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On the simultaneous origin of tektites and other natural glasses
()n the sinmltaneous origin of tektites and other natural glasses R. I.,. FLEISCHER, P. B. PRICE and R. Jf. \VALKER General Electric Research Luboratory, (Received
1.5 October 1964;
Schcnoctady,
Sew York
in rcwiaedform 11 .\‘overn&r
1964)
&&a&--Further f&ion-track
dating experiments have been done to tlsamino the naxociations between tektite falls and various impact glasses. The previous ages found for Libyan desert glass by fission track dating, and for Riea Kessel glass and the Martha’s Vineyard tektito by K-Ar dating, are coniirmed. Although some low ages are found for samples from the Ivory Co-t and the Ashenti Crater, the results support the value of I,3 million years found by GENTLikely mtaimum ages are giwn for sovnral impact NER, LIPPOLT, MILLER and UHRINOER. glasses which are apparently not associated with tektite falls. INTR~DUCTI~H
establishing the chronology of tektite formation, geochronological tools have given two primary types of information: They have shown that the geographical groups of tektites are also chronological groups, samples from a given continental area having the same formation age (GENTNERand ZXHRINOER,1960; ZXHRINGER, 1963; FLEISCHERand PRICE, 1964a, b). Secondly, it has been found that various other natural glasses, generally regarded as impact glasses, have ages that are indistinguishable from those of the various tektite groups (GE~TNER, LIPPOLTand SCHAEFFER,1963; FLEISCHERand PRICE 1964a, b; GRNTNER, LIPPOLT and M~‘LLER,1964). Hence the tektites fell in groups at nearly the same times as much more massive objects created impact glasses. In the present work we report further ages obtained by the fission track method (PRICE and WALKER, 1963). Results will be presented for North American and Ivory Coast tektites and various impact glasses. Some of the measurements substantiate existing K-Ar results, and others are determinations on previously undated materials. Sometimes we find that several samples from the same tektite fall give a spread of ages by fission track dating. Such results are to be expected for tektites which have been heated briefly above 560°C (FLEISCHER and PRICE, 1964a), a temperature which would easily be attained by surface material during a brush or forest fire. When scattered fission track ages are found, we will be able to decide a ‘(true” age in either of two situations: (1) If the highest fission track age is found for several of the samples, as was the case for the australites (FLEISCHERand PRICE, 1964b), or (2) if the highest fission track age agrees with the K-Ar result for the same group of samples. When neither of these situations is the case, we have no basis for giving a firm answer. METHOD IN
In fission track dating, spontaneous fission tracks are displayed by chemical etching and counted in the natural material; the uranium content is found subsequently by a count of tracks induced by neutron irradiation. Although the techniques used for fission track dating of glass have been described in considerable 161
162
R.
L.
FLEISCHER,P. B. PRICE and R. M. WALJCER
detail by FLEISCHER and PRICE (1964a), a few further comments should be made about the procedure followed: Fig. 1 shows an example of fission tracks in an irradiated impact glass. Etching may be carried out either on a clean fracture surface or on an optical-quality, mechanically polished surface. With a reasonably large spontaneous fission track count ( > 1000/cm2) the fracture surface method is generally the easier one. On the other hand, for counting densities of less than a few hundred per cm2, a level, polished surface allows a large area to be scanned without constant refocusing of the microscope. If the uranium distribution is not uniform, it is vital
cl 0
K-Ar AGE FISSION TRACK AGE
Fig. 2. Distribution of tektite groups and associated impact glasses. The numbers are the ages in millions of years of the glasses indicated within the encircled areas. In all cases where both K-Ar and fission track dating have been done (except for the Georgia tektites), the fission track dating confirms the K-Ar results. In no case is there a known contradiction.
to count induced tracks on the same area of material following irradiation as was examined previously for spontaneous tracks. In order to approximate this condition the glass sample is etched in such a way as to remove a layer of only 10-12 p (the range of fission fragments in most glasses); in this procedure very nearly the same surface is available for inspection. If small particles of glass are to be examined, they may be imbedded in plastic, sectioned, polished, and thereafter treated as any bulk sample. The fission decay constant used for the age determination was 6.85 x 10-17yr-l (FLEISCHER and PRICE, 1964c). Neutron doses were measured either by Bar*O counts done by a commercial firm or by fission-track etch pit counts in a calibrated glass (FLEISCHER, PRICE and WALKER, 1964). RESULTS Table 1 presents results on some tektites and chronologically related impact glasses; Table 2 gives results on other impact glasses. The Ries Kessel, Martha’s Vineyard and Libyan glass samples are all in good agreement with previous results
_.-_....._ -
--
Fig. 1. Fission track etch pits in a neutron-irradiated sample of Aouelloul glass, ‘The string of light-bottomed pits is composed of bubbles; the dark ci~les and light-centered
ellipses are fission track pits.
Rose:
5 x HI15 neutranrs/omz,
of
R. R. E. T. E. T. Paris Museum
(USNMI396) (USNM2136) Georgia i (DGA- 1f (E.Y..W.B. (b)) @GA-2) (BSC.TSB)~b)
Libyan Desert
6. Clark 5. Clark A. King Allen A. King Allen Jr.
Jr. Jr. Jr.
8690 (&:e%l
3900 (z&6%)
11,750 ( ;trs %)
12.200 C&8%,
37 (+33%) 3%(zttsO%) 18.5 (&28%)
49 (,tM%) 27 (130%)
42 {&Lti3%)
77 (f9%) 113 (z&9%, 124 (f8%) 103 (f8%)
1.4
(a) 2.4 1‘5
2.0 (a) 2-9
2.0
4-B
1.6 0.61 O-33 0.70 0.72 0.079
0.67 &72 0.69 0.46
37.5 f&2*4)
33.9 f
<0~003 0.57 (*o.lo) 0.60 (*o-13) 3.8 (,tO‘4) 15.2 (fl*5) 16-l (&1*4)
. _ .__ ..__--
..-
has
[33+8 j, 2.61 (i)
1.7 (0)
33.7 & 2.2 (0)
14.8 j, 0.7 (g)
I.3 f O-3 {f)
1.3 (6)
35‘5 (*8*3)
15.2 (;tl.6)
0.16 (&O.Ob) (d) o-49 f fO*O9) 0.50 (&O~l$) 0.32 (&0+14) fd) 0.30 (LtO.19) (d) 1.48 (f0.47)
O-71 (;tO*O7) 0.96 {fiO*lS) 1.10 (;tO*l$) 1.41 (&0*13)
(a) Assumed to be the e&me a~ DGA-1. Note a&&xi in proof: This aagumption is not e valid one, es the result on Georgia&e RSC.TSB shown since thie article wae submitted. [b) Gibbona School, Dodge County. (e) If no track ia found in a surveyed area af tern*. pa < i/cm* is assumed. (d) Discrepancy between Bar‘@ snd glees elide doses added an error of 12 per cent above the etatisticai standard deviation. (0) ZHXfiINoEB (1963). (f) GENTNEI~ cf al. (1984). (g) GENTNE~ ct al. (1963). (b) Hock Branch. (it Ftmsc~~a and PBWE (1964) &ion track result.
gI@J@
lmp6ct
Tektite
R. S. Clark Jr,
Sfartba’e Vineyard (USNM2082)
Tektite
H. Feut
W. Gentner W. Gentner W. Gentner W. Gsntner W. Gentner E. C. T. Chao
R.ies K-l:
(Be&a-84)
(III)
QIVC-3.64)
Impact
J. A. O’Keefe hf. Pelles A. J. Cohen E. C. T. Chao
Ivory &Met
I
(IVC- 13-64) (Paris Mue. 2202) (372)
From
TQkt.its
sample
Type
R. L. FLEISCRER. P. B. PRICEand R. X.
164
WALKER
(GENTNEB,LIPPOLTand SCHAEFFER,1903; ZXHRINGIER,1963; FLEISCHERand PRICE, 1964a). The Georgia tektites, on the other hand, show various ages which all are less than the 34 m.y. found by ZXHRINQER(1963). The scattered and low results suggest heating and support the suggestion of KINK (1964) that Georgia tektites have been stratigraphically disturbed since their fall. The existence of fission tracks in these samples does, however, prove that these are natural rather than manmade glasses. The appearance of heating of tektites is by no means surprising since it is known that at some time in the past they were on the Earth’s surface.
The ages for Ivory Coast tektites and Ashanti Crater (Lake Bosumtwi) glass are of special interest but, unfortunately, have a considerable spread of values, probably for the reason which was discussed above. GENTNER,LIPPOLTand MULLER (1964) found an age of I.3 f O-3 m.y. for the Ashanti Crater glass-in agreement with the age found by Z~RINGER (1963) for the Ivory Coast tektites. The fission track data in each case give a series of ages ranging from O-7 to 1-‘4 (&O-l) m.y. for the tektites and from 0.2 to le.5(f0.5) m.y. for the Aahanti Crater glass. Again, as was found for the Georgiaites and earlier (FLEISCHER and PRICE, 1964b) for australites, heating is the likely cause. Since in each case the highest age is equal to the KAr age, these results must be regarded as supporting their validity. The Ashanti Crater result, however, is regarded as deserving of further work since the highest age (I-5 & 0.5 m.y.) has a large margin of error, because of the exceptional nonuniformity and small volume fraction of glass in the sample used here. Ages of other impact glasses As Table 2 shows, we have dated three samples of Aouelloul glass and placed likely upper limits on the ages of Canon Diablo, Henbury, and Wabar glasses. Table 2. Ages of impact glasses that are apparently From
Aoueiloui Aouetloul Aouelloul C&on Diablo Wabar Wabsr (BM1148) of 1932 Henbury (BMl.546)
unrelated to known toktites
Loaned by
A. J. Cohen D. R. Chapman from W. A. Cassidy D. R. Chapman from E. T. C. Chao A. J. Cohen
123 (5~14%) 86 ( f 14%)
1.6 1.4
0.46 f O-10 0.38 f 0.08
4g (Ltl4%)
I.8
0.16 & 0.06 (8)
(b)
o-7
A. J. Cohen British Museum
<1*5 (b) cl.2 (b)
0.3 0.3
<0.03 <0.03
British Museum
~3 (b)
0.4
<0.04
(8) Discrepancy between BarQo and glass slide dose measurement per cent above the statistical standard deviation. (b) If no track is found in an area of &cm2, pS < 1 is assumed.
added an error of 16
Simclltnnoorw
origin of tektites
nnd other
nntrlrnl
glt~scs
These limits are all consistent with t.he state of preservation of the craters; again a word of caution should be added: Since the craters are surface features, samples could have been exposed to the annealing effect of surface fires.
165
but the
DISCUSSION the completion of this work. samples from all of the four large tektite In all cases except for the occurrences have been dated by fission track dating. Georgiaites, the earlier K-Ar results have been confirmed. For the Georgiaites there is, however. no contradiction, but as discussed in the introduction merely a lack of an answer. For each of the major fields there is at least one impact glass whose age is identical within the experimental uncertainties involved. Figure 2 shows the groupings of tektite and impactite formation ages. With
Geological vs. geochronologicul ages Occasionally geological ages based on stratigraphy have been given lower values than have been found by the fission track and K-Ar methods. One suggested explanation (BAKER, 1960) is that tektites might somehow have been created long before they arrived on Earth, the geochronological ages then being the formation age and the geological age being the time of fall. However, the age of the flange material of australites, which was formed during their descent to Earth, has been shown (FLEISCHER and PRICE, 1964b) to agree with that found for whole samples, so that this suggestion must be incorrect. The alternate view, which we take here, is that because two different methods of age determination give identical results on tektites, differences between these ages and geological ages based on stratigraphy must be resolved by some adjustment in the geological reasoning. Since both K-Ar and fission track dating are performed on the samples themselves while geological dating depends on the environment of the samples, it is the environment which must somehow be irrelevant to the time of arrival of tektites. Relation of dating of impactites to theories of tektite origin
The two major current theories of tektite origin are the lunar (CHAPMANand LARSON, 1963) and terrestrial (BARNES, 1961; COHEN, 1962) theories. In each case the existence of an impact glass of the same age as a tektite fall is a natural consequence of the theory. In the terrestrial theory a body from outer space creates a crater on Earth, thereby throwing out tektites and giving one impact glass per tektite fall. In the lunar hypothesis an impact on the Moon may throw out tektites and larger fragments, each of which could give rise to an impact glass. The experimental observation that there are two impact glasses of the same ages as the North American tektites is thus compatible with the lunar hypothesis.* However, the terrestrial origin is by no means disproved since it is also possible that a second unrelated body created an impact crater at a time within the experimental uncertainty with which the ages are known. A second consideration is the distance between tektite and impactite locations (2000 miles from Clearwater Lakes to l Although not 8eaociated with 8 known impact crater, Libyan glass bee been described 8~ an impact glass (COHEN, 1983). COHEN% speculation that an impact crater could have been obscured by drifting desert aande (and other erosional proceases) is fully consistent with the high age of the glass.
166
R. L. FLEISCHER, P. B. PRICE and R. Y. \VALKER
Texas and 7500 miles from Egypt to Texas). Such a spacing is compatible with a lunar origin but poses a problem for the terrestrial origin theory to explain how tektites could be hurled through an atmosphere over such distances (O’KEEFE, 1963; CHAPMAN, 1964). This reasoning merely adds to the problem presented by the great size of the Far Eastern strewn field, as described by O’KEEFE (1963). CONCLUSIONS 1. The existence of a total of four tektite and impact&e occurrences is confirmed. 2. The agreement previously noted by GENTNER, LIPPOLT and MOLLER (1964) between the age of the Ivory Coast tektites and the Ashanti Crater glass is aupported by fission track dating. 3. The existence of two impactite glasses, each far from the chronologically associated Texas tektites, is evidence which is difficult to explain by the theory of terrestrial origin of tektites. Achmowkdgement-Wo are indobted to those who so graciously suppliod the samplea used in this etudy: ‘I. ALLEN, E. C. T. CHAO. D. CHAP~N, R. S. CLARK JR., A. J. COHEN, H. FAUL, W. GEX~~NER,E. A. KING JR., J. A. O’KEEFE, M. PELLAS and the British Museum. This research was eupported in part by the Air Force Cambridge Rewarch Center. REFERENCES BAKER G. (1980) Origin of tektites. -Nca&re, Land. lS& 291. BARN= V. E. (1961) Tektites. Sci. Amer. 206, 58. CHAPMAND. R. and LARSONH. X. (1983) On the lunar origin of tektites. J. Geophys. Rea. 88, 4306. CKAPXAN D. R. (1964) Amer. Geophys. Union, April 23, unpubliehed. CO-N A, J. (1962) Asteroid impaet hypothesis of tektite origin. f’roc. 3rd. Iti. Space &i. Symp., Waehington. North Holland, Amsterdam. COHEN A. J. (1963) Foe&l giaeses produced by impact of meteorites, asteroid8 and possibly comets with the planet earth. Advanc. Qkaee Tech. Part 2, p. 360. FLEISCHERR. L. and PRXCE P. B. (1964a) Glass dating by fleAion fragment tracltll. ,I. ffeophye. Reu. 6@, 331. FLEISCHERR. L. and PRICE P. B. (1964b) Fiasion track evidence for the simultaneous origin of tektites and other natural glasses. Qeochim. et Coemochim. Acta 22, 755. FLEISCHERR. L. and PRICE P. B. (1964~) Decay constant for spontaneous f&ion of U236. Pfrys. Rev. l=B, 63. FLEISCHERR. L., PRICE P. B. and WALKER R. M. (1964) Neutron flux mt~asurcmonts by fission tracks in solids (in press). GENTNERW. and ZKHRINOERJ. (1960) Dae Kalium-Argon-Aher von Tektiton. 2. Kuturf. &, 93-99. GENTNER W., LIPPOLT H. J. and SCHAEPFER0. A. (1963) Argonbestixnm~mgen am KaliumDie Kalium-Argon-Alter der Gl&aer dea Ndrdlinger Rieaes und der mineralien-XI: Bohmisoh-mahrischen Tektite. Ueochim. et Coemochim. Acta 27, 191. GENTNER W., LIPP~LT H. J. and MILLER 0. (1964) Kalium-Argon-Alter des BosumtwiKraters in Ghana und die Chemisehe Beschaffenheit Seiner Gliieer. 2. Naturf 198, 160.. Actu 28, 915. Kr~o JR. E. A. (1964) New data on Georgia tektites. Qeochim. et Comchim. OXEEFE J. A. (1963) The origin of tektites. Tektites, p. 167. University of Chicago, Chicago. PRICE P. B. and WALKER R. MM.(1963) Fossil tracks of charged particles in mica and the age of minorala. 2. Ueo$yR. Res. $2, 4647. Z~~KRINQER J. (1963) K-Ar measurements of tcktitos. Rrrtlionctiw Dntimj, p, 289. Int. Atomic Enorgy Agency, Vionna.
1.5 October 1964;
Schcnoctady,
Sew York
in rcwiaedform 11 .\‘overn&r
1964)
&&a&--Further f&ion-track
dating experiments have been done to tlsamino the naxociations between tektite falls and various impact glasses. The previous ages found for Libyan desert glass by fission track dating, and for Riea Kessel glass and the Martha’s Vineyard tektito by K-Ar dating, are coniirmed. Although some low ages are found for samples from the Ivory Co-t and the Ashenti Crater, the results support the value of I,3 million years found by GENTLikely mtaimum ages are giwn for sovnral impact NER, LIPPOLT, MILLER and UHRINOER. glasses which are apparently not associated with tektite falls. INTR~DUCTI~H
establishing the chronology of tektite formation, geochronological tools have given two primary types of information: They have shown that the geographical groups of tektites are also chronological groups, samples from a given continental area having the same formation age (GENTNERand ZXHRINOER,1960; ZXHRINGER, 1963; FLEISCHERand PRICE, 1964a, b). Secondly, it has been found that various other natural glasses, generally regarded as impact glasses, have ages that are indistinguishable from those of the various tektite groups (GE~TNER, LIPPOLTand SCHAEFFER,1963; FLEISCHERand PRICE 1964a, b; GRNTNER, LIPPOLT and M~‘LLER,1964). Hence the tektites fell in groups at nearly the same times as much more massive objects created impact glasses. In the present work we report further ages obtained by the fission track method (PRICE and WALKER, 1963). Results will be presented for North American and Ivory Coast tektites and various impact glasses. Some of the measurements substantiate existing K-Ar results, and others are determinations on previously undated materials. Sometimes we find that several samples from the same tektite fall give a spread of ages by fission track dating. Such results are to be expected for tektites which have been heated briefly above 560°C (FLEISCHER and PRICE, 1964a), a temperature which would easily be attained by surface material during a brush or forest fire. When scattered fission track ages are found, we will be able to decide a ‘(true” age in either of two situations: (1) If the highest fission track age is found for several of the samples, as was the case for the australites (FLEISCHERand PRICE, 1964b), or (2) if the highest fission track age agrees with the K-Ar result for the same group of samples. When neither of these situations is the case, we have no basis for giving a firm answer. METHOD IN
In fission track dating, spontaneous fission tracks are displayed by chemical etching and counted in the natural material; the uranium content is found subsequently by a count of tracks induced by neutron irradiation. Although the techniques used for fission track dating of glass have been described in considerable 161
162
R.
L.
FLEISCHER,P. B. PRICE and R. M. WALJCER
detail by FLEISCHER and PRICE (1964a), a few further comments should be made about the procedure followed: Fig. 1 shows an example of fission tracks in an irradiated impact glass. Etching may be carried out either on a clean fracture surface or on an optical-quality, mechanically polished surface. With a reasonably large spontaneous fission track count ( > 1000/cm2) the fracture surface method is generally the easier one. On the other hand, for counting densities of less than a few hundred per cm2, a level, polished surface allows a large area to be scanned without constant refocusing of the microscope. If the uranium distribution is not uniform, it is vital
cl 0
K-Ar AGE FISSION TRACK AGE
Fig. 2. Distribution of tektite groups and associated impact glasses. The numbers are the ages in millions of years of the glasses indicated within the encircled areas. In all cases where both K-Ar and fission track dating have been done (except for the Georgia tektites), the fission track dating confirms the K-Ar results. In no case is there a known contradiction.
to count induced tracks on the same area of material following irradiation as was examined previously for spontaneous tracks. In order to approximate this condition the glass sample is etched in such a way as to remove a layer of only 10-12 p (the range of fission fragments in most glasses); in this procedure very nearly the same surface is available for inspection. If small particles of glass are to be examined, they may be imbedded in plastic, sectioned, polished, and thereafter treated as any bulk sample. The fission decay constant used for the age determination was 6.85 x 10-17yr-l (FLEISCHER and PRICE, 1964c). Neutron doses were measured either by Bar*O counts done by a commercial firm or by fission-track etch pit counts in a calibrated glass (FLEISCHER, PRICE and WALKER, 1964). RESULTS Table 1 presents results on some tektites and chronologically related impact glasses; Table 2 gives results on other impact glasses. The Ries Kessel, Martha’s Vineyard and Libyan glass samples are all in good agreement with previous results
_.-_....._ -
--
Fig. 1. Fission track etch pits in a neutron-irradiated sample of Aouelloul glass, ‘The string of light-bottomed pits is composed of bubbles; the dark ci~les and light-centered
ellipses are fission track pits.
Rose:
5 x HI15 neutranrs/omz,
of
R. R. E. T. E. T. Paris Museum
(USNMI396) (USNM2136) Georgia i (DGA- 1f (E.Y..W.B. (b)) @GA-2) (BSC.TSB)~b)
Libyan Desert
6. Clark 5. Clark A. King Allen A. King Allen Jr.
Jr. Jr. Jr.
8690 (&:e%l
3900 (z&6%)
11,750 ( ;trs %)
12.200 C&8%,
37 (+33%) 3%(zttsO%) 18.5 (&28%)
49 (,tM%) 27 (130%)
42 {&Lti3%)
77 (f9%) 113 (z&9%, 124 (f8%) 103 (f8%)
1.4
(a) 2.4 1‘5
2.0 (a) 2-9
2.0
4-B
1.6 0.61 O-33 0.70 0.72 0.079
0.67 &72 0.69 0.46
37.5 f&2*4)
33.9 f
<0~003 0.57 (*o.lo) 0.60 (*o-13) 3.8 (,tO‘4) 15.2 (fl*5) 16-l (&1*4)
. _ .__ ..__--
..-
has
[33+8 j, 2.61 (i)
1.7 (0)
33.7 & 2.2 (0)
14.8 j, 0.7 (g)
I.3 f O-3 {f)
1.3 (6)
35‘5 (*8*3)
15.2 (;tl.6)
0.16 (&O.Ob) (d) o-49 f fO*O9) 0.50 (&O~l$) 0.32 (&0+14) fd) 0.30 (LtO.19) (d) 1.48 (f0.47)
O-71 (;tO*O7) 0.96 {fiO*lS) 1.10 (;tO*l$) 1.41 (&0*13)
(a) Assumed to be the e&me a~ DGA-1. Note a&&xi in proof: This aagumption is not e valid one, es the result on Georgia&e RSC.TSB shown since thie article wae submitted. [b) Gibbona School, Dodge County. (e) If no track ia found in a surveyed area af tern*. pa < i/cm* is assumed. (d) Discrepancy between Bar‘@ snd glees elide doses added an error of 12 per cent above the etatisticai standard deviation. (0) ZHXfiINoEB (1963). (f) GENTNEI~ cf al. (1984). (g) GENTNE~ ct al. (1963). (b) Hock Branch. (it Ftmsc~~a and PBWE (1964) &ion track result.
gI@J@
lmp6ct
Tektite
R. S. Clark Jr,
Sfartba’e Vineyard (USNM2082)
Tektite
H. Feut
W. Gentner W. Gentner W. Gentner W. Gsntner W. Gentner E. C. T. Chao
R.ies K-l:
(Be&a-84)
(III)
QIVC-3.64)
Impact
J. A. O’Keefe hf. Pelles A. J. Cohen E. C. T. Chao
Ivory &Met
I
(IVC- 13-64) (Paris Mue. 2202) (372)
From
TQkt.its
sample
Type
R. L. FLEISCRER. P. B. PRICEand R. X.
164
WALKER
(GENTNEB,LIPPOLTand SCHAEFFER,1903; ZXHRINGIER,1963; FLEISCHERand PRICE, 1964a). The Georgia tektites, on the other hand, show various ages which all are less than the 34 m.y. found by ZXHRINQER(1963). The scattered and low results suggest heating and support the suggestion of KINK (1964) that Georgia tektites have been stratigraphically disturbed since their fall. The existence of fission tracks in these samples does, however, prove that these are natural rather than manmade glasses. The appearance of heating of tektites is by no means surprising since it is known that at some time in the past they were on the Earth’s surface.
The ages for Ivory Coast tektites and Ashanti Crater (Lake Bosumtwi) glass are of special interest but, unfortunately, have a considerable spread of values, probably for the reason which was discussed above. GENTNER,LIPPOLTand MULLER (1964) found an age of I.3 f O-3 m.y. for the Ashanti Crater glass-in agreement with the age found by Z~RINGER (1963) for the Ivory Coast tektites. The fission track data in each case give a series of ages ranging from O-7 to 1-‘4 (&O-l) m.y. for the tektites and from 0.2 to le.5(f0.5) m.y. for the Aahanti Crater glass. Again, as was found for the Georgiaites and earlier (FLEISCHER and PRICE, 1964b) for australites, heating is the likely cause. Since in each case the highest age is equal to the KAr age, these results must be regarded as supporting their validity. The Ashanti Crater result, however, is regarded as deserving of further work since the highest age (I-5 & 0.5 m.y.) has a large margin of error, because of the exceptional nonuniformity and small volume fraction of glass in the sample used here. Ages of other impact glasses As Table 2 shows, we have dated three samples of Aouelloul glass and placed likely upper limits on the ages of Canon Diablo, Henbury, and Wabar glasses. Table 2. Ages of impact glasses that are apparently From
Aoueiloui Aouetloul Aouelloul C&on Diablo Wabar Wabsr (BM1148) of 1932 Henbury (BMl.546)
unrelated to known toktites
Loaned by
A. J. Cohen D. R. Chapman from W. A. Cassidy D. R. Chapman from E. T. C. Chao A. J. Cohen
123 (5~14%) 86 ( f 14%)
1.6 1.4
0.46 f O-10 0.38 f 0.08
4g (Ltl4%)
I.8
0.16 & 0.06 (8)
(b)
o-7
A. J. Cohen British Museum
<1*5 (b) cl.2 (b)
0.3 0.3
<0.03 <0.03
British Museum
~3 (b)
0.4
<0.04
(8) Discrepancy between BarQo and glass slide dose measurement per cent above the statistical standard deviation. (b) If no track is found in an area of &cm2, pS < 1 is assumed.
added an error of 16
Simclltnnoorw
origin of tektites
nnd other
nntrlrnl
glt~scs
These limits are all consistent with t.he state of preservation of the craters; again a word of caution should be added: Since the craters are surface features, samples could have been exposed to the annealing effect of surface fires.
165
but the
DISCUSSION the completion of this work. samples from all of the four large tektite In all cases except for the occurrences have been dated by fission track dating. Georgiaites, the earlier K-Ar results have been confirmed. For the Georgiaites there is, however. no contradiction, but as discussed in the introduction merely a lack of an answer. For each of the major fields there is at least one impact glass whose age is identical within the experimental uncertainties involved. Figure 2 shows the groupings of tektite and impactite formation ages. With
Geological vs. geochronologicul ages Occasionally geological ages based on stratigraphy have been given lower values than have been found by the fission track and K-Ar methods. One suggested explanation (BAKER, 1960) is that tektites might somehow have been created long before they arrived on Earth, the geochronological ages then being the formation age and the geological age being the time of fall. However, the age of the flange material of australites, which was formed during their descent to Earth, has been shown (FLEISCHER and PRICE, 1964b) to agree with that found for whole samples, so that this suggestion must be incorrect. The alternate view, which we take here, is that because two different methods of age determination give identical results on tektites, differences between these ages and geological ages based on stratigraphy must be resolved by some adjustment in the geological reasoning. Since both K-Ar and fission track dating are performed on the samples themselves while geological dating depends on the environment of the samples, it is the environment which must somehow be irrelevant to the time of arrival of tektites. Relation of dating of impactites to theories of tektite origin
The two major current theories of tektite origin are the lunar (CHAPMANand LARSON, 1963) and terrestrial (BARNES, 1961; COHEN, 1962) theories. In each case the existence of an impact glass of the same age as a tektite fall is a natural consequence of the theory. In the terrestrial theory a body from outer space creates a crater on Earth, thereby throwing out tektites and giving one impact glass per tektite fall. In the lunar hypothesis an impact on the Moon may throw out tektites and larger fragments, each of which could give rise to an impact glass. The experimental observation that there are two impact glasses of the same ages as the North American tektites is thus compatible with the lunar hypothesis.* However, the terrestrial origin is by no means disproved since it is also possible that a second unrelated body created an impact crater at a time within the experimental uncertainty with which the ages are known. A second consideration is the distance between tektite and impactite locations (2000 miles from Clearwater Lakes to l Although not 8eaociated with 8 known impact crater, Libyan glass bee been described 8~ an impact glass (COHEN, 1983). COHEN% speculation that an impact crater could have been obscured by drifting desert aande (and other erosional proceases) is fully consistent with the high age of the glass.
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R. L. FLEISCHER, P. B. PRICE and R. Y. \VALKER
Texas and 7500 miles from Egypt to Texas). Such a spacing is compatible with a lunar origin but poses a problem for the terrestrial origin theory to explain how tektites could be hurled through an atmosphere over such distances (O’KEEFE, 1963; CHAPMAN, 1964). This reasoning merely adds to the problem presented by the great size of the Far Eastern strewn field, as described by O’KEEFE (1963). CONCLUSIONS 1. The existence of a total of four tektite and impact&e occurrences is confirmed. 2. The agreement previously noted by GENTNER, LIPPOLT and MOLLER (1964) between the age of the Ivory Coast tektites and the Ashanti Crater glass is aupported by fission track dating. 3. The existence of two impactite glasses, each far from the chronologically associated Texas tektites, is evidence which is difficult to explain by the theory of terrestrial origin of tektites. Achmowkdgement-Wo are indobted to those who so graciously suppliod the samplea used in this etudy: ‘I. ALLEN, E. C. T. CHAO. D. CHAP~N, R. S. CLARK JR., A. J. COHEN, H. FAUL, W. GEX~~NER,E. A. KING JR., J. A. O’KEEFE, M. PELLAS and the British Museum. This research was eupported in part by the Air Force Cambridge Rewarch Center. REFERENCES BAKER G. (1980) Origin of tektites. -Nca&re, Land. lS& 291. BARN= V. E. (1961) Tektites. Sci. Amer. 206, 58. CHAPMAND. R. and LARSONH. X. (1983) On the lunar origin of tektites. J. Geophys. Rea. 88, 4306. CKAPXAN D. R. (1964) Amer. Geophys. Union, April 23, unpubliehed. CO-N A, J. (1962) Asteroid impaet hypothesis of tektite origin. f’roc. 3rd. Iti. Space &i. Symp., Waehington. North Holland, Amsterdam. COHEN A. J. (1963) Foe&l giaeses produced by impact of meteorites, asteroid8 and possibly comets with the planet earth. Advanc. Qkaee Tech. Part 2, p. 360. FLEISCHERR. L. and PRXCE P. B. (1964a) Glass dating by fleAion fragment tracltll. ,I. ffeophye. Reu. 6@, 331. FLEISCHERR. L. and PRICE P. B. (1964b) Fiasion track evidence for the simultaneous origin of tektites and other natural glasses. Qeochim. et Coemochim. Acta 22, 755. FLEISCHERR. L. and PRICE P. B. (1964~) Decay constant for spontaneous f&ion of U236. Pfrys. Rev. l=B, 63. FLEISCHERR. L., PRICE P. B. and WALKER R. M. (1964) Neutron flux mt~asurcmonts by fission tracks in solids (in press). GENTNERW. and ZKHRINOERJ. (1960) Dae Kalium-Argon-Aher von Tektiton. 2. Kuturf. &, 93-99. GENTNER W., LIPPOLT H. J. and SCHAEPFER0. A. (1963) Argonbestixnm~mgen am KaliumDie Kalium-Argon-Alter der Gl&aer dea Ndrdlinger Rieaes und der mineralien-XI: Bohmisoh-mahrischen Tektite. Ueochim. et Coemochim. Acta 27, 191. GENTNER W., LIPP~LT H. J. and MILLER 0. (1964) Kalium-Argon-Alter des BosumtwiKraters in Ghana und die Chemisehe Beschaffenheit Seiner Gliieer. 2. Naturf 198, 160.. Actu 28, 915. Kr~o JR. E. A. (1964) New data on Georgia tektites. Qeochim. et Comchim. OXEEFE J. A. (1963) The origin of tektites. Tektites, p. 167. University of Chicago, Chicago. PRICE P. B. and WALKER R. MM.(1963) Fossil tracks of charged particles in mica and the age of minorala. 2. Ueo$yR. Res. $2, 4647. Z~~KRINQER J. (1963) K-Ar measurements of tcktitos. Rrrtlionctiw Dntimj, p, 289. Int. Atomic Enorgy Agency, Vionna.