In situ U-Pb dating method for calcite

An in situ U—Pb dating method for calcite, comprising the following steps:

• a) cutting a calcite sample or a rock sample containing calcite to prepare an epoxy resin sample target with a diameter of 1 inch and a thickness of 5 mm to adapt to a size of a laser ablation sample chamber;
• b) placing the sample in the laser ablation sample chamber, and adjusting a position of the sample in an optical axis direction to make a laser beam focus well;
• c) conducting line scanning ablation on calcite in the sample target using the laser beam, loading ablated aerosol into a quadrupole inductively coupled plasma mass spectrometry (ICP-MS) plasma source for ionization using a helium gas as a carrier gas, and measuring ion signal intensity data of 43Ca, 88Sr, 139La, and 238U;
• d) conducting two-dimensional (2D) element imaging using Iolite 4 software, processing data offline, conducting instrument blank deduction for a signal, and conducting instrument drift correction to calculate signal intensities of different elements;
• and calculating an instrument response coefficient according to a signal intensity and content of each element of NIST SRM 612, calculating a content of an unknown sample according to the instrument response coefficient, and imaging a 2D element content distribution map;
• e) according to the 2D element content distribution map, determining a high-U analysis target area, conducting point ablation on the high-U target area using the laser beam, loading ablated aerosol into a double-focusing sector field ICP-MS plasma source for ionization using a carrier gas, and measuring ion signal intensity data of 206Pb, 207Pb, and 238U;
• f) in a measurement process, after every ten unknown samples are tested, repeatedly testing two calibration reference materials NIST SRM 614 for 207Pb/206Pb correction, three calibration reference materials WC-1 for 238U/206Pb correction, and two quality monitoring reference materials Duff Brown Tank to ensure same measurement conditions for the reference materials and the unknown samples;
• g) after the element signal data is obtained, obtaining corresponding fractionation coefficients according to measured values and standard values of 207Pb/206Pb and 238U/206Pb of the reference materials, and correcting 207Pb/206Pb and 238U/206Pb ratios of the unknown sample;
• and
• h) constructing a Tera-Wasserbug diagram through the corrected 207Pb/206Pb and 238U/206Pb ratios, and calculating age data and an initial Pb isotope (207Pb/206Pb) composition of the calcite sample.