Project III.2: Ar/Ar dating and the integrated Cretaceous time scale ( Jonathan Gaylor supervised by Xavier Quidelleur)

Objective: Here we propose to calibrate the upper Cretaceous (65-100 Myr) time scale using 40Ar/39Ar dating of high temperature minerals (sanidine) extracted from volcanic ashes with the aim of constraining the tuning of cyclic successions in Europe to the nearest 405 kyr eccentricity cycle in the astronomical solution. The highest potential to attain the required very high-resolution and high-quality dates for Late Cretaceous time lie within the abundant silicic ashes found in the Western Interior Basin (WIB), which occur intercalated in dominantly shallow marine successions. These projects require a multi-disciplinary approach with radio-isotope dating techniques integrated with magnetostratigraphic, chemostratigraphic and cyclostratigraphic methods.

Background: Recent improvements of the 40Ar/39Ar dating technique have lowered the analytical uncertainties down to 0.1% for best suitable minerals, such as sanidine. On the other hand, systematic uncertainties as high as 1 - 2% remain. This is due to the lack of definitive agreement on the age of flux monitor standards, even for the FCT sanidine, one of most analyzed. One of the objectives of this project will be to compare U/Pb and 40Ar/39Ar ages within the late Cretaceous time interval, where both techniques are optimized in terms of radiogenic production.

Research strategy and methodology: As numerous ash layers are present within the Late Cretaceous successions of the WIB, initial investigations will be conducted on key layers with both large size sanidine and zircon crystals for intercomparison of the 40Ar/39Ar and U/Pb dating methods. Especially bentonites found in Colorado, including Soap Creek, Thatcher, and 3 layers in the Bridge Creek Limestone offer great possibilities for the Cenomanian-Turonian.

Feasibility: The stratigraphic sections where the best cyclostratigraphy is available are located in Italy, south-east France and England. Unfortunately, no volcanic tuff layers without severe weathering can be found and only low temperature minerals (glauconite) provide useful radiometric dating of the Lower Cretaceous (Fiet et al., 2006), but with uncertainties no smaller than about 1%. The aim of this project is to apply the 40Ar/39Ar technique to silicic ash layers from the WIB, which can be correlated precisely with others world wide using in particular the very high resolution ammonite biostratigraphy and carbon isotope stratigraphy. Although only part of the succession is conspicuously rhythmic and has the potential for astronomically tuning (the Cenomanian-Turonian Bridge Creek Limestone), the exceptional resolution of correlation with the cyclic European successions for which floating orbital time scales have been developed allows precise comparison of orbital-induced sedimentary cycles and radio-isotopic dates.

Innovative aspects and relevance: Interlaboratory comparison appears of crucial importance.  Argon dating will be performed at the Orsay (France) geochronology laboratory using two different approaches, and at the Open University (Prof. Dr. S.P. Kelley, UK). Large size (> 200 μ) sanidine crystals from key layers will be dated by the single grain laser fusion technique in both laboratories. In addition, ash layers for which homogeneity has been demonstrated by the single grain approach will also be dated using the multi-collection instrument (Coulié et al., 2004), in order to detect any systematics linked to isotopic measurements or data reduction.

Link with other projects: This project is strongly linked to project III.3, which will be carried out on the same ash layers when possible (i.e. if both zircon and sanidine are found together). In addition, it is also linked to project II.2 and IV.2 for 40Ar/39Ar dating methodology.