Geochronology

In our projects endeavor to measure the age of carefully selected minerals by two "isotopic clocks", the K-Ar and K-Ca radioactive decay, in order to understand if and why the two clocks appear to run at the same or at slightly different speeds. Because the clocks would be expected to run concordantly, discrepancies are most probably due to imprecise or inaccurate determinations of the radioactive decay constants or their ratio (called "branching ratio"). In our pilot study (Nägler and Villa 2000), a determination of the 40K branching ratio yielded values different from those generally used in geochronology, and closer to the values used in the physics community. Very recent advances in analytical protocols and equipment make a substantial improvement of both precision and accuracy possible. The determination of the branching ratio of the decay of 40K to 40Ar and 40Ca will improve the quality of one of the most widely used geochronometers in the Earth Sciences. This branching ratio will be determined on natural rock samples containing K-rich minerals whose geologic history is extremely well known. The rock samples chosen contain minerals that are known to yield concordant ages with other geochronometers (U-Pb and Rb-Sr). Advances in recent years have resulted in our ability to determine the ages of rocks and minerals with a precision of up to 0.1% for some radioactive decay systems. The major problem is now to relate these precise and accurate ages to a geologic process or event. Understanding the relationships between a mineral age and a geologic process is essential to take advantage of the high quality ages and to derive quantitative information on the rates of geologic processes. In some cases, like for the mineral rutile, it is possible to obtain a temperature and a very precise age from a single grain by analyzing its Zr-content and U/Pb ratios. However, it is not clear what the geological significance of the age and temperature are and how they are related, especially for high grade rocks. Therefore we will study the behavior of the Zr content and the U-Pb system in metamorphic rutiles from rocks with a well-constrained thermal history. The results of this project will lead to a better understanding of rutile as a chronometer and thermometer in crustal rocks with different thermal histories and will also yield information on how theses parameters can be related to geologic processes or events.