Direct in Situ Dating of Carbonates by LA-ICP-(MC)-MS and its Applications to Chronostratigraphy
Parrish, Randall R.; Rasbury, Troy
A viable method to provide absolute age constraints in carbonate strata has proven elusive. Direct dating of primary carbonate deposits, and/or early marine cements would constrain rates of carbonate platform growth, allowing refinement of models of sea level and climate change that might have influenced the carbonate depositional environment. Absolute age constraints on later diagenetic elements such as meteoric and burial cements and tectonic veins is very important to a wide range of problems in both the hydrocarbon industry (such as burial history and timing of fluid migration) and earth science more generally. U-Pb dating of carbonate has been shown to be viable but so far has been restricted in application due to use of time consuming ID-TIMS methods that involve dissolution and processing of a sample of carbonate. A major hurdle is that there is currently no a priori way to know if a sample has a viable U/Pb ratio, and many carbonates are not datable.
We have successfully adapted LA-ICP-(MC)-MS methods (used routinely in zircon U-Pb dating) by using a well characterized 254+/-6 Ma calcite standard (dated by ID-TIMS at NIGL and Stony Brook) to correct the U/Pb ratio for matrix effects. This approach allows in situ U-Pb dating with uncertainties as little as ±4% at 95% confidence with minimal sample consumption. We have applied this method to the dating of samples with average uranium concentrations (0.4-5 ppm) such as speleothems, paleosols, lacustrine carbonates, tufa, early marine cements (in ammonite chambers), calcite veins from fractures in MORB and marine fossils. The age of dated materials so far ranges from Permian to late Quaternary, as young as 250ky. Laser ablation allows us to quickly assess the U/Pb ratios of the variety of carbonate phases in a sample to determine if the sample has potential for dating. Carbonate components with favorable U/Pb are subjected to in situ dating using textural petrography and SEM/fluorescence imaging to guide the analysis of coherent zones of growth. Examples of these applications will be presented.
The wealth of applications to explore in the future include structural geology and the dating of structural veins, post-depositional cementation, and direct dating of biogenic aragonite (biogenic calcite usually being too low in uranium to be suitable). The future scope of applicability of this method to earth science, sedimentary geology, and hydrocarbon exploration is very exciting.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013