Calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system

Abstract

The invention relates to the technical field of geological sample dating, in particular to a calcite U-Pb dating method based on quadrupole mass spectrometry and a laser ablation system. The method comprises the following steps: carrying out point denudation pre-screening on a calcite sample, monitoring the ratio of 207Pb / 206Pb to 238U / 206Pb, and judging a potential target sampling point; a high-energy laser ablation system and a quadrupole mass spectrometer are combined in an on-line manner, and nitrogen is assisted to sensitize the instrument. According to the invention, by introducing a quadrupole mass spectrometer with relatively low cost and a laser combined measurement technology, the problems of low sensitivity and high detection limit caused by extremely low U content of calcite, high background noise and substrate effect are effectively solved. The detection sensitivity of the low-U-content sample is remarkably improved, the detection limit is reduced, the sample which is originally difficult to detect becomes analyzable, and the application range of the dating technology is widened. Compared with the prior art, the dating precision is guaranteed, the cost is lower, the testing time is shorter, and the application is wider.

Description

Technical Field

[0001] The invention relates to the technical field of geological sample dating, and in particular to a calcite U-Pb dating method based on a quadrupole mass spectrometer and a laser ablation system. Background Art

[0002] Dating sedimentary strata is a global problem, especially in areas without volcanic ash and magmatic activity.

[0003] At present, although LA-ICP-MS technology has shown great potential in calcite U-Pb dating, due to the extremely low U content in calcite, high background noise and base effect, the technology faces the problems of insufficient sensitivity and high detection limit in practical applications. This limits its ability to accurately analyze low U content samples. The mixed receptor mode used by predecessors, such as the patent publication number: CN115586240A, a high spatial resolution calcite U-Pb dating method, although it can overcome the sensitivity, the high sensitivity requirement of the mass spectrometer increases the detection cost, which requires high-precision and expensive instruments for detection, which relatively limits the promotion of in-situ carbonate U-Pb dating methods. Summary of the invention

[0004] The purpose of the present invention is to provide a calcite U-Pb dating method based on a quadrupole mass spectrometer and a laser ablation system. By adjusting the internal parameters of the mass spectrometer, the stability and reliability of data acquisition are enhanced, ensuring the repeatability and consistency of the analysis results under long-term operation. At the same time, a quadrupole mass spectrometer is used to ensure the sensitivity of low-U content sample detection in the carbonate U-Pb dating process at a relatively low cost, reducing the experimental detection cost, so as to solve the problems raised in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides a calcite U-Pb dating method based on a quadrupole mass spectrometer and a laser ablation system, comprising the following steps: Step 1: Cut the calcite sample to make one of epoxy resin sample targets or common optical thin slices; Step 2: placing the calcite sample into the laser ablation sample chamber, and adjusting the position of the calcite sample in the direction of the optical axis; Step 3: Pre-screen the calcite samples for point erosion and monitor 207 Pb / 206 Pb and 238 U / 206 Pb ratio, determine the potential target sampling point, arrange 3-6 analysis points around it, and screen a total of 40-60 analysis points for each analysis sample; Step 4: Use a high-energy laser ablation system and a quadrupole mass spectrometer in combination, and use nitrogen as an auxiliary gas to enhance the sensitivity of the instrument; use nitrogen as the sensitizing gas to improve the test sensitivity; by introducing auxiliary nitrogen, not only the sensitization effect of the instrument is achieved, but also the gas blank of the system is effectively reduced; this improvement measure makes the U-Pb dating data of calcite at high spatial resolution more reliable, and even if the U content in the sample is as low as 0.03μg / g, it can be directly measured; The target sampling points are point-ablated, and the aerosol produced is carried into the Q-ICP-MS plasma source using helium as carrier gas for ionization and isotope signal data is measured. Using a quadrupole mass spectrometer, the following isotopes were analyzed: 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th, 238 U signal collection and recording; the measurement integration times were 0.002, 0.002, 0.04, 0.08, 0.005, and 0.02 seconds, respectively, and the above isotope signal collection period totaled 0.171 seconds; Step 5: During the measurement process, after testing ten unknown calcite samples, repeat the test with two NIST SRM614, two ARM-3, three calcite international standard materials WC-1 and three calcite standard materials Tarim. 207 Pb / 206 Pb, 238 U / 206 Pb ratio correction; Step 6: After the above correction 207 Pb / 206 Pb, 238 U / 206 Pb ratio, construct the Tera-Wasserbug plot, and calculate the age data of calcite samples.

[0006] As a further improvement of the technical solution, in step 1, the epoxy resin sample target is made to have a diameter of 1 inch and a thickness of 4-8 mm.

[0007] As a further improvement of the technical solution, in the step 1, the thickness of the ordinary optical thin film is 40-60 μm.

[0008] As a further improvement of the present technical solution, in step 4, the nitrogen flow rate is 3 ml / min.

[0009] As a further improvement of the technical solution, in step 4, when performing ablation, the laser beam spot diameter is 80-160 μm, the ablation frequency is 10 Hz, and the energy density is 2.8-3.6 J / cm -2 , the helium flow rate is 550-750ml / min.

[0010] As a further improvement of this technical solution, in step 5, NIST SRM 614 or ARM-3 is first used to 207 Pb / 206 Pb, 238 U / 206 The Pb ratio was corrected for instrument linear drift and laser ablation deep pit fractionation, and then the WC-1 standard material 207 Pb / 206 Pb, 238 U / 206 The Pb measured value and its standard value are used to obtain the corresponding secondary correction coefficient for the unknown calcite sample. 207 Pb / 206 Pb, 238 U / 206 Pb ratio was corrected.

[0011] As a further improvement of the technical solution, the upper limit of the detector pulse mode test of the quadrupole mass spectrometer is changed, so that the upper limit of the test of the quadrupole mass spectrometer is increased from 10E6 to 10E7cps, which greatly improves the test accuracy of the quadrupole mass spectrometer.

[0012] The present invention systematically optimizes the combination of the laser ablation system and the mass spectrometry detection system, adopts a novel NWR193HE 193nm excimer laser ablation system, is equipped with a TV3 third-generation double-volume sample ablation cell and a CoherentCOMPex102F high-energy laser generator; at the same time, an Agilent7900 quadrupole mass spectrometer is connected as an element / isotope detection system; the purpose is to optimize the combination of the laser ablation system and the quadrupole mass spectrometer and improve the sensitivity.

[0013] Replace the material of the connecting pipe for transmitting aerosol between the high-energy laser ablation system and the quadrupole mass spectrometer to reduce gas blank; use E-3603 hose as the connecting pipe for transmitting aerosol between the laser and the quadrupole mass spectrometer. This type of hose exhibits excellent chemical resistance and has a smooth inner surface. This feature not only helps to maintain the cleanliness of the tube wall, but also effectively prevents the accumulation of substances produced by laser ablation in the tube, thereby ensuring the stability and efficiency of material transmission between the laser and the mass spectrometer.

[0014] Compared with the prior art, the present invention has the following beneficial effects: In the calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system, by introducing a relatively low-cost quadrupole mass spectrometer and laser joint measurement technology, the problem of extremely low U content in calcite, high background noise, and base effect, which leads to insufficient sensitivity and high detection limit, is effectively overcome. After optimizing the parameter configuration, the present invention significantly improves the detection sensitivity of low-U content samples, reduces the detection limit, makes samples that were originally difficult to detect analyzable, and broadens the scope of application of the dating technology. More importantly, compared with the existing technology, it not only guarantees the dating accuracy, but also is more economical in cost, faster in test time, and more widely used. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is the Tera-Wasserburg diagram of Test Example 1 of the present invention; Figure 2 The Tera-Wasserbug diagram is a summary of the experimental age results of the Tarim sample under different beam spot conditions of Experimental Example 2 of the present invention. DETAILED DESCRIPTION

[0016] The following will be combined with the accompanying drawings in the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0017] Embodiment: The embodiment of the present invention provides a calcite U-Pb dating method based on a quadrupole mass spectrometer and a laser ablation system, comprising the following steps: Step 1: For calcite samples or rock samples containing calcite, first cut them into a reasonable size and make epoxy resin sample targets (sample target diameter 1 inch, thickness 4-8mm) or ordinary optical thin slices (sample thickness 40-60μm) to adapt to the size of the laser ablation sample chamber.

[0018] Step 2: Place the sample in the laser ablation sample chamber and adjust the position of the sample in the direction of the optical axis so that the laser beam is well focused.

[0019] Step 3: Pre-screen the calcite samples for point erosion and monitor 207 Pb / 206 Pb and 238 U / 206 Pb ratio, such as 207 Pb / 206 If Pb is less than 0.7, the point is determined to be a potential target sampling point, and 3-6 analysis points are arranged around it. A total of 40-60 analysis points are screened for each analysis sample.

[0020] Step 4: Use the new NWR193HE high-energy laser ablation system and Agilent 7900 quadrupole mass spectrometer in combination, and use nitrogen (flow rate: 3mL / min) (Ar-N 2 Plasma) to sensitize the instrument; The target sampling points were ablated at the designated points, with a laser beam spot diameter of 80-160 μm, an ablation frequency of 10 Hz, and an energy density of 2.8-3.6 J / cm -2 The aerosol was loaded into the Q-ICP-MS plasma source using helium as carrier gas (flow rate: 550-750 mL / min) for ionization and isotope signal data was measured.

[0021] Using a quadrupole mass spectrometer, the following isotopes were analyzed: 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th, 238 U signal collection and recording; the measurement integration times were 0.002, 0.002, 0.04, 0.08, 0.005, and 0.02 seconds, respectively, and the total isotope signal acquisition period was 0.171 seconds.

[0022] Step 5: During the measurement process: Laser pre-ablation time: 10s (warmup / gas blank) + 1s pre-ablation + 30s washout; Laser ablation time: 10s (warmup / gas blank) + 20s ablation + 30s washout); After testing every ten unknown samples, repeat the test with two NIST SRM 614, two ARM-3, three calcite international standard materials WC-1 (254±6.4Ma) and three calcite standard materials Tarim (208 Ma) to ensure that the measurement conditions of the standard materials and the unknown calcite samples are the same; After obtaining the isotope signal data, first use NIST SRM 614 or ARM-3 to 207 Pb / 206 Pb, 238 U / 206 The Pb ratio was corrected for instrument linear drift and laser ablation deep pit fractionation, and then the WC-1 standard material 207 Pb / 206 Pb, 238 U / 206The Pb measured value and its standard value are used to obtain the corresponding secondary correction coefficient for the unknown calcite sample. 207 Pb / 206 Pb, 238 U / 206 Pb ratio was corrected.

[0023] Step 6: After the above correction 207 Pb / 206 Pb, 238 U / 206 Pb ratio, construct the Tera-Wasserbug plot, and calculate the age data of calcite samples.

[0024] Test Example 1: In order to further verify the reliability of the age-strong sample in high spatial resolution mode (150μm), this test example used the WC-1 international standard carbonate sample as the test object for an analysis. The ID-TIMS recommended age of the sample was 254±6.4Ma. The specific method is as follows: The new NWR193HE high-energy laser ablation system and Agilent 7900 quadrupole mass spectrometer were used in combination, with a nitrogen flow rate of 3 mL / min and a beam spot diameter of 150 μm. After data processing, a Tera-Wasserburg plot was drawn, with the upper intersection point fixed at 0.85 (Nuriel et al., 2020). Figure 1 The results show that the common lead content of the sample varies greatly (5-90%), the precision of the single-point analysis of 238U / 206Pb is 4-10% (2SE), and the precision of the single-point analysis of 207Pb / 206Pb is 19-30% (2SE). Considering that the uncertainty of LA-ICP-MS calcite U-Pb dating is 3.0-3.5%, these two results are consistent with the ID-TIMS result (254.4±6.4Ma, 2s). The U content varies from 640,000 to 700,000 cps, which verifies the universality of the method of the present invention for young samples.

[0025] Test Example 2: In order to further verify the reliability and optimal beam spot range of this test example for low U content samples in high spatial resolution mode, the method of the present invention used Tarim carbonate samples as the object to perform a change in beam spot diameter analysis, which were five groups of experiments with beam spots of 90μm, 100μm, 110μm, 120μm, 130μm, 140μm, and 150μm. The results of the analysis are shown in Tables 1, 2 and Figure 2, which is consistent with the ID-TIMS age result (208.5±0.6 Ma, 2s) within the error range. The current uncertainty of LA-ICP-MS calcite U-Pb dating is 3.0-3.5%, so the obtained Tarim result is consistent with the ID-TIMS result. This shows that under the parameter conditions provided by the method of the present invention, the beam spot condition of 90-150μm can be applied.

[0026] Table 1: Experimental age results of Tarim samples under different beam conditions Beam spot size Experimental age Error margin 90μm 208.7 ±1.6 100μm 211.0 ±3.9 110μm 208.3 ±3.5 120μm 213.3 ±3.1 130μm 219.3 ±6.6 140μm 222.8 ±3.8 150μm 212.3 ±1.8 Table 2 Experimental results of Tarim samples under different beam conditions

[0027] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. Calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system, characterized in that: The following steps are involved: Step 1: Cut the calcite sample to make one of epoxy resin sample targets or common optical thin slices; Step 2: placing the calcite sample into the laser ablation sample chamber, and adjusting the position of the calcite sample in the direction of the optical axis; Step 3: Pre-screen the calcite samples by point denudation, monitor the 207Pb / 206Pb and 238U / 206Pb ratios, determine potential target sampling points, and arrange 3-6 analysis points around them. A total of 40-60 analysis points are screened for each analysis sample; Step 4: Use a high-energy laser ablation system and a quadrupole mass spectrometer in combination, and use nitrogen to enhance the sensitivity of the instrument; The target sampling points are point-ablated, and the aerosol produced is carried into the Q-ICP-MS plasma source using helium as carrier gas for ionization and isotope signal data is measured. A quadrupole mass spectrometer was used to collect and record the signals of the following isotopes: 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th, and 238U. The measurement integration times were 0.002, 0.002, 0.04, 0.08, 0.005, and 0.02 seconds, respectively. The total acquisition period of the above isotope signals was 0.171 seconds. Step 5: During the measurement process, after every ten unknown calcite samples are tested, two NIST SRM 614, two ARM-3, three calcite international standard materials WC-1 and three calcite standard materials Tarim are tested repeatedly to calibrate the 207Pb / 206Pb and 238U / 206Pb ratios of the unknown calcite samples; Step 6: Use the above-corrected 207Pb / 206Pb and 238U / 206Pb ratios to construct the Tera-Wasserbug diagram and calculate the age data of the calcite sample.

2. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: In the step 1, the epoxy resin sample target is made to have a diameter of 1 inch and a thickness of 4-8 mm.

3. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: In the step 1, the thickness of the common optical thin sheet is 40-60 μm.

4. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: In the step 4, the nitrogen flow rate is 3 ml / min.

5. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: In the step 4, when ablation is performed, the laser beam spot diameter is 80-160 μm, the ablation frequency is 10 Hz, the energy density is 2.8-3.6 J / cm-2, and the helium flow rate is 550-750 ml / min.

6. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: In the step 5, NIST SRM 614 or ARM-3 is first used to perform instrument linear drift and laser ablation deep pit fractionation correction on the 207Pb / 206Pb and 238U / 206Pb ratios, and then the corresponding secondary correction coefficients are obtained based on the 207Pb / 206Pb and 238U / 206Pb measured values ​​of the WC-1 standard material and their standard values, and the 207Pb / 206Pb and 238U / 206Pb ratios of the unknown calcite sample are corrected.

7. The calcite U-Pb dating method based on quadrupole mass spectrometry and laser ablation system according to claim 1, characterized in that: The upper limit of the detector pulse mode test of the quadrupole mass spectrometer is increased from 10E6 to 10E7 cps.

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