Carbonate mineral U-Pb isotope dating method

Abstract

The invention provides a U-Pb isotope dating method for carbonate minerals. The method comprises the following steps: obtaining a representative rock sample of a target carbonate mineral; wherein deposition primary holes filled with target carbonate minerals and / or corrosion holes filled with the target carbonate minerals and / or cracks filled with the target carbonate minerals are developed in the representative rock sample; determining an area in which the 238U content of the target carbonate mineral in the representative rock sample is not lower than the U content detection lower limit and the 238U / 206Pb ratio is greater than 1 as a detection area; and performing laser U-Pb isotope dating on the detection area to obtain the U-Pb isotope age of the target carbonate mineral.

Description

Technical Field

[0001] The invention belongs to the technical field of carbonate rock oil and gas exploration, and particularly relates to a carbonate mineral dating method. Background Art

[0002] Laser in situ U-Pb isotope dating technology has been widely used in the high-precision dating of high-U content minerals (such as zircon, apatite, etc.), and has become the most commonly used dating method in the field of geochronology.

[0003] Like the U-Pb isotope dating technique of zircon and apatite, the decay laws of U and Pb radioactive isotopes in carbonate minerals conform to the theoretical conditions for determining the age of mineral formation through radioactive isotope testing. 204 Pb is a stable isotope. 206 Pb, 207 Pb is a radioactive isotope. The radioactive parent bodies of the two are 238 U. 235 U, and the half-life is 4.4683×10 9 Year, 0.70381×10 9 Years, so through 208 Pb, 207 Pb, 206 Pb, 204 Pb, 238 U and 235 U radioisotope ratios are used to determine the age of mineral formation. 238 U. 207 Pb, 206 Pb ratio.

[0004] The accuracy and success rate of existing carbonate mineral U-Pb isotope dating technology are low. The biggest technical difficulty is the detection of ultra-low U content. The detection limit of U content of existing equipment is 10ppb. U-Pb isotope dating requires that the sample to be tested has a high U content. The higher the U content, the greater the U / Pb ratio, and the higher the dating success rate and accuracy. However, the U content of carbonate minerals is several orders of magnitude lower than that of zircon, making it difficult to obtain effective U content. 238 U information.

[0005] In summary, there is still a need to improve the U-Pb isotope dating technology of carbonate minerals in order to improve the accuracy and success rate of U-Pb isotope dating of carbonate minerals. Summary of the invention

[0006] In view of the shortcomings of the prior art, the object of the present invention is to provide a technical solution with high accuracy and success rate in U-Pb isotope dating of carbonate minerals.

[0007] In order to achieve the above object, the present invention provides a method for U-Pb isotope dating of carbonate minerals, wherein the method comprises:

[0008] Obtaining a representative rock sample of a target carbonate mineral; wherein the representative rock sample has developed sedimentary primary pores filled with the target carbonate mineral and / or dissolution pores filled with the target carbonate mineral and / or cracks filled with the target carbonate mineral;

[0009] Determine the target carbonate minerals in the representative rock sample 238 U content is not less than the detection limit of U content and 238 U / 206 The area where the Pb ratio is greater than 1 is taken as the detection area;

[0010] Laser U-Pb isotope dating is performed on the detection area to obtain the U-Pb isotope age of the target carbonate mineral.

[0011] In the above-mentioned U-Pb isotope dating method for carbonate minerals, preferably, the target carbonate mineral in the representative rock sample is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are considered as detection areas:

[0012] preparing a sample thin slice C corresponding to the representative rock sample;

[0013] The cross section of sample slice C is subjected to trace element laser scanning imaging to determine the target carbonate minerals in the cross section of sample slice C. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 is regarded as the detection area;

[0014] More preferably, the target carbonate mineral in the cut surface of the sample slice C is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with the highest Pb ratio is taken as the detection area; among them, 238 U / 206 The area with the highest Pb ratio 238 U / 206 The Pb ratio is greater than 1;

[0015] More preferably, the thickness of the sample slice C is 100 μm-150 μm;

[0016] More preferably, the diameter of the sample slice C is 1.5-2.5 cm;

[0017] More preferably, the cross section of the sample slice C is subjected to trace element laser surface scanning imaging according to the carbonate rock trace element laser surface scanning method provided in the Chinese patent publication CN112577903A.

[0018] In the above-mentioned U-Pb isotope dating method for carbonate minerals, preferably, the target carbonate mineral in the representative rock sample is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are included as detection areas:

[0019] Prepare parallel samples A and B corresponding to the representative rock samples, and make parallel sample A into sample slice A, and make parallel sample B into sample slice B;

[0020] The cross section of sample slice B is imaged by laser scanning of trace elements to determine the target carbonate minerals in the cross section of sample slice B. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 was taken as the target area;

[0021] Determine in the cut surface of sample slice A an area corresponding to the target area in the cut surface of sample slice B as a detection area;

[0022] Wherein, the sample slice A and the sample slice B satisfy that the target area in the cut surface of the sample slice B and the area in the cut surface of the sample slice A corresponding to the target area in the sample slice B are in a mirror image relationship;

[0023] Compared with using a single sample slice C for both trace element laser surface scanning imaging and U-Pb isotope dating, it is more advantageous to use sample slice B for trace element laser surface scanning imaging and sample slice A for U-Pb isotope dating; when using a single sample slice C for both trace element laser surface scanning imaging and U-Pb isotope dating, if the thickness of sample slice C is set to a thickness suitable for trace element laser surface scanning imaging, then the thickness of sample slice C after completing trace element laser surface scanning imaging is usually less than the thickness suitable for U-Pb isotope dating. The best thickness for dating. If the thickness of sample slice C is increased so that the thickness of sample slice C after trace element laser surface scanning imaging is suitable for U-Pb isotope dating, the thickness of sample slice C will usually be greater than the thickness suitable for trace element laser surface scanning imaging. If sample slice B is used for trace element laser surface scanning imaging and sample slice A is used for U-Pb isotope dating, the thickness of sample slice B can be set to the thickness suitable for trace element laser surface scanning imaging, and the thickness of sample slice A can be set to the thickness suitable for U-Pb isotope dating.

[0024] More preferably, the section of sample slice B is in a mirror image relationship with the section of sample slice A;

[0025] More preferably, the target carbonate mineral in the cut surface of the sample slice B is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with the highest Pb ratio was selected as the target area;

[0026] More preferably, the thickness of the sample sheet A is 95-105 μm (e.g., 100 μm);

[0027] More preferably, the diameter of the sample sheet A is 1.5-2.5 cm;

[0028] More preferably, the thickness of the sample sheet B is 95-105 μm (e.g., 100 μm);

[0029] More preferably, the diameter of the sample slice B is 1.5-2.5 cm;

[0030] More preferably, the preparation of the parallel sample A and the parallel sample B corresponding to the representative rock sample is performed in the following manner: the representative rock sample is cut into a cylinder with a diameter of 1.5-2.5 cm and a thickness of 0.8-1 cm, and two parallel samples are made along both sides of the cut surface, one is the parallel sample A and the other is the parallel sample B;

[0031] More preferably, the cross section of the sample slice B is subjected to trace element laser surface scanning imaging according to the carbonate rock trace element laser surface scanning method provided in the Chinese patent publication CN112577903A.

[0032] Preferably, the U content detection lower limit is 10 ppb.

[0033] Preferably, laser U-Pb isotope dating is performed on the detection area according to the calcite mineral laser uranium-lead isotope dating process provided in Chinese patent publication CN111007141A.

[0034] Preferably, laser U-Pb isotope dating is performed on the detection area according to the laser in-situ uranium-lead isotope dating method for ancient marine carbonate rocks provided in Chinese patent publication CN110376273A.

[0035] In the above-mentioned U-Pb isotope dating method for carbonate minerals, preferably, the U content of the target carbonate mineral is 5ppb-2ppm.

[0036] In the above-mentioned carbonate mineral U-Pb isotope dating method, preferably, in the process of determining the detection area, the target carbonate mineral is selected. 238 U content is not less than the detection limit of U content and238 U / 206 The Pb ratio is greater than 1 and 238 U / 206 The area with the highest Pb ratio was taken as the detection area.

[0037] The existing U-Pb isotope dating technology for carbonate minerals has low accuracy and success rate. The main reason is that U-Pb isotope dating requires the sample to be tested to have a high U content, while the U content of carbonate minerals is low, several orders of magnitude lower than that of zircon and apatite, making it difficult to obtain effective U-Pb dating technology. 238 U information. The inventor is committed to improving the accuracy and success rate of U-Pb isotope dating of carbonate minerals. The inventor has found that carbonate minerals 238 The distribution of U content is extremely uneven. In carbonate mineral samples, there are usually some areas with 238 U content is not less than the detection limit of U content and 238 U / 206 The Pb ratio is greater than 1. Based on this, the U-Pb isotope dating method for carbonate minerals provided by the present invention is proposed. In the U-Pb isotope dating method for carbonate minerals provided by the present invention, first determine 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 is used as the detection area, and the detection area is used for U-Pb isotope dating, thereby improving the accuracy and success rate of U-Pb isotope dating of carbonate minerals. The carbonate mineral U-Pb isotope dating method provided by the present invention can achieve a dating success rate of 90%, and the accuracy is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a diagram of the sparry calcite cement in the dolomite pores in Example 1 of the present invention.

[0039] Figure 2 The spar calcite cement in Example 1 of the present invention 238 Planar distribution diagram of U content.

[0040] Figure 3 The spar calcite cement in Example 1 of the present invention 238 U / 206 Pb ratio plane distribution diagram.

[0041] Figure 4 This is a data diagram for determining the age of the spar calcite cement in Example 1 of the present invention.

[0042] Figure 5 This is a data diagram for determining the age of the spar calcite cement in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0044] The embodiment of the present invention provides a method for determining molecular sieve unit cell parameters, such as Figure 1 As shown, the method comprises the following steps:

[0045] Step 101, obtaining a representative rock sample of a target carbonate mineral; wherein the representative rock sample has developed sedimentary primary pores filled with the target carbonate mineral and / or dissolved pores filled with carbonate minerals and / or cracks filled with carbonate minerals;

[0046] Step 102, determining the target carbonate mineral in the representative rock sample 238 U content is not less than the detection limit of U content and 238 U / 206 The area where the Pb ratio is greater than 1 is taken as the detection area;

[0047] Step 103, performing laser U-Pb isotope dating on the detection area to obtain the U-Pb isotope age of the target carbonate mineral.

[0048] In one embodiment, the target carbonate mineral in the representative rock sample is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are included as detection areas:

[0049] preparing a sample thin slice C corresponding to the representative rock sample;

[0050] The cross section of sample slice C is subjected to trace element laser scanning imaging to determine the target carbonate minerals in the cross section of sample slice C. 238 U content is not less than the detection limit of U content and 238 U / 206 The area where the Pb ratio is greater than 1 is taken as the detection area;

[0051] Furthermore, the target carbonate minerals in the cut surface of sample slice C were determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with the highest Pb ratio was taken as the detection area;

[0052] Further, the thickness of the sample slice C is 100 μm-150 μm;

[0053] Further, the diameter of the sample slice C is 1.5-2.5 cm;

[0054] Furthermore, the cross section of the sample slice C is subjected to trace element laser surface scanning imaging according to the carbonate rock trace element laser surface scanning method provided in the Chinese patent publication CN112577903A.

[0055] In one embodiment, the target carbonate mineral in the representative rock sample is determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are included as detection areas:

[0056] Prepare parallel samples A and B corresponding to the representative rock samples, and make parallel sample A into sample slice A, and make parallel sample B into sample slice B;

[0057] The cross section of sample slice B is imaged by laser scanning of trace elements to determine the target carbonate minerals in the cross section of sample slice B. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 was taken as the target area;

[0058] Determine in the cut surface of sample slice A an area corresponding to the target area in the cut surface of sample slice B as a detection area;

[0059] Wherein, the sample slice A and the sample slice B satisfy that the target area in the cut surface of the sample slice B and the area in the cut surface of the sample slice A corresponding to the target area in the sample slice B are in a mirror image relationship;

[0060] Furthermore, the section of sample slice B is in a mirror image relationship with the section of sample slice A;

[0061] Furthermore, the target carbonate minerals in the cross section of sample slice B were determined. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with the highest Pb ratio is taken as the target area; among them, the area with the highest 238U / 206Pb ratio has a 238U / 206Pb ratio greater than 1;

[0062] Further, the thickness of the sample sheet A is 95-105 μm (e.g., 100 μm);

[0063] Further, the diameter of the sample slice A is 1.5-2.5 cm;

[0064] Further, the thickness of the sample slice B is 95-105 μm (e.g., 100 μm);

[0065] Further, the diameter of the sample slice B is 1.5-2.5 cm;

[0066] Further, the preparation of the parallel sample A and the parallel sample B corresponding to the representative rock sample is performed in the following manner: the representative rock sample is cut into a cylinder with a diameter of 1.5-2.5 cm and a thickness of 0.8-1 cm, and two parallel samples are made along both sides of the cut surface, one is parallel sample A and the other is parallel sample B;

[0067] Furthermore, the cross section of the sample slice B is subjected to trace element laser surface scanning imaging according to the carbonate rock trace element laser surface scanning method provided in the Chinese patent publication CN112577903A.

[0068] In one embodiment, the U content detection lower limit is 10 ppb.

[0069] In one embodiment, laser U-Pb isotope dating is performed on the detection area according to the calcite mineral laser uranium-lead isotope dating process provided in Chinese patent publication CN111007141A.

[0070] In one embodiment, laser U-Pb isotope dating is performed on the detection area according to the laser in-situ uranium-lead isotope dating method for ancient marine carbonate rocks provided in Chinese patent publication CN110376273A.

[0071] In one embodiment, the U content of the target carbonate mineral is 5 ppb-2 ppm.

[0072] In one embodiment, during the determination of the detection zone, the target carbonate mineral is selected. 238 U content is not less than the detection limit of U content and 238 U / 206 The Pb ratio is greater than 1 and 238 U / 206 The area with the highest Pb ratio was taken as the detection area.

[0073] Example 1

[0074] This embodiment provides a method for U-Pb isotope dating of carbonate minerals. The method is used to perform U-Pb isotope dating on calcite cement with ultra-low U content (average U content of about 5 ppb) filling holes in dolomite formations with developed holes in a basin. The method specifically includes the following steps:

[0075] Step S1: Obtain representative rock samples of target carbonate minerals (the target carbonate minerals here are ultra-low U content calcite cements filling the holes in the dolomite formation with developed holes in the basin); specifically, obtain rock samples taken from the dolomite formation with developed holes in the basin, and select rock samples of spar calcite cements filling the holes as representative rock samples of target carbonate minerals (such as Figure 1 shown);

[0076] Step S2: Cut the representative rock sample into a cylinder with a diameter of 1.5 cm and a thickness of 0.8 cm, and make two parallel samples along both sides of the cut surface, one is parallel sample A and the other is parallel sample B; and make parallel sample A into sample slice A and make parallel sample B into sample slice B; the thickness of sample slice A is 100 μm; the thickness of sample slice B is 100 μm; observe sample slice A and sample slice B to ensure that the target carbonate minerals on the cut surface of sample slice B and the target carbonate minerals on the cut surface of sample slice A are in a mirror image relationship.

[0077] Step S3: Perform trace element laser scanning imaging on the cross section of sample slice B to obtain spar calcite cement 238 U content plane distribution (such as Figure 2 shown) and spar calcite cement 238 U / 206 Pb ratio plane distribution (such as Figure 3 The calcite cement content in the cross-section of sample B was determined by 238 The U content is not less than the detection limit of U content 10ppb and 238 U / 206 The Pb ratio is the highest and 238 U / 206 The area with a Pb ratio greater than 1 was taken as the target area;

[0078] Depend on Figure 2 , Figure 3 It can be seen that 238 The distribution of U content in the spar calcite crystals is uneven, with some areas being >10ppb and some areas being <10ppb.

[0079] Step S4: Determine the area corresponding to the target area in the cross section of sample slice B in the cross section of sample slice A as the detection area; perform laser U-Pb isotope dating on the detection area to obtain the U-Pb isotope age of the target carbonate diagenetic mineral. The result is as follows: Figure 5 shown.

[0080] Comparative Example 1

[0081] This comparative example provides a method for U-Pb isotope dating of carbonate minerals, which differs from Example 1 only in that:

[0082] Without determining the detection area of ​​sample slice A, laser U-Pb isotope dating is directly performed on sample slice A to obtain the U-Pb isotope age of the target carbonate diagenetic mineral. The results are as follows: Figure 4 shown.

[0083] When laser U-Pb isotope dating is directly performed on sample slice A without determining the detection area, laser spotting in areas with U content less than 10 ppb will result in dating failure, while spotting in the entire area will result in very low dating accuracy.

Claims

1. A method for U-Pb isotope dating of carbonate minerals, wherein: The method includes: Obtaining a representative rock sample of a target carbonate mineral; wherein the representative rock sample has developed sedimentary primary pores filled with the target carbonate mineral and / or dissolution pores filled with the target carbonate mineral and / or cracks filled with the target carbonate mineral; Determine the target carbonate minerals in the representative rock sample 238 U content is not less than the detection limit of U content and 238 U / 206 The area where the Pb ratio is greater than 1 is taken as the detection area; Laser U-Pb isotope dating is performed on the detection area to obtain the U-Pb isotope age of the target carbonate mineral.

2. The U-Pb isotope dating method for carbonate minerals according to claim 1, wherein: Determine the target carbonate minerals in the representative rock sample 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are included as detection areas: preparing a sample thin slice C corresponding to the representative rock sample; The cross section of sample slice C is imaged by laser scanning of trace elements to determine the target carbonate minerals in the cross section of sample slice C. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 is taken as the detection area.

3. The method for U-Pb isotope dating of carbonate minerals according to claim 2, wherein: Determine the target carbonate minerals in the cross section of sample C. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with the highest Pb ratio was taken as the detection area.

4. The U-Pb isotope dating method for carbonate minerals according to claim 2, wherein: The thickness of the sample slice C is 100 μm-150 μm.

5. The U-Pb isotope dating method for carbonate minerals according to claim 1, wherein: Determine the target carbonate minerals in the representative rock sample 238 U content is not less than the detection limit of U content and 238 U / 206 The areas where the Pb ratio is greater than 1 are included as detection areas: Prepare parallel samples A and B corresponding to the representative rock samples, and make parallel sample A into sample slice A, and make parallel sample B into sample slice B; The cross section of sample slice B is imaged by laser scanning of trace elements to determine the target carbonate minerals in the cross section of sample slice B. 238 U content is not less than the detection limit of U content and 238 U / 206 The area with a Pb ratio greater than 1 was taken as the target area; Determine in the cut surface of sample slice A an area corresponding to the target area in the cut surface of sample slice B as a detection area; The sample slice A and the sample slice B satisfy the relationship that the target area in the cut surface of the sample slice B and the area in the cut surface of the sample slice A corresponding to the target area in the sample slice B are in a mirror image relationship.

6. The method for U-Pb isotope dating of carbonate minerals according to claim 5, wherein: The cross section of sample slice B is a mirror image of the cross section of sample slice A.

7. The method for U-Pb isotope dating of carbonate minerals according to claim 5, wherein: The thickness of the sample sheet A is 95-105 μm.

8. The method for U-Pb isotope dating of carbonate minerals according to claim 5, wherein: The thickness of the sample sheet B is 95-105 μm.

9. The method for U-Pb isotope dating of carbonate minerals according to claim 1, wherein: The preparation of parallel samples A and parallel samples B corresponding to the representative rock sample is carried out in the following manner: the representative rock sample is cut into a cylinder with a diameter of 1.5-2.5 cm and a thickness of 0.8-1 cm, and two parallel samples are made along both sides of the cut surface, one is parallel sample A and the other is parallel sample B.

10. The method for U-Pb isotope dating of carbonate minerals according to any one of claims 1, 2 and 5, wherein: The U content detection lower limit is 10 ppb.

11. The method for U-Pb isotope dating of carbonate minerals according to any one of claims 1, 2 and 5, wherein: When determining the detection area, select the target carbonate minerals. 238 U content is not less than the detection limit of U content and 238 U / 206 The Pb ratio is greater than 1 and 238 U / 206 The area with the highest Pb ratio was taken as the detection area.

12. The method for U-Pb isotope dating of carbonate minerals according to claim 1, wherein: The U content of the target carbonate mineral is 5ppb-2ppm.

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