| Titanite is a common U-bearing accessory mineral which is usually found in magmatic rocks and metamorphic rocks.Compared with zircon,the lower closed temperature of titanite(650 ~ 700 ?)make it possible to record the rapid cooling time of magma.However,it is the high common Pb content that limits the development of U-Pb dating greatly.Combined with the Tera-Wasserburg diagram,the207 Pb calibration method is not always effective for all kinds of Titanite samples.To explore the applicability and optimization potential of the method,we analyzed a series of problems we encountered during the development of the LA-ICP-MS titanite U-Pb dating method.First of all,in terms of instrumental conditions: the accuracy will reduce significantly if there is a deviation between the laser focus and the field focus.Second,it is common to correct the element fractionation with unreasonable method in LA-ICP-MS titanite U-Pb dating.In this study,we optimized the laser optical path system,and analyzed the methods of data processing.Then we drew two conclusions.In one hand,laser focus can affect on U-Pb dating results to a large extent,and it can be reduced by adjusting laser optical path system.In the other hand,it is better to use the standard sample's Pb/U ratio which contains common Pb/U to correct the element fractionation,and the content of common Pb content should be as low and homogenization.After the method optimization above,we developed the LA-ICP-MS titanite U-Pb dating method apply to our lab suitable.After the verification of standard such as titanite BLR-1,MKED1,Ontario and magmatic rock from the Fang Shan,Beijing.We apply the method into some geological sample like metamorphic rock,basic-ultrabasic rock without zircon but with high common Pb content,granite with Pb lost and Proterozoic leucosome.Combine with the regional background to explore the applicability and optimization potential of the LA-ICP-MS titanite U-Pb dating method. |
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