Iron oxidation state in impact glass from the K/Pg boundary at Arroyo El Mimbral (Mexico) by Fe K-edge XANES spectroscopy

We examined the iron oxidation state and coordination number in five green and red impact glasses from the Cretaceous-Paleogene (K/Pg) boundary section at Arroyo El Mimbral, NE Mexico, which formed as the result of impact melting during the Chicxulub impact event. The samples have been analyzed by Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy, and the resulting data on Fe oxidation state and coordination number have been compared with literature data on 9 black impact glasses an 1 High Si-K impact spherule (Giuli et al., 2005) and other five yellow spherules from another K/Pg impact layer at Beloc (Haiti) (Giuli et al., 2008). Although there are several studies on the chemical and isotopic composition of these impact glasses, very few studies on the Fe coordination number and oxidation state have been reported. Such studies, however, can be important to reconstruct the oxygen fugacity and temperature conditions prevailing during impact melt formation. The Fe K-edge high-resolution X-ray Absorption Near Edge Structure (XANES) spectra have been recorded at the BM08 beamline of the ESRF storage ring (Grenoble, F). The pre-edge peaks of our XANES spectra display noticeable variations in intensity and energy, which are indicative of significant changes in the Fe oxidation state, spanning a wide range from about 35 to 100 mole% trivalent Fe. All data plot along a trend, which is compatible with the presence of [6]Fe3+ and [6]Fe3+. The intensity of the pre-edge peak is rather low compared to that of other silicate glasses and, more in particular tektites (e.g., Koeberl et al., 2022; Rochette et al., 2019). Mimbral green and red impact spherules display higher Fe3+/(Fe2+ + Fe3+) ratios when compared to black impact glasses from Haiti (from 20 to 75 mole % trivalent Fe) and high Si-K glass (20 mole % trivalent Fe); and display also a wider range compared to Haiti yellow glasses (from 75 to 100 mole % trivalent Fe) . Our observations can be explained by a very large variety of oxygen fugacity conditions prevailing during impact melt formation. Furthermore, there is a positive relationship between the Fe3+/(Fe2+ + Fe3+) ratio and the Ca content of the studied glasses, suggesting that the Fe oxidation state was affected by a variable contribution of the Ca-sulphate bearing sedimentary rocks overlying the target rock at the impact site.