OOM-7037/88/S3.00+.00

Blue glass: A new variety from Zhamanshin crater, U.S.S.R.

CHRISTIANKOEBERL Instituteof Geochemistry,University af Vienna, ~.-~~-~~r-Ring 1, A-lot0 Vienna, Austria

(Received September 5, 1987;accepted in revisedform December 22, 1987)

Aktmct-A newvariety of impact giass has been discoveredat Zhamansbin (U.S.S.R.). The craterhas been known as the source of differentimpact glassessuch as irghizites and Si-rich zhamanshinites @-rich varieties) and Si-poor (andesitic)zhamanshinites. The newly discoveredimpact glass is ofdistinct blue color and shows a layered structurewith numerous small vesicles. The blue color rangesbetween the layers from opaque turquoiseto very dark blue. The iayers of blue glass are usually connected with layers of greyish or brownish color showing normal Si-rich zhamanshinite composition. The major and tram element chemistry of the blue glass differ from the chemistryof other Sirich impact@asses from the Zhamanshincrater in several ways. One of the ti distinct ftptures is the high CaO content (up to about 7 wt.%),and the different CaO/MgOratios. Volatile trace elements are generafly intemwdiate betwen i&i&es and Sikh zhamanshinites, or even higher than in Si-rich zhamaMini@, refkcting the inhomogpmity of the blueglass. REE abundancxsanz slightly Iar&r than in Si-rich ~rna~init~ and irghizites and show a les$ monounced Eu anomaly. Impactmixing of country rocks present at the crater seems capableof explainingthe chemistry bf the blue glass.

INTRODUCTION pieces of sedimentary precursor rocks are embeddedin some S&richzbamanshinites). The layers contain numerous vesides THE ZHAMANSHINIMPACT CRATER has recently been in the which in some cases show a distortion along the plane of focus of some investigations of impact processes and impact glass flow. Compared to Muong Nong type indochinites, Si- glass,The crater has a diameter of about 6.5 km and is situated rich zhamanshinites are more inhomogeneous. They show near the river Irghiz in the province of Khazakhstan in the interbedded greyish, opaque, white-yellow, brown and black U.S.S.R., approximately 200 km north of Aralsk. The crater glassy layers of variable composition. The other previously is situated in a desert environment and has a rather complex known Si-rich impactite variety, the irghizites, are small and geological setting. The central part of the structure is filled of irreguW shape (wiie-rope-, or droplet-form). They are more with loess and sediments, while the rim consists of outcrops like than like usual impact&, but still display some of Palaeogene and Mesozoic rocks. The bedrocks comprise distinguishing ftatures (like the lack of spherical symmetry, Paiaeozoic metamorphic crystalline rocks, including Upper limited distribution not in a strewn field but in part of an Pa&ozoic vokanic-sedimentary series, and some ultrabasic impact crater, higher water content than tektites). They have material found in veins cutting the bedding. The geology and probably formed by welding smaller particles, the microirgh- geophysics of the crater is described in more detail by ELcr idus (GLASSei al., 1983),@gether to form the Iarger &hi&es RENSKU(1975), ~ORENS~UJef al. (1977,1979), FLQRENWJ (FREDRWBON, 1987). and DABHXZA( 1980) and MXwns et al. ( 1984). The study Si-rich mtes, and to a lesser extent also irghizites, of some drill cores, which have been taken in the crater floor show large variations in chemical composition-not only be- recently, wilI supply additional data on the stratigraphic se- tween different samples, but also within individual pi- quence of the rocks underlying the crater (D. D. BADYUKOV, The major element chemistry of Zhama&$n glasseshas been oral commun., 1986). studied by various authors (e.g. FLORENSKIJand DABIZHA, The occurrence of diff&ent types of impact metamor- 1980; BOUSKAet al., 198f; EHMANNet al., 1977; FRED phosed rocks and impact glasses at one impact crater distin- RIIWON et al., 1977; KOEBERLet a(., 19858; IZOKH, 1986) guishes the ~~nshin impact crater from most other im- and have been used to establish some coneiation between pact craters; only rarely are such a wide variety of country rocks and impact glasses, though the number of found in one location. The impact glasses can be subdivided country rock analyses is rather small (IZOKH,1986). Trace in two major groups: Si-rich and Si-poor glasses.The S&poor element data are sparse, but seem to justify the subdivision impact&es (also called andesitic or basic impactites) occur as of Zhamanshin glasses into several distinct groups. More slag-like, chunky pieces of variable composition. They are complete trace element analyses (induding about 3040 trace partly glassy, partly recrysmllizd and often incorporate more element abundances) have been reported by TAYLORand or less altered (but recognizable) inclusions of country rocks. MCLENNAN (1979) and KOEBERLand FWBRIKSSON( 1986) Chemically they are distinguished by a SiOz content which for three irghizites and four silica-rich zhamanshinites. In is generally less than 55 wt.%. addition, KOE.BERLet al. f 1986) reportedpreliminary data The Si-rich impact glasses are o&n more homogeneous for four additional irghizites. Thus the data base is clearly than the Si-poor varieties and have average Si@ contents not satisfactory as far as trace elements are concerned. Data between 70 and 80 wt.%. S&rich rhamanshinites are glassy for volatile trace elements, which ate useful for the deter- chunks not unlike the Muong Nong type indochinites from mination of volatilization effects during the generation of the Australasian strewn fitid. They show a layered impact melts, are also rare (KOEBERLand STORZER,1987). structure and clear signs of glass flow (more or less intact The discovery of a new impact glass variety among the 779 780 C. Koeberl

Zhamanshin impact glasses is clearly of some importance, fact, there is a correlation between the layering ot the blue since it enables more detailed comparison studies between glass and the chemistry. Parts of the distinct blue color are country rocks and impactites. A short note on the existence Ca-rich, while grey or brown parts are Ca-poor and thus rep- of some blue glass from the Zhamanshin crater was 8iven by resent layers of normal silica-rich zhamanshinite glass. Table FLORENSKIJ and DABIZHA(1980), but without further data. 2 gives data for a microprobe scan across a transition zone Following some chemical studies. KOEBERL etal. (1986) were between a Ca-poor and a Ca-rich region (going from a able to show that these glasses indeed comprise a new im- brownish to a blue layer). pactite variety. The data from the microprobe scan and the analyses given in Table 1 allow the following observations: the iron content ANALYTICAL METHODS shows a positive correlation with calcium. The Fe0 content of the brown parts, which resemble Si-rich zhamanshinites, Major element analyses were performed using staadard electron probe microanalysis techniques Usually 10 x 10 firn areas within is lower than the average Fe0 content of S&rich zhaman- larger chips inea&ng about3X3mmhavebeenanalyzedtostudy shinites (Table 1). In the high-Ca regions the MgO content intrasantple inhomgneitics. The trace element analm were made is generally also higher than in normal Si-rich zhamanshinites. ushtg neutron activation analysis techniques. For details on the but the enrichment is less than for Ca. The CaO/MgO ratio methods see e.g. Kozn~tu ez ol. (1984, 1987). in irghizites averages 0.7, in Si-rich zhamanshinites 1~1 (but with considerably different abundances), while in the Ca-poor NEW BLUE ZHAMANSHINITFS parts of the blue glass the ratio ranges from about 1.6 to 2. I The new impact glass variety has, due to tire color, been (CaO < 2 wt.%), and in the Ca-rich parts (CaO > 2 wt.%) termed “blue zhamanshinites” or “blue glass”. It occurs in the ratio ranges from 2.7 to 6.7. Thus the CaO/MgO ratio in small chunks not unlike Si-rich zhamanshinite glass and has all parts of the blue glass (including the brownish-grey layers) been found to date only at one particular spot within the is different from both S&rich zhamanshinites and irghizites. Zbama&in crater. Although there are some small pieces Si-poor zhamanshinites, on the other hand, show higher CaO (with a maximum of about 2 cm diameter) which consist abundances (average: 7.35 wt.%) and slightly higher MgO entirely of blue glass, it is commonly found to be associated abundances (average: 2.96 wt.%), which yields a ratio of 2.5, with bmwn or grey glass ofthe silica-rich zhama&inite type. which is still different from blue zhamanshinites (IZOKH, Most samples show layeting, where thin (l-5 mm tide) layers 1986). The one analysis which comes very close to S&rich of blue glass are embedded in brown or grey glass layers, or zhamanshinite composition (B-l in Table 1) has a higher difkrent blue layers are alternating with brown glass parts. K20 content, but a lower Fe0 content. Analysis B-4, which The color within or between the glass layers varies from light shows the second-lowest CaO abundance, also has a higher (sometimes opaque) bluish turquoise to dark blue. Most than average KrO abundance, and a lower Fe0 abundance. samples show clear indications of flow structures and vesicles, The same trend can be observed in Table 2. When CaO is with elongated bubbles and flow smlcmms around inclusions low, Fe0 is also low, but KrO is up (Figs. 1, 2). ‘The NarO of different refractive index in the same plane with the layers. content of the blue parts is lower than in the average S&rich As for Si-rich zhama&inites, there is a simiktity with Muong zhamanshinite, but the KzO/NarO ratio shows little variation Nong type indochinites. There are some features, however, and averages about 1.7 (compared to about 1.6 in Si-rich which are different in blue zhamanshinites. They are of zhamanshinites). The incorporation of andesitic material smalkr size, the bubbles are smaller, the flow structures are seems not to be responsible for the K20 variation, since this different in so far as being smaller than in Muong Nong-type would result in an increased NazO content. indochinites and involving opaque glass flows. The chemistry Thus, from the major element abundances we can conclude is also different in blue zhamanshinites. that the blue zhamanshinites are definitively a new group, Major element data for two different blue glass samples which is chemically distinct from the other, previously known, are given in Table 1. Since the samples are very inhomoge- impact glasses at the Zhamanshin crater. They constitute the neous, and compositions vary within one small chip to a third Si-rich impactite variety, together with irghizites and considerable degree, it is not possible to give an average com- Si-rich zhamanshinites. position. Instead, compositions of several small (10 X 10 Trace element data for three different blue zhamanshinite Mm) areas within the samples Zh 3 1/6A and Zh 3 1/6B are samples (Zh 31/6A, Zh 31/6B and BZ 8601) are given in reported. In both samples the large chemical variations be- Table 3. For trace element analysis, small chips from the tween different data points are obvious. One of the most blue layers have been separated, avoiding brown layers. striking features is the unusually high CaO content in most However, the intergrowth of the layers prevents a clean sep- parts of the samples, which ranges up to about 7 wt.%. Some aration. The anbundances of three major elements (Na, K, pmrtsof the sampies have a different chemical composition. and Fe) are also obtained in INAA, and allow some control The analyses El, B-3 and B-4 (Table I) show CaO abun- of how close to the major element composition of the blue dances of less than 2 wt.%, in contrast to another part of the layers, as obtained by microprobe analyses, the INAA samples same sample (B-S), where to CaO abundance is larger than are. Sample BZ 8601 seems to contain some admixture of 5 wt.%. Tabk 1 also gives some comparison data for Si-rich grey or brown glass (higher K, Na, lower Fe), but still rep- zhamanshinites and irghizites (averages as well as ranges), resents the blue glass chemistry well. and a comparison with the blue zhamanshinite analyses Comparison with trace element data for irghizites and Si- mentioned above shows that the low-Ca parts of the blue rich zhamanshinites (TAYLOR and MCLENNAN, 1979; KOE- glass are very similar to normal Si-rich zhamanshinites. In BERL and FREDRIKSSON.1986; KOEBERLet al.. 1986) shows ESiueglass from Zham8nshin crater 781

Table 1. Majorelement analyses of blue zhaunshinites. A-l to A-4 refer to different areas of wple Zh 31/6A. B-l to 8-B to different areas of saagle Zh 31/68. Cmparison data are given for Si-rich zhamn- shinites and irghizites (averages fnm Izokh, 19%; canpositional ranges after Florenskij and Dabizha, 1980, and Boufka et al., l%l). All data in wt.%.

SO* Al~0, FeO" CIO n9g K,O Nat0 TiOI UnO

SW? zh~nshinites A-l 71.3 10.14 4.73 6.30 1.74 2.56 1.44 0.53 0.10 A-2 71.7 10.17 4.96 6.23 1.74 2.58 1.31 0.53 0.08 A-3 71.2 10.29 5.29 7.07 1.92 2.42 1.27 0.62 0.08 A-4 61.8 19.24 5.79 7.31 1.09 2.31 1.73 0.67 0.09

8-f 77.1 12.09 3.34 1.16 0.71 3.33 1.82 0.57 O.OE 8-2 75.4 11.05 4.22 2.91 1.07 2.84 1.61 0.57 0.12 B-3 77.2 10.16 4.07 1.90 0.99 2.84 1.47 0.56 0.09 8-4 74.0 13.16 3.99 1.76 0.85 3.09 1.81 0.61 0.12 B-5 72.0 11.62 5.06 5.61 1.67 2.28 1.44 0.59 0.08 B-6 74.7 11.75 4.18 3.12 1.03 2.77 1.69 0.63 0.06 8-7 74.4 10.60 4.93 4.52 1.47 2.43 1.41 0.57 0.13 B-E 75.0 10.20 4.41 4.35 1.40 2.54 1.49 0.59 0.12

Irghizites (ave.) (15 samples) 72.5 10.9 6.81 2.08 2.92 2.04 1.47 0.80 0.13

Irghitites 70.0 9.45 4;', 1.75 2.16 1.58 0.85 0.69

@awe) 7:s 13:: 6.81 2:5 34075 224 l'$ 0%

Si-rich zhaman- shinites (ave.) 75.7 11.5 5.30 0.94 0.87 2.86 1.83 0.63 0.10 (42 saaplesl Si-rich zhaiaan- 62.9 4.80 1.98 0.55 0.34 0.10 0.57 0.23 shinites (range) 8& 21:; 825 226 1:!6 3:& 1% 1::O

* all Fe as Fe0

some diffencncesbetween them and the blue glass. Nickel is i&i&s, cobalt is also enriched in the blue glass compared con&kmbly lower in the blue glass(probably comparabk to to Si-rich zhamanshinites, Some volatile ekments (As, Rb, Si-rich zhammtes) #an in irghizites.Chromium in the Cs) are intermediatebetween irghiGtc.sand Si-rich zhaman- blue glassis ixBcmx&& betweenSi-rich zbam -8od shin&s, whik chlorine, antimony and gold arc higher in the

Table 2. Microprobe scan across part of a sect&m of blue zh=nskinlta Zh 31/M. All Fe as FeO, and al! data in wt.%. Tha first data point is taken as 0. all distances fron this point we given fn a.

Point No. 1 2 3 4 5 6 7 8 9 10 Distance 0 6.8 12.5 20.0 26.9 34.0 H).9 72.6 102 109

sio, 75.3 75.5 74.2 73.6 11.6 73.4 73.0 72.6 71.9 72.8 Al~0, 10.6 10.4 10.9 10.3 9.83 10.4 10.1 10.1 10.6 10.6 Fto 3.87 3.73 3.85 4.24 5.21 4.04 4.62 4.73 5.03 4.65 CaO 3.21 3.50 4.35 5.28 6.73 5.89 6.13 6.#) 6.37 6.02 MO 1.17 1.16 1.27 1.51 2.02 1.44 1.72 1.74 1.80 1.62 K,O 3.23 3.18 3.03 2.73 2.39 2.66 2.50 2.56 2.61 2.62 Ma,0 1.74 1.71 1.70 1.63 1.34 1.45 1.39 1.44 1.41 1.36 TiOI 0.62 0.65 0.59 0.63 0.57 0.66 0.57 0.61 0.62 0.58 m0 0.11 0.05 0.10 0.07 0.08 0.07 0.07 0.10 0.09 0.06 782 C. Kceberl

Table 3, Trace element abundances in three blue rhamanshinites, ob- tained by INAA. AI1 data in ppm, except for Na, K, and Fe in ut,Z. . . . 6 - . ‘. . Zh 31/6A Zh 31/6B BZ 8601 - . Na 1.14 1 ,I5 1.53 3 . . . . C1 500 400 200 i 0 ’ K 2.1 2.1 3.0 . : . . SC 11"2 9.0 10.3 . Cr 167 111 170

2- Mn 750 695 575 .* Fe 4.43 3.55 3.48 . , 1 1 1 I co 24 14 14.5 3.2 2.6 4.0 4.4 4.8 5.2 5.6 Ni c300 4200 t200 F.0 (wt.%) Ga 20 18 15

RG. I. Correlation diagmm of CaO vs. Fe0 for blue zhamanshin- As 2.5 3.2 5.3 ites. A positivecorrelation is evident. Br

. I 1 I I , DISCUSSION

. The descriptive and chemical data clearly show that blue . zhamanshinites form a dif%rent subgroup of impact glass 5.0 - l , ’ . among the Zhamanshin crater impact&es. They differ in ap- Q . . . pearance as well as in major and trace elements compared j . to Si-rich zhamanshinites and irghizites. There is, however,

0 l . l some similarity to Si-rich zhamanshinites, especially if trace . t 1.0 - . . elements are considered. The layers of brown and grey glass D . . associated with blue layers have a major element chemistry resembiing S&rich zhamanshinites, but with higher KzO and D lower Fe0 abundances. I L I 1 I 2.2 2.4 2.6 2.8 S-0 2.2 2.4 The inhomogeneity of the glass indicates incomplete mix- ing of melts ofdifferent composition. Due to lack of complete It,0 (wt.%) analyms including country rocks, no mixing models for irgh- FIG. 2. Correlation diagram between Fe0 and KrO for blue zha- izites and zhamanshinites are available so far, but it seems manshinites. A negative correlation (I = 0.99) is evident. as if blue zhamanshinites originated from similar country Blueglass from Zhamanshincrater 783

Archeanupper crustal rocks. The (~Lu)cx of 6 (in the range

loo of irghizite and xhamanshinite ratios and comparable with tektites from various strewn fields), a steeper LREE and a flat HREE pattern, the presence of negative Eu-anomalies, and the total abundances all are indicative of a sedimentary signature. fn addition, trace element ratios like Th/Sm, Ba/ Rb, or Th/Sc lead to the &me conclusion (TAYLOR,1973; 10 TAYLOR and MCLENNAN,1979; KOEBERLef al., 1985b; KOEBERL,l&6). Volatile traceelement abundances are sim- ilar to Si-rich zhamanshinites, although a few of them am enriched (Cl, Sb and Au). This seems to reflect the inho- mogeneous mixing of different precutsor melts. Clearly, the blue glass is not as homogeneous and most probably expe- rienced only Iower shock pressures and temperatures than the irghizites.

and !&Poor(an&sitic) zhamanshinites. Data soixces: blue zhaman- CONCLUSIONS shinite, this work;irghizite and Si-rich zbamanshinite,KOEBERL and FREDRIKS+WN( 1986); Si-poor zhamanshinite, TAYLOR and MCLEN- Blue zhamanshinites form a distinct subgroup of impactites NAN(1979). at the Zhamanshin impact crater, U.S.S.R. Their chemistry is sufficiently different from Si-rich zhamanshinites and irgh- izites to justify this conclusion. The glass, which was found rocks to the Si-rich zhamanshiaites, including at least one only in one location within the crater, occurs in small chunks different ~rn~nent. Iron and calcium and calcium and which show blue and brown-grey layers. The chemistry of magnesium show positive correlations in the blue glass, thus different colored layers is difErent. Blue layers are Ca-rich it seems likely that they were introduced by the same com- with C&Oabundance in excess of 7 wt.%,while brown and ponent. On the other hand, incorporation of small amounts grey layers show higher K20 abundances (when compared of one &rich component (such as calcarenaceous sandstone) to Si-rich zhamanshinites). The association of the oolored is just as plausible, because Fe is not anomaleously enriched layers seems to be caused by incomplete mixing of melts of when compared to Si-rich zhamanshinites. Admixture of an- differentcompositiori. Zhamanshin glasses have been formed desitic material (as in Si-poor zhamanshinites) does not seem from more than two diffmnt precwo rrocks. Tnthecaseof likely, beoause no Na20 enrichment is observed in the blue the blue ~man~init~ (the C&-richblue layers),at least two glass. components are differentfrom Si-rich rich ~~n~nit~ Schists with calcite veins (IZOKH, 1986) and limestone (at least concerning their initial relativeabundances), an ul- (FLORENSKIJand DABIZHA,1980) have been reported from trabasiccomponent (Cr, Co), and some carbonate rock (cal- the crater and are probable precursor components for the blue glass. It should be noted that only a small amount of components appear to be similar to Si-rich zhamanshinites. Ca-rich material is required because of the scarcity of blue The sedimentary nature of the majority of the precursor glass and the fact that not all of the blue glass is composed rocks is demonstrated by a sedimentary REE patttem and of C&rich layers (a wet chemical analysis of a larger piece of several trace element ratios (Th/Sm, Ba/Rb). The association a blue glass gave 2.70 wt.% CaO, which is Iower than some of the blue glass with the impact and with other locat impact parts of the samples discus& here, but still higher than av- glasses is also demonstrated by the fact that they have ages erage zhamanshinites (Bou&A, pers. commun., 1986)). Ad- (KOEBERLand STORZER,1987) which are identical to the ditional chemical data on country-rocks are necesmry to ages determined for the crater and other impact glasses clarify the nature of the Ca-rich component involved in the (STORZERand WAGNER,1977, 1979; KOLE~NIKOVet al.. production of the blue glass. 1987). The trace efement chemistry of the blue glass indicates a slight enrichment of siderophile elements such as cobalt and Ackmwfedmts-1 would like to thank M. A. Nazarw and chromium as compared to Si-rich z~rnan~~~. Chro- D. D. Badyukov(Vemadsky Institute of Ge&em&y and AaaIyticaI Chemistry,U.S.S.R. Academy of%iences, Moscow)for many sam- mium in S&rich zhamanshinites usually ranges from about pies,valuable discussions, and ho&a&y duringa visit to Moscow. to 40 to less than 100 ppm, cobalt is often around 10 ppm In addition, I have to thank E. Izokh(Institute of Geologyand Gee- (KOEBERLand FREDRIKSSON, 1986). The introduction of physics, Siberian Branch of the U.S.S.R.Academy of Sciences,No- chromium is also possible with some basic component which vosibirsk) for his hospidity, important discussions,and vaIuaMe may also be responsible for the enrichment in potassium in prepublieationdata on Zhamaushinsamples I am also gratefulto John D&no and an anonymousreferee for helpfulcomments on the brown or grey layers. Despite the fact that some elemental the manuscript. abundances in the blue glass are intermediate between irgh- izites and Si-rich z~manshinit~, little other evidence sup- Editoria! kayoing: S, R. TayIor ports the assumption that they are also genetically (and phys- ically) an intermediate variety. They have varying REE REFERENCES abundances with Eu anomalies which are slightly more pro- BouSKAV.,POVONDRA P., FLORENSKIJP.V. and ~ANDAZ.( 1981) nounced than in Si-rich thamanshinites. Irghiritesand zhamanshinites:Zhamanshin crater. U.S.S.R.Me- The whole REE pattern is sedimentary and typical of post- feorizics 16, 17I - 184. 784 c. Koeberl

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