Coexisting altered glass and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact

Coexisting altered glass and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact

Earth and Planetary Science Letters 182 (2000) 127^136 Coexisting altered glass and Fe^Ni oxides at the Cretaceous^Terti...

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Earth and Planetary Science Letters 182 (2000) 127^136

Coexisting altered glass and Fe^Ni oxides at the Cretaceous^Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact Blanca Bauluz a; *, Donald R. Peacor b , W. Crawford Elliott c a

Departamento de Ciencias de la Tierra, Cristalograf|¨a y Mineralog|¨a, Universidad de Zaragoza, Pedro Cerbuna 12, 50.009 Zaragoza, Spain b Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA c Department of Geology, Georgia State University, Atlanta, GA 30303, USA Received 24 April 2000; received in revised form 2 August 2000; accepted 21 August 2000

Abstract The Cretaceous^Tertiary (K^T) boundary at Stevns Klint, Denmark, is noteworthy for its large Ir anomaly that is taken as evidence of extraterrestrial components, but the origin of the smectite in this marl has been variously interpreted to have a detrital, meteorite impact, or volcanic origin. We have carried out scanning electron microscopy and transmission electron microscopy (TEM)/analytical electron microscopy observations of the impact and contiguous layers within the K^T marl at Stevns Klint. TEM images show abundant smectite, much of which occurs with layers curving around and grading into cores of nanometer-scale glass shards. The smectite composition is unusual in having both significant octahedral Al and Mg. The glass and smectite major element compositions are similar and unique relative to glasses of terrestrial and extraterrestrial origin with the exception for one kind of glass at the K^T boundary in Haiti. Abundant 10^20-nm diameter iron oxides having as much as 10% Ni and minor Zn are intergrown with smectite. We interpret these domains to be altered meteorite fragments, which formed when impact glass was transformed to smectite. The direct association of unique glass and meteorite fragments is unambiguous evidence for meteorite impact. These data may imply fall-out of globally distributed impact-derived particles over an extended time period. The relations imply that TEM observations may be a powerful tool in detecting other impact events in the geological record. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: impactite; K^T boundary; smectite; Stevns Klint; transmission electron microscopy

1. Introduction The discovery of iridium anomalies at the Cre-

* Corresponding author. Fax: +34-976-761088; E-mail: [email protected]

taceous^Tertiary (K^T) boundary led Alvarez and coworkers to propose meteorite impact as a causal mechanism for mass extinctions at the end of the Cretaceous [1]. The high content of Ir and platinum group interelement ratios [2], the presence of unusual spherules [3^8], and impact-derived minerals such as shocked quartz [9] and spinel [10^12] at K^T boundary sites world-

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wide, were explained as the result of an impact with the Earth of an asteroid-sized V20 km diameter, relatively Ir-rich body at Chicxulub, Yucata¨n, Mexico, 65 Ma ago [13]. The causality of mass extinctions by meteorite impact is still a matter of debate. Although micrometer-sized Ir grains were observed in impact melt rocks from Chicxulub that might be derived directly from the projectile [14] and 250-Wm spheroidal debris with chondritic Ir concentrations in K^T sediments from DSDP Site 577 [15], only a single meteorite fragment with su¤cient features to make inferences concerning its origin has heretofore been found in K^T boundary sediments in the Paci¢c Ocean [16]. The Stevns Klint K^T marl was chosen for study by transmission electron microscopy (TEM) because it has one of the highest Ir anomalies, and subsequently, small amounts of shocked quartz grains were found throughout the Fish Clay [2,17]. Smectite is the predominant mineral in the K^T marls in the Denmark sections. However, di¡erent origins for the smectite have been suggested. The lack of illite layers in the smectite, its presence in Cretaceous and Tertiary marls in nearby sections, and its rare-earth element contents were considered evidences of its volcanic origin [18,19]. Other researchers [20] indicated that its major element composition was similar to that of microtektite-like spherules, implying that the smectite was formed from meteorite impact glass. In addition, they noted [20] that its unusually high MgO content attests to an input of chondritic debris from meteorite impact. However, no textural evidence of the precursor phases of smectite in either volcanic or impact glasses was found in these previous studies, glass being the probable precursor inferred by comparison of smectite compositions with those of pyroclastic calcic-feldspars and impact glasses [20,21]. The aim of this research was to look for direct evidence of meteorite impact in the K^T marl at the Stevns Klint, Denmark, by using TEM, because other data obtained from low-resolution techniques were not conclusive in part due to the ¢ne-grained nature of such sediments. The results not only show unambiguously that the smectite is derived from impact glass, but that

fragments of the meteorite are pervasive in this marl. That implies that such high-resolution studies are generally capable of characterizing horizons elsewhere in the rock record corresponding to impact events. 2. Stevns Klint K^T boundary section Following the mineralogical, chemical analytical and petrographic descriptions of the K^T boundary at Stevns Klint, Denmark [22], four distinct layers have been de¢ned within the boundary marl. They are denoted as layers II, IIIa, IIIb and IV. Layer I, below the boundary marl, is Cretaceous chalk. Layer V is Tertiary chalk directly above layer IV. Layer II is the basal layer within this marl and it is light-gray marl. Directly above layer II, layer III was subdivided into layer IIIa, a reddish basal layer named the impact layer by B.F. Bohor and G.A. Izett (see [21]) because of the presence of shocked quartz and pyrite concretions weathered to jarosite, and layer IIIb, a black clayey marl which has the highest Ir content (e.g. 77 ng/g; [19,21]). Layer IV exhibits ¢ne laminae. The color changes upward from dark brown to light-gray. Co, Ni, Cr, Zn and Ir were found to be concentrated in the clay-rich, 6 0.1-Wm size fractions of the marls [19,21]. These concentrations have been inferred to be due to: (1) incorporation into smectite by ion exchange, (2) sorption from seawater enriched in these elements after meteorite impact, (3) concentration by organic carbon, or (4) presence in minerals with abundances below amounts detectable by X-ray di¡raction (XRD) [19,23^25]. 3. Methods Specimens were prepared for optical microscopy, scanning electron microscopy (SEM) and TEM following the methods described in previous papers [26]. SEM observations were made with a Hitachi S570 instrument. TEM data were obtained with a Philips CM-12 scanning TEM (STEM) operated at 120 kV and a beam current of 20 WA. Both the SEM and STEM were ¢tted

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Fig. 1. A: BSE image of the impact layer showing quartz spherules (QS), quartz clasts (QC) and pyrite concretions (Py) in smectite matrix (Sm). B: BSE image of impact layer showing clay areas with typical textures wherein smectite has replaced grains of precursor materials (1). C: BSE image of layer II, which consists of smectite-rich matrix (Sm) with calcite fossil fragments (F), ¢ne-grained calcite (Cal) and quartz clasts (QC).

with Kevex Quantum solid-state detectors and computer systems. Analytical electron microscopy (AEM) chemical analyses were performed in scanning mode over areas of 100 nm2 and EDS data were processed using Kevex software. Resultant intensity ratios were processed following the methods described by the above-cited authors [26]. 4. SEM and TEM results We studied samples corresponding to layers II, IIIa and combined layers IIIb+IV. Undisturbed textures were studied by analyzing ion-milled samples. Backscattered electron (BSE) SEM images showed that the impact layer (layer IIIa) contains abundant pyrite concretions and spherules enclosed within a smectite matrix. Quartz occurs as angular clasts and as 50^150-Wm diameter spherules composed of separate angular grains, which are otherwise featureless, but have the appearance of fragmented single crystals (Fig. 1A). Minor amounts of shocked quartz along with abundant detrital quartz were found at Stevns

Klint [17], but this is the ¢rst observation of spherules of fractured quartz. Although the texture and occurrence of the spherules in the impact layer implies the possibility of an impact-related origin, we were unable to acquire additional data by TEM because of loss of material due to di¡erential ion-milling rates. The impact layer contains packets of nearly pure clay with sharp boundaries, typical of textures where smectite replaces grains of precursor materials (Fig. 1B). Layers II, IIIb and IV are composed largely of smectite and ¢ne-grained calcite (Fig. 1C), plus fossil fragments, quartz clasts, barite grains and rare feldspar. Nano-scale glass shards were directly observed by TEM in samples from layers II^IV. They usually have elongated shapes, with largest dimension 6 700 nm. They were identi¢ed as glass on the basis of the following criteria : the lack of selected area electron di¡raction (SAED) patterns, featureless contrast in TEM images, and composition determined by AEM analyses (Table 1). TEM images showed that these glass shards are in the process of altering to smectite across boundaries with transitional contrast (Fig.

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Fig. 2. A: TEM image of glass (G) altering to smectite (Sm). Altered edges of glass shards are indicated with arrows. B: Lattice fringe image of glass^smectite (G^Sm) boundary. C: SAED pattern of smectite.

2A,B). The clay was identi¢ed as smectite on the basis of typical SAED patterns (Fig. 2C), curved î (001) spaclattice fringes with variable 10^14 A ings, and composition (Table 2). SAED patterns have only two or three weak, di¡use 001 re£ections, and weak, di¡use and non-periodic non-001 re£ections. The texture seen above the impact layer is shown in Fig. 3A. Smectite, glass and a third phase are shown. The third phase is composed of Fe and O, but with signi¢cant Ni and minor Zn from AEM analyses. The Fe:Ni ratio is as

high as 11, and it varies very little from domain to domain. The mottled appearance and SAED patterns exhibiting weak and di¡use re£ections are characteristic of goethite. Smectite and these Fe oxides are intergrown at the scale of micrometers as shown in Fig. 3B. Extra care was therefore used to detect platinum group elements (PGEs) in these goethite domains. No PGEs were detected, but this may be due to their relatively low concentrations (bulk rock 6 80 ng/g [19]) below detection limits for AEM. In smectite-rich areas (Fig. 3C), smectite occurs

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Table 1 Compositions of glasses from the K^T boundary at Haiti, Danish basaltic ashes, tektites and Stevns Klint

Na2 O MgO Al2 O3 SiO2 K2 O CaO TiO2 MnO FeO

Haitian K^T glassesa

Danish basaltic ashesb

Glasses from tektitesc

Stevns Klintd

black glasses (wt%)

yellow glasses (wt%)




3.72 2.55 15.33 63.29 1.62 7.21 0.68 ^ 5.27

2.54 4.02 13.25 48.73 0.65 24.71 0.64 ^ 4.98

2.83 3.77 13.39 50.04 0.71 7.20 3.50 0.22 17.88

1.32 1.85 12.17 74.40 2.61 1.52 0.76 0.11 5.02

0.22 6.22 18.54 64.42 0.79 5.99 ^ ^ 3.83


Koeberl and Sigurdsson [28]. Schmitz and Asaro [29]. c Mazer et al. [30]. d This study. b

as thin packets that are oriented at relatively large angles to one another. The packets are usually curved, discontinuous and have a lens-shaped morphology. Such features are typical of complete alteration of glass shards to smectite [27].

4.1. Glass and smectite compositions AEM data for glasses and smectite are listed in Tables 1^3. Analyses in Tables 1 and 3 are reported in weight percent (wt%) obtained by nor-

Fig. 3. A: TEM image showing glasses (G), Fe^Ni oxides (Ox) and smectite (Sm). B: Lattice fringe image of smectite (Sm) intergrown with Fe^Ni oxides (Ox). C: TEM image of smectite-rich area.

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Table 2 Formulas of smectite from the K^T boundary in Haiti and Stevns Klint Haitia Si AlIV AlVI Fe Mg K Na Ca

Stevns Klintb






3.91 0.09 1.04 0.44 0.62 0.13 0.01 0.36

3.99 0.01 1.15 0.35 0.67 0.01 ^ 0.23

3.96 0.04 1.29 0.24 0.69 n.d. n.d. 0.15

4.01 ^ 1.31 0.13 0.64 0.03 0.26 0.11

3.86 0.14 1.19 0.17 0.70 0.07 0.06 0.35

Normalized on the basis of 11 oxygens; n.d., not determined. a Kring and Boyton [32]; Koeberl and Sigursson [28]. b Kastner et al. [20]; Elliott [19]; this study.

malizing element concentration ratios to 100%. In Table 1, the major element compositions of the Stevns Klint K^T boundary glasses are compared to those of the black and yellow glasses in Haitian K^T boundary sediments [28], Eocene basaltic ash from Denmark [29] and tektites [30]. The black and yellow glasses from Haiti di¡er markedly in SiO2 and CaO contents. The Stevns Klint K^T glasses are similar in major element composition to the black Haitian glass, although those of Stevns Klint have slightly greater Al2 O3 , SiO2 and MgO contents, and smaller CaO, K2 O and Na2 O contents. The major element compositions of K^T glasses from Stevns Klint di¡er signi¢cantly from those of Eocene Danish basaltic ashes. The Eocene basaltic ashes contain much higher FeO and lower Al2 O3 and SiO2 compared to Stevns Klint glasses. In addition, the variety of

smectite ubiquitously observed to be an alteration product of basaltic glass is saponite [31], which has a composition which is very di¡erent from the smectite observed in this study. The K^T glasses from Stevns Klint and black glasses from Haiti are also very di¡erent in composition than Indochinite tektite from Thailand [30], which we have used as a reference for comparison because it has a composition intermediate to those of all tektites. The composition of glass from Stevns Klint is therefore unlike that of any other glass found on Earth, with the exception of the black glass from Haiti. Previous studies of the smectite at the K^T boundary at Stevns Klint noted its unusual major elemental composition [19^21], but its origin has been attributed to a variety of sources. A typical AEM-determined smectite composition is reported in Table 2, compared with analyses of smectite in altered spherules from the Haitian K^T boundary [28,32] and from Stevns Klint reported in previous studies [20,21]. The analyses in Table 2 are all normalized to a total of 11 oxygen atoms for comparison. The analyses reported for this study are typical of all analyses from all samples, very little variation being observed. The total octahedral cation content is slightly greater than 2.0, as consistent with a dioctahedral member of the smectite group. Although Al is the dominant octahedral cation, there is signi¢cant Mg and some Fe, and in that respect it is similar to the celadonite group of micas. Its composition is unusual relative to other members of the smectite group, however, in corresponding to the solid solution gap normally occurring between Al-rich di-

Table 3 Selected AEM analyses of glass and smectite at Stevns Klint

Na2 O MgO Al2 O3 SiO2 K2 O CaO FeO

Gl-1 (wt%)

Sm-1 (wt%)

Gl-2 (wt%)

Sm-2 (wt%)

Gl-3 (wt%)

Sm-3 (wt%)

Gl-4 (wt%)

Sm-4 (wt%)

Gl-5 (wt%)

Sm-5 (wt%)

0.71 4.76 17.07 67.18 1.79 5.96 2.53

0.93 7.02 17.64 65.40 1.27 5.05 2.69

0.00 6.19 17.32 65.15 1.04 7.62 2.67

0.00 9.19 19.96 62.50 0.67 6.31 1.38

0.00 6.03 17.34 68.56 0.54 5.70 1.82

0.00 8.01 17.93 66.85 0.83 4.50 1.88

0.63 6.85 18.73 62.01 0.45 6.94 4.38

1.06 9.17 23.77 59.57 0.39 4.01 2.03

0.00 6.49 20.14 67.93 0.00 3.26 2.19

1.13 9.61 21.80 58.85 1.03 3.02 4.57

Gl, glass; Sm, smectite.

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octahedral and Mg-rich trioctahedral smectite [33]. The composition of Haitian smectite is equally exceptional. The similar compositions of glass and smectite are unusual for their respective classes of material (Table 3). Such a close relation between compositions of volcanic glass and smectite is normal [34]. As required by the stoichiometry of crystalline smectite, however, smectite contains slightly less SiO2 and more MgO than precursor glass. 5. Discussion and conclusions The similarity of the major element composition of Stevns Klint K^T glass to Haitian impact black glass and the dissimilarity with respect to known glasses of terrestrial or extraterrestrial origin implies an impact origin for Stevns Klint glass. Smectite has been shown to be a direct alteration product of the impact glass by the TEM data of this study. The alteration of glass through interaction with pore £uids is expected during sediment diagenesis. Perhaps the only surprising issue is that any glass has been preserved in such ancient rocks. On the other hand, the homogeneous composition of analyzed smectite by AEM is consistent with a single source, and inconsistent with the heterogeneity normally displayed by mixtures of detrital clay minerals, which may be derived from a variety of sources, and diagenetic clays. Relatively high concentrations of elements like Ni, Co, Zn and Ir have been reported in K^T boundary layers at Stevns Klint [19], Caravaca (Spain) [35,36], Woodside Creek (New Zealand) [37] and DSDP Site 465a Paci¢c Ocean [38,39]. Global fall-out of chondritic [23] or unspeci¢ed extraterrestrial matter [38,40] was proposed as the cause of those enrichments. However, normal terrestrial processes have also been proposed. These include exceptional geochemical conditions in reduced sediments [41,42], diagenetic enrichment [43] and precipitation from seawater [23,25,44], although the ultimate origin of some elements was postulated to be through an Earthimpacting asteroid [23]. Previous studies suggested that the metals were hosted by smectite, organic


carbon or pyrite concretions [19,24,42], but the data of this study show that at least some of the Ni and Zn occur in Ni-rich goethite. The goethite is found intimately associated with smectite that is believed to have formed from impact-derived glass. Although Fe oxides have been detected in layers IIIa and IIIb, the association Fe^Ni^Zn was only found in the impact layer (IIIa). XRD patterns of the 6 0.1-Wm fractions of the Stevns Klint samples indicate they are composed of smectite, although the presence of glass or accessory minerals with concentrations below detection limits is also possible [19]; e.g. according to our TEM results, ¢ne-particle fractions should contain nanometer-scale Fe oxides. Chemical analyses of these fractions indicate that the impact layer is enriched in Ni (880 parts per million (ppm)), Zn (820 ppm) and Co (86 ppm), but no enrichment has been observed in other layers. On the other hand, the ¢ne fraction of layer IIIb, a black marl containing pyrite, has a similar Fe content to that of the impact layer, but has only minor Ni and Zn. There is, therefore, no relation between the Ni and Zn content and the presence of organic matter and iron sul¢des. These relations imply that there are two di¡erent modes of occurrence of iron oxides in terms of precursor phase, those found in the impact layer which have signi¢cant Ni and Zn in solid solution and those derived by alteration of pyrite. We interpret the nanometer-scale goethite-like iron oxide with signi¢cant amounts of Ni and trace amounts of Zn to be altered remnants of the metallic component of a meteorite. Its presence implies an origin through oxidation and hydration of the metallic component of meteoritic material. Fe:Ni ratios of the metallic component of some carbonaceous meteorites commonly range up to 10:1, the value observed in this study, although they may vary depending in part on the degree of alteration [45]. Ir, Ni, Co, Cr are elements signi¢cantly enriched in meteorites, and Zn is also enriched in some extraterrestrial materials. These include the Brun£o meteorite, a 10-cm chondritic clast recovered from an Ordovician limestone, whose Zn content varies from 0.2 to 6.8 wt% [46], chondritic interplanetary dust particles [47], and the meteorite fragment found in

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sediments retrieved from K^T boundary sediments in the northern Paci¢c Ocean [16]. The latter is enriched in Ir, Ni, Cr and Zn in comparison with the surrounding sediments. A second possible source of Ni is in Ni-spinel, as observed in other K^T sites [11,48]. Other elements characteristic of spinel were not detected. Complete alteration of metal is to be expected, especially given the evidence for alteration of glass by sediment pore £uids. The presence of glass and the related compositions of glass and altered smectite attest to a relatively closed system, and a low water/rock ratio as noted previously [20]. We infer, therefore, that the Ir-rich K^T boundary layer at Stevns Klint contains direct evidence of meteorite impact heretofore not described, including impact-derived glass which serves as the precursor of smectite, and Ni-rich goethite which is interpreted to be an alteration product of the metallic component of a meteorite. In K^T sites closer to the Chicxulub impact structure, such as Bass River, New Jersey [7], and Blake Nose, Western North Atlantic [8], the deposition of the impact ejecta exactly coincided with the biostratigraphic K^T boundary. The presence of glass and altered smectite throughout the studied sequence from layer II to layer IV at Stevns Klint suggests that there was a continuous contribution of glass to sediments over a considerable time period. The extremely ¢ne-grained nature of both meteorite particles and glass may therefore be consistent with stratospheric travel over long distances, implying a possible global distribution of such material, and prolonged fallout. A study of other K^T sites could test this hypothesis. Alternatively, bioturbation or other syn-sedimentary processes may have redistributed material distributed over a vertical range. The presence of layers [22], macrotextures (such as horsetail laminae) and ¢ssile fabric argue against redistribution of these meteoritic particles by such processes. This study represents the ¢rst attempt to use TEM and ion-milled samples to characterize the ¢ne-grained materials associated with the K^T boundary. Although a meteorite fragment was previously found [16], we have shown that such material is pervasive at the Stevns Klint locality as

nano-sized particles within smectite derived from impact glass, and directly associated with impact glasses. We suggest that similar high-resolution studies should record similar features in occurrences of the K^T boundary elsewhere. These observations suggest that such TEM observations of globally distributed ¢ne-grained material may be generally used as evidence of meteorite impact events. This study has also demonstrated the relation between abundant smectite with a unique composition and impact-derived glass. The association of smectite having such unusual compositions with an Ir anomaly and shocked quartz may therefore generally be used as evidence for meteorite impact at other localities when the smectite is characterized even by relatively low-resolution techniques. Acknowledgements This work was supported by National Science Foundation Grants EAR-9418108 to D.R.P. and EAR-8916473 to W.C.E. We thank J. Smit, P.H. Schultz, M. Kastner, B. Schmitz, F. Kyte and R.C. Reynolds for their comments on earlier versions of this paper.[AH]

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