Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

Abstract

Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditionsAbstract-We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K-edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si-K impact spherule from the same impact layer. The pre-edge peak of our high-resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe 3+ . Yellow K/T glasses have significantly higher Fe 3+ /(Fe 2+ + Fe 3+ ) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe 3+ ) and high Si-K glass (20 mole% Fe 3+ ). In particular, all the pre-edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of [4] Fe 2+ and [5] Fe 2+ ) to [4] Fe 3+ and [5] Fe 3+ , respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of [4] Fe 2+ , [5] Fe 2+ , [4] Fe 3+ , and [5] Fe 3+ . Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe 3+ /(Fe 2+ + Fe 3+ ) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca-sulfate-and Ca-carbonate-bearing sedimentary rocks at the impact site.

Structure of (FexCa1−xO)y(SiODrewitt1, Sanloup2, Bytchkov3 et al. 2013Phys. Rev. B

“…12,13,24,29,31,32,34 In general, ferrous iron (Fe 2+ ) tends to occupy a distribution of four, five, and sixfold coordination sites with oxygen, where interpretations of x-ray absorption spectroscopy (XAS) measurements suggest that four and fivefold coordinated Fe sites dominate. 22,23,25,30,34,36 This result is, however, in contrast to Raman and optical absorption spectra, which report Fe 2+ in sixfold coordination. 6,7,9,17,18,42 By comparison, very little direct information is available on the local coordination environment in iron silicate melts due to the difficulty of performing experiments at high temperatures.…”

“…We also thank Jérome Andrieux and the staff at the ID15 beamline, ESRF. Chris Hayward and Holly Spice are gratefully acknowledged for their contributions to the electron probe microanalysis and we thank Bertrand Guillot for providing the MD partials for An 36 …”

“…In addition to EXAFS, the x-ray absorption near edge structure (XANES) technique has also been commonly employed to determine the local coordination environment of Fe in silicate melts and glasses. 22,34,36 In this technique, the pre-edge intensity can indicate a change in coordination geometry. However, in order to interpret this accurately, the result must be combined with knowledge from the first maximum of the EXAFS oscillation, which is inversely proportional to the Fe-O bond distance.…”

Impact glass spherules in the Chicxulub K‐Pg event bed at Beloc, Haiti: Alteration patterns

“…, ; Bohor and Glass ; Giuli et al. , ). Analyses of whole rock (WR) samples of the Beloc section (Goderis et al.…”

A XANES study of chromophores in archaeological glass

“…4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. XANES spectroscopy has also been successfully applied to the speciation of Mn in minerals and glass [e.g.…”