Project

Discipline

geochronology; K decay constant; molybdenum; decay constant; rutile; K-Ar; U-Pb

Lead
Lay summary
Our project endeavours 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 to other geochronometers (U-Pb and Rb-Sr).Advances in recent years have resulted in our ability to determiner the ages of rocks and minerals with a precision of up to 0.1% of the age 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.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

This project will use natural isotope variations due radioactive decay and fractionation processes in the litho- and hydrosphere to help constrain the dynamics of geologic processes. 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 U-Pb and Rb-Sr ages. The mineral rutile is widespread in medium to high grade rocks and is potentially a useful chronometer that provides high precision ages using the U-Pb system and metamorphic temperatures using the Zr-in-rutile thermometer. It is planned to study the behavior of the U-Pb system and the equilibration of the Zr-contents in natural rutiles from high grade rocks of the petrologically well studied Eastern Ghats belt, India. The results will provide key constraints on the geological meaning of U-Pb rutile ages and the robustness of the Zr-in-rutile thermometer. The distribution of ages in a rutile obtained by in situ spot analyses will be modeled to derive the thermal history of the minerals grains and their host rocks. The result of this project will be the development of a geochronometer for medium to high grade metamorphic rocks that can be used to reconstruct the dynamic evolution of the lithosphere. The fractionation of the stable isotopes of Mo provides key constraint on the redox-evolution of the oceans. Particularly questions related to the timing of the initial oxygen rise in Earth’s atmosphere, and extent of later ocean anoxia are targeted. However, existing models suffer from uncertainties due to a lack of constraints on Mo sources and sinks of the oceanic Mo cycle, and in particular their evolution with time. The dominant sink for isotopically light Mo are pelagic sediments, and they are largely recycled in subduction zones. As subduction zone volcanism significantly contributes to the growth of continental crust, continued recycling of Mo from pelagic sediments into the sources of subduction zone magmas should modify the Mo isotope budget of continental crust. In order to constrain the Mo cycle we will first estimate Mo isotope composition of Bulk Silicate Earth using highly metamorphosed chondrites that are considered the potential building blocks of the Earth. These isotopes then can be compared to juvenile crustal rocks to test for light (pelagic) Mo input. Second, we will study subduction zone related rock suites to evaluate the contribution to their isotope composition from a pelagic source.