On the original rock source of tektites

ON THE

ORIGINAL

ROCK

SOURCE

OF TEKTITES

V L A D I M JR B O U ~ K A

IIOUgKA, V. 1968: On the original rock source of tektites. Litkos I, 102-112. On the K6hler & I(aaz diagram for rock analyses all projection points of tektites and impact glasses, except for the impact glass from Ries, lie inside the field of sedimentogenic rocks. (The Ries glass is just inside the boundar 3" of magmatogenic rocks). This indicates that the majority of tektites derive from terrestrial sedimentogenic rocks. Possible exceptions are the Australian-Asian tektites, which may have originated from weathered rocks of granodioritic to granitic composition. In the author's op!nion these are also the only tektites which might possibly have originated oft the Moon.

Introduction The impact theory of tektites' origin as formulated by Barnes (1961), Cohen (1962) and Chapman & Larson (1963) has been supported by various authors and is now generally accepted. Considerable controversy still exists, however, first as to where such impacts took place, and secondly as to the type of materials involved. Astronomically speaking, tektites must be of fairly local origin, since they show no signs of having been subjected to prolonged cosmic irradiation outside the protection of the Earth's atmosphere (Anders 1960, Fleischer et al. 1965, Chapman & Larson 1963). This does not eliminate, however, the possibility of formation on the Moon. Verbeek (1897) was the first to suggest the Moon as a source of tektites, but it was not until 1963 (Chapman & Larson) that a lunar impact theo, y of tektites generation was first proposed. The possibility that a tektite 'shower' followed a straight trajectory from the Moon has been discussed in detail by Chapman (1964) who postulates that the focusing effect of the Earth's gravitational field would minimize the dispersion of tektites during flight; nevertheless the degree of dispersion to be expected is still much greater than that observed i n most tektite fields. The only exception are Australian-Asian tektites, which apparently originated from one event, and could be of lunar origin. Signs of undoubted two periods of melting have been reported in australites (Baker 1963) and are taken to be indicative of reheating during a flight through the Earth's atmosphere. Since it is unlikely that such small particles could have formed on the Earth's surface and been hurled above the atmosphere for re-entry (Taylor & Sachs 1964), the argument in favour of a lunar origin for australian tektites would appear to be a strong one.

ROCK SOURCEOF TEKTITES

103

The geochemistry of tektites has been studied in great detail (Schnetzler & I'inson 1963, 1964; Chao 1963; Philpotts & Pinson 1966; Z~ihringer 1963; Taylor & Sachs 1964; Taylor 1965; Taylor & Kolbe 1965; Miesch et al. 1966 etc.) and it has been clearly established that not only the macro-compositions but also the trace elements and isotopic compositions of tektites are very similar to those of common terrestrial rocks. The majority of authors assume, therefore, that contamination from the impact body and selective. volatilization are only very minor factors in tektite formation, if in fact they occur at all. Walter (1966), however, performed a laboratory experiment to test the validity of theories of selective volatilization. He annealed silicate glass of approximate tektite composition (type Muong Nong) at 2800~ for up to half an hour and showed that there was practically no change in the alkali content but a decrease in silica and considerable increases in Alz03, FeO, CaO and MgO. If this type of selective volatilization is common during tektite formation, then the parent material of tektites must be very silica rich. The fact that the majority of impact glasses are chemically very similar to tektites, although never identical, again suggests that no major fractionation occurs during flight. The assumption that the chemical composition of tektites is very similar to that of the material subjected to impact, narrows the field of possible parent materials among known rocks to sediments or acid igneous rocks. Tektites were originally assumed to originate from acid igneous rocks; but Barnes (1940), on the basis of chemical criteria, ascribed.moldavites and the Ivory Coast tektites to melting of sediments whilst he considered AustralianAsian tektites to be of igneous rock origin. Pinson & Schnetzler (1962) studied the Rb/Sr ratios, the isotopic composition of Sr, a n d the macrocomposition of bediasites and concluded that they too were of sedimentary origin; but Cuttitta et al. (1962) assumed that they were derived from rhyolitic rocks. Lovering (1960), when postulating a lunar origin for tektites, suggested that there could be granophyric rocks on the surface of the Moon. Taylor (1965) compared Henbury impact glass, Darwin glass, and australites with silica rich sedimentary rocks. Greenland & Lovering (1965), on the basis of MnO/FeO ratios, concluded that moldavites formed from sediments similar to deep-sea clays, bediasites from sandstones, and Pacific tektites from acid granophyres. Here it should be pointed out, however, that reduction of trivalent to bivalent iron as a result of high impact temperatures could alter the original MnO/FeO ratios. Schnetzler & Pinson (1964) think that the high Ca and Sr contents of philippinites imply contamination of the original material by limestone. U and Th contents in tektites lie, according to Adams et al. (1959), between andesite and dacite when compared with igneous rocks, while in comparison with sediments they are in the range shaly sand or shale composition. Heide (1961) compared the U contents of moldavites,, indochinites and

104

v. BOUSKA

some obsidians with that of the shaly sandstone from Sch6ps, S of Jena (73.25% SiO2) and showed that melting of this sandstone (at 1750~ gave a glass similar to moldavites in colour. On the basis of the U content of tektites, Heide considers them to be of terrestrial origin. Taylor & Sach(1964) showed that the rare earth content of tektites is very similar to that of sediments and differs from that of granites. In their paper, there appears the closest resemblance to siliceous shales. More references and detailed discussions concerning the source material of tektites is given in Schnetzler & Pinson (1963) and O'Keefe (1966). The age relationships determined partly by K-Ar method, partly by the fission tract method, of some impact glasses and individual tektite groups, support the impact theory, and strengthen the argument for terrestrial origi n (Gentner & Z~ihringer 1960; Ztihringer 1963; Gentner, Lippolt & Schaeffer 1963; Fleischer & Price 1964; Gentner, Lippolt & Mfiller 1964; Fleischer, Price.& Walker 1965). An age established by the Rb/Sr method refers to the time of formation of the original rocks, and this is apparently the same for the three main tektite areas - the American, the Australian-Asian and the Czechoslovak, between 300-400 rail. years. Faul (1966) noted that this age is in good agreement with the age of the crystalline complex north of the Alps which also forms the base of the Ries crater, but it should be pointed out that the Rb/Sr method would also probably give the same age to the substantially younger sediments of this area which have undoubtedly developed by disintegration of the same crystalline complex.

T h e plotting m e t h o d In 1951, K6hler and Raaz proposed a new graphical presentation of the chemical composition of rocks. They used a modified norm calculation as the basis of a plot in terms of normative quartz (qz), feldspar(F)and ferro-magnesian minerals (fin). A plot of this type is shown in Fig. 1 and detailed information concerning the data plotted is supplied in the text accompanying this figure. This plotting method has several advantages: First, it distinguishes between sedimentogenic rocks (sediments proper and parametamorphites) and igneous rocks and orthometamorphites (strongly dashed boundary); secondly, individual rock series can be distinguished from one another (i.e. curve A arid curve B); thirdly, it is sensitive to the chemical changes induced by weathering and other processes of alteration (as indicated by the separation of the two points k, k', etc.). For the sake of simplicity, Fig. 1 displays few data on terrestrial rocks with compositions unlike those of tektites and impact glasses. On the other hand, for the sake of completeness, data for meteorites have been included. These are all shown to fall within the field of basic and ultrabasic igneous rocks (i, i'") being chondrites nearer to the iron-stone meteorites while achondrites

ROCK SOURCE OF TEKTITES

105

stand apart. A major group of meteorites not shown on the projection are the irons which, since they are not silicate rocks, plot outside the boundaries of this projection at the point qz-lO0, (F-fm)-lO0.

Results All plots of tektites lie in the field of sedimentogenic rocks, and even the dispersion range of moldavites which show considerable variation in chemical composition does not cross the boundary between para- and ortho- rocks. The same is true of australites, whose dispersion field has not been shown for the sake of simplicity, but falls within the field of average tektite values and is delimited by the following values: Sample 31 (qz 69.8; F - f m = + 1.8); sample 25 (qz 64.8; F - f m = - 5 . 4 ) and sample 39 (qz 76.7; F - f i n = - 2 . 7 ) . (Data from the paper of Taylor & Sachs 1964). The average chemical composition of all tektites lies almost exactly on curve E (which indicates the highest concentration of plots of sedimentogenic rocks). In the author's opinio/I this is very strong evidence in favour of a sedimentogenic source material for tektites. Bohemian moldavites contain, on average, more silica than Moravian moldavites, as shown by the projections of their ave?age values. The first to draw attention to the fact was Cohen (1963), who suggested that there was a gradual decrease of silica in moldavites from W to E. Such a regular change has not, however, been confirmed by later study (Bou~ka & Povondra 1964). Statistical investigation of the moldavite colours (Faul & Bou~ka 1963) nevertheless provides confirmatory evidence of differences in the average composition of the two regions; olive green and brown moldavites predominate in Moravia in contrast to bottle green and light (pale) green moldavites in Bohemia. This chemical difference is, however, small and can be explained in two ways: If it is a s s u m e d that all moldavites originated f r o m the Ries crater, t h e n t h e material w h i c h has travelled f u r t h e s t contains least silica. T h i s could be d u e to selective volatilization ( d u r i n g passage t h r o u g h t h e a t m o s p h e r e ) of the type described by Walter (1966); a steady decrease in silica a n d a relative e n r i c h m e n t of t h e r e m a i n i n g c o m p o n e n t s , plus eventually partial oxidation of iron. S u c h a m e c h a n i s m does n o t explain t h e lack of c o n t i n u o u s transition ; or t h e fact that moldavites of all possible colour s h a d e s can be f o u n d side by side at one occurrence. M o r e probable is t h e second hypothesis w h i c h s u g g e s t s that moldavites were p r o d u c e d from an i n h o m o g e n e o u s source material. If pieces of moldavite s u b s t a n c e of a p p r o x i m a t e l y t h e s a m e size are h u r l e d by an equal force from t h e place of impact, t h e n those p o s s e s s i n g an even slightly h i g h e r d e n s i t y should, if t h e v o l u m e s are t h e same, fly farther. T h e fact that the heaviest (largest) moldavites are f o u n d in Moravia, s u p p o r t s this h y p o t h e s i s , as do t h e finds of two-coloured m o l d a v i t e s (Bougka 1965). T h e m o s t acid moldavites are generally f o u n d in t h e w e s t e r n part of t h e B o h e m i a n area (e.g. Radomilice), b u t exceptions occur, s u c h as Ti:ebanice, w h e r e there are an equal n u m b e r of olive green and]or b r o w n - c o l o u r e d pieces. T h i s m a y be explained b y a collision of t h e flying pieces. S u c h a collision m a y h a v e showei'ed d o w n t h e b r o w n pieces a n d caused t h e m to fall sooner t h a n they otherwise w o u l d have done. Naturally, a m a j o r factor is also t h e size of t h e ejected bodies and t h e possibility o f splintering (which is often e n c o u n t e r e d in meteorites). T h e second h y p o t h e s i s in n o way excludes the possibility that selective volatilization was a c o n t r i b u t i n g factor. 8 - - Litos 1:2

106

v. BOU,~KA 9 Ir-

/

f,.)

7""

./

~ da,

"%%. I Pacit'lctack8 i" rnoldavt/os s' s-

~

C.t~tttea l " ultraba*it~s

"(F-fro]

Fig.l. Graphic representation of the chemical composition of rocks, tektites, impact glasses, and meteorites according to the diagram of A. K/Shier & F. Raaz (1951): 9 Curve A - Peralkalic igneous rocks of the Atlantic series; constructed on the basis of the data of KShler & Raaz (1951) and of recalculated analyses from Ilejtman's book (1957). Curve B - Syenlte-monzonite series; constructed according to the data Of K/Shler & Raaz (1951) and of recalculated analyses given by Hejtman (1957). Curve C - Gabbro-granite rocks of the Pacific series; the main dispersion field is denoted by cross hatching; constructed on the basis of the data of K/Shler & Raaz (1951), and of recalculated analyses from the book of Hejtman (1957), plus the data of Sattran (1963), Sattran et al. (1964), and Fiala (1964). Curve D - Approximate boundary of magmatogenie and sedimentogenic rocks (strong, dashed). T h e boundat3" is not sharp; it expresses only the result of interpolation of projeeti6n i~oints between both series. C u r v e E - Course of projections of sedimentogenic rocks (sediments: sandstones, subgreywackes, arkoses, greywackes, shales etc., and metamorphites: phyllites, paragneisses, quartzites etc.). Constructed on the basis of the data of K~Shler & Raaz (1951), and of analyses published in the books of Ilejtman (1962), Petr:inek (1963), plus data given by Sch0wfnek (1965). Tile dispersion field is not given owing to lack of data but its upper boundary appears to lie very close to curve D. T h e majority of analyses of soils and of strongly weathered rocks of the gabbro-granite series fall within the area of sedimentogenic rocks.. Explanation of symbols in Fig. 1 Symbol

qz

F

fm

1 2 3 4

78 69 86 77

14 8 6 12

8 23 8 11

Ref. list no. 8 36 46 8

l,ocality, ~Iaterial, Comments Radomillce moldavite Senohrady moldavite Radomilice moldavite Netolice molda~.'ite

ROCK SOURCE OF TEKTITES Symbol Bo Mo 5 6 7 8 9 I0 11 12 13 14 15 16 17 18 19 20 I

II III 1V g g' a a r

a"

d d' b b' k k'

k':

qz 79.8 76 78 73 67 64 65 71 62 80 69 49 93.3 89 86 98.1 88.1 71.2 -32 - 5 - 7 -44 70 61 54 52.4 57.5 -18 25 -31 -12.5

F 9.4 9.7 10 5 10 13 16 -15 7 4.5 10 24 3.4 2 '0.9 0.8 3.6 6.1 2 15 1.5 - 0.2 25 - 7.2 26 29.2 16 30 -10.5 16 -15

fin Ref. list no.

107

Locality, Material, Comments

10.8 14.3 12 22 23 23 19 14 31 15.5 21 27 3.3 9 13.1 1.1 8.3 22.7 65 80 91.5 55.8 ,5.5 31.7 20 18.4 26.5 52 64.5 53 72.5

17 average value of Bohemian moldavites 17 average value of Moravian moldavites 17 average of moldavites (29 analyses) I0 average of bediasites (21 analyses) 10 average of indochinites (12 analyses) 10 average of philippinites (15 analyses) 10 average of javanites (7 analyses) 10 average of australites (17 analyses) 10 average of Ivory Coast tektites (3 analyses) 30 average of georgianites (3 analyses) 10 average value of tektites 17 average of the Ries glass 51 Wabar, clear impact glass 51 Wabar, black impact glass 55 Darwin glass 3 Libyan glass 49 Aouelloul glass . 53 average of 2 !tenbut3" glasses 34 average content of chondrites 34 average content of pyrox-plag, achondrite 34 average content of hypersthene achondrite 34 average content of pallasites 32 'KarloxT Vat-).' granite 32 Sedlec, kaolin arisen from (g) 56 Cs6diberges, a fresh andesite 56 Cs6diberges, partially weathered andesite 56 Cs6diberges, a disintegrating andesite 16 a fresh diorite 16 a weathered diorite (see d) 9 a fresh basalt 9 laterite arisen from basalt (b) 44 average value of Kru]n~ Hory granites curve of the course of albitization of the Krugn6 tlor3 " granites (Sattran et al. 1964). Other rocks of magmatogenic and sedimentogenic series undergoing albitization show a similar trend. the course of grelsenization, sericitization and kaolinization of the Krugn6 I Iory granites (Fiala 1964, Sattran 1963).

The major difference in chemical composition of moldavite and Ries impact glass is clearly indicated in the present plot on which the Ries glass falls into the field of magmatogenic rocks. This supports the conclusion of Engelhardt & H6rz (1965) that Ries glass formed from the granite andgneiss which constitute the base of the Ries crater. Microscopic investigation of one sample shows a transition of glass into a granite-like rock. Before the Ries explosion the crystalline base must have been covered by about 500 metres of sediments (upper Keuper, Jurassic, Cretaceous and especially Miocene); consisting of marly and calcareous, arenaceous and argillaceous rocks (Preuss 1964). These could have constituted the original material of the moldavites, particularly the soils and miocene sediments, since the older rocks are very rich in calcium. However, on tlie basis of our present knowledge of the chemistry, a derivation from the parametamorphites, forming the base of the crater (quartzites, quartzitic gneisses and gneisses), is equally possible. T h e matter can only be resolved by detailed petrographic and chemical study of

108

v. BOU~KA

both the sedimentary series and the crystalline rocksin the base of the crater. The general nature of the German crystalline complex north of the Alps is not sufficient for solution of the problem. Projections of most impact glasses fall into the field of sedimentogenic rocks, and are, as a rule, more acid than tektites. Volatilization of such glass in the sense of Waher (1966) would, in mostcases, give rise to material of tektite composition. An exception is Henbury glass which plots too low; nor does recalculation on the basis of Taylor's assumption (1965) of contamination by meteoritic Fe, Ni, Co lead to any appreciable improvement (qz 70.6; F-fm = - 17; Taylor & Kolbe 1965). Only art enrichment in alkalis or calcium 9 Would help. As stated previously (p. 104) geochronological studies have revealed that some tektites and impact glasses are of similar age. Other impact glasses have no contemporary tektites; one of these is the glass from the Henbury crater. Weathering processes, kaolinization , sericitization, greisenization etc. can shift the plots of magmatic rocks into tile sedimentogenic field as shown on tile diagram. The sensitivity of this change is best shown in the example of andesite. This is only a case of surface weathering, but the projection points, nevertheless, show considerable separation. The only magmatic rock which could give rise to most acid moldavite (No. 3) is a greisenized granite; but this would be rather a rarity and certainly not a case of general validity. In other cases it is theoretically possible that tektites have formed from koalinized igneous rocks, but these rock types are again so unusual that this explanation is highly unlikely. If we consider the possibility of the impact taking place in strongly altered, kaolinized, sericitized and similar igneous rocks, then such a change would certainly occur over a wider area, and would be known from impact craters. The projections of some soils lie in the field of sedimentogenic rocks. Some of them, randomly selected for illustration, fall straight into the tektite field: the podsol from Adamov (Moravia) developed in the area of Devonian limestones (Smolik 1928) has, for example, tile values qz 67.8, F - f m = - 1 6 . Another podsol formed by a granitic rock near ~eb~tin (Moravia) possesses, according to the analysis given by Pellsek (1964), tile values qz 67.5, F-fm = -24. Although these podsols originated from completely different rocks their compositions are similar. This is due to the fact that changes caused by weathering processes tend to reduce original differences in rock Composition. Theoretically, soils might thus come into consideration as possible sources of tektites (see Schwarcz 1962). The diagram and the foregoing discussion lead to the inference that moldavites, bediasites, georgianites and the Ivory Coast tektites probably originated by impacts into sedimentary rocks. In particular the projections of georgianites, bediasites and Ivory Coast tektites lie so far from the ortho-paraboundary that they could be derived from orthorocks only by postulating complicated processes. In the author's opinion, any attempt to determine more exactly the nature of the original

ROCK SOURCE OF TEKTITES

109

rocks is premature at this stage, when, in many cases, virtuallynothing is known of the distribution and composition of the rocks at the place of impact. In the case of Australian-Asian tektites, a small change in chemical composition would place the plot in the zone of igneous rocks. It is therefore necessary to consider the possibility of their having originated from magmatic rocks of approximately granodioritic to granitic composition, or rocks of similar chemistry. A strong case for lunar origin of these tektites has already been made (p. 102) and it can be postulated that lunar weathering processes would operate somewhat differently from those observed under terrestrial conditions; they would include especially intensive cosmic radiation, streams of solar radioactive radiation, sharp shifts in temperature, impacts of meteorites, plus probably hydrothermal processes and the influence of exhalation, especially in past - processes which could lead to extensive changes in the chemical composition of surface lunar rocks. Nevertheless in terms of chemical composition alone, it is much easier to explain the origin of Australian- Asian tektites by impact on the Earth's surface. The diagram shows that indochinites, philippinites and javanites fall very close to each other, the nearest to australites being javanites. The projections of indochinites and philippinites fall nearer to the curve of sedimentogenic rocks. If it is correct to assume that all Australian-Asian tektites have the same origin, then javanites and australites apparently arose from rocks richer in calcium, possibly through assimilation of a calcareous rock.

Conclusions The observations stated above can be summarized as follows: 1) Plots of tektites show that their chemical composition places them among sedimentogenic rocks, (i.e. sediments or parametamorphites). Impact glasses are more acid in composition, but age determinations reveal a relationship with tektites. If tektites have suffered selective volatilization in the sense of Walter (1966), then they must have originated from material very similar in composition to impact glass. Exceptions are the moldavites and the Ries impact glass. The major chemical differences between these two apparently related rocks can be explained by assuming that moldavites arose by impact from rocks of the sedimentary mantle at Ries, possibly also from the soils, while the Ries impact glass formed from the crystalline rocks in the base of the crater. 2) There appears to be no direct relationship between Henbury impact glass and australites, unless it is supposed that the formation of australites was accompanied by an enrichment in calcium, or that they originated from the same place but from material richer in Ca or alkalis. 3) Australites and possibly all Australian - Asian tektites could have formed from weathered magmatic rocks of granodioritic to granitic composition.

110

V..BOUgKA

4) The Bohemian moldavites are more acid on "an average than the Moravian moldavites. It is probable that moldavites formed from inhomogenous source material. Department of llIineralogy, Geochemistry & Crystallography Faculty of Natural Sciences, Charles University AIbertov 6, Praha 2, Czechoslovakia, ffuly 1967

REFERENCES

AD.~.xls, J.A.S., OS.XIOND, J.K. & ROGEnS, J.J.W. 1959: T h e geochemistry of thorium and uranium. Phys. Chem. Earth 3, 298-348. AN~Ens, E. 1960: T h e record in the meteorites - II. On the presence of aluminium-26 in meteorites and tektites. Geoch. et Cosmoch. Acta 19, 53-62. BA~CER, G. 1959: Tektites. llIem. Nat. 3Ius. Victoria 23, 1-313. B,ucvn, G. 1963:Fore1 and sculpture. Tektites (Ed. John A. O'Keefe), pp. 1-24. Univ. of Chicago Press, Chicago. BARXF_% V.E. 1940: North American tektites. Univ. Texas Publ. 3945, 477-582. BAm,cns, V.E. 1961 : Tektites. Scl. Amer. 205,.(5) 58-65. Bou~I~A, V. 1965: O dvouba(evn~m vhavlnu z Lip[ v ji~.nfch (2ech.4ch. Gas. pro mineralogii a geologii 101 191. (Two-coloured moldavite from Lipl in Southern Bohemia). BOU~KA, V. & Povo.~Dn& P. 1964: Correlation of some physical and chemical properties of moldavites. Geoeh. et Cosmoch. Acta 28, 783-91. CAROLI., D. & WOOF, M. 1951 : Soil Science, 72, No. 2, 87-99, Australia. Cl~.~o, E.C.T. 1963: T h e p e t r o g r a p h i c and chemical characteristics of tektites. Tektites (Ed. John A. O'Keefe), 51-94. Univ. of Chicago Press, Chicago. CIL~P.~L~N, D.R. 1964: On the unity and origin of the Australasian tektites. Geoch. et Cosmoch. Acta 28, 841-80. Ctt.,~PM.,,s, D.R. & LAnsox, tt.K. 1963: On the lunar origin of tektites, ft. Geophys. Res. 68, 4305-58. Co,iF.',;, A.J. 1962: Asteroid impact hypothesis of tektite origin. Proc. 3rd. Int. Space Sci. Syrup. Washington. North IIolland, Amsterdam. Co,w\', A J . 1963: Asteroid- or r hypothesis of tektite origin: T h e moldavite strewn-fields. Tektites. (Ed. J.A. O'Keefe). Chapt. 9, 189-211. Univ. of Chicago Press, Chicago. COTTITTA, F., CAnnox, M.K. & CHAO, E.C.T. 1962: .7. Geophys. Res. 67, p. 3552 (abstract). E.~I.~mNs, W.tl. 1940: The principles of Economic geology. New York, London. EN~VLHARDT W. VO.'~& H6nz, F. 1965: Riesgl~iser und Moldavite. Geoch. et Cosmoch. Acta 29, 609-20. FaUL, H. 1966: Tektites are terrestrial. Science 152, No. 3727, 1341-1345. FAVL, H. & Bous~:a, V. 1963: Moldavite distribution in south-western Czechoslovakia. Second Int. Ss'm. on tektites. Pittsburg, Pennsylvania. (p. 21, abstract). FIALA, F. 1964: T h e chemism of granltoids of the Slavkovsk~' les Nits. (Kaiserwald). Session de l'Azopro en Tchgcoslovaquie. Noyau du ~lassif de la Boh~me. Praha ~ S A V. FLEISCI~ER, R.L. & PRICE, P.B. 1964: Fission tract evidence for simultaneous origin of tektites and other natural glasses. Geoch. et Cosmodt. Acta 28, 755-60. FLEISCHER, R.L., PRICE, P.IL & WaLgER, R.M. 1965: On the simultaneous origin of tektites and other natural glasses. Geoch. et Cosmoch..4cta 29, 161-6. GF-';TNEn, ~V., LIPPOt.T, H.J- & MOLLER, O. 1964: Kalium-Argon-Alter des BosumtwiKraters in Ghana und die Chemische Beschaffenheit Seiner Gl.~ser. Z. Nattm 19a, 150. GENTNER, W., LIPPOLT, H.J. & ScltaErvEa, O.A. 1963: Argonbestimmungen an Kaliummineralien - XI : Die Kalium-Argon-Alter der Gl/ises des N6rdlinger Rieses und der B/Shmisch-mahrischen Tektite. Geoch. et Cosmoch. Acta 27, 191-200. GL~TNER, W. & Z~tmINC;~'R, J. 1960: I)as Kalium-Argon-Alter von Tektiten. Z. Natur. 15a, 93-9~

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Accepted for publication August 1967.

Printed April 1968