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Uranium-lead dating of phosphate minerals: Application to ancient basalts and anthropogenic reconstructions

Final Activity Report Summary - GEOBIOCHRONOS (Uranium-lead dating of phosphate minerals: Application to ancient basalts and anthropogenic reconstructions)

With this project, we intended to develop a high-precision dating technique using the U-Pb isotopic system to date apatites present in basaltic matrix or in fossil biological materials. Because apatites are integral part of almost all geological and biological systems, we expected multidisciplinary applications of our method. The initial goals of the project were to (1) date basalts from Hawaii to gain knowledge on the mantle processes, (2) date basalts from the East- African rift to better constrain the age of the first hominid remains, and (3) modify the technique for direct dating of enamel or dentine of early-hominids. This project was intended to bring together two scientific disciplines, geology and anthropology, by using the same geochronological tool.

We have separated apatites of micrometric size (70-20 µm) from finely crystallised basaltic matrix, thanks to a new minerals separation procedure that we developed. We also developed a new method for analysing Pb, U and Th isotopic compositions and concentrations on single micrometric grains of apatite, which is required for the U-Pb dating. Our results showed, however, that there is almost no U present in the apatite, which contradicts previously published information. Dating method based on the U-Pb decay system requires a high U/Pb ratio (>100) for the age calculation to be meaningful. The U/Pb ratios in our apatites were app.2; consequently we could not calculate the age of the basalts and it made our goal to date recent basalts unachievable.

Despite the impossibility to reach our initial objective, our results brought new insights and raised new questions about the behaviour of U during and after crystallisation of magmatic liquid and after the closure of the system. We adapted our effort in a new challenge of identifying a mineral phase that concentrates U during the last phase of crystallisation. Our results show that U is not present in the non-magnetic minerals, such as apatite, but instead seems to be concentrated in the magnetic mineral phase. This new evidence needs to be further investigated using samples from other locations. However our study is a first step for the re-evaluation of the partition coefficient of U between minerals and basaltic liquid, which is a key point for studying magmatic systems.