80 Earth awl Plauelaty Science Leuers, 78 (1986) 80-88 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Impact glassesfrom Zhamanshin crater (U.S.S.R.) : chemical composition and discussion of origin

Christian Koeberl ’ and Kurt Fredriksson 2

’ Insritrrre of Geochemisny, Uniuersiry oj Vienna, P.O. Box 73. A-1094 Viema (Arcsrriu) ’ Deparanettr of Mineral Scierrces, Snlithsotriatl Jrlstirurion, Washingron. DC 20560 (U.S.A.)

Received October 12, 1985; revised version received February 17, 1986

Three silica-rich zhamanshinites and one irghizite from the Zhamanshin (northern Aral area, U.S.S.R.) have been analyzed for up to 40 major, minor, and trace elements. All data point to a clear distinction between these impact glasses and other or impact glasses. e.g. from the Australasian strewn field. Halogens are generally enriched in the irghizites and zhamanshinites when compared to normal splash for tektites. with zhamanshinites enriched more than irghizites. The same holds also for the alkali metals and a number of other volatile elements like Sb and As. Nickel and cobalt are enriched in the irghizite sample IO a considerable degree, suggesting meteoritic contamination. This view is also supported by gold and selenium data, but for quantifications.other siderophile elements need to be considered. Chromium is not a valid indicator of meteoritic contamination. because small amounts of ultra-basic igneous material may completely alter the picture. The rare earth elements do show a sedimentary pattern, consistent with two or three different source materials and a variation which is probably mostly due to dilution with silica-rich materials. The peak pressure and temperature experienced by the irghizites was lower than for or other splash-form tektites, and even lower for zhamanshinites, which is revealed by the content of volatile elements and lesser homogeneity.

1. Introduction in size and of irregular shape (rope-form, wire- form). They contain particles and The Zhamanshin impact crater is situated near vesicles. Zhamanshinites are of much larger size the river Irghiz about 200 km north of Aralsk in and very often of irregular, blocky appearance. Khazakhstan in the U.S.S.R. and is the source of They are dark in colour and show schlieren, a very interesting varieties of impact glasses. De- layered structure, and numerous bubbles and in- scriptions of the geology of the crater can be clusions. found in the literature [l-6] and need not be Because these impact glasses (zhamanshinites) repeated here. Two principal types of impact and tektites (irghizites) are closely associated with glasses are abundant. The first are called irghizites a specific crater, important conclusions can be [l] and microirghizites [7] and resemble tektites in made concerning the formation process of tektites. composition with an average SiOz content of about However, because of the restricted occurrence of 74%. The second class is called zhamanshinite. the glasses it may, however, not be justified to The zhamanshinites are subdivided into three sub- refer to the irghizite-zhamanshinite area as a fifth groups according to their silica content: (a) the strewn field. Besides a greater inhomogene- silica-rich zhamanshinites (about 70-80s SiO,) ity, there are some other features like higher (b) the zhamanshinites (about 52-57% SiO,), and Fe3+/Fe2+ ratios and water content *, which seem (c) the basic (or silica-poor) zhamanshinites (ca. to put the irghizites between tektites and other 40% SiO,) [5,8]. The irghizites are small, tektite-like impact glasses [9,10]. glasses, a few millimeters up to a few centimeters Chemical analyses of these glasses are of inter- est in providing a basis for deducing the origin of

l One analysis by J. Barrows of a 0.6 g irghirite sample gave and tektites in one event. Major ele- 0.20 wt.% H,O. ment determinations in a number of different

0012-821 X/86/%03.50 0 1986 Elsevier Science Publishers B.V. 81 samples have been performed by Ehmann et al. using the electronprobe at the Smithsonian In- [ll], Fredriksson et al. [9], Taylor and McLennan stitution. Results of homogeneity studies of the [12], Florenskij and Dabizha [5], Boubka et al. [S], 18201 sample have been reported by Koeberl et al. Shaw and Wasserburg [13], and Koeberl et al. [14], [14]. Standard correction methods [15,16] have showing a widespread compositional variation and been applied. Fluorine has been determined using inhomogeneity among the zhamanshinites but a F--sensitive electrode technique (which was in- limited in irghizites. Trace element studies have tercompared to a rapid instrumental neutron been more limited. Ehmann et al. [ll], Florenskij activation analysis method) as described by and Dabizha [5], and BouSka et al. [8] included Koeberl et al. [17,18]. Other trace elements have relatively small numbers of different trace ele- been analyzed using various neutron activation ments. The most complete data set [12] includes analysis procedures. Analytical techniques and data for up to 32 trace elements, but for only one some data for the rare earth elements in the three Si-rich zhamanshinite and two irghizites. To im- silica-rich zhamanshinite samples have been re- prove the data base, we have analyzed four ad- ported by Koeberl et al. [19]. The INAA proce- ditional samples for up to 40 elements. dures for the other elements follow the methods described previously [20]. Se, Ru, and Au in 18201 2. Samples and methods were determined after radiochemical separation, while Au in USNM 6200 was determined via Three silica-rich zhamanshinites (18201, USNM INAA. Ferric/ferrous ratios were determined 6014, and USNM 6015) and one irghizite (USNM wet-chemically. 6200) have been analyzed in the course of this study (see Figs. 1 and 2). One silica-rich 3. Results and discussion zhamanshinite (18201) was analyzed by using the fully automated five spectrometer ARL-SEMQ The results of the major element determina- electron microprobe at the University of Vienna, tions are given in Table 1. Comparison data from while the USNM samples have been analyzed the literature are given in Table 2. The irghizite

Fig. 1. Silksrich zhnmanshinites USNM 6013 (right) and USNM 6014 (left). (Photo courtesy D. Fulrell.) 82

Fig. 2. Silica-rich zhamanshinite USNM 6015 (fragment). (Photo courtesy D. Futrell.) analysis fits well within the range reported by sample of the silica-rich zhamanshinites analyzed BouSka et al. [8] from 31 samples, except for by Taylor and McLennan [12], yielding the suspi- Al,O,, which is slightly lower. Thus our sample cion that we also have to expect slightly lower seems to represent a typical irghizite, at least with contents in many trace elements (especially the respect to major elements. The zhamanshinites are refractories) due to SiO, “dilution”. more inhomogeneous as clearly shown by elec- Trace element data for the samples are reported tronprobe work [14], but all samples fit within the in Table 3. Comparison data from Taylor and range given by Florenskij and Dabizha [5] as well McLennan [12] and BouSka et al. [8] are reported as the smaller major element ranges of BouSka et in Table 4. The agreement between our data and al. [8]. The zhamanshinite samples all have a higher the literature data is excellent especially for the SiO, content (and thus lower Al,O,, FeO, etc., irghizites which appear, relatively, homogeneous due to the inverse relation with SiO,) than the one indicating common parent materials.

TABLE 1 Major element data for three silica-rich zhamanshinites (18201. USNM 6014, and USNM 6015) and one irghizite (USNM 6200). obtained by electron-microprobe analysis (all data in wt.%)

SiO, TiO, Al 2’3 Fe0 * WO CaO Na,O KzO Silica-rich zhamanshinites 18201 16.5 0.65 12.5 4.67 0.90 0.66 1.14 3.06 USNM 6014 75.3 11.1 3.96 0.15 0.59 1.97 2.71 USNM 6015 16.4 11.2 4.65 0.91 0.60 1.87 2.56 lrghizire USNM 6200 73.1 0.75 9.35 6.50 3.40 2.47 1.07 1.75

* All Fe as FeO. 83

TABLE 2 3. I. Major elements Comparison data from the literature for %-rich zhamanshinites and irghizites (from Florenskij and Dabizha [S] and BouSka et Taylor and McLennan [12] claim that there is a al. [8]) (all data in WI.%) very close similarity between their irghizite and Irghizites Silica-rich zhamanshinite data and the three javanites they zhamanshinites analyzed. We can only support this similarity with SiO, 70.0 -79.4 62.9 -88.1 our irghizite analysis, although iron is rather high TiO 0.69- 0.99 0.23- 1.10 and the K,O/Na,O ratio is smaller. If compared Al ,h 9.45-13.6 4.80-21.2 to the large data set of Chapman and Scheiber Fe0 l 4.24- 6.81 1.98- 8.05 [21], the similarity to other tektites from the Md’ 2.16- 3.76 0.34- 1.16 CaO 1.75- 2.85 0.55- 2.16 Australasian strewn field is not entirely convinc- Na,O 0.85- 1.22 0.57- 1.84 ing-if the CaO/MgO ratio is about right, then K,O 1.58- 2.14 O.lO- 3.07 the Al,O, abundance is too high. In most cases, l All Fe as FeO. however, MgO is somewhat less than CaO. The similarity between the silica-rich zhamanshinites and other groups of tektites from the Australasian

TABLE 3 Trace element data for three silica-rich zhamanshinites and one irghizite (data in ppm)

Element Zhamanshinites Irghizite 18201 USNM 6014 USNM 6015 USNM 6200

F 67 72 81 61 Cl 171 128 SC 6.50 7.98 9.06 7.80 V 75 Cr 58 49 62 259 Mn 690 702 820 600 co 10.6 8.6 10.2 91.3 Ni 25 1720 Zn 11 As 4.7 8.7 0.5 Se 0.0018 Br 0.4 0.4 0.3 0.1 Rb 79 118 99 38 Zr 240 220 290 360 RU 0.21 Sb 0.73 0.50 0.51 0.26 CS 4.1 3.72 3.54 2.09 Ba 320 290 390 530 La 21.2 21.8 26.0 15.2 Ce 50.7 50.1 58.3 35.0 Nd 18.3 16.8 31.4 14.0 Sm 3.12 4.1 4.6 2.60 Eu 0.95 0.99 1.08 0.45 Tb 0.75 0.64 0.69 0.56 DY 4.31 3.8 3.0 2.25 Yb 2.20 2.30 2.29 1.75 Lu 0.35 0.35 0.41 0.26 HI 6.3 5.5 6.5 9.93 To 0.75 0.53 0.64 0.82 Au 0.0029 0.0083 Th 6.9 7.8 8.6 5.14 U 1.84 1.50 1.98 0.77 84

TABLE 4 Trace elemenls in zhamanshinites and irghizites-comparison data from Taylor and McLennan [12] (USNM 5932. IRG 1-21, IRG 3-13) and BouVka et al. [8] (1971 and R2556a) (all data in ppm) Element Zhamanshinites Irghizites USNM 5932 R2556a IRG 1-21 IRG 3-13 1971 SC 15 16.8 8.8 9.4 11.6 V 117 250 46 38 150 Cr 92 93 170 200 222 co 16 19.6 73 102 73.3 Ni 40 126 1200 1370 1075 CU 36 24 19 As 13.3 Br 2.5 Rb 46 32 Sr 630 Zr 272 260 351 370 341 Nb 16.8 12.5 10.9 MO 0.73 3.6 0.40 0.23 Sn 3.1 0.75 0.78 Sb 1.16 0.15 cs 7.8 5.0 2.6 1.9 2.5 Ba 314 710 527 508 450 La 33.9 21.6 19.7 19.7 18 Ce 83.6 47 44.2 44.7 44 Pr 8.98 4.53 4.59 Nd 34.1 24.3 18.7 17.3 19 Sm 6.95 5.8 3.78 3.22 2.9 Eu 1.56 1.15 0.80 0.68 0.70 Gd 6.41 6.4 3.46 3.01 3.0 Tb 0.97 0.74 0.60 0.53 0.55 DY 5.63 3.45 2.99 Ho 1.12 0.81 0.67 0.58 Er 3.29 1.89 1.73 Tm 0.26 Yb 3.40 2.36 1.90 1.82 1.5 Lu 0.33 0.40 Hf 6.3 3.97 8.66 8.4 7.3 Ta 0.48 0.53 W 0.73 0.17 0.13 AU 0.0067 TI 0.43 Pb 26 2.9 2.0 Bi 0.12 0.04 Th 10.3 5.95 5.98 5.58 4.7 U 2.97 2.05 1.02 0.83 0.53

strewn field is only vague and fades if we consider more variable than others. The intra-specimen trace element data. One point is the limited homo- variations have been used to give some clues on geneity of the irghizites and the large inhomogene- the parent material of the zhamanshinites [14]. ities among silica-rich zhamanshinites. These inho- mogeneities are present not only within one sam- 3.2. The halogens ple [14], but also between the samples, and in a much higher degree than in or between Australa- The amount of halogens as representatives of sian tektites. Some elements (like Na or K) are the volatile element family is correlated with the 85 degree of shock and the peak temperature and 3.5. Nickel, chromium, and cobalt pressure a sample has received. This is evident from studies of Muong-Nong-type tektites in rela- The irghizite sample is enriched in nickel when tion to splash-form tektites from the Australasian compared to the zhamanshinites and other tektites. strewn field [18,20,22,23]. Fluorine as the least Nickel in some country rocks is between 2 and 48 volatile of the halogens does not show a great ppm [8]; thus the indigenous contribution of nickel difference between irghizites and zhamanshinites, may be near 20 ppm. Cobalt in country rocks was but is somewhat lower in the irghizites than in the between 0.32 and 20.5 ppm [8], and an indigenous zhamanshinite samples, i.e. - 60 ppm and 70-80 contribution of about 10 ppm seems reasonable, ppm respectively. The other halogens show a more although caution is necessary because BouSka et distinctive pattern. Chlorine in the irghizite is at al. [8] analyzed only three different country rocks. about 130 ppm, while it is about 170 ppm in one Especially for chromium (and to a lesser extent zhamanshinite. Bromine is depleted in the also for Ni and Co) one must consider the possi- irghizites by a factor of 3 or 4 compared to the ble incorporations of ultra-basic material which zhamanshinites. This indicates that the irghizites would alter the ratios. O’Keefe ([24] and personal were subjected to a higher peak temperature and communication, 1985) argues that comparisons re- pressure, consistent with the larger inhomogeneity garding cosmic primary material have to be re- of the zhamanshinites, and leads to the conclusion stricted to because of excess chromium that the irghizites and the zhamanshinites (which is negligible in iron ). Consid- originated from different areas of the target, a ering the ultra-basic intrusions at the crater (which conclusion strengthened by considerations of other have not yet been studied in detail) and the possi- trace elements. bility of incorporating such material to an un- known extent in the impact glasses, we find his 3.3. The alkali metals conclusion questionable. The interpretation of the cobalt and nickel Sodium, potassium, rubidium, and cesium show enrichment in the irghizites as a chondritic con- the same patterns as the halogens. They are tamination [11,12,25] may be correct, but our feel- depleted in the irghizites relative to the silica-rich ing is that at present it is difficult to substantiate zhamanshinites by a factor of 1.5-2. The deple- on this ground alone. Glass et al. [25], however, tions seem to be more pronounced for Cs and Rb showed much stronger evidence for a chondritic than for Na and K, implying that selective vola- projectile responsible for the formation of “micro- tilization was involved but because the difference irghizites” and thus the irghizites. Especially the is not large, it was probably not a major factor in strong positive correlation of Mg (and Fe) with Ni the differentiations. Similar trends are apparent in in these particles was cited. Also the composition the data of Taylor and McLennan [12]. of the parent materials (calculated in three inde- pendent ways) indicated an admixture of various 3.4. Volatile elements amounts of chondritic matter. In contrast, Palme et al. [26] and Koeberl (unpublished data) in- Other volatile elements, like As, Sb, Zn, and Se, vestigated siderophile trace elements and found also show an excess when compared to normal hints for an iron projectile. For further conclu- splash-form tektites. The behavior of Sn, Tl, and sions (chondritic or iron) siderophile trace element Pb [12] is also similar. All these elements seem to patterns in the glasses and country rocks should be depleted in irghizites when compared to silica- be investigated. rich zhamanshinites. Although they are low in irghizites, these elements are even lower in all 3.4. Refractory trace elements other tektites, except the Muong-Nong. Unfor- tunately we have Zn and Se data only for one There are some subtle differences in the refrac- sample, so no comparison is possible, but for As, tory element content of silica-rich zhamanshinites Sb, Sn, Pb, and Tl the difference is striking (Ta- and irghizites. For example, the Th/U ratio in bles 3 and 4). zhamanshinites is lower than in irghizites (also 86 evident in Taylor and McLennan’s [12] data), but effects with silica-rich materials like quartzite both are in the range of sedimentary rocks. The present at the crater. BouSka et al. [8] analyzed a same is true for the Zr/Hf ratios, the difference in paleogene quartzite (their sample R3149) from the absolute abundances between zhamanshinites and Zhamanshin crater, which may be a candidate as a irghizites reflects mainly the dilution effect of diluting component, although the REE pattern SiO,, expressed as in inverse correlation with the shows some unexplained anomalies (e.g., in Ce or SiO, content. Gd), which may be due to analytical errors at low concentrations (see for example [8, fig. 21). It has 3.7. Rare earth elements been concluded [12,14,19] that the REE patterns of the Zhamanshin crater glasses are consistent It has been noted previously [8,12,19] that the with a terrestrial sedimentary pattern originating rare earth element (REE) patterns of these impact from two or three different source rocks, a conclu- glasses are almost identical to the post-Archean sion supported by the data presented here. The average sediment (PAAS) pattern range [27-291. spread of the REE patterns in different samples The patterns for the zhamanshinites and the may easily be explained by taking a diluting com- irghizite are displayed in Fig. 3. We note that all ponent (e.g. quartzite) into account. our samples fall slightly below the PAAS pattern range, but also below patterns of the samples 3.8. Gold and selenium analyzed by Taylor and McLennan [12], although the difference is marginal. The variation exhibited Gold and selenium, moderately siderophile ele- by the zhamanshinite samples defines some range ments, have been used to determine the nature of in the patterns (for example the pattern of sample impacting projectiles (see for example [26]), with R2556a from BouSka et al. [8] is virtually identical gold being the most volatile of the siderophiles to ours). This variation may be caused by dilution and selenium being almost chalcophile. An enrich-

I I I I I I. I I I I I I I I I - USNM 6200 700 = - 18207 - -v USNM 6074 -d USNM 6075 -2

I I I I I I I I I I I I I I I 7 La Ce Pr Nd Sm Eu W Tb Dy Ho Er Tm Yb Lu Fig. 3. -normalized REE patterns of the irghizite and the zhamanshinite samples analyzed in this work. All snmplesshow a very pronounced sedimentary pattern. The difference between the samples (with the exception of the extent of the Eu anomaly) is mainly due to different silica contents. a7

TABLE 5 Ferric/ferrous ratios of four impact glass samples from the Zhamanshin crater. Total iron was determined by electron-probe analysis and INAA (except for RSZ-76-1). Fe0 by a wet chemical method (see [31,32])

Sample Fe0 Fe& Fe( III) Total Fe as Fe0 Fe(H) USNM 6200 (irghizite) 5.45 1.17 0.19 6.50 h RSZ-76-l” (irghizite) 4.00 1.73 0.39 5.56 h USNM 6014 (zhamanshinite) 3.41 0.61 0.16 3.95 = USNM 6015 (zhamanshinite) 4.01 0.71 0.16 4.65 ’ ’ Sample RSZ-76-l (USNM). 0.6 g fragment of - 2 g irghizite obtained from P.V. Florensky. Wet chemical analysis by J. Barrows, Smithsonian Institution. h Average of 21 irghizites: FeO: 5.5 f0.5 wt.% [9]. ’ See Table 1. merit in these elements points to a possible 4. Conclusions meteoritic contamination. The determination of the nature of a projectile seems to be clearer if we Data for up to 40 major and trace elements in consider siderophile trace elements rather than three silica-rich zhamanshinites and one irghizite major elements. Also elements like Cr or Co are have been presented. A clear connection between very vulnerable because of potential terrestrial the trace element data for these glasses and the contamination. trace element patterns in sedimentary rocks is Here an enrichment of Au, but not of Se re- present, most prominently shown by the REE. spective to average sediments is present. Also, The Zhamanshin glasses differ in chemical com- gold is enriched in the irghizite respective to the position from Australasian tektites, thus providing zhamanshinite. This may be another indication of further evidence that the two formation events are a meteoritic contamination. Palme et al. [26] also not correlated. Further arguments against a rela- analyzed two irghizites for Au and Se and found tionship between the Zhamanshin and the about 0.4 ppb Au and 7-12 ppb Se, while BouSka Australasian event have been provided previously et al. [8] found 6.7 ppb Au in one silica-rich by Shaw and Wasserburg [13] on the basis of zhamanshinite. Our data are lower in Se but higher isotopic data and by Storzer and Wagner [30] on in Au when compared to the data for Palme et al. the basis of age determinations. It is difficult to [26], but apparently consistent with the suggestion interpret the chemical differences as only due to of an iron projectile ([26], and unpublished data lower heating temperatures, because ratios in geo- on other siderophiles by C.K.). One might expect chemical pairs are not influenced by such effects Se to be less depleted if the projectile was a alone. Hints of a meteoritic contamination are, chondrite. There is, however, strong evidence for a however, clearly present. Our chemical data sup- chondritic origin from microirghizite studies [25], port the concept derived from a recent study of thus the matter is not resolved as yet. microirghizites [25] that the Zhamanshin and associated glasses provide an exam- 3.9. Ferric/ferrous ratios ple for the origin of tektites. Although the irghizites are not as homogeneous as the splash-form Table 5 gives the content of Fe0 and Fe,O, Australasian tektites and do contain more volatile and the Fe(III)/Fe(II) ratios in three of our four elements, indicating a lower peak pressure and samples and another irghizite. In normal splash- temperature during the formation event, it is not form tektites the ferric/ferrous ratio rarely ex- possible to explain the chemistry of the irghizites ceeds 0.10, but here the ratio is larger. This is (and zhamanshinites) as the result of the impact of indicative of lower formation temperature and a homogeneous glassy body as proposed by pressure and is similar to the behaviour of O’Keefe [24]. Muong-Nong-type tektites. 88

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