Method for selecting zircon and dating by U-Pb based on basic rock acid dissolution method

Abstract

The invention relates to the technical field of geology, and discloses a method for selecting zircon and dating by U-Pb based on a basic rock acid dissolution method, which comprises the following steps: step 1, cleaning; step 2, cutting; step 3, crushing; step 4, rough washing; 5, passing through strong magnetism; step 6, fine washing; 7, if the content of zircon in the sample is high, removing other minerals or impurities by using stainless steel pliers, and if the content of zircon and apatite is low, picking out zircon and apatite under a mirror and adhering the zircon and apatite to an oil paper bag; 8, adhering the sample; step 9, preparing a target; step 10, photographing; step 11, selecting points; and step 12, loading on a machine. Compared with manual zircon selection, the acid dissolution method has the advantages that the efficiency is high, the zircon is rapidly separated through chemical dissolution, and the time can be saved; manual errors and interference can be reduced, automatic mineral analysis software is adopted, and the automation degree is high.

Description

Technical Field

[0001] The invention belongs to the technical field of geology, and in particular, relates to a method for selecting zircons and conducting U-Pb dating based on basic rock acid dissolution. Background Art

[0002] Existing technology for analyzing trace defects of zircon in diabase: 1. Manual sorting of zircon particles relies on the physical properties of zircon. Crystals may be lost during the separation process, so it is time-consuming and inefficient.

[0003] 2. The chemical formula of zircon is ZrSiO4. For basic rocks such as diabase, zircon is not easy to grow due to silicon unsaturation. Therefore, when basic rocks are manually selected, even if there are many grams of powder samples, not a single zircon will be picked out.

[0004] 3. For silicon-unsaturated rocks, the zircon content is low, and manual zircon sorting takes a long time and is prone to not picking out zircon. We use chemical reagents to dissolve irrelevant minerals to obtain target minerals, which is shorter than manual screening, has a higher recovery rate, and basically no sample mixing caused by human interference.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] In order to solve the above technical problems, the basic concept of the technical solution adopted by the present invention is: The method for selecting zircons and conducting U-Pb dating based on basic rock acid dissolution method includes the following steps: Step S1: cleaning, after getting the diabase sample, cut off the surface with oxidation or stains, then clean and dry; Step S2: cutting, using a cutting machine to cut the sample into tofu blocks; Step S3: crushing, washing the jaw plate of the machine with a high-pressure water gun, and putting the sample into the jaw crusher for crushing after drying with a hair dryer; Step S4: coarse panning, putting the crushed powder into a panning pan, panning out the light part of the powder, leaving the heavy part to heat and dry; Step S5: If there are magnetic minerals in the heated minerals, they need to be sucked out with a magnet; Step S6: Fine panning: put the powder after passing through the electromagnetic instrument into a small aluminum panning pan, pour alcohol into it, pan out the light part, and observe the remaining heavy part under a microscope; Step S7: If the sample has a high zircon content, use stainless steel pliers to remove other minerals or impurities. If the sample has a low zircon content, pick out the zircon and apatite under a microscope and stick them on the oil paper bag; Step S8: sticking the sample, using tweezers to fix the selected zircon on the double-sided tape under a binocular stereoscope; Step S9: Target preparation: select a suitable mold, inject the prepared resin, draw vacuum, put it into an oven for solidification, grind it with sandpaper, and then polish it; Step S10: taking photos, wiping the sample surface with alcohol, taking transmission photos and reflection photos under an optical microscope, and taking cathode luminescence photos under an electron microscope; Step S11: Select points, combine transflection and CL photos, avoid inclusions, cracks, and radioactive damage sites, and select zircons with good crystal form and ring zones; Step S12: Use a laser ablation-inductively coupled plasma mass spectrometer to perform a dating experiment on the selected zircon particles.

[0007] As a preferred embodiment of the present invention, in step S3, the crushed stone particles have a particle size of 10-80 meshes, and the powder below 60 meshes is sieved out with a sieve, and the remaining samples above 60 meshes are put into a double-roll mill for further crushing until they are crushed to 60-80 meshes.

[0008] As a preferred embodiment of the present invention, the zircon heating temperature in step S4 does not exceed 60 degrees.

[0009] As a preferred embodiment of the present invention, in step S5, if there is no magnetite, a magnet can be used to directly absorb the minerals with relatively strong magnetism. After the magnet treatment, the sample is taken to a clean electromagnetic instrument. Through the difference in the electromagnetic properties of the minerals, the minerals with strong electromagnetic properties such as garnet and pyroxene are electrostatically extracted, and the remaining non-magnetic minerals are left for precise washing with alcohol.

[0010] Compared with the prior art, the present invention has the following beneficial effects: The advantage of the acid dissolution method of the present invention over manual selection of zircon is that it is highly efficient and can save time by quickly separating zircon through chemical dissolution. It has wide applicability and is applicable to a variety of rock types, especially samples with low zircon content or fine particles, such as basic rocks and ultrabasic rocks. It can reduce human errors and interference, and adopts automatic mineral analysis software (TIMA), which has a high degree of automation.

[0011] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the attached picture: Figure 1 Flow chart of the method for selecting zircons and conducting U-Pb dating based on basic rock acid dissolution. DETAILED DESCRIPTION

[0013] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention.

[0014] like Figure 1 As shown, the method of selecting zircons and U-Pb dating based on basic rock acid dissolution method includes the following steps: Step S1. Cleaning: After obtaining the diabase sample, the surface oxidized or stained parts are cut off, then cleaned and dried.

[0015] Step S2: cutting: using a cutting machine to cut the sample into tofu block sizes.

[0016] Step S3. Crushing: rinse the jaw plate of the machine with a high-pressure water gun, and after drying with a hair dryer, put the sample into the jaw crusher and crush it into particles of 40-80 mesh.

[0017] Step S4: coarse panning, putting the crushed powder into a panning pan, panning out the light part of the powder, leaving the heavy part to heat and dry; Step S5: Strong magnetic field, electromagnetic field, if there are magnetic minerals in the heated minerals, they need to be sucked out with a magnet; if there is no magnetite, the magnet can be used to directly suck out the minerals with strong magnetism. After the magnet treatment, the sample is taken to the cleaned electromagnetic instrument, and the electromagnetic difference of the minerals is used to electromagnetize the strong electromagnetic minerals such as garnet pyroxene, and the remaining non-magnetic minerals are left for precise washing with alcohol; Step S6: Fine panning: put the powder after passing through the electromagnetic instrument into a small aluminum panning pan, pour alcohol into it, pan out the light part, and observe the remaining heavy part under a microscope; Step S7: If the sample contains a lot of zircon, use stainless steel pliers to remove other minerals or impurities. If the sample contains less, pick out zircon and apatite under the microscope and stick them to the oil paper bag; weigh the samples, weigh 5g of sample 1 (80 mesh), 0.5g of sample 2 (60-80 mesh), and 0.5g of sample 3 (40-60 mesh) in a clean and dried centrifuge tube; perform chemical experiments, add 5mL, 0.5mL, and 0.5mL of concentrated hydrofluoric acid to the samples for a period of time, add a small amount of concentrated nitric acid and concentrated hydrochloric acid to the centrifuge tube after the reaction, put it in a water bath and heat it at low temperature for about 1 hour. Take it out and let it stand, then pour out the supernatant. Add 15mL, 3mL, and 3mL of aqua regia and 2g, 0.5g, and 0.5g of boric acid to the remaining sample powder, and put the centrifuge tube in a water bath again to heat and react to dissolve the sulfide. The reaction lasts for 2-3 hours, and the sample is constantly observed in the middle to avoid the zircon being completely digested. The reacted samples were washed with pure water and centrifuged several times (Note: let the samples stand and then pour out the supernatant after centrifugation), washed the samples with ethanol into a watch glass, and placed them on a heating plate to evaporate to dryness.

[0018] Step S8. Sticking the sample, fix the selected zircon on the double-sided tape with tweezers under a binocular stereoscope; Step S9. Make a target, select a suitable mold and inject 2g of epoxy resin, take the dried sample and pour it into the epoxy resin, stir while pouring, draw the vacuum for about 1 hour, put it in an oven to solidify, grind it with sandpaper, and then polish it.

[0019] The test was conducted using the automatic mineral analysis software (TIMA) instrument. The resin target was divided into several small squares of 1000×1000μm. The backscattered image was first taken in a single small square, and then the energy spectrum data was obtained point by point. The points were evenly placed inside the mineral and the points were densely placed at the mineral boundary to obtain the final result. The accelerating voltage, current, and working distance were 25kV, 9nA, and 15mm respectively. Before the TIMA test, the BSE signal was calibrated with a platinum Faraday cup, and the EDS signal was calibrated with metal Mn. The standard sample type used was MAC:12925. The dissociation mode was used in the test, the BSE unit pixel size was 5μm, and the energy spectrum test step was 15μm.

[0020] The location of the zircon was automatically identified and marked by TIMA, and cathode luminescence, transmitted light and reflected light photos were taken according to the location, and suitable zircon particles were selected for subsequent dating experiments.

[0021] Step S10: taking photos, wiping the sample surface with alcohol, taking transmission photos and reflection photos under an optical microscope, and taking cathode luminescence photos under an electron microscope; Step S11: Select points, combine transflection and CL photos, avoid inclusions, cracks, and radioactive damage, and select zircons with good crystal shape and ring zones.

[0022] Step S12. On the machine, use the laser ablation-inductively coupled plasma mass spectrometer to perform a dating experiment on the selected zircon particles. Use 5 laser pulses to pre-ablate each ablation area (ablation depth ~0.3μm) to remove possible contamination on the sample surface. The sample is analyzed under the laser conditions of a beam spot diameter of 30μm, an ablation frequency of 5Hz, and an energy density of 3J / cm2.

Claims

1. A method for selecting zircons and conducting U-Pb dating based on basic rock acid dissolution, characterized in that: The steps include: Step S1: cleaning, after getting the diabase sample, cut off the surface with oxidation or stains, then clean and dry; Step S2: cutting, using a cutting machine to cut the sample into tofu blocks; Step S3: crushing, washing the jaw plate of the machine with a high-pressure water gun, and putting the sample into the jaw crusher for crushing after drying with a hair dryer; Step S4: coarse panning, putting the crushed powder into a panning pan, panning out the light part of the powder, leaving the heavy part to heat and dry; Step S5: If there are magnetic minerals in the heated minerals, they need to be sucked out with a magnet; Step S6: Fine panning: put the powder after passing through the electromagnetic instrument into a small aluminum panning pan, pour alcohol into it, pan out the light part, and observe the remaining heavy part under a microscope; Step S7: If the sample has a high zircon content, use stainless steel pliers to remove other minerals or impurities. If the sample has a low zircon content, pick out the zircon and apatite under a microscope and stick them on the oil paper bag; Step S8: sticking the sample, using tweezers to fix the selected zircon on the double-sided tape under a binocular stereoscope; Step S9: Target preparation: select a suitable mold, inject the prepared resin, draw vacuum, put it into an oven for solidification, grind it with sandpaper, and then polish it; Step S10: taking photos, wiping the sample surface with alcohol, taking transmission photos and reflection photos under an optical microscope, and taking cathode luminescence photos under an electron microscope; Step S11: Select points, combine transflection and CL photos, avoid inclusions, cracks, and radioactive damage sites, and select zircons with good crystal form and ring zones; Step S12: Use a laser ablation-inductively coupled plasma mass spectrometer to perform a dating experiment on the selected zircon particles.

2. The method for selecting zircon and U-Pb dating based on basic rock acid dissolution according to claim 1, characterized in that: In step S3, the crushed stone particles have a particle size of 10-80 meshes, and the powder below 60 meshes is sieved out with a sieve, and the remaining samples above 60 meshes are put into a double-roll mill for further crushing until they are crushed to 60-80 meshes.

3. The method for selecting zircon and U-Pb dating based on basic rock acid dissolution method according to claim 1, characterized in that: In step S4, the zircon heating temperature does not exceed 60 degrees.

4. The method for selecting zircon and U-Pb dating based on basic rock acid dissolution method according to claim 1, characterized in that: In step S5, if there is no magnetite, a magnet can be used to directly absorb the minerals with strong magnetism. After the magnet treatment, the sample is taken to a clean electromagnetic instrument. Through the electromagnetic difference of the minerals, the minerals with strong electromagnetic properties are electrostatically extracted, and the remaining non-magnetic minerals are left for precise washing with alcohol.

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