This method is based on the assumption that the closed state of a zircon crystal with uniform primary structure may be damaged by later geological processes (such as radiation damage, weathering and leaching, etc.) , that is, lead or uranium is brought in or out; however, this unstable phase with reduced lead activation energy generally only occurs at the edge of the crystal, while the interior is still a stable high activation energy phase, and its U-Pb system is still in closed state. In addition, if the core of a zircon crystal is an ancient inherited zircon phase and the outside is an accretionary phase, the evaporation activation energy of these lead located in different crystal domains will also be different when they evaporate from the zircon crystal. The required temperature, time and length of diffusion path will all be different. Taking advantage of this difference, through the step-by-step heating stratified evaporation technology, different crystal domains and lead of different compositions can be separated, and the lead isotope compositions of them can be measured at the same time. The 207Pb/206Pb surface of each set of data can be calculated according to formula (86.9) age, and finally the zircon crystallization age is given by excluding inconsistent lead in the form of a histogram. The advantages of this method are that it requires less sample and the analysis process is relatively simple. It does not measure uranium and lead content and does not require chemical separation. It requires zircon crystals to have a large particle size (>120μm), no continuous crystals, no inclusions, no cracks and no molting, and be clean and transparent. According to these conditions, this method is more suitable for zircons from ancient plutonic rocks and ancient metamorphic rocks. It should be used with caution for extremely fine zircons in young volcanic rocks. In addition, since this method does not measure uranium, the U-Pb age cannot be obtained, so the measurement result only has a 207Pb/206Pb age, and the lack of mutual verification is also a shortcoming.
The recent new exploration of this method was published by Wendt in 1991 and 1993. Since 210Pb is an intermediate product in the 238U decay series, the 238U/206Pb ratio is obtained by measuring the 210Pb/206Pb ratio in the zircon evaporation method, and then the U-Pb age is obtained. The difficulty lies in the fact that the half-life of 210Pb is very short and the abundance is very low, often below the detection limit of the mass spectrometer.
Instruments, equipment and materials
Thermal ionization mass spectrometers MAT260, MAT261, MAT262, VG354, TRATON and other equivalent types.
Spot welding machine mass spectrometer supporting equipment.
Mass spectrometer filament preheating device is ancillary equipment for mass spectrometers.
The specifications of the rhenium tape are 18mm×0.03mm×0.8mm, and a boat-shaped groove is pressed out through a special mold.
Lead isotope standards NBS-981, NBS-982, NBS-983.
Experimental Procedures
Lead isotope analysis was performed on a dual-band source mass spectrometer. As mentioned in the previous section, the rhenium ribbon is first dotted on the filament holder using conventional methods and pre-treated to remove impurities in the rhenium ribbon itself, and then the burned rhenium ribbon is made into a boat shape using a special mold. Put the selected zircon into the boat-shaped trough under a binocular microscope, and use tweezers to clamp both sides of the boat trough to cover the zircon, but leave a slit for ion circulation. Finally, install the evaporation belt and the ionization belt on both sides of the ion source turntable respectively. Use shaping to make the two belts parallel and close to each other. They must not touch each other to form a short circuit. Add a shielding cover and send the entire turntable into the mass spectrometer ion source. vacuum.
When the vacuum in the analysis chamber reaches 10-6Pa and then slowly increase the charging current, first wait for a certain period of time at low temperature (900~1000℃) to ensure that the rhenium bands and rhenium bands adhering to the crystal surface are completely removed. Contaminating lead on the stent, and the low activation energy lead located on the surface of the crystal that has lost part of the radiogenic lead. Evaporation of the radiogenic lead in the inner layers of the crystal requires higher temperatures. At this time, the method of alternating heating of two filaments, the evaporation zone and the ionization zone, can be used. First, the lead in the zircon is evaporated from the evaporation zone and condensed on the ionization zone, and then the lead condensed on the ionization zone is evaporated again. Isotope determination. This process can be repeated once when the zircon crystals are large enough. In addition to lead, there are also ZrO2 and SiO2 condensed on the ionization zone, which act as silica gel emitters to stabilize the flow of lead ions. In addition, the lead ion flow evaporated from the evaporation zone can also be measured directly, and the released lead isotope composition can be continuously measured.
The lead ion flow generated by zircon evaporation was received by an electron multiplier and recorded by single-peak jump scan. By adjusting the multiplier high voltage and filament temperature, try to make the lead ion flow intensity large enough and stable to ensure measurement accuracy. Usually, more than 10 blocks of data are collected in one measurement, and each block of data is the summary value of 7 scans. When different crystal domains and 207Pb/206Pb ratios are found, the collected data should be increased as appropriate. If necessary, multiple rounds of alternate evaporation should be performed to obtain information from different crystal domains. Measure the international lead reference material, find the deviation coefficient between the actual measured value and the standard value, and then correct the corresponding ratio of the sample.
Age calculation
After excluding the contaminated lead, theoretically the lead in the zircon sample only includes radiogenic lead and initial ordinary lead. When calculating the 207Pb/206Pb age This part of ordinary lead needs to be deducted from the measured value. The specific formula is:
Rock Mineral Analysis Volume 4 Resource and Environmental Investigation and Analysis Technology
In the formula: the lower right corner is marked with γ, s and c are respectively the radiogenic lead in the sample, the measured value of the lead isotope composition of the sample, and the isotope ratio of ordinary lead. In actual calculations, iterative methods need to be used to solve this equation and obtain the age.
Calculate a 207Pb/206Pb age for each set of data collected, and display these ages on a histogram. Its ordinate is the number of data, and the abscissa is the 207Pb/206Pb age and the corresponding 207Pb/206Pb ratio. , the final age value is determined by the peak value of the histogram, and its mean or median value and the confidence limit at 95% confidence level are calculated.
Appendix 86.1A Diluent solution preparation and concentration calibration
(1) Diluent solution preparation and accurate determination of lead and uranium isotope composition
A.208Pb dilution Preparation of agent solution. Weigh approximately 10 μg of 208Pb-enriched solid lead nitrate [Pb(NO3)2] on an analytical balance into a cleaned small fluoroplastic beaker, add a little 4mol/LHNO3 to dissolve, and transfer to 500mL fluoroplastic reagent with 3.0mol/LHCl. bottle and dilute to approximately 500 mL.
B. Preparation of 205Pb diluent solution. The amount of solid lead nitrate Pb(NO3)2 enriched in 205Pb imported from abroad is very small, generally only 0.5~1μg, which is difficult to see with the naked eye. Open its packaging bottle carefully, add a drop of 4mol/LHNO3 to dissolve it, and dissolve it with 3.0mol/LHCl. Transfer to a 100mL fluoroplastic reagent bottle and dilute to approximately 100mL.
C.235U diluent solution preparation. Weigh approximately 250 μg of 235U-enriched solid uranyl nitrate [UO2(NO3)2·6H2O] on a microbalance into a cleaned small fluorine plastic beaker, add a little 4mol/LHNO3 to dissolve, add 5mL of 3.0mol/LHCl and convert to Pour the hydrochloric acid solution into the anion resin exchange column, remove the impurity lead according to the U-Pb separation procedure of the sample, and collect the uranium. The uranium-containing solution is evaporated to dryness and then dissolved with 3.0 mol/LHCl and transferred to a 500 mL fluoroplastic reagent bottle and continued to be used. 3.0mol/LHCl is approximately diluted to 500mL.
D. Preparation of 205Pb+233U (or 235U) mixed diluent solution and determination of isotope composition. About 0.5μg of solid lead nitrate enriched in 205Pb imported from abroad was first dissolved with a drop of 4mol/LHNO3, and then transferred to a 100mL fluoroplastic reagent bottle with a small amount of 3.0mol/LHCl. Then weigh about 10 μg of uranyl nitrate enriched in 233U (or 235U), dissolve it with a little 4mol/LHNO3, add 5mL of 3.0mol/LHCl to convert it into a hydrochloric acid solution, and then purify the 233U diluent according to the procedure for purifying 235U diluent. Combine the purified 233U diluent solution into the same 100mL fluoroplastic reagent bottle containing 205Pb diluent, and dilute it to approximately 100mL with 3.0mol/LHCl.
After the preparation of the diluent solution is completed, the lead and uranium isotope compositions shall be accurately measured according to the procedure 86.1.2, and each diluent shall be measured in parallel no less than 6 times.
(2) Preparation of standard solution and accurate determination of lead and uranium isotope composition
A. Preparation of lead standard solution and determination of isotope composition. Weigh 200 μg of lead isotope standard material NBS982 (or NBS981) on an analytical balance, place it in a small fluorine plastic beaker, heat and dissolve it with a few drops of 4mol/LHNO3, move it into a weighed 30mL fluorine plastic dropper bottle, and use ultrapure water After rinsing the beaker, put the solution into a dropper bottle, continue to dilute with ultrapure water until the dropper bottle is nearly full, shake well, and weigh the solution mass on a balance with a sensitivity of 0.1 mg. Accurately measure its lead isotope composition according to the procedure of 86.1.2, conduct parallel measurements no less than 6 times, and calculate the 206Pb concentration in the solution.
B. Preparation of uranium standard solution and determination of isotope composition. Weigh 250 μg of the reference substance uranyl nitrate [UO2(NO3)2·6H2O] on an analytical balance into a small fluoroplastic beaker, heat and dissolve it with ultrapure water and a few drops of 4mol/LHNO3, and transfer it into a weighed 30mL fluoroplastic droplet In the bottle, rinse the beaker with ultrapure water and put the solution into a dropper bottle. Continue to dilute with ultrapure water until the dropper bottle is nearly full. Shake well and weigh the solution mass on a balance with a sensitivity of 0.1mg. Theoretically, the 238U/235U ratio of ordinary uranium is 137.88, but in practice this value is often not measured. Therefore, it is still necessary to actually measure its uranium isotope composition according to the 86.1.2 procedure, and measure it in parallel no less than 6 times, and calculate the amount in the solution The amount concentration of 238U and 235U.
(3) Calibration of diluent solution concentration
A.208Pb diluent.
Accurately weigh an appropriate amount of lead standard solution on an analytical balance, put 6 parallel portions (not less than 6 portions) into a small beaker, then weigh accurately according to different proportions, add an appropriate amount of 208Pb diluent solution, mix the two evenly and evaporate Dry, accurately determine the lead isotope composition of the mixture according to the procedure in 86.1.2.
The concentration of 208Pb in the diluent solution is:
Rock Mineral Analysis Volume 4 Resource and Environmental Investigation and Analysis Technology
Where: c208t and c206N respectively is the molar concentration of 208Pb in the diluent solution and 206Pb in the lead standard solution, mol/g; R is the ratio of 206Pb/208Pb; the symbols t, N, and m in the lower right corner represent the 208Pb diluent, lead standard solution, and the mixture of the two respectively. ; mN and mt are the masses of weighed lead standard solution and diluent solution respectively, g.
B.235U diluent. Accurately weigh an appropriate amount of uranium standard solution on an analytical balance, put 6 parallel portions (not less than 6 portions) into a small beaker, then accurately weigh according to different proportions, add an appropriate amount of 235U diluent solution respectively, mix evenly and evaporate to dryness. Accurately determine the uranium isotope composition of the mixture according to procedure 86.1.2.
The concentration of 235U in the diluent solution is
Rock Mineral Analysis Volume 4 Resource and Environmental Investigation and Analysis Technology
Where: c235t and c238N are respectively The molar concentration of 235U in the diluent solution and 238U in the uranium standard solution, mol/g; R is the 238U/235U ratio; the symbols t, N, and m in the lower right corner represent the 235U diluent, the uranium standard solution, and the mixture of the two respectively; mN and mt are the masses of weighed uranium standard solution and diluent solution respectively, g.
C.205Pb+233U mixed diluent. Accurately weigh appropriate amounts of lead and uranium standard solutions on an analytical balance, place 6 parallel portions (not less than 6 portions) of each into a small beaker, and then accurately add an appropriate amount of 205Pb+ to each portion in different proportions. 233U diluent solution, mix evenly and evaporate to dryness, and accurately determine the lead and uranium isotope compositions of the mixture according to the procedure 86.1.2.
The concentration of each isotope in the diluent solution is:
Rock Mineral Analysis Volume 4 Resource and Environmental Investigation and Analysis Technology
The concentration of 233U in the diluent solution The amount concentration is:
Rock Mineral Analysis Volume 4 Resource and Environmental Investigation and Analysis Technology
In the two formulas: R is the ratio of 206Pb/205Pb and 238U/233U respectively; the lower right corner is marked t , N and m respectively represent the corresponding diluent, standard solution and the mixture of the two; c205t and c233t respectively represent the molar concentration of 205Pb and 233U in the mixed diluent, mol/g; c206N and c238N respectively represent the 206Pb and 233U in the lead standard solution. The molar concentration of 238U in uranium standard solution, mol/g; mN and mt represent the mass of the corresponding standard solution and diluent solution respectively, g; (206Pb/205Pb)t, (207Pb/205Pb)t, (208Pb/205Pb)t , (204Pb/205Pb)t respectively represent the corresponding isotope ratio of 205Pb diluent.
Appendix 86.1B Uranium and lead isotope reference materials
Table 86.1 Isotope abundance and isotope ratio of uranium and lead isotope reference materials
References and references
Li Zhichang, Lu Yuanfa, Huang Guicheng. 2004. Radioisotope geological methods and progress. Wuhan: China University of Geosciences Press Isotope geological sample analysis method (DZ/T0184.1-1997～DZ/T0184 .8-1997) [S]. 1997. Beijing: China Standards Press, Chinese Academy of Geological Sciences Isotope Research and G5 Testing Center. 1997. Implementation Rules for Isotope Geological Sample Analysis Methods (ZBGC01-97~ZBGC04-97) (internal information)
The author of this section: Li Zhichang (Yichang Institute of Geology and Mineral Resources, China Geological Survey)