Impact cratering is a geological process that has affected the surface of the Earth crust since its formation. High-energy impacts volatilise and melt the target rocks; in some cases, large volumes of melt are generated. The oxidation state of the transition elements in these melts is important in determining the crystallisation history and trace element distribution of such impact melt rocks. Accurate studies on the oxidation state of Fe (the most common transition element in the Earth’s crust) in glasses resulting from the fast cooling of these melts can provide information on the effect of impact cratering on the oxidation state of the molten target rock.

Tektites and microtektites are unaltered impact glasses produced during impact events from the Cenozoic era (beginning 65 My ago). Despite the availability of geochemical studies on tektites and microtektites, very few studies exist on the Fe oxidation state in such materials [1]. As microtektites constitute a large fraction of the mass of the glass produced by a tektite-generating impact event, such studies are of great importance for a more complete study of impact-generated glasses and, in particular, the effects of impacts on oxygen fugacity of melts produced during these events.

XAS spectroscopy is an excellent technique to study the oxidation state of Fe in such materials. In particular, XAS is the only technique allowing the study of impact glass samples regardless of Fe concentration (down to very low Fe contents) and sample size. We examined the iron oxidation state in a number of microtektites (submillimetre-sized glassy spherules that are distal ejecta of meteorite impact events) from all three known strewn fields of microtektites. namely, the North American, Ivory Coast and Australasian fields. The samples were collected during the Deep Sea Drilling Project (Figure 3). The samples were studied by Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and K- or Kß-detected XANES spectroscopy. The latter techniques offer the advantage of providing spectra of even higher resolution than XAS, thus allowing the detection of minor variations of Fe oxidation state. Experimental data were collected at beamline ID26 using a Si (311) monochromator and with a beam size at the sample of 55 x 300 µm.

Fig. 3: Location of the ocean floor sampling sites (red dots) and source crater (black dots) within the tektites Strewn Fields (green): AA = Australasian, CE = Central European, IC = Ivory Coast and NA = North American strewn field. Modified from [3].

The pre-edge peaks of our XANES spectra display discernible variations in intensity and energy (Figure 4), which are indicative of significant changes in the Fe oxidation state. In the Australasian and Ivory Coast microtektites all Fe is divalent. Small components in the pre-edge peak are detected as relating to Fe3+ at well below 10 mole% level. North American microtektites display a varying Fe3+/(Fe2+ + Fe3+) ratio ranging from 0 to 0.5 (±0.1). All data fall within the same area of the graph, between two mixing lines joining a point calculated as the mean of the group of tektites studied so far (consisting of 4- and 5- coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra show a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+ and [5]Fe3+. There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone.

Fig. 4: Plot of the XANES pre-edge peak integrated intensity versus centroid energy (zero corresponds to the edge energy of metallic iron). The shaded ellipses indicate the field of data occupied by Fe model compounds of known oxidation state and coordination number, whereas the open ellipse shows the range for all tektites studied so far [1]. Also shown are the mixing lines (dashed lines + circles) between [4]Fe3+ or [5]Fe3+ and Fe2+ in a mixture of 4- and 5-fold coordinated sites. K/T glass = Cretaceous/Tertiary boundary impact glass.

The relatively high Fe3+/(Fe2+ + Fe3+) ratio in North American microtektites is a completely new and unexpected result, and poses a problem regarding the mechanism of impact melt reduction. No obvious relationship is evident between Fe oxidation state and glass composition. Possibly several parameters affect the Fe oxidation state, including target rock type, marine versus continental target, amount of energy released, and mass of produced glass.

The North American microtektites data and those of the globally distributed impact glasses from the Cretaceous/Tertiary boundary [2] are compatible with a strong interaction between impact glass microspherules and a CO2-rich plume in the Earth’s atmosphere generated by impact volatilisation of the target rock. Of all the impact glasses studied so far, only those produced during large impact events in marine targets (i.e., with a cover of carbonate rocks) produced such a wide spread of the Fe oxidation state.



[1] G. Giuli, G. Pratesi, E. Paris, C. Cipriani, Geochimica et Cosmochimica Acta 66, 4347-4353 (2002).
[2] G. Giuli, S.G. Eeckhout, E. Paris, C. Koeberl, G. Pratesi, Meteoritics and Planetary Sciences 40, 1575-1580 (2005).
[3] A. Montanari and C. Koeberl, Impact stratigraphy. The Italian record. Springer-Verlag, Berlin-Heidelberg, 364 pp, (2000).

Principal publication and authors

G. Giuli (a), S.G. Eeckhout (b), C. Koeberl (c), M.R. Cicconi (a), E. Paris (a), Meteoritics and Planetary Sciences 42, A56 (2007); G. Giuli et al., Meteoritics and Planetary Sciences, in press (2008). 
(a) Dip. Scienze della Terra, Università di Camerino (Italy)
(b) ESRF
(c) Dept. of Geological Sciences, University of Vienna (Austria)