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Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK
Shell Research, KSEPL, PO Box 60, 2280 AB Rijswijk, The Netherlands
Pb/Pb and U-Pb methods of direct isotopic dating, characteristically associated with igneous and metamorphic rocks, have been applied successfully to well-preserved stromatoporoidal carbonates from Gotland, Sweden. A Pb/Pb date of 432 ± 15 Ma for Wenlock-Ludlow boundary carbonates is in good agreement with independent chronometric estimates and is interpreted as the age of deposition/early diagenesis. A Pb/Pb date of 326 ± 11 Ma for older Middle Wenlock samples is c. 100 Ma too young for its stratigraphic position and is interpreted as a late diagenetic age associated with fluid movement along sub-surface faults. U-Pb systematics in both samples have been disturbed by U remobilization in response to recent weathering associated with post-glacial uplift in the Baltic.
For many years, the direct age determination of sediments has been an irritating problem to earth scientists. Direct Pb/Pb dating of carbonates should be considered a credible source of chronometric information. Application of the method to poorly constrained Precambrian stromatolitic carbonates is potentially powerful since host sediment ages may be only poorly constrained through indirect reference to basement and intrusive rocks. A broad geochemical review is presented to explain the suitability of certain carbonates for this method of isotopic investigation.
