Fragmentation and hydration of tektites and microtektites

Meteoritics & Planetary Science 32,333-341 (1997) 0 Meteoritical Society, 1997. Printed in USA.

Fragmentation and hydration of tektites and microtektites B. P. GLASS1*, D. W. MIJENOW, B. F. BOHOR3 AND G. P. MEEKER4

IDepartment of Geology, University of Delaware, Newark, Delaware 19716, USA 2Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA

3U.S. Geological Survey, Box 25046, MS 973, Denver, Colorado 80225, USA 4U.S. Geological Survey, Box 25046, MS 903, Denver, Colorado 80225, USA

*Correspondence author's e-mail address: [email protected]

(Received 1996 July 8; accepted in revised form 1997 January 8)

Abstract-An examination of data collected over the last 30 years indicates that the percent of glass hgments vs. whole splash forms in the Cenozoic microtektite strewn fields increases towards the source crater (or source region). We propose that this is due to thermal stress produced when tektites and larger microtektites fall into water near the source crater while still relatively hot (>1150 "C). We also fmd evidence (low major oxide totals, frothing when melted) for hydration of most of the North American tektite hgments and microtektites found in marine sediments. High-temperature mass spectrometry indicates that these tektite fragments and microtektites contain up to 3.8 wt?? H20. The H2O-release behavior during the high-temperature mass-spectrometric analysis, plus high C1 abundances (-0.05 wt'??), indicate that the North American tektite hgments and microtektites were hydrated in the marine environment (i. e., the H20 was not trapped solely on quenching fiom a melt). The younger Ivory Coast and Australasian microtektites do not exhibit much evidence of hydration (at least not in excess of 0.5 wt?! H20); this suggests that the degree of hydration increases with age. In addition, we fmd that some glass spherules (with <65 wt?! Si02) fiom the upper Eocene clinopyroxene-bearing spherule layer in the Indian Ocean have palagonitized rims. These spherules appear to have been altered in a similar fashion to the splash form K/T boundary spherules. Thus, our data indicate that tektites and microtektites that generally contain >65 wt% Si02 can undergo simple hydration in the marine environment, while impact glasses (with <65 wf?? Si02) can also undergo palagonithation.

INTRODUCTION

Previous descriptions of Cenozoic microtektites have stressed the splash-form microtektites (i.e., rounded forms produced by solidifi- cation of liquid droplets), and very little quantitative data have been presented on abundance of microtektite fragments in the microtektite layers. In this paper, we present quantitative data on percent of glass fragments vs. whole splash-form microtektites found at various sites within the Australasian, Ivory Coast, and North Ameri- can strewn fields.

Tektites are characterized by having very low H20 contents (<0.02 wt%) (e.g., Gilchrist et al. 1969; Koeberl and Beran, 1988; Beran and Koeberl, 1997). Until recently, most authors agreed that tektites, unlike obsidian, do not undergo hydration (e.g., LaMarche et af., 1984; Barkatt et al., 1984). More recently, Mazer et al. (1992) have proposed that, under conditions of restricted H20 contact, tektites can undergo hydrolysis. Here we report the discovery that some tektite fragments and microtektites recovered from marine sediments can be highly hydrated or palagonitized, depending on the composition of the glass.

SAMPLES We report the percent of glass fragments vs. whole splash-form

microtektites in 42 sites from the Australasian, 9 sites from the Ivory Coast, and 5 sites from the North American strewn fields (Table 1; Fig. 1). Specimens from several of these sites were used for hydration studies, although quantitative data on volatile contents are given only for material from three sites within the North American strewn field. We also describe some glass spherules with palagonitized outer rims from the clinopyroxene-bearing spherule layer in the Indian Ocean, Deep Sea Drilling Project (DSDP) Site 216.

We use the phrase "microtektite layer" to refer to the occurrence of tektite glass in marine sediments, although we realize that at

some sites (e.g., DSDP Site 612 off New Jersey) the tektite glass may occur mostly in the form of tektite fragments rather than in actual splash-form microtektites.

METHODS Percent of Fragments

We define fragments as angular pieces of glass that appear to have formed by fragmentation of larger splash forms. Chipped or broken splash forms were not counted as fragments unless they appeared to be less than half of their original size.

The percent of fragments (by number) at each microtektite-bearing site (Table 1; Fig. 1) was determined from data collected by the senior author (B. P. G.) and his students over a period of 30 years. None of these data have previously been published; however, some quantitative data for the percent of fragments in the North American microtektite layer in Barbados were presented in M.S. theses (Bums, 1986; Lemer, 1986). It is our experience that spherules are easier to spot and recover than are the fragments. The efficiency in counting and recovering merits depends on the sediment type, sample processing, and skill and experience of the observer. Further- more, much of the data were originally collected without any intention of determining the percent of fragments at a given site. As a result, the percent of fragments determined for many of the sites is probably underestimated.

Water Content Electron microprobe analyses of glasses that total significantly 400 wto/o

oxides suggest the presence of H,O (or bad analyses). Low oxide totals were the first evidence that some of the North American and Australasian tektite glass might be hydrated (eg., Thein, 1987; Smit et al., 1991). Major and minor element analyses were obtained on a JEOL 8900 electron microprobe at the U. S. Geological Survey in Denver, Colorado. The analyses were corrected using on-line CITZAF procedures of Armstrong (1995). Analytical conditions were: 15kV accelerating voltage, 20nA beam current (cup), and 20pm probe diameter. Standards used for these analyses included: Tiburon albite (Si, Na), Miyake anorthite (Ca, Al), Springwater olivine (Mg), synthetic TiO, (Ti), synthetic fayalite (Fe), Or-1A orthoclase (K), and Nuevo garnet (Mn). Replicate analyses of secondary standards including GSC glass (Myers ef al., 1976) indicate a relative analytical precision of better than f 1% (la) for major elements and a precision equal to counting statistics for minor elements.

333

334 Glass et al.

TABLE 1. Data for coreslsites used in this study.

Number of microtektites Maximum size @m)

Distance from (>I25 pm) Percent Core/S ite Latitude Longitude crater (km)* per cm2 fragments Spherules Fragments

North American strewn field DSDP 612 38.82N DSDP 94 24.53N DSDP 149 15.10N RC9-58 14.56N Barbados 13.20N Ivory Coast strewn field V19-297 2.62N ODP 663 1.02s V27-239 7.838 K9-56 8.38N RCI 3-2 1 3 10.48s V 19-300 6.88N RC 13-2 10 9.13N K9-57 8.63N ODP 664 0.10N Australasian strewn field ODP 769A ODP 768B ODP 7588 DSDP 292 ODP 767B RC14-46 v19-I 53 V19-158 V19-171 RC 14-24 V20-138 V19-169 V29-39 V29-40

MSN 48G

V29-43

RC 14-23

RC12-33 1

RC12-328 E48-6 RC12-327 RC8-53 E50-2 E45-89 E49-5 1 E49-50 RC8-52 V28-239 RC9-143 V28.238 RC9-142 V16-75 E49-4 V16-76 E45-74 E45-71 LSDH 23G E35-9 RC9- 137 E39-45 V20- 184 V16-70 E35-6

8.79N 8.00N 5.38N

15.82N 4.79N 7.823 8.853

18.18s 7.07s 6.62s

28.87N 10.22s 7.70s

10.48s 9.18s

15.258 2.50N 12.33s 3.95N

34.00s 1.73N

39.383 39.968 39.528 39.93s 40.61s 41.10s 3.25N

41.368 1.02N

42.728 22.22s 46.998 25.158 47.55s 48.02s 31.238 45.05s 45.028 45.16s 25.808 32.10s 53.21s

72.78W 88.47W 69.36W 70.81W 59.51w

12.00w 11.88W 1.52w

15.5ow 2.40W

19.47W 10.62W 22.03W 23.27W

121.22E 121.22E 90.36E

124.65E 123SOE 100.00E 102.12E 99.40E 80.77E 79.44E

135.55E 8 1.62E 77.38E 77.05E 76.768 81.13E 69.87E 75.08E 60.60E 97.54E 57.83E

104.378 104.93E 134.47E 99.94E 99.91E

101.42E 159.18E 114.13E 360.48E 136.89E 58.38E

110.13E 59.90E

134.44E 114.49E 62.97E

328.02E 132.758 133.96E 53.68E 55.85E

128.1 OE

339 1888 2545 2569 3 145

1250 1441 1585 1613 1881 1996 2009 2287 2529

1704 1726 1870 2059 2086 229 1 2346 3417 3510 3600 3605 3661 3858 3982 4009 4091 4126 4369 5084 5172 5444 5687 5750 5773 5783 5858 5896 5947 5969 6147 6167 6496 6543 6548 665 1 6704 6721 6737 6926 6994 7135 7350 7578

n.d. 5750 n.d.

8100 877

80 12 17

n.d. 10 8

14 35 < I

3196 n.d.

3255 448

2980 1056 329

39 20 39 49 56 so

106 55 52 82 82 14

1 13 4 2

1 1 4 5

14 7

18 3

16 2

14 18 2

13 4 2

10 4

10 14 8

99 37 17 24 52t

44t 48

3 7 0 0 0 6 0

25 24 41 30 33 32 30 0

13 4

10 5

10 10 0 0 0 3 0

0 5 0 0 0 0 2 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0

-

-

20005 -1000 - 1000

320 620

950 200 400 700 240 260 270 195 160

480 660 390 340 310 720 500 830 860 800 460 790 640 850 900 830 400 580 340

520 800 240 260 145 520 800 540 550 280 400 320 440 340 220 280 440 22s 800 640 270 400 420

-

70008 n.d. n.d. 280

1030

800 240 1 so 600? - - -

180 -

790 700 700 720 420

1200 320

280 340 320 440 800 480

-

- - -

240 - - -

500 - - - -

400 - - - -

- - - - - - - - - - - -

Fragmentation and hydration of tektites and microtektites 335

TABLE 1. Continued.

Number of Maximum size (um) microtektites

Distance from (>I25 p n ) Percent CorelSite Latitude Longitude crater (h)* per cmz fragments Spherules Fragments

Upper Eocene clinopyroxene-bearing epherule layer DSDP 2 16 1.45N 12.21E - -2000 28 900 820

*We used the Chesapeake Bay structure (Poag et al., 1994), Bosumtwi Crater, and 12"N latitude, 106"E longitude (Glass and Pizzuto, 1994) to determine the distance to the source craterhea for the North American, Ivory Coast, and Austra- lasian strewn fields, respectively. tFrom Burns (1986). tAppears to be due to solution in the marine environment and crushing during laboratory processing. §From Thein (1987). According to Thein, some centimeter-size fragments were observed as a sample was being prepared for geochemical studies. n.d. = not determined.

O'Keefe (1976) pointed out that tektites can be distinguished from terrestrial volcanic glasses (obsidians) by heating them to the melting point with a blowpipe or a blowtorch. Obsidians turn to a foamy glass, while tektites produce only a few bubbles at most because of their much lower contents of HzO and other volatiles. We tested this method by using a propane torch to heat fragments of obsidian and tektite glass and by using a muffle furnace (Thermolyne type F1500) to heat fragments of the same materials to 1100 "C for 5 min. Both methods confirmed that obsidian with as little as 0.07 wt?! H,O foams when heated. On the other hand, the tektite glass melted without vesiculation when heated with a propane torch and neither melted nor vesiculated when heated to 1100 "C in the furnace. We used the above methods to test tektite fragments andor microtektites from all three microtektite layers and from the upper Eocene clinopyroxene- bearing spherule layer.

We determined the content of volatile compounds in tektite glass from three marine sites in the North American strewn field using high-temperature mass-spectrometric analysis (see Byers et al., 1983, for details of the method). In this study, samples were degassed by heating them in an effusion cell at a rate of -5 "Ch in to between 1150 and 1250 "C in a vac- uum of lo-* Pa. The released volatiles were detected and their intensities

measured with a quadrupole mass spectrometer. We have previously measured the H,O content of a Georgia tektite, a bediasite, and a moldavite using this method and found the H,O content of all of these specimens to be <0.01 wt?! (unpublished data). Unfortunately, we did not have enough material from the Australasian or Ivory Coast microtektite layers to determine their volatile content using this method.

Glass spherules from the upper Eocene clinopyroxene-bearing spherule layer were studied using a petrographic microscope and a scanning electron microscope (Cambridge S90B). Their compositions were determined using electron microprobe analysis as described above.

DATA Percent of Fragments

The percent of fragments in the Australasian microtektite layer ranges from 0 to 41% (Table 1). In general, the percent of fragments increases with increasing concentration (number/cm*). The percent of fragments also increases with decreasing distance from the putative source crater (Fig. 2). The location of the Australasian source crater

FIG. 1, Tektite-, microtektite-, and spherule-bearing sites used in the study and proposed source craters or source areas. The Chesapeake Bay structure (Poag el al., 1994), Bosumtwi Crater, and 12"N latitude, 106"E longitude (Glass and Piuuto, 1994) were used as the source craterhea for the North American, Ivory Coast, and Australasian strewn fields, respectively. Crater locations indicated by a circle with an x in it. North American, Ivory Coast, and Australasian core sites are indicated by solid squares, triangles, and circles, respectively. Tektite locations on land are indicated schematically by XS.

336 Glass et al.

microtektites at these sites are highly corroded and fragile. In addition, all the fragments are smaller than the largest microtektites, which indicates that they probably are not fragments of tektites or large microtektites that broke but instead represent fragments of highly corroded, average- sized microtektites that fragmented due to solution and/or laboratory processing of the sediments. Thus, the apparent increase in percentage of fragments closer to the proposed source crater (Bosumtwi crater in Ghana) (Table 1) may not indicate a trend in fragmentation due to increasing thermal stress. With the exception of ODP Site 6638 and core V19-297, the percent of fragments at a given distance from the source crater in the Ivory Coast strewn field is lower than the percentage of fragments at the same distance from the proposed source crater in both the Austra- lasian and North American strewn fields (Table l , Fig. 2). This may be due to the smaller size of the Ivory Coast event, which would result in less shock heating of the atmosphere and faster radiative cooling of the microtektites at a given distance from the source crater.

0 Australasian

o Ivory Coast North b e r i c a n

X Barbados

20 -

0 1000 2000 3000 4ooo 5000 6ooo 7000 8000

Distance from Source Crater (km) FIG. 2. Percent tektite and microtektite fragments vs. distance from proposed source re- gions or craters

is not known; however, most authors agree it is somewhere in Indo- china (e.g., Stauffer, 1978; Ford, 1988; Schnetzler, 1992; Glass and Pizzuto, 1994). We used 12"N latitude and 106"E longitude as the approximate location of the Source crater (see Glass and Pizmto, 1994), but any location in Indochina would give similar results.

The percent of fragments in the North American microtektite layer varies between 17% and 52% by number (Table 1). Assuming that the Chesapeake Bay structure is the source of the North American tektites and microtektites (Poag et al., 1994; Koeberl et al., 1996), it appears that the percent of fragments in the North American microtektite layer also increases with decreasing distance from this Source crater (Table 1; Fig. 2). At DSDP Site 612, there is an -8 cm thick layer of impact ejecta that includes abundant tektite fragments. Whole microtektites are rare; most of the glass occurs as fragments. Nu- merous fragments are several millimeters in size and, thus, are probably fragments of tektites rather than of microtektites. The glass fragments at this site exhibit very strong strain birefringence. The sites in the Gulf of Mexico and Caribbean Sea have a much lower percentage of fragments. However, the percent of fragments at Barbados appears to CorelSite Analyses Range Average Source be too high to fit this model. We note that Barbados is the Australasian only site where the layer is now found above sea level. In ODP 758B addition, the microtektite-bearing sediment from this site ODP 758B 15 99.93 - 101.51 100.44 f 0.43 Smithsonian is semi-indurated and was difficult to disaggregate. We ODP 7 5 8 ~ 1 0 98.87 - 99.38 99.18 f 0.30 this study suggest that tectonism that deformed and uplifted the ODP 769A 19 98.99 - 100.80 99.68 f 0.55 Smithsonian sediments to form Barbados Island, the increased solution RC14-24 4 98.94 - 99.32 99.16 2 0.16 this study

98.47 - 99.81 99.19 f 0.39 this study above sea level (see Glass, 1986), and the harsh treatment MSN-48G V29-39 10 98.86 - 99.85 99.34 f 0.32 this study necessary during processing all resulted in the higher than Rc9-143

expected fragment abundance at this site.

ican strewn fields where the percent of fragments exceeds DSDP 612 97.68 - 99.70 98.60 f 0.89 this study 30%, the maximum size of the fragments is greater than ODP 904A 20 93.14 - 99.34 96.58 f 2.33 this study the maximum size of the splash-form microtektites (Table Barbados 2 94.0 - 97.3 95.65 Ngo er al. (1985) 1). This indicates that at these sites, the larger splash forms Barbados 10 94.16 - 100.10 97.33 f 2.23 this study have been broken by some process (presumably thermal ~vorycoast stress). K9-56 7 99.45 - 100.05 99.70 f 0.24 this study

99.41 - 99.74 99.46 f 0.40 this study The percent of fragments in the Ivory Coast microtektite v27-239 99.42 - 99.78 99.63 & 0.16 this study layer varies from zero to 48% (Table 1). The percent frag- v19-297

lnents in core v19-297 and ODP Site 663B may not indi- *More recently J. Smit (pers. comm., 1996) reanalyzed these specimens using more cate the original percent at the time of deposition. The appropriate glass standards and found that the totals averaged 100.01 f 0.28 wt??.

Hydration Some earlier electron microprobe analyses of North American

microtektites and tektite fragments totaled <98 wt% (Glass et al., 1973; Ngo et al., 1985), but no mention was made of the possibility of hydration. However, Thein (1987) obtained oxide totals ranging from 95.54 to 97.99 wt% for tektite fragments from DSDP Site 612 (Table 2) on the continental slope off New Jersey and suggested that the low oxide sums might be due to hydration. We also obtained low oxide sums for tektite fragments and microtektites from this site. In addition, we obtained low totals for tektite glass from Ocean Drill- ing Project (ODP) Site 904A, close to Site 612, and from Barbados (Table 2).

We found that all the tested tektite fragments from Barbados, Site 612 and Site 904, vesiculated to a froth when heated in a propane

TABLE 2. Major oxide totals for microtektites and tektite fragments from the Australasian, North American, and Ivory Coast Strewn fields.

Number of Major Oxide Totals (wt%)

95.81 f 0.59* Smit ef al. (1991) 94,33 - 96.48

19 98.53 - 101.57 100.15 f 0.71 Smithsonian

96.81 f 0.9 Thein (1987) At those sites in both the Australasian and North Amer- :z! t:erica; 95,54 - 97,99

6 15


torch flame or in an oven at I100 "C. Glass fragments and large microtektites from the North American microtektite layer in the Caribbean Sea (Core RC9-58 and DSDP Site 149) and the Gulf of Mexico (DSDP Site 94) also vesiculated when heated.

The high volatile content of tektite fragments from Barbados and DSDP Site 612 and microtektites from RC9-58 were confirmed by high-temperature mass-spectrometric analysis (Table 3). Of particular importance for this study are the highly elevated H2O abundances relative to those typically found for tektites (<0.02 wt%). This is especially evident for the Barbados tektite fragments and the RC9-58 microtektites (3.821 and 1.216 wt%, respectively). The H20-release behavior, as observed from mass pyrograms from all three samples, resembles that commonly found for abyssal submarine volcanic glasses that have undergone alteration. The onset temperature for H20-release is low (3 15-445 "C), and the release temperature is far too broad (extending up to as high as 850 "C) for what is expected for H2O trapped solely on quenching from a melt. The H20- release envelope also shows many intensity "spikes" (also seen as intensity spikes on an oscilloscope caused by abrupt increases in ion intensity) indicating release from the bursting of vesicles. (The H20-release envelope is the area under the curve on an intensity vs. temperature plot between the onset temperature for volatile release and the temperature when the intensity for that particular volatile drops back down to background levels.) These data, plus the high CI abundances (-0.05 wt%), give strong support that these glasses hydrated in the marine environment. The C 0 2 and hydrocarbon (principally CH4, C2H6, C3Hg) abundances are highly variable among the three samples studied. The origins of these volatiles are uncertain but most appear to be indigenous (i .e. , from the surround- ing pore water). For example, the release envelope for the hydrocarbons observed in the Barbados sample mimics (although at much lower intensity levels) that displayed for H20, thereby indicating a common source. However, small contributions to these C-containing species from a surficial contaminant source cannot be ruled out.

TABLE 3. Content of volatiles in Site 612 and Barbados tektite fragments and RC9-58 microtektites.

Temperature range of release

Sample (weight in mg) Species ("C) WtYO

DSDP 612 H2O 445-682 0.076 (27.236) co2 470-700 0.068

CI 900-1250 0.047 F - n.d.

hydrocarbons - - n.d. Total 0.191

Barbados (37.626)

350-850 3.821 - n.d.

CI 625-890 0.038 F 440-890 0.047

Total 5.026

HZO co2

hydrocarbons 300-880 - 1.120

RC9-58 (1 0.840)

HZO 3 15-830 1.216 co2 450-930 0.244 CI 960-1250 0.052 F - n.d.

hydrocarbons - - n.d. Total 1.512

Analytical uncertainties: H20, C02 f 100 ppm; CI, F ? 50 ppm; hydrocarbons ? 200 ppm. n.d. = not detected.

We do not know if the elevated H 2 0 content in the North Amer- ican tektite merits and microtektites is concentrated in a hydration rind, is uniformly distributed through the glass, or is something in between; however, we have not seen any evidence for a hydrated outer layer in petrographic microscope or SEh4EDS studies.

The fusion experiments and low oxide totals for the Site 612 glasses indicate that most of them have high volatile content. On the other hand, the one mass spectrometric analysis (Table 3) indicates only modest volatile enrichment. In addition, Beran and Koe- berl (1996) analyzed two Site 612 glass fragments by Fourier-trans- form infrared spectrometry (FTIR) and found normal tektite values for H20 content (0.009 and 0.021, wt%) and low C02 content (<0.01 wt%). The relatively low volatile content of one tektite fragment (determined by mass spectrometry) and normal H2O content for two other tektite fragments (determined by FTIR) from Site 612 are difficult to explain, since the oxide totals and fusion experiments all indicate that most of the glasses from this site are volatile rich.

The Australasian microtektites appear to have undergone little, if any, hydration. Smit et al. (1991) found low major oxide contents for six Australasian microtektites from ODP Site 758B in the northeastern part of the Indian Ocean and concluded that "...some volatile constituent (water?) is probably present in the microtektites ....I' However, J. Smit (pers. comm., 1996) stated that he has reanalyzed the six Australasian microtektites from ODP Site 758B using more appropriate glass standards and found that the totals average 100.01 f 0.28 wt%. This is in agreement with unpublished microprobe analyses (made at the Smithsonian Institution) of microtektites from sites ODP 758B and ODP 769A (the Sulu Sea), as well as core RC9-143 (taken south of Australia); all have totals of 100 k 1.5 wt%, which suggests little or no hydration (Table 2). We also obtained totals close to 100 wt% for tektite glasses from Site 758B (Table 2), although two samples were slightly <99 wt%.

We heated glass fragments and splash-form microtektites from several Australasian sites close to the source area (ODP Sites 758B, 768B, and 769A) and four microtektites from a more distal site (RC8-53) (Table I , Fig. I). Sixteen glass fragments from the three sites close to the proposed source area were melted using a propane torch. The largest fragment (-700 p m long) vesiculated, and several up to -500 p m developed a few small vesicles. However, about half of them showed no signs of vesiculation. We also heated seven fragments and nine splash forms in a furnace at 1 100 "C. Again, the largest fragment (Site 768B; -550 pm) showed some bubbling, but the remainder showed no evidence of vesiculation. None of the four microtektites from the more distal site (RC8-53) showed any evidence of vesiculation. The small amount of vesiculation suggests that some of the fragments of tektite glass from the Australasian strewn field may have elevated H 2 0 contents, but the oxide totals indicate that the H 2 0 contents are probably less than a few tenths of a percent. Unfortunately, not enough glass from the Australasian microtektite layer was available for determination of the volatile content using high-temperature mass-spectrometric analysis.

Some earlier electron microprobe analyses of Ivory Coast microtektites indicated major oxide totals significantly <I00 wt% (as low as 95.5 wt%; Glass, 1969; Glass and Zwart, 1979). However, we do not put much faith in these older analyses, and electron microprobe analyses of Ivory Coast microtektites performed during the present study totalled close to 100 wt% (Table 2). Several glass fragments from K9-56 were melted using a propane torch and one fragment and four microtektites were heated in a furnace at I100 "C; none showed any evidence of vesiculation. Although the oxide totals and fusion experiments do not indicate elevated volatile contents, we

338 Glass et al.

cannot rule out the possibility that a few tenths of a percent of H2O are present.

In summary, our studies indicate that most tektite fragments and/or microtektites from all the North American sites contain elevated H20 contents (up to 3.8 wt%), while glasses from the Austral- asian and Ivory Coast microtektite layers show little evidence of elevated H20 contents. Palagonitization of Upper Eocene Glass Spherules

The upper Eocene clinopyroxene-bearing spherule layer at DSDP Site 216 in the Indian Ocean (Table 1, Fig. 1) contains glass fragments (up to 0.5 mm in size) and splash-form microtektites, in addition to the more abundant clinopyroxene-bearing spherules (Glass et al., 1985). The silica-rich glass fragments vesiculate strongly when heated and apparently contain elevated volatile contents.

A small proportion (<5%) of spherules recovered from the upper Eocene clinopyroxene-bearing layer at Site 216 in the Indian Ocean (Keller et al., 1987; Glass et al., 1985) have a blue opalescent appearance; these spherules are generally <200 p m in size. X-ray patterns obtained during this study indicate that no crystalline phase is present. When heated, the outer layer of these spherules shrinks and cracks but does not vesiculate. Several of these spherules were mounted in epoxy and ground and polished to expose an interior

surface. Petrographic and scanning electron microscope studies showed that these spherules have an outer rim (between 5 and 50 p m thick) with a low refractive index that encloses a glass inner core with a scalloped surface (Fig. 3). Polishing showed that the outer rim is softer than the core. Electron microprobe analyses indicate that the outer rims have lower major oxide totals than the glass cores (Table 4); this suggests that the outer rims are hydrated. However, the dif- ference in composition between the outer rims and the cores indicates that the compositions of the rims are not simply a result of hydration. Most of the major oxides have a lower abundance in the rims compared with the cores, but the percent change in oxide content is variable. The most depleted oxide in the rims is always CaO. In contrast with the other oxides, K20 usually is higher in the rims compared with the cores (Table 4). The above data suggest that the rims of these spherules were palagonitized, like the WT boundary spherules (see Bohor and Glass, 1995). The blue opalescent appearance that distinguishes these spherules from others in the layer is apparently due to the palagonite rims.

DISCUSSION The data presented in this paper indicate that the percent of

glass fragments in a microtektite strewn field decreases with increasing distance from the source crater. In other words, the percent

FIG. 3. Scanning electron microscope photomicrograph of glass spherules with palagonitized rims from the clinopyroxene-bearing spherule layer at DSDP Site 216 in the Indian Ocean (see Fig. 1 for location). See Table 4 for compositions (A is 621-13, B is 631-15, and C is 631-1 1).


TABLE 4. Major oxide compositions (wt%) of glass spherules with hydrated rims.*

Sample

63 1-1 1 -core -rim

percent change

62 1-1 3-core -rim

percent change

631-IS-core

percent change -rim

SiO, AI,O, FeOt

61.66 6.23 48.22 5.18 -21.80 -16.85

60.72 7.60 45.66 5.99 -24.80 -21.18

62.89 9.23 57.22 8.19 -9.02 -11.27

9.16 2.26

-75.33

9.42 2.63

-72.09

8.16 2.66

-67.40

8.39 0.64

-92.37

7.84 0.68

-91.33

7.39 0.68

-90.80

CaO N%O

6.95 1.27 0.96 1.03

-86.19 -18.90

6.25 1.28 1.17 0.80

-81.28 -37.50

3.84 1.40 1.74 1.28

-54.69 -8.57

TiO, MnO Cr,O, Total

1.81 2.14

+18.23

2.15 2.21 +5.58

2.26 2.65

+17.26

0.26 0.11 0.07 0.00

-73.08 -100.00

0.35 0.10 0.12 0.04

-65.71 -60.00

0.43 0.09 0.10 0.00

-76.74 -100.00

0.12 95.96 0.00 60.50

-100.00

0.09 95.79 0.00 59.52

-100.00

0.14 95.87 0.00 74.56

-100.00

*The glass spherules are from the clinopyroxene-bearing spherule layer from DSDP Site 216 in the eastern Indian Ocean (see Table 1). tAll Fe reported as FeO.

of tektite fragments increases towards the source crater. Sites with >20% fragments are also the sites with the highest concentrations of tektite glass (tektite fragments plus microtektites). These are also the only sites where unmelted impact ejecta have been found (Glass and Wu, 1993). We hypothesize that the fragmentation occurred as tektites and large microtektites fell into water while still hot. Frag- mentation results from thermal stress; the amount of fragmentation depends on the size, composition, and temperature of the glass when it lands in the water. The temperature of the glass when it lands will depend on the distance from the source crater and size of the glass objects (the larger objects would tend to stay hot longer during transport). Thus, for a given strewn field, there should be a distance from the source crater beyond which the tektites or microtektites will tend not to fragment. Inside that distance, the percent of fragments and the concentration of tektites and microtektites will increase (and the maximum size of unfragmented microtektites will decrease) with decreasing distance from the source crater.

The size of the tektite-producing impact event may also be im- portant. Tektites produced in a larger impact event should be hotter when they land at a given distance from the source crater than would the same size tektites produced in a smaller event. This is because at a given distance from the crater, the atmosphere will be shock-heated to a higher temperature during the formation of a large crater than it will be during the formation of a small crater. The higher temperature of the ambient atmosphere will slow the rate of cooling of materials within it and, as a result, a given size of tektite or microtektite will be hotter. This may explain why the Ivory Coast microtektites generally show less fragmentation, for a given distance from the source crater, than the Australasian and North American tektites.

We carried out some simple laboratory experiments and found that tektites roughly a centimeter in size will fragment when heated to 1150 "C or higher and then dropped into water at room temperature. Smaller tektites will, of course, require higher temperatures before they will fragment under the same conditions. This suggests that the tektites and larger microtektites that underwent fragmentation were hotter than 1 150 OC when they landed in the ocean.

Bums (1990) was unsuccessful in his attempt to use the geographic variation in size of microtektites in the Australasian strewn field to predict the location of the source crater. Between -7500 and 3500 km from the proposed source area in Indochina, the average size of the microtektites increases as expected; but at sites closer than 3500 km from Indochina, the average size decreases. We believe that the drop in average size at sites closer than 3500 km from Indochina is due to fragmentation of the larger microtektites (and

tektites?), which must have still been hot when they landed in the ocean at these more proximal distances. It is obvious that it was the larger glass bodies that broke because the largest fragments in these sites are generally of greater size than the largest whole microtektites.

The high H 2 0 content in some of the North American tektite fragments and microtektites is surprising, since previous authors had concluded that tektites don't hydrate. Mazer et al. (1992) did propose that tektites can hydrate under restricted H20 contact, but even they concluded that in a wet environment, solution would occur at a faster rate than hydration. This would suggest that tektites in the marine environment should not show evidence of hydration. Our data suggest that this is not the case. Our present data indicate that the degree of hydration of tektite glass may increase with length of residence in a marine environment.

The North American tektite fragments and microtektites generally have >65 wt% Si02 (e.g., Glass et al., 1985; DHondt et al., 1987; Koeberl and Glass, 1988; Glass, 1989). Many of these glasses show evidence of simple hydration; none show evidence of palagonitization. Likewise, the silica-rich glasses from the clinopyroxene- bearing spherule layer (Site 216), which are only slightly older than the North American tektites, also only show evidence of simple hydration. In contrast, the glass spherules from the clinopyroxene- bearing layer (Site 216), which have glass cores with <65 wt% silica, have palagonitized rims. This suggests that low silica glasses (with <65 wt% Si02) can undergo palagonitization, while silica-rich glass (with >65 wt?40 Si02) will only undergo simple hydration under the appropriate conditions.

Most of the spherules found at the K/T boundary in the U. S. Western Interior are either hollow or filled with secondary minerals. This and other characteristics led Izett ( I 990) to suggest that they had a diagenetic origin unrelated to impact. On the other hand, Bohor and colleagues argued that they were diagenetically altered microtektites (e.g., Bohor et al., 1987; Bohor and Betterton, 1990). This latter conclusion was supported by the discovery of similar spherules at the WT boundary in Beloc, Haiti, some of which still had relict glass cores composed of impact or tektite glass (e.g., Sigurdsson et al., 1991; Izett, 1991). The outer shells of the Haiti WT spherules have been described as being composed of smectite, but we believe they are composed of palagonite, which has partly altered to smectite (Lyons and Officer, 1992; Bohor and Glass, 1995). After study- ing the Haitian spherules, Bohor and Glass (1 995) proposed that the hollow K/T boundary spherules in the U. S. Western Interior also formed by palagonitization of the rims of original glass bodies, fol- lowed by solution of the glass cores. According to this model, the Western Interior glass spherules fell into shallow water while still

340 Glass et al.

hot, and the outer rims became rapidly hydrated and then palagonitized. AAer burial, the palagonite rims were diagenetically altered to kaolinite, goyazite, or other secondary phases. Later, during ex- posure to ground water, the glass cores were dissolved. In some cases, the hollow interiors were later filled with kaolinite, gypsum, or cal- cite (Bohor et al., 1987; Izett, 1990). The discovery of relict glass cores in spherules from the upper Eocene clinopyroxene-bearing spherule layer supports the above model.

Low H20 content (0.02 wt%) has long been used as a criterion for distinguishing tektites from other naturally occurring terrestrial glasses (King and Arndt, 1977; Koeberl and Beran, 1988). We have shown, however, that tektites also can have high HzO contents under certain conditions. Thus, it can no longer be considered valid to conclude that a given glass object is not a tektite merely because it has a H20 content >0.02 wt%.

CONCLUSIONS

The percent of fragments in a microtektite strewn field increases towards the proposed source crater (or source area). We propose that the fragmentation is caused by thermal stress produced when the tektites and large microtektites land in the water while still hot (>1150 "C). We find, contrary to previous opinions, that tektite glass in the marine environment can hydrate and the amount of hydration appears to increase with increasing age. We also find that under some conditions, tektite or impact glass with low silica content (<65 wtY0 Si02) can undergo palagonitization rather than simple hydration. These findings support the palagonitization model of Bohor and Glass (1995) that explains the hollow K/T boundary spherules as altered microtektites.

AcknowledgementsThe data used to determine the percent of fragments at each site were collected over the last 30 years by the senior author (B. P. G.) and numerous students, working under his supervision. These include: R. N. Baker, J. Barone, N. Barratt, L. Boehm, D. Bullis, P. F. Buis, T. Burgess, C. A. Bums, J. R. Crosbie, J. D. Cressman, D. L. DuBois, C. A. Fernandez, J. L. Gregory, K. M. Keoughan, M. Kranz, D. L. Lerner, C. OShoughnessy, E. Perrone, J. Prosser, J. Sullivan, C. Swarthouth, M. B. Swincki, Jiquan Wu, M. J. Zwart, and P. A. Zwart. We thank Jan Smit and Christian Koeberl for critical reviews that have greatly improved the paper. The research was partly supported by several NSF grants over the last 25 years to B. P. G and by NASA grants to B. F. B. Core samples were supplied by Lamont- Doherty Earth Observatory core lab, Antarctic Core Facility of Florida State University, Deep-sea Drilling Project, Ocean Drilling Project, Scripps Institution of Oceanography, and the Global Ocean Floor Analysis and Research Group of the U.S. Naval Oceanographic Office.

Editorial handling: D. E. Brownlee

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Fragmentation and hydration of tektites and microtektites