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Geochemical Studies of Tektites from East Asia

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Geochemical Studies of Tektites from East Asia Geochemical Journal, Vol. 38, pp. 1 to 17, 2004 Geochemical studies of tektites from East Asia YUNG-TAN LEE,1 JU-CHIN CHEN,1* KUNG-SUAN HO2 and WEN-SHING JUANG2 1Institute of Oceanography, National Taiwan University, Taipei, Taiwan 10617, R.O.C. 2Department of Geology, National Museum of Natural Science, Taichung, Taiwan 40419, R.O.C. (Received March 3, 2003; Accepted June 18, 2003) Thirty tektites from East Asia (including Wenchang and Penglei of Hainan Island, Maoming of Guandong, China; Khon-Kaen of Thailand; Bao Loc of Vietnam; Rizal of Luzon, Philippines) have been analyzed for major and trace ele- ment contents and Rb-Sr isotopic compositions. All the samples studied are splash form tektites. The trace element ratios Ba/Rb (avg. 3.74), Th/Sm (avg. 2.31), Sm/Sc (avg. 0.43), Th/Sc (avg. 0.99) and the rare earth elements (REE) contents of tektites studied are similar to the average upper continental crust. The chemical data of tektites in this study indicate that they were derived from similar target rocks which may be related to post-Archean upper crustal materials. The tektites from East Asia have high positive εSr(0) values-ranging from 164.2 to 198.6, indicating that they were not dominantly derived from recent young sediments, such as soil or loess. The Ar-Ar ages (736.8 ± 55.5~814.6 ± 24.4 ka) of tektites of the present study are consistent with the age of other Australasian tektites, which indicates that all Australasian tektites were derived from a single impact event. Previous studies (Koeberl, 1992; Blum et al., 1992; Schnetzler, 1992) and the present chemical data suggest that these tektites are the result of melting at a single site, which is most probably located in the southern part of the Thailand-Laos border. Mixing calculations based on the model suggested by Ho and Chen (1996) for various amounts and combinations of target rocks indicate that the best fit for East Asia tektites is a mixture of 61% greywacke, 32% sandstone and 7% shale. Keywords: tektites, geochemistry, strewn field, East Asia, 40Ar-39Ar dating Apart from the microtektites found in deep-sea INTRODUCTION sediments (Glass, 1990; Lee and Wei, 2000), tektites on Tektites are relatively homogeneous, dense glass ob- land can be subdivided into three groups: (a) normal or jects found scattered over large areas of the Earth’s sur- splash form tektites, (b) aerodynamically ablated tektites, face called strewn fields. Four strewn fields are known and (c) Muong Nong-type tektites (layered tektites) common names and approximate ages are: North (Koeberl, 1992). The first two groups are only slight dif- American, 34.9 Ma (Storzer and Wagner, 1971); Central ferent in their appearance and physical characteristics. European, 14.5 Ma (Schwarz and Lippolt, 2002); Ivory The shapes of splash form tektites (spheres, droplets, tear- Coast, 1.07 Ma (Koeberl et al., 1997) and Australasian, drops, dumbbells etc.) are often erroneously described as 0.77 Ma (Izett and Obradovich, 1992). Tektites found aerodynamical ablated tektites (flanged button). Muong within a given strewn field are related with respect to their Nong-type tektites are named after a locality in Laos physical and chemical properties besides their age. The where they were first found by Lacroix (1935). They are smallest strewn field is the Central European, and the larg- larger in size, less homogeneous, having higher abun- est is the Australasian field. Some of the strewn fields dances of volatile elements (e.g., B, Cu, Zn, Ga, As, Se, can be divided into different substrewn fields. For exam- Sb and Pb) and water and contain more bubbles and some ple, some of the North American tektites can be relict minerals (e.g., coesite, zircon, corundum, rutile, subclassified as bediasites (Texas) and georgiaites chromite, etc.) which indicate a sedimentary rock as the (Georgia). The Central European strewn field includes source rock (Koeberl, 1992). Besides, based on their tektites named moldavites from Moravia and Bohemia, heavier weights and irregular shapes, some authors whereas the Australasian strewn field consists of several (Stauffer, 1978; Hartung and Rivolo, 1979) suggested that subfields. Muong Nong-type tektites probably have not traveled far from their location of origin, and may therefore be close to the impact crater. The origin of tektites was a scientific puzzle for many *Corresponding author (e-mail: [email protected]) years. Although O’Keefe (1976, 1994) favored a lunar Copyright © 2004 by The Geochemical Society of Japan. volcanic origin, most authors accept that tektites are the 1 product of hypervelocity impact on Earth and represent The purpose of the present study is to analyze the melted target rock ejected during crater formation (Glass, major and trace elements (including rare-earth elements, 1990; Wasson, 1991; Blum et al., 1992; Koeberl, 1994). REE) and the Rb-Sr isotopic ratios and to date the tektites Recently, Futrell (2000) made a case for lunar provenance, by Ar-Ar method in order to shed some light on the ori- based on the theory that tektites could have erupted as gin of the tektites, their parent material and the possible volcanics at the lunar surface, but the case has been re- source crater. torted by Glass (2000), who argued that the silica-rich igneous material in lunar samples is miniature and that it SAMPLES AND ANALYTICAL METHODS is petrographically and compositionally distinct from tektite glass. The tektites were sampled personally by K. S. Ho and Lead isotope data (Wampler et al., 1969) indicate that W. S. Juang from Wenchang (5 samples) and Pengli (5 the parent material of tektites is terrestrial, and consist- samples) of Hainan Island, Maoming (5 samples) in ent with young sediments, including oceanic sediments. Guandong, China, Khon-Kaen (5 samples) in Thailand, Based on 10Be contents of tektites, several authors (Blum Bao Loc (5 samples) in Vietnam and Rizal (5 samples) in et al., 1992; Koeberl, 1992) favor a sedimentary origin Luzon, Philippines (Fig. 1). All the tektite samples are for the parental material of Australasian tektites. Accord- complete pieces and their weights ranging from 3.88 g to ing to 10Be contents (about 1 × 108 atoms) of 7.09 g for Wenchang, 1.64 g to 9.27 g for Pengli, 34.8 g Australasian tektites, Pal et al. (1982) suggested that cos- to 74.5 g for Maoming, 11.45 g to 17.83 g for Khon-Kaen, mic-ray bombardment of the australites cannot produce 20.64 g to 30.02 g for Bao Loc and 11.4 g to 59.3 g for the measured amounts of 10Be either at the earth’s sur- Rizal. The 30 tektites samples in the present study all face or in space, indicating a sedimentary precursor that belong to the splash-form and they are oval, elongated or adsorbed from precipitation 10Be produced in the atmos- dumbbell-shaped. phere. Thirty samples were cleaned with distilled water and Fig. 1. Localities of analyzed East Asia tektites (stars) and the distribution of Jurassic exposures in Southeast Asia: 1, hachured areas denote marine Jurassic exposures; 2, stippled areas show nonmarine Jurassic exposures (after Sato, 1992). 2Y.-T. Lee et al. acetone in ultrasonic cleaner and crushed into chips by a total of the peak heights and their errors from the square hammer while the samples were wrapped with plastic root of the sum of squares of the peak height errors, for sheets. Several larger glass chips were selected and ground all of the temperature steps. The detailed results of 40Ar/ into powders in an agate mortar. 39Ar laser fusion experiments are listed in Table 5. The chemical analyses of tektites from East Asia have been carried out by colorimetry (Si, Al, Ti, P), atomic CHEMICAL CHARACTERISTICS OF absorption (Fe, Mg, Ca, Na, K, Mn) and inductively cou- THE TEKTITES FROM EAST ASIA pled plasma mass spectrometry (Ba, Co, Cr, Cs, Cu, Hf, Ga, Li, Nb, Ni, Rb, Sc, Sr, Ta, Th, U, V, W, Y, Zr, Zn and Major element composition REEs) at the National Taiwan and Tsing-Hua Universi- Koeberl (1990) indicated that tektites are character- ties. ized by high SiO2 content (65 to 85 wt%) and rather high Calibration curves were constructed using U.S.G.S. FeO and low alkali contents. Most tektites from East Asia standard rocks AGV-1, BCR-1, W-2, and G2 and NBS have SiO2 contents ranging from 71 to 81 wt% which is 278 standard obsidian. Values for these rock standards consistent with previous observation on Australasian were adapted from compilations by Govindaraju (1994). tektites (Table 1). Based on the high abundance of silica The precision of the analyses in the present study are as in the tektites and the occurrence of 10Be in the tektites follows: ΣFeO ± 1.46%, MgO ± 1.14%, CaO ± 1.79%, (Tera et al., 1983; Englert et al., 1984; Pal et al., 1982), Na2O ± 0.95%, K2O ± 1.58%, MnO ± 1.88%, Cr ± 1.93%, the lunar volcanic origin for tektites may be excluded. Cu ± 1.04%, Zn ± 1.37%, Rb ± 1.95%, Sr ± 1.16%, The major oxide composition of tektites from different Y ± 1.99%, Nb ± 4.03%, Ba ± 1.14%, La ± 1.31%, localities in the present study, except Al2O3 and CaO, are Ce ± 1.47%, Nd ± 2.65%, Sm ± 1.45%, Eu ± 2.15%, generally uniform. The relative depletion of Al2O3 in Tb ± 3.19%, Yb ± 2.06%, Lu ± 3.45%, Hf ± 4.18% and tektites from Maoming, Guandong, China (Al2O3 aver- Th ± 3.02%. aging 7.06 wt%) and Rizal, Luzon Philippines (Al2O3 Seven tektites (TK-2, W-1, W-2, W-3, BL-12, BL-14 averaging 6.07 wt%) may be related to the lower content and BL-20) were selected for Rb-Sr isotopic composi- of shale in the source rocks.
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