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Solution of naturally-occurring glasses in the geological environment
Journal of Non-Crystalhne Sohds 67 (1984) 265-286 North-Holland, Amsterdam
SOLUTION OF NATURALLY-OCCURRING GEOLOGICAL ENVIRONMENT
265
GLASSES
IN THE
B i l l y P. G L A S S
Department of Geology, University of Delaware, Newark, DE 19711, USA
As part of a study to investigate the feaslbtllty of encapsulating nuclear wastes in glass ingots and burying them on land or dumping them in the ocean, we have made a study of the amount of solution experienced by naturally-occurring glasses from two land sites and tl'urty-four deep-sea sites The glasses used in this study are mlcrotektltes from three strewn fields (Australasian, Ivory Coast, and North Amencan) and from the Zhamansban Impact structure in southern Siberia The mlcrotektltes range in age from 0 7 to 35 m.y and they have a wide range m composmon The weight per cent SIO2, for example, ranges from 44 8 to 81 7. Although several criteria for determining the amount of solution were considered, most of the conclusions are based on two criteria (1) width of cracks, and (2) elevation of sihca-nch inclusions above the adjacent mlcrotektite surface All the measurements were made on scanning electron rmcroscope photomicrographs of the rmcrotektites The amount of solution was determaned for about 140 microtektites; and measured amounts of solution range from 0 2 to at least 28 pro, but most are less than 5 /am. There appears to be no systematic relationship between age and amount of solution Solution amounts are higher for n~crotektltes in terrestrial settings than for mlcrotektites m deep-sea environments, but m both environments the amount of solution increases w~th decreasing SIO 2 content.
1. I n t r o d u c t i o n
The purpose of the proposed research was to study the amount of solution experienced by naturally-occurring glasses in various geological environments i n a n a t t e m p t t o d e t e r m i n e h o w s o l u t i o n v a r i e s w i t h age, c o m p o s i t i o n a n d environment of deposition. Microtektites were used in this study for several reasons: (1) T h e y h a v e a w i d e r a n g e m a g e s - 1.e., 0.7 t o 35 m.y. (2) T h e y w e r e d e p o s i t e d i n a w i d e r a n g e o f g e o l o g i c a l e n v i r o n m e n t s . M o s t o f the microtektites used in this study came from a wide range of deep-sea environments, but two groups came from land areas. One group of particles came from Zhamanshin crater. These glasses, called microirghizites, have compositions and ages similar to the Australasian microtektites found in deep-sea deposits. The second group consists of North American microtektites deposited in a deep-sea environment, but which n o w o c c u r o n l a n d i n B a r b a d o s ( t a b l e 1). (3) T h e y h a v e a w i d e r a n g e i n c o m p o s i t i o n . F o r e x a m p l e , t h e S i O 2 c o n t e n t o f t h e r m c r o t e k t 2 t e s u s e d i n t h i s s t u d y r a n g e s f r o m 44.8 t o 81.7%. 0022-3093/84/$03 00 © Elsevier Soence Publishers B V. (North-Holland Physics Pubhshmg Division)
266
B P Glass / Solution of naturally-occurrmg glasses
(4) Previous work has indicated several criteria that can be used to estimate solution amounts. These criteria will be discussed later in the paper. (5) The microtektites were already available for the study in a collection at the University of Delaware. The present collection at the University of Delaware contains tens of thousands of microtektites ranging in age from 0.7 to 35 m.y., with a wide range in composition [1-5], and from a wide range of deep-sea environments (e.g., physiographic province, water depth, sediment type). Thus it is possible to correlate the amount of solution with age, chemistry, and environment of deposition. In addition, microtektites found on land with similar ages and compositions to those found in deep-sea deposits offer a chance to compare solution in terrestrial versus deep-sea environments.
2. Samples Microtektites belonging to the Australasian strewn field have been found in deep-sea deposits in the Indian Ocean, Philippine Sea, and western equatorial Pacific Ocean [6]. Ivory Coast microtektites have been found in the eastern equatorial Atlantic Ocean [4], and North American microtektites have been found in the Caribbean Sea, Gulf of Mexico, equatorial Pacific Ocean and eastern Indian Ocean [5,7]. The North American microtektites have been found in one piston core and cores from nine Deep Sea Drilling Project (DSDP) sites. Australasian microtektites have been found in thirty piston cores and one DSDP core. Ivory Coast microtektites have been found in five piston cores. For this study, Australasian microtektites were selected from twenty-one cores, Ivory Coast microtektites from five cores, and North American microtektites from one piston core and cores from six DSDP sites (table 2). The cores were taken in water depths ranging from 1793 to 5323 m. The Australasian microtektites were found in a wide range of sediment types including calcareous and siliceous oozes, calcareous and siliceous lutites, red clay and hemipelagic sediments (table 2). The Ivory Coast microtektites were found mostly in calcareous oozes or calcareous lutites. The North American microtektites were
Table 1
Samples used m tlus study Samples
Age (m.y.)
Environment
No of measurements
Australasian nucrotektites Mlcro-irghizites Ivory Coast nucrotektltes North American nucrotektttes North American nucrotektltes (from Barbados)
07 - 1 - 1 35 35
deep-sea continental deep-sea deep-sea deep-sea orlgonally (presently continental)
55 14 25 62 14
B P Glass /Solutton of naturally-occurring glasses
267
found in siliceous or calcareous oozes. Calculated sediment accumulation rates range from 0.12 to 1.68 c m / 1 0 0 0 y. The Australasian, Ivory Coast, and North American microtektites were formed and fell on the ocean floor 0.7, 1, and 35 m.y. ago, respectively (table 1). The micro-irghizites were obtained from P.V. Florensky, at the Academy of Sciences, USSR, via T. Cobleigh, Johnson Space Center, and K. Fredriksson, at the US National Museum, Washington, D.C They apparently were recovered from a stream deposit in or near the Zhamanshln structure [8,9]. T. Cobleigh recovered the micro-irghizltes by pouring small amounts of sample at one corner of a large sloping board with a glass surface and tapping The spherical micro-irghizites rolled down the board and were collected at the bottom. The micro-irghizltes have compositions and ages similar to the Australasian nucrotektites [9]. North American rmcrotektites from Barbados were obtained from W.R. Raedel and A. Sanfihppo, Scripps Institution of Oceanography. These microtektites were originally deposited in deep-sea sediments which were subsequently tectonlcally uplifted above sea level.
3. Methods 3.1. Procedures
The entire collection of tens of thousands of microtektites was searched using a binocular microscope with up to 50X magnification for microtektites that showed evidence of solution. A total of 307 microtektites were selected for study. In addition, 16 North American microtektites from Barbados and 28 micro-irghlzites were selected. The samples were mounted and coated for scanning electron microscope (SEM) study. Photomicrographs were taken of 53 Australasian microtektites, 38 Ivory Coast microtektites, 80 North American microtektites, 21 micro-irghizites, and 16 Barbados microtektites. Measurements of the amount of solution were made on the photomicrographs using the criteria discussed below. After SEM studies, the microtektxtes were removed from the SEM stubs and remounted in epoxy on 2.5 cm diameter glass discs, ground down to expose a flat interior surface, polished, and then coated with carbon. The major element compositions were determined using an energy dispersive X-ray analyzer (EDAX) in combination with the scanning electron microscope. The analyses were made at 20 KV, 45 o tilt angle, 30.2 ° take-off angle, and 2000 counts per second for 200 s. Glasses, with tektite compositions, made by Corning Glass Company and analyzed by the United States Geological Survey were used as standards and mounted on each disc with the microtektltes. The data were corrected for background, absorption, atomic number effect and fluorescence using a computer program ( E D I T / 7 . E P ) by EDAX International, Inc. A
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n o n s t a n d a r d c o m p u t e r p r o g r a m was used a n d the s t a n d a r d s were analyzed as u n k n o w n s . All the analyses were c o n v e r t e d to oxide percents, n o r m a l i z e d using the s a m e s t a n d a r d , and recalculated to total 99.6%. 3.2. Crtterta u s e d to e s t t m a t e the a m o u n t o f s o l u t t o n
Several criteria have been used to estimate the a m o u n t of solution that the m i c r o t e k t l t e s have undergone. (1) Some microtektites have cracks that have been w i d e n e d by solution (fig. 1). If it is a s s u m e d that the cracks f o r m e d at the time of fall, or shortly thereafter, then the width of a crack is twice the a m o u n t of surface solution. O n c e the m i c r o t e k t i t e is buried in the sediment, there a p p e a r s to be no reason for it to break, except p e r h a p s due to stresses g e n e r a t e d b y h y d r a t i o n . A t a n y rate, the cracks can be used to d e t e r m i n e the m i n i m u m a m o u n t o f solution. (2) Microtektites, like tektites, c o n t a i n silica-rich inclusions (including
Ftg. 1 Scanning electron microscope (SEM) photomacrographs of microtektites with cracks. A, Micro-lrgluzite (126-5) with cracks ranging from 28 to 52 pm in width. B, Ivory Coast mlcroteklte (K9-56-26) with a-- 31 pm wide crack. A sdlca-rich protrusion (not visible) is = 16 pm high. C, North American nucrotekUte from Barbados (179-16-11) with a 15 pm wide crack. D, North Amerman microtektlte from Barbados (180-16-16) with a 27 pm wide crack. Scale bars for A-C equal 100/~m, for D equals 40/~m.
B P Glass /Solutton of naturally- occurring glasses
271
lechatellerite). In some cases, the inclusions formed at the surface of the microtekt~te with their outer surface flush w~th the surface of the microtektite. Differential solution has resulted in the mclusion standing above the surrounding surface, producing a protrusion (fig. 2). We believe that the top of these protrusions represents the original outer surface (or close to it) of the microtektite and that the relief between the top of the protrusion and the adjacent microtektite surface indicates the amount of solution that the microtektite glass has experienced. Two observations support tlus conclusion. First, where a microtekUte has two or more slhca-rich protrusions, their outer surfaces are often concentric with the present microtektite surface and are generally elevated
Fig. 2. SEM photormcrographs of rmcrotektltes with sdlca-rtch protrusions A, Australasian nucrotektlte (V20-138-6) with sihca-rich protrusions barely exposed. The slhca-nch protrusions are elevated only 1.3 to 1.8 p m above the surrounding surface. B, Australasian nucrotektlte (E49-4-1) w~th two well exposed s~hca-nch protrusions elevated 9 1 # m above the adjacent nucrotektlte surface. C, Australasmn nucrotekt~te (V16-70-1) w~th sd~ca-nch protrustons elevated 11 # m above the adjacent surface The smaller s~hca-nch particle ~s almost fully exposed D. Ivory Coast rmcrotektite (K9-56) voth slhca-nch protrusions elevated 20 # m above the adjacent surface Note the pedestal of tektite glass under the sdlca-nch particle. The henusphencal shape of the sthca-rlch particle is seen m transnutted hght wRh an opUcal rmcroscope (see insert). The small comcal protrusion at the edge near the top of the rmcrotektlte may m&cate where a smaller sd~ca-nch particle was completely removed by solutton Scale bar for A equals 20 #m, for B equals 50, for C equals 100, and for D and insert m D equals 25 ~,m.
272
B P Glass /Solutton of naturally-occurring glasses
approximately the same distance above the adjacent microtektite surface. Secondly, when a microtektite with silica-rich protrusions is broken and cracked, the cracks are twice as wide as the protrusions are high (fig. 3). The SiO2-rich protrusions have a variety of shapes (fig. 2), but all have a circular outer surface that is concentric with the surface of the microtektite. With only a minor amount of solution, the SiO2-rlch area is slightly elevated above the microtektite surface (fig. 4). With greater solution, more of the protrusion is exposed and the general shape of the protrusion becomes more evident (fig. 4). With still further solution, the S102-rich particle rests on top of a pedestal of microtektite glass which was protected from solution by the overlying S102-rich particle (fig. 4). Further solution may completely undermine the SiO2-rich particle and free it from the imcrotektlte, leaving a comcal-shape protrusion of microtektite glass (fig. 4). Measurements of such features would only indicate
Fig 3. SEM photormcrographs of nucrotektltes with both cracks and slllca-nch protrusions A, Ivory Coast rmcrotektlte (K9-56-13) The slhca-nch protrusions are elevated about 16-17 /Lm above the surrounding surface and the cracks are about 2 8 / t m wtde. Thus the width of the cracks are a httle less than twice the height of the sihca-nch protrusions. B, North American rmcrotektlte from Barbados (179-16-9). The slhca-rlch protrusions stand 8 9 to 1 0 / t m above the surface and the crack is 17 8 to 18.2 # m wide. C, Mlcro-lrgluzate (128-10). The cracks are about 40 # m wide D, Same mlcro-lrghlzlte as in C. The silica-rich particle at the top ts elevated about 18/~m above the surface Scale bars for A - D equal 100/~m
A
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Fig, 4 Sketch showing progressive solution of rmcrotektlte with slhca-rtch hemisphere at surface A, Ongdnal occurrence of sihca-rich inclusion. B, After enough solution to expose only a small part of the redes of the slhca-rich particle Compare with fig 2A C, Solution has exposed almost the entir.~" sdica-nch particle which now stands on a pedestal of tektite glass that was temporarily protected from solution by the overlying sdma-rich particle. Compare with figs. 2 B - C D, Soluuon has freed the sihca-rich particle A comcal-shaped protrusion of tektite glass marks the former location of the silica-rich particle.
Fig. 5 SEM photonucrograph of Ivory Coast microtektlte (K9-56-11) with U-shaped grooves. The grooves are about 80 p m wide. Scale bar equals 100 tim.
274
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the minimum amount of solution that has occurred. (3) U-shaped grooves (fig. 5) occur on the surface of some microtektltes expecially those from the Ivory Coast strewn field. The U-shaped grooves were apparently produced by solution of thermal cracks on the surface of the microtekUtes [10]. Similar U-shaped grooves, except for their size, are found on some tektites, particularly billitonites [11]. Chapman et al. [12] used the widths of the U-shaped grooves on tektites to determine the amount of solution they have experienced. We attempted to use the U-shaped grooves to estimate the amount of solution experienced by the microtektltes assuming that the amount of solution equals ½ the width of the grooves. (4) Studies of microtektites in agglutinated foraminiferal tests (fig. 6) have also been used to determine the amount of solution. One method is to compare the exposed surfaces with those embedded in the test. The second method is to determine the distance between two closely-spaced microtektites in a test. This method gives the maximum amount of solution for the two microtektites combined. The most useful and reliable criteria appear to be the cracks and silica-rich protrusion. Microtekmes with cracks a n d / o r silica-rich protrusions were found at nearly all the sites, and when a mlcrotektite had both features, they both generally indicated the same amount of solution. The U-shaped grooves, on the other hand, are with few exceptions found only on the Ivory Coast microtektites. Where present, the U-shaped grooves often indicate several times the amount of solution indicated by cracks or silica-rich protrusions. Although U-shaped grooves and cracks can grade into each other, we distinguish U-shaped grooves based on their rounded or concave bottoms and width-todepth ratios greater than one. On the other hand, cracks, as used in this study, are much deeper than they are wide and generally have flat bottoms - see figs. 1 and 5. Because the U-shaped grooves generally indicate more solution than the cracks or silica-rich protrusions, and since the U-shaped grooves are essentially confined to the Ivory Coast microtektites, the U-shaped grooves have not been used in this study to determine the amount of solution that the microtektites have experienced. The criteria using agglutinated foraminiferal tests could only be used in a limited number of cases and does not yield quantitative data. Thus most of our conchislons are based on solution est~mates using silica-rich protrusions and cracks.
4. Results Measured amounts of solution range from 0.2 to 28 btm (table 3). When both cracks and silica-rich protrusions were present, the two features often indicated the same amount of solution. However, they do not always agree. In most cases when the two criteria do not agree, the cracks indicate less (generally about ½) the amount of solution as xndicated by the silica-rich protrusions. Generally when the cracks indicate less solution than the silica-rich
B P Glass /Solutton of naturally- occurring glasses
277
protrusions, the cracks are merely surface cracks. Deep cracks that extend a significant distance ( > 1 the radius) into the microtekttte generally indicate solution amounts in agreement with the silica-rich protrusions. Thus most of the questionable solution amounts given m table 3 are based on surface cracks which probably indicate only minimum amounts of solution. Major element analyses were obtained for 46 of the Australasian microtektites, 34 of the Ivory Coast microtektites, and 74 of the North American microtektltes for which we obtained solution measurements. SiO 2 contents range from 44.8 to 76.2, 62.1 to 70.0, and 59.0 to 81.7 for the Australasian, Ivory Coast and North American microtektites, respectively. Most have compositions similar to previously published analyses for microtektites from these strewn fields, but some have unusual or unique compositions.
Table 3 Average values for solution and S l O 2 c o n t e n t Solution ( # m ) a) Range
Average S102 Mean
Content (wt%) a)
Australasmn nucrotekt~tes rehable data only b) rehable + fmrly reliable all data c)
1.8--16 1 8-17 0 4-26
7 2 + 4 3(17) 7 8 + 4.2(28) 6 1 + 5.5(54)
64 0 + 5.9(14) 63 8 + 6.2(23) 64 2 + 6.1(46)
Ivory coast rmcrotektltes rehable data only b) rehable + fairly rehable all data c)
0 9-18 0 8-25 0 8-25
9 7 + 7 . 3 (9) 8 3 + 7 2(20) 9.8 + 7.3(25)
67 3 + 2 . 0 (9) 66 6 + 2 0(20) 66.5 + 1 9(24)
3 2-- 4 9 0.2-28 0 2-28
4 2 + 0 9 (3) 4 7 + 5 6(39) 4.3 + 6.0(57)
64 9 + 0 9 (3) 69.1 + 6.6(34) 71.2 + 6 8(54)
14 0 - 2 8 6 0-28 2 6-28
20 7 + 7 0 (3) 15 6 + 6 9 (9) 13 7 4- 7 2(12)
63.3 + 1 0 (3) 66.8 + 4 4 (9) 67.1 + 4.2(12)
8 0--24 7 0-24 7 0-24
12 2 4- 6.0 (6) 11 9 4- 4.6(14) 11.9 4- 4.6(14)
77 5 4- 2.3 (6) 78 4 4- 2.7(12) 78 4 4- 2.7(12)
North American microtektltes rehable data only b) rehable + fairly reliable all data c) Mlcro-lrgtuzates rehable data only b) rehable + fairly rehable all data o Barbados nucrotektites rehable data only b) rehable + fairly rehable all data c)
a) + one standard deviation. The number m parentheses indicates the number of measurements. b) Usually indicates soluUon values where two or more measurements agree or where two different c m e n a agree c) Includes some fmrly questmnable measurements.
278
B.P Glass /Solutton of naturally-occurring glasses
5. Discussion
5.1. Limitations A discussion of some of the limitations of this study is m order before proceeding to a discussion of the results. First, it should be noted that although most of the microtektites showed some evidence of corrosion, only a small percentage of the microtektites in the collection had obvious features (e.g., cracks or silica-rich protrusions) that could be used to quantify the amount of solution that they have undergone. Secondly, since we only see the end product and have no idea what the original population was like, we have no way of knowing how many microtektites may have been completely removed by solution. Complete dissolution may be the reason why we do not find any microtektites at some sites where otherwise we would have expected them. Finally, we note that some of the microtektites show no signs of corrosion. In these cases, we do not know if the microtektites have not experienced any solution, or if they have undergone uniform solution and have no features present to indicate how much solution they have experienced. Thus the data presented in this report are limited to a small percentage of unique specimens. Nevertheless, we were able to obtain solution amounts for about 170 specimens having a great range in age and composition, and from a rather wide range in depositional environments (tables 1, 2 and 3).
5.2. Relattonship between composttton and amount of solutton The microtektites from a given site show a general decrease in amount of solution with increasing SiO2 content (fig. 7), and at most of the deep-sea sites microtektites with greater than 70% SiO 2 show less than 5 # m of solution. Furthermore, the silica-rich protrusions, which are often nearly pure $102, have apparently undergone little or no solution. This is supported by the observation that when microtektites have both cracks and silica-rich protrusions, the widths of the cracks are almost never greater than twice the heights of the silica-rich protrusions. This is further support for the inverse relationship between solution and SiO 2 content. In all of the strewn fields, however, there are examples of nucrotektites from a given site with similar SiO 2 contents, but with large differences in amount of solution. In some of these cases, the microtektites have similar major oxide compositions and, therefore, the amount of solution does not appear to be related to differences in major oxide compositions. By comparing the compositions of microtektites from a given site with similar SiO 2 contents but with different amounts of solution, we found some possible relationships between amount of solution and major oxide composition other than SiO2. In general, the amount of solution for Australasian microtektites, from a given site with similar SiO 2 contents, increases with increasing CaO and K 2 0 and possibly with decreasing FeO and A1203 contents. The amount of solution for the Ivory
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Coast microtektites appears to increase with increasing N a 2 0 and K 2 0 contents and with decreasing TiO 2 (and possibly FeO) contents. North A m e n can microtektltes from deep-sea sites show increasing solution with increasing A1203, CaO, and N a 2 0 contents and with decreasing MgO contents. In general, then, the amount of solution appears to increase with decreasing SiO 2 content, and for a given SiO 2 content the amount of solution generally increases with increasing CaO and N a 2 0 and K 2 0 contents and perhaps with decreasing FeO a n d / o r MgO contents. These generahzations are, however, based on only a few examples and there are many exceptions. The microtektites from the two land sites (i.e., the micro-irghizites and the Barbados microtektites) also tend to show increasing amounts of solution with decreasing SiO 2 contents (fig. 8 and 9); but again, there is a great deal of scatter. The micro-lrghizites, like the Australasian microtektites, tend to show increasing amounts of solution with decreasing A1203 contents for a given SiO 2 content. However, in contrast to the Australasian microtektites, the microirghizites show increasing amounts of solution with increasing FeO (and perhaps MgO) and decreasing CaO and N a 2 0 and K 2 0 contents. Likewise, the North American n'ucrotektites found on Barbados show different relationships between amount of solution and major oxide contents for a given SiO 2 content as compared to North American microtektites from deep-sea sites. In contrast to the North American microtektites from deep-sea sites, the North American microtektites from Barbados show increasing solution with increasing MgO (and FeO) and decreasing A1203 (and perhaps CaO, N a 2 0 and K 2O) contents. Thus it appears that differences in composition, other than
30
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B P Glass /Solutton of naturally, occurring glasses
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S i O 2 content, have different effects on amount of solution in terrestrial versus deep-sea environments.
5.3. Relattonshtp between age and amount o f solutton
It appears that the 35 m.y. old North American microtektltes have on the average undergone less solution than either the 0.7 m.y. old Australasian microtektltes or the - 1 m.y. old Ivory Coast microtektites (table 3). This is surprising since the North American microtektites are - 3 5 to - 5 0 times older than the Ivory Coast and Australasian microtektites, respectively. A possible explanation is that solution slows down drastically or ceases shortly after burial [13]. A study of microtektites found in agglutinated foraminlferal tests seems to support this. Microtektites are common components of tests of agglutinated Foraminifera found in sediments containing or adjacent to the North American mlcrotektite layer at several deep-sea sites (RC9-58, DSDP 94, D S D P 149, DSDP 166). Since the agglutinated Foraminifera are bottom dwellers and not burrowers, the microtektites must have been incorporated m their tests before burial. Based on the estimated sediment accumulation rates (table 2), a 500 /~m diameter mlcrotektite would be buried in less than 500 y even at the sites with the lowest sediment accumulation rate. Thus the microtektites must have been incorporated into the foraminiferal test shortly after deposition. In the beginning of the study, we hypothesized that the portion of each microtektite enclosed m the test must have been protected from solution; therefore, by removing the microtektites from the tests, we could determine the amount of solution be determining the difference in relief between the exposed and covered (or protected) surfaces. We were not able to see any differences between the exposed and enclosed surfaces (see, for example, fig. 6). The microtektites, however, have undergone solution etching, and in a few cases, microtektites in foraminiferal tests had silica-rich protrusions that indicated comparable amounts of solution to microtektites not found in foramlniferal tests. The microtektites were still tightly bound to the test so that the observed solution could not have taken place after the microtektites were incorporated into the tests. If solution had taken place uniformly around the entire surface of the microtektates, then they would no longer be tightly bound to the adjacent test material, and if solution had affected only the exposed surface of the microtektite, then there should have been an obvious difference in relief between the exposed and enclosed surfaces - and there is not. Thus the observed solution must have taken place before incorporation into the foraminiferal tests. Furthermore, since the microtektites in the foraminiferal tests show as much solution as microtektltes not in the tests (based on only a few measurements), this implies that all of the microtektites underwent solution immediately upon deposition and that solution essentially ceased on or shortly after burial. This study, then, supports Barnes' [13] suggestion that solution ceases shortly after
B.P Glass / Solution of naturally-occurring glasses
282
burial and it seems to explain why 35 m.y. old microtektites do not show any more solution than 0.7 m.y. old microtektites.
5.4. Relationship between emvironment and amount of solutzon Although there is a great deal of scatter in the data, microtektites from a given deep-sea site often show different amounts of solution when compared with microtektites with similar S i S 2 c o n t e n t s from other deep-sea sites within the same strewn field (fig. 10; table 4). There appears to be no correlation between average amount of solution and sediment type or sedimentation rate (table 4; fig. 11), but there does appear to be a tendency for the amount of solution to increase with decreasing water depth (fig. 12). If most of the corrosion takes place at the sediment/water interface, as previously discussed, then the amount of solution should be related to bottom conditions and length of time that the microtektites are on the bottom prior to burial. Length of exposure on the bottom is determined by sedimentation rate. Thus in areas of rapid sedimentation the microtektites should be buried quickly and thus show low amounts of solution. Conversely, in areas of slow sedimentation the microtektites would be exposed for longer periods of time and should, therefore, show greater amounts of solution. However, there
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B.P. Glass /Solutton of naturally- occurring glasses
283
Table 4 Estimated average amounts of solution, depths, se&ment accumulation rates, and se&ment types for selected cores Ave. Soluuon (/~m) a)
Depth b) (m)
Sed Acc. Rate (cm/1000 y)
Se&ment type c)
(3) (9) (2) (3)
4649 4141 3568 2943
0 22 0 46 0 21(?) 11
foram lutlte luute foram, lutlte nanno, chalk
(7) (3) (2) (4) (7)
4346 4479 4152 4020 4687
1 02 1 59 0.84 0 71 1 68
foram, ooze foram, luUte foram, luUte foram, luUte foram, luUte
North American DSDP 149 - 2 (11) DSDP 216 - 5 (4) RC9-58 3 (23) DSDP 94 13 (6)
4242 2525 3456 2209
0 27 0.25 (1) 1.2
rad. ooze nanno, chalk foram, luute nanno chalk
Australasmn V16-70 V20-138 E49-4 DSDP 292 Ivory Coast V27-239 K9-57 V19-300 V19-297 K9-56
0.5 1.5 4.5 17 1 - 1 1 - 5 16
a) Estimated average amount of solution at - 70% StO2 Number in parentheses is number of measurements b) Water depth plus depth to the mlcrotektlte layer m the se&ment c) foram = foranumferal, rad = ra&olanan, nanno. = nannofossd.
a p p e a r s to b e n o c l e a r r e l a t i o n s h i p b e t w e e n s e d i m e n t a c c u m u l a t i o n r a t e a n d a m o u n t o f s o l u t i o n (fig. 11). T h i s m a y b e d u e to o t h e r f a c t o r s s u c h as v a r i a t i o n s in s o l u b i l i t y o f silica w i t h w a t e r d e p t h . T h e s o l u b i l i t y o f silica in sea w a t e r i n c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e , d e c r e a s i n g p r e s s u r e , a n d d e g r e e o f u n d e r s a t u r a t i o n [14,15]. W a t e r t e m p e r a t u r e is h i g h e s t in t h e s u r f a c e l a y e r a n d d e c r e a s e s w i t h d e p t h , w h e r e a s p r e s s u r e i n c r e a s e s w i t h d e p t h . A l t h o u g h silica is u n d e r s a t u r a t e d e v e r y w h e r e in t h e o c e a n s , it is m o r e u n d e r s a t u r a t e d in s h a l l o w water. T h u s silica s o l u t i o n d e c r e a s e s w i t h d e p t h a n d , for e x a m p l e , m o s t o f the d i s s o l u t i o n o f b i o g e n i c silica takes p l a c e in s h a l l o w w a t e r [15]. T h e r e f o r e , as w o u l d b e e x p e c t e d , t h e microtektites generally show decreasing solution with increasing water depth (fig. 12). T h e t e n d e n c y f o r m i c r o t e k t i t e s o l u t i o n to d e c r e a s e w i t h i n c r e a s i n g w a t e r d e p t h m a y at least p a r t l y e x p l a i n the a p p a r e n t l a c k of c o r r e l a t i o n b e t w e e n s o l u t i o n a n d s e d i m e n t a t i o n rate. W e w o u l d e x p e c t t h a t s o l u t i o n w o u l d i n c r e a s e w i t h d e c r e a s i n g s e d i m e n t a t i o n r a t e ; t h u s the a m o u n t s o f s o l u t i o n m e a s u r e d f o r sites D S D P 292, D S D P 94 a n d K 9 - 5 6 a r e m u c h h i g h e r t h a n w o u l d b e e x p e c t e d (fig. 12). B u t the h i g h a m o u n t o f s o l u t i o n at t w o o f t h e sites ( D S D P 94 a n d 292) m a y b e d u e to s h a l l o w d e p t h (see t a b l e 4). C o r e K 9 - 5 6 was f r o m a n a r e a
284
B P Glass
/ Solutzon o f naturally - occurring glasses
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B P Glass / Solunon of naturally-occurring glasses
285
of henupelagtc sedimentation (mixture of pelagic and continental sediments). The remainder of the sites are in water depths greater than 3.5 km (except for D S D P 216) and show a general decrease in average amount of solution with increasing sedimentation rate as would be expected. Although there is some overlap, the micro-irghizites, which have similar age and compositions to the Australasian microtektites, generally have about two to three t~mes as much solution for a given SiO 2 content as the Australasian microtektltes (fig. 9; table 3). Likewise, the North American microtektites found on Barbados generally show more solution than the North American microtekUtes from deep-sea sites (fig. 10; table 3). Since the micro-irghizites and Barbados microtektites are similar in age and composition to the Australasian and deep-sea North American microtektites, respectively, we assume that the greater amount of solution of the micro-irghizites and Barbados microtektites is due to their occurrence in a terrestrial as opposed to a deep-sea enwronment.
6. Conclusion We have measured solution amounts for about 140 microtektites having a large range in age (0.7 to 35 m.y.), composition (e.g., 44.8 to 81.7% SiO2), and environments. Measured amounts of solution range from 0.2 to at least 28 #m, but most are less than 5 #m. We can make the following tentative generalizations: (1) Most of the observed solution took place before the deep-sea microtektites were buried. Solution appears to essentially stop upon burial. Therefore, there is no systematic relationship between age and amount of solution. (2) Solution amounts are higher for rmcrotektites in terrestrial settings than for microtektites in deep-sea environments. (3) Within the marine environment there appears to be a tendency for the amount of solution to increase with decreasing water depth. (4) Solution varies inversely with SiO 2 content. (5) In the marine environment for a gtven SiO2 content the amount of solution generally increases with increasing CaO, N a 2 0 and K 2 0 contents and perhaps with decreasing FeO a n d / o r MgO contents. In the terrestrial environments, however, the relationship between solution and composition appears to be reversed. For a given SiO2 content, the amount of solution seems to increase with increasing FeO, and perhaps MgO, and with decreasing A1203, and perhaps CaO, Na 20 and K 2O.
References [1] W.A Cassldy, B.P. Glass and B C Heezen, J Geophys Res 74 (1969) 1008. [2] B.P Glass, Earth Planet So Lett. 9 (1970) 240 [3] B P Glass, J Geophys. Res. 77 (1972) 7057
286 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
B P. Glass / Solut:on of naturally- occurr:ng glasses B.P Glass, J Geophys. Res. 77 (1972) 7057. B.P. Glass and M.J Zwart, Geol. Soc. Am. Bull 90 (1979) 595. B.P. Glass and P.A. Zwart, Earth Planet. Sci. Lett. 43 (1979) 336. B.P. Glass, M B Swmckl and P.A. Zwart, Proc 10th Lunar Planet SCL Conf, Geocham. Cosmoclum. Acta, Suppl 11 (1979) 2535 B.P. Glass and J.R. Croshie, Bull. Am. Assoc. Petrol. Geol. 66 (1982) 471. P.V. Florensky and A.I. Dabizha, Zhamanshin Meteorite Crater (Nauka Press, Moscow, 1980) p. 127 B.P. Glass, K. Fredriksson and P.V. Florensky, Proc. 14th Lunar Planet. Sci. Conf., J. Geophys. Res., m press. B.P. Glass, Geol. Soc. Am. Bull. 85 (1974) 1305. G. Baker, Victoria Natl. Mus. Mem. 23 (1959) 1. D.R. Chapman, N.B. Zimmerman, F.J. Centolanza and L C Schelber, Abstr. 30th Ann. Meeting Meteontlcal Soc (NASA, Ames Research Center, Moffett Field, CA, Oct. 25-27, 1967). V.E. Barnes, written communication (1973). H.U Sverdrup, M W. Johnson and R.F. Fleming, The Oceans (Prentice-Hall, Englewood Chffs, NJ, 1942). J.P. Kennett, Manne Geology (Prenuce-Hail, Englewood Chffs, N J, 1982).
SOLUTION OF NATURALLY-OCCURRING GEOLOGICAL ENVIRONMENT
265
GLASSES
IN THE
B i l l y P. G L A S S
Department of Geology, University of Delaware, Newark, DE 19711, USA
As part of a study to investigate the feaslbtllty of encapsulating nuclear wastes in glass ingots and burying them on land or dumping them in the ocean, we have made a study of the amount of solution experienced by naturally-occurring glasses from two land sites and tl'urty-four deep-sea sites The glasses used in this study are mlcrotektltes from three strewn fields (Australasian, Ivory Coast, and North Amencan) and from the Zhamansban Impact structure in southern Siberia The mlcrotektltes range in age from 0 7 to 35 m.y and they have a wide range m composmon The weight per cent SIO2, for example, ranges from 44 8 to 81 7. Although several criteria for determining the amount of solution were considered, most of the conclusions are based on two criteria (1) width of cracks, and (2) elevation of sihca-nch inclusions above the adjacent mlcrotektite surface All the measurements were made on scanning electron rmcroscope photomicrographs of the rmcrotektites The amount of solution was determaned for about 140 microtektites; and measured amounts of solution range from 0 2 to at least 28 pro, but most are less than 5 /am. There appears to be no systematic relationship between age and amount of solution Solution amounts are higher for n~crotektltes in terrestrial settings than for mlcrotektites m deep-sea environments, but m both environments the amount of solution increases w~th decreasing SIO 2 content.
1. I n t r o d u c t i o n
The purpose of the proposed research was to study the amount of solution experienced by naturally-occurring glasses in various geological environments i n a n a t t e m p t t o d e t e r m i n e h o w s o l u t i o n v a r i e s w i t h age, c o m p o s i t i o n a n d environment of deposition. Microtektites were used in this study for several reasons: (1) T h e y h a v e a w i d e r a n g e m a g e s - 1.e., 0.7 t o 35 m.y. (2) T h e y w e r e d e p o s i t e d i n a w i d e r a n g e o f g e o l o g i c a l e n v i r o n m e n t s . M o s t o f the microtektites used in this study came from a wide range of deep-sea environments, but two groups came from land areas. One group of particles came from Zhamanshin crater. These glasses, called microirghizites, have compositions and ages similar to the Australasian microtektites found in deep-sea deposits. The second group consists of North American microtektites deposited in a deep-sea environment, but which n o w o c c u r o n l a n d i n B a r b a d o s ( t a b l e 1). (3) T h e y h a v e a w i d e r a n g e i n c o m p o s i t i o n . F o r e x a m p l e , t h e S i O 2 c o n t e n t o f t h e r m c r o t e k t 2 t e s u s e d i n t h i s s t u d y r a n g e s f r o m 44.8 t o 81.7%. 0022-3093/84/$03 00 © Elsevier Soence Publishers B V. (North-Holland Physics Pubhshmg Division)
266
B P Glass / Solution of naturally-occurrmg glasses
(4) Previous work has indicated several criteria that can be used to estimate solution amounts. These criteria will be discussed later in the paper. (5) The microtektites were already available for the study in a collection at the University of Delaware. The present collection at the University of Delaware contains tens of thousands of microtektites ranging in age from 0.7 to 35 m.y., with a wide range in composition [1-5], and from a wide range of deep-sea environments (e.g., physiographic province, water depth, sediment type). Thus it is possible to correlate the amount of solution with age, chemistry, and environment of deposition. In addition, microtektites found on land with similar ages and compositions to those found in deep-sea deposits offer a chance to compare solution in terrestrial versus deep-sea environments.
2. Samples Microtektites belonging to the Australasian strewn field have been found in deep-sea deposits in the Indian Ocean, Philippine Sea, and western equatorial Pacific Ocean [6]. Ivory Coast microtektites have been found in the eastern equatorial Atlantic Ocean [4], and North American microtektites have been found in the Caribbean Sea, Gulf of Mexico, equatorial Pacific Ocean and eastern Indian Ocean [5,7]. The North American microtektites have been found in one piston core and cores from nine Deep Sea Drilling Project (DSDP) sites. Australasian microtektites have been found in thirty piston cores and one DSDP core. Ivory Coast microtektites have been found in five piston cores. For this study, Australasian microtektites were selected from twenty-one cores, Ivory Coast microtektites from five cores, and North American microtektites from one piston core and cores from six DSDP sites (table 2). The cores were taken in water depths ranging from 1793 to 5323 m. The Australasian microtektites were found in a wide range of sediment types including calcareous and siliceous oozes, calcareous and siliceous lutites, red clay and hemipelagic sediments (table 2). The Ivory Coast microtektites were found mostly in calcareous oozes or calcareous lutites. The North American microtektites were
Table 1
Samples used m tlus study Samples
Age (m.y.)
Environment
No of measurements
Australasian nucrotektites Mlcro-irghizites Ivory Coast nucrotektltes North American nucrotektttes North American nucrotektltes (from Barbados)
07 - 1 - 1 35 35
deep-sea continental deep-sea deep-sea deep-sea orlgonally (presently continental)
55 14 25 62 14
B P Glass /Solutton of naturally-occurring glasses
267
found in siliceous or calcareous oozes. Calculated sediment accumulation rates range from 0.12 to 1.68 c m / 1 0 0 0 y. The Australasian, Ivory Coast, and North American microtektites were formed and fell on the ocean floor 0.7, 1, and 35 m.y. ago, respectively (table 1). The micro-irghizites were obtained from P.V. Florensky, at the Academy of Sciences, USSR, via T. Cobleigh, Johnson Space Center, and K. Fredriksson, at the US National Museum, Washington, D.C They apparently were recovered from a stream deposit in or near the Zhamanshln structure [8,9]. T. Cobleigh recovered the micro-irghizltes by pouring small amounts of sample at one corner of a large sloping board with a glass surface and tapping The spherical micro-irghizites rolled down the board and were collected at the bottom. The micro-irghizltes have compositions and ages similar to the Australasian nucrotektites [9]. North American rmcrotektites from Barbados were obtained from W.R. Raedel and A. Sanfihppo, Scripps Institution of Oceanography. These microtektites were originally deposited in deep-sea sediments which were subsequently tectonlcally uplifted above sea level.
3. Methods 3.1. Procedures
The entire collection of tens of thousands of microtektites was searched using a binocular microscope with up to 50X magnification for microtektites that showed evidence of solution. A total of 307 microtektites were selected for study. In addition, 16 North American microtektites from Barbados and 28 micro-irghlzites were selected. The samples were mounted and coated for scanning electron microscope (SEM) study. Photomicrographs were taken of 53 Australasian microtektites, 38 Ivory Coast microtektites, 80 North American microtektites, 21 micro-irghizites, and 16 Barbados microtektites. Measurements of the amount of solution were made on the photomicrographs using the criteria discussed below. After SEM studies, the microtektxtes were removed from the SEM stubs and remounted in epoxy on 2.5 cm diameter glass discs, ground down to expose a flat interior surface, polished, and then coated with carbon. The major element compositions were determined using an energy dispersive X-ray analyzer (EDAX) in combination with the scanning electron microscope. The analyses were made at 20 KV, 45 o tilt angle, 30.2 ° take-off angle, and 2000 counts per second for 200 s. Glasses, with tektite compositions, made by Corning Glass Company and analyzed by the United States Geological Survey were used as standards and mounted on each disc with the microtektltes. The data were corrected for background, absorption, atomic number effect and fluorescence using a computer program ( E D I T / 7 . E P ) by EDAX International, Inc. A
268
B P Glass /Solutton of naturally- occurring glasses
B P Glass / Solunon of naturally-occurring glasses
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B P Glass / Solutton of naturally - occurring glasses
n o n s t a n d a r d c o m p u t e r p r o g r a m was used a n d the s t a n d a r d s were analyzed as u n k n o w n s . All the analyses were c o n v e r t e d to oxide percents, n o r m a l i z e d using the s a m e s t a n d a r d , and recalculated to total 99.6%. 3.2. Crtterta u s e d to e s t t m a t e the a m o u n t o f s o l u t t o n
Several criteria have been used to estimate the a m o u n t of solution that the m i c r o t e k t l t e s have undergone. (1) Some microtektites have cracks that have been w i d e n e d by solution (fig. 1). If it is a s s u m e d that the cracks f o r m e d at the time of fall, or shortly thereafter, then the width of a crack is twice the a m o u n t of surface solution. O n c e the m i c r o t e k t i t e is buried in the sediment, there a p p e a r s to be no reason for it to break, except p e r h a p s due to stresses g e n e r a t e d b y h y d r a t i o n . A t a n y rate, the cracks can be used to d e t e r m i n e the m i n i m u m a m o u n t o f solution. (2) Microtektites, like tektites, c o n t a i n silica-rich inclusions (including
Ftg. 1 Scanning electron microscope (SEM) photomacrographs of microtektites with cracks. A, Micro-lrgluzite (126-5) with cracks ranging from 28 to 52 pm in width. B, Ivory Coast mlcroteklte (K9-56-26) with a-- 31 pm wide crack. A sdlca-rich protrusion (not visible) is = 16 pm high. C, North American nucrotekUte from Barbados (179-16-11) with a 15 pm wide crack. D, North Amerman microtektlte from Barbados (180-16-16) with a 27 pm wide crack. Scale bars for A-C equal 100/~m, for D equals 40/~m.
B P Glass /Solutton of naturally- occurring glasses
271
lechatellerite). In some cases, the inclusions formed at the surface of the microtekt~te with their outer surface flush w~th the surface of the microtektite. Differential solution has resulted in the mclusion standing above the surrounding surface, producing a protrusion (fig. 2). We believe that the top of these protrusions represents the original outer surface (or close to it) of the microtektite and that the relief between the top of the protrusion and the adjacent microtektite surface indicates the amount of solution that the microtektite glass has experienced. Two observations support tlus conclusion. First, where a microtekUte has two or more slhca-rich protrusions, their outer surfaces are often concentric with the present microtektite surface and are generally elevated
Fig. 2. SEM photormcrographs of rmcrotektltes with sdlca-rtch protrusions A, Australasian nucrotektlte (V20-138-6) with sihca-rich protrusions barely exposed. The slhca-nch protrusions are elevated only 1.3 to 1.8 p m above the surrounding surface. B, Australasian nucrotektlte (E49-4-1) w~th two well exposed s~hca-nch protrusions elevated 9 1 # m above the adjacent nucrotektlte surface. C, Australasmn nucrotekt~te (V16-70-1) w~th sd~ca-nch protrustons elevated 11 # m above the adjacent surface The smaller s~hca-nch particle ~s almost fully exposed D. Ivory Coast rmcrotektite (K9-56) voth slhca-nch protrusions elevated 20 # m above the adjacent surface Note the pedestal of tektite glass under the sdlca-nch particle. The henusphencal shape of the sthca-rlch particle is seen m transnutted hght wRh an opUcal rmcroscope (see insert). The small comcal protrusion at the edge near the top of the rmcrotektlte may m&cate where a smaller sd~ca-nch particle was completely removed by solutton Scale bar for A equals 20 #m, for B equals 50, for C equals 100, and for D and insert m D equals 25 ~,m.
272
B P Glass /Solutton of naturally-occurring glasses
approximately the same distance above the adjacent microtektite surface. Secondly, when a microtektite with silica-rich protrusions is broken and cracked, the cracks are twice as wide as the protrusions are high (fig. 3). The SiO2-rich protrusions have a variety of shapes (fig. 2), but all have a circular outer surface that is concentric with the surface of the microtektite. With only a minor amount of solution, the SiO2-rlch area is slightly elevated above the microtektite surface (fig. 4). With greater solution, more of the protrusion is exposed and the general shape of the protrusion becomes more evident (fig. 4). With still further solution, the S102-rich particle rests on top of a pedestal of microtektite glass which was protected from solution by the overlying S102-rich particle (fig. 4). Further solution may completely undermine the SiO2-rich particle and free it from the imcrotektlte, leaving a comcal-shape protrusion of microtektite glass (fig. 4). Measurements of such features would only indicate
Fig 3. SEM photormcrographs of nucrotektltes with both cracks and slllca-nch protrusions A, Ivory Coast rmcrotektlte (K9-56-13) The slhca-nch protrusions are elevated about 16-17 /Lm above the surrounding surface and the cracks are about 2 8 / t m wtde. Thus the width of the cracks are a httle less than twice the height of the sihca-nch protrusions. B, North American rmcrotektlte from Barbados (179-16-9). The slhca-rlch protrusions stand 8 9 to 1 0 / t m above the surface and the crack is 17 8 to 18.2 # m wide. C, Mlcro-lrgluzate (128-10). The cracks are about 40 # m wide D, Same mlcro-lrghlzlte as in C. The silica-rich particle at the top ts elevated about 18/~m above the surface Scale bars for A - D equal 100/~m
A
B
C
D
Fig, 4 Sketch showing progressive solution of rmcrotektlte with slhca-rtch hemisphere at surface A, Ongdnal occurrence of sihca-rich inclusion. B, After enough solution to expose only a small part of the redes of the slhca-rich particle Compare with fig 2A C, Solution has exposed almost the entir.~" sdica-nch particle which now stands on a pedestal of tektite glass that was temporarily protected from solution by the overlying sdma-rich particle. Compare with figs. 2 B - C D, Soluuon has freed the sihca-rich particle A comcal-shaped protrusion of tektite glass marks the former location of the silica-rich particle.
Fig. 5 SEM photonucrograph of Ivory Coast microtektlte (K9-56-11) with U-shaped grooves. The grooves are about 80 p m wide. Scale bar equals 100 tim.
274
B P Glass / Solution of naturally- occurring glasses
B P Glass /Solutton of naturally- occurnng glasses
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B.P Glass /Solutton of naturally-occurrmg glasses
the minimum amount of solution that has occurred. (3) U-shaped grooves (fig. 5) occur on the surface of some microtektltes expecially those from the Ivory Coast strewn field. The U-shaped grooves were apparently produced by solution of thermal cracks on the surface of the microtekUtes [10]. Similar U-shaped grooves, except for their size, are found on some tektites, particularly billitonites [11]. Chapman et al. [12] used the widths of the U-shaped grooves on tektites to determine the amount of solution they have experienced. We attempted to use the U-shaped grooves to estimate the amount of solution experienced by the microtektltes assuming that the amount of solution equals ½ the width of the grooves. (4) Studies of microtektites in agglutinated foraminiferal tests (fig. 6) have also been used to determine the amount of solution. One method is to compare the exposed surfaces with those embedded in the test. The second method is to determine the distance between two closely-spaced microtektites in a test. This method gives the maximum amount of solution for the two microtektites combined. The most useful and reliable criteria appear to be the cracks and silica-rich protrusion. Microtekmes with cracks a n d / o r silica-rich protrusions were found at nearly all the sites, and when a mlcrotektite had both features, they both generally indicated the same amount of solution. The U-shaped grooves, on the other hand, are with few exceptions found only on the Ivory Coast microtektites. Where present, the U-shaped grooves often indicate several times the amount of solution indicated by cracks or silica-rich protrusions. Although U-shaped grooves and cracks can grade into each other, we distinguish U-shaped grooves based on their rounded or concave bottoms and width-todepth ratios greater than one. On the other hand, cracks, as used in this study, are much deeper than they are wide and generally have flat bottoms - see figs. 1 and 5. Because the U-shaped grooves generally indicate more solution than the cracks or silica-rich protrusions, and since the U-shaped grooves are essentially confined to the Ivory Coast microtektites, the U-shaped grooves have not been used in this study to determine the amount of solution that the microtektites have experienced. The criteria using agglutinated foraminiferal tests could only be used in a limited number of cases and does not yield quantitative data. Thus most of our conchislons are based on solution est~mates using silica-rich protrusions and cracks.
4. Results Measured amounts of solution range from 0.2 to 28 btm (table 3). When both cracks and silica-rich protrusions were present, the two features often indicated the same amount of solution. However, they do not always agree. In most cases when the two criteria do not agree, the cracks indicate less (generally about ½) the amount of solution as xndicated by the silica-rich protrusions. Generally when the cracks indicate less solution than the silica-rich
B P Glass /Solutton of naturally- occurring glasses
277
protrusions, the cracks are merely surface cracks. Deep cracks that extend a significant distance ( > 1 the radius) into the microtekttte generally indicate solution amounts in agreement with the silica-rich protrusions. Thus most of the questionable solution amounts given m table 3 are based on surface cracks which probably indicate only minimum amounts of solution. Major element analyses were obtained for 46 of the Australasian microtektites, 34 of the Ivory Coast microtektites, and 74 of the North American microtektltes for which we obtained solution measurements. SiO 2 contents range from 44.8 to 76.2, 62.1 to 70.0, and 59.0 to 81.7 for the Australasian, Ivory Coast and North American microtektites, respectively. Most have compositions similar to previously published analyses for microtektites from these strewn fields, but some have unusual or unique compositions.
Table 3 Average values for solution and S l O 2 c o n t e n t Solution ( # m ) a) Range
Average S102 Mean
Content (wt%) a)
Australasmn nucrotekt~tes rehable data only b) rehable + fmrly reliable all data c)
1.8--16 1 8-17 0 4-26
7 2 + 4 3(17) 7 8 + 4.2(28) 6 1 + 5.5(54)
64 0 + 5.9(14) 63 8 + 6.2(23) 64 2 + 6.1(46)
Ivory coast rmcrotektltes rehable data only b) rehable + fairly rehable all data c)
0 9-18 0 8-25 0 8-25
9 7 + 7 . 3 (9) 8 3 + 7 2(20) 9.8 + 7.3(25)
67 3 + 2 . 0 (9) 66 6 + 2 0(20) 66.5 + 1 9(24)
3 2-- 4 9 0.2-28 0 2-28
4 2 + 0 9 (3) 4 7 + 5 6(39) 4.3 + 6.0(57)
64 9 + 0 9 (3) 69.1 + 6.6(34) 71.2 + 6 8(54)
14 0 - 2 8 6 0-28 2 6-28
20 7 + 7 0 (3) 15 6 + 6 9 (9) 13 7 4- 7 2(12)
63.3 + 1 0 (3) 66.8 + 4 4 (9) 67.1 + 4.2(12)
8 0--24 7 0-24 7 0-24
12 2 4- 6.0 (6) 11 9 4- 4.6(14) 11.9 4- 4.6(14)
77 5 4- 2.3 (6) 78 4 4- 2.7(12) 78 4 4- 2.7(12)
North American microtektltes rehable data only b) rehable + fairly reliable all data c) Mlcro-lrgtuzates rehable data only b) rehable + fairly rehable all data o Barbados nucrotektites rehable data only b) rehable + fairly rehable all data c)
a) + one standard deviation. The number m parentheses indicates the number of measurements. b) Usually indicates soluUon values where two or more measurements agree or where two different c m e n a agree c) Includes some fmrly questmnable measurements.
278
B.P Glass /Solutton of naturally-occurring glasses
5. Discussion
5.1. Limitations A discussion of some of the limitations of this study is m order before proceeding to a discussion of the results. First, it should be noted that although most of the microtektites showed some evidence of corrosion, only a small percentage of the microtektites in the collection had obvious features (e.g., cracks or silica-rich protrusions) that could be used to quantify the amount of solution that they have undergone. Secondly, since we only see the end product and have no idea what the original population was like, we have no way of knowing how many microtektites may have been completely removed by solution. Complete dissolution may be the reason why we do not find any microtektites at some sites where otherwise we would have expected them. Finally, we note that some of the microtektites show no signs of corrosion. In these cases, we do not know if the microtektites have not experienced any solution, or if they have undergone uniform solution and have no features present to indicate how much solution they have experienced. Thus the data presented in this report are limited to a small percentage of unique specimens. Nevertheless, we were able to obtain solution amounts for about 170 specimens having a great range in age and composition, and from a rather wide range in depositional environments (tables 1, 2 and 3).
5.2. Relattonship between composttton and amount of solutton The microtektites from a given site show a general decrease in amount of solution with increasing SiO2 content (fig. 7), and at most of the deep-sea sites microtektites with greater than 70% SiO 2 show less than 5 # m of solution. Furthermore, the silica-rich protrusions, which are often nearly pure $102, have apparently undergone little or no solution. This is supported by the observation that when microtektites have both cracks and silica-rich protrusions, the widths of the cracks are almost never greater than twice the heights of the silica-rich protrusions. This is further support for the inverse relationship between solution and SiO 2 content. In all of the strewn fields, however, there are examples of nucrotektites from a given site with similar SiO 2 contents, but with large differences in amount of solution. In some of these cases, the microtektites have similar major oxide compositions and, therefore, the amount of solution does not appear to be related to differences in major oxide compositions. By comparing the compositions of microtektites from a given site with similar SiO 2 contents but with different amounts of solution, we found some possible relationships between amount of solution and major oxide composition other than SiO2. In general, the amount of solution for Australasian microtektites, from a given site with similar SiO 2 contents, increases with increasing CaO and K 2 0 and possibly with decreasing FeO and A1203 contents. The amount of solution for the Ivory
B.P Glass / Solution of naturally-occurrmg glasses
30
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X
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Fig. 8. A m o u n t of s o l u t i o n versus SzO 2 content for A u s t r a l a s s a n n u c r o t e k t i t e s a n d nucro-irgluzates.
280
B P Glass / So/unon of naturally- occurrzng glasses
Coast microtektites appears to increase with increasing N a 2 0 and K 2 0 contents and with decreasing TiO 2 (and possibly FeO) contents. North A m e n can microtektltes from deep-sea sites show increasing solution with increasing A1203, CaO, and N a 2 0 contents and with decreasing MgO contents. In general, then, the amount of solution appears to increase with decreasing SiO 2 content, and for a given SiO 2 content the amount of solution generally increases with increasing CaO and N a 2 0 and K 2 0 contents and perhaps with decreasing FeO a n d / o r MgO contents. These generahzations are, however, based on only a few examples and there are many exceptions. The microtektites from the two land sites (i.e., the micro-irghizites and the Barbados microtektites) also tend to show increasing amounts of solution with decreasing SiO 2 contents (fig. 8 and 9); but again, there is a great deal of scatter. The micro-lrghizites, like the Australasian microtektites, tend to show increasing amounts of solution with decreasing A1203 contents for a given SiO 2 content. However, in contrast to the Australasian microtektites, the microirghizites show increasing amounts of solution with increasing FeO (and perhaps MgO) and decreasing CaO and N a 2 0 and K 2 0 contents. Likewise, the North American n'ucrotektites found on Barbados show different relationships between amount of solution and major oxide contents for a given SiO 2 content as compared to North American microtektites from deep-sea sites. In contrast to the North American microtektites from deep-sea sites, the North American microtektites from Barbados show increasing solution with increasing MgO (and FeO) and decreasing A1203 (and perhaps CaO, N a 2 0 and K 2O) contents. Thus it appears that differences in composition, other than
30
x North A m e r i c a n
Microtektites
• Barbados Microtektites
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F~g. 9. Amount of soluUon versus S~O2 content for North American rmcrotektltes from deep-sea and from Barbados
s~tes
B P Glass /Solutton of naturally, occurring glasses
281
S i O 2 content, have different effects on amount of solution in terrestrial versus deep-sea environments.
5.3. Relattonshtp between age and amount o f solutton
It appears that the 35 m.y. old North American microtektltes have on the average undergone less solution than either the 0.7 m.y. old Australasian microtektltes or the - 1 m.y. old Ivory Coast microtektites (table 3). This is surprising since the North American microtektites are - 3 5 to - 5 0 times older than the Ivory Coast and Australasian microtektites, respectively. A possible explanation is that solution slows down drastically or ceases shortly after burial [13]. A study of microtektites found in agglutinated foraminlferal tests seems to support this. Microtektites are common components of tests of agglutinated Foraminifera found in sediments containing or adjacent to the North American mlcrotektite layer at several deep-sea sites (RC9-58, DSDP 94, D S D P 149, DSDP 166). Since the agglutinated Foraminifera are bottom dwellers and not burrowers, the microtektites must have been incorporated m their tests before burial. Based on the estimated sediment accumulation rates (table 2), a 500 /~m diameter mlcrotektite would be buried in less than 500 y even at the sites with the lowest sediment accumulation rate. Thus the microtektites must have been incorporated into the foraminiferal test shortly after deposition. In the beginning of the study, we hypothesized that the portion of each microtektite enclosed m the test must have been protected from solution; therefore, by removing the microtektites from the tests, we could determine the amount of solution be determining the difference in relief between the exposed and covered (or protected) surfaces. We were not able to see any differences between the exposed and enclosed surfaces (see, for example, fig. 6). The microtektites, however, have undergone solution etching, and in a few cases, microtektites in foraminiferal tests had silica-rich protrusions that indicated comparable amounts of solution to microtektites not found in foramlniferal tests. The microtektites were still tightly bound to the test so that the observed solution could not have taken place after the microtektites were incorporated into the tests. If solution had taken place uniformly around the entire surface of the microtektates, then they would no longer be tightly bound to the adjacent test material, and if solution had affected only the exposed surface of the microtektite, then there should have been an obvious difference in relief between the exposed and enclosed surfaces - and there is not. Thus the observed solution must have taken place before incorporation into the foraminiferal tests. Furthermore, since the microtektites in the foraminiferal tests show as much solution as microtektltes not in the tests (based on only a few measurements), this implies that all of the microtektites underwent solution immediately upon deposition and that solution essentially ceased on or shortly after burial. This study, then, supports Barnes' [13] suggestion that solution ceases shortly after
B.P Glass / Solution of naturally-occurring glasses
282
burial and it seems to explain why 35 m.y. old microtektites do not show any more solution than 0.7 m.y. old microtektites.
5.4. Relationship between emvironment and amount of solutzon Although there is a great deal of scatter in the data, microtektites from a given deep-sea site often show different amounts of solution when compared with microtektites with similar S i S 2 c o n t e n t s from other deep-sea sites within the same strewn field (fig. 10; table 4). There appears to be no correlation between average amount of solution and sediment type or sedimentation rate (table 4; fig. 11), but there does appear to be a tendency for the amount of solution to increase with decreasing water depth (fig. 12). If most of the corrosion takes place at the sediment/water interface, as previously discussed, then the amount of solution should be related to bottom conditions and length of time that the microtektites are on the bottom prior to burial. Length of exposure on the bottom is determined by sedimentation rate. Thus in areas of rapid sedimentation the microtektites should be buried quickly and thus show low amounts of solution. Conversely, in areas of slow sedimentation the microtektites would be exposed for longer periods of time and should, therefore, show greater amounts of solution. However, there
30
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Fig 10. A m o u n t of solution versus SnO2 c o n t e n t for tmcrotektttes f r o m v a n o u s sntes w~thin the N o r t h A m e r i c a n s t r e w n field
B.P. Glass /Solutton of naturally- occurring glasses
283
Table 4 Estimated average amounts of solution, depths, se&ment accumulation rates, and se&ment types for selected cores Ave. Soluuon (/~m) a)
Depth b) (m)
Sed Acc. Rate (cm/1000 y)
Se&ment type c)
(3) (9) (2) (3)
4649 4141 3568 2943
0 22 0 46 0 21(?) 11
foram lutlte luute foram, lutlte nanno, chalk
(7) (3) (2) (4) (7)
4346 4479 4152 4020 4687
1 02 1 59 0.84 0 71 1 68
foram, ooze foram, luUte foram, luUte foram, luUte foram, luUte
North American DSDP 149 - 2 (11) DSDP 216 - 5 (4) RC9-58 3 (23) DSDP 94 13 (6)
4242 2525 3456 2209
0 27 0.25 (1) 1.2
rad. ooze nanno, chalk foram, luute nanno chalk
Australasmn V16-70 V20-138 E49-4 DSDP 292 Ivory Coast V27-239 K9-57 V19-300 V19-297 K9-56
0.5 1.5 4.5 17 1 - 1 1 - 5 16
a) Estimated average amount of solution at - 70% StO2 Number in parentheses is number of measurements b) Water depth plus depth to the mlcrotektlte layer m the se&ment c) foram = foranumferal, rad = ra&olanan, nanno. = nannofossd.
a p p e a r s to b e n o c l e a r r e l a t i o n s h i p b e t w e e n s e d i m e n t a c c u m u l a t i o n r a t e a n d a m o u n t o f s o l u t i o n (fig. 11). T h i s m a y b e d u e to o t h e r f a c t o r s s u c h as v a r i a t i o n s in s o l u b i l i t y o f silica w i t h w a t e r d e p t h . T h e s o l u b i l i t y o f silica in sea w a t e r i n c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e , d e c r e a s i n g p r e s s u r e , a n d d e g r e e o f u n d e r s a t u r a t i o n [14,15]. W a t e r t e m p e r a t u r e is h i g h e s t in t h e s u r f a c e l a y e r a n d d e c r e a s e s w i t h d e p t h , w h e r e a s p r e s s u r e i n c r e a s e s w i t h d e p t h . A l t h o u g h silica is u n d e r s a t u r a t e d e v e r y w h e r e in t h e o c e a n s , it is m o r e u n d e r s a t u r a t e d in s h a l l o w water. T h u s silica s o l u t i o n d e c r e a s e s w i t h d e p t h a n d , for e x a m p l e , m o s t o f the d i s s o l u t i o n o f b i o g e n i c silica takes p l a c e in s h a l l o w w a t e r [15]. T h e r e f o r e , as w o u l d b e e x p e c t e d , t h e microtektites generally show decreasing solution with increasing water depth (fig. 12). T h e t e n d e n c y f o r m i c r o t e k t i t e s o l u t i o n to d e c r e a s e w i t h i n c r e a s i n g w a t e r d e p t h m a y at least p a r t l y e x p l a i n the a p p a r e n t l a c k of c o r r e l a t i o n b e t w e e n s o l u t i o n a n d s e d i m e n t a t i o n rate. W e w o u l d e x p e c t t h a t s o l u t i o n w o u l d i n c r e a s e w i t h d e c r e a s i n g s e d i m e n t a t i o n r a t e ; t h u s the a m o u n t s o f s o l u t i o n m e a s u r e d f o r sites D S D P 292, D S D P 94 a n d K 9 - 5 6 a r e m u c h h i g h e r t h a n w o u l d b e e x p e c t e d (fig. 12). B u t the h i g h a m o u n t o f s o l u t i o n at t w o o f t h e sites ( D S D P 94 a n d 292) m a y b e d u e to s h a l l o w d e p t h (see t a b l e 4). C o r e K 9 - 5 6 was f r o m a n a r e a
284
B P Glass
/ Solutzon o f naturally - occurring glasses
+DSOP
+
Australasian
o
Ivory
Coast
•
North
American
292
DSOP 84
g g
m o E ¢¢
g (¢
g <
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+
=re eOSDP
149 oDSDp +v20-23e
I 02
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]
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I
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Sedimentation
rate
12 (cm/lO00
t
I
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Fig 11. Average amount of solution of rmcrotekUtes from selected sates versus sedtment accumulation rates at those s=tes
+ Australas=an 15
o Ivory Coast • North American
E
,5 r. o
o
09
10
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E
0~ •
O +
>
<
+ o
o
o +
Depth (km)
~
5
Fig. 12. Average amount of soluUon of rmcrotektRes from selected sites versus water depth of coring sites.
B P Glass / Solunon of naturally-occurring glasses
285
of henupelagtc sedimentation (mixture of pelagic and continental sediments). The remainder of the sites are in water depths greater than 3.5 km (except for D S D P 216) and show a general decrease in average amount of solution with increasing sedimentation rate as would be expected. Although there is some overlap, the micro-irghizites, which have similar age and compositions to the Australasian microtektites, generally have about two to three t~mes as much solution for a given SiO 2 content as the Australasian microtektltes (fig. 9; table 3). Likewise, the North American microtektites found on Barbados generally show more solution than the North American microtekUtes from deep-sea sites (fig. 10; table 3). Since the micro-irghizites and Barbados microtektites are similar in age and composition to the Australasian and deep-sea North American microtektites, respectively, we assume that the greater amount of solution of the micro-irghizites and Barbados microtektites is due to their occurrence in a terrestrial as opposed to a deep-sea enwronment.
6. Conclusion We have measured solution amounts for about 140 microtektites having a large range in age (0.7 to 35 m.y.), composition (e.g., 44.8 to 81.7% SiO2), and environments. Measured amounts of solution range from 0.2 to at least 28 #m, but most are less than 5 #m. We can make the following tentative generalizations: (1) Most of the observed solution took place before the deep-sea microtektites were buried. Solution appears to essentially stop upon burial. Therefore, there is no systematic relationship between age and amount of solution. (2) Solution amounts are higher for rmcrotektites in terrestrial settings than for microtektites in deep-sea environments. (3) Within the marine environment there appears to be a tendency for the amount of solution to increase with decreasing water depth. (4) Solution varies inversely with SiO 2 content. (5) In the marine environment for a gtven SiO2 content the amount of solution generally increases with increasing CaO, N a 2 0 and K 2 0 contents and perhaps with decreasing FeO a n d / o r MgO contents. In the terrestrial environments, however, the relationship between solution and composition appears to be reversed. For a given SiO2 content, the amount of solution seems to increase with increasing FeO, and perhaps MgO, and with decreasing A1203, and perhaps CaO, Na 20 and K 2O.
References [1] W.A Cassldy, B.P. Glass and B C Heezen, J Geophys Res 74 (1969) 1008. [2] B.P Glass, Earth Planet So Lett. 9 (1970) 240 [3] B P Glass, J Geophys. Res. 77 (1972) 7057
286 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
B P. Glass / Solut:on of naturally- occurr:ng glasses B.P Glass, J Geophys. Res. 77 (1972) 7057. B.P. Glass and M.J Zwart, Geol. Soc. Am. Bull 90 (1979) 595. B.P. Glass and P.A. Zwart, Earth Planet. Sci. Lett. 43 (1979) 336. B.P. Glass, M B Swmckl and P.A. Zwart, Proc 10th Lunar Planet SCL Conf, Geocham. Cosmoclum. Acta, Suppl 11 (1979) 2535 B.P. Glass and J.R. Croshie, Bull. Am. Assoc. Petrol. Geol. 66 (1982) 471. P.V. Florensky and A.I. Dabizha, Zhamanshin Meteorite Crater (Nauka Press, Moscow, 1980) p. 127 B.P. Glass, K. Fredriksson and P.V. Florensky, Proc. 14th Lunar Planet. Sci. Conf., J. Geophys. Res., m press. B.P. Glass, Geol. Soc. Am. Bull. 85 (1974) 1305. G. Baker, Victoria Natl. Mus. Mem. 23 (1959) 1. D.R. Chapman, N.B. Zimmerman, F.J. Centolanza and L C Schelber, Abstr. 30th Ann. Meeting Meteontlcal Soc (NASA, Ames Research Center, Moffett Field, CA, Oct. 25-27, 1967). V.E. Barnes, written communication (1973). H.U Sverdrup, M W. Johnson and R.F. Fleming, The Oceans (Prentice-Hall, Englewood Chffs, NJ, 1942). J.P. Kennett, Manne Geology (Prenuce-Hail, Englewood Chffs, N J, 1982).