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Impactor identification is one of the challenges of large-scale impact cratering studies due to the dilution of meteoritic material in impactites (typically < 1 wt%). The nature of the impactor that generated the Australasian tektite/microtektite strewn field, i.e., the largest Cenozoic strewn field (∼15% of the Earth's surface), the youngest (∼0.78 Myr old) on Earth, and the only one without an associated impact crater so far, is an outstanding issue. We identify a chondritic impactor signature in 77 Australasian microtektites (size range: ∼200–700 µm) from within 3000 km from the hypothetical impact location in Indochina (∼17°N, 107°E) based on variations of Cr, Co and Ni interelement ratios in a Co/Ni vs Cr/Ni space (46 microtektites analyzed in this work by Laser Ablation-Inductively Coupled Plasma -Mass Spectrometry and 31 from literature by means of Neutron Activation Analyses with Cr, Co and Ni concentrations up to ∼370, 50 and 680 µg/g, respectively). Despite substantial overlap in Cr/Ni versus Co/Ni composition for several meteorite types with chondritic composition (chondrites and primitive achondrites), regression calculation based on ∼85% of the studied microtektites best fit a mixing line between crustal compositions and an LL chondrite. However, due to some scatter mainly in the Cr versus Ni ratios in the considered dataset, an LL chondrite may not be the best fit to the data amongst impactors of primitive compositions. Eight high Ni/Cr and five low Ni/Cr outlier microtektites (∼15% in total) deviate from the above mixing trend, perhaps resulting from incomplete homogenization of heterogeneous impactor and target precursor materials at the microtektite scale, respectively. Together with previous evidence from the ∼35 Myr old Popigai impact spherules and the ∼1 Myr old Ivory Coast microtektites, our finding suggests that at least three of the five known Cenozoic distal impact ejecta were generated by the impacts of large stony asteroids of chondritic composition, and possibly of ordinary chondritic composition. The impactor signature found in Australasian microtektites documents mixing of target and impactor melts upon impact cratering. This requires target-impactor mixing in both the two competing models in literature for the formation of the Australasian tektites/microtektites: the impact cratering and low-altitude airburst plume models. [ABSTRACT FROM AUTHOR]

Abstract: We determined the fission track age of Transantarctic Mountain microtektites. The plateau method yielded a formation age of 0.85±0.17Ma. This age overlaps within error with that of the catastrophic impact that produced the Australasian tektite–microtektite strewn field ca. 0.8Ma ago. This provides further evidence that Transantarctic Mountain microtektites belong to the Australasian tektite–microtektite strewn field, as previously suggested on the basis of geochemical evidence, Sr–Nd isotope systematics and poorly resolved radiometric data. [Copyright &y& Elsevier]

Abstract: We studied the variations of the volatile major elements Na and K in Transantarctic Mountain microtektites and Australasian microtektites with distance from the putative source crater location in Indochina. The dataset includes 169 normal-type Australasian microtektites (101 from this study and 68 from the literature) from 24 deep-sea sediment cores up to 8000km from Indochina, and 54 Transantarctic Mountain microtektites from northern Victoria Land, 11000km due southeast of Indochina. Normal-type (MgO<5.5wt.% and SiO2 =60–78wt.%) Transantarctic Mountain microtektites and Australasian microtektites share a common volatilization trend with Na and K contents decreasing with distance from Indochina. The average total alkali (Na2O+K2O) concentrations at distance ranges of 1000–2000km, 2000–4000km, 4000–8000km and >8000km are 4.27±0.67wt.% (n =84), 3.20±1.21wt.% (n =50), 2.10±0.25wt.% (n =35) and 1.25±0.25wt.% (n =54), respectively. The trend highlights a relationship between increasing loss of volatiles in microtektites with longer trajectories and higher temperature-time regimes which should be taken into account in microtektite formation modeling. The trend is consistent with a previous hypothesis that Transantarctic Mountain microtektites belong to the Australasian strewn field and that Indochina is the target region for the parent catastrophic impact. [Copyright &y& Elsevier]

We report the occurrence of microscopic inclusions of shocked quartz plus a Zr phase and trace of Fe oxide crystallites in Australasian microtektites recovered from deep-sea sediment cores within 2000 km of Indochina. The shocked quartz and the Zr phase are interpreted as relicts of the target rock. Furthermore, the internal homogeneity of Australasian microtektites in terms of abundance of relict mineral inclusions, vesicles, and schlieren increases with distance from Indochina. This finding strengthens the current hypothesis that the source crater of the largest and youngest tektite strewn field on Earth is located in the Indochina region, as internal heterogeneity characterizes normal impact glass found in or near the source crater. This finding also indicates that the Australasian microtektites with the longest trajectories underwent the highest temperatures or were heated longer. The definition of microtektites should include the possible occurrence of microscopic relict inclusions as an indication of proximity to the source crater. [ABSTRACT FROM AUTHOR]

Abstract: We extended the petrographic and geochemical dataset for the recently discovered Transantarctic Mountain microtektites in order to check our previous claim that they are related to the Australasian strewn field. Based on color and composition, the 465 microtektites so far identified include two groups of transparent glass spheres less than ca. 800μm in diameter: the most abundant pale-yellow, or normal, microtektites, and the rare pale-green, or high-Mg, microtektites. The major element composition of normal microtektites determined through electron microprobe analysis is characterized by high contents of silica (SiO2 =71.5±3.6 (1σ)wt%) and alumina (Al2O3 =15.5±2.2 (1σ)wt%), low total alkali element contents (0.50–1.85wt%), and MgO abundances <6wt%. The high-Mg microtektites have a distinctly higher MgO content >10wt%. Transantarctic Mountain microtektites contain rare silica-rich (up to 93wt% SiO2) glassy inclusions similar to those found in two Australasian microtektites analyzed here for comparison. These inclusions are interpreted as partially digested, lechatelierite-like inclusions typically found in tektites and microtektites. The major and trace element (by laser ablation – inductively coupled plasma – mass spectrometry) abundance pattern of the Transantarctic Mountain microtektites matches the average upper continental crust composition for most elements. Major deviations include a strong to moderate depletion in volatile elements including Pb, Zn, Na, K, Rb, Sr and Cs, as a likely result of severe volatile loss during the high temperature melting and vaporization of crustal target rocks. The normal and high-Mg Transantarctic Mountain microtektites have compositions similar to the most volatile-poor normal and high-Mg Australasian microtektites reported in the literature. Their very low H2O and B contents (by secondary ion mass spectrometry) of 85±58 (1σ) μg/g and 0.53±0.21μg/g, respectively, evidence the extreme volatile loss characteristically observed in tektites. The Sr and Nd isotopic compositions of multigrain samples of Transantarctic Mountain microtektites are 87Sr/86Sr≈0.71629 and 143Nd/144Nd≈0.51209, and fall into the Australasian tektite compositional field. The Nd model age calculated with respect to the chondritic uniform reservoir (CHUR) is TNd CHUR ≈1.1Ga, indicating a Meso-Proterozoic crustal source rock, as was derived for Australasian tektites as well. Coupled with the Quaternary age from the literature, the extended dataset presented in this work strengthens our previous conclusion that Transantarctic Mountain microtektites represent a major southward extension of the Australasian tektite/microtektite strewn field. Furthermore, the significant depletion in volatile elements (i.e., Pb, B, Na, K, Zn, Rb, Sr and Cs) of both normal and high-Mg Transantarctic Mountain microtektites relative to the Australasian ones provide us with further confirmation of a possible relationship between high temperature–time regimes in the microtektite-forming process and ejection distance. [Copyright &y& Elsevier]

We report on the discovery of a microtektite (microscopic impact glass particles) strewn field from the Victoria Land Transantarctic Mountains, Antarctica. Microtektites were found trapped in the local detritus accumulated in weathering pits and in joints of several glacially eroded summits (∼2600 m above sea level [asl]) distributed latitudinally for 520 km. Their physical and chemical properties define a coherent population with a geochemical affinity to Australasian microtektites and compatible Quaternary 40Ar-39Ar formation age. We therefore suggest that Transantarctic Mountain microtektites (TAMM) define the southern extension of the Australasian strewn fi eld. The margin of the Australasian strewn field is thus shifted southward by ∼3000 km and the maximum distance from the putative parent impact site in Indochina by ∼2000 km. This emphasizes the paradox of the missing parent crater of the largest (>10% of the Earth's surface) and youngest tektite strewn field discovered on Earth. Furthermore, TAMM are depleted in volatile elements (i.e., Pb, Na, K, Rb, Sr, Rb, and Cs) relative to Australasian ones, suggesting a possible relationship between high-temperature-time regimes in the microtektite-forming process and high-angle trajectories in the ejecta plume. [ABSTRACT FROM AUTHOR]

High 10Be contents in tektites reported in literature are taken as evidence of a source material, melted at the impact site, enriched in atmospheric 10Be; i.e., a soil or sediment. In 0.8 Ma Australasian tektites, 10Be content increases with distance from the putative impact location in Indochina, with geographic averages from 69 x 106 atoms/g (Indochina) to 136 x 106 atoms/g (Australia). Here we report, for the first time, 10Be contents in microtektites collected from Antarctica and the South China Sea. We show that microtektites are ~30 x 106 atoms/g richer in 10Be than tektites from the same geographic areas. Antarctic microtektites, with an average 10Be content of 184 x 106 atoms/g after correction for in situ production, are the richest impact glass ever measured. The simplest explanation for such systematic size and geographic trends is that the source depth of the melt within the target surface decreases with ejection velocity. Indeed, higher initial kinetic energy implies higher launch distances and higher fragmentation of the ejecta. Antarctic microtektite source depth may tentatively be restricted to the upper tens of centimeters at the impact site. Alternative models invoking a marine or loessic sediment source, or a secondary enrichment in the microtektite (either by atmospheric scavenging, selective fractionation by volatilization, or post-depositional contamination) fail to reproduce the observed relationships. [ABSTRACT FROM AUTHOR]

The authors offer a response to a commentary made on their study on Australasian tektite (AAT) source, soil-covered oil continental sediments. Topics include Beryllium results on microtektites added on previous tektite data by P. Ma et al., analysis about Chinese loes/paleosol source, and by inference the Baidan Jaan desert (BJD), and conclusion on proposed interpretation stands against the Occam's razor test and alternative interpretations based on a thick Quaternary sedimentary target.