A Secondary Ion Mass Spectrometry Uranium-Lead Dating Method for Rock Thin Section Samples

Abstract

The present invention discloses a method for uranium-lead dating of rock thin-section samples by secondary ion mass spectrometry, comprising the following steps: S1, sample scanning: identifying the location of the dating minerals in the sample; S2, sample cutting: cutting and extracting the dating mineral area in the sample to form a circular thin section; S3, sample target preparation: preparing a target for the extracted circular thin section; S4, sample polishing: polishing the thin-section target and depositing a conductive film; S5, secondary scanning: scanning and photographing the area where the dating minerals are located; S6, sample cleaning: cleaning the thin-section target and performing gold plating treatment; S7, uranium-lead dating of the sample target. The present invention can effectively solve the problem of inaccurate dating results caused by the difference in material properties between the sample target and the standard target; avoid sample loss caused by polishing when the sample and the standard are on the same sample target; can retain the original rock mineral information of the sample, perform micro-area in-situ dating on the sample, and obtain dating results with more geological significance.

Description

Technical Field

[0001] The present invention relates to the technical field of mineral dating, and more specifically, to a method for uranium-lead dating of rock thin section samples by secondary ion mass spectrometry. Background Art

[0002] Rock thin sections have multiple significance and values in the research of earth and planetary sciences. They are important research means, and can be used to observe the mineral composition, structure, elements, and tectonic and other characteristics under a microscope to determine the type of rock. They can also be used for popular science and display.

[0003] Uranium-thorium-lead dating is an important means for studying the formation and evolution of the earth and rocky celestial bodies. The uranium-thorium-lead system includes 238 the decay of U (uranium) to 206 Pb (lead), 235 the decay of U to 207 Pb, and 232 the decay of Th (thorium) to 208 Pb. Therefore, the isotopic age of a sample can be obtained by measuring the uranium-thorium-lead isotope ratio of the sample and combining with the decay law.

[0004] High-resolution and high-sensitivity secondary ion mass spectrometry (SIMS) is an important tool for analyzing the radioactive age of uranium-thorium-lead in rock samples. Its characteristics are: 1. It can ensure extremely high test sensitivity on the premise of achieving relatively high mass resolution; 2. Micro-area in-situ analysis, that is, complex chemical treatment is not required, and dating analysis can be directly carried out on rocks or minerals (the partition area can be as small as 5 microns).

[0005] SIMS analysis has relatively high requirements for the physical conditions such as the flatness and conductivity of the sample surface and the content of non-radiogenic lead on the sample surface, and requires standard samples (minerals with known isotopic ages) for fractionation correction. For traditional geological samples, the selected independent minerals after crushing and the standard samples are jointly made into a target, but the above method will lack ore phase information and the occurrence of the dating minerals cannot be obtained, so there will be deviations in the interpretation of the obtained ages.

[0006] Rock thin sections can be directly subjected to SIMS U-Pb dating, and the occurrence of the dating minerals in the host rock can be retained, which can effectively explain the geological significance of the contained age information. However, there are many problems to be solved for rock thin sections to perform SIMS U-Pb dating:

[0007] 1. The shape of the thin section and the specifications of the SIMS sample chamber. "Currently, the mainstream large-scale secondary ion mass spectrometers used in geochronology research mainly have two product lines, SHRIMP and Cameca, and their samples usually use cylindrical sample targets with a shape of 2.54 cm. Rock thin sections are usually rectangular because they need to be observed under a microscope.

[0008] 2. U-Pb dating minerals are usually accessory minerals in rocks. Taking zircon as an example, it has a low content and small particle size in non-acidic rocks, so it is difficult to locate in thin sections.

[0009] 3. For SIMS dating, the flatness, conductivity of the target surface, the angle between the target and the extraction electrode, and the sample position will affect the measured value of the isotope ratio (Williams et al., 1998). Therefore, the prerequisite for correcting the sample to be measured with a standard sample is that the physical conditions during the testing process of the two should be the same. Moreover, special geological samples (such as extraterrestrial samples, deep-sea samples, etc.) will be affected by impacts, alteration, or tectonic actions, resulting in developed fractures. The conductivity and surface flatness vary from sample to sample and cannot be evaluated. Therefore, it is almost impossible to ensure the same physical conditions by independently preparing targets for samples and standard samples, and it is difficult to improve the testing accuracy using traditional methods. Summary of the Invention

[0010] One object of the present invention is to provide a new technical solution for the secondary ion mass spectrometry uranium-lead dating method of rock thin-section samples to solve the problems raised in the above background technology.

[0011] According to the first aspect of the present invention, there is provided a secondary ion mass spectrometry uranium-lead dating method for rock thin-section samples, including the following steps:

[0012] S1. Sample scanning: Identify the location of the dating mineral in the sample;

[0013] S2. Sample cutting: Cut and extract the dating mineral area in the sample to form a circular thin section;

[0014] S3. Sample target preparation: Prepare a target for the extracted circular thin section;

[0015] S4. Sample polishing: Polish the thin-section target and deposit a conductive film;

[0016] S5. Secondary scanning: Scan and photograph the area where the dating mineral is located;

[0017] S6. Sample cleaning: Clean the thin-section target and perform gold plating treatment;

[0018] S7. Perform uranium-lead dating on the sample target.

[0019] Optionally, for the secondary ion mass spectrometry uranium-lead dating method of rock thin-section samples according to the present invention, the S1 includes the following steps:

[0020] S11. Wipe the surface of the thin-section sample with alcohol to remove large particle impurities and organic matter, perform ultrasonic cleaning with ultrapure water to remove the residual polishing paste substances and impurities on the sample, and then place the sample in a dryer for water removal;

[0021] S12. Conduct carbon spraying on the surface of the sample using a carbon spraying instrument for conductive treatment;

[0022] S13. Use a scanning electron microscope and an energy spectrometer to conduct panoramic, secondary electron, backscattered electron, and energy spectrometry element scans on the thin slice to obtain surface morphology, elemental, and mineral information;

[0023] S14. Extract the gray-scale information of the backscattered electron image and identify regions with more than 150 pixel points;

[0024] S15. Conduct energy spectrometry dotting on the extracted dating minerals to identify accessory mineral elements;

[0025] S16. Conduct secondary electron / backscattered electron photography on the identified dating mineral regions to obtain position information at different ratios;

[0026] S17. Take out the sample from the electron microscope, remove the carbon film using polishing paste, and wipe the surface residue using alcohol;

[0027] S18. Observe the dating mineral regions using an optical microscope and mark the regions using a waterproof pen.

[0028] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin slice samples of the present invention, S2 includes the following steps:

[0029] S21. Fix a metal plate with a certain thickness at the bottom of the container and inject ultrapure water;

[0030] S22. Immerse the sample in the container, submerge it in the ultrapure water, and fix the thin slice above the metal plate;

[0031] S23. Use a hollow drill bit to drill the marked region of the sample;

[0032] S24. Clean the drilled circular thin slice using anhydrous ethanol, store it in a specific container, and mark it;

[0033] S25. Clean the tool and repeat steps S22 - S24 until all dating mineral regions are drilled;

[0034] S26. Ultrasonically clean and dry all the drilled samples separately.

[0035] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin slice samples of the present invention, S3 includes the following steps:

[0036] S31. Mark concentric circles with an outer diameter of 2.54 cm and an inner diameter of 1 cm on a square glass plate;

[0037] S32. Arrange the standard samples in a straight line and stick them in a 1-cm inner-diameter circle on the glass plate with double-sided tape.

[0038] S33. Inject the mixture of resin and curing agent into a container and place it in a vacuum chamber. Take the sample target mold, a polytetrafluoroethylene tube, and pre-uniformly apply vaseline to its inner wall for easy demolding after curing.

[0039] S3. Uniformly apply vaseline oil to the inner wall surface at one end of the polytetrafluoroethylene tube, place it vertically on the double-sided tape surface where the samples are already stuck, so that the sample particles are located at the center of the tube and compact them.

[0040] S34. Slowly pour the mixture of resin and curing agent into the polytetrafluoroethylene tube. After placing the whole in the vacuum chamber and evacuating, take out the sample target and let it stand in a constant-temperature drying oven, waiting for the sample target to cure.

[0041] S35. Remove the cured sample target blank from the glass plate, then grind the side without sample particles flat and keep the thickness of the target at 5 - 6 mm.

[0042] S36. Scrub the sample target with absolute ethanol to remove the residues of double-sided tape and vaseline oil.

[0043] S37. Select water sandpaper to polish the sample target according to the particle size of the sample.

[0044] S38. Use a disk saw to cut the standard sample target according to its shape.

[0045] S39. Repeat steps S31 - S38 to make the target, and make the well-surfaced dating mineral area and standard samples on the same target.

[0046] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin-section samples of the present invention, S4 includes the following steps:

[0047] S41. Polish the target containing the thin-section sample and the standard sample on a polishing machine.

[0048] S42. Use a microscope to observe the polishing situation until most of the sample particles expose the center.

[0049] S4,4. Clean and perform surface carbon spraying on the polished sample target.

[0050] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin-section samples of the present invention, S5 includes the following steps:

[0051] S51. Use a scanning electron microscope and an energy spectrometer to perform secondary electron, backscattering, and energy spectrum element scans on the dating mineral area and the standard sample area of the sample target to obtain surface morphology, element, and mineral information.

[0052] S52. Observe the morphological changes of the dating minerals, conduct secondary electron / backscattered electron photography at magnifications of 40 times, 120 times, and 1000 times to obtain position information at different ratios.

[0053] S53. Under the condition of 120 times magnification, perform energy spectrum surface scanning on the dating mineral area to obtain the occurrence of the dating minerals and regional ore facies information.

[0054] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin section samples of the present invention, S6 includes the following steps:

[0055] S61. Place the sample alone in a beaker and rinse it alternately with ultrapure water and absolute ethanol.

[0056] S62. After the ultrasonic cleaning is completed, take out the sample from the beaker, rinse it alternately with ultrapure water and absolute ethanol again, and finally rinse the target with ultrapure water.

[0057] S63. Place the sample in an oven that has been wiped clean and lined with experimental dust-free paper for drying.

[0058] S64. After taking out the dried sample, place it in a covered petri dish and store it in a clean bench.

[0059] Optionally, according to the secondary ion mass spectrometry uranium-lead dating method for rock thin section samples of the present invention, S7 includes the following steps:

[0060] S71. Use different integration times to test the content of the guiding peak of the standard sample and record the peak data.

[0061] S72. Use the electron image in step S5 for comparison to determine the sample position, scan through the guiding peak, and compare the peak data in step S71. If the peak is not reached, finely move the ion beam position through the sample stage, and then scan through the guiding peak again to find the maximum value position.

[0062] The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples disclosed by the present invention has the following beneficial effects compared with the prior art:

[0063] 1. It can effectively solve the problem of inaccurate dating results caused by the difference in the material properties between the sample target and the standard sample target.

[0064] 2. Avoid sample loss caused by polishing when the sample and the standard are on the same sample target.

[0065] 3. Avoid crushing and selecting dating minerals, and retain the original rock mineral information of the sample. Conduct micro-area in-situ dating on the sample to obtain a dating result with more geological significance.

[0066] Other features and advantages of the present invention will become apparent from the following detailed description of exemplary embodiments of the invention with reference to the accompanying drawings. Description of the Drawings

[0067] The drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0068] Figure 1 It is a schematic flow diagram of the secondary ion mass spectrometry uranium-lead dating method for rock thin-section samples disclosed by the present invention;

[0069] Figure 2 It is a schematic diagram of the position for extracting potential uranium-bearing dating minerals by different grayscales according to the present invention;

[0070] Figure 3 It is a schematic flow diagram of sample target preparation disclosed by the present invention. Detailed Embodiments

[0071] Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions and values set forth in these embodiments do not limit the scope of the present invention.

[0072] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the present invention, its application, or its use.

[0073] Techniques, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered as part of the specification.

[0074] In all examples shown and discussed herein, any specific values should be construed as merely exemplary and not as a limitation. Thus, other examples of exemplary embodiments may have different values.

[0075] According to Figures 1 to 3 As shown, the present invention provides a secondary ion mass spectrometry uranium-lead dating method for rock thin-section samples, including the following steps:

[0076] S1, sample scanning: identifying the location of the dating minerals in the sample;

[0077] S2, sample cutting: cutting and extracting the dating mineral area in the sample to form a circular thin slice;

[0078] S3, sample target preparation, preparing a target for the extracted circular thin slice;

[0079] S4, sample polishing, polishing the thin slice target and plating a conductive film;

[0080] S5, Secondary scanning, scanning and photographing the area where the mineral is located;

[0081] S6, Sample cleaning, cleaning and gold plating the thin-section target;

[0082] S7, Conducting uranium-lead (U-Pb) dating on the sample target.

[0083] Furthermore, the purpose of step S1 is to precisely locate the key dating minerals in the thin section and conduct a preliminary analysis of their occurrence (genesis), which specifically includes the following steps:

[0084] S11, Wiping the surface of the thin-section sample with alcohol to remove large-particle impurities and organic substances, performing ultrasonic cleaning with ultrapure water to remove the residual polishing paste substances and impurities on the sample, and then placing the sample in a dryer for water removal. The drying time is 30 minutes and the drying temperature is 60 °C.

[0085] S12, Conducting carbon spraying and conductivity treatment on the sample surface using a carbon spraying instrument, controlling the current intensity (depending on the diameter of the carbon rod) and the carbon film thickness (which should be less than 30 nm).

[0086] S13, Using a scanning electron microscope and an energy spectrometer to conduct panoramic, secondary electron, backscattered, and energy spectrum element scans on the thin section to obtain surface morphology, element, and mineral information; Parameter information: voltage 15 kV, current 6 nA, working distance 8.8 mm, single-image resolution of 1024*768 at a magnification of 120 times.

[0087] S14, Extracting the gray-scale information of the backscattered image (as Figure 2 shown, the gray scale of the backscattered image is related to the average atomic number of the area. The dating accessory minerals in the rock are mostly formed in the late stage, so they are rich in incompatible elements, with a relatively large average atomic number and are brighter in the backscattered image), identifying areas with more than 150 pixel points, and the resolution (ECD equivalent circle diameter) of the sample can be obtained as 6.5 μm (usually the ion beam diameter used for SIMS U-Pb dating is more than 10 μm).

[0088] S15, Conducting energy spectrum dotting on the extracted minerals to identify the dating mineral elements (such as Zr, Ce, etc.), and the energy spectrum count should be greater than 200,000.

[0089] S16, Conducting secondary electron / backscattered photographing at magnifications of 40 times, 120 times, and 1000 times on the identified dating mineral areas to obtain position information at different ratios;

[0090] S17, Taking the sample out of the electron microscope, removing the carbon film using 0.25 μm polishing paste, and wiping the surface residues with alcohol;

[0091] S18. Observe the radiometric dating accessory minerals area using an optical microscope and mark the area with a waterproof pen.

[0092] Furthermore, the purpose of step S2 is to cut and extract the area where the key radiometric dating minerals in the thin section are located and prepare it as a sample to be tested. The specific steps are as follows:

[0093] S21. Prepare a rectangular container, fix a metal plate with a certain thickness at the bottom of the container, and inject ultrapure water into the container.

[0094] S22. Immerse the sample in the container, submerge it in ultrapure water, and fix the thin section on the metal plate.

[0095] S23. Use a hollow drill bit with an inner diameter of 2 mm to drill the marked area of the sample.

[0096] S24. Clean the drilled circular thin section with absolute ethanol, store it in a specific container, and mark it.

[0097] S25. Clean the tools and repeat steps S22 - S24 until all radiometric dating minerals areas are drilled out.

[0098] S26. Ultrasonically clean and dry all the drilled samples separately.

[0099] Furthermore, as Figure 3 shown, the purpose of step S3 is to make a target for the circular thin section where the key minerals are extracted. The sample target consists of two parts. One is the thin section of the extracted circular radiometric dating minerals area with a diameter of 2 mm, and the other is the standard sample. The specific steps are as follows:

[0100] S31. Mark concentric circles with an outer diameter of 2.54 cm and an inner diameter of 1 cm on a square glass plate.

[0101] S32. Arrange the standard samples in a straight line and stick them in the 1 - cm - inner - diameter circle of the glass plate through double - sided tape.

[0102] S33. Inject a mixture of resin and hardener into a container and place it in a vacuum chamber; evacuate for 15 minutes to remove air, take it out and let it stand for 10 minutes. Take the sample target mold, a polytetrafluoroethylene tube, and pre - evenly apply vaseline on the inner wall for easy demolding after curing. The epoxy resin is prepared by mixing two substances, resin EPIREZ COMPOUND (product number: 991,404C) and hardener EPIREZ HARDENER (product number: 991,404H), and the ratio is 25 / 3. Generally, weigh 4.166 g of resin and about 0.5 g of hardener. This ratio determines the physical properties such as the hardness of the target.

[0103] S3; Apply Vaseline oil evenly on the inner wall surface of one end of a polytetrafluoroethylene tube with an inner diameter of 1 inch (2.54 cm), place it vertically on the surface of the double-sided tape to which the sample has been adhered, so that the sample particles are located at the center of the tube and compacted;

[0104] S34, Slowly pour the mixture of resin and curing agent into the polytetrafluoroethylene tube, place the whole in a vacuum chamber, evacuate for 30 - 40 minutes depending on the sample situation, then take out the sample target and let it stand in a constant-temperature drying oven, waiting for the sample target to cure. The curing time is not less than 24 hours, and the curing temperature is 60 °C.

[0105] S35, Remove the cured sample target blank from the glass plate, then grind the side without sample particles to be flat and keep the thickness of the target at 5 - 6 mm;

[0106] S36, Scrub the sample target with absolute ethanol to remove the residue of double-sided tape and Vaseline oil;

[0107] S37, Select water sandpaper to polish the sample target according to the particle size of the sample; Select water sandpaper of appropriate specifications (P1200 - 7000) to polish the sample target according to the particle size of the sample. When polishing, it is required that the sample target is evenly stressed in all directions. Use Milli-Q ultrapure water for cooling and cleaning during the polishing process.

[0108] S38, Use a disk saw to cut the standard sample target according to its shape;

[0109] S39, Repeat steps S31 - S38 to make targets, and make the well-conditioned dating mineral area and standard samples on the same target.

[0110] Further, the purpose of step S4 is to polish the prepared sample target, which specifically includes the following steps:

[0111] S41, Polish the target (including the thin slice sample and the standard sample) on a polishing machine successively with diamond polishing paste of 1 μm, 0.5 μm, 0.25 μm and 0.1 μm; Avoid using chemical polishing fluid (which will dissolve minerals such as apatite). At the same time, only one sample target can be polished on the polishing machine, and different samples need to replace the polishing disc to prevent cross-contamination.

[0112] S42, Regularly use a microscope to observe the polishing situation to avoid excessive loss of the sample. When most of the sample particles are exposed at the center, it indicates that the polishing process is completed.

[0113] S44, Clean and perform surface carbon spraying on the polished sample target. That is, repeat the sample cleaning and carbon spraying processes in step S1.

[0114] Furthermore, the purpose of step S5 is to take microscopic images of the polished sample target, observe the morphological changes and occurrence information of the minerals to be dated, and obtain accurate position information. Specifically, it includes the following steps:

[0115] S51, Use a scanning electron microscope and an energy spectrometer to perform secondary electron, backscattered electron, and energy spectrum element scans on the dating mineral area and the standard sample area of the sample target to obtain surface morphology, element, and mineral information; Parameter information: voltage 15 kV, current 2 nA, working distance 8.8 mm, single-image resolution of 2048*1536 at a magnification of 200 times.

[0116] S52, Observe the morphological changes of the dating minerals, and perform secondary electron / backscattered electron photography at magnifications of 40 times, 120 times, and 1000 times to obtain position information at different ratios;

[0117] S53, Under the condition of 120 times magnification, perform an energy spectrum surface scan on the dating mineral area to obtain the occurrence of the dating minerals and regional ore phase information.

[0118] Furthermore, the purpose of step S6 is to clean the scanned sample target and perform surface conductive treatment to prepare for subsequent dating tests. Specifically, it includes the following steps:

[0119] S61, Place the sample alone in a beaker and rinse it alternately with ultrapure water and absolute ethanol 3 times, 10 minutes each time. Wear brand-new latex gloves before the operation, and only one target can be washed separately in each cleaning process. Use a neutral detergent to ultrasonically clean the sample, and then wash the detergent with ultrapure water.

[0120] S62, After the ultrasonic cleaning is completed, take out the sample from the beaker, and rinse it alternately with ultrapure water and absolute ethanol 3 times again, and finally rinse the target with ultrapure water;

[0121] S63, Place the sample in an oven that has been wiped clean and lined with experimental dust-free paper, and dry it at 60 degrees Celsius for at least 1 hour. Samples to be tested for volatile matter also need to be left standing in a vacuum chamber for 8 hours.

[0122] S64, After taking out the dried sample, place it in a covered petri dish and store it in a laminar flow hood.

[0123] The gold plating material is selected as high-purity gold, and the purity requirement reaches 99.999%. The thickness of the gold plating film should be about 20 nm to ensure the conductivity of the sample target surface and avoid lead contamination caused by excessive gold.

[0124] Furthermore, the purpose of step S7 is to perform uranium-lead isotope dating on the key minerals in the sample target to obtain the formation age of the sample. Specifically, it includes the following steps:

[0125] S71. Test the content of the leading peak of the standard sample (such as the leading peak Zr2SiO4 of zircon) using 10 μm, 1.0 nA, and an integration time of 1 second + , the leading peak CePO4 of monazite + ), and record the peak data.

[0126] Samples that require micro-area dating of thin sections are usually rare samples, with few and small dating minerals. Therefore, for the dating work using secondary ion mass spectrometry, the primary ion beam spot should be less than or equal to 10 μm, and the beam current should be 1.0 nA to avoid excessive loss of longitudinal samples and increase in common lead caused by bombarding other minerals.

[0127] S72. Since the imaging system of secondary ion mass spectrometry is usually reflected light in the visible light band and the optical path is usually relatively simple, it is relatively difficult to identify minerals and structures below 10 μm. Therefore, before testing the dating minerals in the thin section, use the electron image in step S52 for comparison to determine the sample position, and scan through the leading peak (10 μm, 1.0 nA, 1 second integration time). Use the electron image in step S5 for comparison to determine the sample position, and scan through the leading peak. Compare the peak data in step S71. If the peak is not reached, finely move the position of the ion beam through the sample stage, and then scan through the leading peak again to find the maximum position.

[0128] It is necessary to increase 204 the integration time of Pb and the background value, and control the total test time for a single point.

[0129] For a single point, it is necessary to increase the number of analysis groups, reduce the reception time for a single group, and monitor the longitudinal mineral morphology changes through the mineral leading peak (such as the leading peak Zr2SiO4 of zircon + , the leading peak CePO4 of monazite + ).

[0130] Pure gold contains trace amounts of common lead. During analysis and testing, it is necessary to use the primary ion beam to clean the sample surface for 3 - 4 minutes to remove the common lead generated by the surface coating.

[0131] The thin section sample retains the rock information of the rock. However, due to the different hardnesses of different minerals and the effects of impacts and alterations, it usually has more fractures and voids. The gold plating process will cause gold to enter the fractures and pores of the sample. Therefore, during analysis and testing, it is necessary to avoid the fractures and voids to prevent analyzing areas with a high probability of being contaminated.

[0132] Finally, according to the dating formula:

[0133] 206 Pb = 206 Pb0 + 238 U(e λ238t - 1);

[0134] 207 Pb = 207 Pb0 + 235U(e λ235t - 1) ;

[0135] 208 Pb = 208 Pb0+ 232 Th(e λ232t - 1);

[0136] where 206 Pb, 207 Pb, 208 Pb, 238 U, 235 U and 232 Th are the actual contents in the mineral.

[0137] λ 238 、λ 235 and λ 232 , are 238 U, 235 U and 232 Th's decay constants, t is the formation age of the mineral, and e is the natural number;

[0138] 206 Pb0, 207 Pb0 and 208 Pb0 are the initial contents of lead isotopes when the mineral was formed. This part needs to be deducted when calculating the formation age of the mineral to obtain the radiogenic lead calculation age:

[0139] 206 Pb - 206 Pb0 = 206 Pb 放射性成因 ;

[0140] 207 Pb - 207 Pb0 = 206 Pb 放射性成因 ;

[0141] 208 Pb - 208 Pb0 = 206 Pb 放射性成因 ;

[0142] In the actual analysis process of secondary ion mass spectrometry, usually by testing 204 Pb, combined with the ratios of 204 Pb, 206 Pb, 207 Pb and 208 Pb in different eras, calculate 206 Pb0,207 Pb0 and 208 the paired content of Pb0.

[0143] Such as modern (time is 0) 206 Pb / 204 Pb = 18.70, 207 Pb / 204 Pb = 15.63, 208 Pb / 204 Pb = 38.63;

[0144] The ratio 3 billion years ago was 206 Pb / 204 Pb = 12.93, 207 Pb / 204 Pb = 14.34, 208 Pb / 204 Pb = 32.74;

[0145] In the actual analysis process of secondary ion mass spectrometry, due to the relative relationship between the ion beam spot and the sample size, the surface state of the sample (roughness, gold plating thickness), sample cracks and crystal boundaries will increase 206 Pb0, 207 Pb0 and 208 the proportion of Pb0, 204 The content of Pb in uranium-bearing dating minerals is relatively low, so the test error is large, and therefore the error will be transmitted to 206 Pb0, 207 Pb0 and 208 Pb0, thereby reducing the dating accuracy.

[0146] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present invention. Those skilled in the art should understand that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the appended claims.

Claims

1. A secondary ion mass spectrometry uranium-lead dating method for rock thin section samples, characterized in that, It includes the following steps: S1, sample scanning: Identify the location of the dating minerals in the sample; S2, sample cutting: Cut and extract the area of the dating minerals in the sample to form a circular thin slice; S3, sample target making: Make a target for the extracted circular thin slice; S4, sample polishing: Polish the thin slice target and coat it with a conductive film; S5, secondary scanning: Scan and photograph the area where the dating minerals are located; S6, sample cleaning: Clean the thin slice target and perform a gold plating treatment; S7, perform uranium-lead dating on the sample target, wherein, S7 includes the following steps: S71, Use different integration times to test the content of the guiding peak of the standard sample and record the peak data; S72, Use the electron image in step S5 for comparison to determine the sample position, and scan through the guiding peak. Compare the peak data in step S71. If the peak is not reached, finely move the position of the ion beam through the sample stage, and then scan again through the guiding peak to find the maximum value position.

2. The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples according to claim 1, characterized in that, The said S1 includes the following steps: S11, Wipe the surface of the thin slice sample with alcohol to remove large particle impurities and organic substances. Use ultrapure water for ultrasonic cleaning to remove the residual polishing paste substances and impurities on the sample, and then place the sample in a dryer to remove water; S12, Conduct carbon spraying and conductive treatment on the sample surface using a carbon spraying instrument; S13, Use a scanning electron microscope and an energy spectrometer to perform panoramic, secondary electron, backscattered, and energy spectrum element scans on the thin slice to obtain surface morphology, element, and mineral information; S14, Extract the gray-scale information of the backscattered image and identify the area with more than 150 pixel points; S15, Perform energy spectrum dotting on the extracted accessory minerals to identify the dating mineral elements; S16, Perform secondary electron / backscattered photography on the identified dating mineral area to obtain position information in different proportions; S17, Take out the sample from the electron microscope, remove the carbon film with polishing paste, and wipe the surface residue with alcohol; S18, Observe the dating mineral area using an optical microscope and mark the area with a waterproof pen.

3. The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples according to claim 1, characterized in that, The said S2 includes the following steps: S21, Fix a metal plate with a certain thickness at the bottom of the container and inject ultrapure water; S22, Immerse the sample in the container, submerge it in ultrapure water, and fix the thin slice on the metal plate; S23, Use a hollow drill bit to drill the marked area of the sample; S24, Clean the drilled circular thin slice with anhydrous ethanol, store it in a specific container, and mark it; S25, Clean the tools and repeat steps S22 - S24 until all dating mineral areas are drilled; S26, Perform ultrasonic cleaning and drying on all the drilled samples separately.

4. The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples according to claim 1, wherein The said S3 includes the following steps: S31, Mark concentric circles with an outer diameter of 2.54 cm and an inner diameter of 1 cm on a square glass plate; S32, Arrange the standard samples in a straight line and stick them in the 1 cm inner diameter circle of the glass plate through double-sided tape; S33, Inject a mixture of resin and curing agent into a container and place it in a vacuum chamber; Take a sample target mold, a polytetrafluoroethylene tube, and pre-uniformly apply vaseline to the inner wall for easy demolding after curing; S3; Apply Vaseline oil evenly on the inner wall surface at one end of the PTFE tube, place it vertically on the surface of the double-sided tape to which the sample has been adhered, so that the sample particles are located at the center of the tube and compacted; S34, Slowly pour the mixture of resin and curing agent into the PTFE tube. After placing the whole in the vacuum chamber and evacuating, take out the sample target and let it stand in a constant temperature drying oven, waiting for the sample target to cure; S35, Remove the cured sample target blank from the glass plate, then grind the side without sample particles flat and keep the thickness of the target at 5-6 mm; S36, Scrub the sample target with anhydrous ethanol to remove the residue of double-sided tape and Vaseline oil; S37, Select water sandpaper to polish the sample target according to the particle size of the sample; S38, Use a disk saw to cut the standard sample target according to its shape; S39, Repeat steps S31 - S38 to make the target, and make the well-conditioned dating mineral area and the standard sample on the same target.

5. The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples according to claim 1, wherein The said S4 includes the following steps: S41, Polish the target containing the thin slice sample and the standard sample on a polishing machine; S42, Observe the polishing situation with a microscope until most of the sample particles expose the center; S44, Clean and perform surface carbon spraying on the polished target.

6. The method for uranium-lead dating of rock thin section samples by secondary ion mass spectrometry according to claim 1, wherein, The said S5 includes the following steps: S51, Use a scanning electron microscope and an energy spectrometer to perform secondary electron, backscattering and energy spectrum element scanning on the dating mineral area and the standard sample area of the target to obtain surface morphology, element and mineral information; S52, Observe the morphological changes of the dating mineral, and perform secondary electron / backscattering photography at 40 times, 120 times and 1000 times to obtain position information at different ratios; S53, Under the condition of 120 times, perform energy spectrum surface scanning on the dating mineral area to obtain the occurrence of the dating mineral and regional ore facies information.

7. The secondary ion mass spectrometry uranium-lead dating method for rock thin section samples according to claim 1, wherein The said S6 includes the following steps: S61, Place the sample alone in a beaker and rinse it alternately with ultrapure water and anhydrous ethanol; S62, After the ultrasonic cleaning is over, take out the sample from the beaker, rinse it alternately with ultrapure water and anhydrous ethanol again, and finally rinse the target with ultrapure water; S63, Place the sample in an oven wiped clean and lined with experimental dust-free paper for drying treatment; S64, Take out the dried sample and place it in a covered watch glass and store it in a clean bench.

Images