Garnet U-Pb high-precision dating method, device and application based on NIST614 calibration

Abstract

The present invention relates to the technical field of laser ablation-quadrupole mass spectrometer coupling, and specifically, to a method, device and application of high-precision U-Pb dating of garnet based on NIST 614 calibration. It includes preparing a resin target containing NIST 614 standard material and a garnet sample to be tested; using a 193nm excimer laser ablation system to laser ablate the NIST 614 standard material and the garnet sample on the resin target. The present invention improves the test precision and accuracy of the garnet U-Pb dating method. Since the NIST 614 artificial synthetic glass standard is used for Pb / U ratio correction, the method of the present invention can more accurately correct the U and Pb isotope ratios in the sample, thereby obtaining more reliable age data. The economical and practical quadrupole mass spectrometer and the 193nm excimer laser ablation system are used to replace the traditional high-resolution magnetic field mass spectrometer or multi-acceptor isotope mass spectrometer, making the test equipment more economical and easy to operate, and reducing the test cost and complexity.

Description

Technical Field

[0001] The invention relates to the technical field of laser ablation-quadrupole mass spectrometer coupling, and in particular to a garnet U-Pb high-precision dating method, device and application based on NIST 614 calibration. Background Art

[0002] Garnet is a common rock-forming mineral widely distributed in metamorphic rocks, intrusive rocks and volcanic rocks. It is also the earliest skarn mineral in the skarn formation stage. It has a certain uranium content and a high isotope closure temperature (>850℃, Mezger et al., 1989), making it a potential U-Pb dating mineral. The U-Pb isotope dating of garnet can accurately limit the time limit of metamorphism and magmatic activity, and directly limit the starting time of contact metamorphism. Garnet dating has some advantages over other dating methods: (1) For the remote altered skarn, the related intrusive rock is far away from the mineralized area, and the age of the intrusive rock cannot well constrain its mineralization age; (2) Although the ore deposit has a diagenetic relationship with the related intrusive rock, and the intrusive age constrains the diagenetic age, the garnet crystallization occurs after the magma is intruded, so the garnet U-Pb age obtained is the skarn alteration age, which provides a more accurate minimum age for mineralization.

[0003] As a mineral suitable for U-Pb dating, garnet has the characteristics of low U content, complex composition (presence of U-containing mineral inclusions, such as crystalline uraninite, zircon, monazite, etc.), and relatively high non-radiogenic Pb content. These characteristics determine that the in situ U-Pb dating technology of garnet is much more difficult to test than zircon and other commonly used dating minerals. As a result, the in situ U-Pb dating research of garnet is relatively lagging and has not been widely used. Existing testing methods mainly improve the test precision, sensitivity and accuracy from two aspects:

[0004] On the one hand, the high-resolution magnetic field mass spectrometer and multi-acceptor isotope mass spectrometer with high sensitivity and high price are used for online testing with the laser ablation system to improve the determination precision and accuracy of U and Pb.

[0005] On the other hand, more appropriate data correction methods are being sought to correct test deviations caused by instrument drift, mass discrimination, isotope fractionation, etc., mainly including: using matrix-matched natural garnet standards as external standard substances (first, it is necessary to perform ID-TIMS age calibration on natural garnets with standard potential to obtain their "reference age"; then multiple laboratories and multiple methods are used to verify their age accuracy and uniformity, and only then can they be used as standard substances for isotope correction. This process is very time-consuming and expensive).

[0006] Based on the above-mentioned relevant background, the existing technology has the following disadvantages:

[0007] 1. The test equipment used is expensive and the test methods are complicated, which limits the feasibility and universality;

[0008] 2. Lack of universal standard materials: On the one hand, when using zircon 91500 with mismatched matrix as standard, the difference in fractionation effect between standard and sample will produce certain correction deviation; in particular, the difference in the content of the parent U element in zircon 91500 and garnet is too large (zircon 91500 is about 100ppm, while most of the garnet is less than 1ppm, and the U content of Mali garnet is about 1.1ppm), which will produce a large "concentration effect". In other words, under the same laser beam spot size and other ablation conditions, the signal difference between zircon 91500 and garnet samples is more than two orders of magnitude, and the test deviation that may be caused can be as high as On the other hand, if matrix-matched garnets such as Willsboro or Mali are considered as standard samples, the U content is relatively close, but the signal intensity and uniformity of U and Pb cannot fully meet the requirements of U-Pb age determination, thus affecting the final test accuracy. At the same time, the garnet structure itself can easily introduce a certain amount of non-radioactive constituent Pb, which will bring more correction deviations. In addition, the standard samples published in the existing literature are used for internal laboratory quality control and are not commercialized. They are difficult for general laboratories to obtain, which to a certain extent hinders the space for further verification and development. Summary of the invention

[0009] The object of the present invention is to provide a method, device and application of high-precision U-Pb dating of garnet based on NIST 614 calibration, so as to solve the problems raised in the above-mentioned background technology.

[0010] To achieve the above-mentioned purpose, on the one hand, the present invention provides a high-precision U-Pb dating method for garnet based on NIST 614 calibration, by adopting an economical and practical quadrupole mass spectrometer and a 193nm excimer laser ablation system online technology; using the artificially synthesized glass standard NIST 614 (internationally used, U content is 0.823ppm) with a U content that is relatively close to that of garnet, a relatively more consistent matrix, and very uniform U and Pb contents as a Pb / U ratio calibration standard sample, garnet U-Pb age determination and calibration are carried out, and a universal garnet in-situ U-Pb dating technology is established.

[0011] The specific method includes the following steps:

[0012] Step 1: preparing a resin target containing a NIST 614 standard material and a garnet sample to be tested;

[0013] Step 2: Use a 193 nm excimer laser ablation system to perform laser ablation on the NIST 614 standard material and the garnet sample on the resin target;

[0014] Step 3: Setting the parameters of the quadrupole mass spectrometer to detect the U and Pb isotopes of the aerosols produced by the erosion;

[0015] Step 4: Collect, process and analyze the detected data through data processing software to obtain the U-Pb age of the garnet sample;

[0016] Step 5: Use the Pb / U ratio of the NIST 614 standard material to calibrate the U-Pb isotope ratio of the garnet sample and calculate the high-precision U-Pb age of the garnet.

[0017] As a further improvement of the technical solution, the laser energy of the laser ablation system parameters is set to 3-4 J / cm2, and the ablation frequency is 10 Hz.

[0018] As a further improvement of the present technical solution, the mass numbers of U and Pb isotopes detected by the quadrupole mass spectrometer are 238 and 206, 207, 208 respectively.

[0019] As a further improvement of the technical solution, the NIST 614 standard material is an artificially synthesized glass standard sample with known U and Pb isotope compositions.

[0020] As a further improvement of the technical solution, the data processing software is Iolite 4, and the IsoplotR program is used for age calculation and uncertainty assessment.

[0021] As a further improvement of the technical solution, the garnet sample comes from metamorphic rocks, intrusive rocks and volcanic rocks and has a low U content.

[0022] On the other hand, the present invention provides an apparatus for the above-mentioned garnet U-Pb high-precision dating method based on NIST 614 calibration, comprising a 193nm excimer laser ablation system, a quadrupole mass spectrometer, a data processing computer and equipment for preparing resin targets.

[0023] The garnet U-Pb age data obtained by the garnet U-Pb high-precision dating method based on NIST 614 calibration is used for geological research, resource exploration and chronological analysis.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The high-precision U-Pb dating method, device and application of garnet based on NIST 614 calibration:

[0026] 1. Improved the test precision and accuracy of the garnet U-Pb dating method. Since the NIST 614 synthetic glass standard is used for Pb / U ratio correction, the method of the present invention can more accurately correct the U and Pb isotope ratios in the sample, thereby obtaining more reliable age data.

[0027] 2. The economical and practical quadrupole mass spectrometer and 193nm excimer laser ablation system are used to replace the traditional high-resolution magnetic field mass spectrometer or multi-acceptor isotope mass spectrometer, making the test equipment more economical and easier to operate, reducing the test cost and complexity, and facilitating its promotion and use in more geological laboratories.

[0028] 3. By using the internationally universal, homogeneous synthetic standard material NIST 614 for calibration, the method of the present invention is not only applicable to garnet, but can also be extended to the U-Pb dating of other low-U content minerals, providing geologists with a wider range of dating tools. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of isochron analysis in the isotope dating method of the experimental example of the present invention;

[0030] Figure 2 This is a bar graph of the 207Pb / 206Pb weighted average age error of the test example of the present invention. DETAILED DESCRIPTION

[0031] 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. Example

[0032] The embodiment of the present invention provides a garnet U-Pb high-precision dating method based on NIST 614 calibration, wherein the device used includes a 193nm excimer laser ablation system, a quadrupole mass spectrometer, a data processing computer, and equipment for preparing a resin target. The garnet U-Pb age data obtained by the dating method is used for geological research, resource exploration, and chronological analysis. The specific method includes the following steps:

[0033] Step 1: Make two synthetic glass standards, NIST 614 and ARM-3, natural garnet Mali, and zircon 91500 into 1-inch diameter resin targets;

[0034] Step 2: Select natural garnet samples related to geological bodies of known age, and perform data testing using a quadrupole mass spectrometer and a 193nm excimer laser ablation system according to the in-situ U-Pb collection method for accessory minerals;

[0035] Step 3: Use NIST 614 as the first standard to calibrate the Pb / U ratio, and use the natural garnet Mali and zircon standard 91500 as the second standard to check the quality of the data results;

[0036] Step 4: Calculate the ratio and error of 238U / 206Pb and 207Pb / 206Pb by using data processing software Iolite 4, and then calibrate the 238U / 206Pb and 207Pb / 206Pb fractionation of the second sample based on the NIST 614 fractionation curve;

[0037] Step 5. Based on the above test results, the online IsoplotR program is used to project the age of the intersection point under the YW diagram.

[0038] The application of the data processing computer in the high-precision U-Pb dating method of garnet based on NIST 614 calibration includes:

[0039] receiving and storing raw data transmitted from a quadrupole mass spectrometer;

[0040] Use Iolite 4 and other software for data preprocessing, such as background subtraction, signal integration, isotope ratio calculation, etc.;

[0041] The data were calibrated using the calibration curve of NIST 614 standard materials;

[0042] Calculate the U-Pb age of the sample and its uncertainty;

[0043] Output dating results, including tables, graphs and reports.

[0044] Test example

[0045] 1. Sample preparation:

[0046] Natural garnet samples of known age were selected from different geological bodies to ensure the wide applicability of the test results.

[0047] NIST 614, ARM-3, natural garnet Mali, zircon 91500 and natural garnet samples were prepared into a 1-inch diameter resin target to ensure that the position of each standard and sample was evenly distributed on the resin target.

[0048] 2. Test parameter settings:

[0049] A 193 nm excimer laser ablation system was used with the laser energy set to 4 J / cm2 and the ablation frequency set to 10 Hz.

[0050] The parameters of the quadrupole mass spectrometer were set as follows: the detection mass number of U was 238, and the detection mass numbers of Pb were 206, 207, and 208.

[0051] 3. Data collection:

[0052] Laser ablation was performed for at least 30 s on each sample to collect sufficient U and Pb signals.

[0053] When collecting data, first make at least 10 measurements of the NIST 614 standard to establish a calibration curve for the Pb / U ratio.

[0054] 4. Data processing:

[0055] Use Iolite 4 data processing software to import the acquired signal data.

[0056] The ratio calibration curves of 238U / 206Pb and 207Pb / 206Pb were established based on the measurement data of NIST 614 standard.

[0057] The calibration curve was used to calibrate the data of the Mali garnet sample.

[0058] 5. Result analysis:

[0059] After correction, 104 test points of the Mali garnet sample were analyzed, and the weighted average age of the 207Pb / 206Pb ratio was obtained to be 203.1±4.3Ma.

[0060] Comparison of this result with the reference age of Mali, 202.0 ± 1.2 Ma, confirmed that the age results were consistent within the error range.

[0061] according to Figure 1 and Figure 2 As shown, the garnet standard sample Mali tested a total of 104 points, with a fixed upper intersection point of 207Pb / 206Pb = 0.85. The age after projection correction in the TW diagram was 203.1±4.3 Ma, which was highly consistent with the reference age (202.0±1.2Ma, Seman et al., 2017) within the error range; the zircon standard sample 91500 measured a total of 66 points, and the weighted average age of 207Pb / 206Pb was 1073.1±1.6Ma, which was consistent with its reference age value (1065.4±0.3 Ma, Wiedenbecket al., 1995) within the error range.

[0062] 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. A high-precision U-Pb dating method for garnet based on NIST 614 calibration, characterized in that: The following steps are involved: Step 1: preparing a resin target containing a NIST 614 standard material and a garnet sample to be tested; Step 2: Use a 193 nm excimer laser ablation system to perform laser ablation on the NIST 614 standard material and the garnet sample on the resin target; Step 3: Setting the parameters of the quadrupole mass spectrometer to detect the U and Pb isotopes of the aerosols produced by the erosion; Step 4: Collect, process and analyze the detected data through data processing software to obtain the U-Pb age of the garnet sample; Step 5: Use the Pb / U ratio of the NIST 614 standard material to calibrate the U-Pb isotope ratio of the garnet sample and calculate the high-precision U-Pb age of the garnet; The laser energy of the laser ablation system parameter setting is 3-4 J / cm 2 , the erosion frequency is 10Hz; The mass numbers of U and Pb isotopes detected by the quadrupole mass spectrometer are 238 and 206, 207, 208, respectively; The NIST 614 standard material is a synthetic glass standard with known U and Pb isotopic compositions; The garnet samples are from metamorphic, intrusive and volcanic rocks and have low U content.

2. The garnet U-Pb high-precision dating method based on NIST 614 calibration according to claim 1, characterized in that: The data processing software was Iolite 4, and the IsoplotR program was used for age calculation and uncertainty assessment.

3. A device for the high-precision U-Pb dating method for garnet based on NIST 614 calibration according to any one of claims 1 to 2, characterized in that: The invention comprises a 193nm excimer laser ablation system, a quadrupole mass spectrometer, a data processing computer and equipment for preparing resin targets.

4. An application of a high-precision U-Pb dating method for garnet based on NIST 614 calibration, characterized in that: The garnet U-Pb age data obtained by the garnet U-Pb high-precision dating method based on NIST 614 calibration as described in claim 1 is used for geological research, resource exploration and chronological analysis.

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