XAFS study of Zr in Cretaceous–Tertiary boundary clays from Stevns Klint

Abstract

The local structure of K–T boundary clays was studied by Zr K–edge X–ray absorption fine structure (XAFS) in order to provide quantitative data on Zr–O bonding distance, coordination number and oxidation state. The Zr XANES spectra in K–T boundary clays are similar to those from rhyolitic volcanic glass samples (obsidian and pitch stone) and tektite. Since tektite was formed at meteorite impact, those observations suggest that the thermal quenching history at meteorite impact is kept preserved as the local structure of Zr in K–T clays. The Zr–O distance in K–T boundary clays is 2.164 Å (4) and the average coordination number is 6.3. The threshold energy on Zr XANES spectra from the K–T clays is lower than those found on natural glasses, and total effective charge of zirconium ion (formally 4+) estimated from the shift of the spectrum is 3.8+ – 3.9+. The partial reduction of zirconium ion in K–T clays is ascribed to its formation environment, high temperature under reductive environment. Preservation of the partial reduction state also indicates quick quench of K–T clays at their formation, and this observation give as a clue to distinguish sedimentary rocks deposited at periods of so–called extinction events by investigating Zr local structure.