# 10Be in Australasian microtektites compared to tektites: Size and geographic controls

P. Rochette, R. Braucher, L. Folco, C.S. Horng, G. Aumaître, D.L. Bourles, K. Keddadouche

► To cite this version:

P. Rochette, R. Braucher, L. Folco, C.S. Horng, G. Aumaître, et al. 10Be in Australasian microtektites compared to tektites: Size and geographic controls. Geology, 2019, 46 (9), pp.803 - 806. 10.1130/G45038.1. hal-01868053

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<table><tr><td>Manuscript Number:</td><td></td></tr><tr><td>Full Title:</td><td>10Be geographic and size gradients in the Australasian tektite and microtektite strewn field</td></tr><tr><td>Short Title:</td><td>10Be in australasian strewn field</td></tr><tr><td>Article Type:</td><td>Article</td></tr><tr><td>Keywords:</td><td>australasian, tektite, microtektite, Beryllium 10</td></tr><tr><td>Corresponding Author:</td><td>Pierre Rochette<br>Aix-Marseille Universite OSU Institut Pytheas<br>Aix en Provence, not applicable FRANCE</td></tr><tr><td>Corresponding Author Secondary Information:</td><td></td></tr><tr><td>Corresponding Author's Institution:</td><td>Aix-Marseille Universite OSU Institut Pytheas</td></tr><tr><td>Corresponding Author's Secondary Institution:</td><td></td></tr><tr><td>First Author:</td><td>Pierre Rochette</td></tr><tr><td>First Author Secondary Information:</td><td></td></tr><tr><td>Order of Authors:</td><td>Pierre Rochette<br>Regis Braucher<br>Luigi Folco<br>Chorn-Chern Horng<br>ASTER TEAM</td></tr><tr><td>Order of Authors Secondary Information:</td><td></td></tr><tr><td>Manuscript Region of Origin:</td><td>ANTARCTICA</td></tr><tr><td>Abstract:</td><td>Large 10Be content in tektites has been shown to be evidence of a source material enriched in atmospheric 10Be, i.e. a soil or sediment. In Australasian tektites 10Be content increases with distance from the putative source crater in Indochina, with geographic averages from 69 to 136x106 at/g. Here we show that the same trend exists in microtektites by measuring samples from Antarctica and South China Sea. Moreover, microtektites are ~30x106 at/g richer than tektites from the same geographic area. Antarctic microtektites, with an average 10Be content of 184x106 at/g after correction for insitu-production, are the richest impact melt ever measured. The simpler hypothesis for such systematic size and geographic gradient is that the source depth of the melted material in the target soil surface decreases with ejection velocity. A higher initial kinetic energy indeed means a higher launch distance and a higher fragmentation. Alternative models invoking a marine sediment source or a secondary enrichment in the microtektite (either by atmospheric scavenging or host contamination) fail to reproduce the observed relationships.</td></tr><tr><td>Suggested Reviewers:</td><td>GF Herzog<br>Rutgers The State University of New Jersey<br>herzog@rutchem.rutgers.edu<br>specialist of 10Be in tektite<br><br>Phillipe Claeys<br>Vrije Universiteit Brussel<br>phclaeys@vub.be<br>geochemist specialist of impact melts</td></tr></table>
John Wasson
University of California Los Angeles
jtwasson@ucla.edu
geochemist specialist of impact melts

Kieren Howard
Kingsborough Community College
kieren.howard@nhm.ac.uk
geochemist specialist of impact melts
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1 $^{10}$Be geographic and size gradients in the Australasian tektite and

2 microtektite strewn field

3

4 P. Rochette¹, R. Braucher¹, L. Folco², C.S. Horng³ and ASTER Team¹,*

5 ¹ Aix-Marseille Univ., CNRS, IRD, INRA,-Coll France, UM 34 CEREGE, Technopôle de
6 l'Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France

7 ² Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy.

8 ³ Institute of Earth Sciences, Academia Sinica, P. O. Box 1-55, Nankang, Taipei, Taiwan

9 * G. Aumaître, D. L. Bourlès, K. Keddadouche.

10

11 ABSTRACT

12 Large $^{10}$Be content in tektites has been shown to be evidence of a source material
13 enriched in atmospheric $^{10}$Be, i.e. a soil or sediment. In Australasian tektites $^{10}$Be content
14 increases with distance from the putative source crater in Indochina, with geographic
15 averages from 69 to 136x10$^6$ at/g. Here we show that the same trend exists in
16 microtektites by measuring samples from Antarctica and South China Sea. Moreover,
17 microtektites are ~30x10$^6$ at/g richer than tektites from the same geographic area.

18 Antarctic microtektites, with an average $^{10}$Be content of 184x10$^6$ at/g after correction for
19 in situ-production, are the richest impact melt ever measured. The simpler hypothesis
20 for such systematic size and geographic gradient is that the source depth of the melted
21 material in the target soil surface decreases with ejection velocity. A higher initial
22 kinetic energy indeed means a higher launch distance and a higher fragmentation.

23 Alternative models invoking a marine or loessic sediment source or a secondary
24 enrichment in the microtektite (either by atmospheric scavenging or host
25 contamination) fail to reproduce the observed relationships.
26

## 27 INTRODUCTION

28 Tektites are a particular type of natural silicate glass produced by hypervelocity impact
29 and long distance ejection (Glass and Simonson, 2013). They are recovered on continental
30 surfaces mostly as centimetric splash-forms masses dispersed over large strewn-fields. The
31 same material is found as <1mm droplets, mostly in oceanic sediments, and called
32 microtektites. They have recorded extreme high pressure and temperature conditions (Cavosie
33 et al., 2018) and their formation mechanism is still poorly understood. Only four tektite
34 strewn-fields are recognized, compared to nearly 200 impacts structures documented on
35 Earth. These strewn-fields extend over ranges varying from 10² to 10⁴ km and at a minimal
36 distance of 250 km from the source crater. This implies that they were ejected outside the
37 atmosphere at several km/s velocities.

38 The Australasian tektite and microtektite strewn-field is the largest (Folco et al., 2016). It was produced 0.79 Ma ago by an impact hypothetically situated in Indochina and its 39 extension is presented in Fig.1a. The smallest strewn field, called Ivory Coast, was produced 40 by the Bosumtwi crater in Ghana 1.07 Ma ago (Glass and Simonson, 2013).

41 In both strewn fields, tektites have been shown to originate from the near surface 42 layers of the impacted target (likely soil or sediment) based on their high content of 43 cosmogenic nuclides ¹⁰Be (Ma et al., 2004; Sereffidin et al., 2007). This high content cannot 44 be reached through in situ-production since tektite fall and must thus originate from inherited 45 atmospheric ¹⁰Be accumulated at the continental surface melted by the impact. Alternatively, 46 the high concentration may correspond to impact melting of a thicker Quaternary sedimentary 47 sequence, as high atmospheric ¹⁰Be content are observed in such sequence regardless of depth 48 (e.g. Gu et al., 1996; Simon et al., 2016). The two other strewn fields are much older than the 49
half-life of <sup>10</sup>Be (1.39 Ma), thus they cannot be investigated for that purpose (Korschinek et al., 2010).

One major result of the study of the Australasian tektites was that a consistent increase of <sup>10</sup>Be content versus distance (69 to 136x10<sup>6</sup>at/g (Mat/g) from Indochina to Australia averages) was observed, giving possible insight on a relationship between excavation depth of the melted material and the ejection velocity and angle. The first purpose of our study was to verify if this distance relationship extend further by measuring <sup>10</sup>Be content in the Australasian microtektites recently found in Antarctica (Folco et al., 2008). However, it remains to evaluate if microtektites have the same <sup>10</sup>Be content than tektite from the same distance to source. Therefore we include in the present study microtektites from a proximal site. A good candidate for that appeared to be the MD97-2142 South China Sea core (Fig.1) as it has already been demonstrated to be one of the richest core in microtektites (see review in Prasad et al., 2007) with a significant number of large ones (Lee and Wei, 2000).

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## 64 SAMPLE AND METHODS

Antarctic microtektites were extracted under a ZEISS Stemi 2000 stereomicroscope from the 4-00-800 µm size fraction of loose soil sample collected on the flat summit plateau of Miller Butte, Victoria Land Transantarctic Mountains (~72°42'S, 160°14'E; 2600 m a.s.l.) during the 2006 PNRA expedition. They were cleaned in ultrasonic bath and deionized water. Two batches of about 11 and 14 fresh and homogenous microtektites were prepared to produce two samples >3 mg for <sup>10</sup>Be measurements.

Lee and Wei (2000) identified in core MD97-2142 a microtektite peak at 3425 cm depth extending over 20 cm. One-cm thick layers were obtained from the archive half at 3418.5 and 3426.5 cm depth and treated at the Institute of Earth Sciences in Taipei. They were dispersed in water and sieved at 300 µm. Microtektites were picked and shipped to

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74
CEREGE, where a further cleaning was applied using alcohol and ultrasonic bath. A > 3 mg aliquot (with 15-20 spherules of size in the 300-500 µm range) was prepared for each depth, by selecting the clear intact yellow colored spherule, avoiding fragmented, dark and inclusion bearing microtektites.

To test our preparation protocol and check our ability to reproduce Ma et al. (2004) results we also measured a 114 mg aliquot of a large splash form tektite from Vietnam (acquired in Hanoi by P.R.). A sample of the host soil (<100 µm fraction) from Miller Butte was also analyzed.

A carefully weighted amount of microtektite, spiked with ~0.1mg a (3025±9)-ppm in-house ⁹Be carrier (Merchel et al., 2008) was totally dissolved in few ml of 48% HF then fumed in PTFE beaker to dryness. The precipitated was recovered with nitric acid and the Be was purified by solvent extractions and alkaline precipitations. After being oxidized at 800°C for one hour, the BeO was mixed with niobium powder and analyses by accelerator mass spectrometry (AMS) at the French AMS national facility ASTER housed at CEREGE, Aix en Provence, France. All measurements were standardized against the in-house STD11 standard (Braucher et al., 2015).

The collected Antarctic microtektites have resided exposed to cosmic rays on top of Miller Butte since their fall. Therefore, to determine their original ¹⁰Be content, the in situ produced ¹⁰Be accumulated over the last 0.8 Ma has to be subtracted to the measured ¹⁰Be concentrations. Using a local production spallation production rate of 43.52 at/g/a (scaled following Stone polynomial (Stone, 2000), from a SLHL production rate of 4.02 at/g/a (Borchers et al., 2016) and an Antarctic pressure of 702 mbar), the in situ content is estimated to be at most 29 Mat/g. However, accounting from the fact that microtectites were found below about 15 cm of granitic debris (1.8 g/cm³) and a snow cover variable with time (a 10 cm water equivalent layer is likely a minimum), the in situ-production should be reduced to
23 Mat/g. The China Sea microtektites have been shielded from cosmic ray since their fall, thus no correction is need. Ma et al. (2004) discussed the in-situ production in tektites but concluded it must be <10 Mat/g. This is due to the combination of a near zero altitude and low latitude compared to Antarctica. Moreover, the tektites have been buried in soil and sediments for most of their residence time since their fall. We will thus not apply any correction to Ma et al. (2004) data to compare with the microtektite results.

106

## 107 RESULTS

Table 1 lists our results on the four microtektite samples as well as the test tektite from Vietnam and Miller Butte soil, and average tektite data from Ma et al. (2004) ordered according to distance from Indochina. Test tektite sample yields 79.3±2.5 Mat/g, within the average splash form value for Indochina (76±14), ensuring that our preparation procedure reproduce Ma et al. (2004) results.

Analytical uncertainties on our microtektite measurements were around 10 Mat/g. The difference between the two measurements from Antarctica and South China Sea amounts to 18 and 7 Mat/g, i.e. less than two times the analytical error. This shows that the sampling and pooling procedure ensures the determination of a reliable average value for the whole microtektite collection. The weighted mean for the Antarctic microtektites after in situ-production correction (184±8 Mat/g) is 32 and 35% higher than the South China Sea microtektites and Australian tektites, respectively (see Table 1 and Fig.2).

¹⁰Be measurements in microtektites have been reported previously in abstract form by Koeberl et al. (2015). A composite 0.62 mg sample was reported to yield 260±60 Mat/g, not significantly different from our Antarctic uncorrected values, while 13 individual microtektites yield values from 90 to 1,230 Mat/g. As no information is available on the size
124

and provenance sites of the microtektites, as well as on analytical conditions and errors, we
will refrain to comment further these values.

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127

DISCUSSION

128

129

A depth in aerial target surface versus distance and size relationship

130

The fact that SCS microtektites are significantly enriched in ¹⁰Be compared to the
Phillipines tektites (by 15%) indicates that microtektites recovered from approximately the
same locality derive from a precursor material enriched in ¹⁰Be compared to the tektite
source. Therefore when comparing Australian tektites with Antarctic microtektites, one part
of the increase may be linked to the increased launch distance and one part to the contrast
between tektites and microtektites. Although Fig.2 appears to suggest these parts are of the
same order, we should refrain to give too much significance to simplistic interpolations. To
better constrain this point one would have to measure Australian microtektites. However, the
low concentration (100 times less than SCS) and small microtektite size for the sedimentary
cores around Australia make this task hardly achievable (Glass and Pizzuto, 1994).

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A simple interpretation of our results and comparison with tektite results can be put
forward assuming a target surface whose atmospheric ¹⁰Be decreases content with depth, as is
typical for in situ continental soils, formed on old bedrock. From 22 worldwide distributed
soil profiles, Graly et al. (2010) determined an averaged ¹⁰Be concentration in the A and B
soil horizons of ~450 Mat/g, well above our Antarctic microtektite value corrected for decay
over 0.8 Ma (~ 275 Mat/g). With that assumption, microtektites would come from shallower
surface than tektites for a given launch distance and sampling depth will also decrease
according to increasing launch distance. A phenomenological model where sampling depth is
connected to the initial kinetic energy imparted to the ejected material (higher energy toward

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148
the surface) would satisfy this distance scheme. Concerning microtektites, one may assumed that they received higher initial kinetic energy than tektites, but were subsequently slowed down in the atmosphere (to reach the same launch distance than tektite), or that they were simply produced before tektite at the very first contact between continental surface and the impactor. In that case, sequential melting would sample tektites deeper in the soil than microtektites.

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156 Discarding a purely marine or loessic sedimentary target

The whole above scheme falls apart if one assumes that the target is a thick enough sedimentary sequence, either loessic or marine. In such sequences, $^{10}$Be content does not necessarily decrease with depth but may be constant or variable with sedimentation rate, sediment composition and origin. In this case, we would not expect a consistent variation of $^{10}$Be content with launch distance or size. Also, the presence of relatively large residual detrital grains of quartz and zircon in tektites and microtektites (e.g. Glass and Fries, 2008) and the significant chemical heterogeneity observed argue against a homogeneous very fine grained target material, as loess or distal marine sediment. On the other hand, a residual continental soil has the required grain size and chemical heterogeneity to account for all the observations.

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168 Alternative interpretations by host contamination or atmospheric scavenging

Our interpretative model for the microtektite/tektite contrast may be challenged by two alternative ways to obtain higher $^{10}$Be content for object that have a higher surface/volume ratio. The first is that we would partly measure in our microtektites a surface contamination from the embedding soil or sediment. Indeed, Serrefidin et al. (2007) have shown that the first 500 µm of the surface of moldavites tektites is enriched in $^{10}$Be (by 10 Mat/g) from the
surrounding sediment or percolating water. If we can extrapolate these results, it would mean that ¹⁰Be exchange may have occurred down to the center of the studied microtektites. In core MD97-2143, situated 400 km E of our core MD97-2142, an average of 837 Mat/g was obtained (Q. Simon, personal communication) for the period 790-830 ka. In our shallower core, the ¹⁰Be content is likely lower, but by a factor less than 2. Therefore, it could possible to account for the larger ¹⁰Be in SCS microtektite compared to tektite by a sediment contamination. The same mechanism can be invoked for the Antarctic microtektites as their host sediment is extremely enriched (Table 1). However, non-negligible low temperature diffusion of Be in silicates has never been documented and reported.

The other hypothesis is that during their atmospheric flight, microtektites, in molten or hot stage, scavenge the ¹⁰Be content of the atmosphere. The total ¹⁰Be production per surface in an atmospheric column in estimated to be 0.03 atom s⁻¹.cm⁻² (Kovalstov and Usoskin, 2010). Using an atmospheric residence time of 3 yrs (Baroni et al., 2011) and the section of a 500 µm microtektite, the corresponding scavenging potential is 5,700 atoms. With a glass density of 2.5, this translates into 20 Mat/g, i.e. about the difference observed between tektite and microtektite. However, this scavenging mechanism is likely to be efficient only in a narrow range of atmospheric entry velocity and will be counteracted by surface ablation during flight.

In both mechanisms invoked, lower sized microtektites, i.e. with higher specific surface, should yield higher ¹⁰Be contents. Microtektites in sample #2 are smaller than in sample #1 (originating from 400-600 µm and 600-800 µm sieved fractions, respectively). However, their ¹⁰Be content is not significantly different.

Therefore, we conclude that the most likely interpretation of the observed size and geographic gradients is a common mechanism with a decreasing content with depth in the
198 target (as observed in continental emerged surfaces) and an inverse relationship between melt
199 source depth and ejection velocity.

200

201 ACKNOWLEDGEMENTS

202 Work (partially) supported by PNRA (id. PNRA16_0029). Taiwan Ocean Research
203 Institute provided sediments of marine core MD97-2142 for the study. The ¹⁰Be concentration
204 measurements were performed at the ASTER AMS national facility (CEREGE, Aix en
205 Provence) which is supported by the INSU/CNRS, the ANR through the "Projets thématiques
206 d'excellence" program for the "Equipements d'excellence" ASTER-CEREGE action and
207 IRD.

208

209

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280 **Figure captions**

281 Figure 1: a) The Australasian tektite-microtektite strewn field modified after Glass and Koeberl (2006) and Folco et al. (2016). The two locations in South China Sea (MD97-2142)
282 and in the Transantarctic Mountains (Miller Butte) where the microtektites analyzed in this
283 work were found are shown. The possible source impact location in Indochina is also
284 indicated (~17°N, 107°E; Ma et al., 2004); b) and c) Stereomicrographs of the microtektite
285 analysed in this work from MD97-212 and Miller Butte, respectively. Field of view is 2.5
286 mm.
287 mm.

288 Fig.2: ¹⁰Be (Mat/g) contents in tektites (diamond, gray for Muong Nong, after Ma et al., 2004)
289 and microtektites (circles, this work) versus relative distance from assumed impact site. Note
290 that the dashed lines give a visual help but have no physical or statistical meaning.

291

292

293
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>

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<table><caption>Table 1: ¹⁰Be results from this study (underlined is corrected from in situ-production) and
*geographical averages from Ma et al. (2004). D is the estimated distance to source (based on
Glass and Pizzuto, 1994). Splash form = SF; Muong Nong=MN.</caption><thead><tr><th>Tektite</th><th>¹⁰Be (Mat/g)</th><th>s.d.</th><th>N</th><th>D (10³ km)</th></tr></thead><tbody><tr><td>*Indochina MN</td><td>69</td><td>17</td><td>29</td><td>&lt;0.8</td></tr><tr><td>*Indochina SF</td><td>76</td><td>14</td><td>12</td><td>&lt;0.8</td></tr><tr><td>*Philippines</td><td>121</td><td>22</td><td>19</td><td>1.6-2.5</td></tr><tr><td>*Indonesia</td><td>115</td><td>27</td><td>6</td><td>2.5-3</td></tr><tr><td>*Australia</td><td>136</td><td>30</td><td>20</td><td>5-7</td></tr><tr><td>Vietnam SF</td><td>79.3</td><td>2.5</td><td>1</td><td>&lt;0.8</td></tr><tr><th>Microtektite</th><th>¹⁰Be (Mat/g)</th><th>error</th><th>mass (mg)</th><th>D (10³ km)</th></tr><tr><td>China Sea 1</td><td>136</td><td>12</td><td>3.8</td><td>1.6</td></tr><tr><td>China Sea 2</td><td>143</td><td>10</td><td>3.9</td><td>1.6</td></tr><tr><td>Antarctic 1</td><td>199</td><td>11</td><td>3.6</td><td>11</td></tr><tr><td>Antarctic 2</td><td>217</td><td>12</td><td>3.4</td><td>11</td></tr><tr><td><u>Antarctic 1</u></td><td>176</td><td>11</td><td>3.6</td><td>11</td></tr><tr><td><u>Antarctic 2</u></td><td>194</td><td>12</td><td>3.4</td><td>11</td></tr><tr><td>Miller Butte soil</td><td>21115</td><td>330</td><td>525</td><td>11</td></tr></tbody></table>