Geochemistry and age of Ivory Coast tektites and microtektites

Geochimicaet CosmochimicaActa, Vol. 61, No. 8, pp. 1745-1772, 1997 Copyright © 1997ElsevierScienceLtd Printed in the USA.All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97) 00026-4

Geochemistry and age of Ivory Coast tektites and microtektites CHRISTIAN KOEBERL,1,. RICHARDBOTTOMLEY,2 BILLY P. GLASS,3 and DIETER STORZER4 1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 2Department of Physics, Canadian Union College, Box 430, College Heights, Alberta T0C 0Z0, Canada 3Department of Geology, University of Delaware, Newark, Delaware 19716, USA 4Laboratoire de Mineralogie, Museum d'Histoire Naturelle, 61 rue Buffon, F-75005 Paris, France

(Received September 25, 1996;accepted in revised form January 3, 1997) A b s t r a c t - - I v o r y Coast tektites were first reported in 1934 from a geographically restricted area at Ivory Coast, West Africa. Although some additional specimens have been found later, the total number remains small (a few hundred). The Bosumtwi impact crater in Ghana is most likely the source crater for the Ivory Coast tektites, based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions. In addition to tektites on land, microtektites were found in (so far) eleven deep-sea cores off the West African coast, between about 9°N and ll°S and 0 ° and 23°W, defining the extent of the Ivory Coast tektite strewn field. In this study we analyzed eleven Ivory Coast tektites for their major and trace element composition, studied their petrographical characteristics, provided major element data for 111 microtektites, and major and trace element data for four microtektites. We determined the 4°Ar-39Ar step-heating age of five Ivory Coast tektites and four microtektites and obtained fission-track dates for ten tektites and one Bosumtwi impact glass. The tektites have very small intersample and intrasample variations of their major and trace element composition. 111 Ivory Coast microtektites from eleven cores were analyzed for their major element compositional range. Their compositional range is significantly wider than that of the Ivory Coast tektites, but the majority of all microtektites have compositions very similar to those of the tektites (within a factor of 1.2). Trace element compositions of the tektites also show little variation between samples. The samples do not show any distinct Eu anomaly in the REE patterns. This characteristic, as well as the high absolute REE abundances and LaN/YbN ratios of about 8, indicate that Archean rocks are plausible source rocks. The major and trace element contents of four individually analyzed Ivory Coast microtektites show compositions that are very similar to those of the Ivory Coast tektites. However, the microtektites contain >20 rel% higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co, Ni, Sr, Zr, Ba, Hf, Ta, Th, and the REEs. These differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite-forming melt. The Ivory Coast microtektites also have a uniform internal composition. Duplicate 4°Ar-39Ar step-heating age analyses were performed on five tektites. The best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs, resulting in an age of 1.1 _+ 0.05 Ma. We also tried to date four microtektites by 4°Ar-39Ar age analyses, but their young age and small sample size makes it impossible to assign a reliable age to the microtektites. One run yielded satisfactory results that were similar to the tektite age. In addition, we determined the fission-track ages for ten individual Ivory Coast tektite samples and for one impact glass sample from the Bosumtwi crater. The track-size corrected ages for the Ivory Coast tektites ranged from 0.91 to 1.18 Ma, resulting in an average fission-track age of 1.05 _+ 0.11 Ma. This age is, within errors, identical to that of the Bosumtwi impact glass at 1.03 _+ 0.11 Ma, and to the 4°Ar-39Ar age of 1.1 _ 0.05 Ma. The preferred age of the Ivory Coast tektite event is 1.07 Ma. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

On land, tektites are classified into three groups: (a) normal or splash-form tektites, (b) aerodynamically shaped tektites, and (c) Muong Nong-type (blocky, chunky, layered) tektites. The first two groups differ only in their appearance and some of their physical characteristics (see, e.g., O' Keefe, 1963, 1976; Chao, 1963). Muong Nong-type tektites as well as aerodynamically shaped tektites are known mainly from the Australasian strewn field, and neither have yet been found at the Ivory Coast strewn field. The occurrence of tektite glasses is not restricted to land areas. Since the mid-1960s, microtektites have been found in deep-sea cores of three of the four strewn fields (see, e.g., Glass, 1967, 1968, 1969, 1972; Cassidy et al., 1969). Microtektites are generally less than 1 mm in diameter and show a somewhat wider variation in chemical composition

Ivory Coast (IVC) tektites were first reported in 1934 (Lacroix, 1934) from a small area of about 40 km radius within the Ivory Coast (Cote d'Ivoire), West Africa (Fig. 1 ). Further recoveries were made by, e.g., Gentner (1966) and Saul (1969). Tektites are centimeter-sized natural glasses that are found on Earth in four strewn fields: North American (35 Ma old), Central European (15 Ma), Ivory Coast (1.1 Ma), and Australasian (0.78 Ma). Tektites within such strewn fields are related to each other with respect to their petrological, physical, and chemical properties as well as their age.

* Author to whom correspondence should be addressed (christian [email protected]). 1745

1746

C. Koeberl et al.

10 ° N

"I K9-57

GUIN~

" ~ f

ODP 665

'

~.-,",,a., "

{

~' k

:

"'~f"\':

•

TEKTITE STREWNFIELO

'" ".

~

¢) t~-'-

~

".

EQUATORIAL,.,~""-~ "'-~'--""~.GUINEA ,~. _~ (.-~: " °

ODP663B

IODP664

I

•

ATLANTIC OCEAN

I •=Micr°tektites I '

10 ° S

;'

~

• 0°

z

GHANA ~ : ~ ":

.....

~

3oo

6oo

i

klm

i

<

~j.

'~:

"

o

IJt~ GABON

CONGO

GA.,NO,

• •

V27-239

CABINDA

RC13-210

I

I

20 ° W

10 ° W

'~'F~G6 ~

•

RC13-213 0°

I 10 ° E

Fig. 1. Geographical location of the Ivory Coast tektite strewn field, the Bosumtwi impact crater in Ghana, and microtektite-bearingdeep-sea cores. K9-56 and K9-57 are U.S. Navy cores, V19-297, V19-300, V27-239, RC13-210, RC13-213, and RC16-75 are Lamont-Doherty Earth Observatory cores, and ODP 663B, 664C, and 664D are Ocean Drilling Program cores.

than tektites on land, but they have an average composition that is very close to that of normal tektites. In the Ivory Coast strewn field, microtektites have been found in deepsea cores off the coast of Western Africa (Glass, 1968, 1969). The geographical distribution of microtektite-bearing deep-sea sediments has been used to define the extent of the strewn field (Glass and Zwart, 1979, Glass et al., 1979, 1991). So far, Ivory Coast microtektites were found in eleven deep-sea cores (Fig. 1). Regarding the origin of tektites, the scientific current consensus is that tektites have formed during hypervelocity impacts on Earth and represent melts of surfical, predominantly sedimentary, precursor rocks of upper crustal composition (see Koeberl, 1994, for a recent review). For two of those strewn fields, the source craters are known with reasonable certainty. The Central European strewn field is likely derived from the Ries crater in southern Germany, while the Bosumtwi crater in Ghana is the source crater of the Ivory Coast strewn field (see below). The 90-km-diameter buried Chesapeake Bay impact structure, eastern USA, is a likely candidate for the source crater of the North American tektite strewn field (Poag et al., 1994; Koeberl et al., 1996), but no explicit identification has yet been made for the Australasian tektite source crater. The Bosumtwi crater was suggested to be the source crater of the Ivory Coast tektites, based on similar Rb-Sr and SmNd model ages between tektites and crater rocks (e.g., Schnetzler et al., 1966; Shaw and Wasserburg, 1982), similar chemical compositions (Schnetzler et al., 1967; Jones, 1985), and similar isotopic characteristics between the tektites and bedrock at the crater (e.g., Schnetzler et al., 1966; Lippolt and Wasserburg, 1966; Shaw and Wasserburg, 1982). Also, tektites and Bosumtwi impact glasses have similar ages (e.g., Gentner et al., 1964, 1967; Storzer and Wagner, 1977). However, published ages range from 0.71 to 1.2 Ma for Ivory Coast tektites.

The near-circular Bosumtwi crater in Ghana, which is almost completely filled by Lake Bosumtwi, has a rim to rim diameter of 10.5 km and is exposed in 2 Ga old lower greenschist facies metasediments of the Lower Birimian Group (Schnetzler et al., 1966; Kolbe et al., 1967). The origin of the crater was long controversial, but since the early 1960s several lines of argument have developed in favor of an origin by impact. Outcrops of suevitic breccia were found around the crater (e.g., Jones et al., 1981 ). More convincing is the discovery of the high-pressure quartz modification coesite (Littler et al., 1961 ), as well as Ni-rich iron spherules and baddeleyite (the high-temperature decomposition product of zircon) in vesicular glass from the crater rim (El Goresy, 1966; El Goresy et al., 1968), all of which support an impact origin for the Bosumtwi crater. The composition of melt in the suevite is similar to that of the basement rocks (Jones, 1985; Koeberl and Reimold, 1996). However, chemical data for Ivory Coast tektites have been rather sparse and scattered. Major element data were reported by Chapman and Scheiber (1969; five samples), Cuttitta et al. (1972; seven samples), Shaw and Wasserburg (1982; two samples), and Glass et al. (1991; four samples). Trace element data were limited and of disparate quality. Rare earth element data were given by Schnetzler et al. (1967) and Bou~ka and l~anda (1976); Ni and Cr data by Pinson and Griswold ( 1969); U and Th data by Rybach and Adams (1969), Durrani and Khan ( 1971 ), Storzer and Selo (1974), and Yellin et al. (1983); and several other trace elements by Gentner et al. (1964), Chapman and Scheiber (1969), and Cuttitta et al. (1972). Siderophile element data were obtained by Palme et al. (1978). Recently, Koeberl and Shirey (1993) used Re-Os isotope systematics to provide evidence for the presence of up to 0.6% of an extraterrestrial component in Ivory Coast tektites. Major element data for microtektites were reported by Glass (1969), Glass and Zwart (1979), and Glass et al.

Ivory Coast tektite ages and geochemistry ( 1991 ). While Frey et al. (1970) and Frey (1977) published data for a few trace elements in composites o f ten and sixteen Ivory Coast microtektites, no trace element data were so far available for individual Ivory Coast microtektites. In the present paper, we present a set of precise major and trace element data for eleven Ivory Coast tektites and four microtektites, as well as major element data for 111 Ivory Coast microtektites, and age data by 4°Ar-39Ar and fission-track dating on five and ten tektites, respectively. In addition, we attempted to use 4°Ar-39Ar laser probe dating on individual microtektites. 2. SAMPLES AND EXPERIMENTAL METHODS

2.1. Samples Eleven Ivory Coast tektite samples were studied for their petrographical characteristics, major and trace element composition, and age. The Ivory Coast tektite samples (IVC) 2069, 2113, 2114, 3395, 3396, 3397, and 3398 were from the collection of the Museum National d'Histoire Naturelle, Paris; samples 16-3 and 22-5 from the Max Planck Institut, Heidelberg, and 8901 and 8902 from the University, Abidjan, Ivory Coast. With the exception of 16-3 and 22-5, the complete specimens were available, with weights of 7.070.2 g. The samples have a pitted surface, are of dark (black) color, and have little translucence (see Fig. 2 for a macroscopic view of a typical Ivory Coast tektite).

2.2. Major and Trace Element Analysis (IVC) Chips of the Ivory Coast tektite samples were analyzed for their major element composition by electron microprobe analysis, using a Cameca Camebax instrument at NASA Johnson Space Center with standard Cameca data evaluation procedures. To avoid loss of Na during the analyses, the beam was rastered over an area of 50 × 50 #m. Twenty to forty individual area analyses were averaged for each tektite. Trace element contents were determined on cleaned chips or samples powdered with agate or boron carbide mills from interior chips. About 100-200 mg were used for instrumental neutron activation analysis (for information on the INAA procedures, instrumentation, standards, etc., at the Vienna laboratory, see Koeberl et al., 1987 and Koeberl, 1993).

2.3. Microtektites Major element compositions were determined for 111 microtektites from various deep-sea cores (see below) from the Ivory Coast

1747

strewnfield. The compositional data were obtained by energy dispersive x-ray (EDX) analysis (PGT System 4) in combination with a Cambridge S90B scanning electron microscope operating at 15 kV acceleration voltage. Polished sections were analyzed and the spectra were corrected for background, atomic number, absorption, and fluorescence effects using a computer program by Princeton Gamma Tech. Glass prepared by Corning Glass Company and analyzed by the U. S. Geological Survey were used as a standard. The analyses were normalized and then standardized using correction factors obtained from the standard which was analyzed along with the unknowns.

2.4. Mierotektites--Trace Elements Four microtektites, weighing 80-138 #g, were selected to be individually analyzed for their trace element composition. The microtektites were weighed using a Mettler UM2 ultra-microbalance, placed on depressions in high-purity quartz disks, stacked, and packed together with several geological standards in clean aluminum-foil, and sealed in an aluminum tube. Two granites (G-2: U,S. Geological Survey and AC-E: CPGC-CNRS Nancy; Govindaraju, 1987) and the carbonaceous chondrite Allende (Jarosevich et al., 1987) were used as standards and flux monitors. The samples were irradiated in the Astra-research reactor at the Forschungszentrum in Seibersdorf near Vienna (Austria) for about 7 h at a flux of about 6 × 1013 neutrons cm-2s-l. After irradiation, the microtektites were rinsed in distilled water and reweighed. The samples were counted for at least 3-6 h in the first counting period ( ~5 days after irradiation), 6-12 h in the second counting period ( ~10 days after irradiation), and 24-72 h in the third counting period ( 5 - 6 weeks after irradiation), using a high purity germanium detector with 48% relative efficiency and 1.82 keV energy resolution at 1332 keV. For further details on the instrumentation, precision and accuracy, and data evaluation, see Koeberl (1993). After completion of the INAA counting, the four microtektites were mounted in epoxy, polished, and analyzed for their major element composition by wavelengthdispersive electron microprobe (EMP-WDS) analysis, using an ARL-SEMQ instrument at 15 kV acceleration voltage and standard correction procedures.

2.5. 40Ar-39Ar Age Analysis Duplicate 4°Ar-39Ar step heating analyses were performed on five different samples of Ivory Coast tektites. In addition, we also obtained single step analyses on several microtektite samples. All analyses were performed using the Argon Laserprobe system of the University of Toronto. Samples of the North American tektite B242 with an age of 34.7 Ma were used as the irradiation standard for these analyses. For details of the system and the method see Bottomley and York (1988), Bottomley et al. (1990), and Glass et al. (1986b). 2.6. Fission-Track Dating Fission-track analyses were performed on one impact glass from Lake Bosumtwi and on ten different Ivory Coast tektite samples. Aliquants of both the tektite and impactite samples were irradiated with various time-integrated thermal neutron fluences, ranging from 1.29.1015 n/cm 2 to 3.22.1015 n/cm 2, together with aliquants of a 14.81 _+0.26 Ma old moldavite irradiation standard. Following irradiation the irradiated glass discs (containing the neutron-induced fission tracks) and an unirradiated glass disc (containing the fossil fission tracks) of each tektite sample were mounted together in epoxy, polished, and etched in 48 vol% HF at 22°C for 10 s. Fossil and induced fission tracks were counted and their respective sizes (major axes) were determined under an optical microscope under high magnification (up to 1600× ). The apparent fission-track ages were corrected using the track size correction technique (Storzer and Wagner, 1969). For details on methods and analytical data, see Storzer and Wagner (1971, 1982) and Storzer and Selo (1985). 3. RESULTS AND DISCUSSION

3.1. Petrography Fig. 2. Macroscopic view of Ivory Coast tektite IVC 3398. The glass is black with a characteristic pitted surface.

Previous workers have concentrated on the elemental and isotopic compositions o f Ivory Coast tektites; little informa-

1748

C. Koeberl et al.

tion has been available on the petrography of these glasses. Polished sections, about 1 m m thick, were made of seven of the Ivory Coast tektite samples (IVC 2069, 2113, 2114, 3395, 3396, 3397, 3398) that were chemically analyzed in this study. These sections were investigated with a binocular and a petrographic microscope. The volume abundance of bubble cavities and lechatelierite particles were determined for each specimen based on 1000 point counts (Table 1 ). Bubble cavities are all of spherical shape and most are less than 100 # m in diameter. In most of the specimens the largest observed bubble cavity was 0.5 mm or less in size. However, in one specimen (3396) a 3.6 mm diameter bubble cavity was present. Bubble cavities were not uniformly abundant in each specimen. In some specimens regions up to about 1 cm across are free of any visible bubble cavities. In most specimens the bubble cavities were common to abundant, but they generally made up less than 1 vol% of the glass. Lechatelierite particles were generally more abundant than the bubble cavities but still made up only 1.2 vol% or less of the specimens. The lechatelierite particles range from equant to elongate to ribbon-like or filamentous in shape (Fig. 3). A few of the filamentous or ribbon-like particles are highly contorted and up to 2.5 m m in length. Most contain bubble cavities and some were highly vesicular, but none were frothy. There appears to be an inverse correlation between the abundance of bubble cavities and lechatelierite particles and the silica abundance (Table 1 ), but the limited range of silica contents makes it difficult to quantify this trend. The glass is generally dark olive-green in color and fairly homogeneous in appearance except for the lechatelierite particles. Most of the specimens exhibit weak to moderate flow banding due to bands of darker and lighter colored glass. The long axes of the lechatelierite particles are generally parallel with the flow banding (Fig. 3d-f). In one specimen (2069) the flow lines radiate out from a light colored region about 0.5 cm across where there are no bubble cavities and only one large (200 × 900 #m) lechatelierite particle. Strain birefringence is associated with most of the lechatelierite particles and cracks radiate out from many of the larger particles. No crystalline inclusions (or high-pressure polymorphs of

quartz) were observed in any of the polished sections nor in a sample of specimen 2114 that was crushed, sieved, and put through a heavy liquid separation (see below).

3.2. Major Element Composition of Tektites The results of our major and trace element analyses are given in Table 2. The major element composition of all eleven analyzed Ivory Coast tektite samples falls in a rather narrow range. Our results are in agreement with previously published data (e.g., Chapman and Scheiber, 1969; Cuttitta et al., 1972). The abundances of some of the major elements show a total variation of less than about 2.5 tel% for all eleven samples, with other major elements varying by < 10 rel%. This remarkably uniform range in composition is in contrast to the range in elemental contents shown by tektites from other strewn fields. Australasian and North American tektites exist in several groups with distinctly different and diverse compositions, and even for moldavites, which originated from the 24-km-diameter Ries crater in southern Germany, a significant spread in chemical abundances exists (e.g., Koeberl, 1986). It is possible, however, that the narrow range in composition is due to the small geographical area in which Ivory Coast tektites are found. Except for the lechatelierite particles, the Ivory Coast tektites also show remarkably little intrasample compositional variation. Our microprobe analyses of 2 0 - 4 0 50 × 50 # m randomly selected areas across chips of about 5 - 1 5 m m in size show very limited variations between these individual areas (e.g., range for 2113, in wt%:SiO2 66.61-67.93, A1203 16.33-16.81, FeO 5.99-6.43, M g O 3.39-3.71, C a t 1.321.41, Na20 1.74-1.90, K20 1.91-2.03). In order to determine the range in composition within a single specimen, a 1.97 g sample of specimen 2114 was crushed and sieved and the 7 4 - 1 4 9 # m size fraction was put through a series of eleven heavy liquid separations to recover grains of various densities. Grains from the heavier and lighter specific gravity fractions were searched for inclusions and then grains from each heavy liquid fraction were mounted in epoxy on 1" glass disks, ground, and polished for major oxide determination by EDX analysis as described above. The results for eighty-nine analyzed grains are given in

Table 1. Petrography of Ivory Coast Tektites. Lechatelierite particles

Bubble cavities

Sit2 Sample

(wt%)

Avg. diameter (#m)

Max. diameter (#m)

No./cm3

Abundance (vol%) ~

2069 2113 2114 3395 3396 3397 3398

67.8 67.3 67.7 67.3 68.1 67.8 68.5

34 53 54 81 57 55 40

180 500 200 330 3600 360 160

21,470 13,660 19,900 9,160 6,613 4,394 5,730

0.2 0.4 0.2 1.3 4.0z 0.1 <0.1

Max. size (#m) 300 400 150 300 160 500 300

× × x x × × ×

1000 1700 600 700 4000 940 1300

Based on 1000 point counts on each specimen. 2 The high value for this specimen is due primarily to the presence of one large (3.6 mm) bubble cavity.

No./cm3

Abundance (vol%)1

3108 2522 3648 1273 960 1116 1001

0.6 1.1 0.4 1.0 1.2 0.6 0.2

Ivory Coast tektite ages and geochemistry

1749

d

b

Fig. 3. Photomicrographs of polished sections of Ivory Coast tektites. All photos are taken in transmitted light. Each image is 3.23 mm wide. (A) Large (560 × 930 #m) bubbly lechatelierite particle in samples IVC 3397. (B) Several lechatelierite particles in sample IVC 3395. Note two dark cracks that radiate at fight angles to each other around the lechatelierite particle in the center. The dark halo around this particle is due to a crack in the plane of the section. The lechatelierite particle in the lower right also has cracks radiating out from it. (C) Highly elongate and somewhat contorted lechatelierite particles in IVC 3396. The longest particle is about 4 mm long with a maximum width of 160 #m. (D) Lechatelierite particles and spherical vesicles in IVC 3395. Note also elongate patch of darker glass. Long axes of lechatelierite particles are generally parallel to each other and to the zone of dark glass. (E) Lechatelierite particles and flow structure in IVC 3398. The long axes of the lechatelierite particles are generally parallel to the schlieren. (F) Lechatelierite particles and flow structure in IVC 3398. Note the ribbon-like highly folded lechatelierite particle near the center of the image. The long axes of the particles are parallel to the flow structure.

Table 3. Most had SiO2 contents between 67 and 69 wt%, in agreement with the previous observation that Ivory Coast tektites are quite h o m o g e n e o u s in composition; however, a few grains had silica contents as low as 64 wt%, and some

had SiO2 contents greater than 70 wt% (Table 3). In addition to the lechatelierite particles, which are nearly pure silica (not reported in Table 3), grains with SiO2 contents as high as 89 wt% were found. At least some o f the high silica

C. Koeberl et al.

1750

Table 2. Major and trace e l e m e n t composition o f 11 Ivory Coast tektites. 2069

2113

2114

3395

3396

3397

3398

16-3

22-5

8901

8902

Average

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K~O

67.84 0.55 16.74 6.24 0.06 3.40 1.21 1.81 1.96

67.3 0.55 16.60 6.18 0.06 3.58 1.38 2.03 1.97

67.67 0.54 16.38 6.19 0.07 3.61 1.37 1.92 1.96

67.29 0.55 16.63 6.2 0.06 3.58 1.49 1.88 1.83

68.11 0.56 16.71 5.98 0.06 3.30 1.51 1.87 1.92

67.76 0.55 16.44 6.13 0.07 3.51 1.52 1.87 1.89

68.48 0.54 16.28 5.84 0.06 2.98 1.52 2.08 2.04

67.66 0.59 16.91 6.16 0.04 3.21 1.26 2.08 2.13

68.06 0.57 16.75 6.21 0.07 3.38 1.27 1.77 2.05

66.17 0.61 17.72 6.45 0.07 4.39 1.29 1.53 1.73

67.07 0.59 17.03 6.13 0.06 3.13 1.34 2.05 2.02

67.58 0.56 16.74 6.16 0.06 3.46 1.38 1.90 1.95

Total

99.81

99,65

99.71

99.51

100.02

99.74

99.82

100.04

100.13

99.96

99.42

99.79

10.7 1.6 7.9 12.6 228 24.6 143 2.5 26.5 0.23 0.21 0.59 50.3 190 103 0.02 0.16 3.77 280 17.8 35.1 16.3 3.05 1.02 2.5 0.44 2.7 0.24 1.46 0.19 2.96 0.3 0.48 <0.2 1.6 0.25 2.93 0.7

n.d. n.d. n.d. 15,1 250 27.1 184 27 12 0.49 0.11 0.12 70.5 330 120 0.14 0.14 3.22 435 20.8 43.9 22.7 4.56 1.16 3.8 0.65 4.1 0.34 1.98 0.25 3.44 0.35 0.29 0.5 78 0.11 3.88 0.93

5.7 0.8 2.0 13.1 249 32.1 133 2 34 0.17 0.2 0.22 56.4 220 195 0.05 0.28 4.22 260 18.2 37.1 19.9 3.38 1.24 2.8 0.44 2.6 0.24 1.47 0.2 2.91 0.24 0.23 <0.4 1.8 <0.1 2.95 0.52

n.d. n.d. n.d. 14.2 223 26.7 112 3 26 0.26 0.11 0.12 58.6 170 135 0.09 0.14 3.68 280 19.2 40.1 22.9 3.49 1.34 3.4 0.57 3.4 0.25 1.55 0.22 3.14 0.24 1.45 0.4 38 0.16 3.33 0.56

n.d. n.d. n.d. 13.8 227 25.4 100 43.3 13 0.39 0.67 0.97 56.3 210 130 0.08 0.32 4.08 325 18.3 38.7 18.2 3.49 1.16 2.85 0.52 3.1 0.27 1.75 0.21 3.31 0.41 0.62 <0.3 55 0.9 3.13 0.98

n.d. n.d. n.d. 14 255 28.9 94 5 17 0.37 0.22 0.22 61 210 155 0.1 0.19 3.93 270 19.2 39.3 19.3 3.55 1.22 2.75 0.43 2.7 0.24 1.56 0.22 3.18 0.27 0.61 0.4 60 0.08 3.32 0.74

n.d. n.d. n.d. 15.2 227 23.3 123 21.4 18 0.45 0.12 0.14 78.7 350 90 0.04 0.18 3.21 425 21.6 44.1 23.2 4.14 1.18 4.6 0.66 4.1 0.33 1.89 0.26 3.51 0.38 0.7 <0.5 4.5 <0.1 3.74 1.08

n.d. n.d. n.d. 16.4 248 27.4 260 39 20 0.72 0.11 0.21 81.7 390 156 0.03 0.23 3.81 520 24.6 48.7 25.9 4.69 1.26 3.4 0.58 3.8 0.34 2.16 0.31 3.75 0.41 0.67 0.4 2.9 <0.1 4.24 1.39

n.d. n.d. n.d. 14.9 264 29.3 237 60 34 0.63 0.41 0.95 72.1 210 120 0.09 0.45 4.5 340 20.2 43.1 21.7 3.92 1.19 2.95 0.55 3.4 0.31 1.89 0.23 3.65 0.39 0.64 <0.8 89 0.15 3.44 1.12

n.d. n.d. n.d. 16.8 290 25.1 148 31 12 0.63 0.08 0.06 66.4 300 126 0.09 0.22 2.79 485 23.7 46.7 25.2 4.55 1.21 4.4 0.67 4.2 0.34 1.99 0.29 3.79 0.37 0.65 <0.5 53 <0.09 4.07 1.19

n.d. n.d. n.d. 15.3 224 23.8 189 24 15 0.6 0.07 0.08 73.5 285 148 0.09 0.23 3.21 490 24.4 44.3 24.7 4.66 1.19 4.5 0.62 4.2 0.38 2.01 0.28 3.49 0.36 0.61 <0.6 74 <0.08 3.89 1.18

8.2 1.2 5.0 14.7 244 26.7 157 23 21 0.45 0.21 0.33 66 260 134 0.07 0.23 3.67 374 20.7 41.9 21.8 3.95 1.2 3.45 0.56 3.48 0.3 1.79 0.24 3.38 0.34 0.63 0.4 41 0.15 3.54 0.94

Li Be B Sc Cr Co Ni Zn Ga As Se Br Rb Sr Zr Ag Sb Cs Ba La Ce Nd Sm Eu Gd Tb Dy Tm Yb Lu Hf Ta W Ir (ppb) Au (ppb) Hg Th U

±

Range

0.59 66.17-68.48 0.02 0.54-0.61 0.37 16.28-17.72 0.15 5.84-6.45 0.01 0.04-0.07 0.35 2.98-4.39 0.11 1.21-1.52 0.16 1.53-2.08 0.11 1.73-2.13

2.5 0.4 3.0 1.2 20 2.5 52 18 8 0.17 0.17 0.32 9.7 70 27 0.03 0.09 0.49 94 2.4 4 2.9 0.57 0.07 0.73 0.09 0.61 0.05 0.23 0.04 0.29 0.06 0.3 0.2 32 0.25 0.43 0.27

5.7-10.4 0.8-1.6 2.0-7.9 12.6-16.8 223-290 23.3-32.1 94-260 2-60 12-34 0.17-0.72 0.07-0.67 0.06-0.97 50.3-81.7 170-390 90-195 0.02-0.14 0.14-0.45 2.79-4.5 260-520 17.8-24.6 35.1-48.7 16.3-25.9 3.05-4.69 1.02-1.34 2.5-4.6 0.43-0.67 2.6-4.2 0.24-0.38 1.46-2.16 0.19-0.31 2.91-3.79 0.24-0.41 0.23-1.45 0-0.5 1.6-89 0-0.9 2.93-4.24 0.52-1.39

K/U 21548 18190 30769 27976 15901 21059 16049 12470 13170 10714 14477 18393 6109 10714-30769 Th/U 4.19 4.17 5.67 5.95 3.19 4.49 3.46 3.05 3.07 3.42 3.30 4.00 0.97 3.05-5.95 La/Th 6.08 5.36 6.17 5.77 5.85 5.78 5.78 5.80 5.87 5.82 6.27 5.87 0.23 5.36-6.27 Zr/Hf 34.8 34.9 67.0 43.0 39.3 48.7 25.6 41.6 32.9 33.2 42.4 40.3 10.4 25,6-67.0 Hf/Ta 9.87 9.83 12.13 13.08 8.07 11.78 9.24 9.15 9.36 10.24 9.69 10.22 1.42 8.07-13.08 LaN/YbN 8.24 7.10 8.37 8,37 7.07 8.32 7.72 7.70 7.22 8.05 8.20 7.85 0.49 7.07-8.37 Eu/Eu* 1.13 0.85 1.23 1.19 1.12 1.19 0.83 0.96 1.07 0.83 0.79 1.02 0.16 0.79-1.23

Major element data in wt%, by electron microprobe; trace element data in ppm (except as marked), by INAA. All Fe as FeO, Na, K, and Fe data are averages of microprobe and INAA values. Li, Be, B data from Chaussidon and Koeberl (1995). n.d. = not determined.

contents were no doubt due to overlap of the electron beam with lechatelierite particles. Figure 4 shows the variation of major oxides (A1, Fe, Mg, Ca, Na, and K) vs. silica. All of these, except K, show a negative correlation of their abundances with the silica content. Both Na and K abundances scatter much more widely from the correlation compared to the other oxides, but the K content does not follow any linear correlation. Instead, it rises sharply up to about 78 wt% SIO2, after which is seems to follow the normal negative correlation. This could indicate

that the high-silica contents of some particles are not simply due to beam overlap with lechatelierite inclusions, but that up to 78 wt% silica the mixture indicates the presence of a high K-phase, although it is unclear why the K content shows such a bimodal distribution. 3.3. Major Element Composition of Microtektites Microtektite-bearing deep-sea sediment layers have so far been identified from eleven cores (Glass et al., 1991 ). These

Ivory Coast tektite ages and geochemistry

1751

Table 3. Major element composition of grains from IVC 2114. Sample

SiO2

TiO2

A1203

FeO

MgO

CaO

Na20

K20

Total

469-2-9A 469-2-1A 469-2-2A 469-2-3A 469-2-6A 469-2-5A 469-2-0A 469-2-7A 469-2-4A 470-3-1A 470-2-2A 469-2-8A 468-1-8A 468-1-4A 470-3-2A 469-1-6A 468-2-5A 470-2-3A 470-2-9A 468-1-45 470-3-6A 470-3-7A 468-1-3A 470-1-7A 469-3-3A 468-2-7A 468-2-3A 468- I-6A 468-3-1A 470-2-1A 469-3-1A 468-2-1A 469-3-7A 468-3-3A 468-2-2A 469-3-8A 469-1-2A 470-2-4A 468-2-6A 468-2-4A 468-1-1A 470-2-6A 469-3-2A 471-1-1 A 469-1-3A 469-3-5A 468-1-9A 469-1-4A 469-3-6A 468-1-7A 469-3-9A 469-3-4A 468-3-2A 470-3-3A 469-3-0A 469-1-5A 471-2-3A 468-1-2A 470-3-0A 470-2-5A 470-2-8A 470-3-5A 470-3-4A 471-1-5A 470-1-4A 470-3-9A 470-3-8A 470-1-9A 471-2-1A

64.04 65.55 65.55 65.70 65.87 66.05 66.69 66.84 66.86 67.15 67.28 67.36 67.42 67.44 67.45 67.45 67.46 67.46 67.48 67.52 67.52 67.56 67.58 67.58 67.58 67.59 67.59 67.60 67.63 67.64 67.64 67.65 67.65 67.65 67.66 67.67 67.68 67.69 67.69 67.70 67.70 67.78 67.79 67.79 67.82 67.83 67.84 67.86 67.91 67.91 67.96 67.97 68.04 68.04 68.09 68.16 68.21 68.35 68.46 68.54 68.55 68.87 69.21 69.48 70.05 70.66 70.76 70.86 71.13

0.54 0.50 0.55 0.47 0.48 0.42 0.45 0.51 0.50 0.63 0.59 0.52 0.58 0.55 0.52 0.55 0.64 0.50 0.54 0.57 0.66 0.58 0.58 0.58 0.48 0.55 0.48 0.53 0.55 0.55 0.46 0.55 0.49 0.52 0.55 0.50 0.47 0.51 0.61 0.53 0.61 0.55 0.47 0.63 0.54 0.46 0.44 0.49 0.48 0.63 0.50 0.50 0.54 0.49 0.52 0.47 0.54 0.42 0.52 0.56 0.54 0.50 0.56 0.50 0.47 0.45 0.47 0.47 0.44

16.94 16.81 16.39 16.62 16.32 16.55 16.12 16.03 16.09 16.01 16.01 15.99 15.48 15.95 15.94 16.01 15.87 16.05 15.94 15.97 15.79 15.93 15.92 15.73 15.92 15.89 15.86 15.90 15.87 15.81 15.95 15.91 15.83 15.97 16.01 15.80 15.97 15.96 15.80 15.86 15.79 15.91 15.67 15.67 15.66 15.87 15.91 15.88 15.72 15.86 15.91 15.85 15.60 16.20 15.76 15.69 15.84 15.76 15.31 15.50 15.58 14.63 14.88 15.06 14.75 14.51 14.57 14.34 14.18

7.72 7.10 7.01 6.89 7.16 6.97 6.80 6.67 6.68 6.37 6.46 6.30 6.57 6.40 6.23 6.24 6.33 6.32 6.32 6.35 6.33 6.18 6.17 6.24 6.36 6.18 6.38 6.15 6.24 6.19 6.23 6.13 6.22 6.31 6.15 6.34 6.26 6.42 6.18 6.33 6.22 6.27 6.40 6.20 6.34 6.37 6.24 6.11 6.32 6.25 6.25 6.30 6.13 5.85 6.22 5.98 6.07 6.08 6.05 6.07 6.12 6.40 5.77 5.45 5.61 5.31 5.28 5.41 5.15

4.93 4.09 4.34 4.00 4.25 3.92 4.15 4.16 4.02 3.69 3.62 3.69 3.89 3.70 3.62 3.55 3.72 3.52 3.62 3.65 3.59 3.58 3.74 3.64 3.46 3.66 3.72 3.80 3.79 3.64 3.62 3.71 3.62 3.66 3.65 3.56 3.51 3.52 3.66 3.70 3.70 3.63 3.77 3.58 3.58 3.55 3.59 3.55 3.59 3.70 3.50 3.42 3.71 3.47 3.51 3.52 3.51 3.69 3.50 3.47 3.41 3.79 3.40 3.42 3.23 3.15 3.08 3.06 3.15

1.87 1.74 1.71 1.69 1.68 1.68 1.64 1.63 1.55 1.52 1.43 1.62 1.48 1.49 1.45 1.54 1.53 1.47 1.51 1.47 1.49 1.44 1.53 1.53 1.59 1.54 1.45 1.51 1.52 1.49 1.51 1.56 1.53 1.53 1.52 1.52 1.51 1.47 1.52 1.50 1.48 1.45 1.53 1.48 1.50 1.51 1.42 1.56 1.48 1.48 1.49 1.40 1.52 1.41 1.47 1.48 1.39 1.40 1.42 1.43 1.43 1.42 1.42 1.34 1.32 1.30 1.29 1.25 1.23

1.87 1.90 2.06 2.18 1.93 1.99 1.84 1.84 1.91 1.98 1.98 2.00 2.11 1.92 2.10 1.99 1.92 2.07 2.01 1.95 1.98 2.08 1.95 2.04 1.97 2.09 1.96 1.97 1.93 2.01 1.97 1.90 2.00 1.84 1.91 1.99 1.97 1.84 1.97 1.84 1.96 1.87 1.77 2.05 1.98 1.90 2.06 1.97 1.93 1.72 1.83 1.94 1.98 1.89 1.87 2.05 1.83 1.78 2.09 1.81 1.79 1.86 2.01 2.05 1.84 1.88 1.85 1.88 1.98

1.46 1.66 1.74 1.79 1.65 1.76 1.65 1.66 1.73 1.96 1.96 1.82 1.87 1.96 2.02 1.95 1.93 1.92 1.89 2.00 1.93 1.96 1.94 1.96 1.94 1.92 1.98 1.96 1.88 1.99 1.94 2.00 1.94 1.94 1.96 1.92 1.95 1.92 1.99 1.95 1.95 1.86 1.86 1.92 1.90 1.86 1.94 1.90 1.90 1.86 1.89 1.94 1.90 1.98 1.89 1.96 1.97 1.94 1.95 1.94 1.91 1.86 2.02 2.04 2.05 2.07 2.02 2.05 2.06

99.37 99.35 99.35 99.34 99.34 99.34 99.34 99.34 99.34 99.31 99.33 99.30 99.40 99.41 99.33 99.28 99.40 99.31 99.31 99.48 99.29 99.31 99.41 99.30 99.30 99.42 99.42 99.42 99.41 99.32 99.32 99.41 99.28 99.42 99.41 99.30 99.32 99.33 99.42 99.41 99.41 99.32 99.26 99.32 99.32 99.35 99.44 99.32 99.33 99.41 99.33 99.32 99.42 99.33 99.33 99.31 99.36 99.42 99.30 99.32 99.33 99.33 99.27 99.34 99.32 99.33 99.32 99.32 99.32

1752

C. Koeberl et al. Table 3. (Continued)

Sample

SiO2

TiO2

A1203

FeO

MgO

CaO

Na20

K20

Total

470-2-0A 479-1-5A 470-1-2A 470-2-7A 470-1-3A 471-1-2A 470-1-8A 471-1-7A 471-1-0A 470-1-1A 470-1-0A 470-1-9A 471-2-4A 471-1-3A 471-1-4A 471-2-5A 471-2-2A 471-1-8A 471-1-6A 471-2-0A

71.76 72.10 73.19 73.20 74.43 74.76 75.36 75.52 75.83 76.67 76.85 78.73 80.30 80.61 80.78 82.26 83.03 83.38 87.55 89.52

0.43 0.35 0.42 0.38 0.38 0.39 0.39 0.34 0.30 0.33 0.30 0.34 0.15 0.26 0.I 5 0.21 0.22 0.20 0.15 0.04

14.02 13.79 13.25 12.94 12.73 12.33 12.26 12.12 11.81 11.47 11.55 10.42 10.20 9.47 9.62 8.70 8.02 8.08 5.99 5.02

5.10 5.12 4.64 5.17 4.46 4.36 4.21 4.24 4.27 4.00 3.99 3.40 2.59 2.99 3.00 2.53 2.59 2.38 1.82 1.17

2.94 3.00 2.72 2.91 2.57 2.64 2.31 2.48 2.51 2.20 2.09 1.89 1.72 1.88 1.78 1.69 1.64 1.47 1.07 0.86

1.22 1.17 1.09 1.13 0.96 1.04 0.96 1.00 0.92 0.88 0.74 0.75 0.62 0.63 0.64 0.57 0.50 0.47 0.23 0.22

1.73 1.74 1.85 1.63 1.61 1.70 1.66 1.51 1.55 1.51 1.60 1.61 1.62 1.41 1.33 1.28 1.34 1.30 0.90 0.96

2.14 2.06 2.16 1.98 2.21 2.13 2.19 2.13 2.16 2.28 2.24 2.21 2.26 2.09 2.06 2.10 2.01 2.05 1.61 1.53

99.34 99.33 99.32 99.34 99.35 99.35 99.34 99.34 99.35 99.34 99.36 99.35 99.46 99.34 99.36 99.34 99.35 99.33 99.32 99.32

Minimum Maximum

64.04 89.52

0.04 0.66

5.02 16.94

1.17 7.72

0.86 4.93

0.22 1.87

0.90 2.18

1.46 2.28

Data in wt%; all Fe as FeO; grains from 74-149 #m size fraction followed by heavy liquid separation--see text.

cores are: K9-56, K9-57, V19-297, V19-300, and V27-239 (Glass and Zwart, 1979), RC13-210, RC13-213, and RC1675 (Glass et al., 1991), and ODP 663B, 664C, and 664D (Glass et al., 1991 ). Table 4 gives the major element composition of 111 Ivory Coast microtektites from all of these locations. The overall range in composition of the microtektites is significantly wider than that of the Ivory Coast tektites. Silica contents range from about 42 to 82 wt%, and the relative variations for other elements are even larger (Table 4). However, most of the microtektites have compositions that cluster around those of Ivory Coast tektites, as indicated in Fig. 5. In general, negative correlations exist between the silica contents of the microtektites and the abundances of some of the other major oxides, but the Na and K contents are positively correlated with the silica content, and all plots show a considerable scatter. Calculating an average microtektite composition from all 111 analyses leads to the results given in Table 4, which can be compared with the average Ivory Coast tektite data from Table 2. The contents of Si are lower, and the contents of most other major elements higher, in the microtektites compared to the tektites. A better fit is obtained when only microtektite data with silica > 6 0 wt% are used for calculating an average composition (Table 4). This comparison is also shown in Fig. 6, indicating that the average major element compositions of the microtektites and tektites are very similar, with differences being less than a factor of 1.2. Compared to tektites and microtektites from the Australasian and North American strewn fields, this is a small difference (Bohor and Koeberl, 1996). The low-silica microtektites are rich in Mg and Fe and belong mostly to the bottle-green microtektites (Glass, 1972). However, in these microtektites the M g and Fe contents are not well-correlated with each other, indicating that

they may represent a much more heterogenous target rock mixture compared to Ivory Coast tektites (and the Si-rich microtektites).

3.4. Trace Element Composition of Tektites and Microtektites In Table 2, we give the results of the contents of up to thirty-eight trace elements for the eleven Ivory Coast tektites that were analyzed for their major element composition, which is the largest (and most consistent) set of trace element data available so far. In general, our results agree quite well with literature values, where available (e.g., Cuttitta et al., 1972; Pinson and Griswold, 1969; Schnetzler et al., 1966, 1967; Chapman and Scheiber, 1969; Gentner et al., 1964). Some discrepancies between previous datasets existed. For example, Chapman and Scheiber (1969) found 1 7 5 - 2 4 0 ppm Cr in four tektites, while Cuttitta et al. (1972) got a range of 2 6 0 - 3 7 5 ppm Cr for seven tektites, and Pinson and Griswold (1969) found 2 1 3 - 2 7 0 ppm for eighteen samples. Our data range from 223 to 290 ppm, with an average of 244 ppm Cr. Similar differences exist for some of the other trace element data previously available. However, it is remarkable how homogeneous the analyzed Ivory Coast tektite samples are with respect to their trace element composition. For most elements, the variations are <15 rel%, which is significantly less than the intersample variations of tektites from other strewn fields (e.g., Koeberl, 1986). In agreement with the limited compositional range, elemental ratios (Table 2) also show little variation. Compared to tektites from other strewn fields, Ivory Coast tektites have relatively high contents of Cr, Co, and Ni, as already observed before (e.g., Chapman and Scheiber, 1969; Cuttitta et al., 1972), as well as for Ir and some other siderophile

Ivory Coast tektite ages and geochemistry

1753

8-

18-

7-

16-

6-

14 ~

o

~o

Bo 12"

v o

10 ~

04-

o

[.h

[] az

3"

8=

61

2-

'6

7'o

65

7'5

80

a'5

70

65

90

si02 (va%)

5

2 1.8

4"

o

1.6

3.5-

1.4

3-

1.2

2.5-

o%

2"

°B

1

r,.)

El C ~ O O

1.5"

0.8

0

c3

O

0.6

o

1"

2.2

90

85

o

4.5-

0.5

7'5 80 Si02 (wt%)

%

0.4 65

70

75 80 Si02 (wt%)

9O

85

05

o 8

7'0

7'5

2.3 0

o cz

2.2

0

~

[]

2.1 o

9O

o•

85

90

o

[]

~g o

2 Oo°

8'5

o

o o 0o

1.81.6-

s'0

$i02 (wt%)

0

[]

o

6'5

[] o

1.9

o%

0e~ 1.4-

[]

Z

° o%

v

0

1.8

o

o

1.7"

1.2-

1.6" 1.5-

0

0.~

65

70

75 80 SiO2 (wt%)

85

90

1.4

65

70

75 80 SiO2 (wt%)

Fig. 4. Correlation of the contents of the major oxides of A1, Fe, Mg, Ca, Na, and K vs. the silica abundance in eighty-nine representative grains isolated by grinding, sieving, and heavy liquid separation from Ivory Coast tektite 2114 (see Table 3). Note the negative correlation of all oxide contents with silica, except for the K content, which increases up to 78 wt% and only then decreases.

elements (Palme et al., 1978). These high contents of siderophile elements have been interpreted as either being due to an extraterrestrial component (Palme et al., 1978, 1981) or high indigenous contents of the target rocks at Lake Bosumtwi (Jones, 1985). However, our data show widely varying Au content of up to 89 ppb, which are not accompanied by similar high Ir values, and are, thus, difficult to reconcile with the presence of an extraterrestrial component. Rather, the proposal by Jones (1985), that the rocks in the vicinity of Bosumtwi contain various ores, is supported. The relatively high and varying Ag contents are also in agreement with this suggestion (See below for further discussion.) The rare earth elements (REE) are an important group

of trace elements because geochemically they behave very similar to each other, with the possible exception of Eu and Ce, which may show a characteristically different behavior depending on the redox conditions. Their absolute abundances and their chondrite-normalized abundance patterns are characteristic for rock types of different provenance (e.g., Taylor and McLennan, 1985) so that they can be used to infer the type and composition of the tektite parent rocks. In contrast to the chondrite-normalized REE patterns of tektites from the Australasian, Central European, or North American strewn fields, which have the characteristic shape and total abundances of REE distributions in the post-Archean upper crust (e.g., a distinct negative Eu anomaly),

1754

C. Koeberl et al. Table 4. Major element composition of 111 Ivory Coast microtektites.

Sample 11 118 120 117 198-6 119 112 116 10 410-10 409-1 381-1 409-4 477-11 476-11 476-7 110 476-9 111 5-4B 381-4 476-10 477-12 8 410-4 103 5-1B 109 "5-3B 10-7D 381-2 476-2 10-7F 6 7 6-1A 476-3 6-6A 5-4C 410-9 5 108 476-1 6-6B 380-10 5-1D 477-9 380-9 35-6 4 I01 10-7E 198-1 5-7B 410-8 10-7C 5-8A 24A- 1 3 2 476-8 380-8 5-3A 8F 8-6A 10-8A 5-7A 5-2D 35-5

SiO2

TiO2

A1203

FeO

MgO

CaO

Na20

K20

Total

Site

41.80 48.60 51.40 52.30 52.69 53.10 53.40 53.60 53.70 54.12 56.09 57.48 57.49 58.36 58.97 59.10 59.70 60.19 60.50 62.14 62.88 63.44 63.49 63.80 63.89 63.90 63.93 64.30 64.38 64.41 64.57 64.93 64.96 65.00 65.00 65.00 65.05 65.14 65.17 65.26 65.30 65.30 65.36 65.56 65.60 65.71 65.71 65.73 65.90 65.90 66.10 66.11 66.20 66.27 66.30 66.36 66.43 66.50 66.50 66.50 66.60 66.63 66.77 66.80 66.94 66.98 67.03 67.16 67.20

0.15 0.54 0.50 0.49 0.27 0.48 0.49 0.50 0.98 0.76 0.69 0.52 0.65 1.13 0.59 0.59 0.57 0.47 0.67 0.62 0.65 0.65 0.65 0.67 0.79 0.59 0.62 0.50 0.49 0.56 0.65 0.62 0.53 0.64 0.83 0.54 0.59 0.75 0.64 0.50 0.75 0.57 0.57 0.54 0.60 0.58 0.49 0.68 0.49 0.66 0.55 0.45 0.58 0.61 0.57 0.58 0.49 0.54 0.80 0.85 0.59 0.66 0.58 0.52 0.48 0.52 0.39 0.61 0.56

6.0 20.3 16.0 13.4 10.16 14.1 14.0 14.9 23.4 18.79 19.46 15.02 17.29 26.87 14.4 15.59 16.1 13.46 18.4 16.48 15.66 17.09 18.29 17.1 18.47 16.7 17.5 17.3 15.22 16.49 17.37 15.93 15.49 16.4 16.5 17.52 17.23 16.27 16.44 13.68 16.8 16.8 16.82 15.63 14.83 17.1 12.89 16.85 14.5 15.5 16.2 16.06 16.67 16.57 15.45 16.64 16.7 16.9 16.4 17.4 13.35 16.61 14.93 16.8 16.17 15.43 16.04 16.27 16.1

4.2 9.5 10.5 10.6 3.84 10.1 10.8 9.83 6.8 7.98 7.85 9.13 8.6 3.42 10.8 8.73 8.26 10.09 7.68 7.23 8.01 6.84 7.03 7.2 4.56 7.01 7.14 6.9 8.41 6.71 6.04 7.36 7.12 7.6 7.6 6.65 5.91 7.33 6.62 8.49 7.2 6.47 6.44 6.86 6.79 6.51 7.29 5.66 8.0 6.4 6.31 6.59 6.69 6.87 6.79 6.53 8.27 6.8 6.0 6.9 8.1 6.1 7.26 6.21 6.44 7.01 6.39 6.5 7.1

39.6 16.9 20.0 19.1 21.45 20.0 16.6 17.4 12.0 14.62 11.5 13.34 11.63 4.01 9.83 9.64 8.45 10.63 6.55 6.62 6.44 5.00 3.08 6.1 3.77 6.32 4.79 4.83 5.89 5.41 3.03 5.75 5.8 5.6 4.9 6.01 2.68 4.76 5.04 5.2 4.9 3.84 3.61 5.05 5.53 4.43 6.55 3.9 5.2 4.8 3.97 4.88 4.19 4.02 4.7 4.46 3.44 4.4 4.2 3.5 6.29 2.84 4.87 3.2 3.94 4.9 3.91 3.99 3.8

5.6 3.53 2.76 2.48 9.67 2.70 2.32 2.09 3.10 1.87 2.49 2.52 1.81 3.63 1.88 2.21 2.13 1.66 3.31 1.62 2.3 2.14 2.4 0.8 4.32 1.79 1.74 1.6 1.32 1.33 2.66 1.78 1.42 0.8 1.2 1.63 2.74 1.56 1.73 1.48 1.6 1.59 1.70 1.16 2.67 1.52 1.55 2.75 1.1 1.2 1.6 1.48 1.59 1.49 1.57 1.7 1.69 1.4 1.3 1.5 1.09 2.12 1.59 1.57 1.61 1.54 1.89 1.1 0.91

2.6 0.63 0.40 0.42 1.50 0.73 0.71 0.47 1.00 1.12 0.94 0.96 1.34 0.95 1.78 2.15 2.61 1.84 1.15 2.82 1.96 2.35 2.48 2.8 2.32 1.76 2.3 1.61 2.24 2.57 3.02 1.7 2.61 2.4 2.6 1.38 2.91 2.14 2.39 2.73 2.1 1.94 2.53 2.77 1.94 2.16 2.74 2.34 2.2 4.0 2.04 2.43 1.97 2.03 2.23 1.95 2.61 1.9 2.8 1.9 1.83 2.39 2.03 1.86 2.34 2.08 2.03 2.03 1.9

0.02 0.33 0.12 0.14 0.02 0.30 0.31 0.19 0.30 0.38 0.32 0.27 0.51 0.85 1.08 1.38 1.37 1.05 0.94 2.05 1.36 1.82 1.89 1.8 1.33 1.39 1.55 1.4 1.7 2.09 2.04 1.28 1.71 1.8 2.0 0.86 2.21 1.62 1.51 2.22 1.7 1.69 2.30 2.04 1.39 1.57 2.07 1.46 1.8 2.0 1.73 1.61 1.7 1.7 1.85 1.36 1.95 1.5 2.2 1.1 1.52 2.05 1.59 1.93 1.66 1.51 1.88 1.92 1.8

99.97 100.33 101.68 98.93 99.60 101.51 98.63 98.98 101.28 99.64 99.34 99.24 99.32 99.22 99.33 99.39 99.19 99.39 99.20 99.58 99.26 99.33 99.31 100.27 99.45 99.46 99.57 98.44 99.65 99.57 99.38 99.35 99.64 100.24 100.63 99.59 99.32 99.57 99.54 99.56 100.35 98.20 99.33 99.61 99.35 99.58 99.29 99.37 99.19 100.46 98.50 99.61 99.59 99.56 99.46 99.58 101.58 99.94 100.20 99.65 99.37 99.40 99.62 98.89 99.58 99.97 99.56 99.58 99.37

V27-239 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 V27-239 V27-239 ODP664D ODP663B V27-239 ODP663B RC13-210 RC 13-213 K9-56 RC 13 -210 K9-56 V27-239 ODP664C RC13-210 ODP663B V27-239 ODP664D K9-56 K9-56 K9-56 K9-57 V 19-300 ODP663B ODP664D K9-57 V27-239 V27-239 K9-56 ODP664D V27-239 V27-239 V27-239 V27-239 K9-56 ODP664D V27-239 ODP663B K9-56 ODP665B ODP663B K9-57 V27-239 K9-56 V19-300 K9-56 V19-297 V19-297 V19-297 V19-297 K9-56 V27-239 V27-239 RC13-213 ODP663B K9-57 K9-56 K9-56 K9-56 V 19-297 K9-57 K9-57

1755

Ivory Coast tektite ages and geochemistry Table 4. (Continued) Sample 35-1 10-8C 35-4 35-3 5-5B 476-12 35-2 8-2A 477-5 198-7 22A-2 6-1C 409-3 5-1A 198-2 8-2B 8-4 6-1B 198-4 35-3 35-2 6-7 22A- 1 198-5 410-1 378-4 35-1 106 5-4E 6-5A 8-5 477-10 5-6C 198-8 8-3 5-4D 380-11 1 198-3 5-4A 349-6 379-12 380-7 379-4 477-8

SiO2

TiO2

A1203

FeO

MgO

CaO

Na20

K20

Total

67.20 67.28 67.30 67.40 67.45 67.53 67.60 67.61 67.65 67.66 67.80 67.84 67.85 67.85 67.87 67.87 67.97 67.97 67.99 68.00 68.00 68.04 68.10 68.11 68.17 68.18 68.20 68.20 68.28 68.52 68.53 68.83 69.02 69.11 69.43 69.45 69.73 69.80 70.02 70.04 71.30 76.50 76.7 l 77.83 82.10

0.62 0.43 0.44 0.56 0.52 0.53 0.66 0.54 0.55 0.52 0.59 0.69 0.51 0.55 0.60 0.58 0.53 0.61 0.67 0.63 0.56 0.71 0.62 0.55 0.55 0.65 0.68 0.55 0.57 0.45 0.61 0.31 0.27 0.45 0.54 0.59 0.57 0.55 0.47 0.56 0.58 0.70 0.51 0.54 0.24

15.9 15.6 14.8 16.5 16.06 16.7 15.0 16.47 16.61 16.79 15.5 15.5 15.8 16.89 16.07 16.58 16.45 16.25 16.32 15.8 15.8 17.23 15.7 16.32 15.88 16.12 16.2 15.9 16.47 15.73 16.34 11.84 9.83 15.43 15.28 15.63 15.2 14.1 14.51 15.37 14.1 13.05 12.04 11.55 9.52

6.6 5.75 7.2 6.8 6.34 5.94 6.6 5.97 6.37 7.03 6.9 6.61 6.3 6.06 6.63 6.34 6.18 6.00 6.06 6.6 5.9 5.56 7.1 6.15 5.79 5.62 6.5 5.81 5.93 5.97 5.91 6.44 7.67 6.6 6.08 5.97 5.78 6.7 5.73 5.95 4.91 3.88 2.65 3.83 1.29

3.5 4.31 4.2 3.3 4.33 2.8 3.8 3.95 3.15 2.97 3.5 4.01 3.41 3.3 3.06 3.82 3.44 4.00 3.47 3.0 3.1 3.29 3.6 2.83 2.73 2.74 3.5 3.40 2.94 3.41 3.55 2,45 5.48 3.30 3.18 2.87 2.61 3.3 2.96 2.74 2.95 0.95 1.72 0.95 1.06

1.3 1.57 1. l 1.1 1.46 1.36 1.0 1.65 1.2 0.91 1.1 1.13 1.36 1.18 1.46 1.76 1.68 1.38 1.52 1.4 1.0 1.24 1.3 1.40 1.43 2.26 0.89 1.32 0.97 1.61 1.71 1.51 1.5 1.43 1.34 1.14 1.39 0.3 1.23 1.20 2.18 0.49 0.98 0.60 1.16

2.2 2.79 2.2 2.0 1.84 2.3 2.4 2.01 1.91 1.79 2.3 2.1 2.16 2.01 2.04 1.51 1.73 1.97 2.08 2.1 2.6 2.03 1.7 2.30 2.54 2.06 1.8 1.85 2.42 2.31 1.96 3.86 2.98 1.63 2.05 2.05 1.96 2.5 2.5 1.90 1.74 0.95 1.46 1.16 0.45

1.9 1.87 2.0 1.9 1.61 2.16 2.2 1.31 1.94 1.93 2.2 1.7 2.00 1.79 1.85 1.14 1.56 1.45 1.5 1.7 2.2 1.47 1.8 1.93 2.31 1.79 1.8 1.80 2.03 1.64 1.00 4.15 2.89 1.65 1.69 1.83 2.01 2.4 2.17 1.82 1.61 2.87 3.28 2.97 3.67

99.22 99.60 99.24 99.56 99.61 99.32 99.26 99.51 99.38 99.60 99.89 99.58 99.39 99.63 99.58 99.60 99.54 99.63 99.61 99.23 99.16 99.57 99.92 99.59 99.40 99.42 99.57 98.83 99.61 99.64 99.61 99.39 99.64 99.60 99.59 99.53 99.25 99.65 99.59 99.58 99.37 99.39 99.35 99.43 99.49

Site K9-56 K9-56 K9-57 K9-57 V19-297 K9-56 K9-57 K9-56 ODP665A K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 K9-56 V27-239 K9-56 K9-56 ODP664D ODP663B K9-57 K9-56 V27-239 K9-56 K9-56 ODP663B V19-297 K9-56 K9-56 V27-239 ODP663B V27-239 K9-56 V27-239 ODP663B ODP662B ODP663B ODP662B ODP665B

Microtektites with >60 wt% SiO2 Average _+ Minimum Maximum

67.08 3.07 60.19 82.10

0.58 0.10 0.24 0.85

15.80 1.55 9.52 18.47

6.49 1.10 1.29 10.09

4.07 1.39 0.95 10.63

1.52 0.56 0.30 4.32

2.16 0.50 0.45 4.00

1.84 0.50 0.86 4.15

99.53

65.16 5.64 41.80 82.10

0.58 0.13 0.15 1.13

15.86 2.30 6.00 26.87

6.76 1.50 1.29 10.80

5.79 5.22 0.95 39.60

1.75 1.05 0.30 9.67

2.01 0.63 0.40 4.00

1.64 0.69 0.02 4.15

99.56

All Microtektites Average _+ Minimum Maximum

All data in wt%, all Fe as FeO. Data sources: Samples 1-11, 8F, 22A-1, 35-1 to -6, 101-120 from Glass and Zwart (1979) (N = 36); samples with 300 and 400 prefix from Glass et al. (1991) (N = 35); all others (N = 40) this work.

the Ivory Coast tektite R E E patterns do not h a v e any promin e n t E u a n o m a l y (Fig. 7 ) . O u r data s h o w n o n e of the u n u s u a l Tb, Yb, or L u a n o m a l i e s that were present in previous analyses ( B o u g k a and l~anda, 1976). S u c h a n o m a l i e s are m o s t

likely the result o f analytical problems. T h e Ivory Coast tektites s h o w the h i g h absolute a b u n d a n c e s and steep patterns (LaN/YbN ratios o f about 5 - 1 0 ) that are characteristic o f terrestrial continental crustal rocks, but the patterns are

1756

C. Koeberl et al.

5

20

•

•

Ivory Coast Tektites ~IL',o

•

0~15

"'i " •

• o:

<

~:

•.

o'

J /

.=,.'.~;4.

°e •f

(3 3

"•~

•

"

• oi~,~,~

o

I0 •

5

I

12 11 10

o:'•~

I

I

,

I

789

~

,

•

3

I

~

•

~

••

,

4

O

o.O

tTektite: ivoryCoas

1

• • ~,~¢!~

• 2

IvoryCoast Tektites

•

o •

•

•0 • • %0 •'.t'~°oA

ivory s oCoast J Tektite

==0".

I

.

0

rll~-~o +

.

.

t

J

t

t

3

• ,

20

•

• . •

•

Oo+ 2

•

~

IvoryCoast

%Jo-.'~-~.~"~• o l~o L~"III~." o• . i's~4.1, o

Tektites

•

"0 ")."?""

•

•

..-."

•

•

I

15

O0) :~ o~

• •e

°•

1.0

10

•

•

°°

•

•

° •

/ CoastTektites Ivory

00,

~

Ivory Coast Tektites

•

•

~

:

. . . .

•

I

•

•

0o

oS

•

? o°#'''°-o~,. • .:._~-: _.~

•

.......

•

•

;o o

50

.

55

.

.

60

.

65 70 Wt. % SiO2

-

75

80

o

50

55

,

60

-

,

65 70 Wt. % SiO2

;5

80

Fig. 5. Correlation of the contents of (a) A1203, FeO, and MgO, and (b) CaO, Na20, K20, and TiO2 vs. the abundance of SiO2, for Ivory Coast microtektites (see Table 4). The ranges for Ivory Coast tektites (Table 2; Cuttitta et al., 1972) are also plotted.

very similar to those of Archean upper crustal rocks (e.g., Taylor and McLennan, 1985). Thus, we conclude that Archean rocks are plausible source materials for the Ivory Coast tektites, in agreement with the rocks present at Lake Bosumtwi. No Ce anomalies, which are characteristic of laterites and similar weathered rocks that may be present in tropic areas, were found in any of the tektite samples. As no data were available so far on the trace element contents of individual microtektites (bulk analyses of composites of ten and sixteen Ivory Coast microtektites were reported by Frey et al., 1970 and Frey, 1977), we selected four Ivory Coast microtektites for trace element analysis, followed by major element analysis. The results of these

analyses are given in Table 5. Regarding their major element composition, these four microtektites were obviously normal microtektites with an average composition that is very similar to that of average Ivory Coast tektites, as well as that of the majority of the microtektites listed in Table 4. This is also shown graphically in Fig. 6. While the average of the four microtektites shows somewhat different enrichment/ depletion factors in relation to the average tektites compared to the average of ninety-seven microtektites from Table 4, the deviations are also less than a factor of 1.2. The trace element contents of the microtektites (Table 5 ) are in agreement with those of the tektites (Table 2). In addition, the four samples show relatively little intersample

Ivory Coast tektite ages and geochemistry

1.4

97 Microtektites

L) 1.3

.>

1757

are practically identical). Thus, no evidence for fractional vaporization is present. Also, none of the microtektites seem to be hydrated (within error limits of this method), even close (10 /zm) to the rim, which was the outermost point that was measured.

<~ 1.1

3.5. Water and Volatile Content of Ivory Coast Tektites

.~ 0.9 o.)

0.8

Microtektites

M1. M4

0.7

~

0.6 0.5

Si Ti AI Ire MnMg Ca Na K

Fig. 6. Comparison of the major element composition of Ivory Coast microtektites with that of average Ivory Coast tektites. The two lines indicate a comparison with an average of ninety-seven microtektite analyses with >60 wt% SiO2 selected from Table 4 and with the average of the four microtektites that were also analyzed for trace element composition (Table 5).

variation in the trace element contents. In fact, some elemental contents (e.g., Zn, Sb, Au) show less inter-sample variation for the microtektites than for the tektite samples. Even though the silica content is practically the same for the two groups of samples, the microtektites contain slightly higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co, Ni, Sr, Zr, Ba, Hf, Ta, and Th. The enrichments range from about 15 to 80 rel%. The abundances of the alkalies (Rb, Cs) are practically identical, and the U abundances are lower in the microtektites than in the tektites. A comparison between the trace element contents of average microtektites and average tektites is shown in Fig. 8. The REE abundances of the microtektites are slightly higher than those of the average tektite REE contents, as shown in Fig. 9. The microtektite REE range overlaps that of the tektites, but their average is higher by a factor of about 1.2-1.3, in agreement with the slight enrichments of other lithophile elements mentioned above. As none of these enrichments is accompanied by differences in the silica content, or distinct differences in the content of volatile elements, and taking into account that the abundance ratios and patterns are quite similar, it is unlikely that these chemical variations are due to fractional vaporization during impact, or admixture of significantly different types of source rocks. Rather, a slightly higher fraction of accessory trace minerals seems to have been incorporated with the melt that formed the microtektites. To determine the internal homogeneity of Ivory Coast microtektites, we analyzed the major element composition along profiles from the rim to the center of each of the four microtektites that were analyzed for trace element contents. The results are given in Table 6. The internal composition of the microtektites is remarkably uniform, and no significant compositional variations exceeding a few rel% were found. In addition, no compositional rim to center trend exists either, as shown in Fig. 10, which gives the compositional profile for sample IVC-M1 (the profiles for the other samples

Few data on the water content of tektites are available, probably because earlier it was difficult to quantify the low amounts of water present in these glasses. The first reliable measurements, using infrared (IR) spectrometry, were published by Gilchrist et al. (1969). Additional IR analyses were reported by King and Arndt (1977), Engelhardt et al. (1987), and Koeberl and Beran (1988). Tektites in general contain about 0.002-0.03 wt% (20-300 ppm) water; impact glasses range up to about 0.06 wt% (600 ppm) (in some cases up to 0.15 wt%) water. Gilchrist et al. (1969) found an average value of 0.012 __ 0.004 wt% water in twentyfive tektites. Recently, Beran and Koeberl (1997) reported IR data for twenty-six further tektite samples, with an average of 0.012 --_ 0.008 wt% water. The water content of three Ivory Coast tektite samples (IVC 2069, 2113, and 3395) was measured (see Beran and Koeberl, 1997). The results are given in Table 7 in comparison with data for tektites from the other three strewn fields. The Ivory Coast tektites have the lowest water contents of all measured tektite samples, with only 0.002-0.003 wt% ( 2 0 - 3 0 ppm) in the three specimens. No comparison IR data are available from the literature, as these are the first measurements of water in Ivory Coast tektites. Lacroix (1934) gave some wet chemical water data for three Ivory Coast tektites, but his result (0.19 wt%) was clearly erroneous. A low water content is characteristic of impact-derived glasses and can be used as a criterion to help determine the impact origin of natural glasses (Koeberl, 1992). While O'Keefe (1964) cited theoretical reasons for doubting that water can be driven out of wet crustal rocks during the formation of tektite melt, experimental evidence to the con-

100

'

'

I~

z

AverageIvory CoastTektite

Ivory Coa~Tektite Range

LaCe Pr Nd

SmEuGdTb DyHo ErTmYb Lu

Fig. 7. Average and range of chondrite-normalizedrare earth element distribution patterns for the eleven analyzed Ivory Coast tektites. Normalization factors from Taylor and McLennan (1985).

1758

C. Koeberl et al. Table 5. Major and trace element composition of four Ivory Coast microtektites. Sample Weight (#g)

M1 121

M2 138

M3 115

M4 80

SiO2 TiO2 A1203 FeO MnO MgO CaO NazO K20 Total

67.01 0.57 17.33 6.14 0.07 3.25 1.36 2.03 2.21 99.97

65.5 0.61 16.85 6.75 0.08 5.19 1.63 1.77 1.46 99.84

68.37 0.59 17.14 6.44 0.06 3.16 0.9 1.37 1.91 99.94

Sc Cr Co Ni Zn Ga As Se Br Rb Sr Zr Ag Sb Cs Ba La Ce Nd Sm Eu Gd Tb Dy Tm Yb Lu Hf Ta W Ir (ppb) Au (ppb) Hg Th U

16.8 275 32.6 292 12 26 0.31 0.5 0.5 79.9 325 190 <0.3 0.2 3.5 640 23.5 50.1 25 4.68 1.29 4.2 0.69 n.d. 0.28 1.94 0.31 4.07 0.36 0.2 1.1 0.8 <0.4 3.61 0.81

18.7 383 30.1 120 16 11 0.29 0.2 0.3 44.8 364 230 <0.3 0.15 2.9 580 24.2 49.4 23 4.89 1.33 4.5 0.73 n.d. 0.29 1.86 0.29 4.26 0.41 0.3 1.1 1.2 <0.4 4.05 0.66

K/U Th/U La/Th Zr/Hf Hf/Ta LaN/YbN Eu/Eu*

20885 4.46 6.51 46.7 11.31 8.19 0.89

22348 6.14 5.98 54.0 10.39 8.79 0.87

Average

Std. dev.

68.58 0.57 16.96 6.26 0.08 3.18 0.97 1.34 1.87 99.81

67.37 0.59 17.07 6.40 0.07 3.70 1.22 1.63 1.86 99.89

1.23 0.02 0.18 0.23 0.01 0.86 0.30 0.29 0.27

17.9 260 33.3 240 10 19 0.51 0.15 0.4 70.5 277 220 <0.3 0.2 3.1 610 26.4 57.1 31 5.08 1.54 4.3 0.81 n.d. 0.33 2.21 0.31 4.45 0.45 0.3 0.9 0.7 <0.4 4.14 0.59

18.2 248 34.9 242 10 10 0.58 0.28 0.3 71.7 334 220 <0.3 0.3 3.4 650 29.4 63.8 30 5.76 1.56 4.6 0.74 n.d. 0.34 2.28 0.32 4.32 0.45 0.3 0.6 0.5 <0.4 4.16 0.49

17.9 292 32.7 224 12 17 0.42 0.28 0.4 66.7 325 215 <0.3 0.21 3.2 620 25.9 55.1 27.3 5.10 1.43 4.40 0.74 n.d. 0.31 2.07 0.31 4.28 0.42 0.3 0.9 0.8 <0.4 3.99 0.64

0.7 54 1.7 63 2 7 0.13 0.13 0.1 13.2 31 15

19350 7.02 6.38 49.4 9.89 8.07 1.01

22789 8.49 7.07 50.9 9.60 8.71 0.93

21343 6.52 6.48 50.3 10.30 8.44 0.92

0.05 0.2 27 2.3 5.9 3.3 0.41 0.12 0.16 0.04 0.03 0.18 0.01 0.14 0.04 0.0 0.2 0.3 0.22 0.12 1349 1.46 0.39 2.6 0.65 0.32 0.05

All Fe as FeO. Major element data in wt%, by electron microprobe; trace element data in ppm (except as marked), by INAA.

trary exists. Gilchrist et al. (1969) demonstrated that glass formed by melting ( 1 0 - 3 0 seconds) o f wet rocks, soils, and clays in a solar furnace were almost as dry as tektites, and Glass et al. (1986a, 1988) have shown that atomic b o m b glass, formed by melting o f sediments, is very dry (0.007 wt.% H 2 0 ) . The same applies for glasses found at k n o w n impact craters. In addition, Vickery and Browning (1991),

in a preliminary study o f diffusion coefficients for water (as H and O ) in tektites, concluded that the calculated depletions are in agreement with the observed water contents in tektites. Thus, water, as one o f the most volatile components of the target rocks from which tektites have formed, is clearly depleted in the tektites compared to the source rocks. It is also likely that the most volatile elements (e.g., the halogens,

Ivory Coast tektite ages and geochemistry le ~D

.> 1

0.1

ScRbSr ZrBaHfTaTh U

CrCoNiIr

ZnGaAsSbCs

Fig. 8. Comparison of trace element abundances (in three groups: lithophile, siderophile, and volatile elements) between the average of the four analyzed Ivory Coast microtektites (Table 5) and the average of the eleven analyzed tektites (Table 2).

Cu, Zn, Ga, As, Se, and Pb) were volatilized from the source rocks upon melting. While we did not determine the concentrations of all these elements, the low and highly variable amounts of, e.g., Zn, Br, and As indicate that some fraction of the volatile elements was lost during tektite formation. Meisel et al. (1992) reported halogen contents for Ivory Coast tektite sample IVC 2069 (F: 22 ppm, CI: 4.2 ppm, Br: 0.55 ppm, and I: 13 ppb), and Matthies and Koeberl (1991) found 23 ppm F for sample 2069 and 22 ppm F for sample 3395. These values are lower than those for normal crustal rocks (cf. Koeberl, 1994). It should be noted, however, that volatilization was rather limited, because the boron isotopic compositions of Ivory Coast tektites (and other tektites) do not show any evidence for vapor fractionation (Chaussidon and Koeberl, 1995).

3.6. 4°Ar-39Ar Age Analysis The age of the Ivory Coast tektite forming event is still not as well-constrained as would be desirable. So far, K-Ar data of 1.2 _+ 0.2 (Z~ihringer, 1963) and 1.15 _+ 0.15 (Gentner et al., 1969a) Ma, and fission-track ages of 0.71 ___ 1.41 (Fleischer et al., 1965), 0.86 _+ 0.06 (Durrani and Khan, 1971), and 1.01 ± 0.1 (Gentner et al., 1969a) Ma, and ArAr ages of 1.14 _ 0.03 Ma (Bollinger, 1993) were published. To try to improve on the available age data, we selected five of the samples that were studied for their chemical composition (as well a four individual microtektites) for 4°Ar-39Ar age spectrum analyses, as well as ten additional samples for fission-track dating. Some of our results were previously reported in abstract form (Koeberl et al., 1989). The experimental data from our 4°Ar-39Ar age analyses are presented in Tables 8 and 9. The integrated (total gas) ages from the step heating runs fell in the range 0.95-1.7 Ma (with one exception), while the single step ages of the microtektites ranged from -1 to 13 Ma with correspondingly larger errors. Most of the step heating runs showed plateau or near plateau over a significant portion of the released gas when plotted on age spectra diagrams (age vs. fraction of 39Ar released, Fig. 1l a - i ) .

1759

When using the step-heating method on small, young samples ( ~ 1 mg), the small amount of 36Ar released in some steps leads to a large uncertainty in atmospheric argon correction for that fraction. If the corrections for atmospheric contamination form a significant portion of the measured 4°Ar peak, the uncertainty due to the corrections results in large relative errors in the associated age determinations. In general, many of the smaller and lower temperature fractions of the Ivory Coast tektites samples show increased scatter and large relative errors due to this effect. Sample 16-3 (RB 3C- 1, RB3C-3) showed good agreement between both samples analyzed (Fig. l la,b). Each shows a plateau (or near plateau for sample RB3C-3) for 80% of the gas released. The plateau ages and the total gas ages (Table 8) are identical within errors. Sample RB3C-3 displays more experimental scatter because its gas volume was six times smaller than that of RB3C-1. Only one run of sample 22-5 (RB3C-5) was useable (Fig. 1 ld). The replicate analysis (RB3C-4; Fig. 1lc) had a large tailing signal from a low mass contaminant that overwhelmed the 36Ar signal, resulting in a spectrum that does not resemble its replicate run RB3C-5. Except for some small mid-temperature scatter due to fractions with small gas volumes, run RB3C-5 is consistent with its total gas age of 1.43 _+ 0.26. In both runs of sample 3396 (RB3C-7,8; Fig. 1 le,f), the low temperature fractions show more of a tendency to higher ages than the larger, high-temperature fractions that form a plateau. Sample RB3C-7 gives a plateau of 1.06 Ma over the last 45% of the gas released, while RB3C-8 gives a plateau of 1.15 Ma over 90% excluding the last small fraction which drops in age (Fig. 1 If). Sample 2069 (RB 3C-10,11; Fig. 11 g,h) also showed some elevated ages in the lower temperature fractions which elevates the total gas ages as compared to the age of the higher temperature fractions, which show good plateaux with 6 0 80% of the gas in the 1.1-1.2 Ma range. Only one run of the sample 8901 (RB3C-12) had fraction volumes large enough to give useful data. Ninety percent of its higher temperature fractions gave a structured, near-pla-

100 \\A,.

.~ .-~

10

Z

5

[

~

Avg. I V C "--+'- M1 M3

La Ce Pr Nd

~

~

M2

1

M4

SmEuGdTbDyHoErTmYbLu

Fig. 9. Chondrite-normalizedrare earth element distribution patterns for the four analyzed Ivory Coast microtektites, compared to the average pattern of Ivory Coast tektites. Normalization factors from Taylor and McLennan (1985).

1760

C. Koeberl et al. Table 6. Radial compositional profiles across four Ivory Coast microtektites. IVC-M 1

1

2

3

4

5

6

7

8

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20

67.13 0.54 17.3 6.2 0.05 3.3 1.37 1.84 2.34

67.10 0.52 17.5 6.1 0.01 3.3 1.33 1.99 2.29

66.72 0.61 17.5 6.2 0.09 3.1 1.39 2.12 2.21

66.66 0.60 17.2 6.0 0.03 3.2 1.38 1.98 2.15

67.39 0.54 17.1 5.9 0.06 3.3 1.37 2.13 2.18

66.36 0.57 17.4 6.5 0.1 3.3 1.37 2.12 2.17

67.31 0.60 17.3 6.1 0.11 3.3 1.32 2.09 2.21

67.39 0.54 17.3 6.1 0.07 3.2 1.36 1.96 2.09

Total

100.07

100.14

99.94

99.2

99.97

99.89

100.34

100.01

99.95

1

2

7

8

Average

IVC-M2

3

4

5

6

SiO2 TiO2 AlzO3 FeO MnO MgO CaO Na20 K20

66.65 0.57 15.9 6.6 0.06 4.96 1.56 2.04 1.71

65.61 0.61 16.7 6.8 0.09 5.15 1.65 1.91 1.54

65.21 0.64 17.0 6.8 0.08 5.25 1.64 1.82 1.52

65.52 0.62 16.9 6.8 0.11 5.25 1.68 1.74 1.47

64.69 0.65 17.1 7.0 0.07 5.44 1.72 1.76 1.34

65.10 0.60 17.3 6.6 0.05 5.34 1.64 1.69 1.32

65.31 0.60 17.0 6.9 0.08 5.06 1.58 1.62 1.37

65.94 0.56 16.9 6.5 0.06 5.06 1.58 1.58 1.38

Total

100.05

100.06

99.96

100.09

99.77

99.64

99.52

99.56

IVC-M3

1

2

3

4

5

SiO2 TiOz A1203 FeO MnO MgO CaO NazO K20

68.84 0.57 16.8 6.5 0.10 3.0 0.90 1.33 1.95

68.21 0.61 17.1 6.4 0.09 3.2 0.96 1.42 1.95

68.33 0.60 17.2 6.5 0.01 3.3 0.88 1.35 1.88

68.37 0.56 17.2 6.5 0.08 3.1 0.90 1.43 1.91

68.31 0.60 17.4 6.3 0.04 3.2 0.88 1.34 1.86

Total

99.79

99.94

100.05

100.05

99.93

3

4

IVC-M4

1

2

5

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20

68.56 0.57 17.1 6.4 0.03 3.0 0.98 0.98 1.89

68.12 0.56 17.2 6.3 0.05 3.2 0.97 1.45 1.90

68.47 0.59 16.9 6.3 0.10 3.3 0.97 1.41 1.86

68.99 0.55 16.8 6.1 0.14 3.2 0.98 1.43 1.86

68.77 0.56 16.8 6.2 0.10 3.2 0.94 1.42 1.86

Total

99.51

99.75

99.90

100.05

99.85

Average 67.01 0.57 17.33 6.14 0.07 3.25 1.36 2.03 2.21

65.50 0.61 16.85 6.75 0.08 5.19 1.63 1.77 1.46

_+ 0.36 _+ 0.03 _+ 0.13 + 0.17 _+ 0.03 +_ 0.07 + 0.02 -4- 0.10 _+ 0.07

___0.56 _+ 0.03 ___0.39 + 0.16 _+ 0.02 _ 0.15 _+ 0.05 _ 0.14 _+ 0.12

99.83 Average 68.37 0.59 17.14 6.44 0.06 3.16 0.90 1.37 1.91

-4- 0.14 ___ 0.02 _+ 0.20 -4- 0.08 ___0.03 _+ 0.10 _+ 0.03 ___0.04 _+ 0.04

99.95 Average 68.58 0.57 16.96 6.26 0.08 3.18 0.97 1.34 1.87

_+ 0.29 ± 0.01 +__0.16 _+ 0.10 _ 0.04 +__0.10 +_ 0.01 ___ 0.18 ___0.02

99.81

All data in wt%, all Fe as FeO. 1 = rim, 8 or 5 = center; points at about 50 #m distance.

teau that s h o w e d a slight, m o n o t o n i c drop with increasing temperature (Fig. 1 l i ) . T h e experimental data for four r u n s o f the microtektites K - 9 are given in Table 9. T h r e e o f the s a m p l e s were so small that they did not h a v e e n o u g h radiogenic gas for a g o o d analysis (their associated relative errors are 5 0 - 2 0 0 r e l % ) . T h e fourth sample, R B 3 C - 6 , gave an age of 2.00 _ 0.72 Ma, w h i c h overlaps the uncertainty in the age estimate m a d e f r o m the step-heating runs.

To arrive at the best age estimate for the formation age of these tektites, we f o r m e d a w e i g h t e d average o f the ages f r o m the plateau portion of all the runs ( e x c e p t R B 3 C - 4 ) . W e included the larger, h i g h temperature plateau age fractions f r o m s a m p l e s 3396, 2069, and 8901 along with the integrated ages for s a m p l e s 16-3 and 22-5. T h i s gives an age o f 1.1 +_ 0.05 Ma. W e h a v e c h o s e n to u s e all the consistent ages f r o m all five s a m p l e s in our age determination. However, we note that s a m p l e IVC 16-3 h a d plateau ages

Ivory Coast tektite ages and geochemistry

16-18- ~___=

=-----t_

=/.

--.-

14 8"

~

i

~-~ 12 10

6

.

Na

'

~"---+

Rim

Center Profile I V C Microtektite M1

Fig. 10. Major element compositions along a profile from the rim to the center of Ivory Coast microtektite M1, indicating a homogeneous internal composition and no compositional trend from the rim to the center. Points are spaced at approximately 50 #m.

which averaged 0.95 _+ 0.1 Ma. It is possible that IVC 163 may record the actual age of impact with the most fidelity, and the other samples may be slightly contaminated by not being completely outgassed by the tektite forming event, altematively, a thermal effect as described in the next chapter could be responsible for this low value. However, given the present set of data, we believe the best estimate of the age of the impact event, based on 4°Ar-39Ar dating, is 1.1 ___ 0.05 Ma. The combination of young age and small sample size makes it impossible to assign a reliable age to the microtektites. It appears that their apparent age based on run RB3C6 is consistent with the hypothesis that they were formed by the same event which formed the Ivory Coast tektite samples at 1.1 _+ 0.05 Ma.

1761

for dating are the reason for the large relative errors of the fission-track ages. The apparent fission-track ages of the Ivory Coast Tektites do not overlap within their l~r standard deviations, but range between 0.8 and 1.0 Ma. The Bosumtwi impact glass has an apparent fission-track age of only 0.5 Ma. W e have recalculated previously published fission-track ages of Ivory Coast tektites and Bosumtwi crater glasses (Fleischer et al., 1965; Gentner et al., 1967) with the same age constants that are used in this work. For the tektites, seven out of twelve of these apparent fission-track ages are lower than 1 Ma and one is as low as 0.57 Ma. For the Bosumtwi crater glasses the apparent ages can be as low as 0.25 Ma. The Ivory Coast microtektites, on the other hand, have a fission-track age of 1.09 Ma (Gentner et al., 1970). This age scatter could be easily understood if the latent fossil fission tracks in the different tektites and impact glasses had been exposed to higher than ambient temperatures and had partly faded due to these thermal effects. In this case, the number of fossil fission tracks, and, therefore, the age, would be reduced, but also the size of the fossil fission tracks compared to the thermally unaffected induced fission tracks would be reduced (Storzer and Wagner, 1969). A detailed size analysis of fossil and induced fission tracks in all our samples indeed revealed that the fossil tracks are smaller than the induced tracks and, therefore, the apparent fission-track ages have to be corrected for the effect of partial fading of fossil tracks (Storzer and Wagner, 1969). W e believe that the observed thermal perturbance of the fission track clock in the samples is due to natural or agricultural fires in the area. The track-size corrected ages for the ten individual tektite samples ranged from 0.91 to 1.18 Ma and overlap within their h7 standard deviations. The mean corrected average fission-track age of our tektite samples is 1.05 _+ 0.11 Ma, which is, within errors, identical to the age of the Bosumtwi

3.7.Fission-TrackDating Fission-track dating has been successfully applied to tektites and impact glasses (e.g., Gentner et al., 1969a,b, 1970; Storzer and Wagner, 1977, 1979; Koeberl et al., 1994). The method is based on the fact that during the spontaneous fission of 238U (half-life for fission: 8.19 × 1015 years), the high-energy (200 M e V ) fission fragments of U leave a trail of damage (usually about 10 # m long) in their host glass or crystal. These very small tracks are normally invisible under the optical microscope, but they can be enlarged and made visible by etching with various solutions because of the increased solubility of the damaged areas. The experimental data of our fission-track analyses of ten individual Ivory Coast tektites and on impact glass from the Ata river site of the Bosumtwi crater are given in Table 10. In all cases the apparent (measured) ages were too small due to track size and density reduction. The U contents of the tektites (as meaured by the fission-track method; Table 10) are rather low and range between 0.5 and 0.9 ppm. This low U content, together with the low age of the tektites (about 1 M a ) , and the small quantities of material available

Table 7. Water content of Ivory Coast tektites compared to tektites from other strewn fields (by FTIR spectrometry). Sample Ivory Coast tektites [1] IVC 2069 IVC 2113 IVC 3395 Australasian Tektites Muong Nong-type indochinites Range (N = 12) [1, 3] Philippinites (N = 4) [1, 2] Indochina; Thailand; Java (N = 8) [2] Australites (N = 2) [2] Moldavites Range (N = 24) [1, 2, 4] North American tektites Bediasites (N = 9) [1, 2] Georgiaites (N = 3) [1, 2]

Water (wt %) 0.002 0.003 0.002 0.011-0.030 0.011-0.019 0.002-0.015 0.009-0.013 0.006-0.017 0.005-0.030 0.011-0.020

Data from: [1] Beran and Koeberl (1997); [2] Gilchrist et al. (1969); [3] Koeberl and Beran (1988); [4] Engelhardt et al. (1987). N = number of analyses.

1762

C. Koeberl et al. Table 8. Stepwise-release data for 4°mr-39Aranalyses of Ivory Coast tektites.

Temperature (°C)

Atmosphere (%)

37Arca/39ArK

39ARK cumulative

4°mr*/39K

-0.1480 0.3360 0.3226 0.2175 0.3397 0.1703 0.2698 0.2327 0.2356 0.2433 0.2515 0.2509

0.13% 1.14% 2.94% 5.99% 8.37% 11.57% 17.62% 28.69% 44.39% 65.27% 89.01% 100.00%

38.15 5.241 0.514 -0.454 4.528 3.677 -0.243 2.922 2.615 2.027 2.401 1.874

16 2.2 0.2 -0.2 1.9 1.5 -0.1 1.23 1.10 0.85 1.01 0.79 Total age = 0.94

_+ 17 + 2.3 _+ 1.4 _+ 1.3 +_ 1.7 _+ 1.3 + 0.6 _+ 0.35 + 0.17 _+ 0.15 _+ 0.15 +_ 0.40 _+ 0.12

0.1267 0.2542 0.1791 0.2311 0.1490 0.2593 0.2664 0.1817 0.2759 0.2418 0.2457 0.2690

3.84% 9.25% 11.36% 14.51% 17.72% 21.29% 25.00% 29.65% 35.04% 41.96% 64.37% 100.00%

2.462 0.557 2.796 3.011 1.963 4.97 6.573 3.358 3.156 2.299 2.643 1.35

1.0 0.2 1.2 1.3 0.8 2.1 2.8 1.4 1.3 0.96 1.11 0.57 Total age = 0.97

_ 1.5 _+ 1.2 _+ 2.8 _+ 2.2 _+ 1.5 _+ 1.5 _+ 1.7 _+ 1.3 +_ 1.2 _+ 0.93 _+ 0.21 _+ 0.21 _+ 0.21

0.2245 0.0714 -0.0745 0.2015 0.2030 0.2223 0.2285 0.2269 0.2162 0.2181

6.57% 9.64% 11.77% 21.99% 33.16% 49.78% 64.57% 76.39% 90.94% 100.00%

188 39 26 6.0 3.0 -0.2 3.74 2.5 1.97 0.4 Total age = 16.9

_+ 9 + 4 _+ 4 _+ 1.2 _+ 1.2 _+ 0.8 _+ 0.86 _+ 1.1 _+ 0.76 _+ 1.4 _+ 0.4

- 1.2090 0.2636 0.2647 0.8405 - 1.8850 0.3189 0.2624 0.3414 0.3069 0.2827 0.2386 0.2301

0.29% 1.50% 33.12% 33.31% 33.57% 34.78% 39.81% 45.68% 53.71% 69.98% 83.20% 100.00%

25.68 -6.316 3.032 32.66 57.63 8.534 3.316 -2.45 3.188 4.59 4.178 3.333

11 -2.7 1.27 14 24 3.6 1.4 -1.0 1.34 1.92 1.75 1.40 Total age = 1.43

_+ 24 _+ 5.4 _+ 0.20 _+ 41 _+ 24 _+ 7.3 + 1.7 _+ 1.1 _ 0.92 + 0.45 _+ 0.45 _+ 0.58 + 0.26

-0.2890 0.2432 0.3239

1.07% 3.81% 6.70%

18.88 14.16 2.175

Age (Ma)

Sample IVC16-3, run RB3C-I. 600 800 1000 1100 1150 1200 1250 1300 1330 1360 1400 1600

9.71 35.37 88.58 109.7 -15.05 -25.48 107.3 7.62 21.11 43.09 36.05 47.69

Sample IVC16-3, run RB3C-3. 900 1150 1200 1250 1300 1330 1360 1400 1450 1500 1600 1650

75.64 85.62 42.01 12.33 55.00 -83.45 -75.60 -5.60 0.36 32.31 24.16 61.12

Sample IVC22-5, run RB3C-4. 800 900 1000 1140 1250 1350 1400 1450 1500 1600

9.71 - 12.81 -42.30 8.50 -60.89 110.3 - 154.2 -68.78 -33.33 77.92

472.9 95.12 61.30 14.37 7.24 -0.41 8.93 5.93 4.71 0.92

Sample IVC22-5, run RB3C-5. 600 800 1000 1100 1200 1250 1300 1350 1400 1450 1500 1600

66.83 135.0 40.59 1291 -283.4 -84.86 10.00 158.8 31.33 4.56 6.60 27.14

Sample IVC3396, run RB3C-7. 800 1000 1200

-19.56 -160.0 29.29

7.9 _+ 6.6 5.9 _+ 2.5 0.9 +_ 2.4

Ivory Coast tektite ages and geochemistry

1763

Table 8. (Continued) Temperature (°C)

Atmosphere (%)

37Arca/39ArK

39ARK cumulative

4°Ar*/39K

Age (Ma)

Sample IVC3396, run RB3C-7. 1250 1300 1350 1400 1450 1500 1600 1700

-272.3 -154.9 61.09 -71.82 -28.67 -17.50 33.65 20.87

0.3156 0.2526 0.3636 0.2978 0.3231 0.3571 0.3609 0.3341

10.30% 14.49% 19.74% 26.82% 37.68% 55.90% 94.12% 100.00%

8.182 7.361 1.263 5.676 4.053 3.723 2.537 2.537

3.4 3.1 0.5 2.38 1.70 1.56 1.06 1.06 Total age = 1.66

_+ 2.1 _+ 1.2 _+ 1.3 + 0.93 _+ 0.61 + 0.26 +_ 0.17 _+ 0.95 _+ 0.21

-2.9470 0.0526 0.2192 0.3452 0.3375 0.3359 0.2114

-0.13% 1.39% 4.78% 9.14% 61.12% 97.85% 100.00%

33.43 17.10 10.71 7.162 2.588 2.967 -19.33

14 7.2 4.5 3.0 1.09 1.24 -8.1 Total age = 1.22

_+ 70 + 9.6 _+ 4.8 -4- 3.9 _+ 0.25 _+ 0.51 _+ 6.7 _+ 0.40

1.5520 0.3140 0.1851 0.3065 0.2892 0.2466 0.2733 0.2687 0.2492 0.2713 0.2522

0.19% 1.57% 3.86% 7.50% 11.43% 16.64% 24.31% 35.29% 59.26% 88.49% 100.00%

-40.52 7.125 20.88 5.575 7.346 2.913 4.389 3.579 2.630 2.618 3.349

-17 3.0 8.7 2.3 3.1 1.2 1.84 1.50 1.10 1.10 1.40 Total age = 1.53

+ 33 _+ 2.8 + 2.7 + 1.5 _+ 1.2 +_ 1.0 _+ 0.50 +_ 0.52 _+ 0.19 _+ 0.19 _+ 0.46 _+ 0.17

0.2679 0.1504 -0.1959 0.2378 0.3099 0.2407 0.2495 0.2663 0.3101 0.2884 0.2656 0.2464

1.81% 6.27% 7.76% 9.97% 13.73% 19.96% 27.04% 35.03% 43.49% 53.95% 80.41% 100.00%

26.98 -1.700 -6.271 -9.281 4.340 -0.229 7.151 5.020 5.987 1.191 2.796 2.893

11.3 -0.7 -2.6 -3.9 1.8 -0.1 3.0 2.1 2.5 0.5 1.17 1.21 Total age = 1.30

-4- 5.2 _ 2.6 _+ 6.2 -4- 4.1 _+ 2.7 _+ 1.9 _+ 1.2 _+ 1.2 -4- 1.2 _+ 1.1 _+ 0.39 _+ 0.55 _+ 0.35

0.2601 0.2451 0.3760 0.3376 0.3266 0.3235 0.3155

4.03% 7.25% 9.73% 18.39% 36.33% 73.41% 100.00%

5.328 -2.297 9.96 2.972 3.482 2.776 2.345

2.2 -1.0 4.2 1.25 1.46 1.16 0.98 Total age = 1.23

+ 1.5 _+ 2.0 -4- 3.2 _+ 0.90 _+ 0.26 _+ 0.20 _ 0.23 + 0.18

Sample IVC3396, run RB3C-8. 600 800 1000 1200 1400 1500 1600

130.6 -26.94 -145.3 -95.22 21.21 18.32 240.1

Sample IVC2069, run RB3C-10. 600 800 1000 1100 1150 1200 1250 1300 1350 1400 1600

175.1 22.25 -245.1 -17.10 -97.06 23.38 -18.05 6.61 31.68 40.75 16.44

Sample IVC 2069, run RB3C-11. 800 1000 1100 1200 1250 1300 1350 1400 1450 1500 1580 1650

-67.57 140.1 224.9 372.5 -11.61 105.8 -73.76 22.23 -18.98 69.85 33.45 42.70

Sample IVC 8901, run RB3C-12. 1000 1100 1200 1350 1450 1560 1650

62.88 129.4 -116.1 21.71 15.24 36.78 45.00

The value of J for all runs was 2.325.10 -4. Errors are quoted at the lcr level.

1764

C. Koeberl et al.

Table 9. Single-step 4°mr-39Aranalyses of microtektite samples K-9 from the Ivory Coast strewn field. Run RB3C-6 RB3C-14 RB3C-15 RB3C-17

Atmosphere (%) 37Arca/39ArK4°Ar*/39Ar -19.54 -72.49 86.06 118.2

0.3464 -0.3850 0.2737 0.2766

4.762 31.06 6.849 -2.302

Age (Ma) 2.00 _+ 0.72 13.0 _+ 8.4 2.9 + 2.3 -1.0 -4- 2.0

For notes see Table 8.

impact glass at 1.03 _+ 0.11 Ma (Table 10). Both ages are, within errors, indistinguishable from the age of 1.1 +_ 0.05 Ma as determined by the 4°Ar-39Ar method. These findings are consistent with the hypothesis that the formation of the tektites (and microtektites) is coeval with the formation of the Bosumtwi impact crater.

3.8. Origin of Ivory Coast Tektites and Connection to the Bosumtwi Impact Crater Our data clearly confirm that the Ivory Coast tektites are impact glasses that have formed from terrestrial upper crustal rocks. In general, the geochemistry of the tektites is almost identical to the composition of the terrestrial upper crust. This observation is based on the similarity of major and trace element abundances and ratios; e.g., the ratios of Ba/ Rb, K/U, Th/Sm, Sm/Sc, Th/Sc, and K vs. K/U, in tektites are practically the same as those in upper crustal rocks (Taylor and McLennan, 1985). This observation is in agreement with the conclusions of earlier detailed geochemical studies of tektites from other strewn fields (see, e.g., the reviews by Taylor, 1973; and Koeberl, 1986). The REE abundances and patterns of the Ivory Coast tektites are also in agreement with derivation from upper crustal rocks, and the low water content of the tektites is incompatible with a volcanic origin. Thus, we reinforce the conclusions of earlier workers that the Ivory Coast tektites have formed by melting and subsequent quenching of terrestrial rocks during hypervelocity impact on the Earth. The high l°Be content of the Ivory Coast tektites (Tera et al., 1983a,b) requires that the tektites originated from surfical rocks. In the following paragraphs we will briefly review the evidence for an origin of the Ivory Coast tektites from the Bosumtwi impact structure in Ghana. Our chemical data indicate, as mentioned above, derivation of the tektites from Archean crustal rocks, which would agree with the ages of the rocks at Lake Bosumtwi. Furthermore, the Ivory Coast tektites have large negative CNd values of about - 2 0 (Shaw and Wasserburg, 1982), which are typical for old continental crust, yielding mantle extraction Sm-Nd model ages of about 1.9 Ga. This is in agreement with the whole rock Rb-Sr ages of the rocks around the Bosumtwi crater which range from 1.9 to 2.1 Ga (e.g., Schnetzler et al., 1966; Kolbe et al., 1967). Chamberlain et al. (1993) found that the oxygen isotopic characteristics of the tektites are almost indistinguishable from those of the sedimentary country rocks around Bosumtwi. Chaussidon and Koeberl (1995) studied the boron isotopic composition of Ivory Coast tektites and

found low B abundances coupled with slightly negative 6 lIB values, which are distinct from the values of other tektites, and are consistent with granitic and metasedimentary source rocks. Palme et al. ( 1978, 1981 ) tried to detect a possible meteoritic signature in the Ivory Coast tektites. They analyzed two Ivory Coast tektites and found Ir and Os abundances of 0.24 and 0.33 ppb and 0.099 and 0.199 ppb, respectively. These values are higher than those of average crustal rocks, which led Palme et al. ( 1981 ) to suggest the tektites show evidence of an iron meteorite contamination. In contrast, Jones ( 1985 ) concluded that the high siderophile element contents could be derived from the target rocks because the Bosumtwi crater is in an area of known gold mineralization. In an attempt to solve this question, Koeberl and Shirey (1993) determined the abundances and isotopic ratios of Os and Re in four Ivory Cost tektites, two impact glass samples, and five different target rocks from the Bosumtwi crater. They found high Os abundances in the rocks from the Bosumtwi crater (0.020.33 ppb), which seem to confirm the suspicion of Jones (1985) that the siderophile elements in the tektites could have originated from the Bosumtwi country rocks. Thus, abundance data alone do not allow confirmation of a cosmic component. However, the ~87Os/lg8Osratios of the tektites (0.153-0.209) are within the range of chondrites and iron meteorites but are inconsistent with the origin of the Os from crustal rocks, as any significant crustal Os contribution would result in elevated 187Os/188Os and ~87Re/188Os ratios. Bosumtwi crater target rocks were found to have ~87Os/188Os ratios ranging from 1.48 to 4.98 (Koeberl and Shirey, 1993). These values are typical for old continental crust. From the isotopic ratios, and based on chondritic abundances, a meteoritic contribution to the tektite composition not exceeding 0.05-0.1 wt% was estimated. Furthermore, Koeberl and Shirey ( 1993 ) also found that the isotopic composition of the tektites in the 187Os/188Os and 187Re/J88Os diagram can best be explained from mixing of Os from country rocks with Os from a meteorite. This conclusion establishes a further, independent, link between Ivory Coast tektites and the Bosumtwi impact crater. Studies of the distribution and stratigraphic age of Ivory Coast microtektites also help to contrain the age of the event and the relation with the Bosumtwi crater. Glass and Pizzuto (1994) showed that the amount of microtektites in the deepsea cores increases towards the Bosumtwi crater. In addition, using Ivory Coast microtektite abundance data and equations that relate ejecta thickness as a function of distance from the source crater, these authors derived a hypothetic source crater diameter of 12.6 _+ 3.4 km, which is in good agreement with the actual size of the Bosumtwi crater. While earlier studies (e.g., Durrani and Khan, 1971; Glass et al., 1979) argued for a possible relationship between the Ivory Coast tektite event and the onset of the Jaramillo geomagnetic reversal, more recent analyses show that the microtektites were deposited about 8000 y after the onset of the Jaramillo subchron (Schneider and Kent, 1990; Glass et al., 1991). The base of the Jaramillo event has been dated at 1.07 Ma (Cande and Kent, 1995 ), and, thus, the magnetostratigraphic age of the Ivory Coast microtektites (1.06 Ma) is in very

Ivory Coast tektite ages and geochemistry RB3C-t I

20

I

1765

IUCt6-3 I

I

I

I

I

I

I

20

1S-

15

t0-

10

5-

0

o_

iI -5

-t0 0.0

o.I t

0'.2

o.3'

Cumulative RB3C-3 I

20

I

o'.4

o'.s

fraction

0'.6

o.7'

o'.e

o~3

-t0 t.0

o f SgAr r e l e a s e d

IUCt6-3 I

I

I

I

I

I

20

I

15

IS

tO

10

5

O

0

-5

-10

o.o

o'.t

-tO

i

o.2' o'.3 o.4' o.s' o'.s o.7' o'.e o . s C u m u l a t i v e f r a c t i o n o f 89At r e l e a s e d

t.0

Fig. 11. Step-heating age spectra diagrams (age vs. fraction of 39Ar released) for five Ivory Coast tektites. Duplicate runs were performed for each sample. See text for details. (a) sample IVC 16-3, run 1; (b) sample IVC 16-3, run 3; (c) sample IVC 22-5, run 4, which shows the effect of a large spurious signal at mass 36, a tailing artifact from lower mass contaminant; (d) sample IVC 22-5, run 5; (e) sample IVC 3396, run 7; (f) sample IVC 3396, run 8; (g) sample IVC 2069, run 10; (h) sample IVC 2069, run 11; (i) sample IVC 8901, run 12.

good agreement with the tektite and impact glass ages derived in the present work. While a detailed comparison of the compositions of rocks from the Bosumtwi crater and Ivory Coast tektites is beyond the scope of the present paper (and so far still hampered by

incomplete datasets for Bosumtwi crater rocks), it seems safe to support the conclusion that the Ivory Coast tektites have indeed originated during the same impact event that formed the Bosumtwi crater. Our age data are also in agreement with this conclusion.

1766

C. Koeberl et al. RB3C-4

I UC22-5

I

20

I

I

I

I

I

I

I

20

15-

t5

10

t0

5

O-

1~

-5

-5-

L-t0

-1o

-15

-20 0.0

i

O.

0.2

o'.3

i 0.4

o'.s

o

is .

i

0.7

i 0.8

o~9

t.0

Cumulative fraction of 89At released RB3C -5; I

20

I

I UC22-5 I

I

I

I

I

I

I

20

lS

10

t0

A

-5

-t0 0.0

i 0.t

o'.2

o'.3

C~ulative

F i

0.4

-S

i

o.s

o'.6

0 . 7'

o

i .8

o~8

-t0 1.0

fraction of sg/~ released Fig. 11. (Continued)

4. SUMMARY AND CONCLUSIONS Ivory Coast tektites are found in a small strewn field within the West African country of Ivory Coast (Cote d'Ivoire). They were first recognized in 1934 and since then, only a small number of specimens (compared to tektites from the other three strewn fields) were recovered. Microtektites had been recognized in (so far) eleven deep-sea cores

in the Atlantic Ocean south and west of the western part of Africa, between about 9°N and 12°S and 0 ° and 23°W, defining the extent of the Ivory Coast tektite strewn field. In this paper we tried to improve the database for chemical compositions of Ivory Coast tektites and microtektites, as well as to provide new dating information. We analyzed eleven Ivory Coast tektites for their major and trace element composition, studied their petrographical characteristics,

1767

Ivory Coast tektite ages and geochemistry RB3C-7 I

20

IUP-,339(3 T E K T I T E I

I

I

I

I

I

20

tS

t5

tO

tO

S

O

i• -S

-lO I 0.0

o'.1

0:2

0:3

0:4

0:5

o:s

0:7

o'.e

o'.s

,.0

--tO

C u m u l a t i v e f r a c t i o n o f SgAr r e l e a s e d RB3C-8 I

20

I

I UC3396 I

TEKTITE I

I

I

I

I

20

t5

15

t0

10

-5

-10 0.0

o~,

-10 0'.2

0.3 '

0.4 '

o.s

ols

0:7

o'.e

.0

C u m u l a t i v e f r a c t i o n o f 89Ar r e l e a s e d Fig. 11. (Continued)

provided major element data for 111 microtektites, and major and trace element data for four microtektites. In addition, we performed 4°Ar-39Ar step-heating analyses on five Ivory Coast tektites and four microtektites and presented fissiontrack dates for ten tektites and one Bosumtwi impact glass. Our main observations and conclusions are summarized in the following paragraphs. We studied petrographic sections of seven tektites and

determined the volume abundance of bubble cavities and lechatelierite particles. The glass is generally dark olivegreen in color. Bubble cavities were found to be not uniformly distributed with the tektites, but all are of spherical shape (usually < 100 #m in diameter). The average size of the largest observed bubble cavities is ---0.5 ram, but one 3.6 mm diameter bubble cavity was found. They generally made up less than 1 vol% of the glass. Lechatelierite particles

1768

C. Koeberl et al. P,,IB3C-tO

20

I

I

IUC2069 I

I

I

I

I

I

I

20

15-

15

10

tO

A ~a

0

-5

-5

--t0

0.0

o'.i

o'.2

o'.3

o'.4

Cumulative RIB3C - 1 1 I

20

I

o'.s

o'.s

0'.7

o.o'

o'.s

-tO

,'.o

f r a c t i o n o f *'Ar r e l e a s e d

I UC2069 I

I

I

I

i

I

I

20

15

t5

10

tO

ol

o

-5

-5

-10 0.0

o'I.

0'.2 o.~' o.4' 0'.5 o.G' o.7' o'.s C u m u l a t i v e f r a c t i o n o f S'Ar r e l e a s e d

o.s'

I -tO

1.o

Fig. 11. (Continued)

are slightly more abundant, at 1.2 vol%. Their shapes range from equant to elongate to ribbon-like or filamentous, with most being vesicular, but none were frothy. The lechatelierite particles are generally elongate along flow structures. No crystalline inclusions were observed in any of the tektites. All tektites show a remarkably small spread in their major and trace element composition. For most elements, this spread is <10 rel%. The same is true for internal chemical

variations, as determined by electron microprobe analyses. This spread in composition is significantly smaller than for any of the other tektite groups, making the Ivory Coast tektites the most homogenous tektites of all. Furthermore, one tektite sample (2114) was crushed, sieved, and subjected to heavy liquid separation. Of the eighty-nine representative grains that were analyzed for their major element composition, most fall within the range of SiO2 6 7 - 6 9 wt%, with a

1769

Ivory Coast tektite ages and geochemistry RB3C-t2

20

I UC890t

I

I

I

I

I

I

I

20

t5

15

t0

10

F

O

- -5

-Io

I 0.0

Or. 1

0'2

01.3

01.4

OB.5

01.6

01.7

01.8

0'.9

Cumulative fraction o f SgAr released

I -10

It .0

Fig. 11. (Continued)

f e w g r a i n s h a v i n g silica c o n t e n t s as l o w as 6 4 w t % , a n d s o m e h a d SiO2 c o n t e n t s g r e a t e r t h a n 70 w t % . W i t h t h e e x c e p t i o n o f K, all o t h e r m a j o r e l e m e n t a b u n d a n c e s are n e g a t i v e l y

c o r r e l a t e d w i t h t h e silica c o n t e n t . T h e K 2 0 c o n t e n t i n c r e a s e s w i t h t h e silica a b u n d a n c e u p to 78 w t % SiO2 a n d d e c r e a s e s afterwards.

Table 10. Fission-track data and track size corrected ages for Ivory Coast tektites and Bosumtwi impact glass. Reduction Sample

Ivory Coast tektites Bayassou 2201 Daoukro 2206 Akakoumo6krou II 334 Akakoumo6krou II 333 Quell6 II 32 Quell6 2202 Quell4 2109 Daoublo 512a Quell6 136 E B K Seitz Average Ivory Coast tektite (N = 10) Bosumtwi Impact Glass Bosumtwi-Ata

U content (ppm)

0.80 0.66 0.48 0.50 0.85 0.72 0.56 0.78 0.91 0.90

tm (Ma)

0.99 0.94 0.78 0.76 0.85 0.81 0.79 0.83 0.89 0.92

_+ 0.04 -4- 0.05 _+ 0.10 _+ 0.08 4- 0.06 _+ 0.06 ___ 0.05 _+ 0.05 -+ 0.05 _ 0.02

Diameter ps" 102/cm2 [#]

1.61 1.27 0.77 0.78 1.47 1.19 0.90 1.32 1.65 1.68

_+ 0.06 _+ 0.06 -4- 0.08 ___ 0.07 -+ 0.13 + 0.08 _+ 0.05 _+ 0.07 _+ 0.07 -4- 0.03

[723] [467] [98] [123] [131] [217] [327] [389] [480] [2587]

pi/10 tl n [#]

0.81 0.67 0.49 0.51 0.86 0.73 0.57 0.79 0.92 0.91

_+ 0.02 -4- 0.02 _+ 0.03 _+ 0.03 _+ 0.04 _+ 0.03 _+ 0.02 -4- 0.03 -4- 0.03 -+ 0.01

[2807] [1217] [211] [315] [395] [584] [1012] [875] [1023] [5923]

Density

DdDi

pJpl

0.99 0.98 0.86 0.93 0.98 0.89 0.94 0.90 0.94 0.96

0.95 0.93 0.66 0.80 0.93 0.72 0.82 0.74 0.82 0.85

tco~r (Ma)

1.04 1.01 1.18 0.95 0.91 1.13 0.96 1.12 1.09 1.08

_ 0.07 -+ 0.07 -+ 0.16 -4- 0.11 _+ 0.11 _ 0.11 4- 0.08 _ 0.09 -+ 0.08 -4- 0.06

1.05 -4- 0.11 1.22

0.49 -+ 0.03

1.12 -+ 0.05 [502]

1.13 -+ 0.04 [798]

0.77

0.48

1.03 _+ 0.11

Notes: Sample designations include the geographical location and the sample number in the respective collection (the first 7 Ivory Coast tektites are from the M u s e u m Histoire Naturelle, Paris, and the last three tektites and the Bosumtwi impact glass are from the Max-Planck Institut, Heidelberg). Abbreviations: t m = measured (apparent) age; too, = track-size corrected age; Ps = density of fossil fission tracks; Pi = density (normalized to 10 H n/cm z) of thermal neutron-induced tracks ([#] = total number of actually counted fission tracks); DdD~and QJQi = size reduction and corresponding track density reduction of fossil fission tracks; the l~r standard deviation of the apparent fission-track ages is composed of the statistical counting errors of the number of fossil tracks, thermal neutron induced tracks, and the determination of the neutron flux (_+5%). For the corrected ages, an additional error of +5 rel% due to the size correction technique is assumed. The fission-track ages were calculated using the following constants: GrFission_235= 580.2 barn; I (ratio 235U/238U) = 7.253 X 10 -3, and • F i s s l o n - 2 3 8 = 8.46 x 10 ,7 a-l. Track etching conditions: 48 vol% HF, 22°C, 10 seconds.

1770

C. Koeberl et al.

The major element compositions of 111 Ivory Coast microtektites from eleven cores were determined and show a range in composition that is significantly wider than that of the Ivory Coast tektites, with silica contents ranging from about 42 to 82 wt%. Nevertheless, the majority of all microtektites have compositions very similar to those of Ivory Coast tektites. The abundances of major elements, except Na and K, show a scattered negative correlation with the silica contents. Comparison of an average microtektite composition that is calculated from data with silica > 6 0 wt% with an average tektite composition shows that differences for major element abundances are less than a factor of 1.2, which is very little compared to tektites and microtektites from the Australasian and North A m e r i c a n strewn fields. Ivory Coast tektites are also very h o m o g e n o u s with regard to their trace element composition, with most elements showing less than 15 rel% variation between eleven samples. The Ivory Coast tektite REE patterns do not show any distinct Eu anomaly, and the high absolute REE abundances and steep patterns ( L a y / Y b s ratios of about 8), indicate that Archean rocks are plausible source materials, in agreement with the rocks from the B o s u m t w i impact structures. Four Ivory Coast microtektites were individually analyzed for their trace element contents. The major element compositions of these samples indicated a composition very similar to that of average tektites. Also, the trace element contents of the microtektites are almost identical to those of the Ivory Coast tektites, with relatively little variation in the trace element contents between the four samples. However, the microtektites contain slightly higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co. Ni, Sr, Zr, Ba, Hf, Ta, and Th, with most of the enrichments being < 2 0 rel%. The microtektite R E E abundances are also s o m e w h a t higher than those of the average tektite R E E contents, by a factor of about 1.2-1.3. These enrichments are not accompanied by differences in the silica content, or distinct differences in the content of volatile elements, making it unlikely that vapor fractionation played a role, or that significantly different source rocks were involved for the Ivory Coast tektites. Incorporation of a higher abundance of accessory trace minerals with the melt that formed the microtektites seems the most likely explanation. The Ivory Coast microtektites show a remarkably uniform internal composition, with compositional variations not exceeding a few rel%. along microprobe profiles from the rim to the center of each sample. The water content of the Ivory Coast tektites is the lowest o f all tektite groups, at 0 . 0 0 2 - 0 . 0 0 3 wt%. W e performed duplicate 4°Ar-39Ar age analyses on five tektites. Most of the step heating runs resulted in a plateau or near plateau over a significant portion of the gas in age spectra diagrams. A best age estimate for the formation age of these tektites was obtained by calculating a weighted average of the ages from the plateau portion of all but one of the runs, resulting in an age of 1.1 ± 0.05 Ma. Four microtektites were also subjected to 4°Ar-39Ar age analyses, but their young age and small sample size makes it impossible to assign a reliable age to the microtektites. Only one run was satisfactory, and the resulting value is

consistent with the assumption that the microtektites were formed in the Ivory Coast tektite event. W e obtained fission-track ages for ten individual Ivory Coast tektite samples and for one impact glass sample from the Bosumtwi crater. For all samples the apparent ages were too low due to thermal effects on the fossil fission track record and had to be track-size corrected. The corrected ages for the Ivory Coast tektites ranged from 0.91 to 1.18 Ma, resulting in an average fission-track age of 1.05 ___ 0.11 Ma. This age is, within errors, identical to that of the B o s u m t w i impact glass at 1.03 ± 0.11 Ma, and to the 4°Ar-39Ar age of 1.1 + 0.05 Ma. Thus, the preferred age of the Ivory Coast tektite event is 1.07 Ma. Acknowledgments--The research was in part supported by the Aus-

trian Fonds zur Frrdemng der wissenschaftlichen Forschung, Project P09021-GEO (C.K.). We thank the Museum National d'Histoire Naturelle and the University of Abidjan for samples, V. Yang and L. Lee (NASA Johnson Space Center) and F. Brandst~itter (Naturhistorisches Museum, Vienna) for assistance with microprobe work, and Dona Jalufka for preparing some of the line drawings. Perceptive reviews by Ray Anderson and Bevan French, and comments by Ross Taylor, are much appreciated. Editorial handling: S. R. Taylor

REFERENCES Beran A. and Koeberl C. (1997) Water in Tektites and Impact Glasses by FTIR Spectrometry. Meteoritics Planet. Sci. (submitted). Bohor B.F. and Koeberl C. (1996) Are microtektites really microtektites? Meteoritics 31, A17. Bollinger K. ( 1993 ) 4°Ar-39ArDatierung yon Tektites. Diplomarbeit, Max-Planck-Institut, Heidelberg, and Ruprecht-Karls-Universit~it Heidelberg. Bottomley R. and York D. (1988) Age measurement of the submarine Montagnais impact crater. Geophys. Res. Lett. 15, 14091412. Bottomley R.J., York D., and Grieve R. A. F. (1990) 4°Argon39Argon dating of impact craters. Proc. 20th Lunar Planet. Sci. Conf., 421-431. Bougka V. and l~anda Z. (1976) Rare earth elements in tektites. Geochim. Cosmochim. Acta 40,486-488. Cande S. C. and Kent D. V. (1995) Revised calibration of the geomagnetic timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093-6095. Cassidy W. A., Glass B. P., and Heezen B. C. (1969) Physical and chemical properties of Australasian microtektites. J. Geophys. Res. 74, 1008-1025. Chamberlain C. P., Blum J. D., and Koeberl C. (1993)Oxygen isotopes as tracers of tektite source rocks: An example from the Ivory Coast tektites and Lake Bosumtwi crater. Lunar Planet. Sci. XXIV, 267-268. Chao E. C. T. (1963) The petrographic and chemical characteristics of tektites. In Tektites (ed. J. A. O'Keefe), pp. 51-94. University of Chicago Press. Chapman D. R. and Scheiber L. C. (1969) Chemical investigation of Australasian tektites. J. Geophys. Res. 74, 6737-6776. Chaussidon M. and Koeberl C. (1995)Boron content and isotopic composition of tektites and impact glasses: Constraints on source regions. Geochim. Cosmochim. Acta 59,613-624. Cuttitta F., Carron M. K., and Annell C. S. (1972) New data on selected Ivory Coast tektites. Geochim. Cosmochim. Acta 36, 1297-1309. Durrani J.A. and Khan H.A. (1971) Ivory Coast microtektites: Fission track ages and geomagnetic reversals. Nature 3 2 3 , 3 2 0 323.

Ivory Coast tektite ages and geochemistry E1 Goresy A. (1966) Metallic spherules in Bosumtwi crater glasses. Earth Planet. Sci. Lett. 1, 23-24. E1 Goresy A., Fechtig H., and Ottemann T. (1968) The opaque minerals in impactite glasses. In Shock Metamorphism o f Natural Materials (ed. B.M. French and N.M. Short), pp. 531-554. Mono Book Corp. Engelhardt W. V., Luft E., Arndt J., Schock H., and Weiskirchner W. (1987) Origin of moldavites. Geochim. Cosmochim. Acta 51, 1425-1443. Fleischer R. L., Price P. B., and Walker R. M. (1965) On the simultaneous origin of tektites and other natural glasses. Geochim. Cosmochim. Acta 29, 161-166. Frey F. A. (1977) Microtektites: A chemical comparison of bottlegreen microtektites, normal microtektites, and tektites. Earth Planet. Sci. Lett. 35, 43-48. Frey F. A., Spooner C. M., and Baedecker P. A. (1970) Microtektites and tektites: A chemical comparison. Science 170,845-847. Gentner W. (1966) Auf der Suche nach KratergHisern, Tektiten, und Meteoriten in Afrika. Naturwiss.12, 2 8 5 - 289. Gentner W., Lippolt H. J., and Mailer O. (1964) Das Kalium-ArgonAlter des Bosumtwi Kraters in Ghana und die chemische Beschaffenheit seiner Gl~iser. Z. NaturJbrsch. 19A, 150-153. Gentner W., Kleinmann B., and Wagner G. A. (1967) New K-Ar and fission track ages of impact glasses and tektites. Earth Planet. Sci. Lett. 2, 83-86. Gentner W., Storzer D., and Wagner G. A. (1969a) Das Alter yon Tektiten und verwandten Gl~isern. Naturwiss. 56,255-261. Gentner W., Storzer D., and Wagner G.A. (1969b) New fission track ages of tektites and related glasses. Geochim. Cosmochim. Acta 33, 1075-1081. Gentner W., Glass B. P., Storzer D., and Wagner G. A. (1970) Fission track ages and ages of deposition of deep-sea microtekfites. Science 168, 359-361. Gilchrist J., Thorpe A. N., and Senftle F. E. (1969) Infrared analysis of water in tektites and other glasses. J. Geophys. Res. 74, 14751483. Glass B. P. (1967) Microtektites in deep-sea sediments. Nature 214, 372-374. Glass B.P. (1968) Glassy objects (microtektites?) from deep-sea sediments near the Ivory Coast. Science 161,891-893. Glass B. P. (1969) Chemical composition of Ivory Coast microtektites. Geochim. Cosmochim. Acta 33, 1135-1147. Glass B. P. (1972) Bottle green microtektites. J. Geophys. Res. 77, 7057-7064. Glass B. P. and Pizzuto J. E. (1994) Geographical variation in Australasian microtektite concentrations: Implications concerning the location and size of the source crater. J. Geophys. Res. 99, 1907519081. Glass B.P. and Zwart P. A. (1979) The Ivory Coast microtektite strewn field: New data. Earth Planet. Sci. Lett. 43,336-342. Glass B. P., Swincki M. B., and Zwart P. A. (1979) Australasian, Ivory Coast, and North American tektite strewn field: Size, mass and correlation with geomagnetic reversals and other earth events. Proc.lOth Lunar Planet. Sci. Conf., 2535-2545. Glass B.P., Muenow D.W., and Aggrey K.E. (1986a) Further evidence for the impact origin of tektites. Meteoritics 21, 369370. Glass B. P., Hall C. M., and York D. (1986b) 4°Ar/39Ar laser-probe dating of North American tektite fragments from Barbados and the age of the Eocene-Oligocene boundary. Chem. Geology 59, 181-186. Glass B. P., Senftle F. E., Muenow D. W., Aggrey K. E., and Thorpe A. N. (1988) Atomic bomb glass beads: Tektite and microtektite analogs. In Proceedings of the 2nd International Conference on Natural Glasses (ed. J. Konta), pp. 361-369. Charles Univ., Prague. Glass B. P., Kent D. V., Schneider D. A., and Tauxe L. ( 1991 ) Ivory Coast microtektite strewn field: Description and relation to the Jaramillo geomagnetic event. Earth Planet. Sci. Lett. 107, 182196. Govindaraju K. (1987) Compilation report on Ailsa Craig granite AC-E with the participation of 128 GIT-IWG laboratories. Geostand. Newsl. 11, 1-203.

1771

Jarosevich E., Clarke R.S., and Barrows J.N. (1987) The Allende Meteorite Reference Sample. Smithsonian Contrib. Earth Sci. 27, 1-49.

Jones W. B. (1985) Chemical analyses of Bosumtwi crater target rocks compared with Ivory Coast tektites. Geochim. Cosmochim. Acta 49, 2569-2576. Jones W. B., Bacon M., and Hastings D. A. (1981) The Lake Bosumtwi impact crater, Ghana. Geol. Soc. Amer. Bull. 92, 342349. King E. A. and Arndt J. (1977) Water content of Russian tektites. Nature 269, 48-49. Koeberl C. (1986) Geochemistry of tektites and impact glasses. Annu. Rev. Earth Planet. Sci. 14,323-350. Koeberl C. (1992) Water content of glasses from the K/T boundary, Haiti: An indication of impact origin. Geochim. Cosmochim. Acta 56, 4329-4332. Koeberl C. (1993) Instrumental neutron activation analysis of geochemical and cosmochemical samples: A fast and proven method for small sample analysis. J. Radioanal. Nucl. Chem. 168, 4760. Koeberl C. (1994) Tektite origin by hypervelocity asteroidal or cometary impact: Target rocks, source craters, and mechanisms. In Large Meteorite Impacts and Planetary Evolution (ed. B. O. Dressler et al.); Geol. Soc. Amer. Spec. Paper 293, 133-151. Koeberl C. and Beran A. ( 1988 ) Water content of tektites and impact glasses and related chemical studies. Proc. 18th Lunar Planet. Sci. Cove, 403-408. Koeberl C. and Reimold W.U. (1996) Lake Bosumtwi, Ghana, impact structure: Petrography and geochemistry of target rocks. Meteoritics Planet. Sci. 31, A72. Koeberl C. and Shirey S. B. ( 1993 ) Detection of a meteoritic component in Ivory Coast tektites with rhenium-osmium isotopes. Science 261,595-598. Koeberl C., Kluger F., and Kiesl W. (1987) Rare earth element determinations at ultratrace abundance levels in geologic materials. J. Radioanal. Nucl. Chem. 112,481-487. Koeberl C., Bottomley R. J., Glass B. P., Storzer D., and York D. (1989) Geochemistry and age of Ivory Coast tektites. Meteoritics 24,287. Koeberl C., Storzer D., and Reimold W. U. (1994) The age of the Saltpan impact crater, South Africa. Meteoritics 29, 374-379. Koeberl C., Poag C.W., Reimold W.U., and Brandt D. (1996) Impact origin of Chesapeake Bay structure and the source of North American tektites. Science 271, 1263-1266. Kolbe P., Pinson W. H., Saul J. M., and Miller E. W. (1967) Rb-Sr study on country rocks of the Bosumtwi crater, Ghana. Geochim. Cosmochim. Acta 31,869-875. Lacroix A. (1934) Sur la d6couverte de tectites ~t la C6te d'Ivoire. Compt. Rendus Acad. Sci. Paris 199, 1539-1542. Lippolt H.J. and Wasserburg G.J. (1966) Rubidium-StrontiumMessungen an GHisern vom Bosumtwi-Krater und an Elfenbeinktisten-Tektiten. Z. Naturforsch. 21A, 226-231. Littler J., Fahey J. J., Dietz R. S., and Chao E. C. T. (1961) Coesite from the Lake Bosumtwi crater, Ashanti, Ghana. Geol. Soc. Amer. Spec. Paper 68,218. Matthies D. and Koeberl C. ( 1991 ) Fluorine and boron geochemistry of tektites, impact glasses, and target rocks. Meteoritics 26, 4145. Meisel T., Langenauer M., and Krfihenbtthl U. (1992) Halogens in tektites and impact glasses: New results and a critical review. Meteoritics 27,260. O'Keefe J. A. ed. (1963) Tektites. Univ. Chicago Press. O'Keefe J. A. (1964) Water in tektite glass. J. Geophys. Res. 69, 3701-3707. O'Keefe J. A. (1976) Tektites and their Origin. Elsevier. Palme H., Janssens M.-J., Takahasi H., Anders E., and Hertogen J. (1978) Meteorite material at five large impact craters. Geochim. Cosmoehim. Acta 42,313-323. Palme H., Grieve R. A. F., and Wolf R. (1981) Identification of the projectile at the Brent crater and further considerations of projectile types at terrestrial craters. Geochim. Cosmochim. Acta 45, 2417-2424. Pinson W. H. and Griswold T. B. (1969) The relationship of nickel

1772

C. Koebed et al.

and chromium in tektites: New data on the Ivory Coast tektites. J. Geophys. Res. 74, 6811-6815. Poag C. W., Powars, D., Poppe L. J., and Mixon R. B. (1994) Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field. Geology 22,691-694. Rybach L. and Adams J. A. S. (1969) The radioactivity of the Ivory Coast tektites and the formation of the Bosumtwi crater (Ghana) Geochim. Cosmochim. Acta 33, 1101 - 1102. Saul J. M. (1969) Field investigations at Lake Bosumtwi (Ghana) and in the Ivory Coast tektite strewn field. Natl. Geographic Soc. Res. Rept. 1964 Proj., 201-212. Schneider D. A. and Kent D. V. (1990) Ivory Coast microtektites and geomagnetic reversals. Geophys. Res. Lett. 17, 163-166. Schnetzler C. C., Pinson W. H., and Hurley P. M. (1966) RubidiumStrontium age of the Bosumtwi crater area, Ghana, compared with the age of the Ivory Coast tektites. Science 151,817-819. Schnetzler C.C., Philpotts J.A., and Thomas H.H. (1967) Rare earth and barium abundances in Ivory Coast tektites and rocks from the Bosumtwi crater area, Ghana. Geochim. Cosmochim. Acta 31, 1987-1993. Shaw H. F. and Wasserburg G. J. (1982) Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics. Earth Planet. Sci. Lett. 60, 155-177. Storzer D. and Selo M. (1974) Dosage et repartition de l'uranium par la methode des traces de fission dans des tectites et impactites associees. Compt. Rendus Acad. Sci. Paris 278 Ser. D, 19311934. Storzer D. and Selo M. (1985) Chrono-thermom6trie par traces de

fission: Une perspective nouvelle pour la prospection p&roli~re. Rev. Inst. Franc. P~trol. 40,301-321. Storzer D. and Wagner G. A. (1969) Correction of thermally lowered fission track ages of tektites. Earth Planet. Sci. Lett. 5 , 4 6 3 468. Storzer D. and Wagner G. A. (1971) Fission track ages of North American tektites. Earth Planet. Sci. Lett. 10,435-440. Storzer D. and Wagner G. A. ( 1977 ) Fission track dating of meteorite impacts. Meteoritics 12,368-369. Storzer D. and Wagner G. A. (1979) Fission track dating of Elgygytgyn, Popigai and Zhamanshin craters: No source for Australasian or North American tektites. Meteoritics 14,541-542. Storzer D. and Wagner G. A. (1982) The application of fission track dating in stratigraphy: A critical review. In Numerical Dating in Stratigraphy (ed. G. S. Odin), pp. 199-221. Wiley. Taylor S. R. (1973) Tektites: A post-Apollo view. Earth Sci. Rev. 9, 101-123. Taylor S. R. and McLennan S.M. (1985) The Continental Crust: Its Composition and Evolution. Blackwell Sci. Publ. Tera F., Middleton R., Klein J., and Brown L. (1983a) Beryllium10 in tektites. EOS Trans. Amer. Geophys. Union 64,284. Tera F., Brown L., Klein J., and Middleton R. (1983b) Beryllium10: Tektites. Yearb.Carnegie Inst. 82,463-465. Vickery A. M. and Browning L. ( 1991 ) Water depletion in tektites. Meteoritics 26,403. Yellin J., Perlman I., Gentner W., and Miiller O. (1983) High precision elemental abundances for tektites and crater glasses - thorium, uranium, and potassium. J. Radioanal. Chem. 76, 35-47. Zahringer J. (1963) K-Ar measurements of tektites. In Radioactive Dating, pp. 289-305. Intl. Atom. Energy Agency.