾ 0.73 ⫾ 14.81 ⫾ 6.63 ⫾ 0.07 ⫾ 3.44 ⫾ 3.33 ⫾ 2.49 ⫾ 2.02 ⫾ 0.07 ⫾ 0.60 ⫾ 2.60 ⫾ 99.90

[3] 1.08 0.04 0.45 0.46 0.01 0.56 0.28 0.41 0.21 0.02 0.67 0.91

62.74 0.73 14.92 6.78 0.07 3.62 3.81 2.05 2.75 0.08 0.14 1.33 98.95

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.76 0.03 0.69 0.27 0.01 0.26 0.36 0.13 0.26 0.01 0.09 0.70

856 111 26 105 11 11 96 87 242 19 105 31 95 278

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

51 18 4 17 2 1 18 5 13 3 7 2 5 16

736 90 27 85 ⬍10 11 42 82 217 16 113 31 85 216

LOI ⫽ loss on ignition

3. A crustal origin for the PGE in the impact-melt rock is excluded for the following reasons. Fractionation of crustal lithologies does not produce such a flat pattern. Crustal rocks are depleted in Ir and Ru compared to Pd (Barnes et al., 1985). Only mantle rocks display a flat chondritic pattern (e.g., Barnes et al., 1985; Rehkämper et al., 1999). However, it is unlikely that the rare and small mafic bodies present in the Proterozoic target (Vishnevsky and Montanari, 1999; Kettrup et al., 2003) are the source of the PGE detected in the melt rock. PGE concentrations in the mantle range from 0.75 to 0.85% of CI values (Barnes et al., 1985; Rehkämper et al., 1999). To account for the observed enrichment, this ultramafic component should represent ⬃30 wt.% of the Popigai target rock melted during the impact. This high proportion is not supported by the similarity in major and trace element compositions displayed by the target gneisses and the impact melt. Such an important mafic contribution would also be reflected in the Nd and Sr isotopic compositions. However, the melt rock isotopic values plot within the range of the target gneisses (Whitehead et al., 2000; Whitehead et al., 2002; Kettrup et al., 2003). The PGE enrichment in the melt is, thus, clearly the product of the meteoritic contamination by the Popigai projectile, and this contamination appears chondritic based on the flat element pattern. This observation agrees with the chondritic projectile

previously advocated by Masaitis and Raikhlin (1986) based on Ni/Cr, Ni/Co and Cr/Co ratios. 5. DISCUSSION

5.1. Impactor Traces in the Popigai Melt Rock The PGE concentrations correlate well in most of the samples analyzed (Fig. 4). This correlation implies that all PGE in the Popigai impact melt share the same origin and that they were not fractionated during their incorporation in the melt or by later hydrothermal processes. Absence of significant fractionation of the PGE component in impact-melt was also reported for the Morokweng and Clearwater East craters (McDonald et al., 2001; McDonald, 2002). The PGE ratios derived from the Popigai impact melt (Table 5) are compared to those of the different types of chondrites (Fig. 5). The chondrite data comes from the PGE meteorite database (Tagle, 2004). The Popigai values systematically overlap the field of ordinary chondrites (Fig. 5). This is valid for all combinations of elemental ratios (Tagle, 2004). Plotting ratios of elements with different condensation temperatures provides the best discrimination. Ir, Ru, and Pt have the highest condensation temperatures, while Rh and Pd have the lowest

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Table 4. Concentrations of PGE, Ni, Co, Cr and Au in the Popigai impact melt and target rocks. PGE and Au analyzed by NiS fire assay and ICP-MS, Ni, Co and Cr by XRF. The error was calculated after the method of Doerffel (1990). Sample number Impact melt 58 59 102 103 228 230 56 57 107 33 34 35 229 231 106 105 144 134 61 62 154 124 140 152 121 146 136 147 125 143 133 135 Target rocks gneis garnet gneis 1 garnet gneis 2 suevite

Sample name

Ru [ng/g]

Rh [ng/g]

Pd [ng/g]

Ir [ng/g]

Pt [ng/g]

Au [ng/g]

error ⫾

0.06

0.02

0.05

0.05

0.08

0.13

Pop B 92 Pop B 92 Pop F 02 Pop F 10 Pop F 16 Pop F 33 Pop F 33 Pop F 33 Pop F 40 Pop F 41 Pop F 41 Pop F 41 Pop F 53 Pop F 55 Pop F 75 Pop F 98 Pop 4 Pop 4 Pop 5 Pop 5 Pop 6 Pop 6 Pop 9 Pop 10 Pop 10 Pop 11 Pop 11 Pop 12 Pop 12 Pop 13 Pop 13 Pop 19 Average s

1.38 1.65 2.72 2.96 1.78 1.33 1.30 1.26 2.09 1.04 0.96 0.43 1.42 1.23 2.15 3.61 1.52 1.64 1.72 2.18 1.85 1.77 1.80 2.41 2.41 2.16 1.88 1.21 1.06 1.85 1.84 2.35 1.77 0.58

0.31 0.39 0.63 0.65 0.44 0.32 0.29 0.30 0.52 0.21 0.20 0.19 0.38 0.37 0.52 0.83 0.33 0.40 0.35 0.48 0.46 0.44 0.46 0.52 0.54 0.51 0.48 0.26 0.29 0.49 0.43 0.52 0.42 0.14

1.21 1.51 2.52 2.78 2.02 1.60 1.16 1.11 2.04 0.51 0.57 0.61 1.55 1.52 1.71 3.31 1.35 1.69 1.63 2.06 1.92 1.71 1.75 2.04 2.22 2.02 2.15 1.01 1.08 1.89 1.78 1.58 1.75 0.48

0.80 1.01 1.71 1.73 0.97 0.57 0.75 0.82 1.23 0.48 0.55 0.43 0.96 0.65 1.41 2.31 0.68 0.92 0.85 1.19 0.87 0.94 0.99 1.27 1.34 1.13 1.09 0.60 0.72 1.00 0.98 1.16 0.96 0.38

2.14 3.20 4.00 3.89 5.63 1.97 2.07 2.20 2.67 1.97 1.52 1.48 2.49 1.68 3.17 5.54 1.84 2.55 2.25 3.05 2.78 2.63 2.75 3.60 3.46 3.03 3.09 1.52 1.96 2.85 2.57 3.08 2.58 0.84

1.35 1.49 1.27 2.27 0.85 0.78 0.71 1.17 1.32 1.00 0.67 1.72 0.09 0.44 0.65 1.71 0.60 0.43 0.91 1.22 1.00 0.79 0.39 0.92 1.06 1.60 1.18 0.69 0.61 1.17 0.32 0.31 0.87 0.43

0.16 ⬍0.06 0.24 0.26

0.05 0.03 0.07 0.06

0.45 0.21 0.38 0.55

0.16 ⬍0.06 0.14 0.07

0.69 0.40 0.61 0.46

1.63 0.57 0.27 1.54

Ni [␮g/g]

Co [␮g/g]

Cr [␮g/g]

74 73 115 93

29 29 25 29

100 93 96 92

83 n.a.

24 n.a.

87 n.a.

84 n.a. 99 82 147

29 n.a. 32 26 28

95 n.a. 98 102 109

82

23

90

103

28

97

88 112

29 29

86 97

127

28

104

71

32

100

63

24

99

93 84 93 21

32 27 28 3

97 95 96 6

22 64 40 42

24 33 31 27

109 96 91 85

n.a. ⫽ not nalysed.

(Wasson, 1985). Plots such as Ru/Rh vs. Pt/Pd and Ru/Rh vs. Pd/Ir refine the identification of the projectile. The Popigai melt plots most consistently into the field of L-chondrites (Fig. 5). Based on PGE elemental ratios, the projectile that formed the Popigai crater appears to have had an L-chondrite composition. An average L-chondrite contains ⬃490 ng/g Ir according to Wasson and Kallemeyn (1988). The average of 32 Ir values for the Popigai impact melt is 0.96 ⫾ 0.38 ng/g (Table 4). To estimate the meteoritic contamination of the melt, the Ir composition of the target rock must be subtracted from this value. Assuming an average crustal composition of 0.03 ng/g (Peucker-Ehrenbrink and Jahn, 2001) and similar density, the meteoritic contamination of the Popigai melt is approximatively 0.2 wt.%. The concentration of Ir in the melt varies between 0.48 and 2.31 ng/g. If this variation is taken into account, the range of meteoritic contamination lies between 0.10 and 0.45 wt.%, which is in good agreement with contamination values from other craters (Koeberl, 1998). Even for low levels of

contamination of the impact melt it is, thus, possible, using elemental ratios and precise analytical techniques, to infer with some degree of confidence the composition of the impactor. 5.2. Ordinary Chondrites as Impactors 5.2.1. Asteroid or comet shower in the Late Eocene The 85 km diameter Chesapeake Bay crater was formed at approximatively the same time as Popigai; these two structures are dated at 35.5 ⫾ 0.6 Ma and 35.7 ⫾ 0.2 Ma respectively (Koeberl et al., 1996; Bottomley et al., 1997). These two craters fall into a 2.5 Myr period marked in the sediments from the Massignano section (Italy) by a high concentration in extraterrestrial 3He that indicates an increase in the delivery of interplanetary dust particles (IDP’s) onto Earth (Farley et al., 1998). According to these authors, this increase in IDP resulted from an arrival in the inner solar system of a comet shower caused by

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Fig. 3. CI-normalized PGE and Au concentrations in the Popigai melt (average), compared to CI (0.5 wt%), suevite and gneisses from the Anabar Shield.

a perturbation of the Oort cloud. If this scenario is correct the Popigai and Chesapeake Bay craters could represent comet impacts (Farley et al., 1998). Comets are believed to be primitive bodies with a composition similar to that of carbonaceous chondrites. The composition of the Chesapeake Bay projectile is not known. Recently, Kyte et al. (2004) have shown that the Cr isotopic signature of the late Eocene clinopyroxene ejecta layer at ODP site 709c excludes carbonaceous chondrites as a source for the meteoritic component. They favor, instead, an ordinary chondrite source. As Popigai is considered the source crater of this ejecta layer (Whitehead et al., 2000), their interpretation agrees with the composition of the projectile reported in this study. The L-chondrite origin of the Popigai projectile is difficult to reconcile with a comet origin. Based on this argument, Tagle and Claeys (2004) have recently proposed that a major collision in the asteroid belt could perhaps account for the high flux of extraterrestrial material, both dust and larger objects, onto Earth in the Late Eocene. This possibility was not favored by Farley et al. (1998), because they considered that dust bands associated with asteroid families may be over 100 Myr old (Marzari et al., 1995). Consequently, they argued that the IDP production rate decays over much longer periods than 2.5 Myr as observed in the Late Eocene. Recent studies advocate a shorter period for the delivery of extraterrestrial material to Earth. The results of Nesvorny et al. (2003) demonstrated that the three asteroidal dust bands are not associated with ancient asteroid families, as previously expected. They show that the dust bands are primary “byproducts” of small and recent asteroid breakups. As a consequence, the dust bands must decay over relatively short periods, perhaps in a few million years, which is consistent

with the increase of IDP’s detected in the late Eocene (Nesvorny et al., 2003). An even shorter transfer time of some 105 years was recently advocated for the delivery of micrometeorites to Earth after a major asteroidal collision, based on cosmogenic 21Ne preserved in chromite grains from fossil meteorites in early Ordovician limestones from Sweden (Heck et al., 2004). Morbidelli and Gladman (1998) and Burbine et al. (2003) have shown that timescales in the order of 2 to 4 Myr are also possible for the delivery of asteroidal material, as meteoroids, coming from the main inner belt resonance. So far no asteroid family is known with the correct age or in the correct resonance to account for the proposed asteroid shower during the Late Eocene. Orbital simulations should be carried out to evalute the hypothesis of coincident delivery of large objects and IDP to Earth. The simplest scenario is that of a major asteroid breakup in the main belt, injecting numerous fragments into a resonance, which later sends them onto an Earth collision course. In the most efficient resonance, nu6, at the inner border of the belt, only ⬃1% of the bodies can collide with Earth (Morbidelli and Gladman, 1998). This would imply that to form the two large Chesapeake Bay and Popigai craters a few hundred ⬃ 5 km bodies have to be injected into this nu6 resonance; which is rather unlikely but not impossible. Even, if an effective delivery mechanism remains unclear, such an asteroid shower lasting a few million years may also explain the existence of 3 other craters, which seem to have formed around the same time: Wanapitei (37 ⫾ 2 Ma), Mistastin (38 ⫾ 4 Ma) and Logoisk (40 ⫾ 5 Ma) (Grieve, 1991) have ages that overlap within error limits with the Late Eocene event.

An ordinary chondrite impactor for the Popigai crater, Siberia

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Fig. 4. Regression analysis of the measured PGE concentrations in the Popigai impact melt. The slope (B) represents the ratio of these elements in the impactor. The error bars represent the analytical error of the method.

5.2.2. Frequency of ordinary chondrites as crater forming projectiles An L-chondrite as impactor for the Popigai crater adds substance to the hypothesis that ordinary chondrites may represent an unusually high percentage of the projectiles forming craters on Earth. This idea was originally proposed

by McDonald et al. (2001) and McDonald (2002) based on their findings of an ordinary chondrite composition for the projectiles of the Morokweng (70 km in diameter) and Clearwater East (22 km in diameter) craters. Meibom and Clark (1999) suggested that ordinary chondrites were not the most abundant objects in the asteroid belt. Actually, the potential source of ordinary chondrites now assumed to be

Table 5. Elemental ratios calculated after linear regressions of the PGE, Ni, Cr concentrations in the impact melt of the Popigai crater. r ⫽ correlation coefficient. Y-axis

X-axis

Ru Pt Rh Pd Pt Ru Pt Ru Pt Ni Cr Ni

Ir Ir Ir Ir Ru Pd Rh Rh Pd Ir Ir Cr

Slope 1.47 2.07 0.34 1.26 1.32 1.04 6.35 4.45 1.70 28000 6000 2.34

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.08 0.11 0.02 0.11 0.11 0.06 0.40 0.21 0.16 10000 2800 0.87

Intercept 0.32 0.60 0.08 0.43 0.21 0.02 ⫺0.10 ⫺0.10 ⫺0.46 66 89 ⫺149

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.09 0.11 0.02 0.13 0.21 0.11 0.18 0.10 0.31 12 3 99

r

Excluded samples (number/name)

0.96 0.96 0.96 0.90 0.92 0.95 0.95 0.97 0.91 0.57 0.50 0.58

35 228 231 33, 34, 35, 103, 228 228, 35 33, 34, 35, 103, 228 228, 135, 231 33, 34, 35 33, 34, 35; 103, 228 Pop B 92, Pop F 75 Pop B 92, Pop F 75 Pop B 92, Pop F 75

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Fig. 5. Elemental ratios in the impact melt of the Popigai crater compared to the elemental ratios calculated from the database of Tagle (2004) for different types of chondrites. The error bars represent one standard deviation.

the S-type asteroids (e.g., Wetherill and Chapman, 1988; Chapman, 1996; Binzel et al., 2004). S-type asteroids are abundant in the main belt, dominate among the Near Earth Objects (NEO’s), and probably also among projectiles impacting Earth (Ivezic et al., 2001; Morbidelli et al., 2002; Stuart and Binzel, 2004). As a significant compositional variation exists among the observed S-type asteroids, the exact relationship between the different S-type asteroids and ordinary chondrites remains to be fully understood (Binzel et al., 2004). This variation is currently interpreted as a result of “space weathering,” which modified the reflectance spectra of asteroid with time (e.g., Pieters et al., 2000; Binzel et al., 2004). The high abundance of S-type asteroids amongst classified bodies may also be related to their higher albedos compared to those of C-type asteroids (Burbine et al., 2003). According to McDonald et al. (2001), the high frequency of ordinary chondrites colliding with Earth could be due to the position of the S(IV) bodies within the inner asteroid belt. At 3:1 Jovian orbital resonance at 2.5 AU, they appear to be ideally located to be perturbed towards an Earth-crossing trajectory (Gaffey et al., 1993; Farinella et al., 1993; Migliorini et al., 1997a, 1997b). For craters ⬎1.5 km in size, where the projectile has been identified, ordinary chondrites seem to dominate (Table 6). However, this observation is based on relatively few examples and must be taken with caution. So far, of the large craters generated in the last 500 Ma, only the Chicxulub impactor with its carbonaceous chondrite composi-

tion (Kyte, 1998; Shukolyukov and Lugmair, 1998) escapes this trend. It is unlikely that this high frequency could be due to analytical bias favoring the detection of ordinary chondrites. Confusion is improbable, as the compositional fields of the different types of chondrites are clearly distinct (Fig. 5). Moreover, carbonaceous chondrites are also distinguishable from ordinary chondrites by their Cr isotope signatures (Shukolyukov and Lugmair, 1998). 5.2.3. Comparison with the early Ordovician sedimentary rocks from Sweden The early Ordovician (480 Ma) is another stratigraphic interval marked by an increase in the delivery of extraterrestrial material on Earth (Schmitz et al., 1996; Schmitz et al., 1997). In this case, evidence points towards a rain of fine particles, a micrometeorite shower. A major asteroidal disruption event during this period was proposed based on the finding of a large amount of altered fossil meteorites and relict chromite grains in limestone sediments over an area of 250 000 km2 in southern Sweden (Schmitz et al., 2003). These authors estimated an increase in the meteorite flux by a factor of 121 ⫾ 52 compared with today’s rate. The extended stratigraphic interval where the dispersed relict chromite grains occur argues against a major asteroid disruption in the Earth atmosphere (Schmitz et al., 2003). No 3He anomaly was detected, probably because of diagenetic effects (Patterson et al., 1998). Based on the com-

An ordinary chondrite impactor for the Popigai crater, Siberia

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Table 6. Ordinary chondrites as impactors for terrestrial craters in the Phanerozoic. Ages and sizes of the craters from Grieve (2001). IIE Iron is related to ordinary chondrites (Olsen et al., 1994). Crater Name

Diameter [km]

New Quebec Brent Rio Cuatro Bosumtwi Rochechouart Clearwater East Morokweng Popigai Chicxulub

3.44 3.8 4.5 10.5 23 26 70 100 170

Age [Ma] 1.4 ⫾ 450 ⫾ ⬍0.1 1.03 ⫾ 214 ⫾ 290 ⫾ 145 ⫾ 35.7 ⫾ 64.98 ⫾

0.1 30 0.02 8 20 0.8 0.2* 0.05

Impactor type

Ref.

ordinary chondrite (L) ordinary chondrite (L-LL) ordinary chondrite (H) ordinary chondrite (?) IIE Iron (ordinary chondrite H) ordinary chondrite (L, LL) ordinary chondrite (L, LL) ordinary chondrite (L) carbonaceous chondrite

[1] [2] [3] [4] [5] [6] [7] [8] [9]

[1] Grieve (1991); [2] Palme et al. (1981); [3] Schultz et al. (1994); [4] Koeberl et al., 2004; [5] Tagle et al. (2003); [6] McDonald et al. (2002); [7] McDonald et al. (2001); [8] this work; [9] Kyte (1998); Shukolyukov und Lugmair (2000). * Popigai age from Bottomley et al. (1997).

position of the chromite grains, an ordinary chondrite, most likely a L-chondrite, was suggested as the meteoritic source (Schmitz et al., 2003). This rain of meteorites correlates with a possible disruption of the L-chondrite parent body, as many L-chondrites show evidence of strong shock metamorphism and resetting of the K-Ar clock ⬃ 500 million years ago (Heymann 1967; Keil et al., 1994). After the collision, the fragments were placed onto an orbit that became a collision course with Earth within a time frame of ⬃105 years (Heck et al., 2004). Contrary to the late Eocene event, there is no clear evidence, so far, of a significant increase of the impact rate during the early Ordovician. However, Schmitz et al. (2001) noted a clustering of craters in the early to middle Ordovician. Hence, similarities between the late Eocene and the early Ordovician events cannot be completely ruled out at this point. 6. SUMMARY

The Popigai impactor was most likely a L-chondrite. The results presented here show that PGE elemental ratios provide a reliable tool for the identification of projectiles down to the level of a chondrite group. This identification is possible even when the meteoritic contamination in impact melt is ⬍1 wt.%, as it is often the case (e.g., Koeberl 1998). This is one of the main advantages of this method, compared to the identification based on Cr isotopes, which requires high amounts of extraterrestrial Cr to be present in the impact melt (Koeberl et al., 2002). In the case of the late Eocene, a combination of both approaches, direct measurement of PGE from the Popigai impact melt and Cr isotope signature of the ejecta (Kyte el al., 2004) give consistent results. The late Eocene and early Ordovician periods seem to have seen a high delivery of extraterrestrial material to Earth. This increase in the delivery rate could be due to major collisions in the asteroid belt. In all these cases, L-chrondrite material was delivered to Earth in rather short times of the order of 105–106 years. The precursor of the Flora asteroid family (Nesvormy et al., 2002) is a possible parent body for the early Ordovician event (Heck et al., 2004). A candidate for the late Eocene event remains to be found. Acknowledgments—The sampling of the Popigai crater was supported by the German Science Foundation (DFG). We thank John Warme for collecting another batch of Popigai samples in 1999. We acknowledge

Jörg Erzinger for his assistance and providing access to the analytical facilities at the GFZ in Potsdam, and Heike Rothe for helping with the ICP-MS measurements. At the Institute für Mineralogie, we thank Dieter Stöffler for supporting and encouraging these studies, Ralf T. Schmitt for the XRF analyses, Ansgar Greshake for providing meteorite samples, Axel Wittmann and Lutz Hecht for helpful comments, and Silke Weseler for sample preparation. The work was financed by the DFG (grants Cl147/2-1, Cl147/2-2 and STO 11/38). P. Claeys thanks the “Vrije Universiteit Brussel Onderzoeksraad.”We thank the reviewers W.U. Reimold, I. McDonald and S. Morbidelli for their helpful comments. Associate editor: W. U. Reimold REFERENCES Barnes S. J., Naldet A. J. and Gorton M. P. (1985) The origin of the fractionation of platinum-group elements in terrestrial magmas. Chem. Geol. 53, 303–323. Binzel R. P., Rivkin A. S., Stuart J. S., Harris A. W., Bus S. J. and Burbine T. H. (2004) Observed spectral properties of near-Earth objects: results for population distribution, source regions and space weathering processes. Icarus 170, 259 –294. Bottomley R., Grieve R. A. F., York D. and Masaitis V. (1997) The age of Popigai impact event and its relation to events at EoceneOligocene boundary. Nature 338, 365–368. Burbine T. H., McCoy T. J., Meibom A., Glaman B. and Keil K. (2003) Meteoritic parent bodies: Their number and identification. In Asteroids III (eds. W. F. Bottke, A. Cellino, P. Paolicchi and R. P. Binzel), 653– 667. The University of Arizona Press. Chapman C. R. (1996) S-Type Asteroids, Ordinary Chondrites and Space Weathering: The Evidence from Galileo’s Fly-bys of Gaspra and Ida. Meteor. 31, 699 –725. Doerffel K (1990) Statistik in der analytischen Chemie. VEB Deutscher Verlag für Grundstoffindustrie GmbH. Leipzig, 256. Dressler B. O. and Reimold W. U. (2001) Terrestrial impact melt rocks and glasses. Earth-Sci. Rev. 56, 205–284. Ely J. C. and Neal C. R. (2002) Method of data reduction and uncertainty estimation for platinum-group element data using inductively coupled plasma mass spectrometry. Geostand. News. 26, 31–39. Farinella P., Gonczi R., Froeschle C. and Froeschle C. (1993) The injection of asteroid fragments into resonances. Icarus 101, 174 – 187. Farley K. A., Montanari A., Shoemaker E. M. and Shoemaker C. S. (1998) Geochemical evidence for a comet shower in the Late Eocene. Science 280, 1250 –1253. Floran J. R. G. and Dence M. R (1976) Morphology of the Manicouagan Ring-Structure, Quebec and some comparisons with lunar basins and craters. Proc. 7th Lunar Planet. Sci. Conf. 1, Pergamon Press, pp. 2845–2866.

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