In-situ micro-area beryl Rb-Sr dating correction method

Abstract

The invention discloses an in-situ micro-area beryl Rb-Sr dating correction method which comprises the following steps: 1, after a selective reaction active gas reacts with Sr, calculating an 86Sr signal by using a signal of a product, so as to improve the accuracy of a Sr element signal; 2, correcting the ratio of 87Rb / 87Sr and the ratio of 86Sr / 87Sr by adopting a glass standard substance with a uniform Rb / Sr isotope ratio; the method comprises the following steps of: drawing an inverse isochrone graph by adopting a substrate-matched beryl standard substance HJLG241 or XXG251, calculating an apparent age, calculating a substrate effect deviation coefficient according to the apparent age and a recommended age, and applying the coefficient to 86Sr / 87Sr ratio correction of an unknown sample. According to the method, accurate dating of the in-situ micro-area beryl Rb-Sr is realized, accurate dating of the laser micro-area beryl Rb-Sr is realized, and the method has the advantages of high spatial resolution, high dating success rate, short test period and the like.

Description

Technical Field

[0001] This invention relates to the field of geological dating technology, and in particular to an in-situ micro-area beryl Rb-Sr dating correction method. Background Technology

[0002] Isotope geochronology is the most direct quantitative method for determining the timing and duration of deep-seated geological processes. The Rb-Sr isotope dating system is widely used in Earth science research. Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) is often used in this dating system. This technique acquires data through four steps: sample drilling, acid digestion, separation and purification, and instrumental analysis. In recent years, the newly developed laser ablation triple quadruple inductively coupled plasma mass spectrometry (LA-ICP-MS / MS) micro-area Rb-Sr dating technique has further expanded the application scope of this system. It features high spatial resolution (μm level), fast analysis speed (~2 minutes / ablation point), and the ability to separate isotopic interferences online (e.g., ...). 87 Rb to 87 (Sr interference). In LA-ICP-MS / MS micro-area Rb-Sr dating analysis, matrix-matched standard materials are required for fractional calibration of Rb / Sr isotopic values. These standard materials need to have uniform elemental ratios and be free of common Sr. However, such standard materials are extremely rare in nature, and most samples have non-uniform Rb / Sr elemental ratios, which greatly limits the application of this dating technique. Therefore, establishing a calibration method based on non-uniform Rb / Sr elemental ratio standard materials is extremely important and can greatly promote the further application of this technique.

[0003] The chemical formula of natural, pure beryl is Be3Al2[Si6O] 18Beryl is a colorless and transparent mineral. It commonly contains elements such as Cr, Fe, Ti, V, and Mn. Beryl is the most abundant of the 52 known beryllium-bearing minerals globally, with important gemstone varieties including emerald and aquamarine. Its crystals are mostly long prismatic, with alkali-rich crystals often appearing as short prismatic shapes. Common single crystals are predominantly hexagonal prisms, followed by hexagonal bipyramidals, with longitudinal striations on the prism faces, more pronounced in alkali-free crystals. Beryl often tends to form large, well-formed crystals. As a rare beryllium-bearing mineral and a precious gemstone, beryl is not only an important source of beryllium but also a key strategic mineral resource for the nation. Therefore, conducting beryl isotopic dating has significant geological and economic value. Beryl can contain a certain amount of Rb and Sr during crystallization, so the Rb-Sr system can be used for dating. Beryl contains a certain amount of Rb, while containing little or no Sr, indicating its potential for Rb-Sr dating. However, due to its low Rb content, it poses certain challenges to analytical instruments and methods. Therefore, Rb-Sr dating of beryl has not yet been developed and applied.

[0004] Current technologies generally employ isotope dilution-thermal ionization mass spectrometry (ID-TIMS), which acquires data through four steps: sample extraction, acid digestion, separation and purification, and instrumental analysis. However, this technique has the following drawbacks: 1. Long analytical process; 2. Limited sample quantity; 3. Low spatial resolution and low success rate. Summary of the Invention

[0005] This invention provides an in-situ micro-region beryl Rb-Sr dating correction method to address the shortcomings of existing technologies.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0007] An in-situ micro-area beryl Rb-Sr dating correction method includes the following steps:

[0008] Step 1: Cut the beryl sample or beryl-containing rock sample into a sample target or ordinary optical thin section of appropriate size.

[0009] 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.

[0010] Step 3: Perform pre-etching on the target sampling points to prepare for subsequent point etching analysis.

[0011] Step 4: Perform point ablation on the delineated target analysis points using a laser, and then load the ablation products into an ICP-MS / MS plasma source for ionization.

[0012] Step 5: Using SF6 (or N2O) as the reactant gas under ICP-MS / MS, react with... 87 Sr reacts to achieve 87 Sr and 87 Online separation of Rb, with simultaneous acquisition 9 Be, 27 Al, 29 Si, 85 Rb, 84 Sr, 86 Sr, 87 Sr, 88 Sr, (85+19) Rb (or (85+16) Rb), (86+19) Sr (or (86+16) Sr), (87+19) Sr (or (87+16) Sr) and (88+19) Sr (or (88+16) The intensity of the ionic signal of the element Sr.

[0013] Step 6: During the measurement process, periodically test two glass reference materials NIST SRM 610 (or OA-1 / OH-1) and two beryl reference materials HJLG241 (or XXG251) to ensure the consistency and accuracy of the measurement conditions.

[0014] Step 7: After obtaining the elemental signal data from Step 5, since beryl minerals themselves contain little or no ordinary Sr, the sensitivity requirements of in-situ laser micro-area analysis methods pose a significant challenge. Therefore, this invention proposes to... 88 Sr and its product signals were calculated 86 Sr signal, thereby overcoming the limitations imposed by existing sensitivity. 86 The problem of low accuracy in Sr signal data was solved by calculating... 87 Rb / 87 Sr and 86 Sr / 87 The ratio data of Sr ratio, for 87 Rb / 86 Sr and 87 Sr / 86 The Sr ratio was used for instrument drift correction and mass fractionation correction.

[0015] Step 8: Based on the HJLG241 (or XXG251) beryl standard material... 87 Rb / 87 Sr and 86 Sr / 87 Using the Sr ratio and fixing the upper intersection point, construct inverse isochrones and calculate the inverse isochron age; then, using the calculated isochron age and the recommended age, calculate the fractionation coefficient for unknown samples. 87 Rb / 87 The Sr ratio is corrected.

[0016] Preferably, in step 1, the sample target has a diameter of 1 inch and a thickness of about 5 mm, while the thickness of a typical optical sheet sample is 30-50 μm.

[0017] Preferably, in step 3, the target sampling points are pre-etched, and a total of 30-100 points are analyzed for each sample.

[0018] Preferably, in step 4, the diameter of the point ablation laser beam is 60-130 μm, the ablation frequency is 10 Hz, and the energy density is 4.0-8.0 J / cm². -2 .

[0019] Preferably, in step 6, after testing every 10 unknown samples, two glass reference materials NISTSRM 610 and two beryl reference materials HJLG241 are tested repeatedly.

[0020] Preferably, in step 8, the upper intersection point is fixed. 86 Sr / 87 Sr should be a single apatite mineral from the same source region. 86 Sr / 87 The value of Sr.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] 1. Accurate in-situ micro-area dating of Rb-Sr in beryl was achieved: By combining laser micro-area ablation and online mass spectrometry analysis, in-situ analysis of beryl samples can be performed, thus achieving accurate Rb-Sr dating.

[0023] 2. Overcomes the requirement of uniform standard samples for Rb / Sr element ratio in existing calibration methods: Compared with traditional methods, this method no longer needs to rely on uniform standard samples for calibration, reducing the complexity and cost of the experimental procedure.

[0024] 3. Accurate dating of Rb-Sr in beryl achieved by laser micro-area ablation: Laser micro-area ablation technology can selectively ablate the sample surface to obtain Rb-Sr isotope data with spatial resolution, thus achieving accurate dating of beryl.

[0025] 4. High spatial resolution: Utilizing laser micro-area ablation technology, precise local analysis of samples can be performed, achieving high spatial resolution dating of beryl samples.

[0026] 5. High dating success rate: Because beryl itself contains little or no ordinary Sr, this invention proposes to achieve high dating success rates through... 88 Sr and its product signals were calculated 86 This overcomes the limitation imposed by existing sensitivity on the Sr signal. 86 The problem of low accuracy of Sr signal data can be addressed by precisely controlling the laser ablation and analysis process, which can improve the success rate of dating, reduce the failure rate in experiments, and ensure the reliability of data.

[0027] 6. Short testing cycle: Compared with traditional dating methods, this method has a shorter testing cycle, which can obtain the dating results of samples more quickly and improve experimental efficiency. Attached Figure Description

[0028] Figure 1 This is a flowchart of the two-step Rb-Sr correction method according to an embodiment of the present invention.

[0029] Figure 2 This is a diagram showing the Rb-Sr dating results of the inverse isochron of beryl standard material HJLG241 in an embodiment of the present invention.

[0030] Figure 3 This is the inverse isochron Rb-Sr dating result of the first beryl sample in this embodiment of the invention;

[0031] Figure 4 This is the inverse isochron Rb-Sr dating result of the second beryl sample in this embodiment of the invention;

[0032] Figure 5 This is the inverse isochron Rb-Sr dating result of the third beryl sample in this embodiment of the invention;

[0033] Figure 6 This is the inverse isochron Rb-Sr dating result of the fourth beryl sample in this embodiment of the invention;

[0034] Figure 7 This is the inverse isochron Rb-Sr dating result of the fifth beryl sample in this embodiment of the invention;

[0035] Figure 8 This is the inverse isochron Rb-Sr dating result of the sixth beryl sample in this embodiment of the invention;

[0036] Figure 9 This invention is proposed through embodiments. 88 Sr and its product signals were calculated 86 Comparison chart of Sr signal accuracy.

[0037] Figure 10 This is a diagram of the error propagation model in an embodiment of the present invention.

[0038] Figure 11 This is a graph showing the relationship between the Rb content and Rb / Sr ratio of beryl samples and mica group minerals in an embodiment of the present invention.

[0039] Figure 12 This is a graph showing the percentage of different Sr contents in eight beryl samples from embodiments of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0041] This invention provides an in-situ micro-region Rb-Sr dating correction method, which is applied to beryl dating. The method includes the following steps:

[0042] (1) Taking one beryl age standard material, HJLG241 (1797 ± 21 Ma), and six beryl samples as examples, the annual Rb content of the six beryl samples is widely distributed (17.8–531 ppm), and the Rb / Sr content ratio ranges from 9 to 695, fully demonstrating the universality of this method for various beryls. These known ages are used as verification standards in this method to test its accuracy and precision. Information on all beryl samples is shown in Table 1, where Sol represents the solution ID-MC determination and Laser represents LA-ICP-MS.

[0043] Table 1 Information on Beryl Samples

[0044]

[0045] (2) First, cast the above beryl sample into a sample target (1 inch in diameter and about 5 mm thick) using epoxy resin. After slight polishing, expose the beryl cut surface. Then polish, clean and dry for later use, or directly prepare ordinary light thin film (sample thickness 30–50 μm).

[0046] (3) Place the beryl sample target into the laser ablation instrument and purge the sample chamber with helium to remove air and fill it with helium.

[0047] (4) Trace elements (including Rb, Sr, etc.) in beryl were determined by LA-ICP-MS. The laser ablation spot size was set according to the Rb content. When the Rb content was 1–20 ppm, the laser spot size was 60–90 μm, and when the Rb content was 20–600 ppm, the laser spot size was 30–60 μm. The results are shown in Table 2. Figure 2 Trace element data for seven beryl samples determined by LA-ICP-MS are presented, with elemental content data given in ppm.

[0048] Table 2. Major element oxides (wt.%) and trace element composition (ppm) of beryl

[0049]

[0050] (5) The ICP-MS / MS instrument was set to triple quadrupole (TQ) mode, using NIST SRM 610, and the instrument parameters were adjusted in laser line scan mode to achieve the best instrument performance. (88+19) The Sr signal is optimal, making 85 The reaction yield of Rb is less than 0.00025%.

[0051] (6) Pre-etch the target sampling points to remove contamination from the sample surface. A total of 30-100 points are selected for each analysis sample.

[0052] (7) In order to verify the accuracy of this method, the Rb-Sr age of three beryl samples was determined by the ID-MC solution determination method in this embodiment. The results are shown in Table 3.

[0053] Table 3 ID-MS values ​​of beryl samples

[0054] (Sr was tested using TIMS, and Rb was tested using MC-ICP-MS)

[0055]

[0056] (8) Point ablation is performed on the target analysis points. The laser beam spot diameter is 60-130 μm, the ablation frequency is 10 Hz, and the energy density is 4.0-8.0 J / cm². -2 The aerosols etched out are loaded into an ICP-MS plasma source using a carrier gas for ionization.

[0057] (9) Use nitrogen as an auxiliary gas to enhance the sensitivity of the instrument, and adjust the nitrogen flow rate to make it so that (88+19) The Sr signal is optimal.

[0058] (10) Collection 9 Be (2 ms) 27Al (2 ms) 28 Si (1 ms) 85 Rb (1 ms) 84 Sr (50 ms), (84+19 / 84+16) Sr (100 ms) (85+19 / 85+16) Rb (2 ms) 86 Sr (20 ms) (86+19 / 86+16) Sr (50 ms), 87 Sr (20 ms) (87+19 / 87+16) Sr (50 ms) (88) Sr (20 ms) and (88+19 / 88+16) Sr (100 ms) ion signal intensity.

[0059] (11) The data acquisition procedure was as follows: 15 seconds of instrument blanking, 75 seconds of laser ablation line scanning data, and 20 seconds of instrument blanking. During the measurement process, after testing every 10 unknown samples, two glass standard materials NIST SRM 610 and two beryl standard materials HJLG241 were tested repeatedly to ensure that the measurement conditions for the standard materials and unknown samples were the same. Each sample underwent 30-100 point analyses.

[0060] (12) After obtaining the element signal data, calculate 87 Rb / 87 Sr and 86 Sr / 87 The Sr ratio was determined using NIST SRM 610. 87 Rb / 87 Sr and 86 Sr / 87 The Sr ratio is used for instrument drift correction.

[0061] (13) Based on HJLG241 beryl standard material 87 Rb / 87 Sr and 86 Sr / 87 Sr ratio, with a fixed initial Sr value, recommended ( 87 Sr / 86 Sr)0 is a fixed value of apatite from the same source region and the same period (simultaneous and same source). 87 Sr / 86 Sr, construct inverse isochrones, and calculate the inverse isochron age; using the calculated isochron age and the recommended age, calculate the fractionation coefficient for unknown samples. 87 Rb / 87 The Sr ratio is corrected; the two-step correction process is as follows: Figure 1 As shown.

[0062] (14) The results of obtaining the beryl standard material HJLG241 are as follows: Figure 2 As shown.

[0063] To further demonstrate the feasibility of the two-step Rb-Sr correction method proposed in this invention, the data results were verified using six beryl samples. The Rb-Sr dating results are as follows: Figures 3 to 8 As shown.

[0064] from Figure 9 The results clearly show that the accuracy and precision of Rb-Sr inverse isochronous dating with a fixed upper intersection point are significantly better than those with an unfixed upper intersection point or an inappropriate upper intersection point. The Rb-Sr age data with a fixed upper intersection point is consistent with the recommended age within the error range, verifying the reliability of the two-step correction strategy proposed in this invention and demonstrating the practicality of beryl Rb-Sr dating. Therefore, this embodiment recommends fixing the initial Sr value. However, it should be noted that fixing an inappropriate initial Sr value may introduce additional systematic errors and should be handled with caution based on specific examples.

[0065] The two-step Rb-Sr dating correction method proposed in this invention, and the age error propagation model using a fixed upper intersection point for unknown samples, are further clarified, and the results are as follows: Figure 10 As shown.

[0066] To further understand the chemical composition of beryl, we plotted the relationship between the Rb content and Rb / Sr ratio of beryl samples and mica group minerals, as well as plots showing the different Sr content percentages of eight beryl samples. Figure 11 and Figure 12 As shown in the figure, the difference between beryl and mica group minerals can be seen. Beryl has a lower Rb content and a higher Rb / Sr ratio, thus it has the potential to be suitable for Rb-Sr dating, but it also places higher demands on the testing technology.

[0067] Based on the above data, this invention provides an in-situ micro-area beryl Rb-Sr dating correction method, which can provide more favorable technical support for the application of beryl geochronology.

[0068] The data in the above embodiments were all obtained on a LA-ICP-MS / MS system using a Photo Machine Analyst G2 excimer laser in series with an iCap TQICP-MS / MS. The examples provided are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can obtain the same results on similar LA-ICP-MS / MS systems using this method.

[0069] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the implementation methods of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of the present invention.

Claims

1. An in-situ micro-area beryl Rb-Sr dating correction method, characterized in that, Includes the following steps: Step 1: Cut the beryl sample or beryl-containing rock sample into a sample target of appropriate size or a common optical section; Step 2: Place the sample in the laser ablation sample chamber and adjust the position of the sample in the optical axis direction to ensure the laser beam is focused; Step 3: Perform pre-etching on the target sampling points to prepare for subsequent point etching analysis; Step 4: Perform point ablation on the delineated target analysis points using laser ablation, and load the ablation products into an ICP-MS / MS plasma source for ionization; Step 5: Using SF6 or N2O as the reactant gas, react with... 87 Sr reacts to achieve 87 Sr and 87 Online separation of Rb, with simultaneous acquisition 9 Be, 27 Al, 28 Si, 85 Rb, 84 Sr, 86 Sr, 87 Sr, 88 Sr, (85+19 / 85+16) Rb, (86+19 / 86+16) Sr, (87+19 / 87+16) Sr and (88+19 / 88+16) The ionic signal intensity of Sr isotopes; Step 6: During the measurement process, periodically test two glass reference materials NIST SRM 610 or OA-1 / OH-1 and two beryl reference materials HJLG241 or XXG251 to ensure the consistency and accuracy of the measurement conditions. Step 7: After obtaining the elemental signal data from Step 5, the 86Sr signal is calculated using the signals of the products 88+19SrF or 88+16SrO after the selective reactive gas reacts with Sr, and then the 86Sr signal is calculated. 87 Rb / 87 Sr and 86 Sr / 87 The ratio data of Sr ratio, for 87 Rb / 86 Sr and 87 Sr / 86 The Sr ratio was used for instrument drift correction and mass fractionation correction. Step 8: Based on HJLG241 or XXG251 beryl standard material 87 Rb / 87 Sr and 86 Sr / 87 Using the Sr ratio and fixing the upper intersection point, construct inverse isochrones and calculate the inverse isochron age; then, using the calculated inverse isochron age and the recommended age, calculate the fractionation coefficient for unknown samples. 87 Rb / 87 The Sr ratio is corrected.

2. The in-situ micro-area beryl Rb-Sr dating correction method according to claim 1, characterized in that: In step 1, the sample target is 1 inch in diameter and about 5 mm thick, while the thickness of a typical optical sheet sample is 30-50 μm.

3. The in-situ micro-area beryl Rb-Sr dating correction method according to claim 1, characterized in that: In step 3, the target sampling points are pre-etched, and a total of 30-100 points are analyzed for each sample.

4. The in-situ micro-area beryl Rb-Sr dating correction method according to claim 1, characterized in that: In step 4, the diameter of the point ablation laser beam is 60-130 μm, the ablation frequency is 10 Hz, and the energy density is 4.0-8.0 J / cm². -2 .

5. The in-situ micro-area beryl Rb-Sr dating correction method according to claim 1, characterized in that: In step 6, after testing every 10 unknown samples, repeat the testing of two glass reference materials NIST SRM 610 or OA-1 / OH-1 and two beryl reference materials HJLG241 or XXG251.

6. The in-situ micro-area beryl Rb-Sr dating correction method according to claim 1, characterized in that: In step 8, fix the upper intersection point. 86 Sr / 87 Sr is a single mineral of apatite produced in the same source region. 86 Sr / 87 The value of Sr.

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