This method is suitable for zircon in different types of magmatic rocks, and it has more advantages in determining zircon with low uranium and radioactive lead content in basic magmatic rocks and zircon with fine crystals in young volcanic rocks. Because this method allows weighing larger samples (milligram level), it can generate strong lead ion current in mass spectrometry analysis to ensure the determination accuracy. The disadvantage is that a sample may contain many types of zircon, and the measurement result is the average of their different age information, which intuitively shows that the three U-Pb ages obtained by measuring a sample at the same time are obviously inconsistent. Therefore, attention should be paid to the study and reasonable selection of samples before determination.
Firstly, the surface of zircon crystal is treated with dilute acid, and the sample is dissolved with hydrofluoric acid. U and Pb were separated and purified by hydrochloric acid with different concentrations on the colored layer of anion exchange column. Lead isotope analysis was carried out on TIMS, and the concentrations of lead and uranium were determined by isotope dilution method. According to equations (86.9) ~ (86. 12), the time from the formation of U-Pb system in minerals to the present, that is, the crystallization age of minerals, can be calculated directly or by using the graphic method of U-Pb coincidence curve. Because lead pollution is everywhere, in addition to the requirements of measurement accuracy, reducing the whole process background of lead is the key.
The allowable error of this method in determining uranium and lead content is 65438 0.5%, and the determination accuracy of lead isotope ratio of 207Pb/206Pb should be better than 0.05%. When the age of the tested sample is 100 ~ 1000ma, the relative deviation of the age value should be less than 5% at the 95% confidence level.
Instruments and equipment
Thermoelectric ionization mass spectrometers MAT260, MAT26 1, MAT262, VG354, TRITON, etc.
Matching equipment of spot welder mass spectrometer.
The filament preheating device of mass spectrometer is a complete set of equipment of mass spectrometer.
Micro-sampler10μ l and 50μ l
Teflon beaker 10mL and 30mL.
Fluoropolymer (F46) reagent bottle 500mL and 2000mL.
Fluoropolymer (F46) washing bottle 500 ml.
Dropping bottle of fluoropolymer (F46) 30ml.
500 ml of fluoropolymer (F46), used in a double-bottle sub-boiling still.
Timely reagent bottle 2000mL.
Timely Asia is still boiling.
The autoclave consists of a 30 ml PTFE tank, a fluoroplastic heat shrinkable sleeve and a stainless steel jacket.
The ion exchange column adopts quartz tube or fluoroplastic heat shrinkable tube, and the lower part is embedded with timely sieve plate or polypropylene sieve plate to ensure that the resin installed on it does not leak. Specification: the upper part has an inner diameter of 7mm and a height of 50mm, and the lower part (resin bed) has an inner diameter of 5mm and a height of 26 mm
Timely dropper.
Triangular glass bottle 250mL.
Glass beaker 3000mL.
Water purification system.
Paraffin film for laboratory use.
The sensitivity of analytical balance is 0.00001g.
Electric heating plate (temperature controllable).
Ultrasonic cleaner.
Stainless steel constant temperature oven
Ware cleaning
All vessels are soaked and boiled in (1+ 1) pure hydrochloric acid and (1+ 1) pure nitric acid for 24 hours, then soaked and boiled in ultra-pure hydrochloric acid or nitric acid, rinsed with deionized water and ultra-pure water in turn, and finally washed with ultra-pure water at low temperature in an air purification cabinet.
After cleaning the sample dissolving tank in the autoclave according to the above procedures, add 1 ml ultra-pure hydrofluoric acid and a drop of ultra-pure nitric acid, put them in a stainless steel jacket, tighten them, put them in a stainless steel oven, and finally rinse them with ultra-pure water in an ultra-clean cabinet at (180 10)℃ and bake them.
Reagents and materials
The secondary distilled water of deionized water is purified by Milli-Q water purification system.
Ultra-pure deionized water is distilled by a timely distiller.
Ultra-pure hydrochloric acid is purified with (1+ 1) excellent pure hydrochloric acid in a timely still, and the actual concentration is calibrated with sodium hydroxide standard solution. Further prepared into the required concentration.
Ultra-pure nitric acid was purified with (1+ 1) pure nitric acid in a timely still, and the actual concentration was calibrated with sodium hydroxide standard solution. Further prepared into the required concentration.
Ultra-pure hydrofluoric acid is prepared by two bottles of sub-boiling distillers corresponding to fluoroplastics (F46).
The purity of acetone is very high.
The purity of anhydrous ethanol is very high.
235U diluent is dissolved in 3mol/LHCl, and the abundance of 235U is >: 90%. See appendix 86. 1A for concentration calibration.
208Pb diluent dissolved in 3mol/LHCl, 208Pb > abundance: 99.9%. See appendix 86. 1A for concentration calibration.
Strongly basic anion exchange resin Biorad AG 1× 8 (200 ~ 400 mesh) or Dowex 1× 8 (200 ~ 400 mesh) or better resin with similar performance.
Anion exchange resin column Prepare to pour about 100g200 ~ 400 mesh AG 1×8 anion exchange resin into a 250mL beaker, soak it in absolute ethanol for more than 24h, stir it with a glass rod for several times, pour out the ethanol, dry it, and rinse it with deionized water. Soak in Youchun (1+ 1) hydrochloric acid for more than 24 hours, constantly stir with a glass rod, pour out the hydrochloric acid, rinse it with ultra-pure water, and then transfer it into 200mL reagent bottle and soak it in water for long-term use. Suck a small amount of paste resin from reagent bottle with a dropper, and then put it into a cleaned exchange column (or fluoroplastic). The resin bed is 26 mm high, 5 mm in diameter and about 0.5 ml in volume. Eluting dynamically with 20mL( 1+ 1) ultrapure hydrochloric acid and ultrapure water respectively, and finally balancing with 5mL3mol/L ultrapure hydrochloric acid for later use. After each batch of samples is separated, it is necessary to disassemble the chromatographic column, discard the used resin and install new resin according to the above procedures.
Ultrapure phosphoric acid c( 1/3H3PO4)=0.5mol/L was prepared by cation exchange and purification with ultrapure phosphoric acid.
Silica gel is a colloidal solution formed by ultra-fine spectrum pure silica (SiO2 _ 2) and dilute ultra-pure nitric acid under ultrasonic wave.
The saturated solution of borax is dissolved with ultrapure water (Na2B4O7 10H2O).
Isotope reference materials NBS-98 1, NBS-982 and NBS-983.
Uranium isotope reference material uranium -500.
Lead reference material.
Spectral purity uranyl nitrate of uranium reference material.
The annular band of ion source filament is18mm× 0.03mm× 0.8mm. ..
Sample selection and pretreatment
1) sample collection. Zircon and other accessory minerals are generally selected from rock samples, and the collection amount of rock samples depends on the zircon content. For intermediate-acid magmatic rocks (such as granite), if only one zircon can be seen in the rock slice, it will be enough to collect about 10kg, and the sampling amount of basic rocks will increase accordingly. In the place where there is no fresh outcrop due to strong weathering, you can choose to wash the semi-weathered shell with casserole on the spot, select the heavy sand sample bag, and then go back to the room for further mineral processing.
2) Zircon separation.
A. break the sample. Before the sample is crushed, the site should be cleaned strictly, and the dust on the work site and the table should be blown away by high-pressure air. Before each sample crushing, all parts of the sample crusher should be removed, washed with water and scrubbed with alcohol. After recycling, put a piece of white paper under it and let the machine idle for 5 minutes, depending on whether there is any debris falling. If it is unqualified, repeat the operation. Before a large number of rocks start to break, put a small part in, and then throw it away after breaking. The crushing granularity of rock depends on the thickness of rock structure. The principle is that large zircon crystals should not be turned into crystal fragments due to excessive crushing, nor should zircon crystals generally have crystals because of insufficient crushing particle size. For granite, generally through the 0. 1mm and 0.25mm sieve, from
B. shake the table to sort. & lt Put rock powder of 0. 1mm and 0. 1 ~ 0.25 mm on the shaking table respectively, and under the action of running water, use the principle of gravity separation to select heavy minerals. Before loading the sample, brush the surface of the plastic bed with 6mol/LHCl and rinse it with water.
C. heavy liquid separation and electromagnetic separation. The heavy minerals separated by shaking table are first absorbed by U-shaped magnet to remove strong magnetic minerals such as magnetite, and then separated by heavy liquid (diiodomethane and tribromomethane) or washed by small sand table to further enrich zircon. When a large amount of pyrite is mixed in the sample, it is difficult to select pure zircon by the above method. At this time, the sample can be poured into 7mol/LHNO3 and heated slowly. After 2 ~ 3 minutes, pyrite gradually floats to the liquid surface, and zircon still sinks to the bottom of the container. Pyrite floating on the liquid surface can be quickly and accurately poured out many times. This method is very effective for the separation of pyrite and zircon. It is better to use separating funnel and annular electric furnace for heating. Finally, zircon samples are divided into several sub-samples according to electromagnetic intensity and particle size by using electromagnetic instruments, sometimes using pocket screens.
D. selection under binocular microscope. The purity of zircon can reach 100%. At the same time, the mineralogical characteristics of zircon were observed and studied, including color, transparency, luster, crystal morphology, wear degree of crystal surface, crack and metamorphism, inclusion and inclusion characteristics. , and make records. If conditions permit, the cathodoluminescence and backscattered electron images will be further studied, and the external and internal structural characteristics of the crystal will be preserved.
E. cleaning zircon samples. Put the zircon to be tested in a 10mL PTFE beaker, soak it in (1+ 1)HNO3 for 30-60 min, and then treat it in an ultrasonic cleaner for 5min. Pour out nitric acid, rinse it with ultrapure water, treat it with ultrapure acetone for 5min, pour out acetone and slightly heat it with ultrapure water for 30-60min, and then treat it in an ultrasonic cleaner for 5min.
Chemical separation process of uranium and lead
1), dissolving, and adding 238U diluent. Weigh 2 ~ 5 mg (accurate to 0.0 1mg) of pretreated zircon and put it into the sample dissolving tank (americium source can be placed in the balance against the weighing pan to eliminate static electricity, otherwise fine zircon crystals will be easily adsorbed on the container wall by static electricity, which is difficult to handle). Add 2 ~ 3 ml of ultra-pure HF and 2 ~ 3 drops of ultra-pure HNO3, cover it, put it on a heat shrinkable sleeve, tighten it in a stainless steel sleeve, put it in a stainless steel oven, take it out of the oven at (180 10)℃ and cool it to room temperature. Open the stainless steel casing, clean the outer wall of the closed tank with ultra-pure water, and open the closed tank to check whether zircon is completely dissolved. When it is confirmed that zircon is completely decomposed, pat the closed tank carefully to make the liquid beads on the inner wall gather at the bottom, slowly evaporate on the electric heating plate at 1 10℃, cool to room temperature, add 2 ~ 3 drops of 238U diluent, and weigh (accurate to 0.0000 1g) (a film needs to be covered on the closed tank when weighing). Add 2mL3mol/L ultrapure hydrochloric acid into a closed tank filled with 238U diluent, cover it again, put it into a stainless steel sleeve, and then put it into an oven, and heat it overnight at 180℃ to ensure that the sample and 238U diluent can be completely mixed. If it is found that zircon is not completely decomposed, it needs to be restored to its original state and put into the oven again, and the dissolution time of the sample should be appropriately extended.
2) liquid separation. Take two groups of 10mL fluoroplastic beakers and label them with ID and ic respectively. Take out the closed tank according to the above procedure, pour the solution in which zircon has been completely decomposed and completely balanced with 238U diluent into ID and IC beakers according to the ratio of 1: 2, accurately weigh the mass of each solution, add 3 ~ 5 drops of 208Pb diluent to ID, and weigh (accurate to 0.0000 1g). Shake carefully and let the two mix completely. The ID part is used to determine the concentrations of U and Pb, and the IC part is used to determine the isotopic composition of lead.
3) Separation of uranium and lead. Pour ID and IC solutions into two prepared anion exchange columns respectively. After the solution was dried, 3 ml of 3.0 mol/L ultrapure hydrochloric acid was added to leach zirconium plasma. After the solution is dried, add 3mL( 1+ 1) ultra-pure HCl to analyze lead, and then use a 10mL fluoroplastic beaker. Finally, 3ml of ultrapure water was used to analyze uranium, which was changed to 65438. In order to increase the strength, the uranium samples in ID and IC samples can be combined for mass spectrometry analysis. Evaporating the received solution on an electric heating plate at the temperature of 1 10℃, and covering the film for mass spectrometry analysis.
Isotopic analysis of uranium and lead
1) lead isotope determination. The ID of IC samples containing 208Pb diluent and IC samples without diluent were determined respectively. The following operation flow takes MAT26 1 mass spectrometer as an example, and other types of mass spectrometers are similar.
A. load the sample. Pretreatment of rhenium tape: clean the rhenium tape with absolute ethanol, spot weld the rhenium tape on the filament bracket with a spot welder, insert the bracket on the ion source turntable in turn, and put it into the filament preheating device as a whole. After vacuumizing to n× 10-5Pa, the rhenium bars are energized according to the preset procedure, and each rhenium bar is pre-sintered for 0/5 min at the current of 4 ~ 6a to remove uranium and lead impurities in the rhenium bars.
Lead isotope analysis adopts single-band source. Move the turntable burned with rhenium belt to the ultra-clean working cabinet, remove the ionization belt and turn on the power supply of the evaporation belt. A drop of silica gel and a drop of saturated borax solution were added to the central part of the evaporation zone with a microsampler, and then dried at a current of about 65438±0A. Add 2 ~ 3 drops of diluted ultra-pure phosphoric acid to the samples (id and ic) to dissolve the samples, then add the samples drop by drop to the evaporation belt covered with a layer of silica gel-borax, and heat with current to gradually evaporate the water. Increasing the current makes the white smoke on the rhenium belt disappear and the residual acid radicals are completely driven away. Then continue to increase the current to burn the rhenium belt to dark red, and then quickly reduce the current to zero. Turn the turntable to the next position and add the next sample according to the same procedure. After the sample loading procedure is completed, insert the ionization band in situ and clamp the shield. At this time, the ionization zone only plays a supporting role. Send the whole turntable into the ion source of mass spectrometer, and start the vacuum system to vacuum.
B. lead isotope data collection. When the vacuum degree of the ion source reaches n× 10-6Pa, open the isolation valve of the analysis room, apply current to the evaporation belt, and slowly raise the temperature. At this time, the vacuum degree drops, so be careful not to drop too fast. Heating and vacuumizing are carried out alternately. When the vacuum degree in the analysis room is greater than 5× 10-6Pa and the temperature in the evaporation area is about 100 ~ 1300℃, the lead ion current is searched in the range of 204 ~ 208 in the manual state of the measuring system. Carefully adjust the current applied to the evaporation belt, and constantly adjust the peak center, so that the lead ion current can reach enough intensity (10-13 ~10-1a) and remain stable for a long time. Start the automatic program to collect lead isotope ratio data of 204Pb/206Pb, 207Pb/206Pb and 208Pb/206Pb.
Depending on the type of mass spectrometer used, multiple receivers are used to receive the lead isotope ion current at the same time or a single receiver is used for peak-hopping scanning. Each sample collected 4 ~ 6 data blocks, and each data block consisted of 8 ~ 10 scans. The data are automatically processed by computer, and the average value and relative deviation of lead isotope ratio are given.
2) Uranium isotope analysis.
A. load the sample. Uranium isotope analysis adopts single-band source or dual-band source. Add a drop of saturated solution of silica gel and borax in the center of evaporation area with a micro sampler, and then slowly heat and dry it with current in turn. In addition, take 2 ~ 3 drops of phosphoric acid to dissolve the sample with a micro sampler, carefully drop it on the dry propellant, increase the current to drive away the acid radical, burn the rhenium belt to dark red, and quickly reduce the current to zero. In the future, the operation will be the same as lead isotope.
B. u isotope data collection of uranium. The basic operation is the same as lead isotope, but the temperature of the collected data is above 1300℃, the received ion is UO2+, the mass number is 267 ~ 270, and the ratio of the collected isotope is 238 u/235 u.
3) mass fractionation correction. Because the three ratios of Pb isotopes in nature are variable and cannot be used as standard values, it is impossible to directly correct mass fractionation in Pb isotope analysis. The indirect calibration method is to determine the international uranium and lead reference materials, find out the deviation coefficient between the measured value and the standard value, and then correct the corresponding ratio of the samples. The problem of this calibration method is that the standard substance and the sample are measured in two independent operations, and the amount of the sample on the re band (filament) (generally much higher than the former), chemical composition, excitation state, emission temperature, data acquisition time and other conditions are different from each other, so the mass fractionation state is likely to be different, and the calibration effect is uncertain. In addition, double dilution method can be used for mass fractionation correction, that is, two Pb diluents enriched in 204Pb and 207Pb (or 206Pb) are added to the sample at the same time, and the correlation ratio of the mixture is collected at the same time in one determination for correction. This method requires high accuracy of Pb isotope analysis and complicated experimental procedures, so it has not been widely used at present. In view of the above reasons, Pb isotope analysis generally does not do mass fractionation correction, and only collects data according to experience in the best analysis state, and as much data as possible to minimize mass fractionation.
Determination result calculation
Only the basic calculation steps and formulas are involved here.
1)Pb content calculation.
Content of 206Pb in A.id sample:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Where: 206ppb ID is the amount of 206Pb in ID sample, and molC208t is the molar concentration of 208Pb in lead dilution solution, mol/g; M208t is the mass of lead diluent, g; R is the isotope ratio of 206Pb/208Pb; P, T and M in the lower right corner respectively represent the sample (without deducting the background), diluent and their mixture; Id and ic in the upper right corner represent ID and IC samples respectively.
B. Lead isotope content in the whole sample:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Where: 206ppbp, 207ppbp, 208ppbp and 204ppbp are the amounts of 206Pb, 207Pb, 208Pb and 204ppbp in the whole sample (without deducting the background), and molMID and mic are the mass of ID and ic samples respectively, g;
R7/6, R8/6 and R4/6 are the lead isotope ratios of the samples: 207Pb/206Pb, 208Pb/206Pb and 204Pb/206Pb respectively, which were obtained after IC sampling.
C. the amount of Pb isotope in the whole sample after deducting the background:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
In formula (86.18) to formula (8.2 1), the amounts of 206Pb, 207Pb, 208Pb and 204Pb are 206Pb, 207Pb, 208Pb and 204Pb, respectively, in terms of moles; Marks S and P in the lower right corner represent the quantity and actual measurement after deducting background lead, respectively; Pbb is the amount of background lead in the whole process, and mol, Fb206, Fb207, Fb208 and Fb204 are the background lead isotope abundances of 206Pb, 207Pb, 208Pb and 204Pb, respectively, which are obtained through actual measurement.
After deducting the background lead, the lead content of the whole sample is:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Where: wPb is the mass fraction of lead in the sample, μg/g；/g; Ms is the mass of the weighed sample, g; MPb is the molar mass of lead, g/mol.
D. Content of radioactive lead in the sample after deducting ordinary lead:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
In the formula, 206Pbγ, 207Pbγ and 208Pbγ are the amounts of 206Pb, 207Pb and 208Pb in the sample after deducting ordinary lead, and the units are molR(6/4)s, R(7/4)s and R(8/4)s, which are 206Pbs, 207Pbs, 208Pbs and 2000pbs after deducting background, respectively. R(6/4)c, R(7/4)c and R(8/4)c are the ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb respectively, which are contemporary with the sample. In practice, this set of ratios is determined by iterative method according to the evolution model of lead on the earth.
The total amount of radioactive lead in the sample (Pbγ, mol) is:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
2)U content calculation:
A. the amount of 238U and 235U in the sample (mol):
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
B. mass fraction of uranium in the sample:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Formula (86.27) to formula (86.29): 238Us and 235Us are the amounts of 238U and 235U in the sample, respectively, in moles; Wu is the mass fraction of uranium, μg/g；/g; R is the ratio of 238U/235U; The symbols S, T and M in the lower right corner represent samples, diluents and their mixtures respectively; C235t is the molar concentration of 235U in the diluted solution, mol/g; M235t refers to the mass of diluent, g; Ms is the sample mass, g; Ub is the whole process background of u and mol; μ is the molar mass of uranium, g/mol.
In nature, the isotopic half-life of thorium is only 232Th, so the content of thorium can only be determined by general chemical methods, but not by isotope dilution method.
3) Age calculation. There are two methods at present.
A. single sample. Substitute the amounts of radioactive lead 206Pb and 207Pb obtained from (86.23) and (86.24) and radioactive lead 238U and radioactive lead 235U obtained from (86.27) and (86.28) into (86.9) and (86. 10) respectively to obtain a sample.
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Substituting the formula (86. 1 1) to get the age of t207/206. Under the condition that zircon U-Pb system has been evolving in a single-stage closed state since crystallization, the ages of t206/238, t207/235 and t207/2063 should be consistent within the measurement error of 5%. If they are inconsistent, it is difficult to determine the age of minerals, and the age of t206/238 is generally taken as the reference value. B. schematic diagram of uniform curve. When the U-Pb system in minerals does not evolve in a closed state, the ages of t206/238, t207/235 and t207/2063 are obviously inconsistent. For a group of samples, it is more appropriate to use the graphic method of consistent curve at this time. The conditions for applying this method are that the samples have the same crystallization age and evolution history, and the isotopic composition of common lead is the same. At present, the two-stage evolution model of U-Pb system is mature. On the coordinate diagram of 206Pb/238U-207Pb/235U, the samples meeting the above conditions will be able to form a straight line by least square fitting, and the age of the upper and lower intersection of the straight line and the consistent curve is the required age.
At present, the most popular zircon U-Pb coincidence curve age calculation program is Ludwig( 1996) program provided by the US Geological Survey and its latest version. This method is not only in the form of 206Pb/238U-207Pb/235U, but also in the form of 207Pb/206Pb-206Pb/238U, which is suitable for young samples with short age interval in two stages.