D'ORAZIO, MASSIMO

FOLCO, LUIGI

Laurenzi M.

PERCHIAZZI, NATALE

Rochette P.

Tiepolo M.

English

Archivio della Ricerca - Università di Pisa

MICROTEKTITES FROM THE NORTHERN VICTORIA LAND TRANSANTARCTIC MOUNTAINS: AN UPDATE.  L. Folco1, P. Rochette2, N. Perchiazzi3, M. D’Orazio3, M. A. Laurenzi4 and M. Tiepolo5, 1Museo Nazionale dell’Antartide, Via Laterina 8, 53100 Siena, Italy (folco@unisi.it), 2CEREGE, Université d’Aix-Marseille III, France, 3DST, Università di Pisa, Italy, 4IGG-CNR, Pisa, Italy, 5IGG-CNR, Pavia, Italy.   Introduction:  We present an update on the ongo-ing study of microtektites from the northern Victoria Land Transantartcic Mountains. A more detailed ver-sion of the reported results is currently in press [1]. Major changes relative to our preliminary report [2] include enlargment of the strewn field and correction of the formation age based on new Ar-Ar geochro-nological data. Samples and find locations:  During the 2003 Ital-ian Programma Nazionale delle Ricerche in Antartide (PNRA) expedition we discovered eight microtektites ~500 µm in size on top of Frontier Mountain (72° 59’ S – 160° 20’ E; ~2800 m above see level [a.s.l.], ~600 m above ice level). They were found within the fine-grained bedrock detritus accumulated in joints and decimeter-sized weathering pits of flat, glacially eroded granitoid (Granite Harbour Igneous Complex) surfaces. These structures are extraordinary traps for fallout material, as testified by the thousands of mi-crometeorites (microscopic extraterrestrial particles) found therein [3]. During the 2006 PNRA expedition, we also found microtektites in similar traps on two other glacially eroded granitic summit plateaus of the TAM in north-ern Victoria Land: Miller Butte (72° 42’ S – 160° 15’ E; ~2600 m a.s.l., ~400 m above average ice level) and an unnamed nunatak in the Timber Peak area (74° 11’ S – 162° 15’ E; ~2600 m a.s.l., ~300 m above average ice level), ~30 km and ~140 km due north and south of Frontier Mountain, respectively. One additional find-ing can be reported for a sample collected within the fractures of the sandstone (Beacon Supergroup) top of Mistake Peak (77° 26’ S - 160° 11’ E; ~2500 m a.s.l., ~100 m above average ice level), ~520 km due south of Miller Butte in the Dry Valleys area in central Vic-toria Land. So far, a total of ~150 microtektites have been separated from the host detritus in the 400–800 µm size fraction under the stereomicroscope. Their con-centration is of the order of 1 particle per 100 g of granitic detritus and per 20 micrometeorites. Prelimi-nary observations revealed microtektites also in the 200–400 µm size fraction Bulk chemistry: Transantarctic Mountain micro-tektites (TAMM) are transparent glass spheres with a characteristic pale-yellow color, which is unique to some “normal-type” Australasian microtektites (Glass et al., 2004). Rare exceptions have oblate to lenticular shapes. Some particles may contain microbubbles up to 20 µm in diameter. Their external surface is typi-cally smooth and clean, although some particles may be partially covered by thin jarosite and gypsum en-crustations. They are typically devoid of evidence for mechanical chipping, such as impact craters and grooves on their external surfaces, suggesting deposi-tion in situ with negligible subsequent transport. Eleven particles were analyzed by synchrotron X-ray diffraction at the BM8 GILDA beamline of the European Synchrotron Radiation Facility, Grenoble. All these particles were found to be completely amor-phous, as evidenced by their X-ray patterns. The major element composition of 39 sectioned spherules from the four find sites, obtained by EMPA, is somewhat variable. Most major element oxides show negative correlation with silica which ranges from 64.4 to 77.7 wt.%; exceptions are K2O, which increases with SiO2, and Na2O, which is essentially constant. The total alkali (Na2O+K2O) abundances are very low (0.90 – 1.85 wt%) relative to those of vol-canic glasses of similar silica content, and the K2O/Na2O ratio is always >1 (2.7 – 4.4). These com-positional features are characteristically observed in tektites [4]. Overall, major element data define a coherent population of microtektites. With respect to those from the North American, Ivory Coast and Australasian strewn fields [5, 6], the TAMM fall close to the higher and lower compositional extremes for the refractory elements Ti, Al, and Ca, and for the most volatile ele-ments Na and K, respectively, and show a geochemical affinity to Australasian microtektites. The same obser-vations can be made on the basis of the trace element composition obtained by LA-ICP-MS on six spherules.  As characteristically observed in tektites [4], most elements match the average Upper Continental Crust composition [7]. Major deviations from Upper Conti-nental Crust composition include a strong to moderate depletion in Pb, Na, K, U, Rb, Sr and Cs and an en-richment in rare earth elements (REE), Sc, Zr, Hf and Th. Such deviations are likely due to the high-temperature melting and vaporization of crustal target rocks at the time of the impact [4]. 40Ar-39Ar analyses: 40Ar-39Ar total fusion analyses of 25 individual spherules (400–600 µm in diameter) from three different sample sites (Frontier Mountain, Miller Butte and Timber Peak) yielded consistent data indicating that the studied spherules belong to a single population and contain excess radiogenic 40Ar. The Lunar and Planetary Science XXXIX (2008) 1180.pdfisochron gives an age of 1.4 ± 1.5 Ma with a trapped initial 40Ar/36Ar = 409 ± 28. Subsequent multi-grain stepwise heating 40Ar-39Ar analysis of 11 spherules provided a “Plateau-Isochron” age of 0.76 ± 0.98 Ma with an initial 40Ar/36Ar = 422 ± 18. Although poorly resolved, these ages provide evidence that the forma-tion age of the TAM microtektites is essentially Qua-ternary and compatible with the 0.76 ± 0.05 Ma and 1.10 ± 0.05 Ma 40Ar-39Ar ages for the Australasian [8] and Ivory Coast [5] tektites, respectively.  Table 1. Average bulk composition of TAMM.   Major elements (wt %) SiO2 71.6 ± 3.2     TiO2 0.88 ± 0.09     Al2O3 15.3 ± 1.8     FeO 4.07 ± 0.60     MnO 0.09 ± 0.02     MgO 2.91 ± 0.58     CaO 3.43 ± 0.47     Na2O 0.27 ± 0.04     K2O 0.97 ± 0.15     Total 99.5 ± 0.3            Trace elements (µg g-1) Li 36 ± 6            Sm 9 ± 1 Sc 17 ± 2            Eu 1.6 ± 0.2 V  32 ± 5            Gd 7.0 ± 0.9 Cr 60 ± 39            Tb 1.0 ± 0.1 Co 6 ± 2            Dy 6.7 ± 1.0 Rb 45 ± 14            Ho 1.3 ± 0.2 Sr 206 ± 27            Er 3.8 ± 0.5 Y  34 ± 4            Tm 0.5 ± 0.1 Zr 307 ± 25            Yb 3.9 ± 0.5 Nb 20 ± 2            Lu 0.6 ± 0.1 Cs 2.8 ± 1.0            Hf 8.2 ± 0.9 Ba 527 ± 60            Ta 1.7 ± 0.2 La 51 ± 6            Pb 0.02 ± 0.01 Ce 103 ± 12            Th 19 ± 3 Pr 11 ± 1            U 0.6 ± 0.2 Nd 43 ± 5       Discussion: Based on the geochemical composition and the 40Ar-39Ar data, we conclude that the TAM mi-crotektites identify a portion of a strewn field extend-ing latitudinally for more than 520 km in Victoria Land, associated with a catastrophic impact on the Earth’s continental crust in the Quaternary. TAM microtektites share a number of features with the Australasian microtektites. They have the same color of some “normal-type” Australasian microtek-tites and overlapping chemical composition of refrac-tory elements, and have compatible age. We therefore suggest that they likely represent the southern exten-sion of the Australasian strewn field. More precise age and Sr-Nd data will confirm this attribution. Mean-while, however, we cannot rule out that the TAMM identify a new strewn field of Quaternary age. This alternative hypothesis, however, would anomalously increase to three the number of microtektite-generating cosmic impacts in the Quaternary, against the total of four events known from the entire Cenozoic. There would be a number of fascinating implica-tions should the TAMM indeed belong to the Australa-sian strewn field. First of all, the margin of the Austra-lasian strewn field is thus shifted southward by ~3000 km, and the maximum distance from the putative par-ent impact site in Indochina by ~2000 km. This em-phasizes the paradox of the missing parent crater of the largest (>10% of the Earth’s surface) and youngest tektite strewn field discovered on Earth and the puta-tive southward asymmetry of the strewn field. TAMM found at the margin of the Australasian strewn field and at the maximum distance from the hypothetical source crater in Indochina are also the most depleted in volatile elements. This suggests a possible relationship between peak temperature-time regimes at the point of impact and/or during atmos-pheric flight, and high-angle ejection trajectories. Al-ternatively, one may invoke long trajectories outside Earth’s atmosphere and ablation during re-entry. Inves-tigating the possible geographic variation in composi-tion of the Australasian strewn field might therefore contribute to the understanding of the thermal structure of the ejecta plume and ultimately further help to pre-dict source crater location. Lastly, the age of the Australasian microtek-tites suggests that the micrometeorite traps in which the TAMM were found [3] have been accumulating infalling extraterrestrial matter over at least the last ~1 million year, thus providing a unique new approach to meteoritic flux studies over the recent geological past.  References: [1] Folco L. et al. (2008) Geology, in press. [2] Folco L. et al. (2007) Meteoritics & Planet. Sci., 42 (Suppl.), A50. [3] Folco L. (2006) Meteoritics & Planet. Sci., 41, A56. [4] Koeberl C. (1990) Tec-tonophysics, 171, 405–422. [5] Koeberl C. et al. (1997), Geochim. Cosmochim. Acta, 61, 1745–1772. [6] Glass B. et al. (2004) Geochim. Cosmochim. Acta, 68, 3971–4006. [7] Taylor S. R. and McLennan S. M. (1995), Rev. Geophys., 33/2, 241–265. [8] Izett G. A. and Obradovich J. D. (1992), LPSC XXIII, 593-594.  Acknowledgments: This work was supported by PNRA. PR was supported by IPEV.  Lunar and Planetary Science XXXIX (2008) 1180.pdf

Microtektites from the northern Victoria Land Transantarctic Mountains: an update

https://arpi.unipi.it/bitstream/11568/232982/1/1180.pdf

Microtektites from the northern Victoria Land Transantarctic Mountains: an update

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