Geochemistry of impact glasses from the K/T boundary in Haiti: …€¦ · The concentrations of eleven major elements (including S), determined by microprobe, and of ~i~~i~t major,

Geochimica d Cosmochimica Ada Vol. 56, pp. 2113-2129 Copyright Q 1992 Pergamon Press Ltd. Printed in U.S.A.

0016-7037/92/$5.00 + .oo

Geochemistry of impact glasses from the K/T boundary in Haiti: Relation to smectites and a new type of glass

CHRISTIAN KOEBERL’*~ and HARALDUR SIGURDSSON~

‘Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A-1010 Vienna, Austria ZLunar and Planetary Institute, 3303 NASA Road 1, Houston, TX 77058, USA

3Graduate School of Oceanography, University of Rhode Island, Narragansett, RlO2882, USA

(Received December 18, 199 1; accepted in revised form February 19, 1992 )

Abstract-We have individually analyzed twelve black and seven yellow glasses from the K/T boundary section at Beloc, Haiti, plus three smectite mantles around black glasses. The main chemical differences between black and yellow glasses are higher S, Ca, Mg, Zn, As, Br, Sb, and Au, but lower K, Na, and Si abundances in the yellow glasses. We have found high-CaO yellow glasses with low S contents (0.08, 0.17 wt% ), which may be explained by fusion of carbonate-rich sediments. Microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10-100 pm level, while yellow glasses show much more variability and different intra-sample elemental correlations. One of the black glasses has higher SiO, and KrO abundances, but lower concentrations of all other major elements. This suggests the existence of a third glass type, which we have named the high Si-K variety (HSi,K glass). The glass shows areas consisting of pure SiOr (maybe lechatelierite) and some schlieren with lower SiOr content. The chemical composition indicates that the HSi,K glass may have originated from melting of a sedimentary source rock, such as shale or argillite. Severe changes in major and trace element composition occur during alteration of the glass, as documented by analyses of adhering smectites. The REE patterns in the glass arc similar to average upper crust, but the smectites exhibit much lower abundances and flat patterns, not unlike the patterns observed in claystones at numerous K/T boundary sites. The suggestion that the Chicxulub structure in Yucatan may be the source crater for the K/T impact glasses is not fully supported by the currently available data. Impact glasses and tektites are produced from the surface layers of their targets. At Chicxulub, the target stratigraphy comprised mainly carbonates and evaporites. Although we cannot exclude Chicxulub as a source, the present data do not provide any firm chemical evidence for a connection. We conclude, however, that the Haiti glasses have formed by impact and have later undergone alteration by low-temperature hydrothermal processes to produce clays. The boundary claystones at K/T boundary sites may very well be alteration products of these impact glasses.

INTRODUCITON

THE CRETACEOUS-TERTIARY (K/T) boundary is marked by signs of a worldwide catastrophe. Ever since ALVAREZ et al. ( 1980) identified an enrichment of Ir and other siderophile elements in rocks marking the K/T boundary and interpmted it as the mark of a giant asteroid (or comet) impact, researchers have tried to understand the complexities of the K/T boundary event. The impact theory received a critical boost by the discovery of shocked minerals which have so far been found only in association with impact craters (Bo- HOR et al., 1984, 1987). One of the problems of the K/T impact theory was, and still is, the lack of an adequate large crater that is close to the maximum abundance of shocked grains in K/T boundary sections, which was found to occur in sections in Northern America ( BOHOR and IZETT, 1986; IZETT, 1987a; MCHONE et al., 1989; BOHOR, 1990). In this respect, the recent discovery of impact glasses from a K/T section in Haiti ( I~ETT et al., 1990; I~ETT, 199 la; SIGURDS- SON et al., 1991a; KRING and BOYNTON, 1991) is of great importance.

The K/T boundary layer at Beloc, Haiti (Fig. 1). is much thicker than at other known sites and contains large (up to l-2 mm; MAURRASSE and SEN, 199 1) shocked quartz grains as well as altered spherules interpreted to be of impact origin ( HILDEBRAND and B~YNTON, 1990b). The presence of relict

glasses within some of the altered spherules from Haiti was reported by IZETT et al. ( 1990), SIGURDWN et al. ( 199 1 a), IZETT ( 199 la), and KRING and BOYNTON ( 199 1). Micro- spherules have been reported from a variety of K/T boundary sites around the world (e.g., MONTANARI et al., 1983; BOHOR, 1984; BROOKS et al., 1985; NASLUND et al., 1986; IZETT, 1987b) and have in most cases been interpreted as altered impact spherules.

The glass spherules from Haiti provide valuable clues as to the target material of the K/T impact(s). Of great importance also is the finding of glasses within smectite shells, which provides direct data on the alteration of the impact products. Isotopic ( DEPAOLO et al., 1983) and chemical arguments, especially the low abundances of the rare earth elements (REEs) in K/T boundary claystones, have been used to infer an oceanic impact site (e.g., HILDEBRAND and BOYNTON, 1988a,b). Here we attempt to ( 1) provide con- firmation of the chemical composition of the Haiti glasses reported elsewhere and present high-precision data for major and trace elements in individual glass particles; (2) describe, for the first time, the presence of a third type of impact glass at the Haiti K/T boundary layer; and ( 3) describe the chemical relationship of the glasses with their smectite alteration products and relate that alteration to the nature of the K/T boundary layers at other sites.

2113

2114 C. Koeberl and H. Sigurdsson

0 loo 200 so0

km

FIG. 1. Locality map of the impact glass occurrence at the K/T boundary, Reioc, Haiti inset shows Middle America, with Haiti, the Yucatan peninsula, and the Mimbral locality ( bfARGOLiS et al., 199 1) .

SAMPLES

Sample Location and Stratigraphy

The glass pa&lea have been collected ( SIGLJRDSSON et al., 199 1 a,b) from the K/T lxamdary layer that outcrops along the Jacmel highway in the Reloc region, Haiti. The boundary layer is thicker (SO-100 cm) than at other known K/T boundary layers {e.g., HILDEBRAND and ROYNTON, 199Ob, MAURRASSE and SEN, 1991). In most outcrops, the glass is altered to smectite, but relict glasses occur in the interior of smectite spherules. The layer consists of several sub-units, with a 30 cm, smectite-rich basal Iayer (fallout deposit?) with 8 mm to 3 mm diameter spherules, overlain by a poorly stratified lo-40 cm thick unit, consisting of a mixture of smectite spherules in a carbonate/smectite matrix, also containing l-5 cm clasts of carbonate rock. The base of the poorly strati&d unit shows a clear erosional contact with the underlying massive spherule Iayer. The smeetite and glass spherules in the mixed layer are larger than in the basal layer, with clast size of up to 2 cm. The sedimentary features indicate a turbid&e or gravity flow origin for the upper unit. While the deposit is generally altered to smectite, some samples contain up to 25% glass spherules, ranging from 1 to 6 mm in diameter (SIGURD~ON et al., 199 la). The smectite-rich turbidite grades upward into a thinner ( 5- IO cm) unit composed of some mari, calcareous sediment, and smectite, overlain by a 0.5-I cm oxidized reddish layer rich in magnetite microspherules and containing the Ir anomaly ( SIGURDSSON et al., 199la; MAURRASSE and SEN, 1991; JLHANNO et al., 1991).

Sample Description

Most glasses are of the black glass type; brown in thin section and free of crystals or microlites. Many are covered with a smectite shell of varied thickness, the glass surface below the clay shell shows pits and furrows and other corrosion patterns that are similar to those observed on tektites and microtektites (GLASS, 1986, 1989, 1990 ) . Among the glasses, about 2% are amber to yellowish in color and more vesicular than the black glasses. Rarely, the yellow glass occurs as streaks or schlieren within the black glass or is attached to the rim of black glass spherules ( SIGURDSWN et al., 199 1 a). The yellow glass has a distinct chemical composition, being rich in CaO and S, while the black glass is similar in composition to the average upper continental crust (e.g., SIGURD~N et al., 1991a,b; IZETT, 199la,b). Most previous studies of glass com~sition have focussed on major elements and have either provided data only on selected trace elements or relied on analyzing several glasses together rather than individually.

We selected twelve black and seven yellow glass particles, which were cleaned ultrasonically in HCI and distilled water; in three cases the adhering smectite shell (or parts thereof) was preserved and analyzed separately. The sample weights were 1.35- 11.6 mg, 290-7 10 llg, and 4.5-l 1 mg for black glasses ( 1.98 mg for H-8), yellow glasses, and smectite she&, respectively.

ANALYTICAL METHODS

Trace element analysis of all twenty samples was performed by inst~men~ neutron activation analysis (INAA ) , using HpGe de- tectors with up to 48% relative efficiency and counting times of up to three days. All procedures were checked by analyzing international geological reference materials (GQVINDARAKJ, 1984,1987). Precision and accuracy are below 10% for most elements, and in most cases between OS-S%. However, for some elements (especially Ni, Sr, Zr, and Eta in small sampIes) the precision can be as low as 50%. Details on instrumentation and procedures are given by KOEEERL et al. ( 1987) and KOEBERL ( 1992). Alter irradiation and gamma counting, the major elements were determined in polished sections by electron probe microanalysis, using a Camebax microprobe. To determine the hom~eneity of the indi~~l samples, and/or to arrive at a representative average composition, fifteen to twenty-five individual spots were measured on each sample. Each “spot” analysis was in fact an area of about I5 X 15 pm, which was scanned at TY frequency to avoid loss of volatile components. The polished sections were also examined by secondary electron (SE) and back-scattered electron @SE) microscopy, using a Jeol JSM-6400 scanning electron microscope.

RESULTS AND DIscU%~ON

Balk Composition and Comparison with Other Data

The concentrations of eleven major elements (including S), determined by microprobe, and of ~i~~i~t major, minor, and trace elements, obtained by INAA, are given in Table 1 a,b for the twelve black glass samples. The respective compositions for the seven yellow glass samples are given in Table 2. Taking some sample i~orn~nei~ and possible interiaboratory differences into account, our results are in almost perfect agreement with previous data (where avail-

Impact glasses from the K/T boundary in Haiti 2115

able). The major element compositions are in good agreement with averages given by SIGURD~~ON et al. ( 1991a,b),

IZETT et al. ( 1990), MAURRASSE and SEN ( 199 1) , and IZETT (1991b).

For trace elements, the comparison with the INAA data of SIGURDSSON et al. ( 199 1 a,b; averages of several particles ) shows excellent agreement for SC, Co, Cr, Ni, Rb, Zr, REEs, Hf, Ta, Th, and U. Our Ni and Sr concentrations show a large variability in the yellow glasses, which may be due to low precision because of the very small (0.2-0.7 mg) samples. The Ba abundances show some outliers with high values, which is most probably due to baryte crystals which have been discovered in vesicles in a few glasses ( SIGURDSSON et al., 199 la). The only significant difference to averages reported by SIGURDSSON et al. ( 199 1 a,b ) is a consistently higher Cs abundance found in the present analysis. The present Cs data have been double-checked, and no reason for this dis- crepancy was found. A comparison with data reported by IZETT ( 199 1 b ) shows larger differences; this may be because he was using a relatively new procedure (laser ablation ICP) and reported neither standard values nor values for precision and accuracy. For many trace elements, IZETT ( 199 1 b) gives results that show large variations between individual analyses, which is neither confirmed by the present work nor by data given by SIGURDSSON et al. (1991a,b).

The similarity in composition of the individual black glasses (Table 1 a) and, to a lesser degree, the yellow glasses (Table 2) is remarkable and indicates that the approach used in the present work-analyzing individual spherules-essentially confirms the conclusions of SIGURDSSON et al. ( 199 la,b), which were drawn from average analyses. There is a significant difference between the compositions of the yellow and the black glasses, documented in Fig. 2. The main differences are higher S, Ca, and Mg, but lower K, Na, and Si abundances in the yellow glasses, in agreement with SIG- URDSSON et al. ( 199 la,b), IZETT et al. ( 1990), and IZETT ( 199 la,b). There are a number of interesting trace element differences, e.g., enrichment of the more volatile elements Zn, As, Br, Sb, and Au in the yellow glasses.

The ranges of the REE concentrations for the black and yellow glasses, normalized to C 1 chondritic abundances, are shown in Fig. 3a. It is obvious that, although there is overlap in range, the overall concentrations of the yellow glasses are lower than those of the black glasses, and that the glasses have only a limited REE abundance range. Most samples show only small negative Eu anomalies, with larger negative Eu anomalies for the yellow glass samples (cf. Tables 1 a,b and 2). The average Eu/Eu* value is about 0.85 for the yellow, and 0.9 for the black glasses. The slopes (expressed as La~/ybN) are relatively constant, with values around 4.6 and almost no difference between yellow and black glasses. It was proposed (mainly from major element studies) that the glasses (mainly the black glasses) were derived from fusion of continental crust of broadly andesitic composition (e.g., SIGURDSSON et al., 199 1 a,b; IZETT, 199 la,b; MAURRASSE and SEN, 199 1) ; however, the isotopic data are not incompatible with such an assumption ( PREMO and IZETT, 199 1; SIGURDSSON et al., 199 1 b) . The major element chemistry of the black glasses is of andesiticdacitic composition if compared to the composition of various andesites reported by,

e.g., TAYLOR and MCLENNAN (1985, 1988) and ROBIN (1982).

Figure 3b shows the REE range for the black glasses compared with the North American Shale composite (NASC, from TAYLOR and MCLENNAN, 1985), which was cited by Iz~rr ( 1991a,b) as a close analogue to the glasses, and two andesites having major element compositions similar to the black glasses. The REE data for a geographically close andesite from Pica de Orizaba, Mexico, which has also been used for comparison by SIGURDSSON et al. ( 199 1 b ) , are unfortunately limited (ROBIN, 1982), but it is obvious that the Mexican andesite has a REE pattern similar to an Indonesian andesite (TAYLOR and MCLENNAN, 1988). Neither andesite nor NASC patterns fit the black glass range. The main differences are lower HREE concentrations in the andesites by a factor of about 2, steeper slope of the pattern, and usually no Eu anomaly at all. A mixture between NASC-related rocks and andesites is not satisfactory either, given the similarity of the slopes between these materials. Indeed, other trace element concentrations and ratios (such as Co, Th/U, La/Th) do not support a derivation from rocks of a composition closely related to Mexican andesites. A more complex mixture may be required-considerably altering the REE patterns.

The yellow glasses are characterized mainly by high Ca contents (23.3-26.4%, cf. Table 2; which is slightly lower than the average of 27% reported by SIGURDSSON et al., 199 1 b) and by high sulfur contents. For our seven samples we find a range of 0.08-0.32% S, while SIGURDSSON et al. ( 199 1 b ) find an average of 0.33%. For the first time we dem- onstrate here the existence of samples with high CaO, but low S contents (0.08,O. 17 wt%). The higher Ca, Mg, Sr, and Ba abundances in the yellow glass were attributed to a carbonate source, while S is probably derived from evaporites, and the depletion in the alkalies was interpreted to be due to volatilization ( SIGURDSSON et al., 199 1 b ) .

In melting experiments involving the fusion of andesite in presence of gypsum, SIGURDSSON et al. ( 1991b) produced highCa glasses at temperatures of 1300 to 1400°C that have a major element composition comparable to the Haiti yellow glasses, including high S content. Fusion of andesite in presence of carbonate also produced high-Ca glasses, which differ, however, from most of the Haiti yellow glasses in being vir- tually S free. The synthetic black glasses are, however, notably higher in S than the Haiti black glasses. We find about 0.003 wt% in the natural black glass, whereas the synthetic black glass coexisting with yellow glass in the 3-mm-diameter experimental charges contains about 0.05 wt% S. These results suggest that most of the black glass was isolated from, and did not react with, the evaporite sediment, retaining a low S content. In the experimental charges, on the other hand, dif- fusion of S or slight contamination of the black glass occurred at the millimeter scale from the yellow/ black contact.

The REE data presented here put further constraints on the origin of the yellow glass. SIGURDSSON et al. ( 1991 b) found that the yellow glass is systematically depleted by lo- 20% relative to black glass in rare earth and most other trace elements of small ionic radius. They proposed that this depletion is due to dilution of black glass during addition of CaO (and some MgO) during fusion of evaporite to form the yellow glass and estimated about 20% addition of the


TABLE la. Major and trace element data for 12 individual black impact glasses from Haiti. Major element concentrations (upper part of Table) were measured by electron microprobe and are in wt %; minor and trace elements (lower part of Tabfe) were measured by INAA and are In ppm, except where noted.

SiO, 62.25 65.07 65.45 62.82 63.53 61.42 63.30 63.46 63.01 62.86 63.04 86.00

TiO, 0.71 0.62 0.63 0.69 0.63 0.71 0.68 0.71 0.70 0.67 0.67 0.47

A405 15.06 15.31 15.29 15.05 15.61 15.40 15.31 15.71 15.46 15.16 15.26 6.93

Fe0 5.40 5.14 5.03 5.34 4.87 5.32 5.44 5.42 5.26 5.33 5.38 2.45

MnO 0.18 0.17 0.18 0.17 0.17 0.17 0.18 0.18 0.15 0.19 0.16 0.05

M9O 2.77 2.28 2.35 2.63 2.33 2.7s 2.58 2.61 2.54 2.64 2.53 1.15

cao 8.12 5.77 5.65 7.56 6.72 8.39 7.23 7.46 7.40 7.54 7.49 0.38

N%O 3.78 3.73 3.72 3.92 4.08 4.09 3.65 2.79 3.61 3.86 3.69 0.38

KzO 1.59 1.71 1.55 1.65 1.78 1.52 1.50 1.60 1.74 1.59 1.63 2.21

w, 0.07 0.09 0.02 0.09 0.10 0.06 0.06 0.07 0.09 0.08 0.10 0.02

s 0.004 0.003 0.002 0.002 0.002 0.002 0.001 0.002 0.004 0.004 0.001 0.002

Total 99.91 99.90 99.87 99.92 99.84 99.84 99.93 100.00 99.97 99.93 99.95 100.04

Na (X1

K (X1

SC

Cr

Hi-l

Fe (X)

CO

Ni

zn

ca

As

Se

Br

Rb

.?.r

zr

Sb

CS

Ba

LFl

Ce

Nd

Sm

EU

Cd

lb

DY

Tm

Yb

LU

Hf

Ta

u

Ir(ppb)

A'J(F#J)

Hg

Th

u

H-l H-2 H-3 H-4 H-5 H-6 H-7 H-9 H-98 H-IO H-1OB H-8

2.81 2.77 2.76 2.91 3.03 3.04 2.71 2.08 2.68 2.97 2.74 0.28

1.49 1.57 1.62 1.55 1.53 1.46 1.3 1.11 1.38 1.67 1.41 2.39

21.1 19.3 21 20.8 21.4 21.7 20.5 19.3 20.2 21.3 19.9 5.03

23.4 25.7 30.7 27.6 30.2 28.8 23.3 25.9 28.3 30.3 27.5 93.4

1380 1290 1390 1270 1310 1320 1410 1360 1400 1410 1390 390

4.71 4.31 4.38 4.61 4.55 4.87 4.54 4.76 4.68 4.88 4.74 1.94

15.8 13.7 15.7 14.3 15.1 15.9 14.8 12.4 14.8 14.9 15.1 20.4

35 25 <50 40 20 25 40 25 15 <30 20 260

20 20 30 30 <20 35 20 20 45 30 40 30

12 11 14.2 19 13.2 9.3 10.2 11 9.1 12 8.3 5.3

0.16 0.21 0.06 0.12 0.15 0.09 0.2 0.09 0.08 0.15 0.1 0.31

s3 <I cl.4 <I <2 x2 x1.6 qO.8 <I <I <I.8 <2

0.15 0.06 0.1 0.08 0.2 0.06 0.25 0.15 0.08 0.11 0.12 0.38

49.5 67.5 63.3 66.9 66.8 50.8 66.8 39.1 57.9 59.5 62.9 55.1

s350 460 420 560 420 510 350 430 500 280 480 <300

180 160 250 180 100 190 140 140 170 150 170 360

0.33 0.37 0.21 0.36 0.37 0.22 0.3 0.22 0.31 0.14 0.28 0.75

0.91 1.65 1.13 1.86 1.97 2 1.61 1.83 1.29 1.63 1.38 2.18

370 1270 600 610 440 350 310 410 480 280 410 360

20.5 23.6 25.1 22.1 22.7 21.4 22.9 18.3 24.1 23.5 22.3 21.4

51.3 47.1 41.5 46.8 53.8 44.1 45.8 37.1 46.5 50.5 44.8 46.8

26.2 24.7 23 27.8 29.6 24.1 23.6 20 24.3 22.8 23.1 26.5

5.05 5.54 5.27 5.56 5.41 5.28 5.11 4.39 5.43 5.47 5.07 3.97

1.73 1.54 1.61 1.53 1.51 1.63 1.48 1.31 1.41 1.58 1.37 1.34

5.2 5.6 5.3 4.9 4.9 5.1 4.7 5.1 5.4 4.8 5.2 4.95

0.85 1.04 0.84 0.88 0.87 0.89 0.83 0.97 0.91 0.82 0.86 0.84

5.1 5.7 5.7 5.6 5.3 5.3 5.1 5.6 5.6 5.3 5 4.5

0.46 0.48 0.54 0.51 0.44 0.46 0.45 0.45 0.48 0.46 0.47 0.32

2.95 3.18 3.54 3.33 2.99 3.49 3.21 2.75 3.12 3.45 3.15 1.84

0.43 0.52 0.48 0.5 0.36 0.46 0.42 0.41 0.49 0.46 0.43 0.27

4.08 3.75 5.07 4.09 3.98 4.03 4.02 3.63 3.77 4.42 3.86 14.1

0.26 0.27 0.75 0.23 0.35 0.27 0.24 0.34 0.29 0.34 0.31 0.36

1.3 0.9 0.6 0.61 1.1 0.7 0.3 1.1 0.8 2.7 0.8 7.4

<IO <1 x4 x2 <3 <3 <4 Xl.5 <2 e4 q3 ~8

4 2 1 1 2 1 1 1 2 2.3 2.5 2

<2 ~0.8 ~0.6 ~0.8 qO.6 co.9 go.8 so.9 do.8 <I.8 <I.4 e2

5.49 7.74 7.35 6.24 6.95 6.32 5.39 5.69 6.44 6.49 5.88 7.55

0.77 1.41 1.28 1.14 1.08 0.9 0.81 1.08 1.05 1.71 OAB 0.78

K/U 19351

Zr/Hf 44.12

Th/U 7.13

Le/Th 3.73

LaN/YbN 4.70

11135

42.67

5.49

3.05

5.01

0.84

12656 13596 14167 16222 16049 10278 13143 9766 16023 30641

49.31 44.01 25.13 47.15 34.83 38.57 45.09 33.96 44.04 25.53

5.74 5.47 6.44 7.02 6.65 5.27 6.13 3.80 6.68 9.68

3.41 3.54 3.27 3.39 4.25 3.22 3.74 3.62 3.79 2.83

4.79 4.48 5.13 4.14 4.82 4.50 5.22 4.60 4.78 7.86

0.93 0.90 0.90 0.96 0.92 0.85 0.80 0.94 0.82 0.92

H-l H-2 H-3 H-4 H-5 H-6 H-7 H-9 H-96 n-10 H-106 H-8

Eu/Eu* 1.03


TABLE lb. Average, standard deviation, and range of composition for 11 black impact glasses from Haiti (excluding the high Si,K glass H-8). Units see Table la.

sio, TiOt

AWJ Fe0 MnO f&JO cao Na,O

W WA S Total

Avcra~ Std.Ocv. Mini- Maxinun

Na (Xl 2.77

K (Xl 1.46

SC 20.59

Cr 27.43

nn 1357

Fe (X) 4.64

CO lb.77

Ni 22

zn 27.1

GE ii.8 AS 0.13

Se 0.00

Sr 0.12

Rb 59.2

St- CO1

zr 166

Sb 0.28

CS 1.57

SO 503

La 22.41

C.2 46.30

Nd 24.47

Sm 5.23

EU 1.52

Gd 5.11

lb 0.89

OY 5.39

Tm 0.47

Yb 3.20

LU O.b5

nr 4.06

la 0.33

u 0.99

Ir(ppb) e2

AUQJpb) 1.80

no 4

Th 6.36

IJ 1.10

0.25 2.08 3.04

0.15 1.11 1.67

0.79 19.3 21.7

2.48 23.3 30.7

48 1270 1410

0.18 4.31 4.88

0.97 12.4 15.9

13 0 40

11.5 0 45

2.8 a.3 19

0.05 0.06 0.21

0.00 0 0

0.06 0.06 0.25

8.76 39.1 67.5

1b7 0 560

36 100 250

0.07 0.16 0.37

0.34 0.91 2

263 280 1270

1.78 la.3 25.1

4.42 37.1 53.8

2.49 20 29.6

0.32 b.39 5.56

0.12 1.31 1.73

0.26 4.7 5.6

0.06 o.a2 l.Ob

0.25 5 5.7

0.03 O.bb 0.54

0.23 2.75 3.5b

0.04 0.36 0.52

0.u) 3.63 5.07

0.14 0.23 0.75

0.60 0.3 2.7

0.W 4 40

0.90 1 b

0.00 CO.6 <2

0.72 5.39 7.7b

0.27 0.77 1.71

K/U 13853 2769 9766 19351

?P/Hf 40.80 6.74 25.13 49.31

ThlU 5.98 0.93 3.80 7.13

La/Th 3.55 0.32 3.05 4.25

LaN/YbN 4.74 0.30 b.lb 5.22

Eu/Eu* 0.90 0.07 0.80 1.03

Average Std.Dev. Mininun Maxinun

(N=ll)

63.29 1.09 61.42 65.b5

0.68 0.03 0.62 0.71

15.33 0.20 15.05 15.71

5.27 0.17 4.87 5.46

0.17 0.01 0.15 0.19

2.55 0.16 2.28 2.77

7.21 0.82 5.65 a.39

3.72 0.33 2.79 4.09

1.62 0.09 1.50 1.78

0.07 0.02 0.02 0.10

0.002 0.001 0.001 0.006

99.91

evaporite component on the basis of major element data. The new data confirm that the yellow glass is systematically depleted in REEs with respect to black glass by about 15- 25%. The similarity of the REE pattern and slope of the two glasses is striking, but is not directly explained by a straight- forward mixture of andesitic rocks with carbonate and evap orite components. Carbonates and evaporites have relatively low REE abundances (around OS-5 X CI), flat REE patterns, very low Th abundances, and Th/U ratios < 1, to name just a few differences (e.g., HENDERSON, 1984; KIESL et al., 1990; Sharpton and Koeberl, unpubl. data). A 2: 1 or 3: 1 mixture of andesite with carbonate and evaporite rocks, which is nec- essary to reproduce the high Ca, Mg, and S contents, would result in lower La,.,/Yb,., and Th/U ratios than actually observed in the yellow glass. Some elemental fractionation may be required.

Homogeneity of Glasses

For a more detailed characterization of the glasses we also studied the internal homogeneity of each sample. In Table 3a,b we give results that are typical for black and yellow glasses, respectively. A microprobe profile across the black glass sample H-7 shows a good homogeneity at the lo-100 pm range. In the black glasses, SiOr is inversely correlated with, e.g., Al, Fe, Ca, and K. The yellow glasses are generally more variable in composition, as exemplified by sample Y- 5 (Table 3b). Figure 4 shows the variation of some elements in the yellow glass. The SiOr content is positively correlated with Al, Fe, K, Ti, and Mn, but negatively correlated with Ca and Mg. Sulfur and P show no direct correlation. The glass particles analyzed in this study are representative of the main population of the two homogeneous impact glass types from the K/T boundary in Haiti. SIGURDSSON et al. ( 1991a) have also documented rare black glass particles from this locality, which are internally heterogeneous and contain streaks or schlieren of high-Ca yellow glass.

Discovery of a New Glass Type

One of the glass particles (H-8) was found to have a major and minor element composition which is very distinct from the black and yellow glasses and defines a new type of impact glass from the K/T boundary, which we refer to as high Si- K glass. Macroscopically, this sample is a light brown to grey glass shard and is somewhat more translucent than other samples. Figure 5a shows the comparison of the bulk chemical composition of this glass with the other black glasses. Some trace elements are enriched in H-8 (e.g., Cr, Co, Ni, Zr, Sb, Hf, W), while others are depleted compared with other black glasses (e.g., SC, Mn, Rb, Ba, U). The chemical differences cannot be explained by simple admixture of quartz, because enrichment and depletion factors are variable between different elements. Figure 5b shows that the HSi,K glass has a REE pattern that is slightly different from the other glasses in having lower HREE abundances and a higher L+/Yb,., ratio. This is another indication of a different source rock, not just dilution by quartz. MARGOLIS et al. ( 199 1) reported on the presence of a glass with high Si and K at the Mimbral K/T section, and SMIT et al. (1992a) report data for a


TABLE 2. Major and trace element date for 7 indivMual yellow impact glasses from Haiti, Major element concentretkms (upper part d Table) we measured by electron mfcroprobe and are in wt %; minor and trace elements (lower parl of Table) we measured by INAA and we in ppm, except where noted.

SIO, 47.64 49.08 48.24 49.21 49.62 49.34 ha.00 48.73 0.71 47.64 49.62

TiO, 0.63 0.63 0.62 0.64 0.65 0.65 0.64 0.64 0.01 0.62 0.65

A&Q 12.66 13.42 12.58 13.45 13.60 13.62 13.42 13.25 0.4'1 12.58 13.62

Fe0 4.92 4.99 4.91 5.04 5.06 5.04 4.92 4.98 0.06 4.91 5.06

MnO 0.15 0.16 0.15 0.16 0.16 0.16 0.15 0.15 0.00 0.15 0.16

MS@ 4.10 4.04 4.16 4.11 4.00 3.87 3.78 4.02 0.14 3.78 4.18

cao 26.34 24.14 26.22 24.10 23.38 23.70 25.08 24.71 1.10 23.38 26.34

Na,O 2.43 2.61 2.13 2.53 2.57 2.66 2.84 2.54 0.20 2.13 2.8b

K@ 0.55 0.66 0.56 0.59 0.66 0.72 0.80 0.65 o.oa 0.55 0.80

w+ 0.05 0.05 0.05 0.04 0.05 0.07 0.08 0.057 0.013 0.044 0.083

s 0.32 0.25 0.23 0.27 0.22 0.08 0.17 0.220 o.om 0.084 0.322

Total 99.87 100.02 99.85 100.16 99.98 w.91 Pp.89 Pp.%

wa (X)

K (X1

SC

0

l4n

Fe (X1

co

Ni

2n

Ga

AS

SC l?r Rb

Sr

Zr

Sb

CS

Be

L8

Ce

Nd

Bm

EU

Ed

lb

w

Tnl

Yb

LU

Hf

Ta

u

Ir(ppb)

AU(ppb)

H9

Th

u

Y-l Y-18

1.81 2.08

0.62 0.63

21.4 21 .a 22.6 27

1120 1190

4.56 4.12

21.5 20.7

360 doe

100 75

5.4 8

0.75 0.66

i4 ~6

1.1 0.9

28 26

600 800

170 140

2.41 2.29

1.72 1.83

580 510

la.9 17.8

44.9 32.9

22.9 22

4.28 4.11

1.49 1.12

4.9 4.2

0.89 0.82

5.2 4.8

0.43 O.ba

2.66 3.05

0.37 0.36

3.53 3.09

0.74 0.48

1.8 0.9

2 <2.5

12 a <4 4

4.95 4.88

0.94 1.12

Y-2 Y-3

1.58 2.01

0.57 0.72

20.6 23.1

47.3 45.5

1100 1210

4.76 s.07

26.2 28.9

<500 <500

70 60

7.3 6

0.86 0.45

<4 3.7

2.3 2.3

31 21

<loo0 <llOO

270 220

4.5'1 2.57

2.38 3.51

560 750

16.1 19.6

27.b 34.6

16 <30

3.91 4.21

0.83 0.94

3.9 c5

0.71 0.78

4.4 <5

co.5 ~0.8

2.57 3.29

0.25 0.38

3.15 3.01

0.35 1.2

2.7 <3

<lo <a

22 <I5

c4 (9

4.24 6.93

1.09 1.80

Y-4 Y-48 Y-5

1.91 1.98 2.11

0.58 0.61 0.69

20.7 21.1 20.7

22 26 17.8

1190 1160 1090

4.41 4.27 4.36

19.1 18.7 24.1

<zoo <too 210

90 65 60

4 7 7.8

0.3 0.55 0.98

2.1 c3 2.1

1.2 1.1 0.9

26 25 22

680 790 650

250 180 160

1.92 2.21 2.02

3.24 1.91 2.41

240 480 3700

16.8 18.1 17.4

34.2 37.8 39

19 19.5 20.3

3.92 4.04 4.36

1.22 1.18 1.42

4 c5 4.4

0.77 0.74 0.78

4.5 4.7 5.1

qO.4 co.5 10.5

1.86 2.79 3.23

0.33 0.39 0.43

3.17 3.24 3.43

0.9 0.6 0.44

1.2 1.1 2.3

2 c4 ~6

10 8 9

G ~6 <3

4.37 5.14 S.06

1.12 1.2t 1.38

Average Std.Dev. Ilinirm Naximm

1.93 0.17 1.58 2.11

0.63 0.05 0.57 0.72

21.34 0.82 20.6 23.1

29.7 10.9 17.8 47.3

1151 45 10% 1210

4.51 0.30 4.12 5.07

22.74 3.53 la.7 28.9

285 135 0 360

74.3 14.2 60 100

6.5 1.3 4 8

0.65 0.22 0.3 0.98

2.63 1.39 0 3.7

1.60 0.58 0.9 2.3

25.6 3.16 21 31

704 325 0 800

199 4s 140 270

2.56 0.82 1.92 4.51

2.43 0.6S 1.72 3.51

974 1122 240 3700

17.81 1.11 16.1 19.6

35.83 5.07 27.4 44.9

19.95 7.28 0 22.9

4.12 0.16 3.91 4.36

1.17 0.22 0.83 1.49

4.28 1.96 0 4.9

0.1(1 0.05 0.71 0.89

4.78 1.70 0 5.2

0.46 0.21 0 0.48

2.78 0.45 1.88 3.29

0.36 0.05 0.25 0.43

3.23 0.17 3.01 3.53

0.66 0.25 0.35 1.1

1.43 0.84 0 2.7

2 1 0 2

11.5 6.1 0 22

<5 0.00 <3 <9

5.08 0.82 4.24 6.93

1.24 0.26 0.94 1.80

K/U 65% 5625 5229 4000 5179 5041 5000 5239 720 4000 6596

2r/tlf 48.16 45.31 85.71 73.09 78.86 55.56 46.65 61.91 15.66 15.31 85.71

Th/U 5.27 4.36 3.a9 3.85 3.90 4.25 3.67 4.17 0.50 3.67 5.27

ls/Th 3.82 3.65 3.80 2.83 3.84 3.52 3.44 3.56 0.33 2.83 3.84

LaN/YW 4.80 3.94 4.23 4.03 6.04 4.38 3.64 4.44 0.74 3.64 6.04

Eu/Eu* 0.w 0.82 0.65 0.66 0.94 0.88 0.99 0.85 0.13 0.65 0.99

Y-l Y-18 Y-2 Y-3 Y-4 Y-48 Y-5 Average Std.Dev. Ninimun naxinm

Impact glasses from tbe K/T boundary in Haiti 2119

FIG. 2. Comparison of major and minor element contents in average yellow glass vs. average black glass.

K-rich glass which is, however, different from our high Si-K glass (with only about 66 wt% SiO2).

The study of the internal homogeneity or heterogeneity of the HSi,K glass may provide important clues as to its or&in. Figure 6a shows a BSE image of a typical black glass (H-7)) depicting its internal homogeneity. Figure 6b,c shows BSE images of sample H-8; the difference is striking. The darker areas are pure SiOz (maybe lechatelierite) , and a few quartz 8rains at various stages of dissolution in the melt are present. Fi8ure 6c shows clear evidence for flow structures in the glass. The appearance of the HSi,K glass in the BSE image is very similar that observed for some impact glasses, e.g., Aouelloul impact gle To quantify the compositions, microprobe profiles have been measured across this section. Characteristic data are given in Table 4, confirming that the dark areas consist of pure SiOz . The schlieren that appear lighter in the BSE images have lower Si, but all other major elements are higher. Figure 7a is a correlation dia8ram of Al vs. Fe, Mg and Ca which shows only a binary trend, but no clear evidence for multiple source components. Some elemental variations in the HSi,K glass are shown in detail in Fig. 7b. For deter- mination of the average composition (Tables 1 and 4) only the more homogeneous part of the “matrix” was used.

The major element composition of the Iow-Si schlieren in the HSi,K glass is not unlike the composition of some sedimentary rocks, e.g., shales or argillites (TA~OR and MCLENNAN, 1985 ) , but very much unlike any volcanic rocks. As the appearance of the glass is similar to some impact glasses, a similar origin by melting a mixture of shales and sandstones or quart&s is conceivable for the HSi,K glass from. Haiti. The source rocks for the Haiti glasses have tbere- fore incorporated such target rocks.

Relation of Glass to Smectite Mantles

The glass particles are enclosed in a smectite shell, which is probably the alteration product of the glass. In most sections of the Haiti deposit, the glass is completely altered, leaving spherical to ellipsoidal smectite bodies. Table 5 gives the major and trace element data for the fragments of three smectite shells that were directly associated with black glass spherules, providing direct evidence for the alteration of the glass. Al- though some elemental abundances vary by factors of around 2, the smectites have compositions that are relatively similar to each other. The major element compositions of our smectites are generally similar to those of IZE~ ( 199 lb) and JG- HANNO et al. ( 1992). Figure 8a gives the comparison between the compositions of the smectite shells and the black glass cores. The most striking differences am higher abundances of Mg, Fe, Zn, and Br in the smectites, compared to lower abundances for most other elements, especially Na, K, Ca, Mn, Cr, Co, Rb, Sr, Cs, Th, and U. A few elements, such as the major elements Si, Ti, and Al, show little difference. The REEs exhibit an obvious difference. Some of these chemical fractionations are similar to changes observed in alteration of basaltic glasses (e.g., LUTZE et al., 1985; JERCINOVIC et al., 1990). Figure 8b gives the REE patterns for all three smectites, showing variable Eu and small Ce anomalies. We do not see any major negative Ce anomalies such as the one observed by IZETT (1991b).

The chemical comparison between the smectite shells and the glass show directly the changes in composition that occur upon alteration of the glass. Assuming that the glass was produced by impact of the K/T bolide-which is likely, given the age of the Haiti tektites, which is indistinguishable from


1 (4 1. I I I I I I1 * I I I I LaCePrNd SmEuGdTbDyHoErTmYbLu

REE

1 04 1>.3,, IIIIVIIII LaCePr Nd SinEuGdTbDyHoErTmYbLu

REE

et#ac *Ml- -A---

FIG. 3. Chondrite-normalized REE abundance ranges for Haiti impact glasses and Elated rocks. (a) Black glass and yellow glass ranges: (b) Range of black glasses (shaded area) compared with data for-a Mexican-andesite (ROBIN, 1982), an Indonesian andesite (TAYLOR and MCLENNAN. 1988). and the North American Shale composite ( NASC, after TivLoR’and MCLENNAN, 1985 ) . C 1 -nor- malization factors from TAYLOR and MCLENNAN ( 1985).

that of the K/T boundary ( IZETT et al., I99 1; GILLOT et al., 199 1; HALL et al., 199 1 )-it may be expected that similar impact glasses were deposited (and later altered) at many other K/T boundary locations. Some part of the clay layers typically found at K/T boundary locations, e.g., in the we&- em interior of the USA (e.g., IZETT, 1987a; BOHOR, 1990; SHAR~ON et al., 1990), was probably derived by diagenetic alteration of impact glasses similar in composition to the black glasses from Haiti. It has been suggested by BOHOR ( 1988) that the K/T boundary claystones are an alteration product of impact glasses; a direct comparison between K/T boundary claystone and Haiti black glass was made by SIGURDSSON et al. ( 199 1 a), who proposed that the two were related. They attributed the REE depIetion in the claystone to diagenetic alteration. Data on REE mobility during dia- genesis are sparse., but seem to indicate no or limited mobility of the REEs, while some hydrothermal processes are easily capable of removing the REEs (HENDERSON, 1984). Indeed, studies of the alteration of impact melt sheets by low-temperature hydrothermal processes yield clay as the alteration product of impact glass and melt ( GOODING and KEIL, 1978; NEWSOM, 1980; NEWSOM et al., 1986), which provides a

good analogy to the alteration products of the K/T impact glasses.

Some authors have proposed that spherules found at various K/T boundaries are alteration products of either direct condensates of bolide matter or of impact-derived spherules (e.g., SMIT and KLAVER, 1981; MONTANARI et al., 1983; BOHOR, 1984; BROOKS et al., 1985; NASLUND et al., 1986; BOHOR and BETTERTON, 1988). Our data are consistent with the suggestion that impact-derived spherules may be a significant source for clays and alteration spherules found at K/T boundaries.

Another important implication is the explanation of what might be called the “source paradox.” Several authors have concluded, on the basis of isotopic data (DEPAOLO et al., 1983), but also mainly on the basis oflow REE abundances in boundary clays, that the bolide impact occurred into oceanic crust (e.g., HILDEBRAND and BOYNTON, 1988a,b). Even as late as 1990, when describing the Haiti K/T boundary layer, HILDEBRAND and BOYNTON ( 1990a,b) insisted on a “major oceanic contribution,” although evidence from mineral composition ( SHAR~TON et al., 1990) and the occurrence of abundant quartz indicated a continental source. We now know that the alteration of glasses with normal upper crustal REE patterns can yield clays with low REE abundances and flat patterns (Fig. 8b). If we compare the REE patterns observed in claystones from K/T boundaries (e.g., IZETT, 1987a; BOHOR and MEIER, 1990; SHARPTON et al., 1990) with the patterns of the smectites given here, a close similarity is obvious (Fig. 8b). REE patterns of claystones at different K/T boundaries have either slight positive (Starkville South, Brownie Butte: IZETT, 1987a) or negative (Scollard Canyon: SHARPTON et al., 1990) Ce anomalies, indicating diagenetic processes of locally variable redox degree.

Nomenclature and Compuison with Tektites

Several workers have called these glasses “tektites” ( IZETT et al., 1990, 199 1; IZETT, 199 la,b; MAURRASSE and SEN,

1991; MARGOLIS et al., 1991; SMIT et al., 1992a,b). However, tektites are natural glasses that are generally chemically homogeneous on a lo-100 pm scale, are reduced, are almost water-free, contain lechatelitite and bubbles, may show signs of glass flow, are mostly of spherical-symmetric shape (or fragments thereof), and occur in geographically defined strewn fields. Tektites within one strewn field are related in chemistry, isotopic composition, and age. So far, four groups of tektites are known, defined by their occurrence in four strewn fields (listed in order of decreasing age): North Amer- ican, moldavite, Ivory Coast, and Australasian (BARNES, 1963; CHAO, 1963; KOEBERL, 1986). Tektites have most probably originated by hypervelocity impact melting of upper crustal rocks (TAYLOR, 1973; KING, 1977; KOEBERL, 1986, 1990; GLASS, 1990). Sometimes researchers c&l some natural (impact) glasses “tektites,” but often these glasses do not fulfill the criteria mentioned above-not every impactite is a tektite.

Apart from surface alteration and etching, which often re- sembles that observed for microtektites (GLASS, 1974), the black Haiti @asses have surface properties and shapes similar to (small) tektites. They are water-poor, containing on the order of 0.02 wt% Hz0 (Koeberl, unpubl. data; OsKARSSON

urn SIO, TIO,

0 63.23 0.70 15.30 5.47 0.17 2.59 7.52 3.54 1.50 0.053 0.003 100.08

80 63.09 0.68 15.33 5.57 0.18 2.61 7.25 3.68 1.49 0.063 o.ODO 99.94

160 63.07 0.66 15.27 5.54 0.19 2.59 7.34 3.72 1.48 0.083 0.000 99.94

240 63.29 0.70 15.49 5.46 0.18 2.59 6.96 3.71 1.47 0.038 0.004 99.89

320 63.01 0.70 15.20 5.68 0.20 2.61 7.18 3.78 1.50 0.080 0.001 99.94

400 63.69 0.68 15.36 5.47 0.19 2.58 6.89 3.63 1.51 0.038 0.000 100.04

480 63.99 0.68 15.42 5.23 0.16 2.48 6.88 3.53 1.53 0.030 0.000 99.92

560 63.11 0.67 15.29 5.57 0.19 2.59 7.23 3.77 1.46 0.051 0.004 99.94

640 63.73 0.71 15.52 5.21 0.19 2.54 6.83 3.65 1.53 0.039 0.000 99.95

720 62.99 0.71 14.95 5.62 0.17 2.59 7.76 3.58 1.49 0.081 0.000 99.94

800 63.42 0.65 15.28 5.53 0.17 2.53 7.12 3.58 1.55 0.044 0.002 99.87

880 63.16 0.68 15.36 5.57 0.16 2.59 7.18 3.61 1.52 0.071 0.000 99.91

AVERAGE 63.32 0.68 15.31 5.49 0.18 2.58 7.18 3.65 1.50 0.056 0.001 99.95

STD 0.09 0.01 0.04 0.04 0.00 0.01 0.08 0.02 0.01 0.005 0.000

Fe0 MnO MgO CaO Na,O K,O w5 s TOTAL

Impact glasses from the K/T hcundary in Haiti

TABLE 31. Major element composition along a mkroprobe profile across black glass spherule H-7. All data in wt %.

TABLE 3b. Major element composition along a microprobe proMe (see Fig. 4) across yellow glass partide Y-5. All data In wt 96.

“n-l SO, TiO, Al,O, Fe0 MnO MgO Cc10 NqO K,O P.Q S TOTAL

0 47.48 0.62 13.09 5.02 0.15 3.98 26.11 2.43 0.70 0.07 0.16 99.82

30 47.92 0.71 13.82 5.19 0.16 3.47 24.67 2.91 0.88 0.09 0.17 99.98

6D 47.67 0.68 13.50 5.39 0.16 3.62 24.97 2.84 0.77 0.08 0.17 99.86

90 48.09 0.72 13.85 5.18 0.16 3.34 24.82 2.72 0.80 0.12 0.15 99.95

120 45.54 0.44 11.97 3.30 0.11 4.81 29.25 3.32 0.57 0.09 0.20 99.60

150 47.67 0.69 13.35 5.32 0.16 3.37 25.50 2.61 0.74 0.08 0.25 99.74

180 47.18 0.64 12.95 4.94 0.16 3.85 26.52 2.78 0.69 0.10 0.27 100.06

210 48.01 0.61 13.57 4.59 0.16 3.46 25.75 2.65 0.78 0.07 0.24 99.89

240 47.99 0.66 13.26 5.19 0.14 4.03 24.93 2.70 0.76 0.05 0.16 99.88

270 49.56 0.67 13.93 4.98 0.14 3.70 22.70 3.11 1.00 0.09 0.09 99.96

300 50.20 0.63 14.22 4.78 0.17 3.61 22.15 3.07 1.06 0.08 0.04 100.02

330 48.65 0.65 13.54 5.20 0.14 4.10 23.62 2.92 0.81 0.09 0.17 99.89

AVERAGE 48.00 0.64 13.42 4.92 0.15 3.78 25.08 2.84 0.80 0.08 0.17 99.89

STD 0.32 0.02 0.16 0.15 0.00 0.12 0.51 0.07 0.04 0.00 0.02

et al., 1992), which is similar to contents found for other tektites and some impact glasses (GILCHRIST et al., 1969; KOEBERL and BERAN, 1988 ) . Bubbles are present, but so far no lechatelierite grains have been found in the black glasses. Them is some disagmement about the redox state of the black glasses. Izn-rr ( 199 1 b ) cites measuxements by F. Senffle which are said to show that the Haiti glasses are just as reduced as other tektites @DALI et al., 1987), but OSKARSSON et al. ( 199 1, 1992) and Sigurdsson et al. (unpubl. data) find, by Mbssbauer spectrometry, that the glasses are oxidized. So far it has not been shown that the Haiti glasses occur in a strewn field, although the new discovery of glasses at Mimbral, northeastern Mexico ( MARGOLIS et al., 199 1; SWINBOURNE et al., 199 1; SMIT et al., 1992a), .and probably at two DSDP sites (536 and 540) near Yucatan (ALVAREZ et al., 199 la,b), may provide more evidence. At this point we feel it is more pertinent to call the Haiti glass simply “impact glasses.” The yellow glasses are too small for consideration as tektites and lack several of the other criteria; the HSi,K glass qualifies as impact glass.

2121

1:

;

0.1.

o.OlJ, I I I I I I I I I I I 0 30 80 90 120 150 180 210 240 270 300 330

Distanceon Line(um)

FIG. 4. Major element variation in microprobe profiles across yellow glass sample Y-S.

2122 C. Koeberi and H. Sip&son

RG .5. Comparison of the compositions of the HSi,K glass H-8 vs. average black glass, (a) Major and trace elements; (b) REEs for average black and yellow glasses compared with sample H-8.

Tektites generally do not produce any alteration products; tektites (and microtektites) are simply dissolved in the environment. There is abundant literature describing the dissolution of tektite glass by reaction with water (e.g., GLASS, 1974, 1984, 1986; LAMARCHE et al., 1984; BARKATT et al., 1984, 1986, 1989). Tektite glass dissolves more slowly in seawater, which seems to have a buffering effect due to higher Mg concentrations (BARKATT et al., 1986, 1989; GLASS, 1986 ) . Gnly in a fe\; cases has some alteration been observed to be associated with tektites. North American tektite fiag- meats found at DSDP site 612 sometimes have rims of hy- drated glass ( THEIN, 1987; KOEBERL and GLASS, 1988; Koe- berl, unpubl. data). LOVERING et al. ( 1972) observed some zeolites in grooves on tektites, but there is no proof that these are real tektite alteration products. EL-SHAMY ( 1973) and BARKA~ et al. ( 1984) found that the tektite composition is

an important factor-the long-term stability of the glass against dissolution requires a silica content in excess of 67 mol%, most tektites have higher silica contents, although there are a few Australasian tektites with lower silica contents. The Haiti glasses are slightly below this limit; however, even Pre- cambrian glasses contain not much more silica (PALMER et al., 1988). A combination of processes is likely to have brought about the alteration of the Haiti K/T glasses to smectite, including chemical composition, age, and dia8enetic environment.

Origin of the Haiti Glasses and Source Recks

A number of arguments have been used to constrain the type and location oftarget rocks for impact glasses and tektites. Mineralogical, chemical, and isotopic arguments have


FIG. 6. BSE images of sections of two glasses from the Haiti K/T boundary section. (a) Black glass sample H-7; (b) HSi,K glass sample H-8, showing dark areas (pure silica), and lighter schlieren, showing tlow structures; (c) Close-up of section of sample H-8, showing the dissolution of the silica grains in the glass matrix.

shown that post-Archean upper crustal continental rocks, e.g., siltstone, shale, or sandstone, are likely source rocks for tektites (GLASS and BARLOW, 1979; TAYLOR, 1973; KOEBERL, 1986,1990, BLUM et al., 1992 ) . Fu~e~ore, impact glasses and tektites must have been derived from the top surface layers, which follows from ‘@Be studies of Australasian tektites by PAL et al. (1982), YIOU et al. (1984), and ENGLERT et al. ( 1984). The main mass of such glasses must have been derived from a surface layer with a thickness on the order of around 100 m, otherwise the observed “Be content cannot be explained (PAL et al., 1982; BLUM et al., 1992). This observation has impli~tions for the target rock stratigraphy.

Recently, a crater site in Yucatan was proposed as the K/T impact crater (e.g., PENFIELD and CAMARGO, 1991;

KRING et al., 1991; HILDEBRAND~~ al., 1991). The “Chicxu- lub” structure has a diameter of about 180 km (as defined by gravity and magnetic anomalies), but is not well estab- lished as a crater and no reliable age has yet been obtained, thus a connection with the K/T boundary is uncertain at this time ( SHARPTON et al., 199 1). Most other structures or craters suggested thus far as possible “crater candidates” were either not confumed as craters ( HILDEB~ND and BOYSTON, 1990a) or, as in the case of the Kara crater, were found to have the wrong age ( KOEBERL et al., 1990). The Manson Impact Structure is the largest recognized in the USA, 35 km in diameter ( HARTUNG and ANDERSON, 1988), and has a radiometric age in~stin~~hab~e from that of the Cretaceous- Tertiary (K-T) boundary ( KWNK et al., 1989; HARTUNG et

2124 C. Koeherl and H. Sigurdsson

1 2 3 4 5 6 7 a 9 IO 11 12

65.72 66.32 72.90 87.30 86.04 85.60 85.71 93.27 86.21 84.59 99.92 99.89 1.60 1.57 1.22 0.45 0.47 0.47 0.48 0.25 0.47 0.43 0.01 0.01

20.04 19.6s 13.42 6.19 6.80 6.93 7.06 3.11 6.67 7.76 0.02 0.01

5.39 5.30 4.29 2.09 2.44 2.56 2.38 1.12 2.43 2.68 0.03 0.05 0.11 0.11 0.09 0.06 0.05 0.06 0.06 0.00 0.05 0.06 0.00 0.01 2.66 2.61 2.04 0.79 l-23 1.22 1.12 0.45 1.15 1.34 0.00 0.01 0.w 0.94 0.72 0.23 0.39 0.43 0.35 0.12 0.36 0.42 0.00 0.w 0.71 0.69 0.64 0.42 0.32 0.38 0.35 0.38 0.42 0.40 0.00 0.w 2.66 2.66 2.57 2.39 2.18 2.22 2.39 1.22 2.14 2.24 0.02 0.01 0.07 0.07 0.06 0.03 0.04 0.04 0.01 0.01 0.01 0.02 0.00 0.00

0.000 0.000 o.ooo 0.001 0.001 0.000 0.002 0.000 0.002 0.002 0.002 osm8 99.92 Pp.92 99.94 99.96 99.96 W.92 99.91 99.93 99.91 99.93 lDo.Do too.oD

t3 14 15 16 17 10

86.34 86.27 85.04 85.81 85.07 85.91 85.66 86.75 86.18 85.42 86.00

0.44 0.45 0.47 0.48 0.49 0.48 0.47 0.45 0.46 0.48 0.47

6.71 7.05 6.98 6.97 7.1t 6.84 I.04 6.45 6.92 7.19 6.93

2.36 2.42 2.47 2.53 2.39 2.50 2.54 2.39 2.36 2.49 2.45

0.04 0.04 0.05 0.04 0.04 0.05 0.06 0.06 0.06 0.05 0.05

1.11 1.02 1.24 1.17 1.10 1.19 t.22 1.13 1.17 t.20 1.15

0.36 0.35 0.39 0.40 0.38 0.39 0.41 0.35 0.38 0.40 0s

0.38 0.42 0.36 0.37 0.32 0.43 0.36 0.40 0.37 0.41 0.38

2.17 2.25 2.19 2.25 2.30 2.19 2.20 2.13 2.17 2.22 2.21

0.02 0.01 0.04 0.03 0.02 0.01 0.04 0.02 0.03 0.03 0.02

0.005 0.000 0.001 0.001 0.003 0.002 o.oDD 0.0% 0.003 0.001 0.002

Pp.93 too.27 too.03 100.05 lOD.02 99.99 1w.ot 100.12 lW.10 99.90 loo.04

19 20 21 22 AVERAGE STD

0.36 o.ot 0.20

0.07

0.01 0.06

OS!

0.03

0.05

o.ot

0.002

al., 1990). The Manson structure may be considered to be at least one element of the events which led to the catastrophic loss of life and extinction of many species at that time.

The elemental abundances in the black glasses are not incompatible with the ranges observed for target rocks (Eischeid well) and some impact glasses found at the Manson crater ( HARTUNG and ANIXRS~N, 1990, KOEBERL and HARTUNG,

199 1, 1992), and evaporites do exist in the Manson target rock stratigraphy ( WITZKE, 1990). However, no definite conclusion can be made since the isotopic characteristics of the Manson rocks are not yet known.

The Chicxulub crater structure contains abundant carbonate, limestone, and evaporite rocks, and the presence of andesitic rocks has been reported ( KRING et al., 1991; HIL-

DEBRAND et al., 199 1 ), although it is not clear if the “andesite” is a real andesitic bedrock or makes up the proposed melt sheet. The stratigraphy of the crater and the exact succession and age of rocks are not entirely clear at this time, IargeIy because the structure is now buried under about 1 km of Tertiary sediments, mainly limestone, and because of limited sample availability due to the destruction of core samples in a fire. The sedimentary sequence (composed mainly of carbonates and evaporites) overlies a basement at 3-6 km depth that is ir&rred to be composed of metamorphic rocks ( PENFIELD and CAMARGO, 199 1; HILDEBRAND et al., 199 1; SHARON et al., 199 1). There seems to be agreement that, ifchicxulub was formed by impact at a time at or before the end of the Cretaceous, the pm-impact surface consisted largely of rocks for the carbonate-evaporite sedimentary se-

quence (HILDEBRAND et al., 1991; SHARITON et al., 1991;

Sharp&m and Koebml, tmpubl. data), probably releasing large quantities Of Co2 and S@ into the atmosphere ( ~EBRAND

et al., 1991; SIGURIXSON etal., 1991b, 1992). There are a number of difhcuhies with Chicxulub being

the source for the Haiti impact glasses. One problem is that impact gIasses originate, as we have discussed before, from the surface layers of a given target area. However, any “andesitic” rock or other basement rocks at Chicxulub were covered by carhnates and evapotites of up to several kilometers thickness. HILDEBRAND et al. ( 199 1) report glass recovered from a Chicxulub well which is similar in major element composition to some Haiti black glasses. However, other rocks that are present at Chicxulub (limestones, evaporites) that will be mixed in upon impact have trace element signatures that are not compatible with any glass composition (Sharpton and Koeberl, unpubl. data). Some researchers, e.g., IZETT ( 1991c), proposed that two impacts, involving Chicxulub and Manson, might be responsible for the K/T event; in view of the preliminary nature of some data we refrain to speculate on such an origin. Other proposed impact locations, such as near Cuba ( BOHOR and SEITZ, 1990), have not been confirmed ( MCHONE and DIET& 199 1) .

SUMMARY AND CONCLUSIONS

We have presented detailed analytical data for twelve black and seven ydlow glasses from the Haiti K/T boundary section, plm three smectite mantles. From these studies, we de- rive the following observations and conclusions.


:I;,$. : : 1 0 5 10 15 20 25

Al203 (wt. %)

86

5 80

55

0 lDo2oDSao4oo~

50

DistanceonUne(um)

+SI@? +M?O3+FFeo

-=-NaX)*WCJ

FIG. 7. Variation of major element composition in HSi,K glass sample H-8. (a) Correlation diagram of AlSO vs. FeO, CaO, and MgO, showing a positive correlation without a clear indication for more than 2 components; (b) Compositional changes along a microprobe profde across light schlieren, “normal” matrix, pure silica, and “normal” matrix.

1)

2)

3)

4)

We have essentially confirmed the data for bulk black and yellow glasses obtained in previous studies (e.g., SI- GURDSSON et al., 199 la,b; IZETT, 199 la,b) and have added a number of elements to the data base. We have also presented detailed analyses of individual spherules and shards, which are in good agreement with bulk data for several glasses together, as reported before. There are major chemical differences between black and yellow glasses, e.g., higher S, Ca, Mg, Zn, As, Br, Sb, and Au, but lower K, Na, and Si abundances in the yellow glasses. For the 6rst time we have demonstrated the existence of yellow glass samples with high CaO, but low S contents (0.08,0.17 wt%), which might be expected by fusion of various proportions of carbonates and evaporites or carbonates alone. Black glasses are very homogeneous in their composition at the 1 Cl- 100 pm level, while yellow glasses show much more variability. The intra-sample elemental correlations are different for black and yellow glasses. In black glasses, SiO, shows a negative correlation with, e.g., Al, Fe, Ca, and K, while in yellow glasses Si02 is positively correlated with Al, Fe, K, Ti, and Mn, but negatively correlated with Ca and Mg. One of the black glasses was found to have distinct pet- rologic and chemical properties. The sample, having

TABLE 5. Major and traca element composition of chips from three smectite shells around black glass spherules. Major elements (upper part of Table) are in wt %; minor and trace elements (by INAA, lower part of Table) are in ppm, except where noted.

SiO, 63.4 63.9 64.3 63.87 0.37

TIO, 0.68 0.61 0.71 0.67 0.04

A44 15.7 15.9 16.1 15.90 0.16

Fe0 7.03 6.42 6.83 6.76 0.25

MnO 0.01 0.01 0.01 0.01 0.00

MgO 7.25 7.34 7.11 7.23 0.09

cao 3.43 3.26 3.58 3.42 0.13

WO 0.02 0.02 0.02 0.02 0.00

t&O 0.16 0.09 0.10 0.12 0.03

P,O, 0.07 0.04 0.06 0.06 0.01

S 0.01 0.00 0.00 0.00 0.00

TOM 97.76 97.59 98.82 98.06

Y(-1 M-2

0.012 0.013

0.13 O.OS

17.0 15.2

21.1 13.1

100 68

5.49 5.05

11.26 4.42

20 30

76 60

14.5 12

0.18 0.12

1.00 0.53

2.75 1.8

3.25 4.15

67 63

109 78

0.16 0.16

0.12 0.34

240 180

O.TJ 0.43

2.20 1.02

1.30 0.95

0.31 0.26

0.18 0.07

0.51 0.37

0.09 0.07

0.47 0.45

eo.1 (0.06

0.27 0.24

0.043 0.03S

4.27 2.7S

0.44 0.20

0.2 <0.3

<2 s3

18 5

<I *2

4.18 2.41

0.43 0.34

GM-3 Average Std.Dcv.

Na (W

K (XI

SC

Cr

Hn

Fe (X)

CO

Nf

Zn

Ga

As

se

Sr

Rb

Sr

Zr

Sb

CS

ga

La

Cg

Nd

sm

EU

Gd

Tb

DY

Tm

Yb

LU

Nf

la

Y

1roJFw

AU(ppb)

Hg

Th

IJ

0.011 0.012

0.11 0.10

lb.4 16.2

21.1 18.4

100 a9

5.28 5.27

11.26 8.98

<30 25

41 59

14.5 13.7

0.10 0.13

0.60 0.71

0.15 1.57

3.25 3.55

67 66

109 96

0.07 0.13

0.04 0.17

160 193

0.63 0.59

1.75 1 .b5

0.90 1.05

0.24 0.27

0.08 0.11

0.42 0.43

0.08 0.08

0.46 0.46

go.1 co.1

0.31 0.27

0.051 0.044

3.43 3.49

0.16 0.29

0.3 0.25

<2 <2

10 11

<l cl

2.48 3.02

0.33 0.36

0.001

0.02

0.75

3.79

15

0.18

3.22

5

14

1.2

0.03

0.21

1.07

0.42

2

15

0.04

0.13

34

0.13

0.49

0.18

0.03

0.05

0.06

0.01

0.01

0.00

0.03

0.006

0.61

0.11

0.05

5

0.82

0.04

K/U 2941 2568 3231 2847 359

2r/llf 25.41 27.93 31.63 28.32 2.56

Th/U 9.82 7.07 7.62 a.17 1.19

La/Th 0.17 0.10 0.25 0.20 0.04

LaN/YbN 1.01 1.20 1.37 1.46 0.26

WEti 1.39 0.69 0.78 0.95 0.31

W-1 St+2 WI-3 Average Std.Dev.


O.aol’, , I g ( , , , I , ) , , , ( , , ) ( , , ( , , ( , , , , ,’ SITIAlFeMnMgCaNaK P SScCrCoNiZnGsAsBrRbSrZrSbCs6atifToWThU

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

tSt&1 -ntSM-2-=-w-3

RG . 8. Chemical composition of smectite shells around black giass spherules. (a) Comparison of major and trace element composition of average smectites vs. average black glass; (b) REE patterns for the three smectite samples and average black glass, and for a K/T boundary claystone from Brownie Butte, Montana (data fmm IZETT, 1987a).

higher SO2 and K20 abundances, but lower concentrations of all other major elements, thus suggests the existence of a third glass type which we have named the high Si-K variety (HSi,K glass). In BSE images, darker areas, consisting of pure SiOZ (lechatelierite, and a few quartz grains at various stages of dissolution in the melt) and some lighter schlieren (lower SiOz content), are found throughout the section, which is similar to some impact ghS%S.

6)

5) The analysis of smectite shells adhering to black glasses gives direct evidence for the alteration of the glass. Nu- merous changes in major and trace element composition take place during the alteration of the glass. Although the REE patterns in the glass are similar to the average upper crust, the smectites exhibit much lower abundances and

7)

flat patterns, not unlike the patterns observed in claystones at some K/T boundaries. This observation eliminates the need for an oceanic source rock for the K/T boundary clays and suggests that large parts of K/T boundary claystones may have been derived by alteration of such impact glasses. Although the Haiti glasses have been termed “tektites” by several researchers, they do not fulfill all the criteria that characterize tektites (which is mainly a historical term). “Impact glasses” seems a more descriptive and logical term. The andesitic composition of the black glasses, as well as the obvious contribution of carbonates and evaporites to the composition of the yellow glasses, has led some researchers to propose that the Chicxulub structure in Yu-

Impact @asses from the K/T boudary in Haiti 2127

catan may be the source crater for the K/T impact glass. This has to be viewed with caution, since impact glasses and tektites are produced from the surface layers of the taqet areas; it seems that at the time of impact the 6rst l-3 km of the target stratigraphy comprised mainly carbonates and evaporites. Furthermore, admixture of carbonates and evaporites to “andesitic” target rocks (what- ever their provenance) would lower their REE patterns (and alter other trace element ratios) beyond those observed in the yellow glasses.

8) As for an association with the Manson crater, the chemistry of the black glasses is not incompatible with rocks present at Manson, but no clear connection is present either.

9) We conclude that ah varieties of the Haiti glasses have formed by impact and have later undergone alteration by low-temperature hydrothermal processes to produce clays.

Acknowkdgments-CK wants to thank L. Le and V. Yang (SN, NASA Johnson Space Center, Houston) for assistance with the Camebax electron probe, and D. Jalutlca for help with figure prep aration. We also thank F. Bmndstatter and G. Kurat (Natural History Museum. Vienna) for the use of. and help with. the Jeol JSM~~ SEM. We am fpateful to Arch Reid, Horton Ne&m, and two anon- ymous reviewers for very helpful comments on the manuscript. Part of the work was done while CK was a visiting scientist at the Lunar and Planetary Institute, Houston. The Lunar and Planetary Institute is operated by the Universities in Space Research Association under contract NASW-4066 with the National Aeronautics and Space Administration.

Editorial handling: G. Fame

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Geochemistry of impact glasses from the K/T boundary in Haiti: …€¦ · The concentrations of eleven major elements (including S), determined by microprobe, and of ~i~~i~t major,