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Rb-Sr Isotopic Ages of Biotite in the Weathering Profile of Granodiorite, Yecheon  

Jeong Gi Young (Department of Earth and Environmental Sciences, Andong National University)
Cheong Chang-Sik (Geochronology Team, Korea Basic Science Institute)
Lee Bong Ho (Department of Earth and Environmental Sciences, Andong National University)
Publication Information
Journal of the Mineralogical Society of Korea / v.18, no.1, 2005 , pp. 53-59 More about this Journal
Abstract
Rb-Sr isotopic ages of oxidized biotite in the weathering profile of granodiorite, Yecheon area, were measured by thermal ionization mass spectrometry, and compared with their K-Ar ages. A decrease of Rb-Sr isotopic age is well correlated with iron oxidation, and consistent with K-Ar age. Octahedral and interlayer cations including Rb and Sr were partly released from the oxidizing biotite by excess positive charge from iron oxidation. Divalent /sup 87/Sr decayed from monovalent /sup 87/Rb was more easily released from biotite, resulting in the reduction of Rb-Sr isotopic age. Weathered biotite is not suitable for the age dating of parent rocks, but behaviour of radiogenic isotopes provides useful information on the geochemical and structural changes of biotite during weathering.
Keywords
biotite; weathering; Rb-Sr; K-Ar; radiogenic isotope;
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