Xenotime laser-ablation MC-ICP-MS U-Pb and Th-Pb dating

The main objective of the project is to put forward radiometric in-situ U-Pb and Th-Pb dating of xenotime as a geochronological tool. This is aimed at by establishing analytical procedu-res and defining appropriate standard xenotimes for in-situ micro-sampling by combining 193 nm ArF excimer laser ablation and multi-collector inductively-coupled plasma mass spectro-metry (LA-MC-ICP-MS). Xenotime is a widely occurring phosphate with great potential as a petrologically important mineral amenable to U-Pb and Th-Pb dating. In the context of Alpine research two topics are of major interest and form a long term background to the proposed geochronology-specific investigations: a) high-temperature-time (hT-t) evolution of rocks based on U-Th-Pb geochrono-logy of xenotime; b) derivation of sedimentation ages based on U-Pb dating of diagenetic xenotime. However, xenotime is often only found as a micrometer-sized accessory and so can only be successfully dated by in-situ micro-sampling techniques. Also the paragenetic relations of xe-notime to the host rock, which are of extreme importance for data interpretation, can only be successfully established by it-situ examination. A fundamental precondition being indispensable for the application of xenotime in-situ dating is the availability of precise and accurate analytical methods. In order to be able to provide sig-nificant age data for the Phanerozoic, in- situ sampling methods must deliver a resolution (based on internal and external reproducibilities) of better than 5 Ma (2s). In this sense, in-situ xenotime U-Pb and Th-Pb dating is far from being a routine method. For SIMS techniques, possi-ble calibration procedures have been suggested (i.e. matrix cor-rections for the effects of the domi-nant trace elements U, Th and REE). On the contrary, for LA-ICP-MS analysis the lack of inter-nationally accepted analytical procedures, age and mineral chemistry standards all have to be overcome first. This will be tackled by the proposed work by establishing analytical procedures and defining appropriate standard xenotimes for in-situ LA-MC-ICP-MS analysis.

The main objective of the project is to put forward radiometric in-situ U-Pb and Th-Pb dating of xenotime as a geochronological tool. This is aimed at by establishing analytical procedu-res and defining appropriate standard xenotimes for in-situ micro-sampling by combining 193 nm ArF excimer laser ablation and multi-collector inductively-coupled plasma mass spectro-metry (LA-MC-ICP-MS). Xenotime is a widely occurring phosphate with great potential as a petrologically important mineral amenable to U-Pb and Th-Pb dating. In the context of Alpine research two topics are of major interest and form a long term background to the proposed geochronology-specific investigations: a) high-temperature-time (hT-t) evolution of rocks based on U-Th-Pb geochrono-logy of xenotime; b) derivation of sedimentation ages based on U-Pb dating of diagenetic xenotime. However, xenotime is often only found as a micrometer-sized accessory and so can only be successfully dated by in-situ micro-sampling techniques. Also the paragenetic relations of xe-notime to the host rock, which are of extreme importance for data interpretation, can only be successfully established by it-situ examination. A fundamental precondition being indispensable for the application of xenotime in-situ dating is the availability of precise and accurate analytical methods. In order to be able to provide sig-nificant age data for the Phanerozoic, in- situ sampling methods must deliver a resolution (based on internal and external reproducibilities) of better than 5 Ma (2s). In this sense, in-situ xenotime U-Pb and Th-Pb dating is far from being a routine method. For SIMS techniques, possi-ble calibration procedures have been suggested (i.e. matrix cor-rections for the effects of the domi-nant trace elements U, Th and REE). On the contrary, for LA-ICP-MS analysis the lack of inter-nationally accepted analytical procedures, age and mineral chemistry standards all have to be overcome first. This will be tackled by the proposed work by establishing analytical procedures and defining appropriate standard xenotimes for in-situ LA-MC-ICP-MS analysis.