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Ar/Ar and K/Ar geochronology by stepwise dissolution

Final Report Summary - KARSD (Ar/Ar and K/Ar geochronology by stepwise dissolution)

The development of radiometric geochronology is one of the greatest triumphs of 20th century geoscience. Geochronology underpins the study of Earth history and puts important constraints on the rate of biological evolution. However, the ever-increasing analytical precision of geochronological measurements does not automatically translate into better geological understanding. The ‘age’ calculated from the accumulated products of radioactive decay is a mathematical construct that is often difficult to relate to geologically meaningful events. Ideally, it represents a point in time when both the parent and the daughter nuclides became completely immobile, but prior to which they (particularly the daughter nuclide) had been completely ‘mobile’. The conventional way of thinking assumes that the mobility of, say, radiogenic 40Ar in naturally occurring radioactive 40K-bearing minerals is predominantly governed by temperature. Today, this paradigm forms the basis of far reaching Earth systems models regarding the interplay of climate and tectonics which have caused a Copernican revolution in our understanding of large scale geomorphic processes. All these inferences are based on the assumption that diffusion of radiogenic isotopes is controlled by temperature and only by temperature. Given the rapidly increasing importance of ‘thermochronology’ as a research tool for Earth surface science, it is important that this crucial assumption is double-checked. One can only be reasonably confident about the model results if all alternative explanations have been ruled out.

It is a necessary condition for the application of argon thermochronometry that chemical effects can be safely ignored. The KArSD project has established a new method to test this assumption, using an innovative stepwise dissolution technique. By applying different acid strengths to several splits of a mineral separate, the K-Ar ‘reservoirs’ contained in it can be probed independent of temperature. Thermochronological interpretations can only be valid if the acid etching does not reveal any clear correlation between the age and chemical composition of the different dissolution steps. The application of the KArSD method to a 1 billion year old K-feldspar sample from the Limpopo River (South Africa) indicates that this is not the case. Plotting the K-Ar age for each partially dissolved fraction of this complex sample against the degree of partial dissolution yields an age spectrum that mirrors the age spectrum obtained by conventional stepwise heating. Chemical analysis of the dissolved phase show that the K/Na and Si/Al ratios of the solid residue systematically rise with increasing degree of partial dissolution. This confirms that compositionally distinct zones are being progressively ‘mined’ by the acid. The results of our experiments indicate that the K-Ar distribution within the feldspar sample has a chemical rather than purely thermal origin. The reconstruction of the compositional evolution of K-bearing minerals with time is an important step towards the development of ‘hygrochronometry’ as an alternative to thermochronology.

The argon isotopic data for the KArSD project were measured on a sophisticated instrument called a multicollector noble gas mass spectrometer. We have completely and fundamentally revised the way in which such data are processed. All previous studies have ignored the fact that the relative abundances of the argon isotopes are subject to a constant sum constraint, which imposes a covariant structure on the data: the relative amount of any of the isotopes can always be obtained from that of the other ones. This simple fact has far reaching implications for the accuracy of high precision geochronology, and which will eventually lead to a revision of the geologic time scale. In summary, KArSD has successfully tested some of the most basic assumptions behind noble gas geochronology, and geochronology in general. This kind of fundamental research would not have been possible without the support of the ERC.