Method for determining age of carbonate mineral

Abstract

The invention relates to the technical field of geology and thermochronology, and discloses a method for determining the age of carbonate minerals, which comprises the following steps: (1) mass spectrum setting: the flow of sensitizing gas is 3-10ml / s; the sample injection cone group comprises a JET cone and an X cone, and the JET cone and the X cone are expanded by 5-20%; the inner diameter of the mass spectrum central tube is 2.0 mm to 2.8 mm; the pumping speed of the vacuum pump is not lower than 110L / h; (2) reducing the mass spectrum background Pb signal intensity to be not higher than 25cps; (3) cutting the carbonate mineral sample; (4) performing rock-ore analysis on the obtained to-be-detected sample; (5) establishing a dating test analysis method for carbonate laser ablation; (6) debugging the laser-mass spectrometry online, and establishing a test analysis sequence table; and (7) performing denudation and testing according to the sequence of the test analysis sequence table to obtain the age of the carbonate mineral. The accuracy and the success rate of determining the age of the carbonate mineral can be improved by improving the mass spectrum in multiple steps and reducing the background Pb signal intensity of the mass spectrum at the same time.

Description

Technical Field

[0001] This invention relates to the fields of geology and thermochronology, specifically to a method for determining the age of carbonate minerals. Background Technology

[0002] Carbonate minerals (including calcite, dolomite, limestone, and dolomite) are the most widely distributed and common minerals on the Earth's surface. Their research findings can be applied across various professional fields, including geology, paleoenvironment, paleoclimatology, paleoceanography, and even archaeology. However, carbonate minerals or strata often lack directly ageable authigenic minerals, making it difficult to determine their formation age. This is especially true in my country's deep marine carbonate strata, which are characterized by deep burial, old age, and exceptionally complex tectonic processes. Their diagenesis-reservoir formation and reservoir modification processes are complex, with multiple stages of hydrocarbon accumulation and alteration, and diverse sources of deep fluids, further complicating geological issues. Direct dating of carbonate minerals holds promise for solving the aforementioned bottlenecks and technical challenges in deep marine oil and gas exploration and evaluation, providing quantitative chronological evidence for marine oil and gas exploration and evaluation.

[0003] Dating carbonate minerals has always been a challenging problem in geological research both domestically and internationally. Internationally, the main methods attempted for carbonate mineral dating include uranium-series nonequilibrium dating, Rb-Sr isochronous dating, Sm-Nd isochronous dating, and Pb-Pb isochronous dating. However, the carbonate ages obtained by the uranium-series nonequilibrium dating method are generally less than 10 Ma, mainly used for paleoenvironmental and paleoclimate studies. Rb-Sr isotopes are extremely rare in carbonate minerals, making them difficult to detect, and this type of data is rarely reported in the literature. Sm-Nd isotopic dating of carbonate minerals has been developed earlier, but currently only a few articles have reported on it. The main reasons for this are: 1. Sufficient amounts of contemporaneous and different Sm / N isochronous sequences are needed. 1. Carbonate samples with a d-ratio are difficult to obtain; 2. Carbonate minerals have small variations in the Sm / Nd ratio and low REE content, resulting in generally large age errors; Pb-Pb isotope system testing of carbonate minerals has only been reported in a very limited number of articles, mainly due to: 1) expensive equipment, generally requiring large-scale thermal ionization mass spectrometers costing tens of millions; 2) high requirements for instrument operating environment and high maintenance costs; 3) extreme difficulty in obtaining sufficient powder samples: at least 6-8 parallel samples, each 200mg, with samples from the same period and different isotope ratios; 4) obtaining standard dilution solutions with different isotope ratios is extremely difficult, requiring laboratory qualification verification. Laser in-situ U-Pb dating analysis technology for carbonate minerals holds promise for the dating of carbonate rock minerals / strata. This method has a wide age range for testing carbonate minerals, with reports to date ranging from approximately 3 Ma to approximately 500 Ma. Laser sampling is more efficient and faster than traditional thermal ionization mass spectrometry Pb-Pb isochron age and Sm-Nd isochron age solution sampling.

[0004] Currently, laser in-situ U-Pb dating techniques for carbonate minerals are still in the exploratory stage both domestically and internationally. Different laboratories are attempting to conduct laser in-situ U-Pb dating of carbonate minerals using different types of mass spectrometry, such as quadrupole mass spectrometry, high-resolution mass spectrometry, and multi-acceptor mass spectrometry, with high-resolution inductively coupled plasma mass spectrometry being the dominant method. However, due to the relatively low content of U and Pb elements and the relatively high content of common Pb in carbonate minerals, the success rate of carbonate mineral dating is only 20-30%. Summary of the Invention

[0005] The purpose of this invention is to overcome the difficulty in determining the age of carbonate minerals in the existing technology, and to provide a method for determining the age of carbonate minerals.

[0006] To achieve the above objectives, a first aspect of the present invention provides a method for determining the age of carbonate minerals, comprising the following steps: (1) Mass spectrometry setup: The flow rate of the sensitizing gas is 3-10 ml / s; the injection cone group is JET cone and X cone, and the JET cone and X cone are expanded by 5-20%; the inner diameter of the central tube is expanded, and the inner diameter of the mass spectrometer central tube is 2.0 mm-2.8 mm; the vacuum pump speed is not less than 110 L / hour; (2) Reduce the background Pb signal intensity in the mass spectrometer to no more than 25 cps; (3) Cut the carbonate mineral sample to obtain the sample to be tested; (4) Perform rock and mineral analysis on the sample to be tested to determine the dating test area; (5) Establish a method for dating and analyzing carbonates by laser ablation; (6) Debug the laser-mass spectrometer connection and establish a test and analysis sequence list; (7) The standard sample and the sample to be tested were etched and tested in the order of the test and analysis sequence list, and the TW map of the in-situ U-Pb age of carbonate minerals was drawn to obtain the age of carbonate minerals.

[0007] Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: cleaning the laser cell; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: using a flexible tube with Teflon coating, a rigid tube with no or low Pb content, or a stainless steel tube as the gas pipeline; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by changing the gas mixing mode, more preferably by directly mixing the sample gas with argon compensation gas after laser emission, and then mixing it with nitrogen in a counter-current mode; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: removing mercury by carrier gas filtration, and more preferably by filtering helium gas through a mercury trap before it enters the laser. Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: the instrument purge time is set to be no less than 30 minutes before the test, and the helium flow rate is no less than 750 ml / min.

[0008] Preferably, in step (3), the sample to be tested is a sample target or a probe sheet, wherein the diameter of the sample target is ≤2.54cm and the thickness is ≤5mm, and the thickness of the probe sheet is ≥50μm.

[0009] Preferably, step (4) includes: taking pictures of the sample to be tested using transmitted light, reflected light, and cathodic emission, and analyzing the obtained pictures; determining the dating position of the carbonate mineral sample based on the cathodic emission image, selecting the substitution perpendicular to the cathodic emission changes of the carbonate mineral, and conducting laser line scanning or laser dot matrix analysis to test the content of trace rare earth elements such as U, Th, and Pb in the sample of the area to be tested. 206 Pb / 207 Pb, 206 Pb / 238 Different isotope ratios such as U; plotting the planar distribution map of trace rare earth element content and different isotope ratios; determining the dating test area based on the planar distribution map of trace rare earth element content and different isotope ratios, awaiting on-machine testing.

[0010] Preferably, in step (5), establishing the laser ablation carbonate dating test and analysis method includes: selecting the elements to be measured. 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 235 U and 238 U; Preferably, 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 15-30ms. 238 The U test time is 30-50ms; the number of single-point data sets is no less than 100 sets. Preferably, the measurement time interval for each test point is greater than or equal to the time required for the carrier gas purging erosion signal to decrease to the background value, preferably 5-20s.

[0011] Preferably, in step (6), the order of the test analysis sequence list is blank-SRM614, blank-standard sample A, blank-standard sample B, blank-sample to be tested, blank-SRM614, blank-standard sample A, blank-standard sample B; wherein, standard sample A is a carbonate mineral standard sample, a quantitative calibration standard; standard sample B is a carbonate mineral standard sample, a quality monitoring standard; and SRM614 is a standard glass sample.

[0012] Preferably, during the test, 5-10 sample points are inserted between the two sets of standard samples.

[0013] Preferably, in step (6), the method for adjusting the laser-mass spectrometer is as follows: continuously etch a standard glass sample to optimize the sensitivity, stability and oxide yield of the mass spectrometer, so that the mass spectrometer sensitivity reaches the highest level and the oxide content is ≤1%.

[0014] Preferably, in step (7), the laser preheating time is set to 8-20s, the ablation time to 25-40s, the purging time to 10-30s, and the laser energy density to 3-5mJ / cm². 2 .

[0015] Preferably, in step (7), multiple test points are measured. 238 U / 206 Pb and 207 Pb / 206 The ratio of Pb was used to plot the laser in-situ U-Pb age TW map of carbonate minerals.

[0016] The technical solution provided by this invention improves mass spectrometry through multiple steps, reduces the intensity of background Pb signal, and designs the test procedure to effectively determine the age of carbonate minerals, thereby improving the accuracy and success rate of the test. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of one embodiment of the method for determining the age of carbonate minerals provided by the present invention; Figure 2 This refers to the sensitivity characteristics of the improved mass spectrometer in Embodiment 1 of the present invention. Figure 3 This is a laser in-situ U-Pb dating TW diagram of calcite-filled Cambrian weathering crust in the northwest DB section of the sample tower tested in Example 1 of this invention. Figure 4 These are photographs of cracked dolomite samples from the Tailai area of ​​eastern Sichuan, tested in Example 2 of this invention, and a TW diagram of laser in-situ U-Pb dating of the samples. Figure 5The images show photographs of the matrix dolomite of the Maokou Formation in the Tailai area of ​​eastern Sichuan, tested in Example 3 of this invention, and a TW diagram of the sample obtained by laser in-situ U-Pb dating. Detailed Implementation

[0018] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0019] This invention provides a method for determining the age of carbonate minerals; please refer to the following: Figure 1 The method includes the following steps: (1) Mass spectrometry setup: The flow rate of the sensitizing gas is 3-10 ml / s; the injection cone group is JET cone and X cone, and the JET cone and X cone are expanded by 5-20%; the inner diameter of the central tube is expanded, and the inner diameter of the mass spectrometer central tube is 2.0 mm-2.8 mm; the vacuum pump speed is not less than 110 L / hour; (2) Reduce the background Pb signal intensity in the mass spectrometer to no more than 25 cps; (3) Cut the carbonate mineral sample to obtain the sample to be tested; (4) Perform rock and mineral analysis on the sample to be tested to determine the dating test area; (5) Establish a method for dating and analyzing carbonates by laser ablation; (6) Debug the laser-mass spectrometer connection and establish a test and analysis sequence list; (7) The standard sample and the sample to be tested were etched and tested in the order of the test and analysis sequence list, and the TW map of the in-situ U-Pb age of carbonate minerals was drawn to obtain the age of carbonate minerals.

[0020] The method for determining the age of carbonate minerals provided by this invention optimizes the experimental testing methods and improves mass spectrometry in multiple steps, while reducing the background Pb signal intensity. This can increase the high-resolution inductively coupled plasma mass spectrometry signal intensity by 6-15 times, reaching or even exceeding 590,000 cps / ppm. 238 The U content analysis can be performed on samples with U content below 10 ppb, which greatly improves the analytical capability and success rate of in-situ laser U-Pb dating of carbonate minerals.

[0021] In the method described in this invention, in step (1), the sensitizing gas is nitrogen.

[0022] In the method described in this invention, in step (1), the mass spectrometry sensitivity is improved by setting the mass spectrometry parameters. In some embodiments, step (1) specifically includes: ① Nitrogen sensitization, with nitrogen at a flow rate of 3-10 ml / s as the sensitizing gas; ② Replacing the sample injection cone assembly with JET and X cones with larger pore sizes, and further enlarging the JET and X cones by 5-20%; ③ Enlarging the inner diameter of the central tube, increasing the inner diameter of the mass spectrometer central tube from the original 1.75 mm to 2.0 mm-2.8 mm; ④ Replacing the vacuum pump with a high-power vacuum pump, wherein the vacuum pump pumping speed is not less than 110 L / h, preferably 110~200 L / h.

[0023] In some implementations, the vacuum level at the high-mass spectrometer inlet is also increased to enable ignition when using a large-aperture inlet cone assembly.

[0024] In some specific embodiments of the present invention, the injection cone assembly can be a JET cone and an X cone.

[0025] In some other embodiments of the present invention, the injection cone assembly can be a JET cone and an X cone after the borehole has been enlarged by 5 to 20%.

[0026] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by cleaning the laser cell.

[0027] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: using a flexible tube with Teflon coating, a hard tube with no or low Pb content, or a stainless steel tube as the gas pipeline.

[0028] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometer background is reduced by the following method: reducing the diameter and length of the air tube between the laser and the mass spectrometer to improve the transmission efficiency.

[0029] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: changing the gas mixing mode, modifying the gas path, and more preferably, mixing the sample gas directly with the argon compensation gas after the laser is emitted, and then mixing it with nitrogen in a counter-current mode.

[0030] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: mercury is removed by carrier gas filtration, and more preferably, helium gas is filtered by a mercury trap before entering the laser.

[0031] In some specific embodiments of the present invention, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following methods: shortening the tracheal distance, and more preferably setting the instrument purging time to be no less than 30 min before the test; the helium flow rate is no less than 750 ml / min, preferably 750~1000 ml / min.

[0032] In some specific embodiments of the present invention, step (3) includes, before cutting the carbonate mineral sample, selecting a suitable rock sample and location according to the research needs; and describing and photographing the occurrence, distribution characteristics, formation period and co-occurrence relationship of the carbonate mineral with other minerals and surrounding rocks.

[0033] In some embodiments of the present invention, the sample to be tested is a sample target or a probe sheet. Preferably, the diameter of the sample target is ≤2.54cm (1 inch), preferably 1~2.54cm, and the thickness is ≤5mm, preferably 2~5mm; the thickness of the probe sheet is ≥50μm, preferably 50~100μm.

[0034] When the content of U and Pb elements in the carbonate minerals of the sample to be tested is extremely low, randomly selecting the test area may easily lead to no signal intensity during the test, thus making it impossible to obtain the age test results of the sample. In this invention, by performing rock and mineral analysis on the sample to be tested in step (4), the dating test area is determined, thereby improving the success rate of laser in-situ U-Pb age testing.

[0035] In a preferred embodiment, step (4) includes: taking photographs of the sample to be tested using transmitted light, reflected light, and cathodic emission, and performing detailed petrological analysis on the obtained photographs to further clarify the occurrence, distribution characteristics, formation period, and co-occurrence relationship of carbonate minerals with other minerals; based on the cathodic emission images, selecting the substitution perpendicular to the cathodic emission changes of carbonate minerals to determine the dating position of the carbonate mineral sample, conducting laser line scanning or laser dot matrix analysis, and testing the content of trace rare earth elements such as U, Th, and Pb in the sample of the area to be tested. 206 Pb / 207 Pb, 206 Pb / 238 Different isotope ratios such as U; plotting the planar distribution of trace rare earth element content and different isotope ratios; based on the planar distribution of trace rare earth element content and different isotope ratios such as U and Pb, selecting areas with high content and favorable isotope ratios as dating test areas, awaiting on-machine testing.

[0036] In some specific embodiments of the present invention, step (5) of establishing a laser ablation carbonate dating test and analysis method includes: selecting the elements to be measured. 202 Hg, 204Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 235 U and 238 U. Preferably, 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 15-30ms. 238 The U test time is 30-50ms; the number of single-point data sets is no less than 110 sets, preferably 110-160 sets.

[0037] In some embodiments, the measurement time interval for each test point is determined by the time required for the carrier gas purging erosion signal to decrease to the background value. Preferably, the measurement time interval for each test point is greater than or equal to the time required for the carrier gas purging erosion signal to decrease to the background value. More preferably, the measurement time interval for each test point is 5-20 seconds.

[0038] In some specific embodiments of the present invention, in step (6), the blank is measured before each standard sample and unknown sample is tested. The order of the test sequence list is blank-SRM614, blank-standard sample A, blank-standard sample B, blank-sample to be tested, blank-SRM614, blank-standard sample A, blank-standard sample B.

[0039] In some preferred embodiments, establishing a test analysis sequence list includes: using a test procedure of blank-SRM614, blank-SRM614, blank-standard sample A, blank-standard sample A, blank-standard sample B, blank-standard sample B, blank-sample to be tested, ..., blank-sample to be tested, blank-SRM614, blank-SRM614, blank-standard sample A, blank-standard sample A, blank-standard sample B, blank-standard sample B.

[0040] It is understandable that a blank test is performed before each standard sample and the sample to be tested. For example, blank-standard sample A means that a blank test is performed before testing standard sample A.

[0041] In some implementations, 5-10 sample points are inserted between the two sets of standard samples during the testing process.

[0042] In the method described in this invention, standard sample A is a carbonate mineral standard sample, used for quantitative calibration; standard sample B is a carbonate mineral standard sample, used for quality control; and SRM614 is a standard glass sample. Specifically, the standard glass sample SRM614 is used to calibrate the carbonate mineral to be tested. 207 Pb / 206 Pb isotope ratio; Standard sample A is used to calibrate the carbonate minerals to be tested. 238 U / 206 Pb ratio; Standard sample B is a carbonate mineral standard sample with a known recommended value, which is analyzed along with the carbonate mineral sample to be tested to monitor the quality of the test results.

[0043] In some specific embodiments of the present invention, in step (6), the method for adjusting the laser-mass spectrometer is as follows: continuously etch a standard glass sample to optimize the sensitivity, stability and oxide yield of the mass spectrometer, so that the mass spectrometer sensitivity reaches the highest level and the oxide content is ≤1%.

[0044] In some specific embodiments of the present invention, before step (6), a carbonate mineral sample target or probe sheet needs to be loaded. Before loading, the sample surface is wiped clean with anhydrous ethanol to remove possible contamination. The instrument's laser ablation system is started according to the instrument's operating instructions, the carrier gas (He gas) flow rate is increased to the set value, and the sample cell and gas pipeline are continuously rinsed for about 30 minutes to remove any background Pb that may be present in the sample cell and gas pipeline.

[0045] In the method described in this invention, in step (7), there are no special restrictions on the specific parameters of the laser beam spot and the ablation frequency. Appropriate laser beam spot and ablation frequency should be selected based on the U and Pb content of the carbonate minerals. In some embodiments, the laser beam spot is 30-100 μm, and the ablation frequency is 2-15 Hz.

[0046] In some embodiments, in step (7), the laser preheating time is set to 8-20s, the ablation time to 25-40s, the purging time to 10-30s, and the laser energy density to 3-5mJ / cm². 2 .

[0047] In some specific embodiments of the present invention, step (7) includes: ① etching and testing the standard sample and the sample to be tested according to the order of the test analysis sequence list; ② data processing and monitoring: calibrating the test data of the carbonate mineral to be tested by measuring the two sets of standard samples before and after the sample to be tested; the standard glass sample is used to calibrate the carbonate mineral to be tested. 207 Pb / 206 Pb isotope ratio; carbonate mineral standard sample A is used to calibrate the carbonate mineral to be tested. 238 U / 206Pb ratio; Carbonate mineral standard sample B is a carbonate mineral standard with known recommended values, analyzed along with the carbonate mineral sample to be tested, to monitor the quality of the test results; ③ Calibrated values ​​obtained by measuring multiple test points. 238 U / 206 Pb and 207 Pb / 206 The Pb ratio was used to plot the laser in-situ U-Pb age (TW) diagram of carbonate minerals using Isoplot or other graphing software. Finally, based on previous sample observations and petrographic analysis results, the obtained laser in-situ U-Pb ages of carbonate minerals were interpreted.

[0048] In this invention, by ① improving mass spectrometry, the sensitivity of the mass spectrometer is increased to 590,000 CPS / ppm. 238 ① Reduce the instrument's ordinary lead background to 25 cps; ② Optimize the location of the test point by rock and mineral analysis and laser scanning and laser dot matrix analysis to determine the suitable test point location for dating; ③ Optimize the experimental test method to effectively determine the age of carbonate minerals, improving the accuracy and success rate of the test.

[0049] In one specific embodiment, the method for determining the age of carbonate minerals includes the following steps: (1) Improve mass spectrometry sensitivity: ① Nitrogen enhancement, with a nitrogen flow rate of 3-10 ml / s; ② Replace the injection cone assembly with a JET cone and X cone, or with a JET cone and X cone that have been enlarged by 5-20% to increase the sample injection volume; ③ Increase the inner diameter of the mass spectrometer center tube from the original 1.75 mm to 2.0 mm-2.8 mm; ④ Replace with a high-power vacuum pump, with a pumping speed of no less than 110 L / h; improve the vacuum level at the mass spectrometer injection end. After the above operations, the sensitivity of high-resolution mass spectrometry can be improved by 6-15 times, reaching or even exceeding 590,000 CPS / ppm. 238 U.

[0050] (2) Reducing the background signal intensity of mass spectrometry: ① Clean the laser cell; ② Use a flexible tube with Teflon coating, a hard tube with no or low Pb content, or a stainless steel tube for the gas pipeline; ③ Reduce the diameter and length of the gas tube between the laser and the mass spectrometer to improve transmission efficiency; ④ Modify the gas path. The modified gas mixing mode is: after the sample gas exits the laser, it is directly mixed with argon compensation gas, and then mixed with nitrogen in a counter-current mode; ⑤ Helium gas is filtered through a mercury trap before entering the laser; ⑥ Set the instrument purging time to no less than 30 minutes before testing, and the helium flow rate to no less than 750 ml / min. After the above operations, the high-resolution mass spectrometer achieves a sensitivity of 590,000 CPS / ppm. 238 Under the condition of U, the background Pb signal intensity can be controlled at around 25 cps.

[0051] (3) Select appropriate rock samples and locations according to research needs; and describe and photograph the occurrence, distribution characteristics, formation period and co-occurrence relationship of carbonate minerals with other minerals and surrounding rocks; pre-process the samples, cut the selected samples and locations to a reasonable size according to the size of the laser pool, and make epoxy resin sample targets (sample target diameter less than 1 inch, thickness ≤ 5 mm) or probe sheets (sample thickness ≥ 50 μm).

[0052] (4) Photographing: Photograph the prepared sample target or probe slide using transmitted light, reflected light, and cathodic emission. Detailed petrological analysis is conducted to further clarify the occurrence, distribution characteristics, formation period, and co-occurrence relationship of carbonate minerals with other minerals. Based on the cathodic emission images, the substitutional location perpendicular to the cathodic emission changes of carbonate minerals is selected to determine the dating position of the carbonate mineral sample. Laser line scanning or laser array analysis is carried out to test the content of trace rare earth elements such as U, Th, and Pb in the sample of the area to be tested. 206 Pb / 207 Pb, 206 Pb / 238 Different isotope ratios such as U; plotting the planar distribution of trace rare earth element content and different isotope ratios; based on the planar distribution of trace rare earth element content and different isotope ratios such as U and Pb, selecting the optimal testing area for dating, awaiting on-machine testing.

[0053] (5) Establish a laser ablation carbonate dating test and analysis method, and select the elements to be measured. 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 235 U and 238 U, of which 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 15-30ms. 238 The test time is 30-50ms; the number of single-point data sets is no less than 100 sets.

[0054] (6) Load the carbonate mineral sample target or probe sheet; before loading, wipe the sample surface clean with anhydrous ethanol to remove possible contamination. Start the instrument's laser ablation system according to the instrument's operating instructions, increase the carrier gas (He gas) flow rate to the set value, and continuously rinse the sample cell and gas pipeline for about 30 minutes to remove any background Pb that may be present in the sample cell and gas pipeline; turn on the mass spectrometer and enter the testing state, perform laser-mass spectrometry online debugging, and continuously ablate the artificially synthesized silicate glass SRM series standard glass to optimize the sensitivity, stability and oxide yield of the mass spectrometer, achieving the highest mass spectrometry sensitivity and oxide content ≤1%; Compile a sample analysis table: Before testing each standard sample and the sample to be tested, measure the blank first. During testing, adopt the following testing procedure: blank-SRM614, blank-SRM614, blank-standard sample A, blank-standard sample A, blank-standard sample B, blank-sample to be tested, ..., blank-sample to be tested, blank-SRM614, blank-SRM614, blank-standard sample A, blank-standard sample A, blank-standard sample B, blank-standard sample B. Insert 5-10 sample points between each set of standards. Standard sample A is a carbonate mineral standard sample, used for quantitative calibration. Standard sample B is a carbonate mineral standard sample, used for quality control. SRM614 is a standard glass sample.

[0055] (7) Based on the rock and mineral analysis results, select areas with high content and favorable isotope ratios, and set the sample test point locations; select appropriate laser beam spots and ablation frequencies based on the U and Pb content of carbonate minerals and instrument sensitivity; set the laser preheating time to 8-20s, the ablation time to 25-40s, the purging time to 10-30s, and the laser energy density to 3-5mJ / cm³. 2 The measurement time interval for each test point is 5-20 seconds.

[0056] The standard samples and the samples to be tested were etched and analyzed in the order of the sample analysis table; Data processing and monitoring: The test data of the carbonate mineral to be tested are calibrated by measuring two sets of standard samples before and after the sample to be tested; the standard glass sample SRM614 is used to calibrate the carbonate mineral to be tested. 207 Pb / 206 Pb isotope ratio; carbonate mineral standard sample A is used to calibrate the carbonate mineral to be tested. 238 U / 206 Pb ratio; Carbonate mineral standard sample B is a carbonate mineral standard with a known recommended value, which is analyzed along with the carbonate mineral sample to be tested to monitor the quality of the test results.

[0057] Results Presentation and Interpretation: Based on the calibrated carbonate minerals obtained from the tests... 238 U / 206 Pb and 207 Pb / 206 Using the Pb ratio, TW maps of in-situ laser-guided U-Pb ages of carbonate minerals were constructed using Isoplot or other graphing software, and the in-situ laser-guided U-Pb ages of the samples were calculated. Finally, based on previous sample observations and petrographic analysis results, the obtained in-situ laser-guided U-Pb ages of carbonate minerals were interpreted.

[0058] The method for determining the age of carbonate minerals provided by this invention firstly improves mass spectrometry by increasing nitrogen sensitivity, enlarging the aperture of the sample introduction cone, increasing the inner diameter of the central tube, and improving the vacuum at the mass spectrometer injection end, thereby increasing the instrument sensitivity by 6-15 times, reaching or even exceeding 590,000 CPS / ppm. 238 U; By cleaning the laser cell, replacing and shortening the gas pipeline and its inner diameter, changing the gas mixing mode, removing mercury from the carrier gas, and setting an appropriate purging time, the background Pb signal intensity can be controlled below 25 cps; then, based on rock and mineral analysis, using laser line scanning or lattice analysis techniques, the key elements and isotope ratios on the sample surface are tested, and the planar distribution of element content and isotope ratios is plotted; then, based on the distribution of element content and isotope ratios, the locations of favorable dating points for carbonate minerals are determined; next, the testing and analysis methods are optimized, the number of single-point analysis data sets is reduced, and selection is made... 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 238 U、 238 U-test, in which 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 15-30ms. 238The U-test time is 30-50 ms to improve the detection capability for ultra-low concentrations of radiogenic Pb. Then, the sample is injected into a laser ablation cell. A test sequence list is established, and the sensitivity, stability, and oxide yield of the mass spectrometer are calibrated. After the mass spectrometer reaches its optimal state, in-situ laser-guided U-Pb dating of carbonate minerals is performed. The test analysis results are calibrated using standard materials, and the accuracy of the test analysis results is verified. Finally, a TW plot of the in-situ U-Pb age of carbonate minerals is compiled using Isoplot or other plotting software. The method for in-situ laser-guided U-Pb age determination of carbonate minerals proposed in this invention, through improvements to the mass spectrometer, simultaneously reduces the background Pb signal intensity, and through the design of the test procedure, can effectively improve the success rate and accuracy of in-situ laser-guided U-Pb age determination of carbonate minerals.

[0059] The present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following embodiments are commercially available products.

[0060] Example 1 The sample to be tested in this embodiment is calcite filled with the weathering crust of the Cambrian top section of the DB section in the northwest of the Tarim Basin. The U content of this sample is particularly low, only 0.003 ppb.

[0061] The method for determining the age of carbonate minerals includes the following steps: (1) Improve mass spectrometry sensitivity: ① Nitrogen sensitization, with a nitrogen flow rate of 5 ml / s; ② The injection cone assembly consists of a JET cone and an X cone with 6% pore enlargement; ③ The inner diameter of the mass spectrometer center tube is 2.2 mm; ④ Replace with a high-power vacuum pump with a pumping speed of 110 L / h; Increase the vacuum level at the mass spectrometer injection end to 10 -7 mbar. After the above improvements, the sensitivity of high-resolution mass spectrometry is as follows: Figure 2 As shown, its sensitivity reaches 591414 CPS / ppm. 238 U.

[0062] (2) Reducing the background signal intensity of mass spectrometry: ① Clean the laser cell; ② Use a flexible tube with Teflon coating for the gas pipeline; ③ Reduce the diameter and length of the gas pipeline between the laser and the mass spectrometer; ④ Modify the gas path, the modified gas mixing mode is: after the sample gas exits the laser, it is directly mixed with argon compensation gas, and then mixed with nitrogen in a counter-current mode; ⑤ Helium gas is filtered through a mercury trap before entering the laser; ⑥ Set the instrument purging time to 30 minutes and the helium flow rate to 750 ml / min before the test. After the above operations, the high-resolution mass spectrometer achieved a sensitivity of 590,000 CPS / ppm. 238 Under conditions of around U, the background Pb signal intensity can be controlled to around 25 cps.

[0063] (3) Select appropriate rock samples and locations according to research needs; describe in detail and take pictures the occurrence, distribution characteristics, formation period and co-occurrence relationship of carbonate minerals with other minerals and surrounding rocks; pre-process the samples, cut the selected samples and locations to a reasonable size according to the size of the laser pool, and make epoxy resin sample targets (sample target diameter is 1 inch and thickness is 4 mm).

[0064] (4) Photograph the prepared sample target or probe slide using transmitted light, reflected light, and cathodoluminescence for detailed petrological analysis, further clarifying the occurrence, distribution characteristics, formation period, and co-occurrence relationship of carbonate minerals with other minerals; based on the cathodoluminescence images, select the substitution point perpendicular to the cathodoluminescence changes of carbonate minerals to determine the dating position of the carbonate mineral sample, and conduct laser line scanning or laser array analysis to test the content of trace rare earth elements such as U, Th, and Pb in the sample of the area to be tested. 206 Pb / 207 Pb, 206 Pb / 238 Different isotope ratios such as U; plotting the planar distribution of trace rare earth element content and different isotope ratios; based on the planar distribution of trace rare earth element content and different isotope ratios such as U and Pb, determining the dating test area through the distribution of element content such as U and Pb, and waiting for on-machine testing.

[0065] (5) Establish a laser ablation carbonate dating test and analysis method, and select the elements to be measured. 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 235 U and 238 U, of which 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 20ms. 238 The test time is 40ms; the number of single-point data sets is 100.

[0066] (6) Load the carbonate mineral sample target or probe sheet; before loading, wipe the sample surface clean with anhydrous ethanol to remove possible contamination. Start the instrument's laser ablation system according to the instrument's operating instructions, increase the carrier gas (He gas) flow rate to the set value, and continuously rinse the sample cell and gas pipeline for about 30 minutes to remove any background Pb that may be present in the sample cell and gas pipeline; turn on the mass spectrometer and enter the testing state, perform laser-mass spectrometry online debugging, and continuously ablate the artificially synthesized silicate glass SRM series standard glass to optimize the sensitivity, stability and oxide yield of the mass spectrometer, achieving the highest mass spectrometry sensitivity and oxide content ≤1%; Compile a sample analysis table: Before testing each standard sample and the sample to be tested, measure the blank first. The testing procedure is as follows: Blank-SRM614, Blank-SRM614, Blank-Standard Sample A, Blank-Standard Sample A, Blank-Standard Sample B, Blank-Sample to be tested, ..., Blank-Sample to be tested, Blank-SRM614, Blank-SRM614, Blank-Standard Sample A, Blank-Standard Sample A, Blank-Standard Sample B, Blank-Standard Sample B. Insert 5-10 test points between each set of standards. Standard Sample A is a carbonate mineral standard sample, used for quantitative calibration. Standard Sample B is a carbonate mineral standard sample, used for quality control. SRM614 is a standard glass sample.

[0067] (7) Based on the rock and mineral analysis results, select areas with high content and favorable isotope ratios to set the sample test point locations; based on the U and Pb content of carbonate minerals and the instrument sensitivity, select a laser beam spot of 90 μm and an ablation frequency of 3 Hz; set the laser preheating time to 10 s, the ablation time to 30 s, the purging time to 20 s, and the laser energy density to 4 mJ / cm³. 2 The measurement time interval for each test point is 10 seconds.

[0068] The standard samples and the samples to be tested were etched and analyzed in the order of the sample analysis table; Data processing and monitoring: The test data of the carbonate mineral to be tested are calibrated by measuring two sets of standard samples before and after the sample to be tested; the standard glass sample SRM614 is used to calibrate the carbonate mineral to be tested. 207 Pb / 206 Pb isotope ratio; carbonate mineral standard sample A is used to calibrate the carbonate mineral to be tested. 238 U / 206 Pb ratio; Carbonate mineral standard sample B is a carbonate mineral standard with a known recommended value, which is analyzed along with the carbonate mineral sample to be tested to monitor the quality of the test results.

[0069] Results Presentation and Interpretation: Based on the calibrated carbonate minerals obtained from the tests... 238 U / 206Pb and 207 Pb / 206 Using the Pb ratio, TW maps of in-situ laser-induced U-Pb ages of carbonate minerals were constructed using Isoplot or other plotting software, and the in-situ laser-induced U-Pb ages of the samples were calculated.

[0070] Laser in-situ U-Pb dating of calcite in the weathering crust at the top of the Cambrian strata in the DB section of the northwestern Tarim Basin (TW diagram as follows) Figure 3 As shown, this embodiment successfully obtained an age of 479 Ma for calcite filling the weathering crust. Finally, based on previous sample observations and rock and mineral analysis results, the obtained in-situ laser U-Pb ages of carbonate minerals were interpreted.

[0071] Example 2 The sample to be tested in this embodiment is Permian fractured dolomite from the Tailai area of ​​eastern Sichuan.

[0072] The test was performed according to the method described in Example 1, except that the injection cone assembly consisted of a JET cone and an X cone.

[0073] This embodiment obtains Figure 4 The image shows a Permian fractured dolomite from the Tailai area of ​​eastern Sichuan, along with its TW (Thin-Way) laser in-situ U-Pb dating. This embodiment successfully obtained an age of 253.2 ± 8.2 Ma for the fractured dolomite.

[0074] Example 3 The sample to be tested in this embodiment is Permian Maokou Formation dolomite from the Tailai area in eastern Sichuan.

[0075] The test was performed according to the method described in Example 1, and the results were obtained. Figure 5 The image shows the Permian Maokou Formation dolomite and its TW (Track-War) diagram obtained from in-situ U-Pb dating in the Tailai area of ​​eastern Sichuan. In this example, the age of dolomitization was successfully obtained as 251 ± 15 Ma.

[0076] Comparative Example 1 The test sample for this comparative example is calcite infiltrated with the weathering crust of the Cambrian upper layer in the DB section of the northwestern Tarim Basin. The U content of this sample is particularly low, only 0.003 ppb.

[0077] The method described in Example 1 was implemented, except that no modifications were made to the mass spectrometer; conventional mass spectrometer settings were used. Specifically, steps (1) were as follows: ① Nitrogen flow rate 5 ml / min; ② The injection cones were X-cone and H-cone; ③ The inner diameter of the mass spectrometer center tube was 1.75 mm; ④ A conventional vacuum pump was used, with a pumping speed of 30 L / h. The sensitivity of this conventional mass spectrometer was 40,000 CPS / ppm. 238 Around U.

[0078] The results showed that the testing method could not determine the age of the calcite filling in the weathering crust at the top of the Cambrian system in the DB section of the Tarim Basin in the Tarim Basin.

[0079] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for determining the age of carbonate minerals, characterized in that, Includes the following steps: (1) Mass spectrometry setup: The flow rate of the sensitizing gas is 3-10 ml / s; the injection cone group is JET cone and X cone, and the JET cone and X cone are expanded by 5-20%; the inner diameter of the central tube is expanded, and the inner diameter of the mass spectrometer central tube is 2.0 mm-2.8 mm; the vacuum pump speed is not less than 110 L / hour; (2) Reduce the background Pb signal intensity in the mass spectrometer to no more than 25 cps; (3) Cut the carbonate mineral sample to obtain the sample to be tested; (4) Perform rock and mineral analysis on the sample to be tested to determine the dating test area; (5) Establish a method for dating and analyzing carbonates by laser ablation; (6) Debug the laser-mass spectrometer connection and establish a test and analysis sequence list; (7) The standard sample and the sample to be tested were etched and tested in the order of the test and analysis sequence list, and the TW map of the in-situ U-Pb age of carbonate minerals was drawn to obtain the age of carbonate minerals.

2. The method according to claim 1, wherein, In step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by cleaning the laser cell; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: using a flexible tube with Teflon coating, a rigid tube with no or low Pb content, or a stainless steel tube as the gas pipeline; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by changing the gas mixing mode, more preferably by directly mixing the sample gas with argon compensation gas after laser emission, and then mixing it with nitrogen in a counter-current mode; Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: removing mercury by carrier gas filtration, and more preferably by filtering helium gas through a mercury trap before it enters the laser. Preferably, in step (2), the intensity of the Pb signal in the mass spectrometry background is reduced by the following method: the instrument purge time is set to be no less than 30 minutes before the test, and the helium flow rate is no less than 750 ml / min.

3. The method according to claim 1 or 2, wherein, In step (3), the sample to be tested is a sample target or a probe sheet, wherein the diameter of the sample target is ≤2.54cm and the thickness is ≤5mm, and the thickness of the probe sheet is ≥50μm.

4. The method according to any one of claims 1-3, wherein, Step (4) includes: taking pictures of the sample to be tested using transmitted light, reflected light, and cathodic emission, and analyzing the obtained pictures; determining the dating position of the carbonate mineral sample based on the cathodic emission image, selecting the substitution perpendicular to the cathodic emission change of the carbonate mineral, and conducting laser line scanning or laser dot matrix analysis to test the content of trace rare earth elements such as U, Th, and Pb in the sample of the area to be tested. 206 Pb / 207 Pb, 206 Pb / 238 Different isotope ratios such as U; plotting the planar distribution map of trace rare earth element content and different isotope ratios; determining the dating test area based on the planar distribution map of trace rare earth element content and different isotope ratios, awaiting on-machine testing.

5. The method according to any one of claims 1-4, wherein, In step (5), establishing a laser ablation carbonate dating analysis method includes: selecting the elements to be measured. 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th、 235 U and 238 U; Preferably, 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U and 232 The single-point, single-element, single-test time for Th is 15-30ms. 238 The U test time is 30-50ms; the number of single-point data sets is no less than 100 sets. Preferably, the measurement time interval for each test point is greater than or equal to the time required for the carrier gas purging erosion signal to decrease to the background value, preferably 5-20s.

6. The method according to any one of claims 1-5, wherein, In step (6), the order of the test analysis sequence list is blank-SRM614, blank-standard sample A, blank-standard sample B, blank-sample to be tested, blank-SRM614, blank-standard sample A, blank-standard sample B; wherein, standard sample A is a carbonate mineral standard sample, a quantitative calibration standard; standard sample B is a carbonate mineral standard sample, a quality monitoring standard; and SRM614 is a standard glass sample.

7. The method according to claim 6, wherein, During the test, 5-10 sample points are inserted between the two sets of standard samples.

8. The method according to any one of claims 1-7, wherein, In step (6), the method for adjusting the laser-mass spectrometer is as follows: continuously etch the standard glass sample to optimize the sensitivity, stability and oxide yield of the mass spectrometer, so that the mass spectrometer sensitivity reaches the highest level and the oxide content is ≤1%.

9. The method according to any one of claims 1-8, wherein, In step (7), the laser preheating time is set to 8-20s, the ablation time to 25-40s, the purging time to 10-30s, and the laser energy density to 3-5mJ / cm². 2 .

10. The method according to any one of claims 1-9, wherein, In step (7), multiple test points are measured. 238 U / 206 Pb and 207 Pb / 206 The ratio of Pb was used to plot the laser in-situ U-Pb age TW map of carbonate minerals.

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