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Criteria for the source of Australites
Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
CRITERIA FOR THE SOURCE OF AUSTRALITES
S.R. TAYLOR Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T. (Australia) (Received June 17, 1968) (Resubmitted September 20, 1968) SUMMARY Recent data on the composition of the lunar s u r f a c e indicates that m a t e r i a l of tektite composition is not present at the sites sampled. The hypothesis of t e r r e s t r i a l impact origin for the A u s t r a l i a n and east Asian tektite strewn field, which is supported by geochemical evidence, is examined. The apparent absence of a deep c r a t e r (analogous to the A r i z o n a c r a t e r ) indicates that high density (meteorite o r asteroid) objects were not involved. The s u r f a c e evidence expected f r o m impact of a low density comet (analogous to the Tunguska event) is examined. It is suggested that the Australian and east Asian tektites originated in a s u p e r - T u n g u s k a event, and s e a r c h for wide shallow c r a t e r s should be c o n c e n t r a t e d in a r e a s of appropriate composition ( s u b - g r e y w a c k e sandstone). LUNAR ORIGIN OF TEKTITES
Evidence of the composition of the moon obtained by r e c e n t lunar p r o b e s has provided no support f o r a lunar origin of tektites. Direct knowledge of the composition of the lunar s u r f a c e has been obtained by the ~ s c a t t e r i n g a n a l y s e r s c a r r i e d on S u r v e y o r s V, VI, and VII (Turkevich et al., 1967, 1968a,b,c). Indirect information of the abundance of K, U and Th on the lunar s u r f a c e was obtained by a 7 - r a y s p e c t r o m e t e r on Luna-10, the Russian lunar o r b i t e r (Vinogradov et al., 1966). Luna-10 provided the f i r s t information and the data indicated that the concentrations of K, U, and Th were not g r e a t e r than those o b s e r v e d in t e r r e s t r i a l basalts. Vinogradov et al. (1966) gave the following t e r r e s t r i a l basaltic a v e r a g e s : K, 0.83%; U, 0.5 p.p.m.; Th, 3 p.p.m. These values w e r e thought to be upper limits for the lunar surface material. No distinction was o b s e r v e d between the m a r i a and the uplands. In c o n t r a s t to these values obtained f r o m heights of 350-1,000 km above the lunar s u r f a c e the Surveyor data provided d i r e c t s u r f a c e sampling, although only of a r e a s of about 10 cm d i a m e t e r in c o m p a r i s o n with the integrated analysis of l a r g e a r e a s p e r f o r m e d by Luna-10. S u r v e y o r V landed on Mare Tranquilitatis and S u r v e y o r VI on Sinus Medii. Within the r a t h e r l a r g e e r r o r s , the composition at these two m a r i a sites were s i m ilar. This equivalence at two random sampling sites may be extrapolated Chem. Geol., 4 (1969) 451-459
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to i n d i c a t e that t h e s e two m a r i a a r e c l o s e in c o m p o s i t i o n , and by a n a l o g y that the o t h e r m a r i a , which a r e so s i m i l a r in s u r f a c e e x p r e s s i o n , a r e a l s o a l i k e c h e m i c a l l y . The n e a r e s t t e r r e s t r i a l analogue in c o m p o s i t i o n is b a s a l t . Among m e t e o r i t e s the b a s a l t i c a c h o n d r i t e s a r e c l o s e . The following c o m p o s i t i o n s a r e r u l e d out by the S u r v e y o r V and VI data: c h o n d r i t e s , u l t r a b a s i c r o c k s , g r a n i t e s and t e k t i t e s . G i l v a r r y (1968) has r e c e n t l y p o s t u l a t e d that the m a r i a b a s i n s a r e f i l l e d with s e d i m e n t a r y m a t e r i a l , equivalent in c o m p o s i t i o n to t e r r e s t r i a l m u d s t o n e or shale. His c o n c l u s i o n is b a s e d l a r g e l y on the p r e s e n c e of c a r b o n (< 3%), but it should be noted that the f i g u r e by T u r k e v i c h et al. (1967) is an u p p e r l i m i t , and that no p o s i t i v e i d e n t i f i c a t i o n of the e l e m e n t was in fact r e p o r t e d . The heavy e l e m e n t s a r e too low in abundance in the s h a l e s c o m p a r e d with the m a r i a data. G i l v a r r y (1968) a t t r i b u t e s the e x c e s s i r o n to m e t e o r i t i c infall. However, the content of m e t a l l i c i r o n on the m a r i a s u r f a c e is l e s s than one p e r cent as shown by the S u r v e y o r V m a g n e t i c e x p e r i m e n t (De Wys, 1967) and t h e r e is a c c o r d i n g l y insufficient m e t a l l i c i r o n to account f o r the d i s c r e p a n c y . A s e c o n d objection, is that heavy e l e m e n t c o n t a m i n a t i o n f r o m m e t e o r i t i c s o u r c e s , m e t a l l i c o r not, should o c c u r both in m a r i a and uplands. The o l d e r , m o r e h e a v i l y c r a t e r e d uplands should be e n r i c h e d , but a r e d e f i c i e n t in t h e s e e l e m e n t s ( T u r k e v i c h et al., 1968a,c). Thus the i d e n t i f i c a t i o n of the m a r i a m a t e r i a l as r e s e m b l i n g t e r r e s t r i a l s h a l e s is doubtful. In any event, the p r o p o s e d c o m p o s i t i o n is f a r r e m o v e d f r o m that of t e k t i t e s . TYCHO S u r v e y o r VII landed on throwout f r o m Tycho, in the h e a v i l y c r a t e r e d upland region. The c o m p o s i t i o n was c l o s e to that of the m a r i a s a m p l e s , f o r the l i g h t e r e l e m e n t s . The only d i f f e r e n c e which a p p e a r s well e s t a b l i s h e d is "the l o w e r abundance of e l e m e n t s with m a s s e s between 47 and 65 (including the e l e m e n t s Cr, Mn, F e , Co, and Ni - T u r k e v i c h et al., 1968a,c). How r e p r e s e n t a t i v e of the u p l a n d s is t h i s outhrown m a t e r i a l f r o m Tycho ? Tycho is 54 m i l e s (87 km) in d i a m e t e r and 14,000 ft. (4.3 km) deep. Infall will have r e d u c e d the o r i g i n a l depth. The o u t - t h r o w n a p r o n on which S u r v e y o r VII is r e s t i n g is p r o b a b l y d e r i v e d f r o m depths of a few k i l o m e t e r s b e n e a t h the i m p a c t s u r f a c e . The o c c u r r e n c e of what a p p e a r t o b e d i s c r e t e c r y s t a l s p h o t o g r a p h e d by S u r v e y o r VII is c o n t r i b u t o r y evidence f o r a s u b s u r f a c e " p l u t o n i c " o r i g i n of the m a t e r i a l . Is t h i s e x c a v a t e d m a t e r i a l d i f f e r e n t f r o m that c o m p r i s i n g the s u r f a c e in the uplands, that i s , do the uplands have a l a y e r e d s t r u c t u r e ? Tycho is a r e c e n t c r a t e r with a m i n i m u m age of 106 y e a r s . A good i d e a of the p r e i m p a c t s t a t e of the l u n a r s u r f a c e at the s i t e of Tycho may be obtained by studying the uplands a r e a o u t s i d e the throwout a p r o n of d e b r i s f r o m Tycho. This s u r f a c e is c o v e r e d with l a r g e c r a t e r s , s u p e r i m p o s e d on o l d e r c r a t e r s . The s u r f a c e has been s u b j e c t to the heavy c r a t e r i n g t y p i c a l of the uplands. The d i a m e t e r s of the l a r g e r c r a t e r s a r e equivalent to o r g r e a t e r than Tycho. The upland s u r f a c e has thus been dug o v e r and m i x e d by t h i s p r o c e s s . Any o r i g i n a l l a y e r i n g has been d e s t r o y e d and the s u r f a c e m a t e r i a l to a depth of a few k i l o m e t e r s has been t h o r o u g h l y s t i r r e d by the c r a t e r i n g . It is a c c o r d i n g l y r e a s o n a b l e to suppose that the S u r v e y o r VII a n a l y s i s of the s u r f a c e is 452
Chem. Geol., 4 (1969) 451-459
r e p r e s e n t a t i v e . It should be r e c a l l e d that both fine grained material, and blocks, gave s i m i l a r a n a l y s e s at this site. In s u m m a r y , m a t e r i a l of tektite composition does not exist in the m a r i a , if the Surveyor V and VI data a r e at all representative. If the S u r v e y o r VII samples a r e r e p r e s e n t a t i v e of the uplands, as argued above, then these a r e a s likewise do not contain m a t e r i a l of tektite composition. Thus the Luna-10 and Surveyor data have gravely weakened the case f o r a lunar origin of tektites. At present, it is impossible to exclude the existence of small a r e a s of highly siliceous r o c k s , although t h e s e could be detected by a refinement of the Luna-10 experiment. It may be r e c a l l e d (Lowman, 1962) that a m a j o r r e a s o n advanced for p r e f e r r i n g a lunar to a t e r r e s t r i a l origin of tektites was t h e i r r a t h e r uniform and homogeneous composition, thought to be m o r e readily obtainable on the moon in c o n t r a s t to the well-known heterogeneity of the e a r t h ' s surface. Suitable parent m a t e r i a l for tektites will have an R b / S r ratio of about 0.5, and hence will generate high 87Sr/86Sr ratios in short periods of time. The m a t e r i a l c o m p r i s i n g the uplands region is s t r a t i g r a p h i c a l l y older than the m a r i a material. The youngest estimates for the age of the m a r i a a r e about 500-106 y e a r s but they may be up to 3,600.106 y e a r s old (Hartmann, 1965). P r e - m a r i a m a t e r i a l of tektite c o m position will accordingly be labelled with high 87Sr/86Sr ratios, in c o n t r a s t to those observed. The origin of tektites f r o m m o r e r e m o t e m e m b e r s of the s o l a r s y s t e m , or f r o m outside the s o l a r s y s t e m have been refuted by, inter alia, the absence of any evidence of exposure to c o s m i c radiation outside the e a r t h ' s a t m o s p h e r e (Viste and Anders, 1962).
IMPACT ORIGINOF TEKTITES The ultimate origin of tektites due to splash of melted material during impact of a comet, asteroid or large meteorite appears to be accepted by nearly all workers in this otherwise controversial field. The most recent of the tektite events was the formation of the east Asian and Australian strewn field. The very similar chemistry and similarity of K-A ages constitute reasonable grounds for grouping these scattered occurrences as due to one impact event. The case for a terrestrial origin of these tektites has been based largely on geochemical evidence. The chemical composition of anstralites bears a close resemblance to common terrestrial sandstones of sub-greywacke type (Taylor, 1962, 1966; Taylor and Sachs, 1964). Studies of terrestrial impact glass have contributed to our understanding of the chemistry of tektites. The Henbury impact glass is particularly informative (Taylor and Kolbe, 1964,1965; Taylor, 1966). It has been formed by the impact of a medium octahedrite iron meteorite on subgreywacke sandstone, and has a composition very close to that of australites. Apart from loss of volatiles, there is little change in composition between the parental sedimentary rock and the glass. This event provides a small-scale analogy for the tektite producing events. The pt conditions of silicate glass formation will be similar: only the scale of the explosion is different. From the evidence derived from the impact glass, and Chem. Geol., 4 (1969) 451-459
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f r o m i n t e r n a l c o m p o s i t i o n a l e v i d e n c e d e r i v e d f r o m the r a t i o s of v o l a t i l e and n o n - v o l a t i l e e l e m e n t s ( T a y l o r , 1962, 1966) it is c o n c l u d e d that changes in c o m p o s i t i o n of the p a r e n t m a t e r i a l d u r i n g g l a s s f o r m a t i o n (excluding l o s s of H20, CO2 etc.) w e r e s l i g h t and that the p r e s e n t c o m p o s i t i o n of a u s t r a l i t e s is c l o s e to that of the p a r e n t m a t e r i a l , which a c c o r d i n g l y r e s e m b l e s a t e r r e s t r i a l s a n d s t o n e of s u b g r e y w a c k e type. The g e o c h e m i c a l d i f f i c u l t i e s of p r o d u c i n g m a t e r i a l of this c o m p o s i t i o n on the l u n a r s u r f a c e led the author and c o w o r k e r s ( T a y l o r and Sachs, 1960, 1964; T a y l o r 1962, 1966) to p r o p o s e a t e r r e s t r i a l o r i g i n f o r t e k t i t e s .
DISTRIBUTION OF AUSTRALIAN AND EAST ASIAN TEKTITES No a c c e p t e d i d e n t i f i c a t i o n of a t e r r e s t r i a l s o u r c e a r e a f o r this group of t e k t i t e s has b e e n p r o p o s e d and e f f o r t s have b e e n c o n c e n t r a t e d on s e a r c h i n g f o r Canyon Diablo type i m p a c t c r a t e r s of v e r y l a r g e size. A wide v a r i e t y of s i t e s have been s u g g e s t e d (e~g., A n t a r c t i c a - Schmidt, 1962). In the a b s e n c e of any r e a l knowledge of the t e k t i t e p r o d u c i n g event, it is useful to begin t h i s e n q u i r y by looking at the e v i d e n c e which has to be explained. Let us c o n s i d e r the d i s t r i b u t i o n of the A u s t r a l i a n and e a s t A s i a n t e k t i t e s , a s s u m i n g that they a r e the r e s u l t of one event. The f r e q u e n c y of o c c u r r e n c e v a r i e s w i d e l y but t h e r e i s a g e n e r a l tendency f o r the abundance (and m a s s e s of i n d i v i d u a l t e k t i t e s ) to i n c r e a s e in a n o r t h w e s t e r l y d i r e c t i o n a c r o s s the s t r e w n field. The total n u m b e r of a u s t r a l i t e s c o l l e c t e d is between 50,000 and 100,000 and this n u m b e r s e r v e s as an index of the total f r e q u e n c y of a u s t r a l i t e s . In c o n t r a s t to the n u m b e r , B e y e r (1943) c o l l e c t e d 500,000 p h i l i p p i n i t e s in the v i c i n i t y of the city of Manila. F r o m s o u t h e a s t to n o r t h w e s t a c r o s s the A u s t r a l i a n - - e a s t A s i a n s t r e w n f i e l d , the m a s s of the individual t e k t i t e s , as well as t h e i r n u m b e r a p p e a r s to i n c r e a s e . Most d a t a e x i s t f o r the a u s t r a l i t e s , but a t t e m p t s to e s t i m a t e ' t h e a v e r a g e weight of t h i s w e l l d o c u m e n t e d group have p r o v e d difficult. B a k e r and F o r s t e r (1943) gave a value of 1.5 g. A v e r a g e s of v a r i o u s s m a l l e r c o l l e c t i o n s l i e in the r a n g e 1-9 g. A l l o w a n c e has to be m a d e f o r l o s s by a e r o d y n a m i c a b l a t i o n , p a r t i c u l a r l y m a r k e d f o r the a u s t r a l i t e s . It s e e m s safe to conclude that the m o s t f r e quent weight is in the r a n g e 1-10 g, and p r o b a b l y between 1 and 5 g. The weights of the h e a v i e s t s p e c i m e n s of a l l g r o u p s p r o v i d e one of the b e t t e r s e t s of data a v a i l a b l e f o r t h i s enquiry. T h e s e l a r g e s a m p l e s a r e d i s t i n c t i v e , e x c i t e the c u r i o s i t y of the f i n d e r and a r e l i k e l y to find t h e i r way into m u s e u m c o l l e c t i o n s . B e c a u s e of a b l a t i o n l o s s , etching, and difficulty of finding the s a m p l e s , the l o w e s t r e c o r d e d weights a r e not s i g n i f i c a n t , in the p r e s e n t context. The l a r g e s t a u s t r a l i t e w e i g h s 243 g (McCall, 1965) and l e s s than ten weighing m o r e than 100 g have been c o l l e c t e d . In c o n t r a s t , m o r e than 100 p h i l i p p i n i t e s in the r a n g e 200-700 g have been r e c o r d e d , and the l a r g e s t s p e c i m e n weighs 1,070 g. V e r y l a r g e n u m b e r s of t e k t i t e s have b e e n c o l l e c t e d f r o m C a m b o d i a , L a o s and Vietnam. The l a r g e s t r e c o r d e d s p e c i m e n weighs 3,200 g and v e r y l a r g e a m o u n t s of Muong-Nong type m a t e r i a l have been c o l l e c t e d . "In an a r e a of about 100 m2, 67.5 kg of t e k t i t e s w e r e r e c o v e r e d : " ( B a r n e s and P i t a k p a i v a n , 1962.) T h i s m a s s is within an o r d e r of magnitude of the total m a s s of a u s t r a l i t e s c o l l e c t e d o v e r the A u s t r a l i a n s t r e w n field. 454
Chem. Geol., 4 (1969} 451-459
T h i s i n f o r m a t i o n , although sketchy, i n d i c a t e s that both n u m b e r s and m a s s of t e k t i t e s i n c r e a s e t o w a r d the n o r t h w e s t c o r n e r of the c o m b i n e d A u s t r a l i a n - e a s t A s i a n s t r e w n - f i e l d s . T h e r e a p p e a r s to be much l o c a l v a r i a t i o n in d i s t r i b u t i o n s u g g e s t i v e of the a r r i v a l of c l u s t e r s of t e k t i t e s a c r o s s the s t r e w n field. In s u m m a r y , the d i s t r i b u t i o n of t e k t i t e s a c r o s s the s t r e w n f i e l d i n d i c a t e s that the l a r g e s t chunks (Muong-Nong type) the g r e a t e s t n u m b e r and the h e a v i e s t t e k t i t e s o c c u r in the n o r t h w e s t , and the s m a l l e s t and l i g h t e s t f o r m s , which a l s o d i s p l a y the b e s t e v i d e n c e of a e r o d y n a m i c a b l a t i o n . a r e s p a r s e l y d i s t r i b u t e d o v e r the s o u t h e r n half of A u s t r a l i a .
SURFACE EFFECTS OF A TERRESTRIALTEKTITE PRODUCINGEVENT The above m a t e r i a l c o m p r i s e s the total i d e n t i f i e d t e r r e s t r i a l m a t e r i a l f o r m e d d u r i n g the t e k t i t e p r o d u c i n g event. The r e m a i n i n g p a r t of this p a p e r will be devoted to a d i s c u s s i o n of the p o s s i b l e s u r f a c e e f f e c t s of a t e r r e s t r i a l t e k t i t e p r o d u c i n g event. No l a r g e c r a t e r of the Canyon Diablo type has been i d e n t i f i e d a s an a c c e p t e d s o u r c e , although s u g g e s t i o n s of such s i t e s in A n t a r t i c a (Schmidt, 1962) and o t h e r p l a c e s have been made. Lin (1966) has i n v e s t i g a t e d the p o s s i b i l i t y of c o m e t a r y i m p a c t . He c a l c u l a t e s , on the b a s i s of e x t r a p o l a t i o n f r o m the A r i z o n a (Canyon Diablo) c r a t e r , that a s i m p l e s c a l i n g f o r an e x p l o s i o n e n e r g y of 2.1030 e r g s , r e q u i r e d to r e m o v e an a t m o s p h e r i c s e g m e n t , would give a c r a t e r about 300 km in d i a m e t e r and 40 k m in depth. No such c r a t e r has been o b s e r v e d t e r r e s t r i a l l y . L u n a r c r a t e r s as d i s t i n c t f r o m m a r i a , r e a c h 294 k m (Bailly) in d i a m e t e r and a depth of 6 km (Newton). The a p p a r e n t a b s e n c e of such c r a t e r s on the e a r t h , i n d i c a t e s that s i m p l e s c a l i n g f r o m Canyon Diablo type c r a t e r s is not c o r r e c t , if t e k t i t e s a r e t e r r e s t r i a l . The i m p a c t e n e r g y at Canyon Diablo has been e s t i m a t e d by S h o e m a k e r (1963) at 6.8"1022 e r g s (1.7 m e g a t o n T.N.T. equivalent) on the b a s i s of s c a l i n g f r o m the T e a p o t ESS c r a t e r (1.2 ktons). Teapot ESS c r a t e r (300 ft. d i a m e t e r × 100 ft. deep) was p r o d u c e d by the e x p l o s i o n of 1.2 k t o n s 67 ft. below the s u r f a c e . Canyon Diablo (4,000 ft. d i a m e t e r × 570 ft. deep) w a s p r o d u c e d at an e s t i m a t e d depth of 300-400 ft. b e n e a t h the o r i g i n a l s u r f a c e ( S h o e m a k e r , 1963). Further data from large explosions are available from the hydrogen bomb tests, Castle and Ivy Mike (Vaille, 1961; Vortman, 1968). Both devices were approximately 15 megaton T.N.T. equivalent (6" I 023 ergs). The Castle explosion, 7 ft. above the surface produced a crater 3,000 ft. in diameter and 240 ft. deep. In the Ivy Mike experiment, the device was detonated 35 ft. above the surface. A much shallower crater (160 ft. deep) of the same diameter (3,000 ft.) resulted. Thus the height of the explosion is critical for cratering depth. The extreme example of a highly energetic event in the atmosphere is the Tunguska event (60°54' N, 101056' E, June 30, 1908) and a short description of this is instructive (Fesenkov, 1961; Robey, 1963; Krinov, 1966). The height of the explosion was 5 km above the surface. Severe charring of trees occurred in a central area (radius 1.5 km). Scorching occurred up to 17-20 km from the centre. The most spectacular effect was the blowing down of the forest surrounding ground zero. An estimated 80 million trees with trunk diameters greater than 8 inches (20 cm) were Chem. Geol., 4 (1969) 451-459
455
f l a t t e n e d r a d i a l l y f r o m ground z e r o . The r a d i u s of the f e l l e d t r e e s a r e a is 35 km C21 m i l e s ) . P a r t l y d a m a g e d t r e e s a r e found up to 60 k m (37 m i l e s ) f r o m the e x p l o s i o n c e n t r e . A s m a l l a r e a of t r e e s r e m a i n e d standing in the centre. A i r p r e s s u r e waves w e r e r e c o r d e d in England. The e n e r g y r a d i a t e d as v i s i b l e light is e s t i m a t e d at 2.8"1023 e r g s . The total e n e r g y of the object in the a t m o s p h e r e j u s t p r i o r to e x p l o s i o n is e s t i m a t e d at 8.1024 e r g s C200 m e g a t o n s T.N.T. equivalent) CRobey, 1963). The only s u r f a c e effect was the f e l l i n g of the f o r e s t and c h a r r i n g of the t r e e s . No c r a t e r w a s f o r m e d . Many s u g g e s t i o n s have b e e n m a d e of the s o u r c e of t h i s e x c e e d i n g l y e n e r g e t i c event, f r o m p r i m i t i v e n u c l e a r e x p e r i m e n t s to the e n c o u n t e r of the e a r t h with a n t i - m a t t e r (Cowan et al., 1965). P r e d i c t e d i n c r e a s e in 14C a c t i v i t y which would r e s u l t f r o m an a n t i - m a t t e r e x p l o s i o n have not been c o n f i r m e d ( L e r m a n et a l . , 1967) and indeed the p r e s e n c e of a n t i - m a t t e r within the l o c a l group of g a l a x i e s , let alone the s o l a r s y s t e m , a p p e a r s a r e m o t e p o s s i b i l i t y . The c o n s e n s u s is that a s m a l l c o m e t head of m a s s 1012-1013g and of low d e n s i t y c o l l i d e d with the e a r t h and e x p l o d e d at a height of 5-6 km /Krinov, 1966). The s i g n i f i c a n c e of the Tunguska event in the p r e s e n t context i s that a 200 megaton explosion at 5 k m a l t i t u d e p r o d u c e d s u r f a c e e f f e c t s which a r e r a p i d l y d i s a p p e a r i n g 60 y e a r s a f t e r the event. In c o n t r a s t , the e v i d e n c e f r o m an i r o n m e t e o r i t e i m p a c t (of much l o w e r e n e r g y : 1-2 m e g a t o n s ) at a depth of 300-400 ft. at Canyon D i a b l o will r e m a i n until the e r o s i o n has l o w e r e d the s u r f a c e l e v e l by about 1,000 ft. At the p r e s e n t e s t i m a t e d r a t e of l o w e r i n g the s u r f a c e by 2-3 i n c h e s p e r 1,000 y e a r s in u p l i f t e d r e g i o n s (Schumm, 1963; Judson and R i t t e r , 1964) d e t e c t a b l e e v i dence Cfrom o u r p r e s e n t u n d e r s t a n d i n g of c r a t e r s t r u c t u r e ) will r e m a i n f o r at l e a s t 400,000-600,000 y e a r s . It m a y thus be concluded that the s u r v i v a l of evidence of high e n e r g y i m p a c t e v e n t s on the e a r t h ' s s u r f a c e depends c r i t i c a l l y on the height of the e x p l o s i o n , and the d e n s i t y of the i m p a c t i n g object. A t e r r e s t r i a l o r i g i n of t e k t i t e s f r o m an c o m e t a r y i m p a c t s i m i l a r to, but much m o r e e n e r g e t i c than, the Tunguska event has long been a d v o c a t e d by U r e y (1957, 1963). Lin /1966) e x p l o r e d s o m e of the c o n s e q u e n c e s of c o m e t a r y i m p a c t and a t m o s p h e r i c r e m o v a l , using the " d u s t y s n o w b a l l " c o m e t m o d e l of Whipple (1963). The e n e r g y n e c e s s a r y to r e m o v e a s l i c e of the a t m o s p h e r e above the h o r i z o n t a l tangent plane, is 2"103° e r g s . A s s u m i n g an e n e r g y c o n v e r s i o n of 50%, a c o m e t nucleus with d e n s i t y 0.1 g / c m 3, a m a s s of 5"1017 g, with a r a d i u s of 10 k m and an i m p a c t v e l o c i t y of 42 k m / s e c would p r o v i d e the r e q u i r e d amount of e n e r g y (Lin, 1966). T h i s e n e r g y r e q u i r e m e n t can be r e d u c e d . The a b s e n c e of a r a d i a l d i s t r i b u t i o n of t e k t i t e s in the A u s t r a l i a n and A s i a n s t r e w n - f i e l d s i s c o n s i s t e n t with an o r i g i n f r o m s e v e r a l j e t s or s p l a s h e s f r o m the e x p l o s i o n in one d i r e c t i o n . The s t r e w n - f i e l d s can be contained in a s e c t o r of a p p r o x i m a t e l y 15 °. The glancing i m p a c t of a comet nucleus of 10 km d i a m e t e r at 40 k m / s e c can a l s o p r o v i d e e x t r a e n e r g y f o r a t m o s p h e r i c r e m o v a l in the d i r e c t i o n of motion. Although it is difficult to m a k e meaningful c a l c u l a t i o n s , it is p r o b a b l e that the e n e r g y r e q u i r e m e n t to account f o r the total s t r e w n - f i e l d is l e s s than the value of 2 • 1030 e r g s and is p o s s i b l y as low as 1028 e r g s . The l o w e r the d e n s i t y of the i m p a c t i n g o b j e c t , the s h a l l o w e r will be the c r a t e r . Supporting e v i d e n c e f o r low m a s s o r d e n s i t y in the i m p a c t i n g 456
Chem. Geol., 4 (1969} 451-459
body is provided by the very low content of metallic iron spherules found in tektites (Chao et al,, 1962) c o m p a r e d to t e r r e s t r i a l impact g l a s s e s resulting f r o m the impact of iron m e t e o r i t e s . One of the intriguing f e a t u r e s of tektite c h e m i s t r y is the virtual lack of evidence of contamination f r o m the impacting body. Their c h e m i s t r y r e s e m b l e s that of fused sandstones of subgreywacke type without significant selective loss or addition of elements. Identification of m a t e r i a l derived f r o m c o m e t s will be difficult, b e c a u s e of its predicted scarcity. Chemical energy is probably r e l e a s e d in c o m e t a r y i m p a c t s in addition to the kinetic energy r e l e a s e , b e c a u s e of the p r e s e n c e of free r a d i c a l s in comets. A combination of a highly energetic explosion, and a low density body is unlikely to leave much residual m a t e r i a l to identify the impacting object. In addition to the s p e c t r a of CO +, CH, CN, C2, C3, OH, NH 2 etc., r e c o r d e d f r o m c o m e t s the p r e s e n c e of nonvolatile constituents has been established. E m i s s i o n s p e c t r a of Fe, Ni, Cu, Cr, Mn, Ca, Na and K were r e c o r d e d f r o m the comet lkeya-Seki (Preston, 1967). Thus some slight iron and nickel contamination may be expected f r o m a c o m e t a r y impact, which could account for the metallic spherules o b s e r v e d by Chao et al. (1962). Metallic and silicate spherules have been collected at Tunguska, but their identification with the explosion has yet to be proved (Krinov, 1966). Comets a r e plentiful (about 600 per century a r e observed f r o m the earth), but iron m e t e o r i t e s of sufficient size to f o r m large c r a t e r s may be much r a r e r than previously suspected. The low content ( < 1% ) of metallic iron on the lunar m a r i a s u r f a c e (De Wys, 1967), and the low total iron content of the upland s u r f a c e (Turkevich et al., 1968) supports this view. Sun (1968) has proposed that the s t r u c t u r e of the lunar c r a t e r , Copernicus, is consistent with derivation f r o m an explosion at the surface, and he postulates a c o m e t a r y impact as a feasible m e c h a n i s m for this event.
HYPOTHESIS FOR THE ORIGIN OF AUSTRALIAN AND EAST ASIAN TEKTITES
The model proposed here to account f o r the a u s t r a l i t e s and the east Asian tektites follows the suggestions of Urey (1957,1963). A comet collides with the earth and explodes in the a t m o s p h e r e . A thin s u r f a c e l a y e r of s u b - g r e y w a c k e composition is melted, in this s u p e r - T u n g u s k a event, and splashed outwards in s e v e r a l jets. The explosion was of sufficient energy to r e m o v e a segment of the a t m o s p h e r e , permitting escape of the melted material. The solidified m a t e r i a l r e - e n t e r s the atmosphere, when ablation and flange formation occurs. The s u r f a c e effect of the explosion could be limited to a shallow depth o v e r an a r e a of s e v e r a l square k i l o m e t e r s . Such an impact site would be v e r y difficult to detect, except perhaps in d e s e r t a r e a s , where it might be enlarged due to wind export of dust. The following c r i t e r i a a r e p r o p o s e d to aid the s e a r c h for the impact site: (1) A shallow c r a t e r of large d i a m e t e r ( > 10 km). (2) The rock type is sandstone of s u b - g r e y w a c k e type, close to the present chemical composition of the a u s t r a l i t e s . Some variation in c o m p o s i tion is postulated to provide for the v a r i a t i o n s in tektite composition.
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(3) T h e a g e of t h e s o u r c e a r e a s h o u l d b e i n t h e r a n g e 2 0 0 - 4 0 0 . 1 0 6 y e a r s (4) T h e c r a t e r i s r e m o t e f r o m t h e A u s t r a l i a n s t r e w n - f i e l d , t o a c c o u n t f o r t h e e v i d e n c e of i n f a l l a t v e l o c i t i e s b e t w e e n 6 a n d 12 k m / s e c . ( C h a p m a n , 1964.)
REFERENCES B a k e r . G., 1959. T e k t i t e s . Mere. Natl. M u s e u m V i c t o r i a , 23: 5 - 3 1 3 . B a k e r . G. and F o r s t e r , H.C., 1943. T h e s p e c i f i c g r a v i t y r e l a t i o n s h i p s of a u s t r a l i t e s . Am. J. Sci., 241: 3 7 7 - 4 0 6 . B a r n e s . V.E. and P i t a k p a i v a n , K., 1962. O r i g i n of indochinite t e k t i t e s , P r o c . Natl. Aead. Sci. U.S.. 48: 9 4 7 - 9 5 5 . B e y e r . H.O.. 1943. P h i l i p p i n e t e k t i t e s and the t e k t i t e p r o b l e m in g e n e r a l . Arm. Rept. S m i t h s o n i a n Inst., 1942: 2 5 3 - 2 5 9 . Chao, E.C.T., A d l e r , I., Dwornik. E.J. and L i t t l e r , J., 1962. M e t a l l i c s p h e r u l e s in t e k t i t e s f r o m I s a b e l a . P h i l i p p i n e I s l a n d s . Science, 135: 9 7 - 9 8 . Chapman. D.R.. 1964. On the unity and o r i g i n of the A u s t r a l a s i a n t e k t i t e s . G e o c h i m . C o s m o c h i m . Acta, 28: 8 4 1 - 8 8 0 . Cowan. C.. A t l u r i . C.R. and Libby, W . F . . 1965. P o s s i b l e a n t i m a t t e r c o n t e n t of the T u n g u s k a m e t e o r of 1908. N a t u r e , 206: 8 6 1 - 8 6 5 , De Wys. J.N.. 1967. S u r v e y o r V: M a g n e t e x p e r i m e n t . Science, 158: 6 3 2 - 6 3 5 . F e n n e r . C . . 1935. A u s t r a l i t e s ; 2. N u m b e r s , f o r m s , d i s t r i b u t i o n , and o r i g i n . T r a n s . P r o c . Roy. Soc. S. A u s t r a l i a , 59: 1 2 5 - 1 4 0 . F e s e n k o v . V.G.. 1961. On the c o m e t a r y n a t u r e of the Tungus m e t e o r i t e . A s t r o n . Zh., 38: 5 7 7 - 5 9 2 . G i l v a r r y , J . J . , 1968. O b s e r v a t i o n a l e v i d e n c e for s e d i m e n t a r y r o c k s on the moon. N a t u r e . 218: 3 3 6 - 3 4 1 . H a r t m a n n , W.K., 1965. T e r r e s t r i a l and l u n a r flux of l a r g e m e t e o r i t e s in the l a s t two billion y e a r s . I c a r u s , 4: 1 5 7 - 1 6 5 , Judson. S. and R i t t e r , D . F . . 1964. R a t e s of r e g i o n a l denudation in the United S t a t e s . J. Geophys. R e s . , 69: 3 3 9 5 - 3 4 0 1 . Krinov. E . L . , 1966. Giant M e t e o r i t e s . P e r g a m o n P r e s s , Oxford, 540 pp. L e r m a n . J . C . , Mook, W.G. and Vogel, J . C . , 1967. Effect of the T u n g u s k a m e t e o r and s u n s p o t s on r a d i o c a r b o n in t r e e r i n g s . N a t u r e , 216: 9 9 0 - 9 9 1 . Lin. C.S.. 1966. C o m e t a r y i m p a c t and the o r i g i n of t e k t i t e s . J. Geophys. R e s . , 71: 2427-2437. Lowman, P.D., 1962. T e k t i t e s vs. t e r r e s t r i a l r o c k s : a c o m p a r i s o n of v a r i a n c e in c o m p o s i t i o n s . G e o c h i m . C o s m o c h i m . Acta, 26: 5 6 1 - 5 7 9 . McCall. G.J.H.. 1965. The h e a v i e s t r e c o r d e d a u s t r a l i t e . A u s t r a l i a n J. Sci., 27: 267. P r e s t o n , G.W.. 1967. T h e s p e c t r u m of c o m e t I k e y a - S e k i (1965 I). A s t r o p h y s . J . , 147: 7 1 8 - 7 4 2 . R a n k a m a , K.. 1965. O r i g i n of a u s t r a l i t e s . N a t u r e , 207: 1383. Robey, D.H.. 1963. G e n e r a l D y n a m i c s , A s t r o n a u t i c s Rept. No. GD/A 63-0064. Schmidt, R.A., 1962. A u s t r a l i t e s and A n t a r c t i c a . Science, 138: 4 4 3 - 4 4 4 . Schumm, S.A., 1963. T h e d i s p a r i t y b e t w e e n p r e s e n t r a t e s of denudation and o r o g e n y . U.S. Geol. Surv., P r o f e s s . P a p e r s , 454-H. S h o e m a k e r , E.M., 1963. I m p a c t m e c h a n i c s at m e t e o r c r a t e r , A r i z o n a . Chap. 11: T h e moon, m e t e o r i t e s and c o m e t s . In: B.M. M i d d l e h u r s t and G.P. K u i p e r ( E d i t o r s ) , The S o l a r S y s t e m . Univ. Chicago P r e s s , Chicago, Ill., 4: 3 0 1 - 3 3 6 . Sun. J.M.S.. 1968. C o m e t a r y i m p a c t o r i g i n of C o p e r n i c u s . J. Geophys. R e s . , 73: 2721-2728. T a y l o r . S.R., 1962. T h e c h e m i c a l c o m p o s i t i o n of a u s t r a l i t e s . G e o e h i m . C o s m o c h i m . Acta, 26: 6 8 5 - 7 2 2 . T a y l o r , S.R., 1966. A u s t r a l i t e s , H e n b u r y i m p a c t g l a s s and s u b - g r e y w a c k e : A c o m p a r i s o n of the a b u n d a n c e s of 51 e l e m e n t s . G e o c h i m . C o s m o c h i m . A c t a , 30: 1121-1136. 458
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Taylor, S.R. and Kolbe, P., 1964. Geochemical s t a n d a r d s . Geochim. Cosmochim. Acta, 28: 447-454. Taylor, S.R. and Kolbe, P., 1965. The geochemistry of Henbury impact glass. Geochim. Cosmochim. Acta, 29: 741-754. Taylor, S.R. and Sachs, M., 1960. T r a c e e l e m e n t s in a u s t r a l i t e s . Nature, 188: 387-388. Taylor, S.R. and Sachs, M., 1964. Geochemical evidence for the origin of a u s t r a l i t e s . Geochim. Cosmochim. Acta, 28: 235-264. Turkevieh, A.L., F r a n z g r o t e , E.J. and P a t t e r s o n , J.H., 1967. Chemical analysis of the moon at the Surveyor V landing site. Science, 158: 635-637. Turkevich, A.L., F r a n z g r o t e , E.J. and P a t t e r s o n , J.H., 1968a. Chemical a n a l y s i s of the lunar s u r f a c e at the Surveyor landing sites. Science, 160: 443-444. Turkevich, A.L., F r a n z g r o t e , E.J. and P a t t e r s o n , J.H., 1968b. Chemical a n a l y s i s of the moon at the Surveyor VI landing site: P r e l i m i n a r y r e s u l t s . Science, 160: 1108-1110. Turkevich, A.L., F r a n z g r o t e , E.J. and P a t t e r s o n , J.H., 1968c. Chemical a n a l y s i s of the moon at the Surveyor VII landing site: P r e l i m i n a r y r e s u l t s . Science, in p r e s s Urey, H.C., 1957. The origin of tektites. Nature, 179: 556-557. Urey, H.C., 1963. Cometary collisions and tektites. Nature, 197: 228-230. Vaille, R.B., 1961. Pacific c r a t e r s and scaling laws. J. Geophys. Res., 22: 3413-3428. Vinogradov, A.P., Surkov, Yu.A., Chernov, G.M., Kirnozov, F.F. and Nazarkina, G.B., 1966. M e a s u r e m e n t s of g a m m a - r a d i a t i o n f r o m the s u r f a c e of the moon by the c o s m i c station "Luna-10". Geochemistry, 3: 707-715. V i s t e , E. and A n d e r s , E., 1962. C o s m i c - r a y exposure h i s t o r y of tektites. J. Geophys. Res., 67: 2913-2919. Vortman, L.J., 1968. C r a t e r s f r o m surface explosions and scaling laws. J. Geophys. Res., 73: 4621-4636. Whipple, F.L., 1963. On the s t r u c t u r e of the c o m e t a r y nucleus. Chap. 19: The moon, m e t e o r i t e s and planets. In: B.M. Middlehurst and G.P. Kuiper (Editors), The Solar System. Univ. Chicago P r e s s , Chicago, II1., 4: 639-664.
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CRITERIA FOR THE SOURCE OF AUSTRALITES
S.R. TAYLOR Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T. (Australia) (Received June 17, 1968) (Resubmitted September 20, 1968) SUMMARY Recent data on the composition of the lunar s u r f a c e indicates that m a t e r i a l of tektite composition is not present at the sites sampled. The hypothesis of t e r r e s t r i a l impact origin for the A u s t r a l i a n and east Asian tektite strewn field, which is supported by geochemical evidence, is examined. The apparent absence of a deep c r a t e r (analogous to the A r i z o n a c r a t e r ) indicates that high density (meteorite o r asteroid) objects were not involved. The s u r f a c e evidence expected f r o m impact of a low density comet (analogous to the Tunguska event) is examined. It is suggested that the Australian and east Asian tektites originated in a s u p e r - T u n g u s k a event, and s e a r c h for wide shallow c r a t e r s should be c o n c e n t r a t e d in a r e a s of appropriate composition ( s u b - g r e y w a c k e sandstone). LUNAR ORIGIN OF TEKTITES
Evidence of the composition of the moon obtained by r e c e n t lunar p r o b e s has provided no support f o r a lunar origin of tektites. Direct knowledge of the composition of the lunar s u r f a c e has been obtained by the ~ s c a t t e r i n g a n a l y s e r s c a r r i e d on S u r v e y o r s V, VI, and VII (Turkevich et al., 1967, 1968a,b,c). Indirect information of the abundance of K, U and Th on the lunar s u r f a c e was obtained by a 7 - r a y s p e c t r o m e t e r on Luna-10, the Russian lunar o r b i t e r (Vinogradov et al., 1966). Luna-10 provided the f i r s t information and the data indicated that the concentrations of K, U, and Th were not g r e a t e r than those o b s e r v e d in t e r r e s t r i a l basalts. Vinogradov et al. (1966) gave the following t e r r e s t r i a l basaltic a v e r a g e s : K, 0.83%; U, 0.5 p.p.m.; Th, 3 p.p.m. These values w e r e thought to be upper limits for the lunar surface material. No distinction was o b s e r v e d between the m a r i a and the uplands. In c o n t r a s t to these values obtained f r o m heights of 350-1,000 km above the lunar s u r f a c e the Surveyor data provided d i r e c t s u r f a c e sampling, although only of a r e a s of about 10 cm d i a m e t e r in c o m p a r i s o n with the integrated analysis of l a r g e a r e a s p e r f o r m e d by Luna-10. S u r v e y o r V landed on Mare Tranquilitatis and S u r v e y o r VI on Sinus Medii. Within the r a t h e r l a r g e e r r o r s , the composition at these two m a r i a sites were s i m ilar. This equivalence at two random sampling sites may be extrapolated Chem. Geol., 4 (1969) 451-459
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to i n d i c a t e that t h e s e two m a r i a a r e c l o s e in c o m p o s i t i o n , and by a n a l o g y that the o t h e r m a r i a , which a r e so s i m i l a r in s u r f a c e e x p r e s s i o n , a r e a l s o a l i k e c h e m i c a l l y . The n e a r e s t t e r r e s t r i a l analogue in c o m p o s i t i o n is b a s a l t . Among m e t e o r i t e s the b a s a l t i c a c h o n d r i t e s a r e c l o s e . The following c o m p o s i t i o n s a r e r u l e d out by the S u r v e y o r V and VI data: c h o n d r i t e s , u l t r a b a s i c r o c k s , g r a n i t e s and t e k t i t e s . G i l v a r r y (1968) has r e c e n t l y p o s t u l a t e d that the m a r i a b a s i n s a r e f i l l e d with s e d i m e n t a r y m a t e r i a l , equivalent in c o m p o s i t i o n to t e r r e s t r i a l m u d s t o n e or shale. His c o n c l u s i o n is b a s e d l a r g e l y on the p r e s e n c e of c a r b o n (< 3%), but it should be noted that the f i g u r e by T u r k e v i c h et al. (1967) is an u p p e r l i m i t , and that no p o s i t i v e i d e n t i f i c a t i o n of the e l e m e n t was in fact r e p o r t e d . The heavy e l e m e n t s a r e too low in abundance in the s h a l e s c o m p a r e d with the m a r i a data. G i l v a r r y (1968) a t t r i b u t e s the e x c e s s i r o n to m e t e o r i t i c infall. However, the content of m e t a l l i c i r o n on the m a r i a s u r f a c e is l e s s than one p e r cent as shown by the S u r v e y o r V m a g n e t i c e x p e r i m e n t (De Wys, 1967) and t h e r e is a c c o r d i n g l y insufficient m e t a l l i c i r o n to account f o r the d i s c r e p a n c y . A s e c o n d objection, is that heavy e l e m e n t c o n t a m i n a t i o n f r o m m e t e o r i t i c s o u r c e s , m e t a l l i c o r not, should o c c u r both in m a r i a and uplands. The o l d e r , m o r e h e a v i l y c r a t e r e d uplands should be e n r i c h e d , but a r e d e f i c i e n t in t h e s e e l e m e n t s ( T u r k e v i c h et al., 1968a,c). Thus the i d e n t i f i c a t i o n of the m a r i a m a t e r i a l as r e s e m b l i n g t e r r e s t r i a l s h a l e s is doubtful. In any event, the p r o p o s e d c o m p o s i t i o n is f a r r e m o v e d f r o m that of t e k t i t e s . TYCHO S u r v e y o r VII landed on throwout f r o m Tycho, in the h e a v i l y c r a t e r e d upland region. The c o m p o s i t i o n was c l o s e to that of the m a r i a s a m p l e s , f o r the l i g h t e r e l e m e n t s . The only d i f f e r e n c e which a p p e a r s well e s t a b l i s h e d is "the l o w e r abundance of e l e m e n t s with m a s s e s between 47 and 65 (including the e l e m e n t s Cr, Mn, F e , Co, and Ni - T u r k e v i c h et al., 1968a,c). How r e p r e s e n t a t i v e of the u p l a n d s is t h i s outhrown m a t e r i a l f r o m Tycho ? Tycho is 54 m i l e s (87 km) in d i a m e t e r and 14,000 ft. (4.3 km) deep. Infall will have r e d u c e d the o r i g i n a l depth. The o u t - t h r o w n a p r o n on which S u r v e y o r VII is r e s t i n g is p r o b a b l y d e r i v e d f r o m depths of a few k i l o m e t e r s b e n e a t h the i m p a c t s u r f a c e . The o c c u r r e n c e of what a p p e a r t o b e d i s c r e t e c r y s t a l s p h o t o g r a p h e d by S u r v e y o r VII is c o n t r i b u t o r y evidence f o r a s u b s u r f a c e " p l u t o n i c " o r i g i n of the m a t e r i a l . Is t h i s e x c a v a t e d m a t e r i a l d i f f e r e n t f r o m that c o m p r i s i n g the s u r f a c e in the uplands, that i s , do the uplands have a l a y e r e d s t r u c t u r e ? Tycho is a r e c e n t c r a t e r with a m i n i m u m age of 106 y e a r s . A good i d e a of the p r e i m p a c t s t a t e of the l u n a r s u r f a c e at the s i t e of Tycho may be obtained by studying the uplands a r e a o u t s i d e the throwout a p r o n of d e b r i s f r o m Tycho. This s u r f a c e is c o v e r e d with l a r g e c r a t e r s , s u p e r i m p o s e d on o l d e r c r a t e r s . The s u r f a c e has been s u b j e c t to the heavy c r a t e r i n g t y p i c a l of the uplands. The d i a m e t e r s of the l a r g e r c r a t e r s a r e equivalent to o r g r e a t e r than Tycho. The upland s u r f a c e has thus been dug o v e r and m i x e d by t h i s p r o c e s s . Any o r i g i n a l l a y e r i n g has been d e s t r o y e d and the s u r f a c e m a t e r i a l to a depth of a few k i l o m e t e r s has been t h o r o u g h l y s t i r r e d by the c r a t e r i n g . It is a c c o r d i n g l y r e a s o n a b l e to suppose that the S u r v e y o r VII a n a l y s i s of the s u r f a c e is 452
Chem. Geol., 4 (1969) 451-459
r e p r e s e n t a t i v e . It should be r e c a l l e d that both fine grained material, and blocks, gave s i m i l a r a n a l y s e s at this site. In s u m m a r y , m a t e r i a l of tektite composition does not exist in the m a r i a , if the Surveyor V and VI data a r e at all representative. If the S u r v e y o r VII samples a r e r e p r e s e n t a t i v e of the uplands, as argued above, then these a r e a s likewise do not contain m a t e r i a l of tektite composition. Thus the Luna-10 and Surveyor data have gravely weakened the case f o r a lunar origin of tektites. At present, it is impossible to exclude the existence of small a r e a s of highly siliceous r o c k s , although t h e s e could be detected by a refinement of the Luna-10 experiment. It may be r e c a l l e d (Lowman, 1962) that a m a j o r r e a s o n advanced for p r e f e r r i n g a lunar to a t e r r e s t r i a l origin of tektites was t h e i r r a t h e r uniform and homogeneous composition, thought to be m o r e readily obtainable on the moon in c o n t r a s t to the well-known heterogeneity of the e a r t h ' s surface. Suitable parent m a t e r i a l for tektites will have an R b / S r ratio of about 0.5, and hence will generate high 87Sr/86Sr ratios in short periods of time. The m a t e r i a l c o m p r i s i n g the uplands region is s t r a t i g r a p h i c a l l y older than the m a r i a material. The youngest estimates for the age of the m a r i a a r e about 500-106 y e a r s but they may be up to 3,600.106 y e a r s old (Hartmann, 1965). P r e - m a r i a m a t e r i a l of tektite c o m position will accordingly be labelled with high 87Sr/86Sr ratios, in c o n t r a s t to those observed. The origin of tektites f r o m m o r e r e m o t e m e m b e r s of the s o l a r s y s t e m , or f r o m outside the s o l a r s y s t e m have been refuted by, inter alia, the absence of any evidence of exposure to c o s m i c radiation outside the e a r t h ' s a t m o s p h e r e (Viste and Anders, 1962).
IMPACT ORIGINOF TEKTITES The ultimate origin of tektites due to splash of melted material during impact of a comet, asteroid or large meteorite appears to be accepted by nearly all workers in this otherwise controversial field. The most recent of the tektite events was the formation of the east Asian and Australian strewn field. The very similar chemistry and similarity of K-A ages constitute reasonable grounds for grouping these scattered occurrences as due to one impact event. The case for a terrestrial origin of these tektites has been based largely on geochemical evidence. The chemical composition of anstralites bears a close resemblance to common terrestrial sandstones of sub-greywacke type (Taylor, 1962, 1966; Taylor and Sachs, 1964). Studies of terrestrial impact glass have contributed to our understanding of the chemistry of tektites. The Henbury impact glass is particularly informative (Taylor and Kolbe, 1964,1965; Taylor, 1966). It has been formed by the impact of a medium octahedrite iron meteorite on subgreywacke sandstone, and has a composition very close to that of australites. Apart from loss of volatiles, there is little change in composition between the parental sedimentary rock and the glass. This event provides a small-scale analogy for the tektite producing events. The pt conditions of silicate glass formation will be similar: only the scale of the explosion is different. From the evidence derived from the impact glass, and Chem. Geol., 4 (1969) 451-459
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f r o m i n t e r n a l c o m p o s i t i o n a l e v i d e n c e d e r i v e d f r o m the r a t i o s of v o l a t i l e and n o n - v o l a t i l e e l e m e n t s ( T a y l o r , 1962, 1966) it is c o n c l u d e d that changes in c o m p o s i t i o n of the p a r e n t m a t e r i a l d u r i n g g l a s s f o r m a t i o n (excluding l o s s of H20, CO2 etc.) w e r e s l i g h t and that the p r e s e n t c o m p o s i t i o n of a u s t r a l i t e s is c l o s e to that of the p a r e n t m a t e r i a l , which a c c o r d i n g l y r e s e m b l e s a t e r r e s t r i a l s a n d s t o n e of s u b g r e y w a c k e type. The g e o c h e m i c a l d i f f i c u l t i e s of p r o d u c i n g m a t e r i a l of this c o m p o s i t i o n on the l u n a r s u r f a c e led the author and c o w o r k e r s ( T a y l o r and Sachs, 1960, 1964; T a y l o r 1962, 1966) to p r o p o s e a t e r r e s t r i a l o r i g i n f o r t e k t i t e s .
DISTRIBUTION OF AUSTRALIAN AND EAST ASIAN TEKTITES No a c c e p t e d i d e n t i f i c a t i o n of a t e r r e s t r i a l s o u r c e a r e a f o r this group of t e k t i t e s has b e e n p r o p o s e d and e f f o r t s have b e e n c o n c e n t r a t e d on s e a r c h i n g f o r Canyon Diablo type i m p a c t c r a t e r s of v e r y l a r g e size. A wide v a r i e t y of s i t e s have been s u g g e s t e d (e~g., A n t a r c t i c a - Schmidt, 1962). In the a b s e n c e of any r e a l knowledge of the t e k t i t e p r o d u c i n g event, it is useful to begin t h i s e n q u i r y by looking at the e v i d e n c e which has to be explained. Let us c o n s i d e r the d i s t r i b u t i o n of the A u s t r a l i a n and e a s t A s i a n t e k t i t e s , a s s u m i n g that they a r e the r e s u l t of one event. The f r e q u e n c y of o c c u r r e n c e v a r i e s w i d e l y but t h e r e i s a g e n e r a l tendency f o r the abundance (and m a s s e s of i n d i v i d u a l t e k t i t e s ) to i n c r e a s e in a n o r t h w e s t e r l y d i r e c t i o n a c r o s s the s t r e w n field. The total n u m b e r of a u s t r a l i t e s c o l l e c t e d is between 50,000 and 100,000 and this n u m b e r s e r v e s as an index of the total f r e q u e n c y of a u s t r a l i t e s . In c o n t r a s t to the n u m b e r , B e y e r (1943) c o l l e c t e d 500,000 p h i l i p p i n i t e s in the v i c i n i t y of the city of Manila. F r o m s o u t h e a s t to n o r t h w e s t a c r o s s the A u s t r a l i a n - - e a s t A s i a n s t r e w n f i e l d , the m a s s of the individual t e k t i t e s , as well as t h e i r n u m b e r a p p e a r s to i n c r e a s e . Most d a t a e x i s t f o r the a u s t r a l i t e s , but a t t e m p t s to e s t i m a t e ' t h e a v e r a g e weight of t h i s w e l l d o c u m e n t e d group have p r o v e d difficult. B a k e r and F o r s t e r (1943) gave a value of 1.5 g. A v e r a g e s of v a r i o u s s m a l l e r c o l l e c t i o n s l i e in the r a n g e 1-9 g. A l l o w a n c e has to be m a d e f o r l o s s by a e r o d y n a m i c a b l a t i o n , p a r t i c u l a r l y m a r k e d f o r the a u s t r a l i t e s . It s e e m s safe to conclude that the m o s t f r e quent weight is in the r a n g e 1-10 g, and p r o b a b l y between 1 and 5 g. The weights of the h e a v i e s t s p e c i m e n s of a l l g r o u p s p r o v i d e one of the b e t t e r s e t s of data a v a i l a b l e f o r t h i s enquiry. T h e s e l a r g e s a m p l e s a r e d i s t i n c t i v e , e x c i t e the c u r i o s i t y of the f i n d e r and a r e l i k e l y to find t h e i r way into m u s e u m c o l l e c t i o n s . B e c a u s e of a b l a t i o n l o s s , etching, and difficulty of finding the s a m p l e s , the l o w e s t r e c o r d e d weights a r e not s i g n i f i c a n t , in the p r e s e n t context. The l a r g e s t a u s t r a l i t e w e i g h s 243 g (McCall, 1965) and l e s s than ten weighing m o r e than 100 g have been c o l l e c t e d . In c o n t r a s t , m o r e than 100 p h i l i p p i n i t e s in the r a n g e 200-700 g have been r e c o r d e d , and the l a r g e s t s p e c i m e n weighs 1,070 g. V e r y l a r g e n u m b e r s of t e k t i t e s have b e e n c o l l e c t e d f r o m C a m b o d i a , L a o s and Vietnam. The l a r g e s t r e c o r d e d s p e c i m e n weighs 3,200 g and v e r y l a r g e a m o u n t s of Muong-Nong type m a t e r i a l have been c o l l e c t e d . "In an a r e a of about 100 m2, 67.5 kg of t e k t i t e s w e r e r e c o v e r e d : " ( B a r n e s and P i t a k p a i v a n , 1962.) T h i s m a s s is within an o r d e r of magnitude of the total m a s s of a u s t r a l i t e s c o l l e c t e d o v e r the A u s t r a l i a n s t r e w n field. 454
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T h i s i n f o r m a t i o n , although sketchy, i n d i c a t e s that both n u m b e r s and m a s s of t e k t i t e s i n c r e a s e t o w a r d the n o r t h w e s t c o r n e r of the c o m b i n e d A u s t r a l i a n - e a s t A s i a n s t r e w n - f i e l d s . T h e r e a p p e a r s to be much l o c a l v a r i a t i o n in d i s t r i b u t i o n s u g g e s t i v e of the a r r i v a l of c l u s t e r s of t e k t i t e s a c r o s s the s t r e w n field. In s u m m a r y , the d i s t r i b u t i o n of t e k t i t e s a c r o s s the s t r e w n f i e l d i n d i c a t e s that the l a r g e s t chunks (Muong-Nong type) the g r e a t e s t n u m b e r and the h e a v i e s t t e k t i t e s o c c u r in the n o r t h w e s t , and the s m a l l e s t and l i g h t e s t f o r m s , which a l s o d i s p l a y the b e s t e v i d e n c e of a e r o d y n a m i c a b l a t i o n . a r e s p a r s e l y d i s t r i b u t e d o v e r the s o u t h e r n half of A u s t r a l i a .
SURFACE EFFECTS OF A TERRESTRIALTEKTITE PRODUCINGEVENT The above m a t e r i a l c o m p r i s e s the total i d e n t i f i e d t e r r e s t r i a l m a t e r i a l f o r m e d d u r i n g the t e k t i t e p r o d u c i n g event. The r e m a i n i n g p a r t of this p a p e r will be devoted to a d i s c u s s i o n of the p o s s i b l e s u r f a c e e f f e c t s of a t e r r e s t r i a l t e k t i t e p r o d u c i n g event. No l a r g e c r a t e r of the Canyon Diablo type has been i d e n t i f i e d a s an a c c e p t e d s o u r c e , although s u g g e s t i o n s of such s i t e s in A n t a r t i c a (Schmidt, 1962) and o t h e r p l a c e s have been made. Lin (1966) has i n v e s t i g a t e d the p o s s i b i l i t y of c o m e t a r y i m p a c t . He c a l c u l a t e s , on the b a s i s of e x t r a p o l a t i o n f r o m the A r i z o n a (Canyon Diablo) c r a t e r , that a s i m p l e s c a l i n g f o r an e x p l o s i o n e n e r g y of 2.1030 e r g s , r e q u i r e d to r e m o v e an a t m o s p h e r i c s e g m e n t , would give a c r a t e r about 300 km in d i a m e t e r and 40 k m in depth. No such c r a t e r has been o b s e r v e d t e r r e s t r i a l l y . L u n a r c r a t e r s as d i s t i n c t f r o m m a r i a , r e a c h 294 k m (Bailly) in d i a m e t e r and a depth of 6 km (Newton). The a p p a r e n t a b s e n c e of such c r a t e r s on the e a r t h , i n d i c a t e s that s i m p l e s c a l i n g f r o m Canyon Diablo type c r a t e r s is not c o r r e c t , if t e k t i t e s a r e t e r r e s t r i a l . The i m p a c t e n e r g y at Canyon Diablo has been e s t i m a t e d by S h o e m a k e r (1963) at 6.8"1022 e r g s (1.7 m e g a t o n T.N.T. equivalent) on the b a s i s of s c a l i n g f r o m the T e a p o t ESS c r a t e r (1.2 ktons). Teapot ESS c r a t e r (300 ft. d i a m e t e r × 100 ft. deep) was p r o d u c e d by the e x p l o s i o n of 1.2 k t o n s 67 ft. below the s u r f a c e . Canyon Diablo (4,000 ft. d i a m e t e r × 570 ft. deep) w a s p r o d u c e d at an e s t i m a t e d depth of 300-400 ft. b e n e a t h the o r i g i n a l s u r f a c e ( S h o e m a k e r , 1963). Further data from large explosions are available from the hydrogen bomb tests, Castle and Ivy Mike (Vaille, 1961; Vortman, 1968). Both devices were approximately 15 megaton T.N.T. equivalent (6" I 023 ergs). The Castle explosion, 7 ft. above the surface produced a crater 3,000 ft. in diameter and 240 ft. deep. In the Ivy Mike experiment, the device was detonated 35 ft. above the surface. A much shallower crater (160 ft. deep) of the same diameter (3,000 ft.) resulted. Thus the height of the explosion is critical for cratering depth. The extreme example of a highly energetic event in the atmosphere is the Tunguska event (60°54' N, 101056' E, June 30, 1908) and a short description of this is instructive (Fesenkov, 1961; Robey, 1963; Krinov, 1966). The height of the explosion was 5 km above the surface. Severe charring of trees occurred in a central area (radius 1.5 km). Scorching occurred up to 17-20 km from the centre. The most spectacular effect was the blowing down of the forest surrounding ground zero. An estimated 80 million trees with trunk diameters greater than 8 inches (20 cm) were Chem. Geol., 4 (1969) 451-459
455
f l a t t e n e d r a d i a l l y f r o m ground z e r o . The r a d i u s of the f e l l e d t r e e s a r e a is 35 km C21 m i l e s ) . P a r t l y d a m a g e d t r e e s a r e found up to 60 k m (37 m i l e s ) f r o m the e x p l o s i o n c e n t r e . A s m a l l a r e a of t r e e s r e m a i n e d standing in the centre. A i r p r e s s u r e waves w e r e r e c o r d e d in England. The e n e r g y r a d i a t e d as v i s i b l e light is e s t i m a t e d at 2.8"1023 e r g s . The total e n e r g y of the object in the a t m o s p h e r e j u s t p r i o r to e x p l o s i o n is e s t i m a t e d at 8.1024 e r g s C200 m e g a t o n s T.N.T. equivalent) CRobey, 1963). The only s u r f a c e effect was the f e l l i n g of the f o r e s t and c h a r r i n g of the t r e e s . No c r a t e r w a s f o r m e d . Many s u g g e s t i o n s have b e e n m a d e of the s o u r c e of t h i s e x c e e d i n g l y e n e r g e t i c event, f r o m p r i m i t i v e n u c l e a r e x p e r i m e n t s to the e n c o u n t e r of the e a r t h with a n t i - m a t t e r (Cowan et al., 1965). P r e d i c t e d i n c r e a s e in 14C a c t i v i t y which would r e s u l t f r o m an a n t i - m a t t e r e x p l o s i o n have not been c o n f i r m e d ( L e r m a n et a l . , 1967) and indeed the p r e s e n c e of a n t i - m a t t e r within the l o c a l group of g a l a x i e s , let alone the s o l a r s y s t e m , a p p e a r s a r e m o t e p o s s i b i l i t y . The c o n s e n s u s is that a s m a l l c o m e t head of m a s s 1012-1013g and of low d e n s i t y c o l l i d e d with the e a r t h and e x p l o d e d at a height of 5-6 km /Krinov, 1966). The s i g n i f i c a n c e of the Tunguska event in the p r e s e n t context i s that a 200 megaton explosion at 5 k m a l t i t u d e p r o d u c e d s u r f a c e e f f e c t s which a r e r a p i d l y d i s a p p e a r i n g 60 y e a r s a f t e r the event. In c o n t r a s t , the e v i d e n c e f r o m an i r o n m e t e o r i t e i m p a c t (of much l o w e r e n e r g y : 1-2 m e g a t o n s ) at a depth of 300-400 ft. at Canyon D i a b l o will r e m a i n until the e r o s i o n has l o w e r e d the s u r f a c e l e v e l by about 1,000 ft. At the p r e s e n t e s t i m a t e d r a t e of l o w e r i n g the s u r f a c e by 2-3 i n c h e s p e r 1,000 y e a r s in u p l i f t e d r e g i o n s (Schumm, 1963; Judson and R i t t e r , 1964) d e t e c t a b l e e v i dence Cfrom o u r p r e s e n t u n d e r s t a n d i n g of c r a t e r s t r u c t u r e ) will r e m a i n f o r at l e a s t 400,000-600,000 y e a r s . It m a y thus be concluded that the s u r v i v a l of evidence of high e n e r g y i m p a c t e v e n t s on the e a r t h ' s s u r f a c e depends c r i t i c a l l y on the height of the e x p l o s i o n , and the d e n s i t y of the i m p a c t i n g object. A t e r r e s t r i a l o r i g i n of t e k t i t e s f r o m an c o m e t a r y i m p a c t s i m i l a r to, but much m o r e e n e r g e t i c than, the Tunguska event has long been a d v o c a t e d by U r e y (1957, 1963). Lin /1966) e x p l o r e d s o m e of the c o n s e q u e n c e s of c o m e t a r y i m p a c t and a t m o s p h e r i c r e m o v a l , using the " d u s t y s n o w b a l l " c o m e t m o d e l of Whipple (1963). The e n e r g y n e c e s s a r y to r e m o v e a s l i c e of the a t m o s p h e r e above the h o r i z o n t a l tangent plane, is 2"103° e r g s . A s s u m i n g an e n e r g y c o n v e r s i o n of 50%, a c o m e t nucleus with d e n s i t y 0.1 g / c m 3, a m a s s of 5"1017 g, with a r a d i u s of 10 k m and an i m p a c t v e l o c i t y of 42 k m / s e c would p r o v i d e the r e q u i r e d amount of e n e r g y (Lin, 1966). T h i s e n e r g y r e q u i r e m e n t can be r e d u c e d . The a b s e n c e of a r a d i a l d i s t r i b u t i o n of t e k t i t e s in the A u s t r a l i a n and A s i a n s t r e w n - f i e l d s i s c o n s i s t e n t with an o r i g i n f r o m s e v e r a l j e t s or s p l a s h e s f r o m the e x p l o s i o n in one d i r e c t i o n . The s t r e w n - f i e l d s can be contained in a s e c t o r of a p p r o x i m a t e l y 15 °. The glancing i m p a c t of a comet nucleus of 10 km d i a m e t e r at 40 k m / s e c can a l s o p r o v i d e e x t r a e n e r g y f o r a t m o s p h e r i c r e m o v a l in the d i r e c t i o n of motion. Although it is difficult to m a k e meaningful c a l c u l a t i o n s , it is p r o b a b l e that the e n e r g y r e q u i r e m e n t to account f o r the total s t r e w n - f i e l d is l e s s than the value of 2 • 1030 e r g s and is p o s s i b l y as low as 1028 e r g s . The l o w e r the d e n s i t y of the i m p a c t i n g o b j e c t , the s h a l l o w e r will be the c r a t e r . Supporting e v i d e n c e f o r low m a s s o r d e n s i t y in the i m p a c t i n g 456
Chem. Geol., 4 (1969} 451-459
body is provided by the very low content of metallic iron spherules found in tektites (Chao et al,, 1962) c o m p a r e d to t e r r e s t r i a l impact g l a s s e s resulting f r o m the impact of iron m e t e o r i t e s . One of the intriguing f e a t u r e s of tektite c h e m i s t r y is the virtual lack of evidence of contamination f r o m the impacting body. Their c h e m i s t r y r e s e m b l e s that of fused sandstones of subgreywacke type without significant selective loss or addition of elements. Identification of m a t e r i a l derived f r o m c o m e t s will be difficult, b e c a u s e of its predicted scarcity. Chemical energy is probably r e l e a s e d in c o m e t a r y i m p a c t s in addition to the kinetic energy r e l e a s e , b e c a u s e of the p r e s e n c e of free r a d i c a l s in comets. A combination of a highly energetic explosion, and a low density body is unlikely to leave much residual m a t e r i a l to identify the impacting object. In addition to the s p e c t r a of CO +, CH, CN, C2, C3, OH, NH 2 etc., r e c o r d e d f r o m c o m e t s the p r e s e n c e of nonvolatile constituents has been established. E m i s s i o n s p e c t r a of Fe, Ni, Cu, Cr, Mn, Ca, Na and K were r e c o r d e d f r o m the comet lkeya-Seki (Preston, 1967). Thus some slight iron and nickel contamination may be expected f r o m a c o m e t a r y impact, which could account for the metallic spherules o b s e r v e d by Chao et al. (1962). Metallic and silicate spherules have been collected at Tunguska, but their identification with the explosion has yet to be proved (Krinov, 1966). Comets a r e plentiful (about 600 per century a r e observed f r o m the earth), but iron m e t e o r i t e s of sufficient size to f o r m large c r a t e r s may be much r a r e r than previously suspected. The low content ( < 1% ) of metallic iron on the lunar m a r i a s u r f a c e (De Wys, 1967), and the low total iron content of the upland s u r f a c e (Turkevich et al., 1968) supports this view. Sun (1968) has proposed that the s t r u c t u r e of the lunar c r a t e r , Copernicus, is consistent with derivation f r o m an explosion at the surface, and he postulates a c o m e t a r y impact as a feasible m e c h a n i s m for this event.
HYPOTHESIS FOR THE ORIGIN OF AUSTRALIAN AND EAST ASIAN TEKTITES
The model proposed here to account f o r the a u s t r a l i t e s and the east Asian tektites follows the suggestions of Urey (1957,1963). A comet collides with the earth and explodes in the a t m o s p h e r e . A thin s u r f a c e l a y e r of s u b - g r e y w a c k e composition is melted, in this s u p e r - T u n g u s k a event, and splashed outwards in s e v e r a l jets. The explosion was of sufficient energy to r e m o v e a segment of the a t m o s p h e r e , permitting escape of the melted material. The solidified m a t e r i a l r e - e n t e r s the atmosphere, when ablation and flange formation occurs. The s u r f a c e effect of the explosion could be limited to a shallow depth o v e r an a r e a of s e v e r a l square k i l o m e t e r s . Such an impact site would be v e r y difficult to detect, except perhaps in d e s e r t a r e a s , where it might be enlarged due to wind export of dust. The following c r i t e r i a a r e p r o p o s e d to aid the s e a r c h for the impact site: (1) A shallow c r a t e r of large d i a m e t e r ( > 10 km). (2) The rock type is sandstone of s u b - g r e y w a c k e type, close to the present chemical composition of the a u s t r a l i t e s . Some variation in c o m p o s i tion is postulated to provide for the v a r i a t i o n s in tektite composition.
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(3) T h e a g e of t h e s o u r c e a r e a s h o u l d b e i n t h e r a n g e 2 0 0 - 4 0 0 . 1 0 6 y e a r s (4) T h e c r a t e r i s r e m o t e f r o m t h e A u s t r a l i a n s t r e w n - f i e l d , t o a c c o u n t f o r t h e e v i d e n c e of i n f a l l a t v e l o c i t i e s b e t w e e n 6 a n d 12 k m / s e c . ( C h a p m a n , 1964.)
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