Isotopic dating is a critical tool in the earth sciences as it adds the essential dimension of time to a myriad of geological processes. Arguably the most versatile of all the modern dating methods uses the decay of an isotope of potassium into an isotope of argon. The most useful version of this dating method employs nuclear reactions to convert potassium, calcium and chlorine into a variety of argon isotopes. This so-called argon-argon dating method not only provides valuable time information but also gives us important chemical signals from the sample being analyzed. With investigators being able to analyze smaller and smaller mineral samples, it is possible to see that even the most pristine looking mineral often has tiny imperfections, which can be detected and interpreted using the extra chemical data available with the argon-argon method. However, by only looking at elements near argon in mass, there is a significant blind spot because other important major elements cannot normally be measured. This project is an attempt to extend the versatility of the argon-argon dating method by using neon isotopes which are created by nuclear reactions with sodium, magnesium and fluorine. The production of significant quantities of neon isotopes has been demonstrated and the project will do the important work of calibrating the system so that other researchers can adopt this extension to the method.
Specifically, neutron irradiation produces large amounts of 20Ne from fluorine and 21Ne from magnesium. 22Ne is produced both from magnesium and sodium and it appears that sodium can be detected reasonably well for minerals with low magnesium content. Although there are procedural difficulties in analyzing neon and argon isotopes on the same material, modifications to equipment and analytical methods should be possible for virtually any modern argon-argon dating lab. Once calibrated, exploratory tests of the method will be done to demonstrate its potential. Obvious targets include feldspar samples which are made up of K, Ca and Na-rich end members. By adding Na to the two elements currently monitored, it will be possible to directly measure the out-gassing of different feldspar phases within a single crystal. Similarly, being able to monitor F as well as Cl will be a powerful tool for analyzing amphibole samples which typically have significant quantities of these halogens. In addition, the ability to analyze Mg will greatly improve our understanding of the degassing properties of whole-rock samples such as basalt where Mg-rich but Ca-poor minerals will be directly observable for the first time. Part of the project will also involve the dissemination of the results of these measurements and helping other researchers to be able to exploit the potential of this new extension to the already powerful Ar-Ar dating method.
