Method for in-situ U-Pb dating of heterogeneous minerals

A method for in-situ U-Pb dating of heterogeneous minerals, comprising:

• S1, collecting a bedrock sample, wherein the bedrock sample is rich in any one of zircon, rutile or garnet;
• S2, carrying out petrographic analysis of the sample comprising the following sub-steps:
• S21, grinding the collected bedrock sample into a probe piece having a width of 25 mm, a length of 30 mm-35 mm and a thickness of 0.05 mm and a laser sheet having a width of 25 mm, a length of 30 mm-35 mm and a thickness of 0.08 mm, microscopically observing mineral features corresponding to the probe piece and the laser sheet, and recording petrography and paragenetic association features of the probe piece and the laser sheet;
• S22, selecting a single mineral crystal having a desirable crystal form, complete particles and no impurity inclusion as a sample;
• and
• S23, designing a rectangular area greater than a particle size of the single mineral crystal as an experimental test location with the single mineral crystal as a target on the bedrock sample, marking the rectangular area with a marking pen, and carrying out in-situ element mapping analysis on the selected experimental test location by a laser ablation inductively coupled plasma mass spectrometer;
• S3, carrying out mineral phase separation and purification based on mapping data comprising the following sub-steps:
• classifying matrix data of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) mapping obtained in step S23 by K-means semi-automatic supervised multi-channel classification, and hence achieving mineral phase separation to purify the single mineral crystal;
• S4, carrying out data cleaning and filtering analysis comprising the following sub-steps:
• for high-U minerals, carrying out data cleaning with purified and separated high-U mineral data, negative numbers and null values are replaced with 0.1 time of a minimum value, median filtering is carried out on processed data with MineralMAPPING software, and then the matrix data are converted into a two-dimensional element image with XMapTools software to highlight an element distribution law;
• and
• for low-U minerals, carrying out data cleaning with purified and separated low-U mineral data, negative values and null values are replaced with a value 0, median filtering is carried out on processed data with the MineralMAPPING software, then the matrix data are converted into a two-dimensional element image with the XMapTools software, and a compositional zonation having high U238 and low Pb204 is determined;
• and
• S5, carrying out stage determination and spot design comprising the following sub-steps:
• for the high-U minerals, drafting a U-Pb concordia diagram with filtered data, determining whether multiple stages and Pb loss exist, computing a relation between Th/U and the Pb loss, excluding an area in which U238 elements are suddenly changed or gradually changed with an area in which the U238 elements are stably distributed as an area designing LA-ICP-MS SPOT analysis spots, and designing an LA-ICP-MS spot analysis spot distribution diagram according to the above principle;
• and
• for the low-U minerals, searching for the compositional zonation having high U and low Pb, determining whether multiple stages exist, and designing an LA-ICP-MS spot analysis spot distribution diagram for an area in which U238 elements are stably distributed and have a content greater than 10 ppm;
• and
• S6, accurately limiting a mineral formation age comprising the following sub-steps:
• for the high-U minerals, processing and interpreting dating data by using ICPMSDataCal software, and drafting a U-Pb concordia diagram and a U-Pb weighted mean age diagram in combination with ISOPLOTR software;
• and
• for the low-U minerals, processing and interpreting data by using the ICPMSDataCal software, and under the condition that single-spot Pb206/U238 age concordances are greater than 80%, drafting a U-Pb concordia diagram and a U-Pb weighted mean age diagram by using the ISOPLOTR software;
• and under the condition that most of single-spot Pb206/U238 age concordances are less than 80%, drafting a Tera-Wasserburg U-Pb concordia diagram with the ISOPLOTR software.