Neodymium and strontium isotopic study of Australasian ...

Gmhrmica d Cosmuchimm Acta Vol. 56, pp. 483-492 Copyright@ 1992 Pergamon Press pk. Printedin U.S.A.

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Neodymium and strontium isotopic study of Australasian tektites: New constraints on the provenance and age of target materials

JOEL D. BLUM, 1,2 D. A. PAPANASTASSIOU,’ C. KOEBERL,~ and G. J. WA~ERBURG’

‘Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA *Department of Earth Science, Dartmouth College, Fairchild Science Center, Hanover, NH 03755, USA

“Institute of Geochemistry, University of Vienna, A-1010 Vienna. Austria

(Received April 26, 199 1; accepted in revised form October 25, 199 1)

Abstract-Nd and Sr isotopic studies of Australasian tektites provide info~ation on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation. ENd values of australites, splash-form indochinites, and Muong Nong-type indochinites are indistinguishable within analytical uncertainty and average -11.5 cu. Depleted mantle Nd model ages fall within the narrow range of 1490-1620 Ma, indicating that the source material was derived dominantly from a Proterozoic crustal terrane. cSr values are variable and are correlated with the geographic location of the tektite samples. Analyses of four Muong Nong-type (or layered f indo~hinites from a single locality in eastern Thailand yield an isochron age of 167 t 12 Ma ( 2s). A correlation of Rb/Sr fractionation with Sr model ages indicates that the last major Rb/Sr fractionation event experienced by the target materials occurred 175 + 15 Ma ago. We interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in Muong Nong-type tektites to reflect compositional variability inherited from Jurassic sediments. The Nd and Sr isotopic data provide evidence that all Australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 Ma. We suggest that all of the Australasian tektites were derived from a single impact event, and that the australites represent the upper part of a melt sheet ejected at high velocity, whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact. The impact site is inferred to be within an area of Jurassic sedimentary bedrock, which spans the geopolitical boundaries between northern Cambodia, southern Laos, and southeastern Thailand.

INTRODUCTION

CHEMICAL AND ISOTOPIC studies have established that tektites are glasses formed by melting of terrestrial continental crust during meteoritic or cometary impacts (for recent reviews, see GLASS, 1984; KOEBERL, 1986, 1990; BARNES, 1989).

Most tektites belong to one of four identified strewn fields, which include the North American, Ivory Coast, moldavite, and the Australasian fields. Tektites found within a given strewn field are related with respect to their physical and chemical properties as well as their age. SHAW and WASSER-

BURG ( 1982) showed that tektites from each strewn field have well-defined and distinct ranges of Nd and Sr isotopic compositions. The largest strewn field is the Australasian; tektites within this field have been subdivided geographically into the indochinites, australites, philippinites, and javaites. The Australasian impact event has been dated at -0.7 Ma, based on K-Ar ( ZZCHRINGER, 1962; MCDOUGALL and LOVERING,

1969) and fission track ( FLEISCHER and PRICE, 1964; GENT- NER et al., 1969) studies. Major and trace element studies suggest that Australasian tektites are chemically related and were derived from a source with a composition similar to a clay-rich sandstone or loess deposit (TAYLOR, 1962; KOEBERL et al., 1984). The age of the Australasian tektite source material has been suggested previously to be -200-400 Ma ( SCHNETZLER and PINSON, 1964; COMPSTON and CHAPMAN, 1969; TAYLOR, 1969; SHAW and WASSERBURG, 1982).

Australasian tektites have been divided into three mor-

phological subgroups, which include ( 1) aerodynamically ablated tektites (flanged buttons), (2) splash-form tektites (spheres, droplets, dumbbells, etc.), and (3) Muong Nong- type tektites (layered tektites). Muong Nong-type tektites are blocky in appearance and range in mass up to 24 kg (KOEBERL, 1992), in contrast to splash-form and aerody- namic tektites, which are generally much smaller ( l- 100 g). Muong Nong-type tektites are further subdivided into two compositional categories, which are distinguished by high (77-82%) and low (69-72%) Si02 contents (BARNES, 1971; GLASS and KOEBERL, 1989). The high-SiOZ Muong Nong- type tektites contain relict inclusions of refractory minerals, most of which show evidence for shock metamorphism (GLASS and BARLOW, 1979). Other characteristic features of Muong Nong-type tektites include the presence of alternating light and dark layers, enrichment in volatile trace elements, and abundant gas-rich bubbles ( KOEBERL, 1986).

LACROIX ( 1935) first described Muong Nong-type tektite localities in Indochina. BARNES and PITAKPAIVAN ( 1962) and BARNES ( 197 1) interpreted Muong Nong-type tektites as the result of local melting of surficial soils to form puddles of glass. Variations in SiOz content were attributed to compositional heterogeneities in the source material, which flowed along the surface of the ground. According to this scenario, atmospheric turbulence would have whipped puddles of this silicate liquid into the air to produce the splash-form tektites. A difficulty with this scenario is the observation that Muong Nong tektites are found as far as 1100 km apart. This would

483

484 J. D. Blum et al.

require either the impact of a large difise object Like a comet, as suggested by UREY ( 1957) and BARNES f 1989), or multiple meteorite impacts spread throughout Indochina, as suggested by WASSON ( 1987, 199 1) . In either case, Australasian tektites would be expected to show variations in chemical and isotopic compositions that correlate with variabilities in local geology.

An alternate scenario, suggested by GLASS and KOEBERL ( 1989) and based on trace element compositions, is that Muong Nong-type tektites are not the result of local melting but instead originated from incomplete mixing of a single but somewhat heterogeneous target. This would require that some Muong Nong-type tektites traveled up to 550 km from a single impact site. Both the morphology and chemical composition of Australasian tektites appear to be correlated with the distance from Indochina (CHAPMAN, 197 1; STAUF- FER, 1978). An appropriate crater has not been positively identified, but several locations in Indochina have been suggested ( HARTUNG and RIVALO, 1979; SCHNETZLER et al., 1988; HARTUNG, 1990). The occurrence of large (> 1000 g) Muong Nong tektites appears to be restricted to an area with a radius of several 100 km in northern Cambodia, southern Laos, and southeastern Thailand, near the center of all known Muong Nong tektite localities. Splash-form tektites are found in Indochina as well as in Java and the Phillipines, and aero- dynamic tektites are found mainly in Australia.

In this study, we have measured the Nd artd Sr isotopic compositions of flanged australites and Muong Nong-type indochinites. We have analyzed both high- and low?301 Muong Nong-type tektites and have also separated and analyzed light and dark layers of a single Muong Nong-type tektite to investigate whether the compositional heterogeneities among and within Muong Nong-type tektites are re- flected in isotopic heterogeneities. SHAW and WASSERBURG ( 1982 ) previously measured Nd and Sr isotopic com~itions of five splash-form indochinites and two austraiites, and SCHNETZLER and PINSON ( 1964) and COMPSTON and CHAP- MAN ( 1969) measured Sr isotopic compositions in a wide variety of Australasian tektites. We have used Sm-Nd and Rb-Sr isotope systematics to determine the characteristics of the parental material and to evaluate whether all Australasian tektites could have originated from the same target material. A preliminary report was presented at the Twenty-Second Lunar and Planetary Science Conference ( BLUM et ai., 199 f ).

SAMPLES AND METHODS

Australasian tektites were analyzed for Nd and Sr isotopic compositions and Nd, Sm, Sr, and Rb concentrations by isotope dilution thermal ionization mass spectrometry. Included in the samples analyzed were two low-SiOZ Muong Nong-type tektites from a locality in southern Laos (X-102 and X-103), three bighSQ Muong Nong- type tektites from a locality in eastern Thailand near the Laos border ( F- 16,830 I and 83 19), and two flanged tektites from southern Aus- tralia (USNM 2537 and 483 1). The Muong Nong-type tektite samples have previousfy been analyzed for major and trace element chemistry (GLASS and KOEBERL, 1989; KOEBERL et al., 198% KOEBERL, I992). Each sample was cut with a diamond saw to obtain an interior piece free of lateritic alteration weighing - 100 mg. Sample 8319 was crushed to small chips, which were hand-picked under a microscope to obtain a 10 mg sample of material from only the dark layers (83 19D) and a second IO mg sample of material from only the light layers (83193,). Each sampfe was cleaned with acetone and distilled water, crushed in a stainless steel mortar and pestle, and then dissolved in a HF-HCIO, mixture. Chemical separation and mass spectrometric procedures were similar to those outlined previously ( PAPANASTAS- SIOU et al.. 19771. Errors &en for the ‘43Nd/‘44Nd and %r/*% , ratios are 20 of the mean of 150 to 200 and 100 to 150 ratios, respectively. Total procedure blanks are 20 pg Nd and 50 pg Sr and are negligible.

RESULTS

The analytical results are presented in Table 1, Parameters for Nd and Sr isotopic evolution are given in Table 2 where tNd is the measured deviation in parts in lo4 of the ‘43Nd/ ‘@Nd ratio from the present-day chondritic uniform reservoir (CHUR) value of 0.5 11847, and esr is the measured deviation in parts in lo4 of the 87Sr/86Sr ratio from the inferred un- fractionated terrestrial mantle reservoir (UR) reference value of 0.7045. The p~ent-dau~ter ratios are also referenced to the CHUR and UR reservoirs and expressed as enrichment factors (S), which are defined as the fractional deviation of the parent-daughter ratio measured in a sample from the parentdau~ter ratio in the reservoir; where ( ‘47Sm/ ‘44Nd)CHUR = 0.1967 and (*‘Rb/‘?3r)vR = 0.0827. Model ages are a calculation of the time that a sample last had the isotopic composition of a model reservoir. Chondritic reservoir Nd model ages ( ~$~uR), depleted mantle reservoir Nd model ages ( Tgr), and uniform reservoir St model ages (2%:) were calculated for each sample and are given in Table 2.

In Fig. 1, ENd is plotted versus csr for all published analyses of macrotektites from this work and from SHAW and WAS-

Table 1. Sm-Nd and Rb-Sr concentrations and isotopic compositions.

Sample (p$) (pspmm) :zE

143&j Sr Rb s75!z s7sr '44Nd @-vm) (mm) 3% s6Sr

- Low-Si52h4uong Nong-fype induchinites x-102 35.73 6.922 0.1172 0.511227 (+2l) 137.5 123.0 2.581 0.718405 x-103 36.60 7.124 0.1176

(i28j 0.511288 (k25) 141.5 125.8 2.568 0.718426

High-SiO2 Muong Nong-type indochinires (i37)

F-16 33.23 6.381 0.1162 0.511225 (f22) 110.5 108.4 2.830 0.719721 8301 30.92 5.965 0.1167

(i34) 0.511262 (fr18) 105.6 108.2 2.957 0.720063

83191. 34.35 6.605 0.1162 (+36)

0.511227 (122) 109.7 116.7 3.071 0.720325 83190 27.57 5.316 0.1165

(132) 0.511273 (+21) 103.8 98.15 2.725 0.719528

AustfaRes (247)

2537 34.73 6.620 O.tl52 0.511292 (i24) 171.3 103.5 1.743 0.716533 4831 27.47 5.306 0.1168

(i69) 0.511279 (f24) 222.7 83.52 1.082 0.713491 (GO)

Reporteduncertainties are 20. U~e~aintyofco~e~ra~~n ~asurementsforNd,Sm,a~ Sr=O.l%andfor Rb-0.4%. Nd isotopic compositions normalized to 14sN~14*Nd = 0.636151 Sr isotopic compositions normalized to 86Sr/**Sr I 0.1194.

Provenance of Australasian tektites 485

Table 2. Parameters for Nd and Sr isotopic evolution.

Sample sNd f SmMd TNdCk,UR TNdDM W f RbiSr TsrUR

(Ma) (Ma) (Ma)

Low.902 Muong Nong-type indochinites x-102 -12.1 (1?0.4) -0.4042 1190 x-103 -10.9 (kO.5) -0.4021 1080 High-S?02 Muong Nong-type indochinites F-16 -12.2 (f0.4) -0.4093 1180 8301 -11.4 (+0.4) -0.4067 1120 8319L -12.1 (kO.4) -0.4093 1160 8319D -11.2 (kO.4) -0.4077 1100 Austfelites 2537 -10.9 (kO.5) -0.4143 1040 4831 -11.1 (kO.5) -0.4062 1090

1620 1530

1610 1550 1600 1530

1490 1530

197.4 (kO.4) 30.21 392 197.7 (iO.5) 30.05 395

216.1 (k0.5) 33.22 390 220.9 (k0.5) 34.76 381 224.6 (k0.5) 36.13 373 213.3 (i0.7) 31.95 400

170.8 (kO.9) 20.08 510 127.6 (k0.6) 12.08 633

Reported uncertainties are 2a. EN& f Srn,Nd and TfqdCHUR are calculated relative t0 the present day chonciritic Composition Of

147Sm/144Nd = 0.1967, 143Nd/144Nd = 0.511847 and using h(147Sm) = 6.54 x lo-l2 a-l TNdDM is calculated following DePaolo (1961).

~9~. f R~,,s~ and T#R are calculated relative to the present day bulk earth composition of

s7Rb/ssSr = 0.0627, *7S,1ssSr = 0.7045 and using h(s7Rb) = 1.42 x 10-l 1 a-‘.

SERBURG ( 1982) and includes all four known strewn fields.

Each tektite strewn field has an extremely narrow and distinct range of cr.,,, that is within the range of analytical uncertainty.

In contrast, tSr varies considerably within each strewn field, with the largest variability observed for the Australasian strewn field. This variability is shown in detail in Fig. 2, where

tNd is plotted versus tSr for the six Muong Nong-type indo-

chinites, five splash-form indochinites, and four aerodynamically ablated australites that were analyzed by us and by

SHAW and WASSERBURG ( 1982). tNd values range from - 10.9 to - 12.2 with a mean value of -11.5. Thus, the complete range of tNd values is only slightly outside the 20 analytical

uncertainty of I -+0.5 t units (tu). In contrast, tSr values range from 127.6 to 224.6, a variation far in excess of the 2u analytical uncertainty of I kO.9 CU. The fields for Muong Nong-type and splash-form indochinites are overlapping, but

the fields for australites and indochinites are distinct. When all other Sr isotope analyses of australites and indochinites from the literature ( SCHNETZLER and FINSON, 1964; COMP- STON and CHAPMAN, 1969) are considered along with our data, the range of eSr values overlaps only slightly, with aus-

tralites ranging from 127.6 to 19 1.6 and indochinites ranging

from 180.3 to 261.2. tsr values of phillipinites and javaites from the literature ( SCHNETZLER and PINSON, 1964; COMP- STON and CHAPMAN, 1969) span a broad range from 103.6

to 238.5. The Australasian tektite data scatter around a rough cor-

relation line on a Rb-Sr evolution diagram but clearly do not

fall on an isochron (Fig. 3). A linear regression of the data

corresponds roughly to an age of -240 Ma; but since there

is no evidence that the samples had the same initial Sr isotopic composition, a basic assumption of Rb-Sr isochron dating is violated, resulting in an age that probably has no geological significance. A more useful method of interpreting the Rb- Sr data, which does not rely on the assumption of homogeneous initial Sr isotopic composition, is presented in the

following section. The data for the Muong Nong-type tektites alone are plot-

ted on a Rb-Sr evolution diagram in Fig. 4. The two low-

SiOZ samples were collected from a single locality and the four high-SiOz samples were collected from another locality.

The two low-Si02 samples are indistinguishable from one another in both “RbJ8%r and 87Sr/86Sr, indicating that they had a constant initial Sr isotopic composition. The four high-

SiOZ samples show a spread of 87Rb/86Sr but all plot within

-22"' * ’ ” . ' . ' 100 150 200 250 300 350

E Sr

FIG. 1. tNd and tsr analyses from this study and SHAW and WAS- SERBURG ( 1982) for macroscopic tektites from all four known strewn fields.

-11

‘Nd

-1;

120 160 &sr 200 240

FIG. 2. cNd and cSr analyses from this study and SHAW and WAS- SERBURG I 1982) for tektites from the Australasian strewn field, iden-

AUSTRALITES INDOCHINITES

tified by geographic and textural type.


and high Sr contents. We suggest that the compositional lay-

ering in Muong Nong-type tektites reflects compositional variability inherited from a sedimentary rock and that the 167 + 12 Ma (2a) apparent isochron age obtained from the Muong Nong-type tektites (Fig. 4) records the time of the last major Rb/Sr fractionation event, which is probably coincident with the time of deposition of the sediments.

In contrast to the Rb/Sr ratio, Sm/Nd ratios are generally unaffected by weathering processes ( MCCULLOCH and WAS-

SERBURG, 1978). In Fig. 5, we have plotted the concentrations

of Sm versus Nd for the eight samples analyzed by us and

the five samples analyzed by SHAW and WASSERBURG ( 1982),

for which high-precision (-+O. 1%) isotope dilution concentration measurements were made. Although the concentrations of Sm and Nd vary by up to 40%, the Sm/Nd ratio is remarkably constant and varies by less than 2%. The variation in Sm/Nd between the light and dark layer of the Muong Nong-type sample is comparable with the entire variability observed within the indochinites and australites. The narrow range of the Sm/Nd ratio over a large range in concentrations is consistent with dilution of high REE phases, such as clays, with a low REE phase, such as quartz, in a compositionally variable rock. As we will discuss below, the Nd and Sr isotopic data for the Australasian tektites are consistent with a compositionally variable sedimentary target and allow us to con- strain the provenance and depositional age of this putative stratigraphic section.

CHUR Nd model ages ( T”$yUR) for all analyzed Australas- ian tektites range from 1040 to 1190 Ma, and depleted mantle reservoir Nd model ages ( T$‘) range from 1490 to 1620 Ma (Table 2 ), indicating that the source material was derived dominantly from a Proterozoic crustal terrane. The relatively narrow ranges in Nd model ages indicate that the target rocks either had a well-defined mantle separation age or that they were sediments derived from a Proterozoic terrane, and transport and sedimentation were effective at homogenizing materials from sources of different ages (SHAW and WAS- SERBURG, 1982).

Sr model ages ( Tp) for most Australasian tektites range from -300 to -900 Ma (Fig. 6), which are much younger than the values of 7$yuR or TRY, thus requiring an increase in the Rb/Sr ratio of the tektite source materials after the crust-forming event indicated by the Nd model ages. Three

FIG. 3. Rb-Sr isotopic evolution diagram for all analyzed samples of Australasian tektites from this study and from the literature ( SCHNETZLER and PINSON, 1964; COMPSTON and CHAPMAN, 1969; SHAW and WASSERBURG, I982 )

analytical uncertainty on a line that corresponds to an age of 167 -+ 12 Ma (2~) and an initial 87Sr/86Sr ratio of 0.7130 + 0.0005 (2~). The mean square of weighted deviates (MSWD) of the isochron is close to 1 ( 1.57), indicating that the scatter about the best-fit line can be attributed to analytical uncertainty. Thus, although all of the Muong Nong-type samples do not plot on a single isochron, multiple samples from each of two localities had constant initial Sr isotopic compositions.

DISCUSSION

Provenance and Depositional Age of Sedimentary Target Materials

Major, minor, and trace element studies of Australasian tektites are consistent with their derivation by impact melting of terrestrial upper crustal sedimentary rocks with bulk compositions similar to average continental crust (e.g., TAYLOR,

1962, 1966; KOEBERL, 1986, 1992). Major element compositions are variable, with Si02 ranging from 62-82%, and

CaO ranging from l-10% (CHAPMAN and SCHEIBER, 1969; TAYLOR, 1966). TAYLOR (1966) pointed out that the ob-

served compositions are consistent with derivation from a dirty sandstone containing variable amounts of clay and calcite. This would also explain the highly variable Rb/Sr ratios

and Sr contents because clays have high Rb/Sr due to uptake of Rb relative to Sr, whereas calcites have low Rb/Sr ratios

0.721 . MUNG KNGl-YPE

_ TEKTITES DARK LAYER

t fJ?J LOWSO

0.718 ’

2.50

1 I I 2.75 3.00

8’Rbl 86S‘

FIG. 4. Rb-Sr isotopic evolution diagram for the Muong Nong- type tektites analyzed in this study.

INDOCHINITES

0 MVONG NONG-TYPE

b SPLASH FORM

AUSTRALITES

CJ FLANGED

5.0 ’ I

26 28 30 32 34 36 38

Nd (wm)

FIG. 5. Plot of high-precision isotope dilution Sm and Nd concentration analyses of Australasian tektites from this study and SHAW and WASSERBURG ( 1982).


700 - Sr

TUR

500 -

s AUSTRALLTES

FIG. 6. Plot of 1 ifversus TsC uR for analyses of Australasian tektites (identical data set as in Fig. 3). Three analyses of phillipinites with unusually high T’gR of 1150, 13 10, and 1320 Ma plot within a continuation of the wedge were excluded from this figure so that more detail could be displayed within the data set. The wedge intercept is drawn to the - 170 Ma isochron age established in Fig. 4.

phillipinites collected from two localities in the Rizal Province

and analyzed by COMPSTON and CHAPMAN ( 1969) have Tyr, which are above the typical range for Australasian tektites ( 1150, 13 10, and 1320 Ma). These outlaying data points plot within a continuation of the wedge of data points shown in Fig. 6 but were not plotted so that more detail could be displayed within the data set. These samples have low *‘Sr/ “?Sr ratios approaching that of marine carbonates and can be identified on the Rb-Sr evolution diagram (Fig. 3) as the three data points closest to the origin. They have unusually high concentrations of CaO (8.9-9.8s) (CHAPMAN and SCHEIBER, 1969) and Sr (373-440 ppm) (COMPSTON and CHAPMAN, 1969), suggesting that their RbfSr ratio was de- creased during deposition by addition of a Ca- (and Sr-) rich phase, such as calcite.

Based on the available Rb-Sr isotopic data, we consider that an increase in the Rb/Sr ratio of most Australasian tektites occurred - 170 Ma ago as the result of weathering, transport, and deposition of a Proterozoic source terrane. The multistage geologic history that we propose may be elu- cidated by plotting the reciprocal of the Rb/Sr enrichment factor ( 1 /f) versus the TtrR model age ( SHAW and WASSER- BURG, 1982) for each sample (Fig. 6). The systematics of this diagram are such that data points which plot as a line on a Rb-Sr evolution diagram also plot as a line on this diagram, but the y-intercept (rather than the slope) corresponds to the isochron age. In cases where multiple Rb/Sr enrichment events have occurred, data points plot as a wedge-shaped array that points toward the age of the last major Rb/Sr enrichment event on the TgrR axis. While this data treatment does provide a connection between the model age and the actual times of multiple disturbances, the y-intercept gives an entirely model-independent age of last disturbance ( SHAW and WASSERBURG, 1982).

Heterogeneities in the initial *‘Sr/“Sr ratio result in ex- cessive scatter of the data points for Australasian tektites on the Rb-Sr evolution diagram (Fig. 3). For a given unce~nty in the initial “Sr/‘%r ratio, the uncertainty in the age of a sample will be greatest if it has a low 87Rb/86Sr ratio and will decrease with increasing 87Rb/86Sr ratio. At the limit of an infinite 87Rb /‘%r ratio (or 1 /f = 0)) the age uncertainty due

to uncertainty in the initial 8’Sr/86Sr ratio tends toward zero. The 1 /f versus Tsr UR diagram (Fig. 6) takes advantage of this relationship and allows the extrapolation of data to 1 /f = 0. A wedge-shaped region, spanning an angle 0 and emanating from a TyrR intercept point, will encompass a set of data points that share a common age of Rb/Sr fmctionation. The slope of the lines connecting each individual data point to the common TgR intercept value will be proportional to the initial 87Sr/86Sr ratio of each data point. The wedge that best fits the data set (i.e., the wedge with the minimum angle 8) yields the best estimate of the age of iast Rb-Sr fractionation experienced by the set of samples. A sample that has infinite enrichment of Rb at the time of fractionation would, therefore, plot on the TyrR axis.

Figure 6 shows that all of the data for Australasian tektites plot within a wedge that appears to point toward the - 170 Ma age defined by the apparent isochron from the four high- SiOZ Muong Nong-type tektites (Fig. 4). To quantify the wedge intercept value that best fits the data, we determined the angle of wedges which completely encompass the data (e) for each value of the wedge intercept (i.e., age) and then divided each angle by the minimum angle (tI min). The ratio of these angles (8/0 min) is a measure of how well the wedge fits the data with the minimum value (0 f f? min = 1) repre- senting the best-fit intercept. The actual wedge is defined by the extreme outlying data points. The results of an iterative calculation of the best wedge intercept arc plotted as wedge intercept age (in Ma) versus 0/e min in Fig. 7. The minimum wedge angle occurs at an intercept value of 115 f 1.5 Ma, where the given uncertainty corresponds to the flat region of the minimum on Fig. 7. This age is in close agreement with the 167 f 12 Ma (2~) Rb-Sr isochron age from the high- SiOZ Muong Nong-type tektites but is significantly lower than the -240 Ma age derived from a linear regression of the same data set on a conventional Rb-Sr evolution diagram.

The age of - 170 Ma is an indication of the time of the last major RblSr fmctionation event experienced by the target materials. In agreement with previous studies, this suggests that the impact melting event at -0.7 Ma did not result in significant volatilization of Rb. Our interpretation of the Nd and Sr isotopic data strongly suggests that all Australasian tektites were derived from a single sedimentary formation with a narrow range of stmti~aphic ages close to - 170 Ma. The systematics of the Rb-Sr evolution diagram for the target

x

e/e x

min x 1.2:

x x 1.1 : x x

x xxx x

100 125 150 175 200 225 250 intercept age (Ma)

FIG, 7. Plot of the wedge intercept age (in Ma) versus the wedge angle (e/0 min) for the data plotted in Fig. 6. The minimum wedge angle corresponds to an intercept age of 175 + 15 Ma.


a7 Rbl aa Sr

FIG. 8. A model for the evolution of the observed scatter of points on the Rb-Sr evolution diagram for Australasian tektites (Fig. 3) within the context of the proposed three-stage history for the sedimentary target rocks. See text for further explanation.

sediments are illustrated in Fig. 8, on which the data for all Australasian tektites is plotted. We suggest that the broadly scattered array of data can best be explained by the following history, which in its simplest form includes at least the following three stages:

1) Derivation of a crustal source terrane from a mantle reservoir during the Proterozoic, followed by isotopic evolution (possibly including multiple weathering-erosion cycles) until - 170 Ma ago, when it was exposed at the Earth’s surface and subjected to chemical weathering.

2) After the source terrane was weathered, detritus was transported and deposited in a sedimentary basin, where it evolved isotopically from - 170 to -0.7 Ma ago.

3) The sedimentary rock was exposed near the Earth’s surface -0.7 Ma ago, where it was shock-melted by a meteoritic or cometary impact producing melts that created the Aus- tralasian strewn field.

The observed scatter of data points for the Australasian tektites on the Rb-Sr evolution diagram is explained on Fig. 8 within the context of the preceding three-stage history. At the time of crystallization of the source terrane, whole-rock samples would have had a homogeneous mantle Sr isotopic composition (“Sr/‘%r s 0.7045) and variable “Rb/‘%r ratios and would have plotted on Line 1 (L, ) on Fig. 8. Iso- topic evolution for a period of - 900 Ma (from - 1100 to

170 Ma ago) would have followed the vertical arrow resulting in the whole-rock samples plotting on the isochron line LZ and resulting in the variability of “Sr/‘%r ratios that existed at the time of weathering. Chemical weathering would not have changed the “Sr/?Sr ratios but would have resulted in significant loss of Sr and retention of Rb as feldspars and mafic minerals were converted to clay minerals. This would result in large increases in the “Rb/%r ratio of the source rocks (indicated by the horizontal arrows) and yield weathered detritus, which would plot along a family of lines des- ignated L3. We assume that the detritus was transported and deposited into a sedimentary basin soon after being weathered.

With the passage oftime, variabilities in the 87Sr/86Sr ratio of the sedimentary rocks that are correlated with the “Rb/ ‘%r ratios would develop. Isotopic evolution for a period of

- 170 Ma would follow a path defined by vertical arrows and would result in the observed data, which fall between isochron lines Lq. Samples that were closely related strati- graphically (such as the alternating compositional layers in Muong Nong-type tektites) probably had homogeneous %r/ “%r ratios at the time of deposition and, therefore, form apparent isochrons, such as the one that has been used above for the high-SiOz Muong Nong-type tektites (Fig. 4) to pre- cisely determine the stratigraphic age of the target sediments. The two low-SiOZ Muong Nong-type tektites (Fig. 4) had a lower 87Sr/86Sr ratio at the time of deposition than the high- SiOz Muong Nong-type tektites but did not have varying *‘Rb/%r ratios. Muong Nong-type tektites from other localities that have variable “Rb / %Sr ratios would be expected to fall on lines parallel to the apparent isochron shown on Fig. 4.

Single or Multiple Impact?

Several workers have rejected the single impact hypothesis for the origin of Australasian tektites in favor of local melting of soil (BARNES and PITAKPAIVAN, 1962; BARNES, 1989) or aeolian silt ( WASSON, 1987, 199 1) by a diffuse impact or multiple impacts over a large area of Indochina. However, the Rb-Sr data presented here rule out any material that has suffered a recent increase in the Rb/Sr ratio as the predom- inant source material. Soil developed by weathering of local bedrock is not a likely source for the following reasons:

1) The tNd values of Muong Nong-type indochinites from several localities, splash-form indochinites, and aerodynamically shaped australites are indistinguishable. This is indicative of a homogeneous target material rather than local derivation from soils, which would be expected to reflect local variabilities in bedrock geology.

2) The Rb-Sr isotope systematics indicate tha; the last major Rb-Sr fractionation event that affected the Australasian tektite source material occurred - 170 Ma ago. The soil- forming process leaches and removes Sr while retaining Rb in the soil by incorporation into clay minerals. The net result is a large increase in the Rb/Sr ratio during weathering. If soils were the source material, the last Rb/ Sr enrichment recorded by the Rb-Sr isotopic system would have been -0.7 Ma ago, rather than the - 170 Ma value observed (Figs. 4 and 6).

Aeolian silt (or loess) is a more plausible source material than soil but still faces several problems and fails, in our view, to be the simplest explanation for the source material. We will first describe the following features of loess that make it an attractive potential source material:

1)

2)

Loess deposits throughout the world have a relatively homogeneous composition, close to that of average Aus- tralasian tektites as well as to average upper continental crust and shales. The tNd value of Chinese loess collected at Nanking (-2000 km from Indochina) has a value of -10.2 cu (MCCULLOCH and WASSERBURG, 1978), which is very close to the value of - 11.5 tu for Australasian tektites. The tNd values of loess deposits throughout the world range


from -5.1 to - 15.1 eu ( MCCULLOCH and WASSERBURG, 1978; TAYLOR et al., 1983).

Although there are chemical and Nd isotopic similarities between loess and Australasian tektites, these other features of loess may disqualify it as a potential source material:

1)

2)

3)

The northern Chinese loess plateau, which is one of the most massive accumulations of loess in the world and ranges in age from 2.4 Ma to -0.01 Ma, is restricted to a belt between - 30-50”N latitude ( KUKLA, 1987). The region of Indochina in which tektites occur is between - 12- 17”N latitude, a distance of over 2000 km. TAYLOR et al. ( 1983) demonstrated that for several loess deposits worldwide, multiple samples of each deposit are homogeneous with respect to major-element and Sr isotopic composition. In contrast, Australasian tektites have highly variable major-element (e.g., 62-82% SiOz) and Sr isotopic compositions ( eSr = 128-225 ) The Rb-Sr isotope systematics require that if loess is the source material, it was derived from rock with an age of - 170 Ma and that significant Rb/Sr fractionation did not occur during a) the production of silt-sized grains from a bedrock source, b) the aeolian transport of the silt, or c) the exposure of the loess to the weathering environment after deposition. The stratigraphy of the Chinese loess deposits consists of a sequence of alternating layers of loess and paleosols, which are differentiated based on both magnetic susceptibility and the silt-to-clay ratio (e.g., KUKLA, 1987 ) . The time of the Australasian impact event ( -0.7 Ma) was marked by extensive soil devel- opment with a soil/loess volume ratio of - 1.4 ( KUKLA, 1987). The higher clay content of the paleosols suggests that mineral weathering (and concomitant Rb/Sr fractionation) occurred soon after the time of loess deposition. This is inconsistent with the Rb-Sr isotope systematics of the Australasian tektites, which indicate that major Rb/ Sr fractionation has not occurred in the past 170 Ma.

Appropriate Target Materials in Indochina

The depositional age of - 170 Ma that we have suggested for the target materials corresponds to the Middle Jurassic stratigraphic period ( 187-163 Ma). The geologic maps of Thailand ( JAVANAPHET, 1969) and Vietnam, Cambodia, and Laos ( FROMAGET, 197 1) indicate that Jurassic and Creta- ceous marine sedimentary rocks outcrop over much of In- dochina and are largely sandstones interbcdded with shales and carbonates. For example, the Jurassic Phu Phan and Phra Wihan Formations outcrop near the Muong Nong-type sample localities close to the Thailand-Laos border and are described as massive sandstone and conglomerate with in- terlayered micaceous shale and siltstone ( JAVANAPHET, 1969). Appropriately aged sedimentary target materials outcrop over a large portion of southeastern Thailand and southern Laos near localities where large Muong Nong-type tektites have been found. Cambodia, including the region near Lake Tonle Sap, which has been suggested as a possible crater location by HARTUNG ( 1990), is largely covered by Quaternary alluvium, but outcrops of Jurassic marine sediments protrude through the Quaternary alluvial cover at sev-

era1 localities, indicating that much of Cambodia may be underlain by appropriate target sediments. Jurassic sedimentary rocks do not outcrop in southern Vietnam; therefore, the Muong Nong-type tektite locality near Dalat is not an appropriate target area.

Although our model requires Jurassic sedimentary bedrock as a precursor to the Australasian tektites, suticial deposits derived from Jurassic sedimentary rocks without any concomitant Rb/Sr fractionation could provide an adequate source. The presence of “Be in Australasian tektites suggests that the source rocks were exposed near the Earth’s surface in the zone of meteoric water percolation. “Be concentrations in australites and indochinites are 0.8-2.0 X lo* atom g-’ and 0.4-0.9 X lo8 atom g-’ , respectively (PAL et al., 1982; TERA et al., 1983; RAISBECK et al., 1988), which is -5 times lower than modem fluvial sediments and soils and - 50 times lower than modern deep-sea sediment (BROWN et al., 198 1). PAL et al. ( 1982) estimated that exposure of 10 g pieces of tektite at the Earth’s surface for the 0.7 Ma since the impact event could account for only -8% of the observed “Be by cosmic ray and neutron induced reactions, thus requiring a major contribution from atmospherically produced “Be. This leaves open the question of whether the “Be was added to the tektite in the soil environment or was present in the target materials prior to the impact event.

The average global fallout rate for “Be from the atmosphere (q) is 1.2 X lo6 atom cm-* year-’ (MONAGHAN et al., 1985), which is high enough that the surface of a 10 g tektite resting at the earth’s surface would be struck by the equivalent of 1 X lo8 atom g-’ in only -300 years. However, PAL et al. ( 1982) conducted an acid leaching experiment on an australite that indicated there is no surface enhancement of “Be and suggested instead that “Be was present in the target material prior to tektite formation. If the weathering surface in the target area was exposed to meteoric water for longer than the half-life of “Be ( 1.5 Ma), and no “Be was lost through erosion or in solution, concentrations would be close to the equilibrium value q/X = 2.6 X lOI* atom cm-‘, where q is the fallout rate given above and X is the decay constant of “Be (4.6 X lo-’ yr-’ ). In this case, there would be a sufficient quantity of “Be that it could be mixed with a -200 m column of bedrock to yield the average “Be content of Australasian tektites. Alternatively, the source of “Be could have been a young surficial cover, in which case the ratio of young sediment to underlying rock would have had to be - 1:4 for average fluvial sediments.

Several locations in Indochina have been suggested as possible sites for the Australasian impact crater. Estimates of the expected diameter of the crater range from 17 km (BALDWIN, 198 1) to several hundred kms (CHAPMAN, 197 1). GLASS et al. ( 1979) estimated the mass of the Australasian tektites (based on the distribution and mass of microtektites in deep- sea sediment cores) to be - 10 I4 g. Although this is only a minimum estimate, it is worth noting that even for the small- est estimate for the source crater diameter ( 17 km), only the top 0.2 m of target material over the entire crater area would need to be melted to produce a tektite mass of 10 I4 g. Based on the distribution of tektites in the strewn field and the proximity to the Muong Nong-type tektite localities, STAUF- FER ( 1978) suggested that the crater may be concealed


beneath alluvial deposits ofthe lower Mekong valley in Cam- bodia. A circular structure in Cambodia was suggested as a possible crater by HARTUNG and RIVOLO ( 1979) but this has been identified as a volcanic caldera ( LACOMBE, 1967; KOE- BERL, 1992). A depression on the continental slope 175 km offshore of Vietnam was suggested as a possible impact crater by SCHNETZLER et al. ( 1988). Finally, HARTUNG ( 1990) proposed that the 35 X 100 km Lake Tonle Sap in central Cambodia is a reflection of the Australasian tektite source crater that has been covered with alluvial sediments. Our study of the tektite isotopic compositions excludes a source from young oceanic sediments and is in agreement with sug- gestions of STAUFFER ( 1978) and HARTUNG ( 1990) that the impact was in central Cambodia and has been covered by alluvium.

Evidence Concerning the Impact Process

CHAPMAN ( 197 1) showed that the chemical composition of Australian tektites varies ~stemati~y with the distance from Indochina. The Sr isotopic composition and Rb/Sr ratios display a similar trend, i.e., the indochinites have higher tsr values and higher Rb/Sr ratios than the australites, and the geographically intermediate phillipinites and javaites span the entire range of variability (Fig. 3). Australites have also been completely melted and homogenize by intense shock, whereas the Muong Nong-type subgroup of the indochinites display compositional layering and contain unmelted relict crystals. “Be contents also show systematic variability with australites containing a mean concentration of 0.65 X lo8 atom g-’ and indochinites containing a mean concentration of 1.4 X lo8 atom g-i. It is our view that the observed textural, compositional, and isotopic variations within the Australasian strewn field are consistent with derivation from variable depths of target material in a single impact event.

information about the relationship between tektite origin and the stratigmphy of layered target materials has come from studies of the Ries crater and associated moldavite strewn held (-270-440 km away); conclusions have been applied to the North American tektite strewn field ( STECHER et al., 1989) and may also be applicable to understanding the Aus- tralasian impact event. Reconstructions of the stratigraphy prior to the Ries impact suggest that there was a -50 m thick layer of middle Miocene fluvial sediments (OSM sands) at the very top of the stmti~phic column, underlain by -600 m of Triassic and Jurassic sedimentary rocks (sandstones, limestones, marls, shales), which were underlain by late Pa- leozoic crystalline basement rocks ( POHL et al., 1977 ). Only the thin cover of fluvial sediment has chemical abundances and isotopic compositions that correspond to those of the molda~tes (GRAUP et al., 198 1; HORN et al., 198.5; VON ENGELHARDT et al., 1987), suggesting that tektites may be formed exclusively from thin, surficial deposits without in- corporating materials from deeper levels at the impact site (SHAW and WASSERBLJRG, 1982). Impact glasses (which represent melts of deeper stratigraphic layers) are restricted to a radial distance of only -40 km from the center of the Ries crater ( POBL et al., 1977).

High-velocity jets of ejected surficial material are often produced by large impacts, and oblique impacts are likely to produce directional jets ( K~EFFER, 1977; KIEFFER and SI-

MONDS, 1980). It has been suggested that high-velocity jets may account for the origin of moldavites exclusively from the upper 50 m of target material ( SHAW and WASSERBURG, 1982; VON ENGELHARDT et al., 1987). VICKERY (1990), however, recently questioned the jetting model for the origin of tektites based on theoretical calculations that require jetted material to contain roughly half projectile material, which is at odds with the Iack of any detectable projectile contami- nation in most tektites.

Direct comparisons between the Ries crater / moldavites and the Australasian tektites are speculative because there may be a large difference in the scale of the two impact events. However, the qualitative observations from studies of the Ries crater that ( 1) tektites can be formed from thin surficial deposits in the target area and (2) impact glasses found close to an impact site may sample deeper levels of target material than tektites found at greater distances, have direct application to studies of Australasian tektites. We suggest that australites (which have traveled >5000 km from the postulated impact site in Indochina) formed from intense shock melting of near- surface materials in the initial stages of the impact event. In contrast, we suggest that indochinites were formed by less- severe shock melting of rocks from a somewhat deeper stratigraphic level of the target material. Higher concentrations of “Be and CaO in australites compared to indochinites indicates that the near-surface australite target material had more interaction with meteoric water and had a higher calcite content than the deeper indochinite target material. Phillipinites and javaites appear to represent a mixture of the two com- ponents. Muong Nong-type indochinites represent the deepest and least-shocked impact glasses and, as a result, preserve detrital minerals and compositional banding.

CONCLUSIONS

Nd and Sr isotopic studies of Australasian tektites provide info~a~on on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation. CNd values of australites, splash-form indochinites, and Muong Nong-type indochinites are indistinguishable within analytical uncertainty and average - 11.5 CU. TNd DM fall within the narrow range of 1490- 1620 Ma, indicating that the source material was derived dominantly from a Proterozoic crustal terrane. esr values are variable and are correlated with the geographic location of the tektite samples. Australites are the least radiogenic with esr ranging from 128 to 192 ELI and indochinites are the most radiogenic with cssr ranging from 180 to 26 1 CU. Phillipinites and javaites {which are found in a geographic location intermediate between Australia and Indochina) have highly variable esr vafues ranging from 104 to 239 cu, which spans nearly the entire range observed in australites and indochinites.

Analyses of four Muong Nong-type (or layered) indochinites from a single locality in eastern Thailand yield an apparent isochron age of 167 ? 12 Ma (25). We interpret the age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in Muong Nong-type tektites to reflect compositional variability inherited from Jurassic sediments. When Rb-Sr data for all Aus-


tr&&an tektites are plotted as 1 /fversus T.$i , the data points plot within a wedge with an intercept value emanating from 175 -t_ 15 Ma, which is indicative of the time of the last major Rb/Sr fractionation event experienced by the target materials. The Nd and Sr isotopic data provide evidence that all Aus- tralasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 Ma.

The simplest geologic history for the target materials that is consistent with the isotopic data includes at least the following three stages:

1) Derivation of a crustal source terrane from a mantle reservoir during the Proterozoic, followed by isotopic evolution until - 170 Ma ago, when it was exposed at the Earth’s surface and subjected to chemical weathering.

2) After the source terrane was weathered, detritus was transported and deposited into a sedimentary basin, where it evolved isotopically from - 170 to -0.7 Ma ago.

3) The sedimentary rock was exposed near the Earth’s surface -0.7 Ma ago, where it was shock-melted by a meteoritic or cometary impact, pr~u~ing melts that created the Australasian strewn field.

Nd and Sr isotope data are most easily explained with a scenario whereby all of the Australasian tektites were derived from a single impact event. We suggest that the australites represent the upper part of a melt sheet ejected at high velocity and the indochinites represent melts formed at a lower level in the target material, which were distributed close to the area of impact. The impact site is inferred to be within an area of Jurassic sedimentary bedrock that spans the geopolitical boundaries between northern Cambodia, southern Laos, and southeastern Thailand.

icknowle;dgments-laboratory work was performed while J. D. Blum was a visiting faculty member at Caltech. We thank D. Futrell for providing the Muong Nong-type tektites, G. MacPherson of the US Museum of Natural History for providing the australites, C. L. Bium for help with pro~mming, H. Ngo for generously sharing his time and expertise on laboratory procedures, M. C. Monaghan and B. P. Glass for helpful discussions, and T. Esat and an anonymous referee for reviews. This work was supported by NASA Grant NAG 9-43 and by a Burke Research Award from Dartmouth College. Division Cont~bution No. 5007 (740).

Editorial handling: S. R. Taylor

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