Method for obtaining age of Rb-Sr in potassium-rich mineral microcell

Abstract

The invention relates to the technical field of geological exploration, in particular to a method for obtaining the age of Rb-Sr in a potassium-rich mineral microcell. The technical operation process is simple, and complicated chemical pretreatment is not needed; various sample carriers (such as sample targets or rock slices and the like) can be directly analyzed, and high spatial resolution (50-100 microns) is achieved; the analysis speed is high (1t, 2 minutes); the Rb-Sr dating result has relatively high accuracy (100.1%-101.6%) and precision (superior to + / -2.3%).

Description

Technical Field

[0001] This invention relates to the field of geological exploration technology, and in particular to a method for obtaining Rb-Sr ages of potassium-rich mineral micro-regions. Background Technology

[0002] Rb-Sr radioisotope dating is an important technique for studying the formation and evolution of geological bodies. Its principle is based on... Radioactive beta decay generation By accurately measuring the content of minerals or rocks and The ratio is used to calculate the age using the isochron method. However, the development of this technique has long been limited by sample pretreatment and analytical techniques.

[0003] Traditional solution-based Rb-Sr dating methods mainly employ isotope dilution-thermal ionization mass spectrometry (ID-TIMS). This typically involves the following steps: (1) Sample selection and crushing: manually selecting single mineral particles or whole-rock powder in the range of hundreds of milligrams to grams; (2) Chemical dissolution: using strong acids such as hydrofluoric acid (HF) and nitric acid (HNO3) to digest the sample under high temperature and pressure for an extended period; (3) Chemical separation and purification: using ion exchange chromatography to separate and purify Rb and Sr elements from other matrix elements (especially K, Ca, rare earth elements, etc.) in the sample solution to obtain pure Rb and Sr fractions; (4) Thermal ionization mass spectrometry (TIMS) determination: coating the purified Rb and Sr solution onto a metal filament and performing high-precision isotope ratio determination using TIMS. This method has the following inherent defects: (1) The process is cumbersome and time-consuming: The entire process, from sample processing to data acquisition, usually takes at least two weeks, which is extremely inefficient and cannot meet the needs of modern efficient geological research. (2) The sample requirement is large: A large amount (milligram level) of pure minerals is required, which is extremely difficult to select for fine-grained, rare, or coexisting samples with other minerals, and may even be impossible. (3) Spatial information is completely lost: This is one of the most fatal defects. The entire mineral grain or rock is crushed and dissolved to obtain an average age, which completely destroys the micro-regional age information recorded by different zonations, different stages of inclusions, or alteration edges within the mineral. This micro-regional information is crucial for understanding complex geological processes (such as multi-stage mineralization, hydrothermal alteration, metamorphic accretion, etc.).

[0004] To overcome the shortcomings of solution methods, researchers began to try to combine laser ablation (LA) systems with inductively coupled plasma mass spectrometry (ICP-MS) for in-situ micro-area Rb-Sr dating. However, before the maturity and application of tandem mass spectrometry (MS / MS) technology, this method faces a series of insurmountable technical bottlenecks: (1) severe isotope interference: this is the primary technical obstacle to micro-area Rb-Sr dating. right Direct overlapping interference: and With the same mass-to-charge ratio (m / z = 87), they are indistinguishable on a standard single quadrupole ICP-MS. Large variations in the Rb / Sr ratio within the sample can severely interfere with... Accurate determination of the ratio is the most critical parameter in age calculation. (2) Significant matrix effect: Different minerals (such as mica and feldspar) have different physical properties (hardness, thermal conductivity, laser absorption efficiency) and chemical composition. Under the same laser ablation conditions, the particle size distribution, transmission efficiency and ionization efficiency of the generated aerosol particles are significantly different. This matrix effect makes it difficult to accurately correct all types of minerals using a universal standard material (such as NIST SRM 610 glass), thus introducing systematic errors and affecting the accuracy of dating. (3) Scarcity of standard materials: Internationally recognized mineral standard materials suitable for micro-area Rb-Sr dating are extremely scarce. In particular, for common dating minerals such as mica and potassium feldspar, there is a lack of standard materials that have been fully homogeneously verified and accurately valued, which makes it impossible to effectively carry out data quality control and inter-laboratory comparison, seriously restricting the promotion of this technology and the reliability of geological applications. Summary of the Invention

[0005] To address the aforementioned problems, this application discloses a method for obtaining Rb-Sr ages of potassium-rich mineral microregions, by targeting instrument sensitivity, and The research focuses on the accuracy of isotope ratio testing, optimization of instrument parameters and calibration strategies for matrix effects, and aims to solve the problem of how to achieve high spatial resolution, high precision and high accuracy Rb-Sr dating of potassium-rich minerals using femtosecond laser ablation-tandem mass spectrometry.

[0006] To achieve the above objectives, this application discloses a method for obtaining Rb-Sr ages of potassium-rich mineral microregions, employing laser ablation coupled with inductively coupled plasma mass spectrometry (ICP-MS), comprising the following steps:

[0007] S1. Using a laser, the aerosols obtained after pre-etching and formal etching of the sample carrier of the sample to be tested and the natural mineral standard material of the same mineral type as the sample to be tested are loaded into the Q-ICP-MS / MS plasma source. The elemental signal data of 85 / 85Rb, 86 / 102Sr, 87 / 103Sr and 88 / 104Sr of the natural mineral standard material and the sample to be tested are measured in the bar mode.

[0008] S2. Based on the above elemental signal data, the values ​​of the natural mineral standard material and the sample to be tested are calculated respectively. 87 Rb / 86 Sr、 87 Sr / 86 Sr ratio;

[0009] S3. First calibration: based on the glass standard reference material. 87 Rb / 86 Sr、 87 Sr / 86 By comparing the measured and standard values ​​of Sr, the corresponding fractionation coefficients are obtained for the sample to be tested. 87 Rb / 86 Sr、 87 Sr / 86 The Sr ratio is corrected;

[0010] S4. Secondary calibration: The sample is calibrated using natural mineral standard materials after the initial calibration. 87 Rb / 86 The Sr ratio is corrected twice;

[0011] S5. After secondary correction 87 Rb / 86 Sr、 87 Sr / 86 Sr ratios were used to construct isochron plots, and the age data and initial Sr isotopes of the samples were calculated. 87 Sr / 86 Composition of Sr.

[0012] The types of natural mineral standard substances are consistent with the types of minerals in the sample to be tested, and the quantity is N, where N≥1.

[0013] Furthermore, in S1, line scanning ablation is used during both laser pre-ablation and formal ablation.

[0014] Furthermore, the laser frequency for the pre-abrasion is 8-10 Hz, and the energy density is 2-3.5 J / cm². 2 The etch diameter is 50-60 micrometers, the line scan speed is 50 micrometers / s, the line scan length is 100 micrometers, and the etch time is 2s.

[0015] Furthermore, the laser ablation beam diameter for the formal ablation is 50-60 micrometers, the line scan speed is 2 micrometers / s, the line scan length is 100 micrometers, and the laser energy density is 3.5 J / cm². 2 The erosion frequency is 8-10Hz.

[0016] Furthermore, before measuring elemental signal data using the Q-ICP-MS / MS plasma source as described in S1, the ICP-MS instrument conditions need to be optimized, specifically as follows:

[0017] The instrument conditions for ICP-MS in single-bar mode were optimized using NIST 610 glass, resulting in... 206 Pb and 238 The U signal is highest, while ensuring oxide yield (ThO) + / Th + The secondary ion yield (Ca) is less than 0.5%, and the secondary ion yield (Ca) is less than 0.5%. 2+ (Ca+) less than 2.0%;

[0018] N₂O reactive gas was introduced into the collision cell. The gas flow rate and collision cell voltage were optimized using a NIST 610 glass pair in Mass / Mass mode to maximize the 85 / 85Rb and 88 / 10⁴Sr signals, ensuring... 88 Sr + Transform into 104 SrO + Its efficiency is higher than 80%.

[0019] Furthermore, the integration time for measuring the element signal data in S1 is set to 85 / 85Rb: 10 milliseconds; 86 / 102Sr: 25 milliseconds; 87 / 103Sr: 25 milliseconds; and 88 / 104Sr1: 25 milliseconds, respectively.

[0020] Furthermore, when measuring the elemental signal data of the natural mineral standard material and the sample to be tested as described in S1, after testing 7-8 samples to be tested, one glass standard material, one natural mineral standard material, and one reference sample are repeatedly tested. The reference sample is a sample of known age and is used for data monitoring.

[0021] Furthermore, in step S1, when the aerosol is loaded into the Q-ICP-MS / MS plasma source, nitrogen gas is added to the laser carrier gas He, wherein the nitrogen gas flow rate is 4 mL / min and the Ar atomizing gas flow rate is <0.9 L / min.

[0022] Furthermore, the first-order correction formula described in S3 is:

[0023] ;

[0024] In the formula,

[0025] Furthermore, the secondary correction formula described in S4 is:

[0026] ;

[0027] In the formula, The secondary calibrated rubidium-strontium ratio of the sample under test; This is the initial calibrated rubidium-strontium ratio of the sample to be tested; The rubidium-strontium ratio is a reference value for natural mineral standard references; This refers to the rubidium-strontium ratio measured using natural mineral standard references.

[0028] Beneficial effects: The technical operation process of this invention is simple and does not require complicated chemical pretreatment; it can directly analyze various sample carriers (such as sample targets or rock thin sections) with high spatial resolution (60-100 micrometers); the analysis speed is fast (single point <2 minutes); the Rb-Sr dating results have high accuracy (100.1% - 101.6%) and precision (better than ±2.3%). Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 The effects of ablation frequency (5, 8, 10 Hz) on the sensitivity of Rb, Sr and reaction products of NIST 610, phlogopite powder tablets (MicaMg), potassium feldspar powder tablets (FK-N), muscovite (ZMT04) and biotite (MD4B).

[0031] Figure 2 For energy densities (2.5, 3.5 J / cm³) 2 The effects of ) on the sensitivity of Rb, Sr and reaction products of NIST 610, phlogopite powder tablets (MicaMg), potassium feldspar powder tablets (FK-N), muscovite (ZMT04) and biotite (MD4B);

[0032] Figure 3 The laser parameters are as follows (where AH represents point ablation, IJ represents line scanning; A: 50 micrometers, 3.5 J / cm). 2 8Hz, Heonly; B: 50 micrometers, 3.5J / cm 28Hz, He + 2mL N2; C: 50 micrometers, 3.5J / cm 2 8Hz, He + 4mL N2; D: 50 micrometers, 3.5J / cm 2 8Hz, He + 6mL N2; E: 50 micrometers, 3.5J / cm 2 10Hz, He + 4mL N2; F: 50 micrometers, 3.5J / cm 2 5Hz, He + 4mL N2; G: 50 micrometers, 2.5J / cm 2 8Hz, He + 4mL N2; H: 30 micrometers, 3.5J / cm 2 8Hz, He + 4mL N2; I: Linear scan rate 1 μm / s, 50 μm, 2.5 J / cm 2 10Hz, He + 4mL N2; J: Linear scan rate 2 μm / s, 50 μm, 3.5 J / cm 2 8Hz, He + 4mL N2) for NIST 610, phlogopite powder tablets (MicaMg), potassium feldspar powder tablets (FK-N) 87 Rb / 86 Sr and 87 Sr / 86 The impact of Sr repeatability and internal accuracy;

[0033] Figure 4 For residence time, NIST 610, phlogopite powder tablets (MicaMg), and potassium feldspar powder tablets (FK-N) were tested. 87 Rb / 86 Sr and 87 Sr / 86 The influence of Sr on the internal accuracy and repeatability of a single point. The error is 2s (internal error).

[0034] Figure 5 The results show the influence of matrix effects on the accuracy of Rb-Sr ages. (A) Rb-Sr isochron ages of ZBH-15 biotite, (B) MD4B biotite, and (C) WCM muscovite are shown. The green, blue, and red circles represent external standard corrections using NIST 610 glass, phlogopite powder (MicaMg), and potassium feldspar powder (FK-N), respectively. The error ellipse is 2s.

[0035] Figure 6 The results are isochron ages, including the Rb-Sr isochron ages of (A) ZBH15 biotite, (B) WCM muscovite, and (C) ZMT04 muscovite; the error ellipse is 2s. Detailed Implementation

[0036] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0037] The "potassium-rich minerals" mentioned in this application refer to silicate minerals with high potassium content, such as biotite [chemical formula: ,muscovite Potassium feldspar In silicate minerals, potassium and rubidium readily form isomorphous substitutions. Therefore, minerals with higher potassium content usually also have higher rubidium content, making them suitable for laser micro-area Rb-Sr dating.

[0038] The "natural mineral standard material consistent with the mineral species of the sample to be tested" mentioned in this application, also known as "natural mineral standard material," refers to a natural mineral selected that is consistent with the mineral species to be tested, and whose Rb-Sr isotopes are known. Examples include MD4B biotite.

[0039] The “glass standard material” mentioned in this application refers to any glass standard material with homogeneous Rb-Sr isotopes and known accurate isotopic ratios, such as the trace element analysis standard material 610 glass issued by the National Institute of Standards and Technology (NIST) and the trace element analysis standard material BHVO-2G glass issued by the United States Geological Survey (USGS).

[0040] This embodiment uses MD4B biotite (100.3 ± 1.9 Ma) as a natural mineral standard material (for secondary calibration) consistent with the mineral species of the sample to be tested, ZBH-15 biotite (132.8 ± 1.3 Ma), WCM muscovite (306 ± 1 Ma), and ZMT-04 muscovite (1772 ± 3 Ma) as the samples to be tested, to describe the specific implementation of the technical method of this invention. The age of the above mica is used as the verification standard in this method to test the accuracy and precision of this method.

[0041] This method employs a combination of laser ablation and inductively coupled plasma mass spectrometry (ICP-MS) for measurement. The ICP-MS must be a multiple quadrupole ICP-MS (Q-ICP-MS / MS). In this application, the Perkin Elmer NexION 5000 multiple quadrupole ICP-MS was used in the experiment, and the laser ablation system used was the NWRfemto fractional-second laser ablation system from ESL Corporation.

[0042] S1. Using a laser, the aerosols obtained after pre-etching and formal etching of the sample carrier of the test sample and the natural mineral standard material of the same mineral type as the test sample are loaded into the Q-ICP-MS / MS plasma source. The elemental signal data of 27 / 27Al, 85 / 85Rb, 86 / 102Sr, 87 / 103Sr and 88 / 104Sr of the natural mineral standard material and the test sample are measured in the bar mode.

[0043] In this embodiment, the above steps specifically include:

[0044] 1. MD4B biotite, ZBH-15 biotite, WCM muscovite, and ZMT-04 muscovite were cast into sample targets (1 inch in diameter, approximately 5 mm thick) using epoxy resin. After slight grinding to expose the mica cut surface, the targets were then polished, cleaned, and dried for later use. It should be noted that the sample target is the sample carrier used in this application. Furthermore, the sample carrier described in this application can include various sample forms suitable for laser ablation, such as sample targets and thin rock sections with a thickness greater than 50 micrometers.

[0045] 2. Place the natural mineral standard material and the sample target of the sample to be tested into the laser ablation instrument chamber, and purge the sample chamber with helium gas to fill it with helium. Adjust the position of the sample in the optical axis direction to ensure good laser beam focusing.

[0046] 3. Pre-etching: A laser beam is used to pre-etch the analytical points in the sample target to remove surface impurities. The pre-etching laser parameters are set as follows: frequency: 8Hz, energy density: 2-3.5J / cm². 2 The erosion diameter is between 50 and 60 micrometers, the line scan speed is 50 micrometers / s, the line scan length is 100 micrometers, and the erosion time is 2s.

[0047] 4. Formal Abrasion: After pre-abrasion, formal abrasion analysis is performed on the analysis points using a laser beam. The laser abrasion beam diameter is 50-60 micrometers, the line scan speed is 2 micrometers / s, the line scan length is 100 micrometers, and the laser energy density is 3.5 J / cm². 2 The ablation frequency is 8 Hz. The ablated aerosol is loaded into a Q-ICP-MS / MS plasma source using a carrier gas for ionization and ion signal reception.

[0048] In order to perform high spatial resolution, high precision, and high accuracy Rb-Sr dating on the test sample described in this application, this application first optimized the sensitivity of laser etching.

[0049] (1) Optimization of instrument sensitivity

[0050] Effect of gas flow rate on sensitivity: Adding trace amounts of nitrogen (N2) to the laser carrier gas (He) can improve sensitivity. This application investigated the effects of different trace N2 flow rates on the sensitivity of U, Pb, Rb, and Sr, the Th / U ratio, and the Rb / Sr ratio under a He flow rate of 750 mL / min. The study showed that the sensitivity of U and Pb isotopes changed similarly with increasing N2 flow rate. At lower nebulizer flow rates (0.75–0.87 L / min), the sensitivity increased. When the nebulizer flow rate continued to increase to 1 L / min, the sensitivity continued to increase from 0 to 4 mL / min. When the N2 flow rate continued to increase to 6 mL / min, the sensitivity decreased slightly, and continued to decrease with further increases in nebulizer flow rate. When the nebulizer flow rate was greater than 1 mL / min, regardless of the N2 flow rate, further increases in nebulizer flow rate decreased the U-Pb sensitivity. The Th / U ratio decreased with increasing N2 and nebulizer flow rates. The trends in sensitivity variation for Rb and Sr are similar. Unlike U and Pb, at an N2 flow rate of 4 mL / min, when the nebulizer gas flow rate is between 0.75 and 0.9 L / min, the sensitivity increases with increasing nebulizer gas flow rate; as the nebulizer flow rate continues to increase, the sensitivity decreases. At lower N2 flow rates (0–3 mL / min), the Rb / Sr ratio initially increases with increasing nebulizer gas flow rate, then decreases at an nebulizer gas flow rate of 0.9 L / min; when the N2 flow rate is between 4 and 6 mL / min, the Rb / Sr ratio shows an increasing trend. At an nebulizer flow rate of 0.9 L / min… 85 Rb / 88 The Sr ratio is closest to the reference value of 0.35. Based on the above results, the optimal N2 flow rate is 4 mL / min, and the Ar atomizing gas flow rate is <0.9 L / min.

[0051] (2) Effect of laser parameters on sensitivity (using 610 glass, phlogopite powder tablets (MicaMg), potassium feldspar powder tablets (FK-N), muscovite (ZMT04) and biotite (MD4B) as test samples)

[0052] Effect of erosion frequency on the sensitivity of Rb and Sr reaction products

[0053] (i) This application investigated the changes in the sensitivity of Rb, Sr, and reaction products within the 5-10 Hz ablation frequency range. Experimental results are as follows: Figure 1As shown, for 610 glass, increasing the laser ablation frequency from 5 Hz to 10 Hz increases the sensitivity of Rb and SrO by approximately 1.5 times, but the RSD of cps is slightly higher at 10 Hz. For MicaMg, the sensitivity difference between 8 Hz and 10 Hz is not significant, and the RSD of cps is slightly higher at 10 Hz. For muscovite ZMT04, 8 Hz has the highest sensitivity and the lowest RSD. For biotite MD4B, although 10 Hz has the highest sensitivity, the RSD is very large, and the RSD is the smallest at 8 Hz. The experimental results indicate that the ablation frequency in the range of 8 Hz to 10 Hz can improve sensitivity, but at 10 Hz, the Rb and SrO signals decay rapidly with ablation time, leading to an increase in RSD. Therefore, 8 Hz is the optimal ablation frequency.

[0054] (ii) Effect of energy density on the sensitivity of Rb and Sr reaction products

[0055] The energy density in the range of 2.5–3.5 J / cm² was investigated. 2 Changes in the sensitivity of Rb, Sr, and reaction products within the specified range. For example... Figure 2 As shown, with 2.5 J / cm 2 In comparison, using 3.5J / cm 2 The sensitivity is approximately 1.3 times higher than that of NIST 610 or 2 times higher than that of MicaMg and ZMT04, and the repeatability is good. Therefore, 3.5 J / cm² was used in subsequent experiments. 2 Energy density.

[0056] (3) Optimization of isotope ratio accuracy

[0057] (i) Laser parameters 87 Rb / 86 Sr ratio and 87 Sr / 86 Effects of Sr ratio on accuracy and repeatability

[0058] Comparison of point erosion and line scan erosion 87 Rb / 86 Sr ratio and 87 Sr / 86 Sr ratio within-range precision and repeatability, such as Figure 3 As shown, parameter AH represents the point ablation mode. Except for the very low sensitivity when using a 30-micron beam spot, different laser parameters affect the pressing of FK-N and MicaMg powder tablets. 87 Rb / 86 Sr ratio and 87 Sr / 86 The Sr ratio has a negligible impact on internal accuracy; the lack of a consistent repeatability indicates that, given the required sensitivity, changes in laser parameters (energy density, ablation frequency, and trace nitrogen flow rate) have little effect on the accuracy of the repeatability.87 Rb / 86 Sr ratio and 87 Sr / 86 The accuracy and repeatability of the Sr ratio were not significantly affected. An attempt was made to overcome the fractionation effect of the femtosecond laser using line-scan ablation. Figure 3 As can be seen, parameters I and J were obtained using line scan ablation at speeds of 1 μm / s and 2 μm / s, respectively. 87 Rb / 86 Sr ratio and 87 Sr / 86 The accuracy of Sr ratios has been improved for all values. 87 Rb / 86 The Sr ratio was optimized to 0.32-0.41% for 610 glass, 0.56-0.67% for MicaMg powder tablets, and 0.53% for FK-N powder tablets; for 87 Sr / 86 The Sr ratio was optimized to 0.22% for 610 glass, 0.48-0.58% for MicaMg powder pellets, and 0.47% for FK-N powder pellets. Therefore, subsequent experiments selected line ablation scanning mode to obtain Rb-Sr isotope ratios and ages.

[0059] (ii) Effect of residence time on accuracy and repeatability within the 87Rb / 86Sr and 87Sr / 86Sr ratios

[0060] By altering the residence time on the analyte peak, the accuracy of counting statistics can be improved, directly impacting the detection limit and measurement uncertainty. Typically, optimal results are achieved by combining a sufficiently long acquisition time with rapid scanning to offset the noise characteristics of the ICP ion source and laser ablation sampling system. We used a femtosecond laser with an 8Hz frequency, a 50µm beam size, and 3.5J / cm². 2 The energy density of NIST 610, MicaMg, and FK-N powder tablets was measured, and different residence times were compared. Figure 4 ) 87 Rb / 86 Sr and 87 Sr / 86Within-range accuracy and repeatability of Sr ratios. Typically, potassium-rich minerals such as mica and potassium feldspar have significantly higher Rb content than Sr content, therefore the Rb residence time used is longer than that of SrO. Four residence times were compared in the experiment: A: Rb (1ms) + SrO (25ms), B: Rb (10ms) + SrO (25ms), C: Rb (20ms) + SrO (50ms), and D: Rb (40ms) + SrO (80ms), corresponding to complete scan cycles of 0.082s, 0.091s, 0.151s, and 0.231s, respectively. Since the erosion time remained constant (81s), as the residence time increased, the collected... 87 Rb / 86 Sr and 87 Sr / 86 The Sr ratio data will decrease. No significant difference was observed in the signal or background count rate (CPS) between the residence times used. Changing the residence time from A to B, i.e., increasing the residence time of Rb, resulted in [data missing] in NIST 610, Mica-Mg, and FK-N. 87 Rb / 86 The Sr ratio has improved both the single-point internal accuracy and repeatability, while 87 Sr / 86 The Sr ratio did not change significantly. For example, NIST 610's... 87 Rb / 86 Sr and 87 Sr / 86 The mean internal precision of the Sr ratios were 1.38% and 0.45% (A residence time), and 0.95% and 0.49% (B residence time), respectively. 87 Rb / 86 Sr and 87 Sr / 86 The repeatability of the Sr ratio was 0.83% and 0.45% (dwell time A), and 0.54% and 0.45% (dwell time B), respectively. Further increasing the dwell time of Rb and SrO did not improve the single-point accuracy or repeatability, possibly because the limited data collection prevented the optimal maximum dwell time from being achieved. Therefore, the optimal conditions obtained in this experiment were Rb (10 ms) + SrO (25 ms).

[0061] Therefore, the integration times for measuring element signal data are set as follows: 27 / 27Al: 1 ms; 85 / 85Rb: 10 ms; 86 / 102Sr: 25 ms; 87 / 103Sr: 25 ms; 88 / 104Sr1: 25 ms.

[0062] The above experiment improved the accuracy of element signal data counting by setting the laser ablation parameters.

[0063] In a preferred embodiment, before measuring elemental signal data using a Q-ICP-MS / MS plasma source as described in step S1, the ICP-MS instrument conditions need to be optimized, specifically as follows:

[0064] Furthermore, before measuring elemental signal data using the Q-ICP-MS / MS plasma source as described in S1, the ICP-MS instrument conditions need to be optimized, specifically as follows:

[0065] The instrument conditions for ICP-MS in single-bar mode were optimized using NIST 610 glass, resulting in... 206 Pb and 238 The U signal is highest, while ensuring oxide yield (ThO) + / Th + The secondary ion yield (Ca) is less than 0.5%, and the secondary ion yield (Ca) is less than 0.5%. 2+ (Ca+) less than 2.0%;

[0066] N₂O reactive gas was introduced into the collision cell. The gas flow rate and collision cell voltage were optimized using a NIST 610 glass pair in Mass / Mass mode to maximize the 85 / 85Rb and 88 / 10⁴Sr signals, ensuring... 88 Sr + Transform into 104 SrO + Its efficiency is higher than 80%.

[0067] In a preferred embodiment, when measuring the elemental signal data of the natural mineral standard material and the sample to be tested in S1, after testing 7-8 samples to be tested, one glass standard material (for 87Sr / 86Sr ratio and trace element correction), one natural mineral standard material (for 87Rb / 86Sr correction), and one reference sample are repeatedly tested. The reference sample is a sample of known age and is used for data quality monitoring.

[0068] S2. Based on the above elemental signal data, the values ​​of the natural mineral standard material and the sample to be tested are calculated respectively. 87 Rb / 86 Sr、 87 Sr / 86 Sr ratio; The above element signal data were processed offline using Iolite laser data processing software. First, the signal was subjected to instrument blank subtraction and instrument sensitivity drift correction over time.

[0069] S3. First calibration: based on the glass standard reference material. 87 Rb / 86 Sr、 87 Sr / 86By comparing the measured and standard values ​​of Sr, the corresponding fractionation coefficients are obtained for the sample to be tested. 87 Rb / 86 Sr、 87 Sr / 86 The Sr ratio is corrected using the following formula:

[0070] ;

[0071] In the formula, The first-calibrated rubidium-strontium ratio value of the sample under test; The measured rubidium-strontium ratio of the sample to be tested; The reference rubidium-strontium ratio for standard substances; The rubidium-strontium ratio is used for measuring the standard substance.

[0072] S4. Secondary calibration: The sample is calibrated using natural mineral standard materials after the initial calibration. 87 Rb / 86 The Sr ratio is corrected twice (the purpose is to eliminate the matrix effect). 87 Rb / 86 The effect of Sr), the specific calculation formula is as follows:

[0073] ;

[0074] In the formula, The secondary calibrated rubidium-strontium ratio of the sample under test; This is the initial calibrated rubidium-strontium ratio of the sample to be tested; The rubidium-strontium ratio is a reference value for natural mineral standard references; This refers to the rubidium-strontium ratio measured using natural mineral standard references.

[0075] The two corrections mentioned above can eliminate the influence of matrix effect on the accuracy of Rb-Sr age, as demonstrated by the following experiments.

[0076] Because Rb-Sr elemental fractionation occurs during laser ablation, matrix-matched standard materials are typically used to calibrate the target material to obtain accurate elemental ratios. However, in some cases, differences in physical or chemical properties between the target material and the calibrator lead to varying degrees of elemental fractionation. Calibration in such cases will introduce systematic biases in the ratios, thus affecting the accuracy of Rb-Sr dating results. We compared the Rb-Sr ages of ZBH-15 biotite, MD4B biotite, and WCM muscovite using NIST610 glass, MicaMg powder pellets, and FK-N powder pellets as standards. Figure 5As shown in Figure A, using NIST 610 glass as an external standard, the Rb-Sr isochron age of ZBH-15 biotite was 115.3 ± 2.8 Ma (2 s), and the age using MicaMg phlogopite powder pellets as an external standard was 128.0 ± 3.1 Ma (2 s), which are ~15% and ~5% younger than the reference age of ZBH-15 biotite, respectively. However, using FK-N potassium feldspar powder pellets as an external standard, the age was 150.7 ± 3.6 Ma (2 s), which is ~10% older than the reference age. This inconsistency in age is mainly due to the measurement... 87 Rb / 86 The deviation between Sr calibration and the reference value is due to the choice of standard material. Using MicaMg and FK-N as external standards is better than using NIST 610 for external standard correction. 87 Rb / 86 Sr was -10% and -23% lower than normal, respectively, while the three... 87 Sr / 86 The Sr ratio variation is less than 0.9%, which is less than the single-point internal error. The initial Sr isotope ratios obtained from the three isochrons are consistent within the error range. The ages of the MD4B biotite Rb-Sr isochrons with different external standard corrections also deviate from the reference age. Figure 5 B), using NIST610 glass as an external standard, the Rb-Sr isochron age of MD4B biotite was 88.4±9.8 Ma (2s), and the age of MicaMg phlogopite powder pellets was 97±11 Ma (2s), which were ~14% and ~5% older than the reference age (100.3 Ma), respectively. The age of FK-N potassium feldspar powder pellets was 112±12 Ma (2s), which was ~9% older than the reference age. The initial Sr isotope ratios were consistent within the error range.

[0077] A similar pattern exists for muscovite. Figure 5 C) Using NIST 610 glass as an external standard, the Rb-Sr isochron age of WCM muscovite was 270±20 Ma (2s), and the age of MicaMg phlogopite powder pellets was 295±21 Ma (2s), which are ~13% and ~5% older than the reference age (306 Ma), respectively. Using FK-N potassium feldspar powder pellets as an external standard, the age was 344±25 Ma (2s), which is ~11% older than the reference age. The initial Sr isotope ratios obtained from the isochrons varied from 0.17 to 0.36, which may be due to the lack of low-density phosphite. 87 Rb / 86The lower end of the isochron is determined by the Sr (<100) ratio. The above age results indicate that there is a matrix effect due to differences in chemical composition and material properties between NIST 610 glass, potassium feldspar powder compacts, and mica minerals, leading to significant deviations (-14% to +11%) in Rb-Sr isochron dating results. Even though phlogopite powder compacts have similar chemical compositions to natural mica minerals, the obtained isochron ages still have a deviation of ~5%. This is because differences in light absorption, density, and hardness between powder compacts and natural minerals cause a matrix effect during laser-sample interaction, affecting the accuracy of dating. This suggests that physically and chemically compatible mica minerals need to be used for correction to obtain more accurate ages.

[0078] For potassium feldspar, using NIST 610 glass as an external standard, the Rb-Sr isochron age of Yunxiao potassium feldspar was 73.5 ± 2.6 Ma (2 s), and the age using MicaMg phlogopite powder pellets as an external standard was 81.5 ± 2.9 Ma (2 s), which are ~23% and ~14% older than the reference age (95 Ma), respectively. Using FK-N potassium feldspar powder pellets as an external standard, the age was 96.0 ± 3.5 Ma (2 s), only ~1% older than the reference age. The initial Sr isotope ratios were consistent within the error range. Figure 5 (D) This indicates that there is a matrix effect due to differences in chemical composition and material properties among NIST 610 glass, phlogopite powder compacts, and potassium feldspar minerals, which significantly affects the accuracy of Rb-Sr isochron dating. However, when potassium feldspar powder compacts with the same chemical composition are used as external standards, the matrix effect due to differences in material properties can be ignored. Using potassium feldspar powder compacts as external standards can yield more accurate Rb-Sr isochron ages for potassium feldspar minerals.

[0079] S5. After secondary correction 87 Rb / 86 Sr、 87 Sr / 86 Sr ratios were used to construct isochron plots, and the age data and initial Sr isotopes of the samples were calculated. 87 Sr / 86 The composition is Sr. The type of the natural mineral standard reference is consistent with the type of mineral in the sample to be tested, and the quantity is N, where N≥1.

[0080] like Figure 6 As shown, ZBH-15 biotite (132.8±1.3 Ma), WCM muscovite (306±1 Ma), and ZMT-04 muscovite (1772±3 Ma) (with existing age reference values) were inserted into the test sequence as unknown samples. The Rb-Sr isochron ages obtained from the tests were compared with the reference values ​​to evaluate the reliability of the method. The experimental results are as follows: Figure 6 As shown, the accuracy of Rb-Sr age is between 100.1% and 101.6%, with a precision (age accuracy better than ±2.3%) and a spatial resolution of 60-100 micrometers; indicating that this technique is reliable and can directly date fine minerals with high precision.

[0081] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for obtaining Rb-Sr age of micro-area of potassium-rich mineral, using laser ablation and inductively coupled plasma mass spectrometry, characterized in that, Comprise the following steps: S1. The laser is used to load the aerosol obtained by pre-erosion and formal erosion of the sample carrier of the sample to be tested and the sample carrier of the natural mineral standard material consistent with the mineral species of the sample to be tested into the Q-ICP-MS / MS plasma source, respectively, and measure the 85 / 85Rb, 86 / 102Sr, 87 / 103Sr, 88 / 104Sr element signal data of the natural mineral standard material and the sample to be tested in the string rod mode. S2. According to the above element signal data, the natural mineral standard substance and the sample to be measured are calculated respectively 87 Rb / 86 Sr、 87 Sr / 86 Sr ratio; S3. One-time correction: according to the glass standard material 87 Rb / 86 Sr、 87 Sr / 86 Sr measured value and standard value, the corresponding fractionation coefficient is obtained, and the 87 Rb / 86 Sr、 87 Sr / 86 Sr ratio of the sample to be measured is corrected; S4. Secondary correction: The primary corrected value of the sample to be tested is corrected again by the ratio of Rb / Sr of the natural mineral standard material. 87 Rb / 86 Sr S5. Through the secondary correction of the 87 Rb / 86 Sr、 87 Sr / 86 Sr ratio, isochron diagram is constructed, and the age data and initial Sr isotope of the sample to be measured are calculated 87 Sr / 86 Sr composition; Wherein, the type of the natural mineral standard material is consistent with the mineral type of the sample to be tested, and the number is N, N≥1.

2. The method of claim 1, wherein, The laser pre-erosion and formal erosion in S1 adopts line scanning erosion.

3. The method of claim 2, wherein, The pre-erosion laser frequency is 8-10 Hz, the energy density is 2-3.5 J / cm 2 , the ablation diameter is 50-60 microns, the line scanning speed is 50 microns / s, the line scanning length is 100 microns, and the ablation time is 2 s.

4. The method of claim 2, wherein, The formal ablation laser ablation beam spot diameter is 50-60 microns, the line scanning speed is 2 microns / s, the line scanning length is 100 microns, the laser energy density is 3.5 J / cm 2 , and the ablation frequency is 8-10 Hz.

5. The method of claim 1, wherein, Before measuring the element signal data by using the Q-ICP-MS / MS plasma source in S1, the ICP-MS instrument conditions need to be optimized, specifically: The ICP-MS instrument conditions in single rod mode were optimized using NIST 610 glass, so that 206 Pb and 238 U signals were the highest, while ensuring that the oxide yield (ThO + / Th + ) was less than 0.5%, and the secondary ion yield (Ca 2+ / Ca+) was less than 2.0%; In the collision reaction cell, N2O reaction gas is passed, and NIST 610 glass is used to optimize the reaction gas flow rate and collision cell voltage in the string-pole mode Mass / Mass, so that the 85 / 85Rb and 88 / 104Sr signals are the highest, and the efficiency of conversion to 88 Sr + is higher than 80%. 104 SrO + .

6. The method of claim 1, wherein, When measuring the 85 / 85Rb, 86 / 102Sr, 87 / 103Sr, 88 / 104Sr element signal data of the natural mineral standard material and the sample to be tested in S1, the integral time is set to 85 / 85Rb: 10 milliseconds; 86 / 102Sr: 25 milliseconds; 87 / 103Sr: 25 milliseconds; 88 / 104Sr1: 25 milliseconds, respectively.

7. The method of claim 1, wherein, When measuring the element signal data of the natural mineral standard material and the sample to be tested in S1, after testing 7-8 samples to be tested, a glass standard material, a natural mineral standard material and a reference sample are repeatedly tested, the reference sample is a sample with known age, which is used for data monitoring.

8. The method of claim 1, wherein, When loading the aerosol into the Q-ICP-MS / MS plasma source in S1, nitrogen is added to the laser carrier gas He, wherein the nitrogen flow rate is 4mL / min, and the Ar atomizing gas flow rate is <0.9 L / min.

9. The method of claim 1, wherein, The once correction formula is: ; In the formulae, 10. The method of claim 1, wherein, The quadratic correction formula is: ; wherein represents the secondary correction rubidium-strontium ratio value of the sample to be measured; represents the primary correction rubidium-strontium ratio value of the sample to be measured; represents the reference rubidium-strontium ratio value of the natural mineral standard material; represents the measured rubidium-strontium ratio value of the natural mineral standard material.

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