Synthesis Method of Monazite Standard Sample and Its Application in Dual Dating

Abstract

The present invention relates to the technical field of monazite dating, specifically to a synthesis method of monazite standard samples and the application of dual dating methods. It includes the following steps: weighing raw materials according to the stoichiometric ratio of U, Th, and Pb concentrations of the standard sample to be prepared, processing the raw materials into powder, and then grinding, compressing, and calcining them multiple times; fully grinding the sample after heat cycle treatment, mixing it with a flux and grinding evenly, and then pouring it into a crucible; placing the crucible in a high-temperature furnace to fully melt and uniformly mix all substances, quickly taking out the crucible, and rapidly quenching it to form a uniform yellow transparent phosphate glass, thereby artificially synthesizing a phosphate glass standard sample to be used for measuring the U concentration in monazite by laser ablation-inductively coupled plasma mass spectrometry, and further achieving the simultaneous and accurate acquisition of the U-Pb age and fission track age of monazite.

Description

Technical Field

[0001] The present invention relates to the technical field of monazite dating, and specifically, to a synthesis method of a monazite standard sample and its application in dual dating methods. Background Art

[0002] Monazite is a phosphate mineral rich in radioactive elements such as U and Th. By measuring the concentrations of uranium element isotopes and their decay product Pb isotopes in monazite, the U-Pb age of monazite can be obtained using decay equations, thereby obtaining information related to mineral crystallization, evolution, etc. In the field of earth sciences, the age and thermal history information of geological bodies can be obtained by measuring the concentrations of radioactive elements and their decay products in monazite.

[0003] For example, CN116609421B involves a dating method based on monazite fission tracks. The method includes: etching multiple monazite samples; placing the etched multiple monazite samples under a fission track analyzer for observation, counting the track densities of each monazite sample, and marking the corresponding test position coordinates; according to the marked test position coordinates, using a laser ablation inductively coupled plasma mass spectrometer to perform laser ablation on the monazite samples and selected standard samples, and calculating the 238 U content in the monazite samples based on the laser ablation data; based on the track densities of each monazite sample and the 238 U content in each monazite sample, analyzing and determining the fission track ages of multiple monazite samples. This dating method breaks through the technical barrier that monazite cannot be used to measure the quantity of radioactive parent bodies by the external detector method due to its high Gd content strongly absorbing neutrons, realizes the determination of the fission track ages of monazite minerals, and makes up for the blank in the existing technology.

[0004] In the above dating method, laser ablation is performed using a laser ablation inductively coupled plasma mass spectrometer. Laser ablation inductively coupled plasma mass spectrometry (ICP-MS) is a widely used isotope measurement method. When using the ICP-MS method to measure the concentration of a specific isotope in a sample, a sample with a known concentration of this element (i.e., a standard sample) is required to correct its intensity. In addition to concentration determination, it is also required that the distribution of this element in the standard sample is uniform. Currently, commonly used standard samples for measuring silicate minerals include silicate glass standard sample NIST610, etc. However, there is currently no such standard sample with a known U element concentration for monazite. Due to matrix effects, glass NIST610, etc. cannot accurately calibrate the U element content in monazite (CePO4).

[0005] In summary, the technical difficulty in accurately determining the U concentration in the above dating method is due to the lack of a standard sample for determining the content of the fission track parent U that matches the monazite matrix.

[0006] In order to overcome the deficiencies of the prior art, a phosphate glass with a known U content and good uniformity is prepared as a standard sample to calibrate the relationship between the ICP-MS signal intensity of monazite and the U element content, and to correct experimental errors such as instrument signal drift and mass fractionation of lasers and mass spectrometers, so as to achieve accurate measurement of the uranium content of monazite. Therefore, a synthesis method of monazite standard sample and its application in double dating are proposed. Summary of the Invention

[0007] The purpose of the present invention is to provide a synthesis method of monazite standard sample and its application in double dating to solve the problems raised in the above background technology.

[0008] To achieve the above purpose, the present invention aims to provide a synthesis method of monazite standard sample, including the following steps:

[0009] S1. Weigh cerium phosphate, uranium oxide, thorium dioxide, lead carbonate and phosphorus pentoxide according to the stoichiometric ratio of U, Th, and Pb concentrations of the standard sample to be prepared, and then grind them thoroughly in a mortar until they become a uniform and delicate flour-like powder.

[0010] S2. After compacting with a tablet press and then calcining in a heating furnace, grind and compact it again and then put it into the heating furnace for calcination, repeating 3 - 4 times.

[0011] S3. After thoroughly grinding the sample that has completed the thermal cycle treatment, mix it with a flux in a ratio of 15:85 and grind it evenly, then pour it into a crucible.

[0012] S4. Place the crucible in a high-temperature furnace to fully melt and uniformly mix all substances, then quickly take out the crucible while keeping the heating furnace at 1400 °C, and quickly quench it to form a uniform yellow transparent phosphate glass.

[0013] As a further improvement of this technical solution, in S1, thoroughly grind in a mortar to form a uniform and delicate flour-like powder with a mesh size of 280 - 320.

[0014] As a further improvement of this technical solution, in S2, grind and compact the powder and then put it into a heating furnace for calcination at 1200 - 1600 °C for 24 h.

[0015] As a further improvement of this technical solution, in S3, the flux is potassium metaphosphate.

[0016] As a further improvement of this technical solution, in S3, the crucible is a platinum crucible.

[0017] As a further improvement of this technical solution, in S4, place the platinum crucible in a high-temperature furnace and keep it at 1200 - 1500 °C for 24 hours.

[0018] Correspondingly, the application of the method for synthesizing monazite standard samples in double dating methods.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] In the method for synthesizing monazite standard samples and its application in double dating methods, raw materials are weighed according to the stoichiometric ratios of U, Th, and Pb concentrations of the standard samples to be prepared as needed, and after being processed into powders, they are ground, compacted, and calcined multiple times, and then fully melted and mixed with a flux to form a yellow transparent phosphate glass, thereby artificially synthesizing a phosphate glass standard sample for measuring the U concentration in monazite by laser ablation-inductively coupled plasma mass spectrometry, and further accurately obtaining the U-Pb age and fission track age of monazite simultaneously.

[0021] In the method for synthesizing monazite standard samples and its application in double dating methods, a correction scheme for U content data of monazite is also proposed. By adopting a secondary correction strategy, the relationship between the ICP-MS signal intensity of monazite and the U element content is calibrated, and experimental errors such as instrument signal drift and mass fractionation of the laser and mass spectrometer are corrected, thereby achieving accurate measurement of the uranium content of monazite. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a flowchart of the present invention;

[0023] Figure 2 is a test result diagram of the U content of the samples of the present invention;

[0024] Figure 3 is a schematic diagram of the homogeneity of the U content of the standard glass of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0026] In monazite 238 Spontaneous fission of U will form damage in minerals, namely fission tracks. After the tracks are enlarged to the micron level by chemical etching methods, the track density and length information can be counted under an optical microscope. Since fission tracks are formed by the 238 fission of U in minerals, the number of fission tracks also conforms to the fission equation with the change in the number of 238 U in minerals. The fission track age of monazite can be obtained by measuring the number of 238 U in minerals and the number of spontaneous fission tracks.

[0027] In addition, when fission tracks are heated, the track length will be shortened or even disappear (density decreases), which is called fission track annealing. By measuring the degree of fission track annealing, the thermal history of minerals can be inverted. Therefore, this method is also called the fission track thermochronology method. Thus, whether obtaining the monazite U-Pb age or the monazite fission track age, it is necessary to accurately measure the 238 U content.

[0028] Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a widely used method for measuring trace element content. Due to various error sources such as matrix effect, signal drift, and mass discrimination effect during the testing process, accurate correction analysis of the signals collected by the mass spectrometer is required to obtain accurate trace element content data of the sample. In actual work, the most commonly used data correction method is the combined correction of external standard + internal standard element + standard sample-sample cross method (SSB).

[0029] Among them, the external standard currently mostly uses the silicate glass standard samples NIST610 / 612 / 614 series developed artificially by the National Institute of Standards and Technology of the United States (Na, Al, Si, and Ca are the main elements in NIST610, and the contents of Na2O, Al2O3, SiO2, and CaO are 13.4%, 1.95%, 69.7%, and 11.4% respectively, and the contents of most of the other more than sixty trace elements are about 500 ppm); the internal standard element generally selects relatively light main elements that are evenly distributed and have known concentrations in minerals as the internal standard (Longerich & Jackson, 1996; Paul et al., 2023; Walas et al., 2014), such as Si in zircon (ZrSiO4) and Ca in apatite (CaPO4).

[0030] Taking the determination of U content in zircon as an example, the specific testing and correction methods are as follows:

[0031] (1) Measure the signal intensities CPS of the internal standard element Si and the element to be measured U in the external standard substance NIST610 through the mass spectrometer 610-Si and CPS 610-U , as well as the signal intensities CPS of the internal standard element and the element to be measured in the sample sam-Si and CPS sam-U ;

[0032] (2) Through the above signal intensities, combined with the content C of the internal standard element in the external standard substance NIST610 610-Si and the content C of the internal standard element in the samplesam-Si , the matrix effect difference coefficient K between the sample and the external standard substance is calculated, where K = (CPS sam-Si / C sam-Si ) / (CPS 610-Si / C 610-Si );

[0033] (3) Apply the above difference coefficient to the calculation of the element U to be measured, that is, K = (CPS sam-U / C sam-U ) / (CPS 610-U / C 610-U ). Since the U content in the external standard NIST610 is a known quantity, and the signals of the element U to be measured in NIST610 and the sample have been measured by the mass spectrometer, the content C sam-U of U in the sample can be calculated through the above equation.

[0034] Therefore, there are the following technical defects in the above test and correction scheme, which will lead to deviations in the test results:

[0035] 1. Monazite is a phosphate mineral containing cerium (La) and lanthanum (Ce), with the molecular formula (Ce, Y, La, Th)PO4. Compared with the above NIST610 series of reference materials, the matrix difference is very large. Whether the above correction scheme can accurately correct the matrix effect deviation must be considered;

[0036] 2. The main elements (%) in monazite are mainly Ce, La, Th, and P. By comparing the content information of the NIST standard glass, it can be found that the main elements in monazite are trace elements (ppm) in the NIST series of reference materials, and the content difference can be 4 - 5 orders of magnitude. This difference will inevitably bring correction deviation to the above correction scheme;

[0037] 3. At present, there is a lack of a standard sample of monazite with a known U element concentration and uniform content. There are only two natural single mineral reference samples 44069 and RW - 1 (Ling Xiaoxiao et al., 2017) reported in the literature for U - Pb isotope dating of monazite. Among them, 44069 is a small - particle sample with a diameter of only dozens of microns, and it has been exhausted internationally and is basically unavailable; RW - 1 was published by the Institute of Geology and Geophysics, Chinese Academy of Sciences. The particles are slightly larger and it is also difficult to use on a large scale. Moreover, there are inevitable defects in the homogeneity of natural minerals, making it difficult to carry out the above correction scheme.

[0038] In summary, the existing SiO2 glass reference materials NIST610 and NIST612 cannot accurately calibrate the U content in the phosphate mineral monazite. The present invention aims to overcome the existing deficiencies and prepare a phosphate glass with confirmed U content and good homogeneity as a standard sample to calibrate the relationship between the ICP-MS signal intensity of monazite and the U element content, and correct experimental errors such as instrument signal drift and mass fractionation of the laser and mass spectrometer, so as to achieve accurate measurement of the uranium content in monazite.

[0039] Therefore, please refer to Figure 1 As shown, the object of the present invention is to provide a method for synthesizing a monazite standard sample, which includes the following steps:

[0040] S1. After weighing cerium phosphate (CePO4), uranium oxide, thorium dioxide (ThO2), lead carbonate (PbCO3) and phosphorus pentoxide (P2O5) according to the stoichiometric ratio of the U, Th, and Pb concentrations of the reference sample to be prepared, grind them thoroughly in a mortar until a uniform and delicate powdery substance with a particle size of 280-320 meshes is formed;

[0041] S2. After compacting with a tablet press and then placing it in a heating furnace for calcination at 1200-1600 °C for 24 hours, grind and compact it again and then place it in the heating furnace for calcination, repeating 3-4 times;

[0042] S3. After thoroughly grinding the sample after the heat cycle treatment, mix it with a flux such as potassium metaphosphate (KPO3) in a ratio of 15:85 and grind it evenly, then pour it into a platinum crucible. Mixing in a ratio of 15:85 can avoid the formation of CePO4 crystals during cooling and quenching;

[0043] S4. Place the platinum crucible in a high-temperature furnace and keep it at 1200-1500 °C for 24 hours to allow all substances to melt and mix evenly. Then, quickly take out the platinum crucible from the heating furnace while keeping the furnace at 1400 °C to make it quickly quench to form a uniform yellow transparent phosphate glass, that is, the monazite standard glass;

[0044] Then divide the prepared phosphate glass into multiple batches, and then conduct data testing and homogeneity inspection on different batches of phosphate glass. Among them, the specific processes of data testing and homogeneity inspection are as follows:

[0045] Step 1. Conduct random U content determination experiments on different batches of phosphate glass. The testing process adopts the conventional laser in-situ testing process of monazite fission track and U-Pb double dating. The instrument for on-machine testing is a laser ablation quadrupole inductively coupled plasma mass spectrometer (LA-ICP-MS, models are ESL 193NWRUC and Agilent 7500a respectively);

[0046] Step 2: Using the digestion method, perform the solution method for U content determination on the standard glass sample;

[0047] Step 3: Verify the U content homogeneity of the standard glass through in-situ random experiment results; then, verify the accuracy and precision of the U content determination results by laser in-situ through the comparison with the results of the digestion sample solution method.

[0048] The following further illustrates the synthesis method and dual dating application of the monazite standard sample provided by the present invention by preparing the standard sample required for a dating method based on monazite fission tracks with the patent publication number CN116609421B. Example

[0049] The purpose of this example is to prepare a batch of monazite standard samples with U and Th contents of approximately 145 ppm. The specific steps are as follows:

[0050] S1: Weigh cerium phosphate (CePO4), uranium oxide, thorium dioxide (ThO2), lead carbonate (PbCO3), and phosphorus pentoxide (P2O5) according to the stoichiometric ratio of the U, Th, and Pb concentrations of the standard sample to be prepared, and then thoroughly grind them in a mortar until a homogeneous and delicate flour-like powder of 300 mesh is formed;

[0051] S2: After compacting with a tablet press and placing it in a heating furnace for calcination at 1400 °C for 24 h, grind and compact it again and then place it in the heating furnace for calcination, repeating this cycle 3 times;

[0052] S3: After thoroughly grinding the sample that has completed the thermal cycle treatment, mix it with a flux such as potassium metaphosphate (KPO3) in a ratio of 15:85 and grind it evenly, then pour it into a platinum crucible;

[0053] S4: Place the platinum crucible in a high-temperature furnace and maintain it at 1400 °C for 24 hours to allow all substances to fully melt and mix evenly. Then, quickly take out the platinum crucible from the heating furnace while maintaining the temperature at 1400 °C to make it rapidly quench to form a uniform yellow transparent phosphate glass, that is, the monazite standard glass, thus completing the preparation of the standard sample;

[0054] The masses of some raw materials used in this experimental example and the concentrations of the U and Th elements to be calibrated are shown in Table 1.

[0055] Table 1: The masses of some raw materials used in this experimental example and the concentrations of the U and Th elements to be calibrated

[0056] Raw material Weighing mass (g) Concentration of element to be calibrated (ppm) <![CDATA[Cerium phosphate (CePO4)]]> 5.0003 - <![CDATA[Uranium oxide (U3O8)]]> 0.0057 143 <![CDATA[Thorium dioxide (ThO2)]]> 0.0057 148 <![CDATA[Potassium metaphosphate (KPO3)]]> 28.3 -

[0057] After complete preparation, randomly take out some small pieces of glass solid from the crucible and use LA-ICP-MS to perform laser in-situ micro-area U content determination to verify the U content homogeneity of the monazite standard glass:

[0058] Randomly select the prepared standard glass pieces and perform the following sample preparation process for in-situ laser microarea determination:

[0059] (1) First, heat the heating stage to 170 °C (the temperature that can melt the glue);

[0060] (2) Stick the sample onto the polished sample stage;

[0061] (3) Make the height of the sample level with the screws on both sides of the polisher;

[0062] (4) Gently use the "8" - shaped grinding method on the polishing paper with the polisher to first polish a flat surface. First, rough - polish with 1200 - mesh sandpaper, and then perform fine - polishing successively with 2400 - mesh and 4000 - mesh sandpapers;

[0063] (5) Finally, polish with the polishing paper to obtain a scratch - free shiny surface;

[0064] (6) Take down the polished sample, melt the glue on the heating stage with the glass slide, place the sample on the glue, and then take down the glass slide from the heating stage.

[0065] From September 2024 to December 2024, within nearly three months, randomly select some standard glass sample particles for in - situ laser microarea U content determination experiments and data correction:

[0066] (1) On September 23, 2024, randomly select a small piece of sample, randomly establish a test sequence with a total of 20 measurement points. The original data number is 2024I23p00.B. The average value of the U content of the 20 measurement points after test correction is 167.00 ± 3.58 ppm;

[0067] (2) On October 18, 2024, randomly select two small pieces of samples. One is prepared into a conventional 1 - inch circular resin target, and the other is directly stuck on the glass slide. Randomly establish a test sequence. The two samples are respectively tested at 30 measurement points in total. The original data number is 24J18r00.B. The U content test results of the two samples are consistent. The average value of the 60 measurement points after correction is 154.39 ± 3.60 ppm;

[0068] (3) On October 28, 2024, randomly select three small pieces of samples. After sticking them on the glass slide and polishing, they are respectively randomly tested at 30 measurement points on the machine. The original data number is 24J28k00.B. The U content test correction results of the 90 measurement points are consistent, and the average value is 163.56 ± 1.92 ppm;

[0069] On November 12, 2024, two small pieces of samples were randomly selected, stuck on the glass slides and polished, and then 30 measuring points were tested on the machine respectively. The original data number was 24K12o00.B. The corrected results of the U content test for 60 measuring points were consistent, and the average value was 173.80 ± 1.36 ppm;

[0070] On November 13, 2024, two small pieces of samples were randomly selected, stuck on the glass slides and polished, and then 30 measuring points were tested on the machine respectively. The original data number was 24K12o00.B. The corrected results of the U content test for 60 measuring points were consistent, and the average value was 156.55 ± 1.44 ppm;

[0071] On November 14, 2024, two small pieces of samples were randomly selected, stuck on the glass slides and polished, and two testing sequences were established on the machine. The original data numbers were 24K14p00.B and 24K14r00.B respectively. 30 sample measuring points were completed respectively. The average value of the U content of the 30 measuring points in the first sequence was 159.33 ± 2.17 ppm, and that of the second sequence was 155.40 ± 1.33 ppm;

[0072] On December 9, 2024, two small pieces of samples were randomly selected, stuck on the glass slides and polished, and then 30 measuring points were tested on the machine respectively. The original data number was 24L09o00.B. The corrected results of the U content test for 60 measuring points were consistent, and the average value was 166.07 ± 5.93 ppm;

[0073] In summary, in the past three months, a total of 13 small pieces of samples were selected, 8 testing sequences were randomly completed, and a total of about 410 measuring points were measured. The test results are as Figure 2 、 Figure 3 shown. The average value of the 8 testing sequences was 161.94 ppm, the absolute external precision was 0.67 ppm, and the relative external precision was 0.41%, effectively proving that the U content of the standard glass was well homogeneous.

[0074] Accuracy test for the determination of U content in standard glass:

[0075] Two small pieces of standard glass samples were randomly selected and completed in the Rock Mineral Sample Preparation and Analysis Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. The testing instrument on the machine was an icap RQ type inductively coupled plasma mass spectrometer of ThermoFisher Company. Two parallel experiments were carried out, and the test results were 161.455 ppm and 158.075 ppm respectively; the determination results were completely consistent with the above laser in-situ microarea determination results within the error range.

[0076] In the present invention, raw materials are weighed according to the stoichiometric ratios of U, Th, and Pb concentrations of the reference samples to be prepared as needed. After the raw materials are processed into powders, they are ground, compacted, and calcined multiple times, and then fully melted and mixed with a flux to form a yellow transparent phosphate glass, thereby artificially synthesizing a phosphate glass standard sample for measuring the U concentration in monazite by laser ablation-inductively coupled plasma mass spectrometry, and further accurately obtaining the U-Pb age and fission track age of monazite simultaneously.

[0077] In addition, the present invention also proposes a correction scheme for the U content data of monazite, adopting a secondary correction strategy: First, NIST610 silicate glass is used as an external standard, and Ce is used as an internal standard element (the Ce content of the monazite standard glass is 88087 ppm calculated from the original formula). The Iolite data analysis software is used to obtain the U content result of the first correction. Then, using the UO2 content of 0.3% in the monazite U-Pb dating reference sample RW-1 (the Ce content of the internal standard element is 25.22%) reported in the Ling (2017) literature, the U content result corrected by NIST610 is corrected for the second time, that is, the relationship between the ICP-MS signal intensity of monazite and the U element content is calibrated, and experimental errors such as instrument signal drift and mass fractionation of the laser and mass spectrometer are corrected, so as to achieve accurate measurement of the uranium content of monazite.

[0078] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification are only preferred examples of the present invention and are not used to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing a monazite standard sample, characterized in that It includes the following steps: S1. After weighing cerium phosphate, uranium oxide, thorium dioxide, lead carbonate and phosphorus pentoxide according to the stoichiometric ratios of U, Th, and Pb concentrations of the reference sample to be prepared, grind them thoroughly in a mortar until they become a homogeneous and delicate powdery substance like flour; S2. After compressing with a tablet press and then placing it in a heating furnace for calcination at 1200 - 1600 °C for 24 h, grind and compress it again and then place it in the heating furnace for calcination, repeating 3 - 4 times; S3. After thoroughly grinding the sample that has completed the heat cycle treatment, mix and grind it evenly with a flux in a ratio of 15:85 and then pour it into a crucible; S4. Place the crucible in a heating furnace and keep it at 1200 - 1500 °C for 24 hours to fully melt and uniformly mix all substances. Then, quickly take out the crucible from the heating furnace maintained at 1400 °C to perform rapid quenching to form a uniform yellow transparent phosphate glass; In S3, the flux is potassium metaphosphate.

2. The synthesis method of the monazite standard sample according to claim 1, characterized in that: In S1, thoroughly grind in a mortar to form a homogeneous and delicate powdery substance like flour with a mesh size of 280 - 320.

3. The synthesis method of the monazite standard sample according to claim 1, characterized in that: In S3, the crucible is a platinum crucible.

4. Application of the synthesis method of the monazite reference sample according to any one of claims 1 - 3 in double dating.

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