Local Structure of Sb in Cretaceous-Tertiary Boundary Clays from Stevns Klint By the XAFS Method

Abstract:

The Cretaceous-Tertiary (K-T) mass extinctions has been thought to be due to the asteroid impact since Ir anomalies was found by Alvarez et al. (1980) . The boundary clay is also enriched in Cr, Co, Ni, Cu, Zn, As and Sb. Especially concentrations of Sb and As are unusually large. However, the origins and concentration processes of Sb are unknown. In this study, local structure around antimony atoms in K-T boundary clay from Stevns Klint, Denmark, was determined by Sb K-edge XAFS spectroscopy. The XAFS analyses give the information about the chemical state and coordination environment around Sb atoms and help identify of the concentration phase, and also may provide various kinds of information about the asteroid impact and mass extinction. The XAFS measurements were performed at the BL-NW10A beamline at the Photon Factory in KEK, Tsukuba, Japan. The XANES spectra and radial structure function (RSF) showed that Sb in K-T boundary clays is high oxidation state Sb⁵⁺ and occupies a SbO₆ octahedral site. The Sb-O interatomic distance in K-T clay sample is 2.08(1) A. It is known that Sb⁵⁺ is stable form in soil and soil water under an equilibrium situation within the Earth’s surface environment. Antimony belongs to group 15 in the periodic table below arsenic, and the chemical behavior of Sb⁵⁺ is similar to that of As⁵⁺. Because there is a close correlation on co-precipitation between As and Fe (Ebihara and Miura, 1996; Sakai et al., 2007) , it is considered that Sb also correlates closely with Fe compounds (e.g., ferric hydroxides). Abundant ferric hydroxides occur in K-T boundary clays. It is considered that one of the reasons of abnormal high concentrations of Sb and As in K-T boundary clays is a lot of dust from impact ejecta falls with iron ions and deposits on surface of the Earth for a short period of time after the asteroid impact.

References

L. W. Alvarez, Science, 208, 1095-1108 (1980)

M. Ebihara and T. Miura, Geochimica et Cosmochimica Acta, 60, 5133-5144 (1996)

S. Sakai et al., The American Institute of Physics, Conference Proceeding, 882, 274-276 (2007)