HR: 1330h
AN: V32C-1037    [PDF]
TI: Pb-Pb geochronologic study on the carbonaceous rocks in the Krai area, Altai, Russia: V-C boundary or Snowball Earth event?
AU: * Nohda, S
EM: snohda@sci.kumamoto-u.ac.jp
AF: Department of Environmental Sciences,Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555 Japan
AU: Uchio, Y
EM: uchio@esa.c.u-tokyo.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguroku Ookayama2-12-1, Tokyo, 152-8551 Japan
AU: Kani, T
EM: kani@sci.kumamoto-u.ac.jp
AF: Department of Environmental Sciences,Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555 Japan
AU: Isozaki, Y
EM: isozaki@chianti.c.u-tokyo.ac.jp
AF: Dept. Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo, Meguroku, Komaba 3-8-1, Tokyo, 153-8902 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguroku Ookayama2-12-1, Tokyo, 152-8551 Japan
AB: We have analyzed the limestones occurred in the Kurai area, Altai district, Russia to define Pb-Pb isochron age and examine their variation of Sr isotopic compositions through time. The limestones are inferred to have deposited at the boundary of the Vendian-Cambrian from stratigraphic analyses. In this study, We have newly collected samples for the purpose of the present Pb isotopic study on the basis of the reinvestigated stratigraphic map. Samples are also available from various horizons to examine isotopic variation of Sr through time. We made a minor modification on the method of Pb extraction process from the samples to maintain a stable and higher yield, which made it possible to obtain reliable Pb isotopic data. At present, we have obtained an isochron age of 598 + - 25 Ma (MSWD=1.06) for 9 samples. The present age is evidently older than the boundary of the Vendian-Cambrian, and seems to be correlated with the snowball Earth event. Sr isotopes show substantial variation from 0.7059 to 0.7077 which is interpreted in terms of environmental change during the snowball Earth event. Besides the Pb isotopic age, homogenous 208Pb/204Pb ratios of the samples are suggestive that Th/Pb ratio had been uniform within the seawater or through the formation process of the limestone.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 9619 Precambrian
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting