A calibration method for Lu-Hf dating of in-situ micro-area allanite

Abstract

The present invention discloses an in-situ micro-area allantois Lu-Hf dating calibration method, comprising: using a glass standard material with a uniform Lu / Hf element ratio to calibrate 176 Lu / 176 Hf and 176 Hf / 177 Hf ratio; use the matrix-matched allanite standard material LE2808 to draw an inverse isochron diagram, calculate the apparent age, and calculate the matrix effect deviation coefficient based on the apparent age and the recommended age. This coefficient is applied to the unknown sample. 176 Lu / 176 Hf ratio correction. This method achieves accurate dating of Lu-Hf in micro-area allantois in situ, overcomes the requirement of existing calibration methods for standard samples with uniform Lu / Hf element ratio, and achieves accurate dating of Lu-Hf in micro-area allantois in laser, with the advantages of high spatial resolution, high dating success rate, and short testing cycle.

Description

Technical Field

[0001] The invention relates to the technical field of geological dating, and in particular to an in-situ micro-area allanite Lu-Hf dating correction method. Background Art

[0002] Isotope geochronology is the most direct quantitative method to determine the time and duration of deep geological processes. The Lu-Hf isotope dating system is widely used in the field of earth science research. Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) is often used in this dating system. This technology obtains data through four steps: sample drilling, acid dissolution, separation and purification, and on-machine testing. In recent years, the newly developed laser ablation triple quadrupole inductive coupled plasma mass spectrometer (LA-ICP-MS / MS) micro-area Lu-Hf dating technology has further developed the application field of this system. It has high spatial resolution (μm level), fast analysis speed (~2 minutes / ablation point) and the ability to separate isotopic interferences online (such as 176 Lu 176 Hf interference). In LA-ICP-MS / MS micro-area Lu-Hf dating analysis, matrix-matched standard substances are required to calibrate the Lu / Hf element ratio fractionation. Such standard substances need to have uniform element ratios and do not contain common Hf. However, there are very few such standard substances in nature, and most samples have non-uniform Lu / Hf element ratios, which greatly limits the application of this dating technology. Therefore, it is extremely important to establish a calibration method based on standard substances with non-uniform Lu / Hf element ratios, which can greatly promote the further application of this technology.

[0003] Allanite, its chemical formula is: A2M3(SiO4)(Si2O7)(O,F)(OH), where A position = Ca, Sr, Pb 2+ ,Mn 2 + ,Th,REE 3+ and U,M position = Al,Fe 2+ ,Fe 3+ ,Mn 2+ ,Mn 3+ ,Mg,Cr 3+ and V 3+. Allanite is one of the most important rare earth element carrier minerals in crustal rocks, especially light rare earth elements. At the same time, allanite is widely exposed in various geological environments. Therefore, the development of allanite isotope chronology has important geological significance. Allanite can contain a certain amount of U and Th during the crystallization process, so it can be dated using the U-Th-Pb system. However, due to its high Th characteristics, samples are often severely radioactively damaged, making the U-Th-Pb system open and difficult to provide reliable U-Pb age data. Allanite is a rare earth-rich mineral that contains a certain amount of Lu and no or very little Hf. Therefore, it has the potential for Lu-Hf dating. However, Lu-Hf dating of allanite has not yet been developed and applied.

[0004] Prior art related to the present invention

[0005] Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) is a technology that obtains data through four steps: sample drilling, acid digestion, separation and purification, and on-machine testing.

[0006] Disadvantages of the prior art 1

[0007] 1. The analysis process is long;

[0008] 2. Small number of samples analyzed;

[0009] 3. Low spatial resolution and low success rate. Summary of the invention

[0010] The present invention aims at the defects of the prior art and provides an in-situ micro-area allanite Lu-Hf dating correction method.

[0011] In order to achieve the above invention object, the technical solution adopted by the present invention is as follows:

[0012] An in-situ micro-area allanite Lu-Hf dating calibration method comprises the following steps:

[0013] Step 1: Cut the allantite sample or the rock sample containing allantite to make a sample target or ordinary optical slice of appropriate size.

[0014] Step 2: Place the sample into the laser ablation sample chamber and adjust the position of the sample along the optical axis to ensure good focusing of the laser beam.

[0015] Step 3: Pre-strip the target sampling points to prepare for subsequent point stripping analysis.

[0016] Step 4: Perform point ablation on the circled target analysis point using laser ablation, and load the ablation product into the ICP-MS / MS plasma source for ionization.

[0017] Step 5: Use pure ammonia as the reaction gas in the ICP-MS / MS instrument. 176 Hf reacts to achieve 176 Lu, 176 Online separation of Yb and simultaneous collection 27 Al, 43 Ca, 89 Y, 90 Zr, 172 Yb, (172+82) Yb, 175 Lu, (175+82) Lu, (176+82) Hf, (177+82) Hf and (178+82) Ion signal intensity of Hf element.

[0018] Step 6: During the measurement process, two glass standard materials NIST SRM 610 and two allanite standard materials LE2808 were tested regularly to ensure the consistency and accuracy of the measurement conditions.

[0019] Step 7: After obtaining the element signal data from step 5, calculate 176 Lu / 176 Hf and 176 Hf / 177 The ratio data of Hf ratio is 176 Hf / 176 Lu and 176 Hf / 177 The Hf ratio was corrected for instrument drift and mass fractionation.

[0020] Step 8: According to the LE2808 allanite standard material 176 Lu / 176 Hf and 176 Hf / 177 Hf ratio, fix the upper intersection point, construct the anti-isochron, and calculate the anti-isochron age; calculate the fractionation coefficient through the calculated isochron age and the recommended age, and 176 Hf / 176 Lu ratio correction.

[0021] Preferably, in step 1, the sample target has a diameter of 1 inch and a thickness of about 5 mm, and the thickness of an ordinary optical thin film sample is 30-50 μm.

[0022] Preferably, in step 3, the target sampling points are pre-eroded, and a total of 30-100 points are analyzed for each analysis sample.

[0023] Preferably, in step 4, the spot diameter of the ablation laser beam is 60-130 μm, the ablation frequency is 10 Hz, and the energy density is 3.0 J cm -2 .

[0024] Preferably, in step 6, after every 10 unknown samples are tested, two glass standard materials NISTSRM 610 and two allanite standard materials LE2808 are tested repeatedly.

[0025] As a preferred method, in step 8, fix the upper intersection point 177 Hf / 176 Hf is 0.282.

[0026] Compared with the prior art, the advantages of the present invention are:

[0027] 1. Accurate in-situ micro-area Lu-Hf dating of allantite has been achieved: Through the combination of laser micro-area ablation and mass spectrometer online analysis, the allantite samples can be analyzed in situ, thus achieving accurate Lu-Hf dating.

[0028] 2. It overcomes the need for a uniform standard sample of Lu / Hf element ratio in existing calibration methods: Compared with traditional methods, this method no longer needs to rely on a uniform standard sample for calibration, reducing the complexity and cost of the experimental process.

[0029] 3. Accurate laser micro-area Lu-Hf dating of allantite has been achieved: Through laser micro-area ablation technology, the sample surface can be selectively ablated to obtain Lu-Hf isotope data with spatial resolution, thus achieving accurate dating of allantite.

[0030] 4. High spatial resolution: Laser micro-ablation technology can be used to accurately analyze samples locally, achieving high spatial resolution dating of allantois samples.

[0031] 5. High dating success rate: By precisely controlling the laser ablation and analysis process, the dating success rate can be improved, the failure rate in the experiment can be reduced, and the reliability of the data can be ensured.

[0032] 6. Short test cycle: Compared with traditional dating methods, this method has a shorter test cycle, can obtain the dating results of samples more quickly, and improves experimental efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is an information diagram of the allantite sample used in the embodiments of the present invention.

[0034] Figure 2 1 is a graph showing trace element data of nine allanite samples determined by LA-ICP-MS in an embodiment of the present invention.

[0035] Figure 3 Schematic diagram of the Yb and Lu interference subtraction simulation experiment and its influence on the accuracy of Lu-Hf dating of allantite in the embodiment of the present invention, wherein (a) is when the Lu reaction efficiency is 0.0025%, 176 Hf / 177 The impact of Hf ratio; (b) when Yb reaction efficiency is 0.00025%, 176 Hf / 177 The impact of Hf ratio; (c) Lu content of allantite, xenotime, apatite, and garnet 175 Lu / 177 Hf ratio relationship diagram; (d) Lu content of allantoisite, xenotime, apatite, and garnet 172 Yb / 177 Hf ratio relationship diagram.

[0036] Figure 4 1 is a flow chart of the two-step Lu-Hf correction method according to an embodiment of the present invention.

[0037] Figure 5 This is the inverse isochron Lu-Hf dating result diagram of the allanite standard material LE2802 according to the embodiment of the present invention;

[0038] Figure 6 is a graph of inverse isochron Lu-Hf dating results of 8 allanite samples in the embodiment of the present invention;

[0039] Figure 7 : It is a simulation diagram of the system error introduced by the fixed upper intersection point of the embodiment of the present invention; wherein (a) is the age error introduced by the fixed upper intersection point (0.280-0.284) when the proportion of ordinary Hf is 20%; (b) is the age error introduced by the fixed upper intersection point (0.280-0.284) when the proportion of ordinary Hf is 50%; (c) is the age error introduced by the fixed upper intersection point (0.280-0.284) when the proportion of ordinary Hf is 80%; (d) is the age error introduced by the fixed upper intersection point (0.280-0.284) when the proportion of ordinary Hf is 90%; (e) is the relationship diagram between the age uncertainty caused by the fixed upper intersection point (0.280-0.284) and ordinary Hf; (f) is the age uncertainty of Rb-Sr, Lu-Hf and U-Pb introduced by the fixed upper intersection point when the proportion of ordinary Hf / Sr / Pb is 50%.

[0040] Figure 8 2 is a diagram of an error transfer model according to an embodiment of the present invention.

[0041] Fig. 91 is a graph showing the chemical composition of the allantite of the embodiment of the present invention, wherein (a) is a graph showing the relationship between the Lu content and the Hf content of 15 allantite samples; (b) is a graph showing the relationship between the Lu content and the Y content of 15 allantite samples; (c) is a graph showing the rare earth distribution curves of 9 allantite samples; and (d) is a graph showing the proportions of different Hf contents of 9 allantite samples. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples.

[0043] The present invention provides an in-situ micro-area Lu-Hf dating correction method, which is applied to allanite dating. The method comprises the following steps:

[0044] 1) Taking an allanite age standard material: LE2808 (1080.4±15.4Ma) and eight allanite samples as examples, the Lu contents of the nine allanite samples have a wide distribution (1.6–133 μg g -1 ), the Lu / Hf ratio range is 6–206, which fully demonstrates the universality of this method for various allanite. These known ages are used as verification standards in this method to test the accuracy and precision of this method. The information of the 9 allanite samples is as follows Figure 1 As shown, 1 represents ID-TIMS, 2 represents LA-ICP-MS, and 3 represents SIMS;

[0045] 2) First, the allanite sample is cast into a sample target (1 inch in diameter and about 5 mm thick) using epoxy resin. After slight grinding, the allanite cross section is exposed. After polishing, cleaning and drying, it is ready for use or directly prepared into an ordinary optical thin section (sample thickness 30-50 μm).

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

[0047] 4) LA-ICP-MS was used to determine the trace elements (including Rb, Sr, rare earth elements, Th and U) of allantite. The laser ablation spot size was set according to the Lu content, and the Lu content was 1–10 μg g -1 When the laser beam spot is 90–130 μm, the Lu content is 10–200 μg g -1 When the laser beam spot is 60–90 μm, the measured results are as follows: Figure 2 shown. Figure 2 The trace element data of 9 allanite samples determined by LA-ICP-MS. The element content data are expressed in μg g -1 is given in the form of .

[0048] 5) The ICP-MS / MS instrument was set to triple quadrupole (TQ) mode, using NIST SRM 610, and in laser line scanning mode, the instrument parameters were tuned to optimize the instrument status, i.e. (178+82) The Hf signal is optimal, making 176 The reaction yield of Lu is less than 0.0025%, 176 The reaction yield of Yb is less than 0.00025%;

[0049] 6) Pre-strip the target sampling points to remove contamination on the sample surface, and screen a total of 30-100 points for each analytical sample;

[0050] 7) To illustrate 176 The reaction yield of Lu is less than 0.0025% and 176 Under the condition that the reaction yield of Yb is less than 0.00025%, for allanite, it is not necessary to use it for interference correction of Lu and Hf. This embodiment makes the following simulation, such as Figure 3 shown.

[0051] 8) Perform point ablation on the target analysis point, with a laser beam spot diameter of 60-130 μm, an ablation frequency of 10 Hz, and an energy density of 3.0 J cm -2 , using carrier gas to carry the aerosol produced by the ablation into the ICP-MS plasma source for ionization;

[0052] 9) Use auxiliary gas nitrogen to enhance the sensitivity of the instrument and adjust the nitrogen flow rate so that (178+82) Hf signal is optimal.

[0053] 10) Collection 27 Al(2ms), 43 Ca(2ms), 89 Y(1ms), 90 Zr(2ms), 172 Yb(1ms), (172+82) Yb(100ms), 175 Lu(1ms), (175+82) Lu(50ms), (176+82) Hf(300ms), (176+82) Hf(100ms) and (178+82) Ion signal intensity of Hf (100ms);

[0054] 11) The acquisition procedure is: instrument blank for 5 seconds, laser ablation line scan data for 30 seconds, and instrument blank for 10 seconds. During the measurement process, after every 10 unknown samples are tested, two glass standard materials NIST SRM 610 and two allanite standard materials LE2808 are tested repeatedly to ensure that the measurement conditions of the standard materials and unknown samples are the same. Each sample is analyzed 30-100 times;

[0055] 12) After obtaining the element signal data, calculate 176 Lu / 176 Hf and 176 Hf / 177 Hf ratio, using NIST SRM 610 176 Hf / 176 Lu and 176 Hf / 177 The Hf ratio was used to correct the instrument drift.

[0056] 13) According to LE2808 allanite standard material 176 Lu / 176 Hf and 176 Hf / 177 Hf ratio, fixed upper intersection point ( 177 Hf / 176 Hf is 0.282), construct anti-isochrons, and calculate anti-isochron ages; calculate fractionation coefficients based on the calculated isochron ages and recommended ages, and 176 Hf / 176 Lu ratio is used for correction; the two-step correction process is as follows Figure 4 shown.

[0057] 14) The results of the allantite standard substance LE2802 obtained are as follows Figure 5 shown.

[0058] In order to further demonstrate the feasibility of the two-step Lu-Hf calibration method proposed in this invention, eight allanite samples were used to verify the data results. Figure 6 As shown,

[0059] from Figure 6 It is obvious that the accuracy and precision of the Lu-Hf anti-isochron age with a fixed upper intersection is significantly better than that of the data without a fixed upper intersection point. The Lu-Hf age data with a fixed upper intersection point is consistent with the recommended age within the error range, which verifies the reliability of the two-step correction strategy proposed in this patent and proves the practicality of Lu-Hf dating of allanite. Therefore, the recommended data in this embodiment all have fixed upper intersection points.

[0060] Then, fixing the upper intersection point may introduce additional system errors. This embodiment systematically evaluates the system errors introduced by fixing the upper intersection point. The simulation results are as follows Figure 7 As shown in the figure, when the normal Hf content is less than 80%, the introduced system error is less than 2.8%.

[0061] Further clarifying the two-step Lu-Hf dating correction method proposed in the present invention, and the age error propagation model using a fixed upper intersection point for unknown samples, the results are as follows Figure 8 shown.

[0062] In order to further understand the chemical composition information of allanite, Lu content, Lu / Hf ratio, Y / Lu ratio, rare earth distribution and ordinary Hf diagram are plotted, such as Fig. 9 As shown in the figure, Y / Lu has a stable ratio relationship, so the high Lu area can be quickly located by using the Y content.

[0063] Combined with the above data, this patent provides an in-situ micro-area allantois Lu-Hf dating correction method, which can provide more favorable technical support for the application of allantois geochronology.

[0064] The data in the above examples were all obtained on a LA-ICP-MS / MS of a Photo Machine Analyst G2 excimer laser in series with an iCap TQICP-MS / MS. The examples are listed only for the purpose of illustrating the present invention, but not for limiting the present invention. A person skilled in the art can obtain the same results on a similar LA-ICP-MS / MS according to the present method.

[0065] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the implementation methods of the present invention, and should be understood that the protection scope of the present invention is not limited to such special statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. An in-situ micro-area allanite Lu-Hf dating calibration method, characterized in that: The following steps are involved: Step 1: Cut the allanite sample or the rock sample containing allanite to make a sample target or a common optical slice of a suitable size; 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 good focusing of the laser beam; Step 3: Pre-stripping the target sampling points to prepare for subsequent point stripping analysis; Step 4: Perform point ablation on the circled target analysis point using laser ablation, and load the ablation product into the ICP-MS / MS plasma source for ionization; Step 5: Use pure ammonia as the reaction gas in the ICP-MS / MS instrument. 176 Hf reacts to achieve 176 Lu, 176 Online separation of Yb and simultaneous collection 27 Al, 43 Ca, 89 Y, 90 Zr, 172 Yb, (172+82) Yb, 175 Lu, (175+82) Lu, (176+82) Hf, (177+82) Hf and (178+82) Ion signal intensity of Hf element; Step 6: During the measurement process, two glass standard materials NIST SRM 610 and two allanite standard materials LE2808 were tested regularly to ensure the consistency and accuracy of the measurement conditions; Step 7: After obtaining the element signal data from step 5, calculate 176 Lu / 176 Hf and 176 Hf / 177 The ratio data of Hf ratio is 176 Hf / 176 Lu and 176 Hf / 177 The Hf ratio is used for instrument drift correction and mass fractionation correction; Step 8: According to the LE2808 allanite standard material 176 Lu / 176 Hf and 176 Hf / 177 Hf ratio, fix the upper intersection point, construct the anti-isochron, and calculate the anti-isochron age; calculate the fractionation coefficient through the calculated isochron age and the recommended age, and 176 Hf / 176 Lu ratio correction.

2. The in-situ micro-area allanite Lu-Hf dating calibration 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, and the thickness of an ordinary optical thin film sample is 30-50 μm.

3. The in-situ micro-area allanite Lu-Hf dating calibration method according to claim 1, characterized in that: In step 3, the target sampling points are pre-eroded, and a total of 30-100 points are analyzed for each analytical sample.

4. The in-situ micro-area allanite Lu-Hf dating calibration method according to claim 1, characterized in that: In step 4, the spot diameter of the ablation laser beam is 60-130 μm, the ablation frequency is 10 Hz, and the energy density is 3.0 J cm -2 .

5. The in-situ micro-area allanite Lu-Hf dating calibration method according to claim 1, characterized in that: In step 6, after every 10 unknown samples are tested, two glass standard materials NIST SRM 610 and two allanite standard materials LE2808 are tested repeatedly.

6. The in-situ micro-area allanite Lu-Hf dating calibration method according to claim 1, characterized in that: In step 8, fix the upper intersection point 177 Hf / 176 Hf is 0.282.

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