~~hernis~y and origin of muong nong-type tektites*...american, moldavite, ivory coast, and...

Geochimica ef Cosmochimica ACM Vol. 56, pp. 1033-1064 Copyright 0 1992 Pergamon Press plc. Printed in U.S.A.

0016-7037/92/$$.00 + 00

~~hernis~y and origin of Muong Nong-type tektites*

CHRISTIAN K~EBERL

Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring I, A- 10 10 Vienna, Austria

(Received January 3 1, I99 1; accepted in revised form December 20, 199 1)

Abstract-Muong Nong-type tektites are one of three tektite groups occurring on land (the others being splash-form and ~r~yn~i~ly shaped tektites). They differ in appearance from splash-form tektites by having irregular, blocky shapes and a layered structure. In thin sections, dark and light colored layers alternate, with dark layers being less abundant and embedded in a lighter glass matrix. The dark layers contain fewer bubbles than the lighter zones, in which bubbles are much more abundant than in splash- form tektites. Lechatelierite is often frothy, indicating that homogenization with the surrounding glass was not as efficient as in splash-form tektites. All nineteen samples studied here belong to the high silica group. The major element contents show an inverse correlation with the SiOz content. Additionally, forty-four trace elements have been determined in all samples, using various methods. Muong Nong- type tektites are enriched in volatile elements compared to splash-form tektites. The halogens F, Cl, Br, and I, and several other volatile elements (e.g., B, Cu, Zn, Ga, As, Se, Sb, and Pb), show enrichment factors that vary between about 1.5 and 25, with the highest enrichments being shown by Cl, Br, and Zn. Compared to volatile element contents of possible target rocks, Muong Nong-type tektites are only slightly depleted compared to the target rocks, while splash-form tektites show considerable depletions. Some volatilization and selective element loss affected the tektites during their production, but only the volatile elements were affected, in contrast to the suggestion that volatilization of silica took place. The water contents are also slightly higher in Muong Nong-type tektites than in splash-form tektites (0.014 wt% Hz0 vs. 0.008 wt% HzO). Trace element ratios such as K/U, Th/U, La/Th, or Zr/Hf of Muong Nong- type tektites are very similar to those of the average upper continental crust. The chondrite-normalized REE patterns of the Muong Nong-type tektites are very similar to those of post-Archean upper crustal sediments. Local soil samples have different REE patterns, La/M, slopes, and Ce and Eu anomalies. Mixing of local soils, or with some related Ioess samples, cannot reproduce the tektite REE patterns, and any basaltic, oceanic, or extraterrestrial rocks can be excluded as source rocks as well. The La/Th ratio of Muong Nong-type tektites is additional evidence for an origin from post-Archean sediments.

Major and trace elements have been analyzed in chips of dark and light layers, showing that a distinct chemical difference exists between the layers. Light layers have higher contents of A1203, FeO, TiO*, and MgO, and lower contents of SiOz, but the enrichment is not in linear correlation with the SiOz content, thus simple dilution with silica cannot account for these differences. Trace element abundances, element ratios (e.g., K/U, ThfU, and La/Yb), and REE patterns show marked differences between layers. This indicates incompIete mixing of different (but not completely di~imil~) parent rocks. Fe~c/fe~ous iron ratios were determined in all samples, yielding an average of 0.133, which is slightly higher than the ratio determined for two thailandite samples (0.07), but not different from the average ratio of 0.14 that was determined by previous analyses for australites.

Muong Nong-type tektites differ in the following criteria from splash-form tektites: ( 1) higher concentrations of volatile elements (e.g., Cl, Br, Zn, Cu, Pb); (2) chemically inhomogeneous on a millimeter scale; (3) dark and light layers with different chemical compositions; (4) may contain relict mineral inclusions (e.g., zircon, chromite, Wile, quartz, monazite); (5) large and more abundant bubbles that may be elliptical, showing glass how; (6) large and irregular sample size with no sign of ablation, Muong Nong-type tektites have most probably originated during impact melting from a mixture of post-Archean sediments with compositions close to that of the upper crust (e.g., greywacke, sandstone, shale, etc.). Local loess and soil mixtures may reproduce the major element chemistry of average Muong Nong-type tektites, but the trace element ratios and REE patterns differ, and Sm/Nd-Rb/Sr isotopic studies of Muong Nong-type tektites exclude recent young sediments such as soil or loess as tektite source materials. The data are in agreement with older sediments (with a sedimentation age of about 167 Ma) such as shales or greywacke. The chemical and isotopic data also do not support an origin of Muong Nong-type tektites from a multitude of very small impact craters. A single large impact, maybe occurring at an oblique angle, was probably responsible for all tektites in the Australasian strewn field. The crater is likely to be situated on or near Indochina, e.g., unde~ater, on the ~ontinen~l slope east of Vietnam, or on land (i.e., the Cambodian lake of Tome Sap). The production of tektites seems to require special impact conditions because otherwise there should be more than four tektite strewn fields. Muong Nong-type tektites have not travelled far from the site of the impact, which most probably occurred somewhere in Indochina into post-Archean upper crustal sediments.

* Presented at the symposium for S. R. Taylor, “Origin and Evolution of Planetary Crusts,” held October 1-2, 1990, at the Research School of Earth Sciences, ANU.

1033

1034 C. Koeberl

aer~ynamic~ly shaped tektites, and (c) Muong Nong-type tektites (sometimes also called layered tektites). The first two

TEKTITES ARE A group of natural glasses that have been groups differ only in their appearance and some of their known to humankind for many centuries. They are chemi- physical characteristics. The aerodynamic ablation results tally homogeneous, often spherically symmetric objects that from partial re-melting of glass during atmospheric passage are several centimeters in size, and occur in four known after it was ejected outside the terrestrial atmosphere and strewn fields on the surface of the earth (Fig. 1): the North quenched from a hot liquid. Aerodynamically shaped tektites American, moldavite, Ivory Coast, and Australasian strewn are known mainly from the Australasian strewn field, where fields (BARNES, 1963a; O’KEEFE, 1963). Tektites found they occur as flanged-button australites. The shapes of splash- within such strewn fields are related to each other with respect form tektites (spheres, droplets, teardrops, dumbbells, etc., to their petroIogical, physical, and chemical properties as well or fragments thereof) are often erroneously described as as their age. A discussion of the ge~herni~~ and origin of aer~ynami~ forms; they, however, result from the solidi- the tektites, among other things, has to explain the similarity fication of rotating liquids in the air or vacuum. of tektites in respect to age and certain aspects of isotopic Muong Nong-type tektites are named after a locality in and chemical composition within one strewn field as well as Laos where they were first found by Lacroix. He was the first the variety of tektite materials present in each strewn field to describe them in his paper about the “tektites without (TAYLOR, 1973; KING, 1977; KOEBERL, 1986a, 1988a, 1990). shapes” ( LACROIX, 1935). They differ from the other two

In most of the strewn fields, the occurrence of tektite glasses groups in several ways. They are usually considerably larger is not restricted to the continents. Microtektites have been than normal tektites and are of chunky, blocky appearance. found in deep-sea cores. They are generally less than 1 mm The largest Muong Nong-type tektite described in the liter- in diameter, although the new finds of tektite material on ature weighs 12.8 kg (BARNES, 1971), but recently other Barbados and in DSDP Site 6 12 samples (e.g., KOEBERL and Muong Nong-type tektites of similar size, and two samples GLASS, 1988; GLASS, 1989) seem to blur this traditional dis- each weighing 24.1 kg, have been offered at a Bangkok deal- tinction between microte~it~ and “macro” tektites. Apart ership. Muong Nong-type tektites show a layered structure from the microtektites, tektites on tand can be subdivided with abundant vesicles. Microscopic e~ination of the Iayers into three proups: (a) normal or splash-form tektites, (b) shows bands of lighter and darker color (which should not

V IV AUST~~SIA~ \ STREWN FIELD

\ \

\ -2,

----______----

FIG. 1. World map showing the approximate extent of the Australasian tektite strewn field as outlined by tektite finds on land and microtektite finds in deep sea cores. The sample location of Ubon Ratchathani, the source of the present samples, is indicated. The two full squares indicate the locations where tektite samples have been found on the ocean floor (near Vietnam: SAURIN and MILLIES-LACROIX, 1961: in the Indian Ocean: PRASAD and RAO, 1990).

Geochemistry of Muong Nong-type tektites 1035

be confused with schlieren ) . Some mineral inclusions (zircon, chromite, rutile, corundum, cristobalite, etc.; GLASS, 1970, 1972; GLASS and BARLOW, 1979) and coesite (WALTER, 1965) have bee.n described from Muong Nong-type tektites.

The average major element chemistry of Muong Nong- type tektites, and their age, is (within normal variations) identical to normal tektites. Muong Nong types are abundant in the Australasian strewn field, but recently a few samples that are most probably of Muong Nong type have also been found in samples from the moldavite ( MEISEL et al., 1989; GLASS et al., 1990) and North American ( KOEBERL and GLASS, 1988; WITTKE and BARNES, 1988) strewn fields. There are some important differences in the chemical composition of Muong Nong-type tektites: they are chemically less homogeneous than normal tektites, and there are differences in composition between the layers. Furthermore, water and other volatile elements are enriched in Muong Nong- type tektites-or just less depleted (see, e.g., MOLLER and GENTNER, 1973; KOEBERL and BERAN, 1988; and later).

Taylor and co-workers (see, e.g., TAYLOR, 1962a,b, 1966, 1973; TAYLOR and SACHS, 1964; TAYLOR and SOLOMON, 1964; TAYLOR and KAYE, 1969) have contributed essential details that led to a probable theory for the origin of tektites. Because of their chemical studies, it is now commonly accepted that tektites are the product of melting and quenching of terrestrial rocks during hypervelocity impact on the Earth. The chemistry of tektites is in many respects identical to the composition of upper crustal material (TAYLOR, 1973; KOE- BERL, 1986a). Trace elements are particularly useful in this respect: the ratios of, e.g., Ba/Rb, K/U, Th/Sm, Sm/Sc, Th/Sc, and K vs. K/U in tektites are indistinguishable from upper crustal rocks. The chondrite-normalized REE patterns of tektites are very similar to shales or loess, and have the characteristic shape and total abundances of the post-Archean upper crust. None of the trace element ratios or REE patterns are anywhere near lunar or other extraterrestrial values. Rb/ Sr and Sm/Nd isotopic studies further confirm the connection with terrestrial rocks ( SHAW and WASSERBURG, 1982).

The exact target rocks from which tektites were produced are, however, not known. The reason for this is manifold: for some fields, the exact source crater is not known, the extension of the strewn fields is very large, a variety of inhomogeneous target rocks was sampled by the impact, the exact physico-chemical processes which may alter the chemical composition during the impact are not known. Muong Nong-type tektites seem to be the key to many of these ques- tions. They are less homogeneous than splash-form tektites and may therefore preserve the original target rock compositions much better. They are, in some ways, the missing link between target rocks and tektites. Impact glasses that are similar to Muong Nong-type tektites, with similar petrographic and chemical characteristics (e.g., KOEBERL and FREDRIKS SON, 1986)) have been found at the Zhamanshin impact crater in the USSR, thus confirming the link with an impact event. Because of the size and shape of Muong Nong-type tektites, it can be assumed that they have not travelled far from their location of origin, and may therefore lead to the crater.

The present report is based on the detailed study of nineteen Muong Nong-type tektite specimens. The petrographical

characteristics and the chemical compositions of these samples were studied in great detail, using a variety of techniques. Electron microprobe studies have been performed to inves- tigate the chemical differences between layers and within layers. Furthermore, light and dark layers have been separated from some samples for trace element analyses. Some preliminary data have been reported previously in a series of ab- stracts ( KOEBERL et al. 1984c,d,e,g; WEINKE and KOEBERL, 1984; KOEBERL, 1985 ) and used in two related papers ( KOE- BERL et al., 1984f; KOEBERL and BERAN, 1988). Here I present and discuss the complete data set, and try to put it in context with related observations to arrive at some conclusions regarding the origin of the Muong Nong tektites.

SAMPLES

Nineteen Muong Nong-type tektites of various size (ca. 20-600 grams weight) were made available by D. Futrell (Whittier, CA). The samples all originated from Ubon Ratchatani in East Thailand, near the border to Laos (Fig. 1) , which is close to the original location described by LACROIX ( 1935). The samples were cleaned ultrason- ically and broken; only interior parts were used for analysis to exclude any surficial contamination or parts of the laterite crust which adhered to some parts of the samples. Several larger glass chips (totalling about 5-20 g for each sample) were selected and carefully ground in an automatic agate ball mill. The sample powder was then used for various analyses, e.g., neutron activation analysis, atomic absorption spectrometry, and ion selective electrode analysis. Small chips (several mm in size) were used for making polished sections and thin sections for electron microprobe work and optical microscopy.

ANALYTICAL METHODS

Major Element Analysis

The major elements were determined by electron probe micro- analysis, using an automated five-spectrometer ARL-SEMQ microprobe.. Standard Bence-Albee correction procedures were applied. On the polished section of each tektite sample, individual small areas (about 5 X 5 pm) were scanned for analysis with TV frequency (to avoid loss of Na) . About twenty to thirty such areas were measured on each section to provide a good statistical average of the major element composition. Several thin sections were prepared to study the chemistry of dark and light layers by electron microprobe. Several major element profiles were analyzed across alternating dark and light layers, using the ARL microprobe (Vienna) and a fully automated CAMEBAX microprobe (NASA Johnson Space Center, Houston). The agreement between measurements at the two sites was excellent.

Neutron Activation Analysis

About 250-350 mg of the powdered samples were weighed into clean polyethylene vials and heat sealed. For the determination of trace elements in dark and light layers, individual chips (about IO- 30 mg) of adjacent dark and light layers were selected under the stereo microscope by breaking (not sawing) larger sample chips. The samples were packed together with natural (U.S. Geological Survey) and synthetic multielement standards into larger irradiation con- tainers. Synthetic multi-element standards have the advantage ofbeing virtually interference-free (i.e., peak or isotope interferences in the gamma spectrum can be kept to a minimum.

Two irradiations were performed at the Triga Mark II reactor of the Atominstitut der &ezreichischen Universit&en at a neutron flux of 2.10 I2 n cm-* SK’. The first irradiation. usinn a uneumatic tube __ system, had a duration of 1 min and was done to determine sbort- lived isotopes. The second irradiation, several days later, bad a duration of about 8 h. ARer the first irradiation, two counts of the samples were done to determine 52V ( Tliz = 3.75 min) and “Cl ( Tl12 = 37.2 min), and 56Mn (T,,z = 2.56 b) and ‘65Dy ( T1,2 = 2.35 h), respectively. For the first counts, on-site equipment (Nuclear

1036 C. Koeberl

Data ND-66 GeLi detector) was used; the second counts (Mn/Dy) were performed at the Institute of Geochemistry, using a Canberra 8 100 System with GeLi detectors.

Following the second irradiation, the samples were first counted for about @IO-3,000 set starting about 1-2 days after the end of the irradiation to determine Na, K, Ga, As, Br, Sb, W, and Au. A second measuring cycle, during which the samples were counted for about 3000-10,000 set, followed about 4-5 days after the end of the first cycle, and led to the determination of Na, Cr, Fe, As, Rb, Zr, Ba, La, Ce, Nd, Sm, lb, Yb, Lu, Hf, Ta, Au, Th, and U. The third counting period, with counting times of 5000-50,000 set, was begun two weeks aBer the end of the second cycle, and a fourth set of measurements was performed about 2-4 weeks affer the end of the previous counts. with durations of20.~1~,~ sec. The last two -rne~u~rnen~ involve the domination of !& Cr, Fe, Co, Ni, Zn, Se, Rb, Sr, Sb, Zr, Cs, Ce, Nd, Eu, Gd, Tb, Tm, Yb, Lu, Hf, Ta, Ir, Hg, and Th.

Many elements are determined in more than one counting cycle, and several gamma lines are used where possible; the final results are averages of up to ten individual values per element. These measurements were done at the Institute of Geochemistry, using GeLi detectors and a Canberra 8 100 MCA system controlled by a PDP 1 1 / 34 microcomputer. Recently (starting in 1989), some samples and/ or elements have been reanalyzed with a new activation analysis system, consisting of two EG&G Ortec HpGe detectors ( 15% rel. efficiency, 1.6 keV resolution at 1332 keV), a gated integrator am- plifier, and 1.5 ps ADC connected via acquisition interlace module and Ethernet to a MicroVAX computer system (Nuclear Data ND9900 system). All procedures were checked and corrected by an- alyzing intemation~ geological reference materials (~VINDARA~U, 1984). Both precision and accuracy are below 10% for almost all elements, and in most cases between OS-5%. More details on the analytical procedures are given in KOEBERL et al. ( 1987) and KOEBERL (1992).

Atomic Absorption Spectrometry

About SO-200 mg of the sample powder was treated with con- centrated HF/HrSO, and then converted to a 0.5-M HCI solution (50 ml). Alternatively, a HF/HNOr digestion was performed, followed by conversion into 0.5 M HNOr solution. These solutions were used for all atomic absorption spectrometry (AAS) determinations. Two AAS varieties were used: Ilame-AAS for the detem& nation of Li (see also KOEBERL et al., 1984a), Cu, and Zn [and K and Mg, as reported already by KOEBERL et al. ( 1984f); these results are not discussed here], and ~phite-fuma~ AAS for the determination of Be and Pb. The instrument used was a computer~ntmlled Perkin-Elmer 3030 atomic absorption spectrometer with a HGA- 400 graphite furnace and an AS-1 autosampler.

For all elements determined by flame-AAS, an air-acetylene flame was used. For the construction of standard curves, at least four standard solutions with different concentrations were used. Each sample was measured five times for averaging, and at least two independent runs were made for determining the final averages. Precision and accuracy were checked with international standards. For the determination of Pb. 20 UL of a 1% solution of I NH, hHPOd was added .,_ as a matrix modifier to 20 pl of sample solution. precision was better than 2% for Li and Cu. 4% for Zn (flame technique), and 10% and 20% for Be and Pb, respectively (g~phite-fuma~ technique). The accuracy was between 2-5s (Li, Cu. Zn ) and 5- 10% (Be, F’b) .

Ion Selective Electrode Teehniques

Fluorine and boron were determined by ion selective electrode techniques. For fluorine, our method is describe3 in detail by KLUGER et al. (1975) and KOEBERL et al. ( 1984b). In short, a NaOH digestion is done in a Ni crucible with about 200 mg of the powdered sample. The fusion cake is dissolved with 1 M citric acid and the pH adjusted to 6.0. The determination is done after the standard addition method and the values are calculated from the Nemst equation. Boron is determined potentiometrically via the tetrafluoroborate complex. Method and data are given by KLUGER and KOEBERL ( 1985).

Ferrous /Ferric Iron Ratios

A standard wet chemical technique was employed for the determination of ferrous iron. About 200-500 mg of the powdered sample and a measured quantity of ammonium metavanadate are treated for one day with cold cont. HF in a polyethylene vessel until the sample is dissolved. After adding 10 N HzSO., and a saturated boric acid solution, the excess vanadate is titrated with l/30 N ferrous ammonium sulfate solution and barium diphenylaminesulfonate in- dicator. For total iron, average values from electron probe micro- analysis were taken rather than INAA results (as earlier reported; KOEBERL et al., 1984~) because they were determined to be more precise. Ferric iron was calculated by difference.

RESULTS AND DISUNION

Petrography

The samples were of blocky, chunky shape and showed signs of corrosion on some surfaces. Thin sections showed abundant layering, with closely spaced alternating dark and light layers, while other samples (at least in the chips studied) were of more uniform color, and only showed gradual changes within about 5- 10 mm. Figures 2a,b shows two examples of dark layers in samples MN8302 and MN83 11. The dark Iay- ers are usually very pronounced and are less abundant than the lighter “matrix” (or the “light layers”) _ Dark layers o&en amplify the existing flow blurs and make them more easily visible. Fig. 2a shows clearly that larger bubbles have diverted the flow of the glass. Bubbles are abundant in all samples: they are estimated to be at least one (but probably two) order(s) of magnitude more abundant in the present samples than in splash-form indochinites. The largest bubbles found in the present samples were up to 1 cm in diameter. Bubbles are generally less abundant in dark layers. Along ffow zones, bubbles are sometimes elliptical, indicating formation at the time of the viscous flow. BARNES and PITAK-

PAIVAN ( 1962) even state that bubbles in Muong Nong-type tektites are predominantly elliptical; they too interpreted this as being indicative of glass flow.

Fig. 2c shows the corroded interior of two bubbles in sample MN83 11. The inside of the bubbles looks similar to the outside of most splash-form tektites, showing etch pits and grooves. This indicates that the inner bubble walls may have been subjected to corrosion at least during one phase of their history, probably during their formation. BARNES and Rus- SELL ( 1966) describe devitrification of glass around partly collapsed bubbles in artificially heated philippinites. They interpreted their findings as providing evidence that the bubbles in normal tektites have formed from the absorption of water vapor into the surrounding glass, leaving low-pressure bubbles behind. This also leads to higher water contents in the glass around the bubbles.

KOEBERL and BERAN ( 1988) have shown (see later) that Muong Nong-type tektites have higher water contents than splash-form tektites; JESSBERGER and GENTNER ( 1972) determined the abundances and isotopic composition of gases in bubbles in Muong Nong-type tektites and found that, apart from a COZ overabundance, the ratios are consistent with an origin from the terrestrial atmosphere and that pressures within the bubbles range up to one third of the atmospheric pressure, much higher than in splash-form tektites. It seems likely that at the time of bubble formation (i.e., at the time


(4

(b) (4

FIG. 2. Photomicrographs of Muong Nong-type indochinite thin sections. (a) Dark layer and flow structure around bubbles in a thin section of MN8302. (b) Dark layer in MN83 11. (c) Corroded interior in two larger bubbles in MN83 11. (d) Two lechatelierite inclusions showing bizarre flow structures in MN83 11. Widths of pictures: (a,b): 2.4 mm; (c,d): 0.8 mm.

of glass solidification), wet carbonate-rich sediments were incorporated into the fluid glass, with the water being partly absorbed in the glass and partly escaping with other volatiles during the viscous flow. For Muong Nong-type tektites, this process seems to have been less effective than for splash-form tektites, leaving higher gas pressures and a COz enrichment in the bubbles. It is likely that the etching of the inside of the bubbles occurred at this time.

Lechatclierite is generally more abundant in Muong Nong- type tektites than in splash-form tektites. Fig. 2d shows two adjacent lechatelietites which clearly show evidence of glass flow and mixing at a microscopic scale. Lechatelierite in Muong Nong-type tektites is usually of a more frothy structure than in splash-form tektites, indicating lower formation temperatures for Muong Nong-type tektites (BARNES, 1989)

[earlier, BARNES and RUSSELL ( 1966) thought that Muong Nong-type tektites were subjected to higher temperatures]. Brecciation or welded breccia composed of different layers as reported by FUTRELL and FREDRIKSON ( 1983) and Fu- TRELL ( 1986a) were not observed in any of the present thin sections, probably because these structures are rare. Some indication of faulting (BARNES, 1963b) of layers is present

on a macroscopic and microscopic scale in some of the samples.

Relict Mineral Grains in Muong Nong-Type Tektites

No systematic mineral grain studies have been made for the present samples; however, GLASS and KOEBERL ( 1989) studied the refractive indices in these samples. It is well established that Muong Nong-type tektites contain relict inclusions, in contrast to splash-form tektites (BARNES, 1963~; GLASS, 1970, 1972; GLASS and BARLOW, 1979). The phases identified included zircon, chromite, quartz (GLASS, 1970), corundum (plus SiOz ) , rutile, and monazite (GLASS, 1972 ), all showing evidence of various degrees of shock metamor- phism. The type of mineral inclusions present, as well as their size and shape, suggests that a fine-grained, well-sorted sediment was the tektite parent material. Another evidence for shock was provided by WALTER ( 1965 ), who discovered the presence of coesite in Muong Nong tektites (which was previously seen in Darwin glass; REID and COHEN, 1962). The presence of coesite in tektite glass was confirmed by GLASS et al. (1986). GLASS (1987) and later BOHOR et al.

1038 C. Koeberl

Table 1. Major element composition of 19 thong Nong type tektites. obtained by electron microprobe analysis (in wt.%).

MN- 0301 8302 8303 8304 6305 8306 0307 8308 8309 8310

SiO, 01.73 80.37 77.37 78.03 80.9 78.91 77.80 78.42 78.57 79.74

TiO, 0.53 0.59 0.67 0.67 0.59 0.62 0.65 0.72 0.63 0.60

AR 8.61 9.64 Il.41 10.69 8.89 10.34 11.06 10.18 10.65 9.53

Cr20, 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01

Fe0 3.18 3.27 3.95 4.15 3.24 3.60 4.05 3.75 3.92 3.97

MnO 0.08 0.09 0.08 0.10 0.09 0.09 0.09 0.07 0.08 0.10

MgO 1.20 1.32 1.57 1.51 1.33 1.40 1.49 1.65 1.47 1.38

CaO 1.17 1.14 1.25 1.20 1.39 1.10 1.04 1.63 1.09 1.21

Na,O 1.01 1.04 1.01 0.82 0.87 0.97 0.77 0.96 0.77 0.87

$0 2.25 2.36 2.50 2.49 2.26 2.47 2.55 2.33 2.46 2.37

Total 99.77 99.83 99.82 99.67 99.56 99.71 99.51 99.72 99.65 99.78

MN- 8311 0312 0313 8314 0315 8316 0317 0310 8319 Ave.

sio,

TiO,

A12o3

cr203

Fe0

MnO

MgC

CaO

Na,O

K20

Total

78.03 78.20 77.50 81.35 79.72 77.08 77.47 01.24 77.28 78.93+1.50

0.66 0.66 0.67 0.56 0.58 0.68 0.66 0.59 0.65 0.63kO.05

10.97 10.21 11.19 8.60 9.39 11.26 11.22 8.58 11.13 10.19*0.98

0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01+0.00

4.04 3.77 4.10 3.18 3.48 4.06 4.02 3.23 4.00 3.75t0.35

0.08 0.09 0.07 0.09 0.10 0.09 0.08 0.07 0.07 0.06~0.01

1.51 1.50 1.53 1.19 1.33 1.52 1.58 1.26 1.48 1.43+0.13

1.11 1.44 1.16 1.19 1.36 1.16 1.26 1.03 1.15 1.21kO.15

0.82 1.07 0.95 0.86 1.03 0.92 0.95 0.92 0.95 0.92+0.09

2.52 2.39 2.50 2.28 2.37 2.51 2.52 2.24 2.52 2.42~0.10

99.75 99.33 99.68 99.30 99.32 99.29 99.76 99.17 99.24

( 1988) found coesite and shocked minerals associated with tektites in an impact debris layer at DSDP Site 612 in the North American tektite strewn field, lending further support to the association of tektites with shocked minerals. GLASS and BARLOW ( 1979) found that, based on relict

inclusions, Muong Nong-type indochinites can be divided into two groups: those with low refractive indices (~1.503) contain mineral inclusions, while those with high refractive indices ( > 1.5 13 ) do not. In a preliminary report, FUDALI et al. ( 1984) claimed not only to have found relict mineral in a high-R.I. Muong Nong-type tektite, but also that they found low-temperature hydrous minerals (e.g., diaspore, biotite, muscovite) in this tektite. However, these preliminary observations were not substantiated and were probably caused by contamination. GLASS and KOEBERL ( 1989), who determined the refractive indices of sixteen of the present nineteen samples (all except MN83 12, MN8314, and MN8315), found that they all belong to the low refractive index group, with an average R.I. of 1.495, and thus contain mineral inclusions. This group also has higher silica contents. Regarding trace elements, GLASS and KOEBERL ( 1989) found that only Ta

shows a significant difference between the two groups, being higher in the low-R.I. group.

Major Element Chemistry

The major element compositions of all nineteen samples, together with an overall average and standard deviation, are given in Table 1. As each composition represents an average of about twenty to thirty small-area analyses, it can be assumed that the data represent the real composition of the samples with reasonable accuracy. All samples belong to the (more common) Si-rich variety of Muong Nong-type indochinites. The data are in good agreement with comparable analyses (i.e., of Si-rich Muong Nong tektites) by, e.g., CHAPMAN and SCHEIBER ( 1969) and BARNES ( 1989). All samples show the familiar inverse correlation between the SiOz content and the content of most other major elements (e.g., CHAO, 1963; KOEBERL, 1990). For example, Fig. 3a shows a Harker diagram of SiOz vs. A&O3 and MgO, clearly depicting the inverse relationship. Fig. 3b shows that although K20 is inversely correlated with SiOl , NazO is obviously un-


14

13 1

12-l

p ll- 4+

g lo-

++.: .

8

++.+lp= ++ +

9 9-

I=

++ = I.

+ 8,

++

1 +

7 ++

6 1

t

5-1 I1 1 I I I I I I 74 75 76 77 70 79 e.0 at 02 83 :

SiO2(wt%)

2

1.9

i .a

1.7

1.6 g

1.5 :

1.4 P

1.3

1.2

1.1

1

2.7

1 t 1.1

I

74 75 76 77 70 79 80 81 82 83 84 SiO2(wt%)

(b)

FIG. 3. Harker diagrams of major element contents in Muong Nong-type indochinites. (a) SiOz vs. A&O3 (left scale, filled squares) and MgO (right scale, crosses). (b) Si02 vs. K20 (left scale, filled triangles) and NazO (right scale, crossed open squares). All elements, with the exception of Na*O, show the characteristic inverse relationship with Siq. The scatter in the sodium content might be due to analytical uncertainty of microprobe analyses.

correlated. It cannot be excluded that some of the irregular behavior of Na is an analytical artefact due to possible volatilization of Na during electron probe analysis because the Na contents determined by INAA (see next section) are slightly higher.

Variations in chemical composition within each sample lead to a compositional range for each sample. During analysis of each sample, total variations of up to about 20 rel% (about 5- 10 rel% standard deviation) were encountered for certain elements (because of the data quantity, these numbers, as well as individual point analyses, are not given in Table 1). The deviations from mean composition are largely due to the abundance of dark and light layers within the chip that was analyzed by microprobe because of the compositional differences between these layers (see later), Therefore analyses of different chips of the same sample may well give slightly different chemical compositions. Because of the relatively narrow range in composition we can assume that the present composition is typical for Muong Nong-type tektites of the Ubon Ratchathani location, and probably representative for

the Si-rich variety. Muong Nong-type tektites from other locations often have different compositions (although most still fall in the range observed here), thus showing a compositional variation with location in the strewn field.

Trace Element Chemistry

The results of the analyses of three major and forty-four

trace elements in all nineteen samples are given in Table 2.

The data are generally in good agreement with available lit-

erature data. CHAPMAN and SCHEIBER ( 1969) have performed a thorough study of numerous Australasian tektites, including the major and selected trace elements (B, Ba, Co, Cr, Cu, Mn, Ni, V, Y, and Zr). Among their samples they also analyzed seven Muong Nong-type tektites which they named the HCu,B indochinites (because of their unusually high contents of copper and boron). In contrast to the present samples, which all belong to the highSi group, they included some samples from the low-Si group ( 7 l-75 wt% SiOz ) . The data in Table 2 agree very well with the trace elements reported by CHAPMAN and SCHEIBER ( 1969), with the excep tion of Ni, where their data are lower by about 30%. A similar agreement is present with data for seven Muong Nong-type indochinites reported by BARNES ( 1989 ) , who reports results for twenty trace elements, nineteen of which were also analyzed in the course of this work. The contents for most elements agree within the reported standard deviations, but some minor differences are present. For example, the average LREE contents are higher in the BARNES ( 1989) analyses than in the present data set. A larger difference is present for Zn, for which BARNES ( 1989) reports an average of 150 ppm, which he attributed to the possibility of contamination for two specimens.

The present data are also consistent with analyses reported by GLASS and KOEBERL ( 1989). They studied four Muong Nong-type indochinites with high refractive indices (low silica) that do not contain relict mineral inclusions and compared them to the analyses of the current nineteen low-RI. samples (containing inclusions) plus one more highSi sample from Ubon Ratchathani. Most elements do not show any significant differences between the two data sets, except Ta, as reported by GLASS and KOEBERL ( 1989 ) . The concentra- tion of Ta is less than 1 ppm for the high-R.I. group, and greater than 1 ppm for the low-RI. group. However, GLASS and KOEBERL ( 1989) did not consider the complete data set as presented in Table 2. Even though most ranges overlap, there is a clear trend for higher LREE contents in the low silica, high-RI. group compared with the present high silica, low-RI. group. Other refractory elements do not show any clear trend.

Volatile trace elements show a slightly larger range than incompatible refractory elements. As mentioned before, CHAPMAN and SCHEIBER ( 1969 ) found high contents of Cu and B in Muong Nong-type tektites, with B and Cu contents ranging from 32 to 72 and from 11 to 21 ppm, respectively, compared to a B range of 28-88 ppm and a Cu range of 8- 22 ppm (excluding the anomalous value for MN8302) in the present data set. The boron data agree with data for AUS- tralasian tektites reported by MILLS ( 1968) and TAYLOR and KAYE ( 1969). Recently, MATTHIES and KOEBERL ( 199 1) remeasured the B content of MN8302 and other (splash-

1040 C. Koeberl

Table 2. Major, minor, and trace element contents of 19 Muong Nong type tektites. plus average and standard deviation. Data obtained by neutron activation analysis, atomic absorption speetrometty, plasmaspe&ometry, and lonselectiveelectrodes(seetextfordatalls);in ppm exceptwhers noted.

MN- 8301 8302 8303 8304 8305 8306 8307 8308 8309 8310

Li Be

z Na (wt.%) cf K (wt.%) SC v Cr

2(Wl.%) co

:: Zn Ga As Se Br Rb Sr Zr As

:: Ba

& Nd Sm

?!4 Tb DY

2 Lu

:: W Ir (Ppb) Au (wb)

2 Th U

K/U Zr/Hf Th/U La flbN EuyEu*

57.5 35.0 36.1 37.2 26.5 41.0 44.5 46.0 49.6 50.5 4.31 3.19 3.76 2.65 3.05 4.40 3.42 4.38 3.72 3.39

42.8 30.8 30.5 31.7 31.2 40.4 27.6 30.6 50.6 77.5 126 90.3 109 0.92 0.97 0.66

7:; s:z* '"L6 'A:L 559 1:06

903 0:90

72.5 0.79

330 155 140 190 150 200 300 120 200 355 2.14 1.80 1.99

7.09 7.95 I.79 I.42 2.07 2.21 2.02 2.11 2.14

8.78 7.16 6.40 8.46 7.76 7.81 9.27 5.94 65 77 59 98 65 69 73 89 54 39 68.6 63.9 62.4 54.6 54.8 65.4 60.4 64.7 72.9 53.9

691 715 649 757 667 732 725 541 625 790 3.34 3.19 4.13 3.37 3.63 3.57 3.61 3.34 3.94 3.07 12.4 12.9 13.2 13.2 II.4 13.3 11.8 12.0 14.2 12.6 29 37 48 39 49 49 54 36 28 60 17.7 151 16.9 20.1 16.9 12.5 12.5 22.1 11.4 10.0 90 70 57 39 65 66 64 74 66 22 23 3': 19 19 I7 23 34 17 21 6.21 4.87 6.91 2.44 2.24 5.41 5.39 4.01 6.42 4.81 0.1 co.1 0.12 0.22 0.17 0.08 0.1 ~0.08 0.07 0.09 8.7 3.8 2.0 3.2 2.2 3.3 6.6 0.6 4.7 9.9

129 115 109 107 90 124 113 112 127 100 130 125 170 160 130 98 290 190 115 65 150 170 320 250 290 360 110 340 170 260 0.2 CO.1 0.1 ~0.25 0.1 CO.2 0.15 0.06 co.1 <O.I 0.58 0.38 1.57 0.68 0.66 1.12 0.69 0.75 1.15 0.65 5.65 4.70 4.96 4.82 4.00 5.79 5.03 4.75 6.27 4.91

465 290 390 320 280 340 250 365 360 290 31.3 29.0 28.5 25.6 25.5 30.4 27.0 30.3 30.5 26.2 55.6 59.7 60.9 59.5 75.1 64.9 66.9 64.8 62.8 61.0 29.1 26.8 30.2 27.2 25.9 31.8 25.8 31.5 31.4 26.6 5.82 5.03 4.92 4.31 4.24 5.53 4.24 5.71 5.23 4.76 1.14 0.96 0.98 1.00 0.94 1.09 0.95 1.06 1.07 0.96 4.7 4.1

;:36 ::: 3.9 4.75 4.4 4.9 4.7 3.8

0.66 0.72 0.69 0.81 0.74 0.66 0.64 0.67 5.31 4.39 4.95 4.02 4.19 5.12 5.45 5.51 5.52 4.15 0.47 0.41 0.42 0.37 0.36 0.45 0.46 0.49 0.51 0.37 2.95 2.55 2.56 2.55 2.40 2.60 2.82 3.12 3.22 2.32 0.46 0.36 0.32 0.34 0.30 0.52 0.40 0.52 0.54 0.36 8.41 7.83 8.03 7.42 8.55 8.08 7.03 11.0 9.48 6.89 I.17 1.07 1.12 1.16 1.08 1.24 1.01 1.14 1.33 1.23 0.6 0.9 1.1 0.9 0.8 1.0 0.8 0.8 0.7 1.5

<l ~0.8 <l <I CO.7 <2 cl.5 Cl <1.5 <2 1.8 I.5 2.5 1.7 1.0 2 2.6 I.2 3 1.8

SO.8 <I <I <3 <2 <4 <I CO.8 0.8 <I 5.8 6.3 8.5 5.7 9.5 6.5 6.7 3.6 6.8 9.6 12.4 10.1 11.3 9.44 9.41 12.1 10.2 11.9 14.2 10.2 2.38 2.13 2.41 1.77 2.50 2.91 2.59 3.63 2.86 2.21

8991 8451 8257 10112 5680 7113 8532 5564 7378 9683 17.8 21.7 39.9 33.7 33.9 44.6 15.7 30.9 17.9 40.6 5.21 4.74 4.69 5.33 3.76 4.15 3.94 3.27 4.97 4.61 7.18 7.67 7.52 6.78 7.20 7.33 6.47 6.55 6.40 7.64 0.67 0.65 0.65 0.75 0.70 0.65 0.67 0.62 0.66 0.69

form) tektites and found a content of 44 ppm, compared with 3 1 ppm in this work. As the deviation is larger than the precision it seems possible that the contents of B (and other volatile elements) vary between different parts of the samples, similar to variations observed for refractory elements (see later).

An important observation was made by MOLLER and GENTNER ( 1973), who found that chlorine and bromine are enriched in Muong Nong-type tektites compared to splash- form tektites. BECKER and MANUEL ( 1972) determined Cl contents of I .4-4.3 ppm and Br contents of 0.09-0.23 ppm in five splash-form tektites, while MULLER and GENTNER

( 1973) reported ranges of i- 14 ppm Cl and 0.0 I S-O. I.5 ppm

Br for splash-form tektites, and ranges of 100-330 ppm Cl and 1.1-4.6 ppm Br in Muong Nong-type indochinites. MULLER and GENTNER ( 1973 f also reported Zn and Cu enrichments in Muong Nong-type tektites. KOEBERL et al. ( 1984~) found that not only Cl and Br but also F is enriched in Muong Nong-types. Some radiochemical data (Koeberl, unpubl. data) show that I is also enriched in Muong Nong- type tektites compared to the range of 0.17-0.56 ppm I reported by BECKER and MANUEL (1972). The present data (Cl: 120-355 ppm; Br: 0.7-9.9 ppm) agree with data reported by MULLER and GENTNER(~~~~), and with the 170 ppm Cl ppm determined in a single Muong Nong tektite by MOORE et at. ( 1984).

Geochemistry of Muong Nong-type tektites

Table 2 (continued).

MN- 8311 8312 8313 8314 8315 8318 8317 8318 8319 Ave.* Std.dev.

Li Be B

La(wt.%) Cl ;e(vJW

V Cr

;$Wx0,

:I

2 As Se

!%I Sr Zr ALI Sb cs Ba Le Ce Nd Sm

k Tb DY Tm Yb Lu Hf Ta W fr (ppb) Au (ppb)

FE Th U

K/U Zr/Hf Th/U

42.9 35.1 41.2 36.4 41.0 43.9 47.2 32.3 46.9 42.127.17 3.98 3.80 4.05 3.25 2.66 4.42 4.28 1.95 5.51 3.701tO.78 66.2 60.9 71 51.7 85.5 41.2 46.3 46.8 37 47.7217.2 101 95.3 122 110 90.5 122 124 89.8 121 97.3e20.8 0.66 0.64 0.64 0.87 0.67 0.66 0.90 0.92 0.91 090~0.06

150 210 240 195 210 260 120 195 170 204.0+85 2.02 2.04 2.06 2.04 2.07 2.10 2.11 1.65 2.15 2.01+0.18 7.95 8.23 7.39 7.59 7.85 8.40 8.30 8.57 9.56 7.70+0.96

103 59 78 66 80.7 48.8 59.2 59.0

91 E.1 57.7

ii.8 zz.8 69 723215.7 72.7 80.6t8.4

853 730 820 722 754 870 822 800 546 874.0+86 3.45 3.12 3.78 3.60 2.57 3.43 3.81 2.58 3.78 3.4320.41 11.8 11.3 12.8 13.2 12.1 12.8 12.5 11.2 14.3 12.8?0.68 71 34 z.2 29 39 60 57 81 42 48.6el7.2 11.7 9.30 20.2 11.5 12.9 19.3 7.90 10.8 14.3?4.2 63 89 85 65 86 88 82 58 79 88.7+10.2 29 18 34 31 29 25 19 22 27 24.2t5.5 2.78 2.98 8.45 4.24 4.77 8.04 3.72 4.78 5.79 4.7521.39 0.12 0.25 0.18 0.3 co.1 0.1 co.3 0.21 co.2 0.2t0.1 2.0 5.3 5.8 3.6 4.7 5.2 0.7 3.8 2.0 4.12*2.39

99 90 106 108 102 109 124 99 121 109.8+11.3 110 100 99 96 100 120 150 180 150 135.8?48 500 290 300 270 260 310 260 180 500 260.5~101 co.2 co.2 0.24 co.3 <0.2 <0.2 <O.l co.3 co.2 0.1 to.2 0.78 0.85 0.74 0.87 0.87 0.77 1.01 0.79 0.97 0.62zO.25 4.90 4.18 5.24 4.96 5.10 5.43 5.45 4.20 8.41 5.09kO.63

345 300 320 350 360 360 300 290 470 341.3*57 28.8 24.3 28.9 26.7 26.5 29.8 27.6 27.3 31.8 28.2+2.05 56.3 52.0 58.1 61.0 85.5 68.4 84.0 89.8 87.2 80.7+4.02 27.9 28.8 28.2 27.1 31.8 31.7 28.1 29.2 34.2 29.1~2.35 4.55 4.22 4.28 4.34 4.87 5.32 4.80 4.84 5.63 4.6520.55 0.94 0.90 1.01 1.01 0.94 1.09 1.08 0.96 1.12 1.01+0.07 3.9 4.35 4.04 3.65 3.70 4.7 4.1 4.3 4.9 4.3t0.4 0.89 0.77 0.71 0.86 0.63 0.64 0.75 0.78 0.69 0.75t0.08 4.22 4.57 4.35 4.05 4.20 5.34 4.40 4.60 5.74 4.75kO.57 0.39 0.34 0.37 0.35 0.39 0.48 0.41 0.42 0.53 0.42+0.05 2.47 2.15 2.46 2.42 2.73 3.04 2.81 2.89 3.47 2.71+0.33 0.44 0.29 0.34 0.38 0.36 0.51 0.41 0.47 0.58 0.42+0.05 7.76 7.24 7.05 7.48 7.82 7.82 7.86 9.98 9.14 8.13+1.08 1.09 1.18 1.29 1.17 1.08 1.09 1.20 1.14 1.53 1.17+0.11 1.2 1.4 0.9 2.1 1.4 0.9 0.7 0.8 0.9 1.02+0.35

<I cl.5 <2 <1.5 <2 <l <l cl.3 cl.8 <1.5 1.0 2.5 1.8 3.1 1.8 1.8 1.8 2 2.5 2.1kO.9

<2 c3 <l <I Cl ~0.8 <2 <3 <2 <I 5.4 4.9 6.8 7.5 8.3 7.4 8.4 5.9 8.8 8.65t1.52 10.8 8.81 10.7 10.3 9.72 11.9 11.8 11.0 14.5 ll.lk1.46 2.14 1.86 2.59 2.11 2.09 2.89 2.55 2.72 2.92 2.46kO.45

9439 12142 6030 9886 9904 7286 8275 8601 7363 6350.?1574 84.4 40.1 42.8 36.1 38.8 39.7 38.5 18.0 54.7 34.9512 5.04 5.24 4.13 4.66 4.85 4.12 4.54 4.04 4.97 4.5420.55 7.87 7.28 7.34 7.45 7.05 6.82 8.84 8.64 8.19 7.071to.49 0.86 0.64 0.74 0.75 0.89 0.66 0.78 0.87 0.64 0.86t0.04

1041

Additional data: Pin MNt3301:550 ppm;Sin MN6319:96 ppm. &@ MN6302 excluded foraverage Cuvalue

The F contents given in Table 2 are similar to data reported by MCKIRE et al. ( 1984) and MAYTHIES and KOEBERL ( 199 1). From comparison with data for splash-form tektites, it is evident that F abundances are higher in Muong Nong-type tektites compared to splash-form tektites ( KOEBERL et al., 1984b; MOORE et al., 1984). In a study of F and Cl in tektites, BAILEY ( 1986) reports considerably higher F contents than the ones found here and by MOORE et al. ( 1984), but he now considers that his reported high F values may be too high due to lab- oratory contamination (Bailey, pers. comm., 1990). Another “volatile” element, nitrogen, seems to be enriched in Muong

Nong-type tektites compared to splash-form tektites ( MURTY et al., 1989).

Refractory trace elements show less variation between samples (and probably also between different parts of the same sample) than volatile elements. The agreement between the present data and data from CHAPMAN and SCHEIBER ( 1969), who determined a few refractory trace elements, is generally good. Very good agreement is also present with the newer data of BARNES ( 1989). A very important group of incompatible lithophile trace elements is the group of rare earth elements (REE), as the chondrite-normalized REE

J-----J La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

REE

FIG. 4. Chondrite-normalized REE diagram showing the range of Muong Nong-type tektite compositions compared to the composition of the average upper continental crust (dashed line). C 1 -normalization values after EVENSEN et al. ( 1978) and TAYLOR (1982); upper crust data from TAYLOR and MC LENNAN ( 1985 )

pattern is usually characteristic of the parent rocks (see later). Not much data on REE abundances in Muong Nong-type tektites were available until recently. CHAPMAN and SCHEIBER ( 1969) presented a ligure with a REE pattern for their HCu,B indochinites, but no data. Their figure, however, seems to indicate analytical problems with the REEs in general, if compared to more modem REE analyses in tektites (e.g., TAYLOR and MCLENNAN, 1979; KOEBERL~~ al., 1985). Fig- ure 4 shows the range of chondrite-normalized REE patterns

1042 C. Koeberl

for the present nineteen samples, compared to the pattern of the present upper crust. The range of REE compositions in Muong Nong-type indochinites is rather restricted, indicating a well-defined group of parent rocks. The agreement with the data presented by BARNES ( 1989) is good, but there is a tendency for higher LREE values in the BARNES ( 1989 ) data set compared to the present data.

Comparison with Splash-Form Tektites and Local Terrestrial Rocks

Table 3 gives a comparison of the average and range of Muong Nong-type tektite major element compositions from the present study with an average indochinite (TAYLOR and MCLENNAN, 1979) local surface sediments (TAYLOR et al., 1983; BARNES, 1989), and average continental crust (TAYLOR and MCLENNAN, 1985). Figure Sa shows the major element compositions of average Muong Nong tektite vs. average indochinite (i.e., splash-form tektite). Most elements show a good correlation, but slightly larger deviations occur for MgO and CaO, which are higher in the average indochinite. The comparison between average continental crust and average Muong Nong-type tektite is shown in Fig. 5b, and it is evident that the fit is not very good for most elements. CaO and NazO show the largest deviation, being lower in the tektites due to weathering of the source rocks.

Figure 5c shows the comparison of the compositions of average Muong Nong-type tektite vs. two different loess samples (TAYLOR et al., 1983 ) , as loess or related materials have been considered as a possible precursor material for Muong Nong-type tektites (e.g., BARNES, 1989, 1990; BARNES and

Table 3. Major element composition for average and range of Muong Nong type tektites, and comparison data for an average splash-form indochinite (TAYLOR and McLENNAN. 1979), two typical loess samples (Nanking and Banks Peninsula; TAYLOR et al., 1993), two soil samples found with tektiies in Thailand and Cambodia (BARNES, Xi@), and the average continental crust (TAYLOR and McLENNAN, 1985).

AVWagl3 Muoq Nong Ave. LoeM Lows Soil Soil Average Mucmg Nong toktlte4

z& Nanking Bank4

Ml&g Mixing Thailand

HMng Cambodia CentI- Model Model Mod4l

Mixing Modei

mdlwa P*zzork,

Pwdtwula KLD-SC K-SO crust Ml M2 M3 M4 [thls work] Ill I21 121 I31 I31 I41

SiO,

TIO,

AI2o3

Fe0

MnO

MgO

CaO

Na,O

$0

L.O.I.

Total

7s.93+1.50

0.63+0.05

10.18+0.98

3.74+0.35

0.081rO.01

1.43+0.13

1.21+0.15

0.92+0.09

2.4220.10

77.0881.73 72.7 72.8 73.1

0.53-0.72 0.78 0.78 0.58

8.88-11.4 13.37 15.4 15.0

3.18-4.15 4.85 4.31 3.11

0.07-0.10 0.08 0.12 0.054

1.19-1.65 2.14 1.59 0.99

1.03-1.63 1.98 0.95 1.43

0.77-1.07 1.05 1.28 3.29

2.24-2.55 2.62 2.21 2.37

99.57 99.44 99.92

89.14 62.21 66.0 75.7

0.48 0.70 0.50 0.59

5.33 16.02 15.2 11.7

0.65 6.53 4.50 3.59

0.01 0.05 0.08 0.03

0.22 1.05 2.2 0.64

0.18 0.61 4.2 0.40

0.01 2.05 3.9 1.03

0.17 1.42 3.4 0.80

3.30 5.80

99.42 98.44 99.98

78.6 78.7 75.2

0.63 0.59 0.54

11.4 11.1 11.6

3.07 2.88 3.41

0.06 0.06 0.04

1.01 1.16 1.12

0.53 1.05 1.48

0.95 1.26 1.80

1.63 1.88 1.81

References: [I] TAYLOR and McLENNAN (1979) [2] TAYLOR et al. (1993) [31 BARNES (MS) [4] TAYLOR and McLENNAN (1995).

Mixincr Models: Ml: sO%Soil KLDSO (Thailand) + 5o%sOil K-SO(Csmbodia). M2:40% Nanking Lows + .XJ%sOil K-SO (Cambodia) + 4O%ave.Ouertzite (notintabls,from MEISEL

et al., 1990). M3: 35% Laess Nanking + 10% Soil K-SO (Camlxdia)+ 40% BVB. Ouartzite + 15% ave. Continental Crust. M4: 10% Soil KLDSO (Thailand) + 30% Soil K-SO (Cambodia)

+ 30% ave. Continental Crust + 30% we. Ouartzite.


PITAKPAIVAN, 1962; WASSON, 1987, 1989, 1990). The Nanking loess, which occurs closer to the area in which Muong Nong tektites are found, shows a better agreement than Banks Peninsula loess. Some deviations are, however, present, but we can note that the major element composition of loess is not unlike the composition of Muong Nong-type tektites. Despite this rather close agreement, there are, however, severe problems with loess as tektite parent material (see section entitled “Isotopic and Rare Gas Evidence”).

Table 4 gives comparison data of trace elements for average and range of Muong Nong-type tektites and the same terrestrial materials as listed above, where available. Figure 6 depicts enrichment or depletion factors for all trace elements in average Muong Nong-type tektites vs. average indochinites and average upper continental crust, respectively. Most refractory elements show a ratio close to unity, but a number of clear deviations are present. Table 4 also gives some characteristic element ratios, e.g., K/U, Zr/Hf, or Th/U. The ratios of the respective sediment samples (and the upper crustal values) are closely related to the average Muong Nong tektite ratios, thereby indicating that upper crustal sediments are the most likely parent rocks of the Muong Nong-type tektites.

The REEs are a group of elements of great genetic significance because they can be used to infer the type and composition of the parent rocks. Fig. 7a shows the chondrite- normalized REE pattern of average Muong Nong-type tektites in comparison with the REE patterns of an average indochinite (TAYLOR and MCLENNAN, 1979) and the average upper crust. The patterns are clearly very similar, with almost identical slopes, LREE/HREE or La/Yb ratios (Table 4), and Eu anomalies. The indochinite has slightly higher REE abundances which is due to its lower silica content, assuming a dilution effect.

Ce anomalies are prominent in laterites (BRAUN et al., 1990), but it can be concluded from the absence of Ce anomalies in Muong Nong-type tektites that laterites, and similarly weathered rocks, did not contribute significantly to the composition of Muong Nong-type tektites. It should be noted that MURTY et al. ( 1989) claim to have found evidence for Ce anomalies in Muong Nong-type tektites. However, they only determined Ce, Eu, and Tb in their samples; in absence of any data on La and Nd, a claim regarding Ce anomalies (which they ascribe to a change in redox conditions!) is rather absurd.

Figure 7b shows the range of Muong Nong-type tektite REE patterns in comparison with two local soils and two loess samples. The REE patterns of both soils show some significant differences from the Muong Nong tektite pattern. Most notably, the Thailand soil has much lower REE abundances and a different HREE pattern, and the Cambodian soil has a less pronounced Eu anomaly but a marked negative Ce anomaly which shows clear influence of weathering. It is evident that mixing of the soils with each other or with any of the loess samples would not be able to reproduce the REE pattern range of the Muong Nong-type tektites. A combi- nation of loess samples would also not yield the Muong Nong patterns.

Figure 8 is a plot of the average and range of Muong Nong- type tektite REE concentrations normalized to the average

upper continental crust. The REE pattern of an average indochinite is plotted for comparison. It is evident that the tektite REE compositions are very similar to the average crnstal REE pattern. The small deviations (especially the slight increase in HREE contents) are probably due to the incorporation of a slightly different relict mineral suite. Most rock types (such as basalts, ocean crust, deep-sea sediments) other than upper crustal sediments can be excluded as source rocks because of their different REE patterns (e.g., HENDER- SON, 1984). In particular, there are no extraterrestrial rocks that show sedimentary REE patterns and also agree with the other trace element contents in Muong Nong-type tektites. It is well known (e.g., HENDERSON, 1984; TAYLOR and MCLENNAN, 1985) that the REE pattern that is defined by rocks such as shale, sandstone, greywacke, granites, and related rocks results from geochemical processes on or close to the surface of the earth, through mixing, weathering, erosion, and transport. These processes would not operate in absence of the hydro- and atmosphere of the earth and are therefore impossible on water- and atmosphereless bodies. The REE patterns of tektites are therefore an elegant and rather un- refutable argument against any extraterrestrial origin as ad- vocated by, e.g., O’KEEFE (1976, 1987), and are only in agreement with a terrestrial origin.

Furthermore, there is a difference in average upper crustal REE patterns between Archean and post-Archean sediments, with post-Archean sediments showing a characteristic negative Eu anomaly that is absent from Archean sediments (e.g., TAYLOR and MCLENNAN, 1985). From Figs. 7 and 8 we therefore conclude that Archean sediments are implausible as tektite source materials (which would also be unlikely because of their limited surface expression). Figure 9 shows the correlation between Th and La in Muong Nong-type tektites. It is clear that the Muong Nong-type tektite compositions cluster around the correlation line defined by post-Ar- chean sediments (La/Th = 2.8) and are different from the Archean sediment correlation line (La/Th = 3.5; data from TAYLOR and MCLENNAN, 1985); this supports the evidence for a recent sedimentary source.

Volatile Element Enrichments and Volatilization

An important observation can be made by comparing volatile element abundances in Muong Nong-type indochinites and in splash-form indochinites. An enrichment of several volatile elements in Muong Nong-type tektites was already observed by CHAPMAN and SCHEIBER ( 1969) and MOLLER and GENTNER (1973). KOEBERL (1986a,b, 1988b) has as- sessed that the enrichment in volatile elements is a determining factor in ascribing Muong Nong-type character to tektite samples. Figure 10 shows clearly that most volatile elements are strongly enriched in Muong Nong-type tektites compared to splash-form tektites. Of the eleven elements depicted, only Se and Sb show enrichment factors below 2, while for Cl, Zn, and Br, the enrichment factor is greater than 10, and for other elements, it is between 2 and 10. It is in- teresting to note that among the halogens F shows a smaller enrichment than Cl and Br, which is probably due to the smaller ionic radius of F, which may allow a stronger binding to the silicate chains of the glass structure.

1044 C. Koeberl

100 1

J

10:

1:

0.1:

d Si02

Al203

i/ n Fe0

0.1

Ave. Muong Noig Tektites (G%)

100

B SiO

Al203

/

n /A20

WO

/

’ Ti02

/ MnO

(4

(b)

FIG. 5. Comparison of average Muong Nong-type tektite major element composition with related terrestrial materials. (a) Muong Nong tektites vs. average indochinite (TAYLOR and MCLENNAN, 1979). (b) Muong Nong-type tektites vs. average upper crust (TAYLOR and MCLENNAN, 1985 ). (c) Muong Nong-type tektites vs. Nanking and Banks Peninsula loess (TAYLOR et al., 1983).

Geochemistry of Muong Nong-type tektites

100, M

Si02

n Nanking Loess 0 Banks PI. Loess

0.01 I I IT1llll I I1111111 1 I I , I,,, I I I I lllll 0.01

Aw~‘Ivluong b&g Tektites $t %) 1

FIG. 5. (Continued)

1045

I

Because the most prominent enrichments are shown by Cl, Zn, and Br, the abundance of these elements can be used to determine the character of doubtful samples (i.e., Muong Nong-type vs. splash form). The volatile element enrichment in Muong Nong-type tektites could be due to incorporation of slightly different parent rocks; however, it seems that a more logical and straightforward explanation would be that Muong Nong-type tektites have been exposed to lower temperatures of formation than splash-form tektites. They have probably formed from the same or a very similar source material, but were less thoroughly heated and have therefore lost less of their volatile element content than the splash-form tektites. Unfortunately, data for volatile element contents of suitable parent rocks (such as the ones used for comparison in Table 4) are rather sparse, but the data available indicate that volatile element contents in Muong Nong-type tektites are comparable to or only slightly lower than the ones in upper crustal sedimentary rocks (see, e.g., TAYLOR and MCLENNAN, 1985 ) , while splash-form tektites show considerable depletions. A similar observation has been made by MATTHIES and KOEBERL ( 199 1) regarding the F and B content of impact glasses vs. target rocks at different craters.

It should be noted that WASSON et al. ( 1990) suggested that uranium may behave as a volatile element during tektite formation. They found that the average Th/U ratios in splash- form tektites are higher than in upper crustal rocks and Muong Nong-type tektites, which is due to a lower U content in splash-form tektites. The data in Table 4 allow a similar conclusion, although the possible difference in U content is relatively small compared to the effects shown by other volatile elements (Fig. 10). The data on %-rich and Si-poor Muong Nong-type tektites given by GLASS and KOEBERL

( 1989) show slightly lower U contents in the Si-rich group, but no difference in Th/U ratios.

It is therefore likely that some kind of selective volatilization has affected the composition of tektites in the Austral- asian strewn field. However, the extent of volatilization and vapor fractionation from the melt that took place has been a subject of controversy. Walter and co-workers (WALTER and CARRON, 1964; WALTER, 1967; WALTER and CLAYTON, 1967; WALTER and GUITRONICH, 1967 ) have suggested that vapor fractionation played an important role in tektite formation, and that Si-poor tektites can be formed by volatilization of silica at high temperatures and oxygen fugacities. This interpretation has been questioned by other authors, e.g., CHAPMAN and SCHEIBER ( 1969). In addition, MOLINI- VESKO et al. ( 1982) have shown that the behavior of silicon isotopes in tektites is contrary to what would be expected if selective silica volatilization has taken place, and that therefore vapor fractionation may not have played a significant role. This is also acknowledged by WALTER ( 1989). RIDEN- OUR ( 1986) analyzed K, Rb, and Li in indochinites and suggested that some element correlations are in favor of selective volatilization, but he also acknowledged that imperfect mixing of different source materials would be equally consistent with the data, as already suggested by, e.g., TAYLOR and KOLBE ( 1964) for Henbury impactites. This was in fact also the conclusion of MOLINI-VESKO et al. ( 1982).

Muong Nong-type tektites exist in silica-poor and silica- rich varieties, and both groups show the characteristic features of Muong Nong-type tektites (e.g., volatile element enrichment, layering, bubbles) that indicate a lower temperature origin than for splash-form tektites. It seems highly unlikely that Si-poor Muong Nong-type tektites have originated from

1046 C. Koeberl

Table 4. Average and range of the major, minor, and trace element composition of 19 Muong Nong type tektites, with comparison data from the literature for an average indochinlte, two different types of loess, soil found near Moung Nong-type tektites, and the average upper continental crust (see text). Data in ppm. except where noted.

AVdaN MN mu MN min. Ave.lodo- Lceaa Lows 8otl Soil AWLUOtW

Li Be

F Na (wt.%) cl K (wt.%) SC

lr

F&t.%) co Ni cu

2 As Se Br Rb Sr Zr As Sb cs Ba

2 Nd Sm

E Tb DY

z

2 Ta w Ir (svb) Au (W

2 Th U

K/U Zr/Hf Th/U

La fib, EuyEu*

42.127.17 57.5 26.5 47.1 38 3.70+0.70 5.51 1.95 2.2 2.4

47.72 17.2 88.2 27.6 24 - 97.3c20.8 126 55.9 34

0.90+0.06 1.07 0.79 0.78 0.96 204.266 355 120 8 -

2.01 r0.18 2.21 1.42 2.18 1.84 7.7o+o.S6 9.68 5.94 10.5 11.9

72.32 15.7 103 39 : 99 60.8+6.4 72.9 46.8

674.0268 790 541 690 9z 3.43t0.41 4.13 2.53 3.57 3.34

12.620.86 14.3 11.2 11 18 48.6t17.2 81 28 19 34 14.3t4.2 151 7.9 4 30 66.7+10.2 90 39 5.7 78 24.2t5.5 34 17 % 17

4.75 2 1.39 6.91 2.24 - 0.220.1 0.3 0.07 0.1 - 4.12t2.39 9.9 0.7 0.23 -

109.8t11.3 129 98 130 108 135.248 65 90 104 285.+101 z 110 252 330

0.1 to.2 0.24 0.08 - - 0.821to.25 1.57 0.38 0.5 - 5.0Qt0.63 6.41 4.0 6.5 7.7

341.+57 470 250 360 480 28.222.05 31.8 24.3 36.5 38.5 60.7+4.02 67.2 52.0 73.1 84.8 29.1 k2.35 34.2 25.8 33.2 35.2

4.8620.55 5.83 4.22 6.6 6.66 1 .Ol to.07 1.14 090 1.22 1.28 4.3kO.4 4.9 3.7 5.24 4.98 0.76~0.08 0.89 0.63 0.85 0.92 4.76 f 0.57 5.74 4.02 5.68 5.49 0.42t0.05 0.63 0.34 - - 2.71 to.33 3.47 2.15 2.9 2.86 0.42~tO.05 0.58 0.29 - - 8.13kl.06 11 .o 6.89 6.95 10.1 1.t710.11 1.63 t.ot 1.6 - t .02t0.35 2.1 0.6 0.29 2.6

ct.5 - _ 0.02 - 2.1+0.9 3.1 1.0

<t _ _ Go02 . 6.86+1.52 9.6 3.6 I:8 12

11.1 tt.48 14.5 8.81 14.0 13.6 2.48t0.46 3.63 1.68 2.07 2.51

8350.*1574 12143 5565 10530 7330 34.9212 64.4 t 5.6 36.3 32.7

4.64+0.55 5.33 3.28 6.76 5.41 7.07t0.49 7.87 6.19 8.51 9.12 0.6.8t0.04 0.76 0.62 0.63 0.68

32 16 2.3 0.3

2.44

1.98 8.1

65 31

420 2.35 9.6

13.2 12.4 56 14

83.4 304 366

3.8 572

35.4 72 36.5

6.56 1.25 4.5 0.78 4.38

2.45

10.4

1.3

7

13 20 to.2 to.7 2.66 2.8

‘444 35.2

3.83 9.76 0.70

20.3 0.59

- 1ooo0 32.8

3.82 t1.7 9.21 0.80 0.65

CO.02

0.14

43

51 0.60

10

z;:

77 12 24

6

A:f 0.9

::A 0.08 0.4 0.06

40 20 3.5 3

15

1.53 2.89

1.18 2.8 11

160 60 35

420 600 5.W 3.5

10 45 20 33 25 81 71

17 1.5 0.05

- 112 230 350 65 190

0.05 0.2 3.7

730 550 38 30 41 64 27 26

5.9 4.5 1.4 0.88 4.8 3.8 0.7 0.64 3.7 0.3 %3 2.2 2.2 0.3 0.32

5.8 2.2 2 0.02 1.8

A&& MN8302 (151 ppm) excluded fmm awtqy Co value Mrsnces: [l] TAYLOR and McLENNAN (1979) 121 KOEBERL (W&a) 13) TAYLOR et al. (1983) 141 BARNES MS)

[Sl TAYLOR and McLENNAN (1925).

Si-rich Muong Nong-type tektites by vapor fractionation of silica. The model of imperfect mixing of different source rocks (as probably evident by layering and different relict mineral compositions) seems to explain the data much better. Some volatilization and vapor fractionation has certainly taken place during tektite formation, but has probably been limited to volatile elements (e.g., Fig. 10) and was not responsible for major element variations.

Water Content of Muong Nong-Type Tektites

Data on the water content of Muong Nong-type tektites are sparse. GILCHRIST et al. ( 1969) analyzed a number of tektites using the reliable infrared spectrometry method (ear- tier analyses with other methods yieIded erratic results), in- eluding two Muong Nong-type tektites. They found contents of 0.0 17 and 0.008 wt% HZ0 and concluded that these num-


1 m Ave.MN vs. indochin m Ave. MN vs. crust

RG. 6. Ratio of trace element contents in Muong Nong-type tektites vs. average indochinite (TAYLOR and MCLENNAN, 1979) and average upper crust (TAYLOR and MCLENNAN, 1985).

bers are almost identical to the water contents of splash-form tektites. KOEBERL and BERAN ( 1988) measured six more Muong Nong-type tektites with infrared spectrometry, yielding the following water contents (in wt%): MN8302: 0.015, MN8304: 0.013, MN8308: 0.017, MN8310: 0.017, MN8317: 0.009, and MN83 19: 0.0 11. This gives an average content of 0.014 -t 0.003 wt% HZ0 in these samples, compared to an average of 0.008 + 0.003 w-t% HZ0 in three indochinites (and 0.011 f 0.005 wt% Hz0 in twelve tektites from the Austral- asian strewn field; GILCHRIST et al., 1969).

The water content in Muong Nong-type tektites is thus slightly higher compared to the water content in splash-form tektites (enrichment factor of 1.75 compared to splash-form indochinites) , However, the enrichment is not as pronounced as for some other volatile elements. KOEBERL and BERAN ( 1988 ) noted that the enrichment factor for water is close to the enrichment factor of fluorine, and that this might be due to the similarity of the behavior of OH- and F- in silicate structures. Opponents of the impact origin of tektites (e.g., O’KEEFE, 1976) have stated that it is not possible to drive water out of the parent sediments (containing up to several percent water) in the short time available for the tektite production. However, GLASS et al. ( 1986, 1988) have shown that atomic bomb glass, which originated in a short-time, high-temperature event from local sediments, is very dry (0.007 wt% HzO).

Chemistry of Light and Dark Layers

Muong Nong-type tektites show a characteristic layered structure, as already mentioned, composed of alternating dark

and light layers (see also Fig. 2a,b). The occurrence of layering is not uniform in all Muong Nong-type tektites; some samples show abundant layering, others are more homogeneous or show layers whose thicknesses approach the sample size. The layering can also be described as the presence of occasional dark layers within a light matrix (because dark layers are less abundant than the lighter zones). A difference in chemical composition would be expected between different layers; BARNES and PITAKPAIVAN ( 1962) report chemical differences between (overall) darker and lighter samples (but they did not analyze layers). On the other hand, YAGI et al. ( 1982) failed to detect any chemical differences between the layers. However, FUTRELL and FREDRIKSSON ( 1983) noticed chemical differences between layers which was confirmed by WEINKE and KOEBERL ( 1984).

The chemical compositions of several layers were studied in the course of this work. The results of two different microprobe traverses, composed of individual microprobe point analyses, of different regions of sample MN831 1 are given in Tables 5a and 5b. The first profile, in Table 5a, shows alternating zones of higher and lower silica. The chemical variations of some elements across this profile are depicted in Fig. 1 lab. The locations of the dark and light layers, which were located by optical microscopy, are indicated in Fig. 1 la,b in relation to the data points. The profile clearly intersects several dark and light layers and shows a complicated structure. Table 5b gives the results from a profile across only one (thicker) dark/light layer boundary. The elemental variations of some of the elements in this profile are shown in Fig. 12a,b. This figure shows clearly the relation between layering and chemical composition.

1048 C. Koeberl

I-- ---I.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE

MuonQ NonQ lektilss

T+ NankinQ bell

- Sanka P1. beas

--A- So&t Thailand

Soil Cmnbodis

(a)

fb)

La Ce Pr Nd Sm Eu Qd Tb Dy Ho Er Tm Yb Lu

REE

FIG. 7. Chondrite-normalized REE diagrams of Muong Nong- type tektites and various comparison materials. For normalization values see Fig. 4. (a) REE patterns of average Muong Nong-type tektite compared to average indochinite (TAYLOR and MCLENNAN, 1979) and average upper crust (TAYLOR and MCLENNAN, 1985). (b) Range of Muong Nong-type tektite REE compositions compared to two soil samples (BARNES, 1989) and two loess samples (from Nanking and Banks Peninsula: TAYLOR et al., 1983).

Y

2.5 -

2-

1.5

1

0.5

,‘I $5 / I / ’ I I ” / ’ I / La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

REE

Ftcr. 8. Average and range of REE compositions of Muong Nong- type tektites normalized to average upper continental crust. The REE pattern of an average indochinite (TAYLOR and MCLENNAN, 1979) is shown for comparison (dashed line). Normalization values from ‘TAYLOR and MCLENNAN ( 1985 ).

I? 1

Th @Pm) 10

FIG. 9. Correlation diagram of Th vs. La in Muong Nong-type tektites. It is evident that the data points for Muong Nong-type tektites cluster around the correlation line defined by post-Archean sediments, which is clearly distinguished from the Archean sediments correlation line. Data for Archean and post-Archean sediments from TAYLOR and MCLENNAN (1985).

More major element compositions for five different adjacent dark and light layers in different samples are given in Table SC. From Figs. 1 la,b and 12a,b and Tables Sa,b,c a number of observations can be made. There is a clear variation in chemistry between dark and light layers, which is shown by almost all elements. There is a positive correlation between A&OS, TiOZ , FeO, MgO, CaO, and K20, but a negative correlation between these elements and SiOt. This is similar to the major element correlations observed in whole samples (e.g., Fig. 3a,b). The most pronounced chemical differences were observed for A1203, FeO, TiOz, and MgO,

1 B F CICuZnGaAsSeBrSbPb

FIG, 10. Diagram showing the enrichment of eleven volatile elements in Muong Nong-type tektites (average of nineteen samples; this work) compared to average splash-form indochinites (from KOEBERL, 1986a).


Table 5a. Major element composition of indMdual points (spaced about 50-100 pm apart) along an electron microprobe traverse across a series of colored layers (dark/light) in sample MN831 1 (see also Fig. 1 la,b). All data in wt.%.

# sio, Ti02 A’203 Fe0 MnO MgO CaO Na,O $0 Total

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

81.2 0.50 9.20 3.41 0.06 1.13 0.89 1.11 2.47 99.97

76.1 0.72 12.76 4.43 0.11 1.34 1.11 0.98 2.47 100.02

76.0 0.72 12.42 4.54 0.13 1.46 1.23 1.08 2.33 99.91

75.0 0.76 12.65 5.20 0.10 1.74 1.35 1.02 2.35 100.15

81.6 0.55 9.18 3.47 0.08 1.11 0.86 0.96 2.37 100.18

81.7 0.66 8.83 3.32 0.08 1.01 0.85 1.03 2.32 99.65

77.8 0.60 10.98 3.98 0.15 1.37 1.24 0.99 2.41 99.52

81.3 0.51 8.92 3.57 0.11 1.20 0.98 0.88 2.29 99.76

82.0 0.50 8.47 3.41 0.11 1.15 0.98 0.93 2.20 99.75

81.5 0.52 9.08 3.30 0.13 1.15 1.06 0.98 2.30 100.02

82.4 0.46 8.42 3.34 0.11 1.07 0.92 1.05 2.30 100.07

81.0 0.52 9.34 3.40 0.10 1.17 0.95 1.12 2.31 99.89

81.6 0.54 8.79 3.64 0.08 1.14 0.86 1.05 2.23 99.93

76.9 0.70 11.9 4.26 0.13 1.44 1.20 1.11 2.47 100.11

78.3 0.63 11.42 3.76 0.10 1.26 1.02 1.04 2.43 99.96

78.2 0.61 11.31 3.93 0.10 1.22 1.02 1.12 2.45 99.92

while the K20 concentrations vary to a lesser extent. CaO shows a change at the dark/light boundary, but the difference within the adjacent layers is less pronounced.

The distribution of the major elements between the layers is somewhat unexpected: A1203, Ti02, FeO, MgO, and K20 are enriched in the light layers and depleted in the dark layers.

Table 5b. Major element composition of indivkfual points (spaced about 50-l 00 pm apart) along an electron microprobe traverse across a well-defined transition from a dark to a light layer in sample MN831 1 (see also Fig. 12a.b). AJI data in wt.%.

# sio, TiO, 403 Fe0 MnO MgO CaO Na,O K,O Total

1 77.7 0.61 11.4 4.22 0.14 1.40 1.15 0.95 2.59 100.16

2 77.1 0.68 11.9 4.26 0.12 1.37 1.17 0.98 2.63 100.21

3 76.5 0.68 12.1 4.19 0.12 1.41 1.17 1.07 2.64 99.88

4 77.5 0.66 11.6 3.95 0.11 1.40 1.09 0.99 2.66 99.96

5 77.6 0.62 11.4 3.85 0.13 1.30 1.05 0.89 2.64 99.48

6 77.6 0.65 11.6 3.93 0.11 1.25 1.03 0.98 2.62 99.77

7 77.1 0.68 11.8 4.08 0.12 1.30 1.07 0.95 2.68 99.78

8 77.9 0.65 11.4 3.94 0.09 1.32 1.07 1.05 2.59 100.01

9 78.3 0.64 11.3 3.90 0.09 1.27 1.05 1.12 2.63 100.30

10 78.5 0.62 11.1 3.79 0.13 1.27 1.03 0.99 2.58 100.01

11 78.1 0.64 11.5 3.95 0.13 1.22 1.06 1.08 2.66 100.34

12 78.1 0.64 11.5 3.88 0.11 1.32 1.14 1.09 2.65 100.43

13 78.1 0.61 11.3 3.95 0.09 1.31 1.13 1.14 2.61 100.23

14 78.1 0.59 11.1 4.04 0.11 1.36 1.17 0.99 2.54 100.00

15 82.1 0.51 9.04 3.15 0.13 1.06 0.86 0.89 2.43 100.17

16 81.9 0.52 8.61 3.43 0.12 1.15 0.93 0.92 2.43 100.01

17 81.9 0.54 8.60 3.45 0.08 1.21 1.02 0.94 2.42 100.16

18 81.3 0.53 8.65 3.43 0.05 1.21 1.10 0.97 2.44 99.68

19 82.1 0.48 8.49 3.34 0.11 1.17 1.09 0.98 2.48 100.24

20 81.3 0.54 8.56 3.35 0.10 1.22 1.14 0.99 2.47 99.67

21 82.0 0.50 8.46 3.31 0.11 1.19 1.15 0.93 2.39 100.04

22 81.1 0.48 8.48 3.29 0.12 1.20 1.17 0.95 2.45 99.24

23 81.9 0.52 8.43 3.16 0.10 1.15 1.24 0.91 2.42 99.83

24 81.9 0.51 8.49 3.12 0.09 1.12 1.26 0.93 2.42 99.84

1050 C. Koeberl

Table 56. Average major element composition of 5 adjacent light and dark layers in Muong Nong type tektites. The values given for each layer are averages of at least fh/e lndivkfual points, measured by electron microprobe analysis (in wt.%).

MN- 8302 8302 a304 a304 a310 a310 8311 8311 831 i 8311 lfgm dark tffht dark lfght dark light dark light dark

SiO, 76.3 at.3 77.3 81.4 76.5 80.6 77.7 81.8 75.8 al.9

TiO, 0.75 0.55 0.66 0.53 0.71 0.59 0.64 0.51 0.73 0.53

v3 12.26 9.28 11.41 8.64 12.53 9.43 11.5 0.58 12.51 8.46

Fe0 4.38 3.33 4.22 3.31 4.33 3.22 4.00 3.30 4.70 2.87

MnO 0.10 0.09 0.07 0.12 0.09 0.08 O.lf 0.10 0.13 0.12

MgO I.68 I .23 1.46 l.f3 1.71 1.34 1.32 1.17 1.57 1.03

CaO 1.06 1.04 1 .Ol 0.90 0.92 0.94 1.10 1.10 1.11 1.18

Na,O 0.99 0.93 1 .O? 0.90 0.93 1.03 1.02 0.94 0.96 1.04

K20 2.51 2.40 2.53 2.46 2.41 2.39 2.62 2.43 2.60 2.52

Total 100.05 100.15 99.75 99.56 100.14 99.82 100.01 99.93 100.11 99.65

Si& shows the opposite behavior: the dark layers are the high-silica zones, and the light layers are the low-silica zones. This is clearly visible from Figs. 1 la,b and 12a,b, but also evident from the data for several other layers given in Table 5c. Figure 13 summarizes the data: it shows the enrichment factors of all major elements in the light areas compared to the dark layers. This figure quantifies the above statements: the most prominent differences are shown by A1203, TiO2, and FeO, while by calculating an average the scatter in Na20 contents disappears and shows that there is co-variation with k&O. A simple dilution of the tektite material with silica (i.e., quartz) cannot explain the chemical differences between the layers because the other major elements are enriched or depleted to various degrees. It is more likely that we observe incomplete mixing of different parent materials (of similar provenance).

In order to determine if the chemical differences between light and dark layers extend to the trace elements, a set of carefully isolated adjacent layers were analyzed by INAA. The results of the analyses of six such pairs are given in Table 6. It is evident that there are differences in trace element contents between individual light and dark layers, in agreement with major element dam. The Fe content in the layers was taken as indication of the quality of separation between light and dark layers. Almost all trace elements are enriched in the light layers and depleted in the dark layers. This is in exact agreement with the variation of the major element concentrations. The variations are not exactly the same for all six layer pairs, but the tendencies are the same for all.

Figure 14 shows the enrichment of the trace elements in a light layer of sample MN8302 compared to an adjacent dark layer. The REEs, for example, show some slight intra- group variations (i.e., different slopes or La/Yb ratios). This is illustrated in Fig. 15, which shows the REE patterns of two light/dark layer pairs. The REE abundances in the light layem are distinctly higher than in the dark layers. The Eu anomalies and the Th/U ratios also show some differences between the layers. Another consistent difference is shown by the K/U

ratio: it is always higher in the light layer. The difference in Eu anomaly and in the Th/U and K/U ratios persists throughout all samples. As for the major element variations, a simple variation in silica content (as proposed in the selective volatilization model of WALTER, 1967 ) would not be able to explain the observed variations of trace element contents and ratios. The present data indicate incomplete mixing of different parent rocks. Liquid immiscibility, as suggested by ZOLENSKV and KOEBERL ( 199 I), may also play some role in the formation of the colored layers.

Another example of zones of different color in Muong Nong-type tektites was mentioned by ~L~TRELL ( 1988, 1991), who described white vesicular glass within a closed fold in a Muong Nong tektite from the Laos/Thailand border. Small samples of the white and dark glass (courtesy D. Futrell) were analyzed for trace elements, and preliminary results were reported by MEISEL et al. ( 1989). The final data are given in Table 7. Similar to the major element data, the trace element results show that there are only small chemical differences between the two zones. FUTRELL ( 1988 ) suggested that the vesicular glass originated from a different stage of tektite production which was subsequently, in yet another high- temperature event, welded to the dark glass. The present data (and our understanding of tektite formation) do not support this interpretation. It seems more likely that the white color is mainty due to the foamy nature of this oscular glass zone, which was entrapped by the closing fold while it was still degassing. Similar vesicular zones exist in zhamanshinites; these impact glasses, which are found at the Zhamanshin crater in the USSR, have other general similarities to Muong Nong-type tektites ( KOEBERL and FWDRIKSSON, 1986).

Ferric/Ferrous Iron Ratios

The ratio of Fe2+ to Fe3+ in tektites and impact glasses is rarely discussed in these days of modem electron microprobe analyses, but may have important implications for the determination of tektite source rocks and impact processes.


Microprobe Traverse

-.-Fe0 +,%?03+6,02

Microprobe Traverse

-.-Fe0 +AI203+MgO -B-‘&O

FIG. 11. Major element compositions along a microprobe profile across several alternating light and dark layers in a thin section of sample MN831 1 (see also Table 5a). The tick marks on the x-axis mark individual electron microprobe points, spaced about 100 pm apart. (a) SiOl shows a variation that is in negative correlation with Fe0 and Al2O3. (b) FeO, A&OS, MgO and (to a lesser extent) CaO all show a positive correlation with each other.

From a study of correlation of elements of different valency in moldavites, DELANO et al. ( 1987) concluded that the source materials of the tektites contained iron predominantly in the +3 state, which is in accordance with the chemistry of sediments. This implies that the tektite production process, i.e., the impact, leads to a shift in the redox state: the glasses are more reduced than the parent rocks. This is also discussed by FLORENSKI et al. ( 1978) on the basis of quenching and its influence on structural chemistry. A number of tektite analyses including ferric/ferrous iron ratios have been pub lished, mainly in the 196Os, and have shown that the iron in tektites is predominantly in the +2 state. TAYLOR ( 1962b) and TAYLOR and SACHS ( 1964 ) found an average Fe3+/Fe2+ ratio of 0.14 f 0.06 for thirty-one australites and SCHNETZLER and PINSON ( 1964) reported a ratio of 0.14 f 0.04 in thirty- four australites. The results for the present Muong Nong- type tektite samples are given in Table 8. An average ratio of 0.133 f 0.04 1 was found.

In contrast to these wet-chemical determinations, EVANS and LEUNC ( 1979) reported that they found Fe3+/Fez+ ratios in tektites of less than 0.001, by MiiBbauer spectrometry, while some impact glasses show an Fe3+ component. This result is in contradiction with MbBbauer measurements of GRASS et al. ( 1983), who found a ferric/ferrous ratio of0.06 for a thailandite, and more recent MiiBbauer measurements (Lottermoser and Koeberl, unpubl. data) that show results comparable to the earlier wet-chemical analyses. On the other hand, SCHREIBER et al. ( 1984), using wet-chemical techniques, failed to find any Fe3+ in four Australasian tektites. However, RAYKHLIN et al. (1986) used MSBbauer spectrometry again, and electron spin resonance (ESR), and found measurable quantities of Fe3+ in tektites.

Because of the contradictory results, FUDALI et al. ( 1987) used wet-chemical and instrumental methods on the same tektite samples. Their results were somewhat disappointing: they found that the agreement of the results obtained by four wet-chemical methods and two instrumental methods (MZiBbauer spectrometry and ESR) is rather bad, which they attributed to random analytical errors and systematic errors of the methods they used. FUDALI et al. ( 1987 ) analyzed two splash-form and three Muong Nong-type tektites and did not find any significant difference in Fe3+ / Fe2+ ratio between the different tektite types. Earlier, KOEBERL et al. ( 1984g) reported preliminary data which suggested that Muong Nong- type tektites have a higher ferric/ferrous iron ratio than splash-form tektites. The data in Table 8 are the final data for the present nineteen samples which have been obtained by reanalyzing some samples and using average electron probe data for total Fe rather than INAA data (some of the earlier INAA Fe numbers were less precise than the electron probe results). There does not seem to be a significant difference between the present data (average ratio 0.13 + 0.04, range 0.06-0.26) and the wet-chemical data for splash-form tektites published in the 1960s (two thailandites which were analyzed by the same method gave ratios of 0.06 and 0.07, respectively, but two samples probably do not prove much). The data are also in agreement with Muong Nong-type tektite analyses by BARNES and P~TAKPAIVAN ( 1962). There seem to be quite a few analytical problems with determining Fe3+/Fe2+ ratios: FUDALI et al. ( 1987) report Fe’+ values ranging from 0.05 to 0.40 wt% in one sample, depending on the method! There- fore, no firm conclusions regarding any differences between splash-form and Muong Nong-type tektites seem possible at this time.

DISCUSSION REGARDING THE ORIGIN OF MUONG NONG-TYPE TEKTITES

Age of Muong Nong-Type and Splash-Form Australasian Tektites

The formation age of the Australiasian tektites has been determined by various methods (fission track, K-Ar dating) and authors, e.g., GENTNER et al. ( 1969a,b) and STORZER and WAGNER ( 1977, 1979), to be 0.7 Ma. Muong Nong- type indochinites were dated by K-Ar and fission track analysis and have an age that is indistinguishable from the splash- form tektites ( GENTNER et al., 1969a,b; STORZER and WAG-

NER, 1977). Some irregular Muong Nong tektite ages

1052 C. Koeberl

16

14-7

8 - LIGHT LAYER __) - DARK LAYER +

3 13- 0

Microprobe Traverse

1 + SiO2 l 0.2 + Al203 1

3.5

3

8 G 2.5 0

8 2

z

(4

FIG. 12. Major element compositions along a microprobe profile across well-pronounced boundary between a light and a dark layer in a thin section of sample MN83 I I (see also Table 5b). The tick marks on the x-axis mark individual electron microprobe points, spaced about 50-100 Nrn apart. (a) Variation of SiOz (to amplify the effect, the data have been multiplied by 0.2) and A1203. (b) Variation of FeO, KzO, MgO, CaO, and Tiq, showing the very pronounced chemical changes at the dark/light boundary. Silica is depleted in the light layer, while all other elements are enriched.

(spreading from about 0.4 to 0.9 Ma) were determined by fission track analysis by KASHKAROV et al. (1985). These ages have never been described any closer or confirmed by independent work and seem to be due to analytical problems.

GENTNER et al. ( 1970) showed that microtektites found in the Australasian tektite strewn field have the same fission track age of 0.7 Ma. This is in perfect agreement with the strati~phic age of Australasian microtektites which are found in deep sea drift cores in layers that are closely related to or coinciding with the Brunhes-Matuyama reversal boundary, indicating that they fell 0.7 Ma ago (e.g., GLASS, 1978, 1979b; GLASS et al., 1979). On the other hand, the

stratigraphic age of the tektites found on land is much youn- ger, around 7000-24,000 years. Several authors have suggested that this argues against a relationship between microtektites and tektites, and that tektites only fell around 7000- 24,000 years ago (e.g., LOVERING et al., 1972; CHALMERS et al., 1976,1979). This was termed the so-called “age paradox.”

Several observations were cited that were suppotiy in favor of an age paradox; for example, that australites appear too delicate to have undergone heavy erosion and transport or that no microtektites have been found on land. These arguments have already been questioned by GLASS ( 1978, 1979b), who pointed out the many fmgments present in the


1.6

F

!!7

1.4

$ 1.2

s 6 1

2

$j O.6

t; 0.6 ‘6

f 0.4

0.2

0 Si02 T02A1203 Fe0 MnO MgO CaO Na20 K20

FIG. 13. Enrichment factor of major elements in light layers compared to dark layers. Only silica is depleted in the light layers. Average of data from Table 5b,c.

australite collections, and that many tektites obviously did not survive erosion. He also reiterated evidence for the connection between microtektites and tektites and concluded that tektites found in late to post-Pleistocene deposits must have been transported and eroded. It seems to me that, if certain facts are considered, no age paradox remains. It is beyond any doubt that tektites were eroded, mainly by interaction with water. Many tektites show etch pits and grooves (see photos in, e.g., O’tiEFE, 1963, 1976); Muong Nong- type tektites are sometimes deeply eroded. Because of the very slow dissolution rate of tektites (LAMARCHE et al., 1984), it is highly unlikely that these etching patterns could have originated during only a few thousand years. Further- more, recent leaching studies on tektite glass ( BARKATT et al., 1989) have demonstrated that in seawater, tektite glass corrodes much more slowly (by at least 2 orders of magnitude) than on land. This explains readily why no microtektites have been found on land: they have long been dissolved.

From all chemical, isotopic, and age studies, the relationship between microtektites and tektites cannot be seriously doubted. In the other three strewn fields, tektites are generally not found in recent sediments, but sometimes occur in the stratigraphic position corresponding to the formation age. Of great significance is the discovery of the tektite locations in Barbados and DSDP Site 6 12 in the North American strewn field (e.g., NGO et al., 1985; THEIN, 1987; KOEBERL and GLASS, 1988; GLASS, 1989). At these locations, microtektites and tektites (tektite fragments) occur together. At DSDP Site 6 12, shocked minerals found in the same layer (e.g., THEIN, 1987; BOHOR et al., 1988; GLASS, 1987, 1989) provide the immediate link with the impact event. It can be argued that the collective occurrence of tektites, microtektites, and shocked minerals not only provides strong evidence for the association of microtektites and tektites (KOEBERL and GLASS, 1988; KOEBERL, 1989), but also eliminates the age paradox. It is suggested that geological processes that were responsible for the erosion and transport of the Australasian tektites have to be explored, and that an age paradox does not exist in reality.

Another puzzle was noted by STORZER and WAGNER ( 1980a,b). Their detailed fission track work suggests that

australites may have a slightly different age than other Aus- tralasian tektites. They found a corrected plateau age of 0.83 Ma for australites, but only 0.69 Ma for indochinites, and therefore suggested that two different impact events were responsible for the two tektite groups. These ages have report- edly been supported by 40Ar- 39Ar ages of one australite (0.89 Ma) and one indochinite (0.69 Ma) as given by STORZER et al. ( 1984). However, GLASS ( 1986) finds no evidence for a second, older (0.8-0.9 Ma) microtektite layer in deep sea cores, and SHAW and WASSERBURG ( 1982) find no isotopic difference between australites and other Australasian tektites. In absence of independent confirmation of two different ages, more Australasian tektites need to be dated reliably to obtain further evidence in favor of or against any such difference.

Isotopic and Rare Gas Evidence

In an important study, SHAW and WASSERBURG ( 1982) have shown that the crustal materials which weathered to form the parent sediments for the Australasian tektites have Nd model ages of about 1.15 Ga, and that Rb-Sr data point to a final sedimentation of their parent material around 250 Ma ago. Further studies have expanded our knowledge mainly of the North American strewn field (NGO et al., 1985; STECHER et al., 1989). Recently, BLUM et al. (1991, 1992) have studied the Rb-Sr and Sm-Nd isotopic systematic of Muong Nong-type tektites and found that the Muong Nong data agree with the data for splash-form tektites, and that the combined data now allow the conclusions that the source material was Precambrian crustal terrane (from Nd model ages) and that the sediments that were later melted to form tektites were weathered and deposited about 167 Ma ago. These results indicate that loess, which was suggested as source materials by, e.g., WASSON ( 1987, 1989, 1990), is an unlikely parent material (see also later section). Older stratigraphic units (Jurassic sediments), which are not uncommon throughout Indochina, seem to have been the tektite source rocks.

More evidence for a sedimentary precursor, and further proof of a terrestrial origin of tektites, comes from the study of cosmogenic radionuclides. PAL et al. ( 1982) first reported that the “Be content of Australasian tektites cannot have originated from direct cosmic ray irradiation in space or on earth, but can only have been introduced from sediments that have absorbed “Be that was produced in the terrestrial atmosphere. This conclusion was supported by further studies of 26A1 and 53Mn by YIOU et al. ( 1984) and ENGLERT et al. ( 1984). The isotopic studies thus support the geochemical conclusions, namely, that the Muong Nong-type and other Australasian tektites were produced from local terrestrial sediments in the same event.

MULLER and GENTNER ( 1968) demonstrated that gas bubbles in tektites contain residues of the terrestrial atmosphere at low pressures. JESSBERGER and GENTNER ( 1972) have measured the gas content and composition in bubbles in Muong Nong-type tektites and found that the N2/Ar ratio, and the isotopic ratios of 40Ar /36Ar, 36Ar/38Ar, 82Kr/s“Kr, ‘29Xe/‘32Xe, 84Kr/ ‘32Xe, and others, agree perfectly with the respective atmospheric ratios, providing further evidence for an origin of tektites within the terrestrial atmosphere. Re-

1054 C. Koeberi

Table 6. Trace element contents of 6 selected pairs of fight and dark layers from Muong Nong type tektites. Light layers are designs (L) and dark fayers (D). All data obtained by neutron Austin analysis; In ppm except where noted.

MN8302 MN8302 MN8362 MNWX? MNB306 MN8306 MN8310 MN8310 MNWG MN8316 MN8319 MN8319

&I PI (4 (W 0-I (01 04 (0) (11 m 0.) (0)

Na (wt.%) Cl K (wt.%) SC Cr Mn Fe (~.%) co Ni Zn Ga As Se Br Fib Sr Zr

AS Sb cs f3a La Ce Nd Sm ELI Gd Tb

4 Tm Yb Lu Hf Ta W

fr (wb) Au (wb) Hg Th U

K/U tr/Hf Th/U

$lub,

1.26 1.29 1.11 1.20 0.96 0.90 1.13 1.08 1.06 0.97 1.01 1.18 270 250 260 <2x! 290 250 230 190 210 230 ~250 280

1.93 1.89 1.88 1.91 2.07 2.11 2.14 2.10 2.11 2.10 2.13 2.15 9.78 7.49 10.0 7.02 7.%4 7.14 11.9 6.57 10.1 6.04 10.0 7.72

66 54 67 52 67 50 120 60 68 61 76 61 620 690 640 695 360 696 990 730 620 665 672 645

3.57 2.73 3.73 2.85 2.76 2.52 4.37 2.60 3.68 2.27 3.71 2.86 14.3 11.4 15.5 12.2 10.4 9.44 19.9 9.92 13.9 9.38 14.5 10.9 39 30 4% 37 42 31 56 33 49 37 51 40 62 49 71 46 60 51 76 55 56 51 71 58 21 14 22 16 19 13 22 12 19 17 18 12

4.61 2.76 6.62 2.28 4.52 3.61 5.42 2.61 5.89 1.91 7.16 1.58

<O.S <0.4 0.15 co.2 0.28 0.25 0.27 <0.3 0.12 0.18 co.2 <0.2

3.8 3.5 3.6 2.5 4.08 3.1 4.12 3.9 3.1 2.8 2.5 2.2

170 140 IS0 110 120 91 240 87 175 130 145 160

140 120 165 115 146 130 125 148 160 150 190 145 280 160 210 I60 480 350 310 276 480 290 510 280

qo.1 CO.1 co.1 CO.1 <O.i <O.l 0.1 <O.l co.2 ccl.15 <0.2 <O.l

0.45 0.35 0.4% 0.37 1.15 0.65 0.77 0.61 0.85 0.64 1.08 0.86 6.40 4.52 7.38 5.95 5.51 4.24 9.6% 3.82 7.35 3.85 6.24 4.62

410 300 395 370 460 240 420 260 450 230 490 340

37.9 30.2 41.9 31.1 31.0 28.0 39.0 28.9 39.6 27.9 37.1 31.9

81.4 67.1 93.6 65.0 58.1 56.3 81.0 66.1 86.1 61.5 77.0 67.2

42.0 32.8 43.1 31.2 28.1 26.9 41.0 30.1 45.2 28.8 36.1 30.0

6.47 5.21 5.59 4.96 5.07 4.46 7.23 4.28 5.76 4.09 5.90 4.85

1.02 0.61 0.8% 0.58 0.76 0.69 0.95 0.71 0.98 0.6% 0.85 0.91

5.1 3.7 5.6 3.6 4.1 3.7 6.3 3.9 6.0 3.6 6.5 5.1

0.91 0.62 0.99 0.59 0.73 0.64 1.12 0.68 1.06 0.64 1.18 0.91

6.01 3.44 5.46 3.55 4.41 4.01 6.36 4.61 5.81 3.65 7.21 5.25

0.45 0.32 0.46 0.35 0.36 0.35 0.57 0.43 0.47 0.37 0.53 0.46

2.87 2.20 2.95 2.31 2.29 2.15 3.86 2.66 3.25 2.73 3.42 3.12

0.42 0.34 0.43 0.39 0.3% 0.36 0.61 0.44 0.45 0.35 0.44 0.40

9.33 6.52 9.89 6.87 6.12 5.64 9.42 6.6% 8.08 6.44 10.5 7.7%

1.43 0.94 1.66 1.04 1.36 1.05 I.50 I.60 1.68 1.01 1.7% I.33

0.9 0.7 0.9 0.7 1.1 0.9 0.8 0.8 1.2 0.8 1.0 0.8

<1.2 cl.5 <I <l ~0.8 <I.2 <2 cl.6 <2 <2 <I cl.5

1.8 2.9 2 1.6 2 1.6 2.5 2 1.5 2 <2 <2

<1 <l CO.6 cl <l <0.9 <3 <4 <2 <l <3 ~2.5

15.8 10.0 16.2 10.8 12.4 11.1 17.5 10.5 15.8 10.2 17.0 12.5

2.15 1.88 2.21 1.95 2.65 I.85 3.32 2.t8 3.05 1.65 4.2 2.45

8980 10050 6610 9790 7810 11400 6460 9330 6920 12730 5070 8780 30.0 24.5 21.2 21.8 76.4 59.9 21.9 41.1 59.4 45.0 48.6 36.0

7.35 5.32 7.33 5.54 4.6% 6.00 6.27 4.61 5.16 6.18 4.04 5.10

6.92 9.28 9.60 9.10 9.19 8.80 6.66 6.83 8.23 9.91 7.33 6.91

Eu/Eu* 0.54 0.42 0.4% 0.41 0.53

cently,M~'rw~~etal.( 1989)and MATsUBAKALZ~~~.(I~~~)

have shown that the ratios of rare gases in impact glasses and tektites are consistent with an origin from the terrestrial atmosphere, and that, because of its higher mobility, during the long terrestrial residence times, Ne has predominantly diffused into the glass.

Muong ~ang-T~e Tektites at other Strewn Fields

Muong Nong-type tektites occur mainly in Kndochina (Thailand, Cambodia, Laos, Vietnam), although some occur in China (BARNES, 1969) and the Philippines (CHAPMAN

0.52 0.43 0.63 0.51 0.54 0.42 0.56

and SCHEIBER, 1969 ). Some earlier reports of Muong Nong- type tektites from other strewn fields, e.g., the moldavite field (ROST, 1966), remained un~onfi~ed because only ambig- uous petrological, and no chemical, data were given. KOEBERL ( 1986b) studied the chemistry of a moldavite and a North American tektite which showed some evidence for a possible layered structure, but the criteria that distinguish Muong Nong-type tektites from splash-fog tektites (which will be discussed in the last section) were not fulfilled.

Only recently, unambiguous Muong Nong-type tektites were identified among samples from other strewn fields. Some North American tektite fmgments found at DSDP Site 612


FIG. 14. Enrichment factor of trace elements in light layers compared to dark layers, determined by INAA of individual chips of sample MN8302. Most elements are enriched in the light layer, but to a variable degree.

are layered and show some enrichment in volatile elements ( KOEBERL and GLASS, 1988; GLASS, 1989) and have been interpreted as Muong Nong-type tektites ( KOEBERL, 1989 ) . In tbe Australasian and in the North American strewn fields, Muong Nong-type tektites apparently show an asymmetric distribution in the strewn field, occurring on only one end, which is probably close to the source crater ( KOEBERL, 1989).

Geochemistry of Source Rocks

In the Results and Discussion section, it was demonstrated that the source rocks of Muong Nong-type indochinites are most probably some recent (i.e., post-Archean) upper crustal sediments such as shale, greywacke, other sandstones, and similar weathered sediments. The major and trace element chemistry, in particular the REE abundances, the ratios of, e.g., K/U, Th/U, Zr/Hf, and La/Yb, support this interpretation. Figure 16a,b,c,d shows several attempts to reproduce the major element chemistry of Muong Nong-type tektites by mixing different local country rocks. The major element data of the components, and the results of the mixing calculations, are given in Table 3.

Figure 16a shows a simple approach by mixing 50% each >f two different soil types found in Thailand and Cambodia t v BARNES ( 1989). Although five major elements show a good fit, other major elements do not. Trace elements would not fit either; in particular the REE pattern cannot be repro- duced (see Table 4 and Fig. SC). Figure 16b shows the results of a mixture of loess, soil, and quartzite, which yields a better, but still not acceptable, fit. Fig. 16c shows a mixture that incorporates less soil but some average continental crust. In

this mixture, which also incorporates loess and quartzite, the fit is better than for models Ml and M2. In M4 (Fig. 16d) the loess is replaced by a higher proportion of soil and average crust. This fit is comparable to model M3. Sodium seems to

100

10

I,,,,,,,,,,,,,,, Ln Ce Pr Nd Sm Eu Gd Tb tiy Ho Er Tm WJ Lu

REE

FIG. 15. Chondrite-normalized REE patterns for two different sets of light and dark layers. The upper line for each sample is the light layer, the lower line for each sample is the dark layer. Solid lines: layers in sample MN8302; dotted lines: layers in MN83 10. There is a consistent and significant difference between the light and dark layers.

1056 C. Koeberl

Table 7. Abundances of selected major and trace elements fn a whke (foamy) zone and adjacent dark matrix in a folded Muong Nong tektite.

Foam Matrix

Na W 1.64 I.39

SC 10 9.3 Cr 117 103 Mn 667 620 Fe (%I 3.80 3.30 CO 18.5 15.3 Zn 80 75 Ga 19 15 AS 3.4 3.4 Rb 182 143

:;1 368 0.98 210 0.74 CS 10.2 8.8 Ba 400 388 La 50.3 46 Ce 102 86 Nd 43.7 41.8 Sm 7.86 7.9 EU 3.76 1.5 Tb 0.8 0.8 DY 7.2 7.12 Yb 3.5 3.5 Lu 0.53 0.46 Hf 9.8 9.4 Ta 1.5 1.0 Th 18.2 17.6 u 4.0 3.3

Anslvst: T.Meisel AN data in Pam, except where nolal. Sample Courtney D. FutmN (FlJ?RELL, 1988).

be too high in all models, but this may because Na could have been removed from the glass by hydrothe~al solutions ( SHIRAKI and IIYAMA, 1990), maybe during a post-impact hydrothermal activity phase. Variations in Ca and Mg content could be due to variable carbonate. It should be emphasized that the mixtures are only examples: by substituting average sediments for average crust, a slightly better fit is obtained. The main point is that the chemical signature is sedimentary.

It was previously shown that the chemistry of the dark and light layers in Muong Nong-type tektites is compatible with incomplete mixtures of different target rocks. It is reasonable to assume that more than just two different target rock varieties (which need not differ significantly in overall composition) were involved. The simple mixing models shown in Fig. 16 demonstrate that it is relatively easy to reproduce (at least approximately) the major element chemistry of average Muong Nong-type tektites. It should not be forgotten, however, that the present average refers to Si-rich Muong Nong-type tektites, and that a slightly different proportion of target rocks needs to be mixed to reproduce the low-Si Muong Nong-type indochinites.

On the other hand, the more difficult task is to match the trace element abundances. I have just mentioned that some of the mixtures Ml-M4 may yield a good approximation of the major element chemistry but fail to reproduce details of the trace element abundances, e.g., the REE pattern. Wi~out knowledge of the exact target rocks (which requires knowledge

of the crater; see later), any exact solutions regarding mixing models seem somewhat hazardous. We can, however, conclude that local sediments, e.g., shale, sandstone, and quartzite, have been involved in the tektite production. Loess or soil seem to provide an acceptable fit of major elements, but trace elements are more difficult to match. Loess is problem- atic as a source for a number of reasons. The closest large loess deposit, the Chinese loess plateau, is about 2000 km from Indochina; no loess deposits are known in Indochina. Also, such recent sediments as loess or soil are not favored by the Rb/Sr and Sm/Nd isotopic data because loess shows usually a homogeneous Sr isotopic composition worldwide, while the tektites have a variable Sr isotopic composition (TAYLOR et al., 1983; BLUM et al., 1992). However, there is reason to believe that the loess and soil precursor rocks (i.e., before weathering) also provide a better match (e.g., no Ce anomalies due to weathering).

The Question of the Source Crater

Over the history of tektite research, and a&er the realization that tektites have most probably originated during a hypervelocity impact, numerous suggestions and educated guesses have been made regarding the location of the source crater for the Australasian tektites. Relatively reliable links between a crater and the tektite strewn field have been established for the Ivory Coast and the moldavite fields, but no large crater of the required age is known (GRIEVE, 1987) for the Austra- lasian strewn field. Several proposals for possible craters were made and later discounted. SCHMIDT ( 1962) and WEIHAUFT ( 1976) suggested that the Wilkes Land gravity anomaly in

Table 8. Contents cf Fe*’ and Fe3’ (fn wt.%) and ferric/fe~ous Iron ratios In 19 Muong None type tektftes.

Fe (tot.)

Fe*’ Fe3’ Fe3’/Fe2’

MN8301 2.47 2.22 0.26 0.104

MN6302 2.54 2.24 0.30 0.119

MN6303 3.07 2.57 0.68 0.162

MN6304 3.23 2.72 0.51 0.157

MN8305 2.52 2.34 0.18 0.071

MN6686 2.95 2.68 0.27 0.092

MN8387 3.15 262 0.53 0.168

MN8388 2.92 2.47 0.44 0.152

MN6309 3.06 2.85 0.19 0.964

MN8310 3.09 2.68 0.48 0.131

MN831 1 3.14 2.78 0.36 0.114

MN8312 2.93 2.49 0.44 0.151

MN8313 3.19 2.75 0.44 0.137

MN6314 2.47 2.16 0.31 0.126

MN8315 2.71 2.37 0.33 0.124

MN8316 3.16 2.85 0.30 0.096

MNS317 3.13 2.62 0.51 0.162

MN8318 2.51 2.14 0.37 0.149

MN6319 3.11 2.32 0.79 0.255

Average

Range

0.133t0.641

0.064-0.266


Antarctica may be evidence for a crater, but this was disputed by BENTLEY (1979). DIETZ (1977) proposed that the Elgy- gytgyn crater in Siberia may be the source for the Australasian tektites, while GLASS ( 1979a) preferred the Zhamanshin crater. Both versions are unlikely because of geographic location, chemistry ( KOEBERL and FREDRIKSSON, 1986), size, and especially age (e.g., STORZER and WAGNER, 1979; KOEBERL and STORZER, 1988). HARTUNG and RIVOLO (1979) suggested that a circular structure in Cambodia could be the source crater, but this structure rather seems to be a volcanic caldera ( LACOMBE, 1967).

More recently, WASSON ( 1987, 1991) has renewed the earlier suggestion of Barnes that the Muong Nong-type tektites may have originated in a multitude of small craters scattered over all of Indochina. There seem to be many problems with this idea; some objections include: small craters produce small to negligible quantities of relatively inhomogeneous impact glasses, as we know from many impact craters, and not large amounts of homogeneous material; small impact events are unable to provide the energy to launch the (associated) splash- form and aerodynamically shaped tektites; the isotopic data do not seem to be in agreement with the multitude of different source rocks that are required by a multiple impact theory; the crater problem has been multiplied-instead of one missing crater, there would be a multitude of missing craters. In agreement with most other studies I therefore prefer to discuss the question for a single large impact crater.

STAUFFER ( 1978) analyzed the pattern of distribution of tektites and microtektites within the Australasian strewn field and found that the pattern does not show a homogeneous distribution. It rather shows both radial and concentric patterns, and he suggests that the common way of drawing the outer border of the strewn field around the locations of deep sea cores containing microtektites (e.g., Fig. I ) may not be exact enough because there are zones that do not contain microtektite-bearing deep sea cores. These radial distribution patterns are also found in chemical distribution patterns observed by CHAPMAN ( I97 I ). From the distribution patterns, and considering that Muong Nong-type tektite occurrences indicate proximity of the crater, STAUFFER ( 1978) suggested a crater that may be concealed beneath alluvial deposits of the lower Mekong Valley area.

A possible off-shore impact location was suggested by SCHNETZLER et al. ( 1988 ) , who analyzed combined Seasat / Geos 3 altimeter data of sea surface heights in the northern part of the strewn field to search for negative gravity anomalies on the continental shelf and slope which could be related to a source crater. They found a large negative gravity anomaly consistent in magnitude with that expected by a crater of about 100 km diameter at a location on the continental slope. The center of the structure (named the Qui Nhon Slope Anomaly) is about I75 km to the east of the Vietnam seashore (not far from the location where four tektites were dredged up from the seafloor at a depth of 1270 m; SAURIN and MIL- LIESLACROIX, I96 I ). Underwater craters must exist on earth, but, with one exception (JANSA and PE-PIPER, 1987), they have not yet been found.

A new suggestion for the source crater location was made by HARTUNG ( 1990 ) , who proposed that the lake Tonle Sap in Cambodia is the result of the Australasian tektite source

crater. The lake is about 100 km long and up to 35 km wide and is divided into two parts, giving an appearance similar to an elongated double crater. The dimensions are probably minimum values, as the structure is almost completely filled with alluvium. The estimate of BALDWIN ( 1981), only I7 km diameter for the Australasian tektite source crater, is almost certainly too small, considering the volume of impact material (GLASS et al., 1979). The structures proposed by SCHNETZLER et al. ( 1988) or HARTUNG ( 1990) are of a more realistic size. Tonle Sap would be in agreement with chemical and isotopic data for tektites, but because of the sparse knowledge of the composition and distribution of rock out- crops in this area of Cambodia, more detailed studies are necessary.

The exact mechanism of tektite production is still not known, despite detailed calculations by, e.g., DAVID ( 1966, 1972 ) . Obviously, the production of tektites requires special conditions because otherwise more than just four tektite strewn fields would be associated with the known impact craters (GRIEVE, 1987). One possibility would be oblique impact. A numerical model of tektite dispersal was studied by JONES and SANDFORD ( 1977). The suggestion that tektites are formed by jetting during the very early phases of an impact was discounted by VICKERY ( 1990). Material originating from jetting would be composed predominantly of projectile material. Any projectile signatures in tektites (e.g., platinum group element enrichments) are not very pronounced and do not allow unambiguous identifications (e.g., MORGAN et al., 1975; PALME, 1980; PALME et al., 198 I ). Recently, MEI- SEL and KOEBERL ( 1990) studied platinum group element abundances in some Muong Nong tektites and found contents of about 0.2-0.3 ppb Ir and 0.03-o. IO ppb OS, excluding a major projectile component. MELOSH ( 1990) suggested that the expanding vapor plume after an impact may be important in distributing the tektite material. GLASS et al. ( 1979) estimated the total mass of microtektites in the Australasian strewn field to be around lost, but the total mass of tektite material in the strewn field must be higher. A recent impact model calculation by RODDY et al. ( 1987) showed that after the impact of a IO km asteroid at 20 km/s into a continental target, I .65 - IO 14t (or 85.8% of the total ejecta) is composed of sedimentary material. The projectile contributes only 0.5% to the total ejecta, and for an oceanic impact, 5 I .9% of the ejecta is water and only 47.3% crust. The continental crater ended up to be about 80 km in diameter, and the oceanic crater 105 km.

CONCLUSIONS

A detailed petrological and chemical study of Muong Nong-type tektites from Thailand has provided a large data set of chemical data and allows some conclusions regarding their origin. Muong Nong-type tektites are one of three general groups of tektites which occur on land, the others being splash-form and aerodynamically shaped tektites. The latter type is found predominantly among the australites (with the exception of one poorly documented possible australite re- covered from the seafloor of the Central Indian Basin; PRASAD and RAO, 1990). Muong Nong-type tektites at the AustraI- iasian strewn field are restricted to the northern part of the

1058 C. Koeberl

50% Soil (Thailand)

50% Soil (Cambodia)

n ‘Fe0

Na20 /

/

n K20 Ti02 n n MgO

n CaO

100 Si02.

40% Lows (Nanking)

20% Soil (Cambodia)

Al203 n

/

/

n Fe0

n K20

Na20H n Mg” Ti02 n ’

n CaO

(4

(b)

FIG. 16. Mixing models aimed at reproducing the major element chemistry of Muong Nong-type tektites. (a) Mixture of two local soils (BARNES, 1989). (b) Mixture of loess (TAYLOR et al., 1983), soil (BARNES, 1989), and average quartzite (MEISEL et al., 1990). (c) Mixture of loess, quartzite, soil, and average continental crust (TAYLOR and MCLENNAN, 1985). (d) Mixture of soils, quartzite, and average crust.


10:

1:

0.1 z

35% Loess (Nanking) 40% Quartzita 10% Soil (Cambodia) 15% Average Crust Al203.

’ /.’

n Fe0

n K20

Na20 ’ g MgO

Ti02 w / CaO

/

n MnO

Si02.

0.01 vv 0.01 0.1 1 10 1

Ave. Muong Nong Tektites (wt %)

10:

1:

0.1:

/

/ n Fe0

Na20 n ,““/“‘k K20

n MgO

/ Ti02 h

/

.//

n MnO

Ave. Muong Nong Tektites (wt %)

FIG. 16. (Continued)

(4

(4

strewn field and are found predominantly in Thailand, Laos, layered structure. Because of their large size it has been ques- Cambodia, and Vietnam. tioned whether they formed in one piece or are accumulations

Muong Nong-type tektites differ in appearance from splash- of small particles, but in a study of the viscous flow and form tektites; they have irregular, blocky shapes and show a crystallization behavior of the glass, KLEIN et al. (1980)

1060 c. Koeherl

showed that Muong Nong material is an excellent glass former and could occur in even larger bodies than the ones found so far. Because of the slow dissolution rate of tektites during interaction with water (LAMARCHE et al.. 1984; BARNES, 1990), the surface structures indicate that the samples have been lying on earth since their origin 0.7 Ma ago. The erosion often amplifies the layered structure and clearly shows flow structures, sometimes even faulting. In thin sections, dark and light colored layers alternate, with dark layers being less abundant and embedded in a lighter glass matrix. The dark layers amplify the flow structures and contain less bubbles than the lighter zones in which bubbles are very abundant. Lechatelierite is common in Muong Nong-type tektites. It is often frothy, indicating a lower temperature origin than for splash-form tektites.

Muong Nong-type indochinites can be roughly divided in two groups; a high silica, low refractive index (RI.) group, and a low silica, high-RI. group. The first group was found to contain relict mineral grains, while the latter one does not (GLASS and BARLOW, 1979). All samples of this study belong to the low-RI. group and may contain relict mineral grains (GLASS and KOEBERL, 1989). Minerals found in Muong Nong-type tektites include zircon, chromite, quartz, corundum, rutile, and monazite (GLASS, 1970, 1972; GLASS and BARLOW, 1979), which all show sign of shock. Furthermore, shock consistent with impact was indicated by the presence of coesite (WALTER, 1965; GLASS et al., 1986). The type, size, and shape of mineral inclusions suggest well-sorted sediments as precursor rocks.

All samples of the present study belong to the high silica group. The contents of almost all major elements show an inverse correlation with the SiOz content (Fig. 3a,b). In addition to the major elements, forty-four trace elements have been determined in all nineteen samples, using various methods. Volatile elements in Muong Nong-type tektites show higher abundances compared to splash-form tektites. The halogens F, Cl, Br, and I, and several other volatile elements (e.g., B, Cu, Zn, Ga, As, Se, Sb, and Pb), show enrichment factors that vary between about I.5 and 25. The highest enrichments are shown to be Cl, Br, and Zn. It seems that the volatile element contents in Muong Nong types are only slightly depleted compared with hypothetical target rocks, while normal tektites show considerable depletions.

It is likely that some degree of volatilization and selective element loss has affected the tektites during their production. However, it is unlikely that more than the volatile elements have been affected, which is in contrast to the suggestion by WALTER ( 1967, 1989) that silica was volatilized from tektite material at high temperatures. No isotopic fractionation effects, which would be expected for such extreme fractionation, was observed for Si isotopes ( MOLINI-VESKO et al., 1982) or Mg isotopes ( ESAT and TAYLOR, 1987). It is unlikely that Si vapor fractionation was responsible for the difference in Si content between the Si-rich and Si-poor Muong Nong- type tektite varieties.

The water contents are also slightly higher in Muong Nong- type tektites. Abundances of about 0.014 wt% Hz0 were found in six Muong Nong-type tektites by KOEBERL and BERAN ( 1988), compared to 0.008 wt% Hz0 in three indochinites (GILCHRIST et al., 1969). The enrichment of water

is similar in extent to enrichment in F, which may be due to similar bonding behavior of OH- and F-. It was shown by GLASS et al. (1986, 1988) that despite all theoretical glass forming arguments ( O’KEEFE, 1976), dry glass can be made from sediments in a short-time, high-temperature event.

Trace element ratios such as K/U, Th/U, La/Th, or Zr/ Hf in Muong Nong-type tektites are very similar to those in the average upper continental crust and related sedimentary rocks. The Cl-normalized REE patterns are impo~ant because they are indicative of the source rocks. The REE patterns of the Muong Nong-type tektites are very similar to that of the post-Archean upper crust. Significant differences exist between the patterns exhibited by Australasian tektites and patterns of local soil samples (BARNES, 1989), e.g., different La/ Yb slopes, Ce, and Eu anomalies. Mixing of soils with each other, or with some related loess samples, would not reproduce the tektite REE patterns. On the basis of the upper crustal REE patterns of the Muong Nong tektites, any basaltic or oceanic (or extraterrestrial) rocks can be excluded as source rocks. The La/Th ratio of Muong Nong-type tektites is additional evidence for an origin from post-Archean sediments, because Archean sediments have a different La/Th ratio.

Major and trace element analyses have been performed on samples of dark and light layers from within several Muong Nong tektite samples. A distinct chemical difference exists between the layers. The most pronounced differences for major elements are higher contents of A1203, FeO, TiOl, and MgO and lower contents of Si02 in the lighter layers, and the opposite for dark layers (i.e., the dark layers are thus high-silica zones). The enrichment or depletion factors are not in direct correlation with the SiOz content; therefore, a simple difference in silica content cannot be responsible for these differences. This is also shown by trace element abundances. Not only the absolute abundance but also some element ratios (e.g., K/U, Th/U, and La/Yb) and the REE patterns show differences between the layers. This indicates incomplete mixing of different parent rocks.

Ferric/ferrous ratios were determined in all samples (average: 0.133) and are slightly higher than the ratio determined for two thailandite samples (0.07), but not different from the average ratio of 0.14 that was determined for numerous splash-form australites ( SCHNETZLER and PINSON, 1964; TAYL.OR and SACHS, 1964). In view of obvious analytical problems associated with the determination of the ferric/ ferrous iron ratio (FUDALI et al., 1987, found Fe3’ values ranging from 0.05 to 0.40 wt% in one sample, depending on the method used), no conclusions regarding a difference between Muong Nong and splash-form tektites can be made.

Some criteria allowing the distinction between Muong Nong-type tektites and splash-form tektites can be summa- rized. Muong Nong-type tektites should fulfill the following criteria: ( 1) higher concentrations of volatile elements (e.g., Cl, Br, Zn); (2) chemically less homogeneous on a millimeter scale; (3) dark and light layers with different chemical composition; (4) possibility of relict mineral inclusions; (5) generally large and more abundant bubbles; (6) large and irregular sample size and no sign of ablation; (7) otherwise similar chemical, isotopic, and age characteristics as splash-form tektites.


Muong Nong-type tektites seem to have originated from impact melting of a mixture of post-Archean sediments with compositions close to those of typical post-Archean sediments. Some loess and soil sample mixtures (Fig. 16) may be able to reproduce the major element chemistry of average Muong Nong-type tektites, but the trace element patterns differ sometimes significantly. In addition, Nd-Sr isotopic data of Muong Nong-type tektites do not favor young sediments such as soil or loess as tektite source materials, but are indicative of Jurassic sedimentary rocks (with a sedimentation age of about 167 Ma) with some smlicial deposits derived from Jurassic sediments and containing “Be ( BLUM et al., 199 1, 1992). The chemical and isotopic data also seem at variance with the proposal that Muong Nong-type tektites have originated in a multitude of very small impact craters. A single large impact, probably occurring at an oblique angle, in Indochina, into (most likely) Jurassic sedimentary rocks was most probably responsible for the production of all tektites in the Australasian strewn field.

Acknowledgments-I am grateful to D. Futrell (Whittier, CA, USA) for most of the samples, and to many colleagues, most notably K. Fredriksson, B. P. Glass, J. Hartung, E. A. King, J. A. O’Keefe, and J. T. Wasson, for various discussions on Muong Nong-type tektites over the many years of this study. Detailed reviews by Billy Glass and Scott McLennan were essential in clearing up errors and mis- conceptions in the first draft. I appreciate the help of D. Jalufka with the figure preparations, and I thank V. Yang (NASA JSC) for help with analyses with the Camebax microprobe.

Editorial handling: S. M. McLennan

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~~hernis~y and origin of muong nong-type tektites*...american, moldavite, ivory coast, and...

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