A high spatial resolution calcite U-Pb dating method

Abstract

The present invention provides a high-spatial-resolution calcite U-Pb dating method, which adopts a mixed receiver mode, that is, a Faraday cup is used to receive U and Th, and an ion counter is used to receive Pb for calcite U-Pb dating. By optimizing the cone set and using nitrogen as the auxiliary gas to enhance the sensitivity of the instrument, accurate dating of calcite U-Pb with a high spatial resolution (≤60 μm) is achieved. Samples with a U content as low as 0.03 μg g-1 can be directly measured without switching the detector mode. At the same time, a mode of screening while dotting is adopted to improve the success rate of dating. The screening conditions for samples are 207Pb / 206Pb < 0.6 and 238U / 206Pb > 1.0. The present invention realizes the U-Pb dating technology for high-spatial-resolution calcite, overcoming the disadvantages of poor spatial resolution and low dating success rate in the prior art.

Description

Technical Field

[0001] The present invention belongs to the field of geological technologies, and particularly relates to a high-spatial-resolution calcite U-Pb dating method. Background Art

[0002] Isotope geochronology is one of the most fundamental directions in earth science research, providing quantitative constraints for determining the occurrence and duration of deep-time geological processes, mainly including dating systems such as U-Pb, Lu-Hf, Sm-Nd, Rb-Sr, Re-Os, and Ar-Ar. Calcite is widely formed in various geological environments such as diagenesis, biological processes, and hydrothermal processes. During the crystallization process, a certain amount of U (<10 μg g -1 ) and a small amount of Pb can be incorporated, which makes it have the potential for U-Pb dating. The calcite U-Pb dating technique has great application potential, especially in aspects such as paleoenvironment, sedimentation process, diagenesis, tectonic deformation, ore deposit genesis, and carbohydrate migration. The U content in calcite is low (generally below 5 μg g -1 ), and the common lead content is high. The traditional calcite U-Pb dating is mainly based on isotope dilution-thermal ionization mass spectrometry (abbreviated as ID-TIMS). This technique obtains data through four steps: sample drilling, acid dissolution and digestion, separation and purification, and on-machine testing. ID-TIMS can provide high-precision data quality, but the operation process of this technique is complex, time-consuming, and the quantity obtained is limited. As an overall analysis technique, the spatial resolution of ID-TIMS is low, which makes it have great limitations in the analysis of zoned samples.

[0003] In addition, there is also the LA-(Q,SF)-ICP-MS dating technique, but the spatial resolution of this method is low, and the laser beam spot needs to be more than 100 μm. Summary of the Invention

[0004] In view of the above technical problems, the present invention uses a mixed receiver mode (i.e., a Faraday cup to receive U and Th, and an ion counter to receive Pb) for calcite U-Pb dating. By systematically optimizing the cone set and using nitrogen as the auxiliary gas to increase the sensitivity of the instrument, accurate dating of calcite U-Pb with a high spatial resolution (≤60 μm) is achieved. For samples with a U content as low as 0.03 μg g -1 , direct determination can be carried out without switching the detector mode. At the same time, a mode of screening while laying out points is adopted to improve the success rate of dating. The screening conditions for samples are 207 Pb / 206 Pb < 0.6 and 238 U / 206Pb > 1.0。

[0005] The technical solution of the present invention is as follows:

[0006] A high-spatial-resolution calcite U-Pb dating method, comprising the following steps:

[0007] Step 1: For calcite samples or rock samples containing calcite, first cut them into reasonable sizes to make epoxy resin sample targets (sample target diameter 1 inch, thickness about 5 mm), or ordinary optical thin sections (sample thickness 30 - 50 μm) to fit the size of the laser ablation sample chamber;

[0008] Step 2: Place the sample into the laser ablation sample chamber and adjust the position of the sample in the optical axis direction to ensure good focusing of the laser beam;

[0009] Step 3: Conduct spot ablation pre-screening on the calcite sample, monitor 207 Pb / 206 Pb and 238 U / 206 Pb ratios. If 207 Pb / 206 Pb < 0.6 and 238 U / 206 Pb > 1.0, then determine that this point is the target sampling point, and arrange 5 analysis points around it. A total of 30 - 40 analysis points are screened out for each analysis sample;

[0010] Step 4: Conduct spot ablation on the defined target sampling points. The laser beam spot diameter is 20 - 60 μm, the ablation frequency is 10 Hz, and the energy density is 2.0 J cm -2 , and use the carrier gas to introduce the ablated aerosol into the MC-ICP-MS plasma source for ionization, and measure the ion signal data;

[0011] Step 5: In the low-resolution mode of the MC-ICP-MS instrument, adopt the mixed receiver mode (i.e., the Faraday cup receives U and Th, and the ion counter receives Pb), 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th and 238 U ion signal intensities. The central cup virtual mass number is set to 223.95, and H3 and H4 respectively receive 232 Th and 238 U, and IC4, IC5, IC3A, IC2, IC1B1 respectively receive 202 Hg, 204 Pb, 206 Pb, 207Pb, 208 Pb. IC3, IC2, and IC1B1 are traditional secondary electron multipliers (SEM), and IC4 and IC5 are small electron multipliers (CDD);

[0012] Step 6: Use a high-sensitivity interface cone (Jet + X cone set) and assist with nitrogen gas (Ar-N2 plasma) to sensitize the instrument;

[0013] Step 7: During the measurement process, after every ten unknown samples are tested, repeat the test of two NIST SRM 614, two ARM-3, three calcite reference materials WC-1, and two calcite reference materials Duff Brown Tank to ensure that the measurement conditions for the reference materials and unknown samples are the same;

[0014] Step 8: After obtaining the elemental signal data, first use NIST SRM 614 or ARM-3 to 207 Pb / 206 Pb, 238 U / 206 Pb ratio for instrument linear drift correction, and then according to the 207 Pb / 206 Pb, 238 U / 206 Pb measured values and their standard values of NIST SRM 614 and WC-1 reference materials, obtain the corresponding fractionation coefficients, and correct the 207 Pb / 206 Pb, 238 U / 206 Pb ratio of the unknown samples;

[0015] Step 9: Through the 207 Pb / 206 Pb, 238 U / 206 Pb ratios after the above corrections, construct a Tera-Wasserbug diagram, and calculate the age data of the calcite samples.

[0016] The purpose of the present invention is to establish a high-spatial-resolution calcite U-Pb dating method through a series of technical measures, providing more favorable technical support for related geochronology applications.

[0017] The present invention realizes the high-spatial-resolution calcite U-Pb dating technology. It overcomes the disadvantages of poor spatial resolution and low dating success rate in the prior art. Brief Description of the Drawings

[0018] Figure 1 is the flowchart of the present invention;

[0019] Figure 2aIt is the peak set diagram of U-Pb dating in the solution mode of the embodiment;

[0020] Figure 2b It is the peak set diagram of U-Pb dating in the laser mode of the embodiment;

[0021] Figure 3 It is for the four cone combinations (S+H, S+X, Jet+H and Jet+X) of the embodiment under the condition of introducing different amounts of nitrogen gas for 238 U and 206 Pb signal intensities and Th + / U + ratio;

[0022] Figure 4 It is the comparison diagram of U and Pb sensitivities for the four cone combinations (S+H, S+X, Jet+H and Jet+X) of the embodiment under the condition of introducing different amounts of nitrogen gas, where the data of Element XR are cited from Wu et al. (2022);

[0023] Figure 5 It is the Tera-Wasserburg diagram of the LA-MC-ICP-MS data of ASH-15D in the embodiment. The upper intersection point is fixed at 0.832. The instrument conditions of using the Jet+X cone group and introducing a small amount of N2 are adopted. In the figure, n = a / b, where "a" represents the total number of data after removing outliers, and "b" represents the total number of analysis times. The drawing is carried out by Isoplot software (Ludwig, 2003);

[0024] Figure 6 It is the Tera-Wasserburg diagram of the LA-MC-ICP-MS data of JT in the embodiment. The upper intersection point is fixed at 0.839. The instrument conditions of using the Jet+X cone group and introducing a small amount of N2 are adopted. In the figure, n = a / b, where "a" represents the total number of data after removing outliers, and "b" represents the total number of analysis times. The drawing is carried out by Isoplot software (Ludwig, 2003);

[0025] Figure 7 It is the Tera-Wasserburg diagram of the LA-MC-ICP-MS data of Duff Brown Tank in the embodiment. The upper intersection point is fixed at 0.738. The instrument conditions of using the Jet+X cone and introducing a small amount of N2 are adopted. In the figure, n = a / b, where "a" represents the total number of data after removing outliers, and "b" represents the total number of analysis times. The drawing is carried out by Isoplot software (Ludwig, 2003). Detailed implementation manners

[0026] The following further describes in combination with specific samples and the technical solutions involved in the present invention, but it does not limit the content of the present invention. As described above, the present invention provides a high-spatial-resolution calcite U-Pb dating method, as Figure 1 shown, the method includes the following steps:

[0027] Taking four calcite age reference materials: WC-1 (254±6.4 Ma), Duff Brown Tank (64.04±0.67 Ma), JT (13.797±0.031 Ma), ASH-15 (2.965±0.011 Ma), the specific implementation of the technical method of the present invention is introduced. The U content distribution of the four calcite reference samples is relatively wide (0.5–20 μg g -1 ), fully demonstrating the universality of this method for various calcite. These known ages are used as verification criteria in this method to test the accuracy and precision of this method.

[0028] First, the above-mentioned calcite sample is cast into a sample target (1 inch in diameter and about 5 mm in thickness) using epoxy resin. After slight polishing, the calcite section is exposed, and then it is polished, cleaned, dried and reserved for use, or directly prepared into a common optical thin section.

[0029] Put the calcite sample target into a laser ablation instrument, and purge the sample chamber with helium to remove air and fill it with helium.

[0030] In the low-resolution mode of MC-ICP-MS, the mixed receiver mode is adopted to receive 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th and 238 U ion signal intensities. The virtual mass number of the central cup is set to 223.95. H3 and H4 respectively receive 232 Th and 238 U, and IC4, IC5, IC3A, IC2, IC1B1 respectively receive 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb. IC3, IC2, IC1B1 are traditional secondary electron multipliers (SEM), and IC4, IC5 are small electron multipliers (CDD). The ion current is introduced into the SEM or CDD through a guiding device; the cup structure is shown in Table 1 below:

[0031] Table 1 Cup structure information of the MC-ICP-MS instrument

[0032]

[0033] In the solution mode, a multi-receiver peak fitting was performed using a mixed standard tuning solution. Under laser ablation, the accuracy of the peak fitting was verified by ablating NIST SRM614 or ARM-3. The results are as Figure 2a and Figure 2b .

[0034] Yield correction for different receivers. The dark noise and operating voltage of IC1B1, IC2, IC3, IC4, and IC5 were determined using the PCL program in the instrument operation software. The dark noise of each individual IC did not exceed 0.013 cps, and the operating voltages of these 5 ICs were all between 2500 V and 2800 V. The yield of each IC was determined by the dynamic peak skipping method in the solution injection mode. The signals of these 5 ICs and the central Faraday cup were successively received at approximately 120000 cps, with an integration time of 33.554 s, and 5 cycles were collected. Finally, the yield of each IC was normalized according to the average value of its signal intensity and the average value of the signal intensity of the central Faraday cup (1 mv = 62500 cps). The yields of the 5 ICs were between 80% and 90%.

[0035] In the laser ablation mode, the instrument conditions of the MC-ICP-MS were optimized using the NIST SRM 614 glass reference material (including sample gas flow rate, auxiliary gas flow rate, sampling cone, skimmer cone, etc.) to make 206 Pb and 238 U signals optimal, while ensuring that the oxide yield (ThO / Th) was less than 1.0%, the secondary ion yield (Ca 2+ / Ca + ) was less than 2.0%, and the Th + / U + ratio was higher than 0.3. Under the conditions of a laser beam spot of 60 μm, an ablation frequency of 10 Hz, and an energy density of 2.0 J cm -2 , 238 the U sensitivity was higher than 550000 cps / μg g -1 .

[0036] In the spot ablation mode, the laser beam spot was adjusted to be circular with a diameter of 60 μm, the laser energy density was 2.0 J / cm 2 , the ablation frequency was 10 Hz, and a spot ablation pre-screening of the sample was carried out to monitor 207 Pb / 206 Pb and 238 U / 206 Pb ratios. If 207 Pb / 206 Pb < 0.6 and 238 U / 206If Pb > 1.0, then determine that this point is the target sampling point, and arrange 5 analysis points around it. A total of 30 - 40 analysis points are screened out for each analysis sample;

[0037] Use a laser beam to perform point ablation on the delineated sampling point area (laser ablation diameter 20 - 60 μm, laser energy density 2.0 J / cm 2 , ablation frequency is 10 Hz), use the carrier gas to load the ablated aerosol into the MC - ICP - MS plasma source for ionization, and measure the ion signal data;

[0038] The acquisition program is as follows: instrument blank for 5 seconds, laser ablation line scan data for 30 seconds, and instrument blank for 10 seconds. After every 10 unknown samples, repeat the test of 2 NIST SRM 614 (for 207 Pb / 206 Pb calibration), 3 WC - 1 (for 238 U / 206 Pb calibration), 2 Duff Brow Tank (for data quality monitoring), to ensure that the measurement conditions for standard substances and unknown samples are the same. Each sample is subjected to 30 - 40 point analyses for constructing an isochron;

[0039] In the present invention, when performing U - Pb dating experiments, the ions to be measured and the mass numbers to be characterized are respectively 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th, 235 U, 238 U, and the measurement integration time is 0.131 seconds for all.

[0040] Use Iolite software (version 3.7) to process the data offline. First, subtract the instrument blank from the signal, calculate the ratios of 207 Pb / 206 Pb and 238 U / 206 Pb and their errors. Use NIST SRM 614 to correct the instrument drift for the ratios of 207 Pb / 206 Pb and 238 U / 206 Pb. Simulate the element fractionation curve according to NIST SRM 614 and use it to correct the element fractionation of unknown samples. Then, calculate the ratios of 207 Pb / 206 Pb, 238 U / 206The fractionation coefficient of the Pb instrument is shown in the following formula:

[0041]

[0042]

[0043] where Pb is the element lead, U is the element uranium, 206, 207, and 238 are the mass numbers of the elements, and k1 and k2 are 207 Pb / 206 Pb and

[0044] 238 U / 206 Pb fractionation coefficients of the ratio. Correct the unknown sample's 207 Pb / 206 Pb, 238 U / 206 Pb ratio according to the fractionation coefficient, as shown in the following formula:

[0045]

[0046]

[0047] Then, based on 207 Pb / 206 Pb, 238 U / 206 Pb ratio, construct a Tera-Wasserbug diagram to calculate the age data and initial lead composition of the calcite sample.

[0048] High-spatial-resolution calcite U-Pb dating requires high instrument sensitivity. Therefore, this invention explores the sensitization effect of the instrument with different cone sets and auxiliary nitrogen addition. In eight modes (S+H cone set+Ar plasma, S+H cone set+Ar-N2 plasma, S+X cone set+Ar plasma, S+X cone set+Ar-N2 plasma, Jet+H cone set+Ar plasma, Jet+H cone set+Ar-N2 plasma, Jet+X cone set+Ar plasma, Jet+X cone set+Ar-N2 plasma), adjust the instrument parameters to make 206 Pb and 238 U signals optimal, while ensuring that the oxide yield (ThO / Th) is less than 1.0%, the secondary ion yield (Ca 2+ / Ca + ) is less than 2.0%, and the Th + / U + ratio is higher than 0.3. The results are shown in Figure 3 and Figure 4 . It can be seen from the figure that the signal intensity is the highest under the combination of the Jet+X cone set and Ar-N2 plasma.

[0049] To further verify the reliability of this method for young samples in the high spatial resolution mode (60 μm), the present invention conducted an analysis on ASH-15D. The recommended ID-TIMS age of this reference sample is 2.965 ± 0.011 Ma. One analysis was performed on this sample using Jet+X cone combined with 4 mL min -1 nitrogen instrument parameters. Figure 5 A Tera-Wasserburg diagram was plotted, and the upper intersection point was fixed at 0.832 (Nuriel et al., 2020). The results showed that the common lead content of this sample varied greatly (5% - 90%), 238 U / 206 The precision of single-point analysis of U / Pb was 4% - 10% (2SE), 207 Pb / 206 The precision of single-point analysis of Pb / Pb was 2% - 20% (2SE). After fixing the upper intersection point ( 207 Pb / 206 Pb0 = 0.832, Nuriel et al., 2020), the lower intersection point ages were: 2.94 ± 0.07 Ma (2s, n = 30 / 30). Considering 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 (2.965 ± 0.011 Ma, 2s). The content range of U is 1 - 5 μg g -1 , verifying the universality of this method for young samples.

[0050] To further verify the reliability of this method for low-U content samples in the high spatial resolution mode (60 μm), the present invention conducted an analysis on JT using Jet+X cone set combined with 4 mL min-1 nitrogen instrument parameters. It can be seen from the figure that when using Jet+X cone set combined with 4 mL min -1 nitrogen instrument conditions. Figure 6 A Tera-Wasserburg diagram was plotted, and the upper intersection point was fixed at 0.839 (Guillong et al., 2020). The results showed that the common lead content of this sample varied greatly, ranging from 35% to 90%. The analysis result was 13.8 ± 0.8 Ma (2s, n = 27 / 30), which was consistent with the ID-TIMS age result (13.797 ± 0.031 Ma, 2s) within the error range. The U content of this sample was low, with a variation range of 0.03 - 1.72 μg g -1 , and the lowest U content was ~0.03 μg g -1 , which proved that this study is effective for U content as low as ~0.03 μg g -1 .

[0051] To further verify the reliability of this patent in a higher resolution mode (20 μm), the present invention used the Duff Brown Tank for method verification. This sample was analyzed once, using Jet + X cone combined with 4 mL min -1 nitrogen. Figure 7 A Tera-Wasserburg diagram was plotted, with the upper intersection fixed at 0.738 (Hill et al., 2016). The common lead content of this sample was 18% - 40%, 238 U / 206 The precision of the U / Pb single-point analysis was better than 3% (2SE), 207 Pb / 206 The precision of the Pb / Pb single-point analysis was better than 2% (2SE). The age results obtained by fixing the upper intersection were 66.0 ± 0.4 Ma (2s, n = 50 / 50) respectively, slightly higher than the previously reported ID-TIMS results (64.04 ± 0.67 Ma, 2s). Considering the ID-TIMS calibration error of WC-1 (~2.5%), the current uncertainty of LA-ICP-MS calcite U-Pb dating is 3.0% - 3.5%. Therefore, the obtained Duff Brown Tank results are consistent with the ID-TIMS results.

[0052] Combining the above data, the present invention provides a calcite U-Pb dating method with high spatial resolution (≤60 μm), which can provide more favorable technical support for the application of calcite geochronology.

[0053] The data in the above examples were all completed on a LA-ICP-MS consisting of a Photo Machine Analyst G2 excimer laser in series with an ElemnetXR ICP-MS. The examples are listed only for illustrating the present invention and not for limiting it. Those skilled in the art can obtain the same results on similar LA-ICP-MS according to this method.

[0054] Those skilled in the relevant art can make various changes and modifications without departing from the scope of the present invention. Therefore, all equivalent technical solutions should also fall within the scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A high-spatial-resolution calcite U-Pb dating method, characterized in that, For accurately dating calcite U-Pb with a resolution ≤ 60 μm, direct determination can be carried out for samples with a U content as low as 0.03 μg g -1 ; Including the following steps: Step 1: For calcite samples or rock samples containing calcite, first cut them to make epoxy resin sample targets, or use ordinary optical thin sections to fit the size of the laser ablation sample chamber; Step 2: Place the sample in the laser ablation sample chamber and adjust the position of the sample in the optical axis direction to focus the laser beam; Step 3: Conduct spot ablation pre-screening on the sample and monitor 207 Pb / 206 Pb and 238 U / 206 Pb ratios, select the target sampling points, arrange 5 analysis points around them, and a total of 30 - 40 analysis points are screened out for each analysis sample; Step 4: Perform spot ablation on the delineated target sampling points. The diameter of the laser beam spot is 20 - 60 μm, the ablation frequency is 10 Hz, and the energy density is 2.0 J cm -2 ; Use the carrier gas to introduce the ablated aerosol into the MC-ICP-MS plasma source for ionization, and measure the ion signal data; Step 5: In the low-resolution mode, the MC-ICP-MS instrument adopts the hybrid receiver mode, 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th and 238 the U ion signal intensities. The virtual mass number of the central cup is set to 223.

95. H3 and H4 respectively receive 232 Th and 238 U. IC4, IC5, IC3A, IC2, and IC1B1 respectively receive 202 Hg, 204 Pb, 206 Pb, 207 Pb, 208 Pb. IC3, IC2, and IC1B1 are traditional secondary electron multipliers SEM, and IC4 and IC5 are small electron multipliers CDD; Step 6: Use Jet + X cone and assist with nitrogen, namely Ar-N2 plasma, N2 = 4.0 mL min -1 , to sensitize the instrument signal; Step 7: During the measurement, after every ten unknown samples are tested, repeat the test of two NIST SRM 614, two ARM-3, three calcite reference materials WC-1, and two calcite reference materials Duff Brown Tank to ensure that the measurement conditions for the reference materials and unknown samples are the same; Step 8: After obtaining the elemental signal data, first use NIST SRM 614 or ARM-3 to 207 Pb / 206 Pb, 238 U / 206 perform instrumental linear drift correction on the Pb ratio, and then according to the 207 Pb / 206 Pb, 238 U / 206 measured values of Pb and their standard values, obtain the corresponding fractionation coefficients, and correct the 207 Pb / 206 Pb, 238 U / 206 Pb ratio of the unknown sample; Step 9: Through the 207 Pb / 206 Pb, 238 U / 206 Pb ratio, construct a Tera-Wasserburg diagram, and calculate the age data of the calcite sample.

2. A high-spatial-resolution calcite U-Pb dating method according to claim 1, characterized in that, In Step 1, the sample target has a diameter of 1 inch and a thickness of 5 mm, or an ordinary optical thin section with a sample thickness of 30 - 50 μm.

3. A high-spatial-resolution calcite U-Pb dating method according to claim 1, characterized in that, In Step 3 207 Pb / 206 Pb < 0.6 and 238 U / 206 Pb > 1.0, then it is determined that this point is the target sampling point.

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