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 socalled extinction events by investigating Zr local structure.

XAFS study of Sb and As in Cretaceous–Tertiary boundary sediments: an index of soiling of the global environment with dust and ashes from impact ejecta falls

“…Furthermore, Miyano et al (2016) determined the local structures of Ca, Cr, Mn, Fe, and Ni atoms in K-T boundary sediments and clarified the effects of weathering, diagenesis, and life activity under the sedimentation. Tobase et al (2015) reported that the local structures of Zr in K-T sediments maintain a glass structure and keep the thermal quenching records by the asteroid impact in the Zr XAFS study in K-T boundary sediments. The samples and XAFS measurement points of K-T boundary sediments used in Sakai et al (2007), Okube et al (2012), Miyano et al (2016), and Tobase et al (2015) were the same as those used in this study.…”

“…Tobase et al (2015) reported that the local structures of Zr in K-T sediments maintain a glass structure and keep the thermal quenching records by the asteroid impact in the Zr XAFS study in K-T boundary sediments. The samples and XAFS measurement points of K-T boundary sediments used in Sakai et al (2007), Okube et al (2012), Miyano et al (2016), and Tobase et al (2015) were the same as those used in this study. In this study, we similarly determined the local structure around Sb atoms to obtain information about the chemical state and coordination environment of Sb atoms in K-T boundary sediments by Sb Kedge XAFS spectroscopy.…”