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Age and Chemical Composition of Тне Zнaмanshin Crater Impactites and Tektites Cqmparison with Australasian Tektites

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Age and Chemical Composition of Тне Zнaмanshin Crater Impactites and Tektites Cqmparison with Australasian Tektites E.Izokh, L.каshkаrоv, N .коrоtkоvа AGE AND CHEMICAL COMPOSITION OF ТНЕ ZНAМANSHIN CRATER IMPACTITES AND TEKTITES AND CQMPARISON WITH AUSTRALASIAN TEKTITES Novosibirsk 1993 RUSSIAN ACADEMY OF SCIENCES SIВERIAN BRANCI:I UNIТED INSTIТUTE OF GEOLOGY, GEOPНYSICS AND MINERALOGY E.Izokh, L.Kashkarov, N.Korotkova AGE AND CHEMICAL COMPOSITION ОР ТНЕ ZHAМANSHIN CRATER IMPACTITES AND TEKTITES. AND COMPARISON WITH AUSTRALASIAN TEKTITES NOVOSIВIRSK 1993 E.Izokh, I,.каShkагоv N .Korotkova. A�e . and chemical composition of the Zhamanshin Crater ...lmpactites and Tektites and comparison with Australasian Tektites. NovosibIrSk, 1993. 94 р. ISBN 5-7323-1784-6 ТЬе Zhamanshin impact ci'ater is the only impact crater оп the Earth where impactites, various tektites and microtektites coexist. Тhus t�e Crater becomes the best object to solve the old tektite puzzle. Published before and new data оп the radiogenic ages and chemistry of the Zhamanshin tektites and Australasian tektites summarized to demonstrate their close genetic relations. ТЬе tektite age-paradox �erves as the base evidence in favor of the exstraterrestrial origin of tektites. Тhe petrographical and petrochemical features of tektites demonstrate their most рсоЬаЫе volcanic origin. ISBN 5-7323-1784-6 © E.P.Izokh, UIGGM, 1993 L.Firsov (Novosibirsk) was the first to obtain < 1 т.у. K-Ar age of the Zhamanshin acid glasses. V.Perelygin фиЬпа Inst. of Nuclear Physics) obtained 0.81 +0.16 т.у. fission track age of the same glasses (Florensky et аl, 1979). 1.07+0.05 and 1.07+0.06 fission track ages of the acid zhamanshinites and irghizites respectively were obtained Ьу Storzer and Wagner (1977, 1979). Following Florensky and Dabizha (1980, р. 32), the age of the Zhamanshin crater was determined between 0.69-0.85 т.у. Ьу five independent methods: Ьу stratigraphy (Iess than 25 т.у.); Ьу geomorphology (05-1 т.у.); Ьу K-Ar dating « 1 т.у.); Ьу fission .track dating (0.65-0.75 т.у.), and from reverse magnetization of the impactites (0.69-0.85 т.у.). Al1 these data served as the base for the most recent judgement оп the crater age because по other il1terpretation was permitted Ьу the EIТ. S.Taylor and S.McLennan (1979, р. 1551) noted а close compositional resembIance between Zhamanshin acid glass and НепЬигу impact glass, which "has, Ьу chance, а composition identical for most elements to australites". They saw in the irghizites "а parental material similar {о terrestrial subgraywacke, Iike that at the НепЬигу target rocks, while basaltic andesite is а Iikely source for the basic zhamanshinites" (ор. cit., р. 1558). "The chemical and possibIe age similarities between the irghizites and the Australasian tektites are sufficiently close to warrant а detailed investigations" (ор. cit., р. 1555), and "estabIish the Zhamanshin structure as а possibIe source for the Australasian strewn field" (р. 1563). V.Bou�ka also studied P.Florensky's samples, but suggested that platform cover, i.e. the Paleogene sands and clays, served as the precursor of the various Zhamanshin glass (Bouska et al,1981). After Р .Florensky's researches the investigations of the crater were supported Ьу the Siberian Branch of the Academy of Sciences and continued Ьу E.Izokh (1984). Later, V.Masaitis, V.Feldman, Ja.Boiko and other geologists were invited to participate in the research Program. Three bore holes of -1 km deep were drilled within the crater fu nnel Ьу regional geological survey (Aktyubinsk) under the auspices of the аЬоуе Program. From the beginning E.Izokh noted unusually young morphological features of the crater and the close resembIance between acid zhamanshinites and Muong Nong-type tektites (MN-tektites) from Vietnam, as we]J as the absence of proper target rock for tektites (lzokh, 1984; Izokh et al, 1984) . V.Masaitis, оп the other hiшd, as а staunch supporter of the EIТ, tried to find compositional analogues in the Paleozoic beds, but not in the sediment cover, which, from his point of view, did not participate in the impact melting process 4 INTRODUCTION ТЬе Zhamanshin impact crater (North of Aral Sea) is the only one where at least 3 different types of melted glass are found 'together: 1 - impactites, 2 - various layered Muong Nong-type tektites, 3 - tektites-irghizites associated with microtektites, ТЬе well known tektite age-paradox (the great difference betwee� the age of tektite formation and their geological position) is widely recognized as an argument in favor of the extrateri'estrial origin of tektites, Thе close genetic relation between the Zhamanshin crater and the Australasian tektite strewn fieId (ААTSF), as evidence of their simultaneous origin, is also clearly recognized, ТЬе currentIy dominant Earth Impact ТЬеосу (ЕП) does not' explain the аЬоуе and many other data, Thеalternative extraterrestrial volcanic hypothesis proposed Ьу J.O ' Keefe (1976) seems more suitable. Some corrections based оп the tektite age-paradox are necessary, however. Thus, the discussion, now more than l00-year old, оп the origin of tektites continues. ТЬе Zhamanshin structure represents in itself an excellent testing ground оп which to develop the discussion, and to check and to compare the rival ideas of impact cratering, the origin of the tektites, catastrophic global changes connected with the tektite faB, etc. HistoryоС the Zhamanshin Crater Study ТЬе basic data оп the Zhamanshin crater: geology, age, target rocks and various glass composition are to Ье found in Florensky and Dabizha (1980); Вошо (1983); Izokh (1984-1991); Masaitis (1987-1991).1t was Florensky (1975) who first recognized the crater as an impact structure and distinguished three main types of glass: basic zhamanshinites, acid zhamanshinites and tektites� irghizites. Florensky suggested that the basic zhamanshinites originated from various target rocks, including those from the paleozoic and tertiary formations. Не considered the acid zhamanshinites as completely melted paleozoic quartzite-slates, while the irghizites were regarded as formed Ьу condensation and accretion of the melt droplets within an incandescent gas cloud, where evaporated slates and meteorite were mixed (Florensky ' and Dabizha,1980). 3 (Masaitis and Selivanovskaya, 1987; Masaitis, 1989). In general, Ье fo11owed the рзth laid out Ьу Р j<'lorensky. V.Feldman used а statistical cluster method to compare the chemicaI composition of target rock and glass. Не concltided that' а11 types of the Zhamanshin glass Ьауе chemical analogues in the basement and in the platfol"ffi cover, as well. ТЬе most likely source material for tektites, Ье considered, was the superficial loess-like cover. V.Feldman underlined the most unusual feature of the Zhamanshin crater: that is, the formation of various compositional groups of gIass, quite distinct from the homogenization of the target rock соттоп to other well studied impact structures. At present, аН participants of the �rogram unanimously admit the impact origin of the basic zhamanshinites Ьу rrielting of the carbonic (С1) andesite volcanicS. With respect to the origin of the acid gIass, significant disagreement remains. Previous Age Determinations ТЬе first age determinations were applied only to the acid gIass (table 1, 1-4). ТЬе basic impact gIasses were analysed later'by L.каshkагоv et аl (1986) оп the request of E.Izokh. As it was anticipated, the fission track age of the basic impactites turned out to Ье one or two orders of magnitude Iess than that of the acid gIass « 0.1 т.у. vs - 1 т.у.; tables 1, 2). ТЬе same speciniens were analysed Ьу Е.коlеsпikоv (Moscow U!1iversity) and Ьу D.Storzer (Museum of Nat. History, Paris). As L.Kashkarov's results were doubted Ьу these researchers, а dramatic discussion arose; the course of this, discussion is ihstructive. E.Kolesnikov et аl (1988) found 153-1.66 т.у. K-Ar age of'the basic impact gIass (table 2). Nevertheless, 1.1 т.у. age was inferred, following the К­ Ar isochron. Unfortunate\y, incompatible data were plotted оп the diagram to align the isochrone. As fig. 1 shows, only acid gIass data Ьауе s\gnificantweight to give the slope of the isochron confirming 1 т.у. age of the tektites alone, but not impactites. Matsubara et aI. (1991) also found rather oId 0.99 т.у. age of the basic gIass, which, as they write, "is in agreement with the corrected fission track age of 1.07- т.у." obtained Ьу Storzer and Wagner (1979) and Ьу Koeberl and Storzer (1987). However,the value of К2О = 2,67% was erroneously used for the age calculation in this case. In fact, К2О = 1.43+0.16% (average from 27 analysis) is tyPical of the Zhamanshin basic impact glass, while К20 = 5 2.86 +0.19 (average from 42) is typical only of acid Muong Nong-type g1ass (Izokh, 1986). Therefore, the calculated age for the sample Zh62j3b wШ Ье - 2.2 т.у., which is much older in comparison with апу fission .track ages obtained for Zhamanshin glasses. Fig. 1. K-Ar isochron of the Zhamanshin glasses: 1 - basic (impactites), 2 - acid (tektites), (see tables 1, 2) (Kolesnikov et al, 1987) . / 02 The discrepancies between fission track ages and K.Ar age& are the соmтоп feature of various impactites. This. feature сап Ье easily explained "Ьу incomplete degassing or contamiriation Ьу old source material" (Matsubara et al, 1991, р. 2954). As to tektites, such discrepancies are either absent or less significant; tabIe 1 represents а good example. The basic zhamanshinites, indeed, practically always have numerous target rock inclusions; therefore, misinterpretation of their K-Ar age is easily understood. Contrary to basic zhamanshinites, the acid MN-type zhamanshinites with. rare exceptions (see below), have по impurities of this kind. Thus, their K-Ar ages near equal to the fission track ages could Ье interpreted as the age of tektite formation. From this point of view, the confirmation of the 5.2 т.у.
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