Tektites and microtektites Fe oxidation state and water content

Asteroid or cometary impacts onto the Earth surface are known to have played an important role in modifying the composition of the earth crust. Impact glasses, resulting from the rapid cooling of the molten target rock, are clues of the complex melting and metamorphic processes taking place during an impact. Tektites and micro-tektites are a sub class of impact glasses formed during the very first stages of the cratering process by high temperature melting of the target rock. They usually display rounded shapes and can be found over wide areas called strewn fields. As Fe oxidation state could be a useful probe to obtain information on the formation conditions of tektites, it has been the focus of many studies. However, the difficulties in analysing samples with small dimensions and high Fe dilution have so far hindered the possibility to systematically study the Fe oxidation state in these glasses. To this aim, XANES is an ideal technique as it allows to determine the Fe oxidation state also in small samples even at very high dilution without deteriorating the error in the Fe3+/(Fe2++Fe3+) ratio. Fe K-edge XANES spectra have been collected in fluorescence mode at the ID26 beamline of ESRF using a Si (311) monochromator focusing the X-ray beam down to about 50 x 200 μm. The excellent energy reproducibility (±0.03 eV) allowed to obtain a small error in the determination of the Fe oxidation state. Micro-IR data have been collected in transmission mode at the LNF (Frascati, Italy). Areal analyses (50 x 50 μm) have been collected for 16 moldavites, 7 North American tektites and 5 microtektites. Tektite glasses display Fe3+/(Fe2++Fe3+) ratios close to 0.05 (±0.03). With few exceptions (moldavites from the Moravian area), no significant variations have been found in the Fe oxidation state of tektite samples according to impact age or target rock composition [1]. Even for very large impacts events, where the target rock presumably displayed a wide range of Fe oxidation states, tektites have been homogeneously reduced to almost exclusively Fe2+ [2]. Similar behaviour has been observed in molten rock from the first atomic bomb test (Alamogordo, USA) [3]. Contrary to tektites, North American microtektites show a wide variation in the Fe oxidation state raising the issue of a possible difference with the formation mechanism of tektites [4]. Water content of all the tektites and microtektites are in the range of already published tektite water content data and display no correlation with the Fe oxidation state. The low water content of North American microtektites studied suggests that there has been no sea-water induced alteration in these samples, thus strongly suggesting that the variation of the Fe oxidation state in the North American microtektite samples studied here is not due to secondary alteration. We maintain that the mechanism responsible for NA microtektite oxidation is not sea-water alteration, nor oxidation in air. [1] Giuli, G. et al. (2002) Geochim. Cosmochim. Ac., 66, 4347- 4353. [2] Giuli, G. et al. (in press) Geol. S. Am. S. [3] Giuli, G. et al. (in press) Geol. S. Am. S. [4] Giuli, G. et al. (in prep).