Microtektites from Antarctica

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Joe McCall reports on a shower of glass from the sky over Antarctica

Geoscientist 18.8 September 2008

Folco et al. ¹ have reported the discovery of microtektites in the Frontier Mountain area of the Transantarctic Mountains, Antarctica. These objects were found within bedrock detritus, in joints and decimetre-sized weathering pits, in flat glacially eroded, granitoid rock outcrops. Folco et.al ² later reported similar discoveries at Miller Butte to the north and Timber Peak and Mistake Peak to the south, in the same range. Thus, the microtektites occur over a length of about 400km (Figure 1). This discovery is remarkable because microtektites are normally only recovered from, deep-sea cores, whereas the larger tektites of the four post-K/T strewn fields can occur on land or in the sea; though most have been recovered on land³. The reason for this is that microtektites are rapidly destroyed by the chemical action of solutions within terrestrial rock sequences. Microtektites occur in under-sea extensions of three of the four strewn fields, but not in the Central European strewn field3. Clearly the peculiar conditions in Antarctica have allowed preservation.

The Antarctic microtektites, recovered from 600-800μm fraction, are of two varieties: yellow and rare green. Most are spherical but a few are lenticular (Figure 2). They are smooth, except for some thin encrustations of jarosite and gypsum. In a later publication, Folco et al.4 report that the yellow variety have Mg0 >5% and silica 64-78 wt%; the green variety have MgO 11-13% and silica 59-61%. In one microtektite there was a development of high silica material (lechatelierite).

Age determinations by the 40Ar/36Ar method gave consistent but poorly resolved results, which indicated that these microtektites could come from either the Australasian strewn field (0.76 + or –0.05 Ma) or the Ivory Coast strewn field (1.10 + or- 0.05 Ma). Hpwever the latter has no extension south of the equator3 so these microtektites must form an extension of the Australasian field.

Major and rare-earth element compositions are entirely consistent with an attribution to the Australasian strewn field, as are Sr and Nd isotopic compositions. This immense strewn field has no known source, though it must lie somewhere in Southeast Asia. Glass⁵ wrote that one tenth of the Earth’s surface was showered with glass at the time, and these new finds extend the strewn field considerably.

Folco et al.⁶ in another article suggest that an international programme should be set up to find further occurrences. Whereas such a programme would undoubtedly discover new micrometeorite occurrences, from the point of view of the microtektites, it might be difficult to justify the cost. All that it would be likely to produce would be a further extension of the huge Australasian strewn field, and there is already plenty of material so far recovered for further laboratory research.

References

1. Folco,. L., Rochette, P., Suavet, C. and Gattacceca, J. 2006. Micrometeorites from Frontier Mountain (Antarctica). Meteoritics and Planetary Science 41(Abstracts); A56.
2. Folco, L., Rochette, P., Perchazzi, N., D’Orazzio, M., Laurenzi, M.A. and Tiepolo, M. 2008a. Microtektites from Victoria Land, Transantarctic Mountains. Geology 36(4); 291-294
3. McCall, G.J.H. 2001. Tektites in the Geological Record. Geological Society, London 256 pp.
4. Folco, L., D’Orazzio, M., Ottolini, L., Tonarini, S., and Rochette, P. 2008b. Transantarctic Mountain meteorites: new petrographic data, water content and Nd and Sr isotopic composition. Meteoritics and Planetary Science 43 (Supplement); A44.
5. Glass, B.P. 1997 Tektites. In: Shirley, J.H. & Fairbridge, R.W. (eds.): International Encyclopedia of Planetary Sciences. Chapman & Hall, London; 802-805.
6. Folco, L. and Rochette, P. 2008c. Extending searches for Transatlantic Mountain tektites Meteoritics and Planetary Science 43 (Supplement); A181