Element Fractionation and Correction Method for U-Pb Dating of Titanite by Laser Ablation-Inductively Coupled Plasms-Mass Spectrometry
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ABSTRACT
BACKGROUNDIsotope geochronology is a basic subject to explore the temporal and special evolution of geological bodies and geodynamics. The U-Pb dating technology of accessory minerals is the most widely used in isotope geochronology. Titanite has a relatively low closed temperature for U-Pb isotopic system and is common in magmatic rocks, metamorphic rocks, hydrothermal-related rocks and a few sedimentary rocks. This indicates that it is an ideal mineral for U-Pb dating to constrain the medium to high temperature geological event.OBJECTIVESTo understand the element fractionation behavior during laser ablation-inductively coupled plasms-mass spectrometry (LA-ICP-MS) U-Pb dating and propose corresponding correction methods.METHODSIn situ U-Pb dating were performed using a Geolas Pro laser-ablation system and a 7700x quadrupole ICP-MS. A stationary laser ablation spot with a beam diameter of 24μm was used for the analyses. The ablated aerosol was carried by helium and then combined with argon via a T-connector before being introduced to the ICP-MS plasma. After smoothed, the sample gas will go into quadrupole ICP-MS for U-Pb dating. Each analysis incorporated a background acquisition of approximately 10s (gas blank) followed by 40s data acquisition from the sample. After the experiments, the fractionation behavior of elements of titanite during the laser ablation were compared for using different fractionation correction methods to correct zircon, monazite and titanite separately. These correction methods were based on different mathematical model such as quadratic curve, power function and so on. The data was processed with different softwares such as GLITTER and BUSTER based on different mathematical equations, in order to look for the appropriate correction methods for different uranium-rich minerals based on different fractionation characteristics.RESULTSThe inconsistent fractionation behavior of elements between different titanite particles was revealed. After comparison, it was proposed that the 'Exponential Function Method' and the 'Average Data method' can only be used for uranium-rich minerals minerals that have consistent fractionation behavior, but it was not pragmatic for inconsistent ones. For these uranium-rich minerals minerals such as tianite, the 'Intercept Method' was an improvement on the current method, in order to ascertain the correct age.CONCLUSIONSIt is indicated that the 'Intercept Method' can avoid inaccurate correction caused by inconsistent fractionation behavior.
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