Sr-Nd-Pb isotope systematics of Australasian tektites: Implications for the nature and composition of target materials and possible volatile loss of Pb

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Abstract

The Australasian tektite (AAT) strewn field is the largest strewn field on the Earth with about ∼10–30% coverage, both land and ocean, but a clearly identified source impact crater is absent despite the young age of AAT of ca. 790 ka. A genetic link between the Australasian tektites and their unequivocal parental materials is therefore largely impossible to establish. Nevertheless, the nature of the parental materials and the extent of volatilization can be constrained using the splash form tektites, carrying the chemical signatures of high-temperature processes, and the layered (so-called Muong Nong-type) tektites, which are less chemically homogenized and exceptionally abundant in the AAT field. New high-precision Sr, Nd and Pb isotopic measurements were obtained for a chemically and petrographically well-characterized suite of AAT, which included the Muong Nong-type (MN-AAT) with precisely known field locations in Laos and splash forms (SF-AAT) from different parts of the strewn field. In addition, optically dark and light zones of the MN-AAT were also separately analyzed. Homogeneous εNd values from −11.8 to −11.2, combined with a narrow range of two-stage Nd model ages from 1.67 to 1.72 Ga for the entire AAT suite, point to a well-mixed source, in terms of REE, of the crustal segment from which the sedimentary material for tektites was ultimately derived. The Sr isotopic data largely overlap for SF-AAT and MN-AAT (87Sr/86Sr = 0.71636–0.72021) and indicate Paleozoic to Mesozoic sedimentary parentage. However, late Neogene to early Quaternary re-deposition and formation of a thick silt-sized sedimentary section with vertical stratification is required to comply with 10Be data. Lead isotope systematics documents at least three different components which can perhaps be represented by different mineral phases, such as feldspar, zircon, organic matter adsorbed on young sediments etc., sorted during fluvial transport and final deposition. In addition, the SF-AAT have systematically lower Pb contents than the MN-AAT, and generally show isotopically heavier Pb isotopic ratios. This is theoretically consistent with a preferential volatilization of lighter Pb isotopes during evaporation and considerably larger Pb loss from SF-AAT when compared to MN-AAT. Nevertheless, further experimental work would be necessary to unambiguously distinguish kinetic fractionation from source mixing.

Introduction

Studies of spatial distribution, physical properties, chemistry and isotopic compositions have established that tektites, distal silica-rich glassy ejecta of impact events, were formed almost exclusively from terrestrial target materials, mainly from sediments of continental origin, whose composition was close to average upper continental crust, and that the admixture of an extra-terrestrial component in tektites is largely below resolution for most methods (Koeberl, 2014 and references therein). So far only partly understood tektite formation processes indicate their formation during early phases of the impact process. The most widely accepted theory of tektite formation involves (1) extreme compression and heating of the target materials followed by high-velocity ejection of the target matter accompanied by melting and/or partial vaporization, (2) melt fragmentation, (3) partial volatilization, and probably also partial re-condensation of some elements and/or compounds (e.g., Stöffler et al., 2002, von Engelhardt et al., 2005, Koeberl, 2014, Žák et al., 2016).

The location of the source impact crater of the Australasian tektite (AAT) strewn field dated at 788.1 ± 3.0 ka (Ar–Ar; Jourdan et al., 2019) remains unknown despite concerted efforts and a panoply of methods and approaches dealing with various aspects of AAT and their strewn field, such as geophysics, petrography and chemical composition of the possible target sediments (e.g., Schnetzler et al., 1988, Ma et al., 2004, Mizera et al., 2016, Schwarz et al., 2016, Goderis et al., 2017, Cavosie et al., 2018, Folco et al., 2018, Rochette et al., 2018, Van Ginneken et al., 2018, Ackerman et al., 2019, Jourdan et al., 2019, Žák et al., 2019, Sieh et al., 2020).

The geographic distribution of the individual AAT types differs significantly. The less chemically homogenized layered tektites (Muong Nong type; MN-AAT), which (i) are in general much larger in size, (ii) preserve a greater number of mineral inclusions (or unmelted products of mineral decomposition processes), (iii) contain more bubbles in the glass, and (iv) exhibit a lesser extent of loss of volatile species, occur in a geographically limited area in Laos, Thailand, Vietnam, and the southernmost part of China. It is assumed that MN-AAT were deposited closer to the impact target area and were sourced from deeper parts of a sedimentary pile (e.g., Blum et al., 1992, Ma et al., 2004, Rochette et al., 2018). The more widely distributed splash form tektites (SF-AAT) cover a large part of SE Asia and the ablated SF-AAT forms (partly re-melted during atmospheric re-entry) occur as far as in South Australia. Tektites <1 mm in size, so-called microtektites, cover an even larger area of Earth’s surface and have been collected especially from ocean-floor sediments (see Fig. 1 in Folco et al., 2018). Spatial distribution of these tektite types collectively hints at positioning the AAT source crater to SE Asia (Schnetzler et al., 1988, Koeberl, 1992, Glass and Koeberl, 2006, Goderis et al., 2017, Folco et al., 2018, Sieh et al., 2020, see Mizera et al., 2016 for alternative localizations) although the impact structure itself remains elusive.

Current interpretations suggest formation of the entire AAT strewn field by a single impactor, probably of chondritic or primitive achondritic composition (Goderis et al., 2017, Folco et al., 2018, Ackerman et al., 2019). Location of the impact structure in SE Asia, now accepted by majority of researchers, is also supported by compositional trends of AAT within the strewn field, where cosmogenic isotopes, e.g., 10Be contents (with the highest 10Be content in the most distant AAT and microtektites and the lowest in MN-AAT; Ma et al., 2004, Rochette et al., 2018), agree with the model of a 10Be-stratified sedimentary target, deposited during the Late Neogene or Quaternary.

Radiogenic isotopic compositions of Sr, Nd and Pb are a useful tool in determining the nature of possible target rocks, considered to be precursors for the AAT. Strontium and Nd isotope systematics of AAT have been studied earlier (Compston and Chapman, 1969, Shaw and Wasserburg, 1982, Blum et al., 1992, Lee et al., 2004), although most data are represented by SF-AAT. These investigations revealed a range of Rb/Sr ratios, combined with variable 87Sr/86Sr values, whereas Sm/Nd ratios and εNd values were rather constant, with a mean Nd model age of ∼1.15 Ga. This has been taken as evidence for a Precambrian crustal segment as a precursor for sediments that were ultimately sourced by AAT. The last Rb/Sr fractionation event calculated at ∼0.25 Ga (Shaw and Wasserburg, 1982) or ∼0.17 Ga (Blum et al., 1992) for AAT was considered as the time of deposition of the sedimentary target. Moreover, Shaw and Wasserburg (1982) did not exclude continental shelf as a target area from their Sr isotopic data, an idea elaborated recently on the basis of the highly siderophile elements (HSE) and Os isotope systematics (Ackerman et al., 2019). Sole Pb isotopic data for AAT were reported by Tilton, 1958, Wampler et al., 1969. These studies provided evidence for terrestrial origin of Pb and elevated Th/U, possibly resulting from the accumulation of Th over U in an argillaceous sedimentary precursor. Deloule et al. (2001) reported on a strong discordance and complete resetting of U–Pb systematics in relic zircons from several MN-AAT as a consequence of the impact.

Since the publication of these papers focused on radiogenic isotope systems, a wealth of new data on the distribution, properties, element chemistry and isotopic compositions of AAT accumulated, which raises a need for a new interpretation of the Sr, Nd and Pb isotopic compositions for AAT. In this contribution, the available radiogenic isotope data were supplemented with new high-precision Sr, Nd and Pb isotope measurements performed on a chemically and petrographically well-characterized suite of AAT, which included a subset of MN-AAT samples with precisely known field locations in Laos and a representative set of SF-AAT from different parts of the strewn field, with the aim to provide a fresh look at the behavior of lithophile radiogenic isotopic systems during terrestrial impacts.

Section snippets

Samples and methods

The Laotian Muong Nong (MN-AAT) and splash form (SF-AAT) Australasian tektites were collected during the 2017 field campaign in the Muong Nong area, south Laos. Details on their field appearance, major/trace element compositions as well as triple oxygen isotope systematics have been reported by Žák et al. (2019); highly siderophile element systematics and Re–Os isotopic compositions are given in Ackerman et al. (2019). Optically dark and light zones of MN-AAT (see Žák et al., 2019 for their

Major and trace element compositions

The analyzed AAT display a wide range of major element compositions (Table 1 and Žák et al., 2019). The Laotian MN-AAT display moderately variable SiO2 contents from 71.9 to 81.7 wt.%, paralleled by Al2O3 variation from 8.2 to 13.1 wt.%. There are tight negative correlations between SiO2 and Al2O3, FeO, MgO and CaO, to a lesser extent, whereas two studied samples from the Belitung Island (Indonesia) depart from the general trends delineated by the other samples in having higher FeO, MgO and CaO

Discussion

The interpretation of the previously published and newly obtained Sr, Nd and Pb isotopic data for AAT needs to be in agreement with the existing information on the major/trace element chemistry, other isotopic data (e.g., δ18O, 10Be), and the grain size and mineralogy of the probable precursor sediments. Major and trace element chemistry of AAT unequivocally documents that the precursor materials were predominantly similar to the average UCC (Koeberl, 2014), whose composition is generally based

Conclusions

We present the first integrated study of the Rb–Sr, Sm–Nd and U–Th–Pb isotopic systems on a set of well-characterized suite of Australasian tektites (AAT) including the Muong Nong (MN-AAT) and splash form (SF-AAT) types with precisely known field positions of the samples. The obtained Rb–Sr and Sm–Nd isotope results are in a good agreement with previously published data collected mostly for the SF-AAT. While the interpretation of the Nd isotopic compositions is consistent with earlier

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Czech Science Foundation (project No. 17-27099S) and the Scientific Programme RVO67985831 of the Institute of Geology of the Czech Academy of Sciences. RS and ŠK are grateful to Milan Trnka for generous donation of some samples for this study and his guidance during the fieldwork in Laos. The manuscript benefited from the discussion with Klaus Mezger, grammar correction made by Jiří Adamovič and the constructive criticism of Renaud Merle and an anonymous

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