Objective: This sub-task will, in combination with task IV.2, continue to explore the development of astronomically tuned mineral standards for 40Ar/39Ar geochronology. It will obtain precise and accurate U-Pb age constraints on astronomically tuned and 40Ar/39Ar -dated volcanic ash beds. The study will furthermore address fundamental problems of U-Pb dating and quantify the bias between the two dating techniques.
Background: Direct comparison of high-precision 40Ar/39Ar and U-Pb age data at permil age precision is compromised by a systematic 0.6-1% bias, most likely due to inaccuracy of the 40K decay or physical constants. This offset is generally accepted but there are only few well-investigated cases and has to be thoroughly quantified. 40Ar/39Ar sanidine ages of volcanic ash beds in astronomically tuned succession exhibits the same offset. The U-Pb dating of astronomically tuned volcanic ashes also provides the opportunity for exploring two issues related to the U-Pb systematics of zircon in volcanic rocks: (I) the pre-eruptive residence time of zircon in a magmatic melt has the potential to be a limiting factor in the accuracy of precise U-Pb dates from young volcanic rocks. Some studies suggest that zircon may crystallize ten thousand to a few hundreds of thousands of years prior to eruption and ash bed deposition (Crowley, 2006 and in press). (II) The second issue relates to quantifying intermediate daughter product disequilibria in minerals used for the U-Pb dating (i.e., 230Th deficiency and 231Pa excess in zircon, leading to excess 206Pb and 207Pb, respectively), which is an important limitation for age accuracy.
Research strategy and methodology: Priority targets are late Miocene ash layers in astronomically tuned, land-based marine sections in the Mediterranean, such as the tephras from the Melilla peninsula of NE Morocco or elsewhere in the Mediterranean area (van Assen et al. 2006; Kuiper et al. 2004). Precise and accurate 40Ar/39Ar ages are already available from those sections and will be refined in the course of tasks I.2. and IV.2. Using a well-calibrated U-Pb tracer we will achieve permil precision and accuracy and be able to quantify the offset between the two techniques, and detect effects of prolonged residence time of analyzed zircon. The quantification and correction of intermediate daughter product disequilibrium (i.e., 230Th deficiency and 231Pa excess in zircon) will be investigated via the study of melt inclusions as recorders of magma composition.
Feasibility: The availability of the new EARTHTIME mixed-tracer solution as well as lowest procedural blanks obtained in the Geneva laboratory make such analyses possible. Intensive exchange of experiment protocols and data duplication in the other laboratories will ensure highest level of accuracy and allow interlaboratory comparison. Melt inclusions will be studied using the LA-ICPMS laboratory at University of Lausanne (Switzerland; Prof. O. Muentener, subcontractor).
Innovative aspects and relevance: This project extends the analytical limits for U-Pb zircon age determination of young rocks to unprecedented levels of precision and accuracy. The quantification of residence times and of intermediate daughter disequilibria is of general importance for U-Pb chronometry. The intercalibration of the two decay schemes is paramount for achieving absolute accuracy in isotopic dating.
Link with other projects: The project adds information to the age of 40Ar/39Ar calibrated standards and will be carried in close collaboration with tasks I.2 and IV.2. Laboratory and technology exchange with projects II.3, III.3 and IV.3 allows direct intercomparison between different laboratories and analytical equipment.