TEM study of meteorite impact glass at New ZealandCretaceous^Tertiary sites: evidence for multiple impacts or

di¡erentiation during global circulation?

Blanca Bauluz a;�, Donald R. Peacor b, Christopher J. Hollis c

a Cristalograf|¤a y Mineralog|¤a, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spainb Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1063, USA

c Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand

Received 26 August 2003; accepted 15 December 2003

Abstract

Study by transmission electron microscopy of samples from the Cretaceous^Tertiary (K^T) boundary clay atFlaxbourne River and Woodside Creek, New Zealand, has revealed the occurrence of nanometer-sized meteoriteimpact-derived glass. The average glass composition is exceptionally Ca-rich and is distinct from other glass found onEarth, apart from glass inferred to be of impact origin at Mexican and Haitian K^T sites. The glass shards arepartially altered to montmorillonite-like smectite, with the dominant interlayer cation, Ca, reflecting the compositionof the parent glass. The data imply a heterogeneous global distribution in composition of K^T boundary impactglass: Si-rich and Ca-rich in Mexico and Haiti, Si-rich in Denmark, and Ca-rich in New Zealand. This heterogeneousdistribution may relate to dispersal processes similar to those used to account for the asymmetric distribution ofclastic debris from the Chicxulub impact site. However, recent discovery of an impact crater of K^T boundary age inUkraine raises the possibility of impact clusters which produce material of heterogeneous composition.; 2004 Elsevier B.V. All rights reserved.

Keywords: impact glass; cretaceous^Tertiary; transmission electron microscopy; clays

1. Introduction

The discovery of an iridium anomaly at theCretaceous^Tertiary (K^T) boundary led Alvarezand co-workers to propose meteorite impact as acausal mechanism for mass extinctions at the endof the Cretaceous [1]. The high content of Ir and

associated platinum-group element ratios, thepresence of unusual spherules, shocked quartz,and spinel at K^T boundary sites worldwide areconsistent with an impact of a chondritic meteor-ite ([2], and references therein).

The Chicxulub crater in Yucatan, Mexico, isthe only crater to be con¢rmed with an isotopicage within error of the accepted K^T boundary[3] and large enough, V180 km in diameter [4], toproduce ejecta of signi¢cant volume and to dis-tribute them worldwide. The impact ejecta com-prise two macroscopic layers in North America

0012-821X / 04 / $ ^ see front matter ; 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(04)00011-1

* Corresponding author. Tel. : +34-976-761103;Fax: 34-976-761106.E-mail address: bauluz@unizar.es (B. Bauluz).

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and a single layer in Europe, northern Africa,New Zealand, the south Atlantic Ocean, and thePaci¢c Ocean. The lower layer in and adjacent toNorth America has been interpreted to consist ofrelatively low-energy proximal ejecta which weredeposited from a ballistically emplaced ejecta cur-tain [5]. The upper layer is interpreted to be com-prised of higher-energy ejecta (also called ‘¢reballlayer’ [6]) that were carried in a vapor-rich plumethat rose far above Earth’s atmosphere and dis-tributed material globally [6^11].

Previous studies indicated that the high-energyejecta consisted of shocked grains from the target(most of them less than 30 Wm in size) [10,12^14],30- to 500-Wm spherules [2,15^18], which appearto be condensates, and some additional materialin the form of diamonds [19,20], soot [21,22], andpossibly some kind of ¢ne-grained glassy compo-nent [23]. The amount of material in the sub-mi-crometer fraction is particularly important be-cause it can result in signi¢cant atmosphericopacity causing decrease in surface temperaturesand reducing the rate of sunlight-induced photo-synthesis [1,24^25].

In a transmission electron microscopy (TEM)study of sub-micrometer particles in undisturbedsamples from the K^T boundary at Stevns Klint,Denmark [26], images showed relict, oxidizedfragments of Ni-rich iron-meteorite and micro-meter-sized, impact-glass shards partially trans-formed to smectite. Assuming a source at Chicxu-lub, this demonstrated the extremely ¢ne-grainednature of both meteorite particles and glass, con-sistent with stratospheric travel over long distan-ces, and a global distribution and prolonged fall-out of such material.

In order to test the hypothesis that such mate-rial was distributed globally [26], to relate thematerial to possible sources, and to characterizethe components of the high-energy ejecta, we haveexamined samples from New Zealand, a SouthernHemisphere site distant both from the probableimpact site and from Denmark. Undisturbed sam-ples from the K^T boundary clay layer at Flax-bourne River and Woodside Creek were selectedfor study by TEM. The K^T boundary layers inthese sites have large Ir anomalies, 134 ng/cm2 atthe Flaxbourne River [27] and 153 ng/cm2 at

Woodside Creek [28], and are enriched in Crand Ni which are inferred to be of meteoritic ori-gin [27,28]. Previous studies of these sites identi-¢ed abundant soot [22], polycyclic aromatic hy-drocarbons [29], and fullerenes [30], implyingthat a bolide impact caused local wild¢res. How-ever, subsequent re-examination [31] of a K^Tboundary sample from Woodside Creek castsdoubt on the presence of those C60 hydrocar-bons. Studies of fossil pollen and spores con-¢rmed that a bolide impact caused catastrophicdestruction of New Zealand forests [32].

Recent studies have shown that the Boltysh im-pact crater in Ukraine, which is 25 km in diame-ter, is 65 Myr old [33] and that the Silverpit struc-ture in the North Sea, 20 km in diameter, mightbe 60^65 Myr old [34,35]. Such data suggest thatmore than one impact could have occurred at theK^T boundary. However, the size of those struc-tures, which are much smaller than that of Chic-xulub, implies that the e¡ects of the impactswould have been much smaller and the distribu-tion of their ejecta limited. Although more studiesare needed to con¢rm the age and other aspects ofthese two minor impacts, their discovery revivesthe hypothesis of impact clusters at the end of theCretaceous, as has been inferred for the Frasnian^Famennian boundary [36], the mid-Norian [37],and the end of the Jurassic [38].

2. Materials and methods

We studied ¢ve samples from K^T boundarylayers at the Flaxbourne River and WoodsideCreek by scanning electron microscopy (SEM)and TEM. Samples from the Flaxbourne Riverwere obtained from 0.001 m (CQ03), 0.039 m(CQ04), and 0.095 m (CQ08) above the base ofthe 0.02-m-thick K^T boundary clay. Samples atWoodside Creek were located 0.005 m (WC01)and 0.009 m (WC02) above the base of the0.025-m-thick boundary clay.

The K^T boundary at Woodside Creek andFlaxbourne River occurs within the Mead HillFormation, which consists of medium-bedded sili-ceous limestone and chert with thin marl or mud-stone interbeds, deposited at upper bathyal depths

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on the £anks of a terrigenous sediment-starvedcarbonate platform [39]. The K^T boundarymarks a signi¢cant change in lithology from me-dium-bedded siliceous limestone to thin-beddedsiliceous mudstone and calcareous chert. In bothsections the basal Paleocene succession consistsof: 0.02^0.03 m thick boundary clay, 0.3^0.5 mthick clay-rich chert or siliceous mudstone, and a10^30-m interval of calcareous chert that gradesinto siliceous limestone. A paleoenvironmental in-terpretation of this lithofacies succession and as-sociated changes in microfossils and geochemistrywas provided by Hollis et al. [39] and is reviewedin relation to other New Zealand K^T boundaryrecords by Hollis [40].

Thin sections of the ¢ve samples were preparedwith surfaces normal to bedding, and ¢rst exam-ined by optical microscopy. Typical areas wereremoved for SEM and TEM observations via at-tached Al washers, thinned in an ion mill andcarbon-coated. This method of sample prepara-tion permits the unmodi¢ed sample texture to beimaged in the SEM and TEM. The SEM obser-vations were made with a Hitachi S570 instru-ment. The TEM data were obtained using a Phi-lips CM12 scanning TEM (STEM). Both theSEM and TEM were ¢tted with Kevex Quantumsolid-state detectors and computer systems, thedetector having a boron-composite window per-mitting analysis of low-atomic-number elements.The STEM was operated at 120 kV and a beamcurrent of 20 WA. A selected-area aperture 10 Wmin diameter was used to obtain selected area elec-

tron di¡raction (SAED) patterns. Quantitativeanalyses (analytical electron microscopy, AEM)were obtained from thin edges using a 100 nm2

scanning area. The EDS data were processed us-ing Kevex software, with resultant intensity ratiosbeing corrected with k-values determined for well-characterized samples [41]. The concentration ra-tios were normalized to 22 negative charges forsmectite and to oxides (wt%) recalculated to100% for glasses. Errors of reported analyticalvalues are V3^5% of the amount present.

3. SEM and TEM results

Backscattered electron (BSE) SEM imagesshowed that all samples are very ¢ne-grained(Fig. 1) and that they are calcareous siliceousclaystones comprised of clay minerals, calciteand quartz. Microfossil studies indicated thatmuch of the quartz is biogenic, with diatoms, ra-diolarians and sponge spicules common in theCretaceous and abundant in the Early Paleocene[40,42]. Woodside Creek samples (Fig. 1B) con-tain abundant Fe oxide spherules up to 20 Wmin diameter. No elements beside Fe and O weredetected by EDS analysis. The spherules are in-ferred to be alteration products of diagenetic py-rite, as other studies concluded [43].

Glass shards were observed only in TEM im-ages, in samples from both the Flaxbourne River(CQ03, CQ04) and Woodside Creek (WC01,WC02) (Figs. 2 and 3). They are commonly elon-

Fig. 1. BSE images of representative ion-milled areas corresponding to samples: (A) CQ03 (Flaxbourne River) and (B) WC01(Woodside Creek). Cal = calcite, Qz= quartz, Fe ox=Fe oxide.

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gated, ranging from 45 to 135 nm in length, butwith most 80^90 nm. That size range is smallerthan that of the shards at Stevns Klint [26] (6 700nm). The shards were identi¢ed as glass becausethey did not give SAED patterns, had featurelesscontrast in TEM images, and had compositions(AEM analyses) consistent with the unique com-positions of other K^T boundary glasses (e.g.,Mimbral and Beloc) (Table 1). TEM imagesshowed that shards were partially altered to smec-tite across boundaries with transitional contrast(Fig. 3A). Smectite was identi¢ed on the basis oftypical SAED patterns (Fig. 3D), curved latticefringes with variable 10^14 AW (001) spacings(Fig. 3C), and composition (Table 2). SAED pat-terns have only two or three weak, di¡use 00lre£ections, and weak and di¡use and non-periodicnon-00l re£ections (Fig. 3D). The variability offringe spacing is inferred to be a result of partialcollapse of smectite interlayers by dehydrationand/or beam damage in the TEM environment.

In smectite-rich areas (Fig. 3B), smectite occursas thin packets that are oriented at relatively largeangles to one another. The packets are usuallycurved, discontinuous, and lens-shaped. Such fea-tures are typical of smectite formed by completealteration of glass shards, where smectite layerscurve around the surface of altering shards ofvolcanic origin, as commonly observed in benton-ites [44].

3.1. Glass and smectite compositions

Selected AEM-determined compositions forFlaxbourne River and Woodside Creek glassesare listed in Table 1 in order to illustrate theirvariation ranges. The average compositions, andthose of glasses from other K^T sites such asMimbral (Mexico), Beloc (Haiti), and StevnsKlint (Denmark), and melted rocks from the Bol-

Fig. 2. Low-magni¢cation TEM image showing smectite(Sm) packets surrounding relatively featureless glass cores(Gl).

Fig. 3. (A) TEM image showing a glass shard (Gl) fromsample CQ3 altering to smectite (Sm). White triangles indi-cate the boundaries between glass and alteration products.(B) In smectite-rich areas, smectite occurs as thin packetsthat are oriented at relatively large angles to one another.The packets are usually curved, discontinuous, and lens-shaped. (C) Wavy and discontinuous lattice fringes of smec-tite, with spacing from 10 to 11 AW , depending on the degreeof dehydration and layer collapse caused by interaction withthe electron beam or the TEM vacuum. (D) SAED patternof smectite, with di¡use low-order 001 re£ections and ill-de-¢ned and non-periodic non-001 re£ections with di¡usenessparallel to c*.

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tysh crater are shown in Table 2. AEM analysesare reported as weights per cent (wt%), obtainedby normalization to 100 wt% of relative atomicproportions from AEM analyses. Table 3 lists theaverage compositions for smectite in the NewZealand samples, compared with those of Belocand Stevns Klint. The analyses reported for thisstudy are typical of all analyses of all samples.

The average glass composition for the Flax-bourne River and Woodside Creek samples hasan exceptionally large Ca content of 25%. Thus,the overall composition is unique relative to otherglasses found on Earth, except for samples fromMimbral, Mexico and Beloc, Haiti, which wereinferred to be of impact origin on the basis oftheir elemental and isotopic compositions andwater content [45^50]. Two types of impact glass

were described as occurring at Mimbral and Be-loc, one Si-rich (black) and the other Ca-rich (yel-low). The latter type has average CaO content of23^24%. The unusual composition determined inthis study for New Zealand glasses is strikinglysimilar to that of Ca-rich glass from Mimbraland Beloc, implying a common composition oftarget rocks and origin.

The normalized AEM-determined compositionfor smectite has an octahedral cation contentclose to the ideal dioctahedral value (2.0), andAl is the principal octahedral cation, as in thereported smectite compositions from other sites.The Fe and Mg contents are relatively large, ac-counting for nearly all of the net negative layercharge. Ca is the dominant interlayer cation, re-£ecting the large Ca content of the parent glass.

Table 1Selected AEM-determined compositions (wt%) for glasses from the K^T boundary at Flaxbourne River and Woodside Creek inNew Zealand

Flaxbourne River Woodside Creek

Na2O 0.00 0.86 0.37 0.50 0.25 0.45 0.10 0.95MgO 1.50 1.65 1.32 1.05 1.90 1.15 1.25 0.95Al2O3 13.11 17.11 15.20 15.90 14.10 15.50 14.15 17.25SiO2 48.10 49.50 49.10 49.50 49.50 49.50 49.10 49.50K2O 4.38 3.85 4.01 3.99 4.05 3.65 4.01 4.08CaO 26.01 24.05 26.10 25.20 25.30 24.95 25.30 25.50FeO 6.90 2.98 3.90 3.86 4.90 4.80 6.09 1.77

100 100 100 100 100 100 100 100

Table 2Major element concentrations (wt%) for glasses from the K^T boundary at Mimbral (Mexico), Beloc (Haiti), Stevns Klint (Den-mark), the Flaxbourne River and Woodside Creek (New Zealand), and melted rocks from the Boltysh crater

Mimbral (Mexico)* Beloc (Haiti)2 Boltysh(Ukraine)3

Stevns Klint(Denmark)#

Flaxbourne R., Woodside C. (NZ)**

black gl. Ca-rich gl. black gl. yellow gl.

Na2O 3.34 2.02 3.72 2.54 3.55 0.2 0.4 (0.4)MgO 3.01 3.90 2.55 4.02 1.24 6.2 1.3 (0.3)Al2O3 15.73 12.40 15.33 13.25 14.34 18.5 15.2 (1.6)SiO2 62.99 52.20 63.29 48.73 68.75 64.4 49.2 (0.5)K2O 1.50 0.58 1.62 0.65 3.98 0.8 4.0 (0.2)CaO 6.88 22.96 7.21 24.71 1.96 6.0 25.3 (0.1)TiO2 0.70 0.56 0.68 0.64 0.41 ^ ^MnO 0.13 0.14 ^ ^ 0.04 ^ ^FeO 5.32 4.73 5.27 4.98 3.06 3.8 4.6 (1.4)Total 99.6 99.49 99.67 99.52 97.33 100.00 100.00

Note: standard deviation in parentheses.*Smit et al. [51], 2Koeberl and Sigursson [46], 3Grieve et al. [52], #Bauluz et al. [26], **this study.

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4. Discussion and conclusions

As noted above, analysis of meteorite impact-derived glass from Mexico [51] and Haiti [46] re-vealed the occurrence of two kinds of glass withsimilar morphologies in both K^T sites: Si-richglass, described as dark-black with dacitic to an-desitic compositions; and Ca-rich glass, describedas yellow from Haiti, with smaller SiO2 contentand often containing high-Ca silicate and sulfateinclusions [50]. Most workers [45,47^50] agreethat both types of glass were formed by impact/shock melting, and that the Ca-rich glasses wereprobably formed by impact melting and mixing ofsur¢cial carbonate (and minor anhydrite) rockswith more deeply buried crystalline parent rocksof the black glass. Some authors inferred a com-positional gap between the black and yellowglasses [47]. However, glass compositional analy-ses with values extending throughout this inter-mediate range were reported [50] lending supportto the impact-mixing model. They showed thatthe yellow glass is also present as light-coloredstreaks or schlieren in black glass or as mixtureswith the black glass which are not visible by eye.Black glass far exceeds yellow glass in amountand grain size. Study of Haitian material indicatesthat yellow glass comprises V6% of the total re-covered glass [50].

The Stevns Klint K^T glass shards [26] are re-markably similar in composition to the Si-richglasses from Haiti and Mexico, as consistent

with a common origin. All of the glass shardsobserved by Bauluz and co-workers had similarcompositions, none being Ca-rich. In addition,the melted rocks from the Boltysh crater [52] areenriched in Si with a major element compositionsimilar to those of dark Haitian and Mexicanglass, and that of Danish glass.

In marked contrast, however, the glass shardsfrom the Flaxbourne River and Woodside Creeksites have compositions which are strikingly sim-ilar to those of the Ca-rich glasses from Mexicoand Haiti. Despite the use of very di¡erent ana-lytical techniques, analyses show a similar rangeof variation [46,53].

Fig. 4 compares the New Zealand glass compo-sitions with those of the Mexican, Haitian, andDanish glasses and those of the melted rocksfrom the Boltysh crater. The remarkable featureof these plots is the close correspondence in majorelement compositions among the Ca-rich glasses,especially considering the wide range of glasscomposition derived from volcanic and other ter-restrial sources. Taking into account that impactsusually generate Si-rich glass because crustal ma-terial is the most likely target, such a Ca contentindicates the existence of a thick carbonate coveras the cause of the exceptional CaO composition.

The Boltysh impact probably was not largeenough to produce an ejecta layer as signi¢cantas generated at Chicxulub, and the di¡erences incomposition among the melted rocks from theBoltysh impact and the Si-rich glasses from Mim-

Table 3Formulae of smectite from the K^T boundary in Beloc (Haiti), Stevns Klint (Denmark) and Flaxbourne River and WoodsideCreek (New Zealand)

Beloc (Haiti) Stevns Klint (Denmark) Flaxbourne River, Woodside Creek (New Zealand)

* 2 3 # ** 22

Si 3.91 3.98 3.96 4.01 3.86 3.9 (0.1)AlIV 0.09 0.01 0.04 ^ 0.14 0.1 (0.1)AlVI 1.04 1.15 1.29 1.31 1.19 1.4 (0.2)Fe 0.44 0.35 0.24 0.13 0.17 0.2 (0.1)Mg 0.62 0.67 0.69 0.64 0.70 0.3 (0.1)Ti ^ 0.03 ^ ^ ^ 0.0 (0.1)K 0.13 0.01 ^ 0.03 0.07 0.1 (0.1)Na 0.01 ^ ^ 0.26 0.06 ^Ca 0.36 0.23 0.15 0.11 0.35 0.3 (0.1)

Note: Compositions normalized on the basis of 11 oxygen atoms. Standard deviation in parentheses.*Kring and Boyton [61], 2Koeberl and Sigursson [46], 3Kastner et al. [62], #Elliott [63], **Bauluz et al. [26], 22this study.

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bral, Beloc and Stevns Klint indicate that theirtarget rocks had di¡erent average compositions.Therefore, the impact at Boltysh could not beresponsible for generation of such Si-rich glass.Indeed, the entire ballistic trajectory of such smallimpacts occurs within Earth’s atmosphere [23],with no possibility for development of high-en-ergy ejecta.

A number of factors have constrained the typesof target rocks for K^T boundary impact glasses.The dacitic to andesitic compositions of the relictblack impact glass indicate that the target rockswere mainly siliceous and not oceanic basalts [54].These authors further suggested that variation inCa content of the black glasses and the occur-rence of high-Ca yellow glass resulted from mix-ing of melted basement rocks with sur¢cial lime-stones and anhydrite that occur at the putative

Chicxulub impact site. The presence of largeamounts of S in some glass analyses [50^54] fur-ther supports this mixing hypothesis. The discov-ery of CaSO4 inclusions in the yellow glasses alsolends credence to this scenario. However, recentisotopic studies of the Haitian glasses constrainthe anhydrite component of this mixture to6 10% [47,48,55].

Assuming that the Si-rich glasses found atStevns Klint represented global distribution of im-pact-derived materials from Chicxulub, we hadexpected that impact glass at the New Zealandsites would also be Si-rich. The data of this studywhich indicate the occurrence only of Ca-richglasses are therefore unexpected. However, thepresence of some amount of Si-rich glasses inthe New Zealand samples cannot be completelyruled out. If these glasses were present in verylow concentration they might not have been de-tected by TEM if the selected areas for the studywere not completely representative of the entiresample. On the other hand, the absence of Si-rich glasses in the New Zealand samples couldbe a consequence of a lower preservation rate ofthese glasses in comparison with the Ca-richglasses. However, in that case the TEM studyshould have detected two di¡erent smectite com-positions (e.g., signi¢cant di¡erences in the inter-layer cation/octahedral composition) or at least amuch larger range in smectite composition thanthat shown in Table 3.

Some calculations [23] show that high-energyejecta were distributed globally, although theyare concentrated around the Chicxulub impactsite and at the antipode (corresponding to India,and the Indian Ocean 65 Myr ago). The distribu-tion is also elongated in a longitudinal directionbecause of the Earth’s rotation. On the otherhand, the world-wide heterogeneity in composi-tion of the high-energy ejecta may simply be afunction of an original heterogeneity of ejecta.The mineral and melt components entrained inthe vapor plume could not be distributed homo-geneously over the surface of the Earth as a resultof the mechanics of the vapor plume expansionprocess and factors that are a function of particlediameter and density [23].

Pope [56] discussed a mechanism of dispersal of

Fig. 4. Plot of major element compositions (wt%) of ana-lyzed glasses in New Zealand and those of Mimbral (Mexi-co), Beloc (Haiti), Stevns Klint (Denmark), and the Boltyshcrater (Ukraine).

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high-energy ejecta from the Chicxulub impact siteby asymmetrical dispersal patterns of stratospher-ic winds. Such stratospheric winds would havebeen the primary agent of dispersal of clastic de-bris and impact glasses to distal sites such as NewZealand and Denmark. Pope suggested that pre-vailing westerly £ows of stratospheric winds andthe limited ability of such winds to transport de-bris across latitudes could explain the low abun-dances of clastic debris to the east (Italy) and farsouth (New Zealand) of the Chicxulub impactsite.

The glasses from Haiti and Mexico are similarin size, up to 5 mm in diameter. The Stevns Klintglass shards, on the other hand, are irregular inshape and only up to 700 nm in diameter. TheNew Zealand shards are also irregular in shape,but the largest observed in this study was only 135nm in size. There is thus a regular decrease in sizewith distance from the inferred impact site inMexico. The presence of glass and altered smec-tite at least up to the ¢rst 0.039 m of the studiedsequence, in the case of Flaxbourne River, sug-gests that there may have been a continuous con-tribution of glass to sediments over a considerabletime period, up to 20 kyr if the age models ofHollis et al. [39] are valid. However, it is alsopossible that the upper occurrences of glass inthis section are the result of local remobilizationand redeposition of fall-out debris. Such processescould also produce the two additional peaks in Irabundance above the boundary clay [27]. The Iranomalies are 8.3 ng/g at 0.02^0.03 m above thebase of the boundary and 1.2 ng/g at 0.09^0.12 m.

An alternative explanation for the distributionof compositions of impact glass is related to theconcept of impact clusters at the end of the Creta-ceous. Although this hypothesis is supported bythe discovery of new craters with K^T boundaryages such as the Boltysh in Ukraine (25 km indiameter), it is unlikely that that impact producedthe New Zealand ejecta, as discussed above. Thepresence of more than one crater of K^T age,however, implies that still others might remainundiscovered, especially if such sites are in oceansediments. The Ca-rich glasses found in New Zea-land might therefore have originated in a still-un-discovered impact site, perhaps in the Southern

Hemisphere. Although we consider this hypothe-sis to be very speculative it is a possibility whichmust at least be considered. Especially in the lightof the lack of direct observation of additionalimpact sites, however, di¡erentiation of glassesof di¡erent compositions as derived from Chicxu-lub must be considered the more likely explana-tion of the data regarding heterogeneity of distri-bution of glass compositions.

Smectite from Haiti was observed to occur asan alteration product of glass. The compositionsof the smectite from Haiti, Stevns Klint and NewZealand (Table 2) are similar. They are montmor-illonite-like in having virtually no tetrahedral Al,with net negative charge derived almost entirelyfrom Fe and Mg substitution in octahedral sites.With the exception of some Stevns Klint smectite,Ca is the dominant interlayer cation, and even forthe exception there is signi¢cant Ca. Some au-thors [57] emphasized that smectite which directlyalters from glass has a composition which re£ectsthat of the glass. The Al, Mg and Fe contents ofall source glasses are similar, even though thereare major di¡erences in Ca and Si contents, asconsistent with the similar octahedral composi-tions of all smectite. Even the glasses with smallerCa contents have Ca values which are greaterthan those of K and Na, giving rise to Ca-richinterlayer compositions. Such smectite composi-tions are relatively unusual. Where parent glasseshave been entirely altered to smectite, such com-positions could therefore serve as indicators ofimpact events [26]. In addition, the homogeneityin atypical smectite compositions shown by thisstudy suggests that a large portion of the K^Tboundary layer was originally comprised of im-pact glasses, rather than smectite of typical sedi-mentary origin. Some researchers [58] suggest that1% of nanometer-sized particles (silicate and sootparticles) in the K^T layer is su⁄cient to haveprevented photosynthesis. These relations implythat even in New Zealand, nanometer-sized glassgrains were more than abundant enough to pro-duce the atmospheric opacity that caused surfacetemperatures to decrease, preventing sunlightfrom reaching the surface where it is needed forphotosynthesis [1,24,25,59,60].

The data of this study support the hypothesis of

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a global distribution of nanometer-scale impactmaterial previously suggested in the TEM studyof samples from Stevns Klint [26], and furtherdemonstrate the usefulness of TEM in character-izing impact events. On the other hand, the het-erogeneity in the global distribution of composi-tions of impact glasses remains an enigma, in thatit appears to be consistent either with di¡erentia-tion of impact glass from only the Chicxulub site,or with the concept of impact clusters at the endof the Cretaceous period. These questions may beresolved with similar high-resolution studies ofglobally distributed samples, so that patterns ofthe distribution of glass compositions may beidenti¢ed.

Acknowledgements

This work was supported by NSF Grant EAR-98-14391 to D.R.P. B.B. is grateful to C.E. Hen-derson for technical assistance at the Electron Mi-crobeam Analysis Laboratory, the University ofMichigan. We thank P. Claeys, W.C. Elliott andK. Koeberl for their thorough reviews and K.Farley for comments and editorial handling ofthe manuscript.[KF]

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