Dissolving method and (uranium-thorium) / helium age measuring method for single particle sphene

Abstract

To provide a dissolving method and a (uranium-thorium ) / helium age measuring method for a single particle sphene.SOLUTION: This dissolving method comprises: a step of mixing a single particle sphene, a hydrofluoric acid, and a concentrated nitric acid, and putting the obtained mixture in an autoclave to heat and digest, thereby obtaining an initial dissolved sample, the temperature of heating and dissolving being 180°C and time being 24 hours; a step of heating, evaporating and drying the initial dissolved sample to obtain an evaporated and dried sample; and a step of mixing the evaporated and dried sample with a concentrated hydrochloric acid, and putting the obtained mixture in the autoclave to dissolve the mixture again, thereby obtaining a redissolved sample, the temperature of redissolving being 180°C and time being 24 hours. Further, the (uranium-thorium ) / helium age measuring method for the single particle sphene obtains contents of uranium, thorium, and helium by measuring the particle sample, and an age value can be obtained by substituting a (uranium-thorium ) / helium age formula for these contents.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to the technical field of mineral isotope dating, and in particular to a method for dissolving single-grain sphene and a method for (uranium-thorium) / helium dating of single-grain sphene. [Background technology]

[0002] Isotope thermochronology technology has been rapidly developed in recent years, which can not only provide information on the age of geological bodies, but also provide information on the temperature and depth at the time of the formation of minerals and geological bodies, and therefore has great potential in the study of the formation and evolution of geological bodies and structures. Because the (uranium-thorium) / helium system is sensitive to low-temperature conditions, the (uranium-thorium) / helium dating method is currently one of the most commonly used dating techniques in low-temperature thermochronology research. The (uranium-thorium) / helium dating technique has provided a very important and effective means in the dating of geological bodies, the evolution study of the thermal history of basins and regions, the reconstruction and evolution study of paleotopography, tracing of sediment sources, the thermal history analysis of the structure of the source area, the history study of the uplift and denudation of structures, and the study of the formation, uplift and denudation process of mineral deposits. The (uranium-thorium) / helium dating method, like other isotope dating methods, is based on the principle of radioactive decay. 238 U, 235 U, 232 Th undergoes α decay to become a daughter isotope 4 He is produced, and the decay equations are (I) to (III). 238 U → 206 Pb+8α( 4 He) + 6β - (I) 235 U → 207 Pb+7α( 4 He)+4β - (II) 232 Th → 208 Pb+6α( 4 He)+4β - (III)

[0003] In reality, other elements, including Sm, also decay to produce radioactive He, but in most cases, 4 The content of He is much less than the other three elements. In the laboratory, it is usually found remaining in minerals. 238 U, 235 U, 232 Only the Th content was measured, and the amount of the three elements produced was 4 Under the right conditions, He can be partially or completely retained within minerals and gradually accumulate within the minerals. 4 The He content was measured, and the above radioactive decay formula and existing parameters (λ 238 =1.551×10 -10 ;λ 235 =9.848×10 -10 ;λ 232 =4.947×10 -11 ), we can calculate the (uranium-thorium) / helium age of a mineral, where the natural U isotope ratio is 235 U / 238 U=1 / 137.88. From the above formula, 4 The He age formula is as follows:

[0004]

number

[0005] The condition for formula (1) to hold is that the mineral to be dated contains pristine 4 The helium content in the atmosphere is very low (about 5×10 -6 ), and in most cases, atmospheric helium contamination can be neglected.

[0006] Due to the different diffusion dynamics of helium in different minerals, the closure temperature is also different, therefore, the helium ages of different minerals can provide the thermal history evolution information in different temperature ranges, and the cooling history of the geological body can be obtained by utilizing the (uranium-thorium) / helium system. Although many minerals are potentially suitable for (uranium-thorium) / helium dating, only a few minerals, such as zircon and apatite, have been studied in more detail. Sphene is a monoclinic island silicate mineral that contains uranium and thorium and is widely distributed in various rocks such as igneous rocks and metamorphic rocks as an accessory mineral, so it has attracted attention as a potential (uranium-thorium) / helium dating mineral. Through the study of the helium diffusion characteristics in sphene, the closure temperature of the sphene-helium system has been determined to be 100-180 °C, and this temperature range is not well constrained by other existing thermochronological techniques. The fission-tracking temperature range of apatite is about 60–120 °C, and the argon-argon system of feldspar minerals can provide information for a temperature range above 170 °C, so the application of the (uranium-thorium) / helium system of sphene can fill a gap in low-temperature thermochronology research.

[0007] There have been many research results on uranium-thorium / helium dating techniques both in Japan and overseas, but relatively few on sphene-thorium / helium dating methods. Published research results are sporadic, and either uranium-thorium / helium dating is performed using samples such as sphene, or the uranium-thorium / helium age of sphene is measured by completely imitating the uranium-thorium / helium dating method for zircon.

[0008] The specific method of (uranium-thorium) / helium dating using an equal-part sample of sphene is to prepare two equal-part sphene samples, one of which is used to measure the helium content, and the other is used to measure the uranium and thorium content. A major drawback of this method is that there is a minimum mass limit for each equal-part sample. Because the uranium and thorium content in the sample particles is not uniform, each equal-part sample must be large enough to balance the difference in uranium and thorium in each sphene particle. Meanwhile, the amount of the equal-part sample must be predicted by studying the sample in detail, but the accuracy of the predicted required sample amount is low, and usually one equal-part sample requires at least a few milligrams of sample amount, and the required amount of sphene sample is very large. Uranium-thorium / helium dating using equal-partition samples is a relatively primitive method among the Uranium-thorium / helium dating methods, and as experimental processes improve, this less accurate and more sample-intensive method is gradually being replaced by single-particle Uranium-thorium / helium dating.

[0009] At present, single particle (uranium-thorium) / helium dating methods for minerals such as zircon and apatite have been established at home and abroad, and the uranium, thorium and helium contents can be obtained by measuring the same grain sample. Here, the dissolution process is an important process of the single particle (uranium-thorium) / helium dating method. Common dissolution methods include atmospheric pressure dissolution, high pressure dissolution, ultrasonic dissolution, etc. Apatite is a mineral that is relatively easy to dissolve, and apatite can be completely dissolved by adding an acid reagent to the sample and using ultrasonic waves. On the other hand, silicate minerals such as zircon and sphene are difficult to dissolve, and need to be dissolved multiple times using multiple acid reagents. When using atmospheric pressure dissolution, an open vessel is usually used, but this causes some difficult-to-dissolve elements to not completely dissolve, and a large amount of acid volatilizes under high temperatures, which may pollute the environment and harm the health of the experimenter. At the same time, some elements are easily lost due to volatilization, which has a significant impact on the accuracy of the analysis results. In the (uranium-thorium) / helium dating method for zircon, high-pressure dissolution is used to dissolve zircon samples, and highly corrosive acids such as hydrofluoric acid are used. The dissolution time for a single time is more than 48 hours, and the dissolution temperature reaches more than 200°C. In addition, re-dissolution is required, which lengthens the overall sample dissolution step time. If the zircon dissolution process is directly used to dissolve sphene, firstly, the dissolution time is long and the dissolution efficiency is low, and secondly, it cannot be guaranteed that the sphene sample is completely dissolved.

[0010] Currently, there are no reports of an established experimental method for (uranium-thorium) / helium dating of single-grain sphene. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to provide a method for dissolving single-particle sphene and a method for (uranium-thorium) / helium dating of single-particle sphene, and to accurately measure the (uranium-thorium) / helium age and fill the gap in the (uranium-thorium) / helium dating technology of single-particle sphene. [Means for solving the problem]

[0012] In order to achieve the above objectives of the present invention, the present invention provides the following technical solutions: The present invention provides a method for dissolving single-particle sphenes, Mixing single-particle sphene, hydrofluoric acid and concentrated nitric acid, and then putting the mixture into a high-pressure cooker for thermal digestion to obtain an initial dissolution sample, the temperature of the thermal digestion is 180°C, and the time is 24 hours; evaporating the initially dissolved sample to obtain an evaporatively dried sample; The method includes the steps of mixing the evaporated and dried sample with concentrated hydrochloric acid, and then re-dissolving the mixture in an autoclave to obtain a re-dissolved sample, the temperature of the re-dissolving is 180° C., and the time of the re-dissolving is 24 hours.

[0013] Preferably, the amount of hydrofluoric acid added is 350 μL, the amount of concentrated nitric acid added is 25 μL, and the volume concentration of concentrated nitric acid is 50%.

[0014] Preferably, the amount of concentrated hydrochloric acid added is 300 μL.

[0015] Preferably, the metal impurity contents of the hydrofluoric acid, concentrated nitric acid and concentrated hydrochloric acid are all less than 0.01 ppb.

[0016] Preferably, the temperature of the heat evaporation is 60°C.

[0017] The present invention provides a method for (uranium-thorium) / helium dating of single-grain sphenes, Step S1 of selecting a single particle sphene sample; Heating the single particle sphene sample 4 He is extracted and purified to obtain a purified gas, and the He in the purified gas is analyzed by isotope dilution using a helium isotope mass spectrometer. 4 The He content was measured in single-particle sphene samples. 4Step S2 of setting the He content; The single particle sphene sample is dissolved according to the dissolution method described in the above technical solution to prepare a mixed solution to be measured, in which the step of heating and dissolving is replaced by the following: the single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, the resulting mixture is heated and dissolved, and the isotope dilution method is used to measure the amount of sphene in the single particle sphene sample by using an inductively coupled plasma mass spectrometer. 238 U and 232 The content of Th is obtained, and the diluent solution is 235 U, 238 U, 232 Th, 230 Step S3, which is a concentrated nitric acid solution containing Th; Measured single particle sphene sample 4 He, 238 U and 232 The Th content is substituted into the age formula (1) and the sphene (uranium-thorium) / helium age is calculated.

[0018]

number

[0019] In formula (1), 4 He, 238 U and 232 Th is the measured atomic number, and t is the daughter isotope produced by radioactive decay. 4 is the time He is accumulated, and λ 238 , λ 235 , λ 232 teeth 238 U, 235 U, 232 are the decay constants of Th and 1.55125×10 -10 a -1 , 9.8485×10 -10 a -1 , 4.9475×10 -11 a -1 The method includes step S4.

[0020] Preferably, the minimum width of the single particle sphene sample is >80 μm.

[0021] Preferably, 4 Heating and extraction of He was performed with a 970 nm diode laser. 4 The laser current for He heating and extraction is 15 A and the time is 10 min.

[0022] Preferably, in the step S2, 4 To measure the He content, The purified gas and a diluent 3 He is mixed to obtain a sample mixed gas, and the sample mixed gas is analyzed using a helium isotope mass spectrometer. 4 He / 3 Measure the He ratio and ( 4 He / 3 He) Spiked Sample and recording Known amount of 4 He standard gas and diluent 3 He was mixed to obtain a standard mixed gas, and the standard mixed gas was analyzed using a helium isotope mass spectrometer. 4 He / 3 Measure the He ratio and ( 4 He / 3 He) Spike Q standard and the diluent used to prepare the sample gas mixture and the standard gas mixture. 3 The volume of He is the same, and Based on equation (2), 4 The He content was calculated as follows:

[0023]

number

[0024] Preferably, step S3 includes: 235 U, 238 U, 232 Th and 230 A concentrated nitric acid solution containing Th is provided as a diluent solution, 235 U / 238 U ratio and 230 Th / 232 The Th ratio has already been determined. 238 U and 232 The Th content is known, 230 A nitrate solution not containing Th was provided as a standard solution, and the 235 U / 238 The U ratio has already been determined, The single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and re-dissolved in concentrated hydrochloric acid to obtain a mixed solution to be measured; The diluent solution is mixed with the standard solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and re-dissolved in concentrated hydrochloric acid to obtain a diluent-standard solution mixture, and the volume of the diluent solution used to prepare the mixture solution to be measured and the diluent-standard solution mixture is the same; Using an inductively coupled plasma mass spectrometer, the mixture solution to be measured and the diluent-standard solution mixture were 235 U / 238 U ratio and 230 Th / 232 Measuring the Th ratio; In diluent solution 238 The U content was calculated by equation (3), 238 U Spike was recorded, and then the 238 The U content was calculated by equation (4), 238 U Sample It was recorded that

[0025]

number

[0026] In formula (3), 238 U Standard is the exact amount of standard solution added to the diluent-standard mixture. 238 is the number of U atoms, ( 235 U / 238 U) Standard is the amount of the standard solution 235 U / 238 U ratio, ( 235 U / 238 U) Spike is the amount of the standardized diluent solution 235 U / 238 U ratio, ( 235 U / 238 U) mix is the concentration of the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry. 235 U / 238 U ratio,

[0027]

number

[0028] In formula (4), 238 U Spike is the exact concentration of the diluent solution added to the diluent-standard solution mixture calculated by equation (3). 238 is the number of atoms of U, i.e., the exact concentration of the diluent solution when it is added to the mixture to be measured. 238 It is also the number of U atoms, ( 235 U / 238 U) Spike is the standardized diluent solution 235 U / 238 U ratio, ( 235 U / 238 U) Sample is the natural abundance of the sphene sample. 235 U / 238 U ratio, ( 235 U / 238 U) spike-sample is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer.235 U / 238 It is U ratio, in the diluent solution according to equation (5). 232 Calculate the Th content, 232 Th Spike and then use equation (6) to calculate the 232 Calculate the Th content, 232 Th Sample And recorded,

[0029]

number

[0030] In formula (5), in the standard solution 230 Th / 232 The Th ratio is 0. 232 Th Standard is the exact amount of standard solution added to the mixture. 232 is the number of Th atoms, ( 230 Th / 232 Th) Spike is the amount of the standardized diluent solution 230 Th / 232 Th ratio, ( 230 Th / 232 Th) mix is the concentration of the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry. 230 Th / 232 is the Th ratio,

[0031]

number

[0032] In equation (6), the sphene sample 230 Th / 232 The Th ratio is 0. 232 Th Spike is the exact concentration of the diluent solution when it is added to the diluent-standard mixture, calculated based on equation (5). 232 is the number of atoms of Th, i.e. the exact number of atoms when the diluent solution is added to the mixture to be measured. 232It is also the number of Th atoms, ( 230 Th / 232 Th) Spike is the amount of the standardized diluent solution 230 Th / 232 Th ratio, ( 230 Th / 232 Th) spike-sample is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer. 230 Th / 232 This includes being a Th ratio.

[0033] The present invention has established a complete dissolution process for sphene particles. In the (uranium-thorium) / helium dating method, one of the most important steps is the dissolution method for mineral particles. The conventional sphene (uranium-thorium) / helium dating method basically imitates the experimental process of zircon, and the temperature used to dissolve the sample is high and takes a long time, making the process complicated, time-consuming, and inefficient, and there are also risks in the experimental process. The present invention provides a dissolution method specific to single-particle sphene. Regarding the use of acid, considering that sphene is a silicate mineral, hydrofluoric acid can effectively dissolve sphene, but the use of hydrofluoric acid may produce insoluble fluorides, so after dissolving with hydrofluoric acid, it was heated, evaporated, dried, and then redissolved with concentrated hydrochloric acid.

[0034] The present invention selects a method of decomposing sphene samples using acid in a closed vessel, and the boiling point of the acid increases with increasing pressure, which strengthens the decomposition ability of the acid. In addition, the use of a closed vessel allows the volatile components to be quantitatively kept in solution, and the dissolution temperature of the sample is relatively low. The dissolution method of single-particle sphene established by the present invention is carried out in a high-pressure cooker, and the high pressure in the high-pressure cooker further improves the decomposition effect of the acid. Compared with the dissolution process of zircon, the present invention reduces the dissolution temperature, and further shortens the dissolution time significantly, improving the dissolution efficiency.

[0035] Furthermore, since the present invention uses a purified acid with high purity, interference caused by sample decomposition can be ignored.

[0036] The present invention provides a method for measuring the (uranium-thorium) / helium age of a single particle of sphene. Compared with the conventional (uranium-thorium) / helium dating method, which uses an equally divided sample method to measure the contents of uranium, thorium, and helium, the present invention measures the same grain sample to obtain the contents of uranium, thorium, and helium, which can be directly substituted into the (uranium-thorium) / helium dating formula to obtain the age value, which can avoid the trouble of examining and measuring the weight in sample preparation, making the measurement more convenient and effectively saving valuable sphene samples.

[0037] In the (uranium-thorium) / helium dating method for single-particle sphene provided by the present invention, the isotope dilution method is used to measure the daughter isotope helium in sphene and the parent isotopes uranium and thorium. The isotope dilution method has the following advantages: by simply adding an equal amount of diluent to the standard gas / standard solution and the sphene sample, the content of the isotope to be measured (uranium, thorium, helium) in the sphene sample can be obtained without knowing the exact amount of diluent. The method is relatively accurate, and the measurement error for uranium and thorium is usually about 1% to 2%, and the measurement error for helium is less than 1%. Through error propagation, it has been calculated that the dating error for single-particle sphene is less than 3%. [Brief description of the drawings]

[0038] [Figure 1] 1 is a flow chart of the single particle sphene (uranium-thorium) / helium dating method of the present invention. [Diagram 2] FIG. 1 is a measurement diagram of sphene particles. [Diagram 3] FIG. 2 is an actual diagram of a niobium capsule. [Figure 4] FIG. 1 is an actual diagram of the Alphachron helium isotope mass spectrometer used in the examples. [Diagram 5] Photograph of the laser chamber and the sample holder with the sphene sample. [Figure 6] This is an actual diagram of a high-pressure cooker. [Figure 7] This is an actual image showing heating, evaporation and drying. [Figure 8] 1 is a photograph of a sample undergoing remelting. [Figure 9] FIG. 1 is a diagram of an inductively coupled plasma mass spectrometer. [Figure 10] FIG. 1 is a relationship diagram between the ages of FCT sphenes in Comparative Examples 1 to 4 and U / Th. [Figure 11] FIG. 1 is a relationship diagram between the age and U / Th of FCT sphenes in Comparative Examples 5 and 6. [Figure 12] FIG. 1 is a relationship diagram between the age and U / Th of FCT sphenes in Comparative Examples 7 and 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present invention provides a method for dissolving single-particle sphenes, Mixing single-particle sphene, hydrofluoric acid and concentrated nitric acid, and then putting the mixture into a high-pressure cooker for thermal digestion to obtain an initial dissolution sample, the temperature of the thermal digestion is 180°C, and the time is 24 hours; evaporating the initially dissolved sample to obtain an evaporatively dried sample; The method includes the steps of mixing the evaporated and dried sample with concentrated hydrochloric acid, and then re-dissolving the mixture in an autoclave to obtain a re-dissolved sample, the temperature of the re-dissolving is 180° C., and the time of the re-dissolving is 24 hours.

[0040] In the present invention, unless otherwise specified, all the raw materials used are commercially available products well known in the art. In the present invention, the hydrofluoric acid is commercially available pure hydrofluoric acid, and the concentrated hydrochloric acid is commercially available concentrated hydrochloric acid well known in the art, with a mass fraction of 36-38%.

[0041] In the present invention, an initial dissolution sample is obtained by mixing single particle sphene, hydrofluoric acid, and concentrated nitric acid, and then placing the resulting mixture in a high pressure cooker and dissolving it by heating.

[0042] In the present invention, the minimum width of said single particle sphene is preferably greater than 80 μm.

[0043] In the present invention, the hydrofluoric acid and concentrated nitric acid are preferably purified acids, and the metal impurity content is less than 0.01 ppb. In the present invention, the use of purified high-purity acids can prevent the introduction of new interferences due to sample decomposition. In the present invention, the volume concentration of the concentrated nitric acid is preferably 50% (i.e., HNO 3 and water volume ratio is 1:1). In the present invention, the amount of hydrofluoric acid added is preferably 350 μL, and the amount of concentrated nitric acid added is preferably 25 μL. In the present invention, by controlling the amount of hydrofluoric acid and concentrated nitric acid added within the above range, the complete dissolution of single particle sphene can be guaranteed, and the safety of the experiment can be guaranteed.

[0044] In the present invention, preferably, single particle sphene, hydrofluoric acid, and concentrated nitric acid are added to a Teflon (registered trademark) dissolution bottle, the Teflon dissolution bottle is placed on the lining of a high-pressure cooker, hydrofluoric acid and concentrated nitric acid with a volume concentration of 50% are added to the lining of the high-pressure cooker, and the high-pressure cooker is sealed to perform heating and dissolving. In the present invention, the lining of the high-pressure cooker is preferably a polytetrafluoroethylene lining. The amount of hydrofluoric acid added to the lining of the high-pressure cooker is preferably 9 mL, and the amount of concentrated nitric acid is preferably 420 μL. The inside of the high-pressure cooker is a high-temperature and high-pressure environment, and in the present invention, the balance of the internal pressure can be maintained by adding concentrated nitric acid and hydrofluoric acid to the lining of the high-pressure cooker.

[0045] In the present invention, the temperature of the heating and dissolving is 180° C., and the time is 24 hours. In the present invention, sphene is dissolved using hydrofluoric acid and concentrated nitric acid, so that it has good solubility, but there is a possibility that poorly soluble fluorides are generated during the dissolving process.

[0046] After obtaining the initial dissolution sample, the present invention heats and evaporates the initial dissolution sample to obtain an evaporatively dried sample. In the present invention, the temperature of the heat and evaporative drying is preferably 60° C., and the heat and evaporative drying is preferably performed on a hot plate.

[0047] After obtaining the evaporated-dried sample, the present invention mixes the evaporated-dried sample with concentrated hydrochloric acid, and the resulting mixture is placed in an autoclave for re-dissolution to obtain a re-dissolved sample.

[0048] The concentrated hydrochloric acid is preferably purified concentrated hydrochloric acid, and the metal impurity content is less than 0.01 ppb. In the present invention, the amount of concentrated hydrochloric acid added is preferably 300 μL. In the present invention, the redissolution is performed using concentrated hydrochloric acid, so that the poorly soluble fluoride can be effectively dissolved.

[0049] In the present invention, concentrated hydrochloric acid is preferably added to the evaporated and dried sample, the evaporated and dried sample to which concentrated hydrochloric acid has been added is placed inside an autoclave, concentrated hydrochloric acid is added to the autoclave, and the autoclave is sealed to perform redissolution. In the present invention, the amount of concentrated hydrochloric acid added to the autoclave lining is preferably 9 mL. The inside of the autoclave is a high-temperature and high-pressure environment, and in the present invention, the balance of the internal pressure can be maintained by adding concentrated hydrochloric acid to the autoclave lining.

[0050] In the present invention, the remelting temperature is 180° C. and the remelting time is 24 hours.

[0051] After obtaining the re-dissolved sample, the present invention preferably further comprises removing the re-dissolved sample from the acid. In the present invention, the temperature of the acid removal is preferably 80° C. In the present invention, there is no special requirement for the process of the acid removal, and any acid removal process well known in the art can be adopted. In the embodiment of the present invention, the re-dissolved sample is transferred into a 7 mL Teflon dissolution bottle using a pipette, and the 7 mL dissolution bottle is placed on a heating plate and heated, and the heating is stopped when the solution in the dissolution bottle evaporates to 100 μL.

[0052] The present invention provides a unique dissolution method for single-particle sphene. Regarding the use of acid, considering that sphene is a silicate mineral, hydrofluoric acid can effectively dissolve sphene, but the use of hydrofluoric acid may produce insoluble fluorides, so after dissolving with hydrofluoric acid, the material is heated, evaporated, and dried, and then redissolved with concentrated hydrochloric acid. Compared with the dissolution process of zircon, the present invention not only reduces the dissolution temperature, but also greatly shortens the dissolution time and improves the dissolution efficiency.

[0053] The present invention provides a method for (uranium-thorium) / helium dating of single-grain sphenes, Step S1 of selecting a single particle sphene sample; Heating the single particle sphene sample 4 He is extracted and purified to obtain a purified gas, and the He in the purified gas is analyzed by isotope dilution using a helium isotope mass spectrometer. 4 The He content was measured in single-particle sphene samples. 4 Step S2 of setting the He content; The single particle sphene sample is dissolved according to the dissolution method described in the above technical solution to prepare a mixed solution to be measured, in which the step of heating and dissolving is replaced by the following: the single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, the resulting mixture is heated and dissolved, and the isotope dilution method is used to measure the amount of sphene in the single particle sphene sample by using an inductively coupled plasma mass spectrometer. 238 U and 232 The content of Th is obtained, and the diluent solution is 235 U, 238 U, 232 Th and 230 Step S3, which is a concentrated nitric acid solution containing Th; Measured single particle sphene sample 4 He, 238 U and 232 The Th content is substituted into the age formula (1) and the sphene (uranium-thorium) / helium age is calculated.

[0054]

number

[0055] In formula (1), 4 He, 238 U and 232 Th is the measured atomic number, and t is the daughter isotope produced by radioactive decay. 4 is the time He is accumulated, and λ 238 , λ 235 , λ 232 teeth 238 U, 235 U, 232 are the decay constants of Th and 1.55125×10 -10 a -1 , 9.8485×10 -10 a -1 , 4.9475×10 -11 a -1 The method includes step S4.

[0056] The present invention first selects a single particle sphene sample.

[0057] In the present invention, the single-particle sphene sample is preferably selected as a sphene particle with a perfect crystal shape, clean, crack-free, and inclusion-free. In most cases, if the crystal is partially missing or severely damaged, the measured age may be overestimated, and the inclusions in the sphene may cause a component separation effect inside the sphene crystal, so that the pure sphene particle without inclusions is preferably selected as the sample for (uranium-thorium) / helium dating. In the present invention, the minimum width of the single-particle sphene sample is preferably >80 μm, more preferably 90-120 μm. The present invention is advantageous for the accuracy of the measurement by adopting the single-particle sphene sample of the above-mentioned size, and the accuracy of the final age result is significantly reduced due to the excessive calibration factor in the case of a mineral particle with a small size. In the present invention, the sphene sample is preferably observed using a microscope and the size is measured to select it.

[0058] After selecting a single particle sphene sample, the present invention includes heating the single particle sphene sample to obtain 4 He is extracted and purified to obtain a purified gas. 4 The He content was measured by the isotope dilution method using a helium isotope mass spectrometer, and this was used to calculate the He content in single-particle sphene samples. 4 Used as the He content.

[0059] In the present invention, preferably, the single particle sphene sample is loaded into a niobium capsule and heated, 4 He is extracted, and in the present invention, there is no special requirement for the size of the niobium capsule, and it is sufficient to adopt the size familiar in the art, and in the embodiment of the present invention, the length and diameter of the niobium capsule are about 1 mm. The present invention loads a single particle sphene sample into a niobium capsule, which can prevent particle loss during sample transfer and evaporation of uranium and thorium elements during helium content analysis. The present invention adopts a niobium capsule to prevent melting during the helium gas extraction process, and also prevents interference with the analysis of uranium and thorium content due to melting during the subsequent melting process.

[0060] In the present invention, 4 The thermal extraction of He is preferably performed using a 970 nm diode laser, 4 The laser current for the He thermal extraction is preferably 15 A, the time is preferably 10 min, and the vacuum level is preferably 1.0×10 -8 The present invention achieves the above-mentioned results by controlling the heating and extraction conditions within the above-mentioned range. 4 This is advantageous for achieving sufficient extraction of He.

[0061] In the embodiment of the present invention, specifically, the sphene sample encapsulated in the niobium capsule is placed in the laser chamber of the helium isotope mass spectrometer, and in order to load the sample and expose the laser chamber to the atmosphere, the laser chamber must be returned to a vacuum state before heating and gas extraction can begin. After 3 minutes of gas extraction with a mechanical pump and 18 hours of gas extraction with a turbomolecular pump, the vacuum level in the system is 1.0×10-8 Once the vacuum level is below Torr, the experimental requirements are met and heating and gas extraction commences.

[0062] In the present invention, in order to eliminate the system error and the error caused by the niobium capsule and ensure the accuracy of the measurement, the present invention preferably 4 Cold and hot sole tests are performed before the heating extraction of He.

[0063] The cold bottom is a bottom temperature measured by loading a sphene sample contained in a niobium capsule into the laser chamber and measuring the temperature inside the instrument piping without laser heating under the same experimental process. 4 The thermal bottom refers to the He content in the piping when an empty niobium capsule is laser heated under the same experimental process (laser current 15A, time 10 minutes). 4 Refers to the He content.

[0064] If the test results for the cold sole and hot sole are within 0.0040 ncc, the sphene sample 4 It indicates that there is no effect on He measurement. If the cold bottom exceeds the above value, it is necessary to check the vacuum level of the equipment, whether the pipeline is damaged, whether gas leakage occurs, etc., and if the hot bottom is too high, it is necessary to check whether the empty niobium capsule is contaminated.

[0065] In the present invention, preferably, at least two heating extractions are performed, and the second heating extraction 4 The amount of He gas used for the first 4 If the amount of He gas is less than 1% or below the thermal bottom, it is determined that the gas extraction is sufficient; if not, extraction continues and stops when the final gas amount is less than 1% of the first gas amount or below the thermal bottom.

[0066] In the present invention, the purification is preferably performed by zirconium-aluminum pump purification, and the purification time is preferably 60 to 120 seconds. 2 , O 2 , H 2 O, CO 2 , and S.O. 2can be effectively removed by purification. The purified gas enters a helium isotope mass spectrometer for testing. In an embodiment of the present invention, a quadrupole mass spectrometer is specifically used for analysis and testing.

[0067] In the present invention, 4 The measurement of the He content preferably includes the following steps: The purified gas and a diluent 3 He is mixed to obtain a sample mixed gas, and the sample mixed gas is analyzed using a helium isotope mass spectrometer. 4 He / 3 Measure the He ratio and ( 4 He / 3 He) Spiked Sample and recording Known amount of 4 He standard gas and diluent 3 He was mixed to obtain a standard mixed gas, and the standard mixed gas was analyzed using a helium isotope mass spectrometer. 4 He / 3 Measure the He ratio and ( 4 He / 3 He) Spike Q standard and the diluent used to prepare the sample gas mixture and the standard gas mixture. 3 The volume of He is the same, and Based on equation (2), 4 The He content was calculated as follows:

[0068]

number

[0069] In formula (2), 4 He Sample in the refined gas 4 is the content of He, 4 He Q Standard teeth 4 He standard gas 4 The step includes a step of He content.

[0070] In the present invention, the calculated purified gas 4 He content, i.e., in a single-particle sphene sample 4 The amount of He is expressed as a volume and must be converted to moles using the ideal gas equation PV=nRT for use in calculating the age of the sphene.

[0071] In a single particle sphene sample 4 After measuring the He content, in the present invention, 4 The single particle sphene sample contained in the niobium capsule that has been inspected for He content is dissolved according to the dissolution method described in the above technical proposal to prepare a mixed solution to be measured, in which the heating and dissolving step is replaced by the following: the single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, the resulting mixture is heated and dissolved, and the He content in the single particle sphene sample is detected by isotope dilution using an electrochemically coupled plasma mass spectrometer. 238 U and 232 The content of Th was measured, and the diluent solution was 235 U and 230 The specific steps preferably include: 235 U, 238 U, 232 Th and 230 A concentrated nitric acid solution containing Th is provided as a diluent solution (commercially available), and 235 U / 238 U ratio and 230 Th / 232 The Th ratio has already been standardized, and the substrate of the diluent solution is preferably concentrated nitric acid with a volume concentration of 50%, i.e., HNO 3 The volume concentration of is 50%. 238 U and 232 The Th content is known, 230 A nitrate solution not containing Th was provided as a standard solution (commercially available), and the 235 U / 238 The U ratio has already been standardized, and the substrate of the standard solution is preferably nitric acid at a volume concentration of 10%, i.e., HNO3 The volume concentration of is 10%. The single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and then re-dissolved in concentrated hydrochloric acid to obtain a mixed solution to be measured; The diluent solution is mixed with the standard solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and re-dissolved in concentrated hydrochloric acid to obtain a diluent-standard solution mixture, and the volume of the diluent solution used to prepare the mixture solution to be measured and the diluent-standard solution mixture is the same; Using an inductively coupled plasma mass spectrometer, the mixture solution to be measured and the diluent-standard solution mixture were 235 U / 238 U ratio and 230 Th / 232 Measuring the Th ratio; In the diluent solution according to the formula (3) of the present invention, 238 Calculate the U content, 238 U Spike and then calculate the concentration of the single-particle sphene sample by Eq. (4). 238 Calculate the U content, 238 U Sample And recorded,

[0072]

number

[0073] In formula (3), 238 U Standard is the exact amount of standard solution added to the diluent-standard mixture. 238 is the number of U atoms, ( 235 U / 238 U) Standard is the amount of the standard solution 235 U / 238 U ratio, ( 235 U / 238 U) Spike is the amount of the standardized diluent solution 235 U / 238 U ratio, ( 235 U / 238 U) mixis the concentration of the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry. 235 U / 238 U ratio,

[0074]

number

[0075] In formula (4), 238 U Spike is the exact concentration of the diluent solution added to the diluent-standard solution mixture calculated by equation (3). 238 is the number of atoms of U, i.e. the exact amount of diluent added to the mixture being measured. 238 It is also the number of U atoms, ( 235 U / 238 U) Spike is the standardized diluent solution 235 U / 238 U ratio, ( 235 U / 238 U) Sample is the natural abundance of the sphene sample. 235 U / 238 U ratio, ( 235 U / 238 U) spike-sample is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer. 235 U / 238 It is U ratio, in the diluent solution according to equation (5). 232 Calculate the Th content, 232 Th Spike and then calculate the concentration of the single-particle sphene sample by Eq. (6). 232 Calculate the Th content, 232 Th Sample And recorded,

[0076]

number

[0077] In formula (5), in the standard solution 230 Th / 232 The Th ratio is 0.232 Th Standard is the exact amount of standard solution added to the mixture. 232 is the number of Th atoms, ( 230 Th / 232 Th) Spike is the amount of the standardized diluent solution 230 Th / 232 Th ratio, ( 230 Th / 232 Th) mix is the concentration of the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry. 230 Th / 232 is the Th ratio,

[0078]

number

[0079] In equation (6), the sphene sample 230 Th / 232 The Th ratio is 0 (natural sphene 230 Since it does not contain Th, all of the sphene samples 230 Th / 232 Th ratio is 0), 232 Th Spike is the exact concentration of the diluent solution when it is added to the diluent-standard mixture, calculated based on equation (5). 232 is the number of atoms of Th, i.e. the exact number of times the diluent solution is added to the mixture to be measured. 232 It is also the number of Th atoms, ( 230 Th / 232 Th) Spike is the amount of the standardized diluent solution 230 Th / 232 Th ratio, ( 230 Th / 232 Th) spike-sample is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer. 230 Th / 232 This includes being a Th ratio.

[0080] In order to provide a blank bottom in the testing process and to monitor for contamination of various laboratory equipment, such as reagents, niobium capsules, and containers used throughout the testing, the present invention preferably analytically tests the reagents and empty niobium capsules to provide the bottom. Each bottom test includes all the reagents used in the dissolution process, but does not include any diluents (i.e. 235 U and 230 No uranium or thorium (containing substances containing Th) is added, so that the contents of uranium and thorium elements in the bottom cannot be quantitatively obtained, but the bottom level can be evaluated based on the signal levels (cps) of uranium and thorium isotopes measured by a plasma mass spectrometer. When calculating the contents of uranium and thorium in the sphene sample, the present invention preferably first subtracts the isotope signal levels of uranium and thorium in the bottom from the corresponding isotope signal levels in the actually measured sphene sample solution. The experimental results show that the isotope signal levels after dissolution of the sphene sample with the addition of diluent are about 10 times the isotope signal levels in the bottom. 5 This is more than double the original value, indicating that it has little effect on the age results of the sphene samples.

[0081] The method for dissolving single-particle sphene and the method for (uranium-thorium) / helium dating of single-particle sphene provided by the present invention will be described in detail below using examples, but these should not be construed as limiting the scope of the present invention.

[0082] [Example 1] According to the process shown in Figure 1, sample preparation, helium content analysis, sample dissolution and uranium-thorium content analysis are carried out sequentially, and the specific steps are as follows: (1) Sample preparation Five clean grains of Fish Canyon Tuff (FCT) sphene, free of cracks or inclusions, were selected under a microscope, photographed and measured (Figure 2), and then each grain of sphene was loaded into a niobium capsule approximately 1 mm in length and diameter (Figure 3).

[0083] (2) Helium content analysis Sphene sample4 Extraction and analysis of He is performed with an Alphachron helium isotope mass spectrometer (Figure 4) using a 970 nm diode laser to heat and extract the gas, and a QMG quadrupole mass spectrometer to measure the helium content.

[0084] Five sphene samples, each encased in a niobium capsule, are placed in the laser chamber of a helium isotope mass spectrometer (Figure 5). After loading the samples and exposing the laser chamber to the atmosphere, the laser chamber must be returned to a vacuum before heating and gas extraction can begin. After 3 minutes of gas extraction with a mechanical pump and 18 hours of gas extraction with a turbomolecular pump, the vacuum level in the system is 1.0 x 10 -8 Once the vacuum level is below Torr, the experiment requirements are met and the experiment can begin.

[0085] Sphene sample 4 Before measuring He, we first conduct cold and hot bottom tests. Cold bottom is the bottom of the equipment piping without laser heating under the same experimental process. 4 The thermal bottom refers to the He content in the piping when an empty niobium capsule is laser heated under the same experimental process. 4 He content. Tests of multiple batches of soles have shown that the cold soles and hot soles are always maintained at about 0.0010 to 0.0040 ncc (Table 1). On the other hand, the helium content in the sphene samples is 4 The He content is 100 to 1000 times higher than that of the bottom, so the sphene sample 4 It can be seen that there is no effect on the He measurement.

[0086] [Table 1]

[0087] Note: In Table 1, CB-n (n is 1 to 5) is a cold sole, and NbHB-n (n is 1 to 5) is a hot sole.

[0088] Start heating and gas extraction, set the laser current to 15A for each grain sample, and the heating time to 10 minutes. Perform two gas extractions for each grain sample. After two gas extractions are completed, perform analysis calculations, and if the gas volume in the second extraction is less than 1% of the gas volume in the first extraction, the gas extraction is considered to be complete.

[0089] (3) Sample dissolution 1) Transfer five sphene samples to 4.5mL Teflon dissolution bottles, and the dissolution bottles containing the sphene samples are called sample bottles. At the same time, add one bottom bottle and put an empty niobium capsule in it to use for bottom testing of the entire process. In addition, add one standard solution bottle and add 25μL of standard solution. The standard solution contains 25×10 -9 of 238 U and 25×10 -9 The solution contains 232Th and the substrate is 10% nitric acid by volume.

[0090] 2) Add 25 μL of blank solution to the bottom bottle. The blank solution is concentrated nitric acid with a volume concentration of 50%.

[0091] 3) Add 25 μL of diluent solution to each sample bottle and standard solution bottle. 235 U and 230 It contains Th, and the substrate of the solution is concentrated nitric acid with a volume concentration of 50%.

[0092] 4) Add 350 μL of purified hydrofluoric acid to each of the sample bottle, bottom bottle, and standard solution bottle.

[0093] 5) Place the sample bottle, bottom bottle, and standard solution bottle in the autoclave (Figure 6) and add 420 μL of concentrated nitric acid and 9 mL of hydrofluoric acid to the autoclave.

[0094] 6) Place the sealed autoclave in an oven and heat it for 24 hours at a heating temperature of 180°C.

[0095] 7) After the heating is complete, cool the autoclave and remove it from the oven. Take out the sample bottle, bottom bottle, and standard solution bottle and place them on a heating plate to evaporate and dry the samples at 60°C (Figure 7).

[0096] 8) After the solution in each bottle has evaporated to dryness, add 300μL of concentrated hydrochloric acid (Figure 8) to each bottle, and add 9mL of concentrated hydrochloric acid to the autoclave. Seal the autoclave again, place in an oven and heat for 24 hours at a heating temperature of 180℃.

[0097] 9) After redissolution is complete, use a pipette to transfer the solution from each bottle to a 7 mL Teflon dissolution bottle, place the 7 mL dissolution bottle on a heating plate, and heat at 80°C. When the solution in the dissolution bottle evaporates and 100 μL remains, stop heating. After cooling, add 300 μL of ultrapure water to each dissolution bottle to dilute and obtain the solution to be analyzed for uranium-thorium content. Transfer the solution from the dissolution bottle to a 1.5 mL centrifuge tube and prepare for mass spectrometry.

[0098] (4) Uranium-thorium content analysis The uranium and thorium content analysis was performed using an electrochemically coupled plasma mass spectrometer (Figure 9), which mainly measures the isotope counts of mass numbers 230, 232, 235, and 238. The bottom count was subtracted from the isotope count in the sample to obtain the ratios of 230 / 232 and 235 / 238, and the ratios were then calculated using standard solutions. 238 U and 232 Th was calculated.

[0099] Measured 4 He, 238 U and 232 The FCT sphene (uranium-thorium) / helium ages were calculated by substituting Th into the age formula. The FCT sphene (uranium-thorium) / helium age results are shown in Table 2.

[0100] [Example 2] Example 2 is almost the same as Example 1, except for the different grain sizes of the FCT sphenes. The (uranium-thorium) / helium age results are shown in Table 2.

[0101] [Example 3] Example 3 is almost the same as Example 2, except for the different grain sizes of the FCT sphenes. The (uranium-thorium) / helium age results are shown in Table 2.

[0102] [Table 2]

[0103] The results in Table 1 show that the present invention has good applicability to different sphene samples. Specifically, in Examples 1 to 3, sphene particles of different particle sizes were selected, and the obtained age results all agreed within the error range, and also agreed with the reference values ​​within the error range. This shows the reliability of the method of the present invention, and proves that the ages measured by the present invention are reproducible. In addition, the present invention can completely dissolve sphene particles of various particle sizes, including large sphene particles with a radius of more than 250 μm.

[0104] In Table 2, the source of reference value 1 is: Reiners PW, Farley KA, 1999. Helium diffusion and (U-Th) / He thermochronometry of titanite. Geochimica et Cosmochimica Acta, 63(22): 3845-3859. Reference value 2 source: AlexandraM.Hornea,MatthijsC.van Soest,KipV.Hodges,et al,2016.Integrated single crystal laser ablation U / Pb and(U-Th) / He dating of detrital accessory minerals-Proof-of-concept studies of titanites and zircons from the Fish Canyon tuff. Geochimica et Cosmochimica Acta, 178:106-123.

[0105] In order to obtain the optimal heating temperature, heating time, and re-dissolving conditions for the sample dissolving process, several comparative examples were set up. Comparative Examples 1 to 6 are control tests for the initial dissolving conditions, and Comparative Examples 7 and 8 are control tests for the re-dissolving conditions.

[0106] [Comparative Example 1] 1) Transfer five sphene samples to 4.5mL Teflon dissolution bottles, and call the dissolution bottles containing the sphene samples sample bottles. At the same time, add one bottom bottle and put an empty niobium capsule in it to perform bottom inspection of the entire process. Add one more standard solution bottle and add 25μL of standard solution. 2) Add 25 μL of blank solution to the bottom bottle. 3) Add 25 μL of diluent solution to each sample bottle and standard solution bottle. 4) Add 350 μL of purified hydrofluoric acid to each of the sample bottle, bottom bottle, and standard solution bottle. 5) Place the sample bottle, bottom bottle, and standard solution bottle in the autoclave and add 420 μL of concentrated nitric acid and 9 mL of hydrofluoric acid to the autoclave. 6) Place the sealed autoclave in an oven and heat to dissolve for 60 hours at a heating temperature of 220°C. 7) After the heating is complete, cool the autoclave and remove it from the oven. Take out the sample bottle, bottom bottle, and standard solution bottle and place them on a heating plate to evaporate and dry the samples at 60°C. 8) After the solution in each bottle has evaporated to dryness, add 300μL of concentrated hydrochloric acid to each bottle, and add 9mL of concentrated hydrochloric acid to the autoclave. Seal the autoclave again and place in the oven to redissolve. Heat for 24 hours at 180℃. 9) After redissolution is complete, use a pipette to transfer the solution from each bottle to a 7 mL Teflon dissolution bottle, place the 7 mL dissolution bottle on a heating plate, and heat at 80°C. When the solution in the dissolution bottle evaporates and 100 μL remains, stop heating. After cooling, add 300 μL of ultrapure water to each dissolution bottle to dilute and obtain the solution to be analyzed for uranium-thorium content. Transfer the solution from the dissolution bottle to a 1.5 mL centrifuge tube and prepare for mass spectrometry. The final FCT sphene (uranium-thorium) / helium age results are shown in Figure 10.

[0107] [Comparative Example 2] Comparative Example 2 is almost the same as Comparative Example 1, except that the initial melting conditions were set at 220° C. for 48 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 10.

[0108] [Comparative Example 3] Comparative Example 3 is almost the same as Comparative Example 1, except that the initial melting conditions were set at 220° C. for 36 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 10.

[0109] [Comparative Example 4] Comparative Example 4 is almost the same as Comparative Example 1, except that the initial melting conditions were set at 220° C. for 24 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 10.

[0110] [Comparative Example 5] Comparative Example 5 is almost the same as Comparative Example 1, except that the initial melting conditions were set at 220° C. for 12 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 11.

[0111] [Comparative Example 6] Comparative Example 6 is almost the same as Comparative Example 1, except that the initial melting conditions were set at 180° C. for 12 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 11.

[0112] [Comparative Example 7] 1) Transfer five sphene samples to 4.5mL Teflon dissolution bottles, and call the dissolution bottles containing the sphene samples sample bottles. At the same time, add one bottom bottle and place an empty niobium capsule in the bottom bottle to perform bottom inspection of the entire process. Add one more standard solution bottle and add 25μL of standard solution. 2) Add 25 μL of blank solution to the bottom bottle. 3) Add 25 μL of diluent to each sample bottle and standard solution bottle. 4) Add 350 μL of purified hydrofluoric acid to each of the sample bottle, bottom bottle, and standard solution bottle. 5) Place the sample bottle, bottom bottle, and standard solution bottle in the autoclave and add 420 μL of concentrated nitric acid and 9 mL of hydrofluoric acid to the autoclave. 6) Place the sealed autoclave in an oven and heat for 12 hours at a temperature of 180°C. 7) After the heating is complete, cool the autoclave and remove it from the oven. Take out the sample bottle, bottom bottle, and standard solution bottle and place them on a heating plate to evaporate and dry the samples at 60°C. 8) After the solution in each bottle has evaporated to dryness, add 300μL of concentrated hydrochloric acid to each bottle, and add 9mL of concentrated hydrochloric acid to the autoclave. Seal the autoclave again and place in the oven to redissolve. Heat for 24 hours at 220℃. 9) After redissolution is complete, use a pipette to transfer the solution from each bottle to a 7 mL Teflon dissolution bottle, place the 7 mL dissolution bottle on a heating plate, and heat at 80°C. When the solution in the dissolution bottle evaporates and 100 μL remains, stop heating. After cooling, add 300 μL of ultrapure water to each dissolution bottle to dilute and obtain the solution to be analyzed for uranium-thorium content. Transfer the solution from the dissolution bottle to a 1.5 mL centrifuge tube and prepare for mass spectrometry. The final FCT sphene (uranium-thorium) / helium age results are shown in Figure 12.

[0113] [Comparative Example 8] Comparative Example 8 was almost the same as Comparative Example 1, except that the remelting conditions were set at 220° C. for 12 hours. The final FCT sphene (uranium-thorium) / helium age results are shown in FIG. 12.

[0114] The control experiment revealed the following: When the heating time in the initial dissolution is more than 24 hours and the heating temperature is more than 180 ° C, the obtained FCT sphene (uranium-thorium) / helium ages are all consistent with the reference values ​​within the error range, and the degree of discretization is low. When the heating time in the initial dissolution is 12 hours, even if the heating temperature is 180 ° C or 220 ° C, the degree of discretization of the obtained ages is large, and there is a tendency that the ages increase with the increase of the U / Th ratio, which explains that the sphene particles may not be completely dissolved, and as a result, the contents of uranium and thorium obtained in the experimental analysis are lower than the actual contents. If the heating time is more than 24 hours and the heating temperature is more than 180 ° C, sphene can be completely dissolved, but in consideration of the experimental efficiency and environmental protection and energy saving, the heating temperature of 180 ° C and the heating time of 24 hours are the optimal conditions for the initial dissolution in this invention.

[0115] Similarly, control experiments were set up for the remelting process to obtain the optimal conditions. When heating at 220 ° C for 12 hours during remelting, the obtained sphene ages all coincide with the reference values ​​within the error range, but the degree of discretization is large, and the ages tend to increase slightly with the increase in the U / Th ratio. In order to ensure the accuracy and precision of the experimental results, the condition of heating at 220 ° C for 12 hours was abandoned. The ages obtained by heating at 180 ° C for 24 hours coincide with the reference values ​​within the error range and have a low degree of discretization, so in this invention, heating at 180 ° C for 24 hours is the optimal remelting condition.

[0116] It should be noted that the above is only a preferred embodiment of the present invention, and those skilled in the art may make some improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be regarded as within the protection scope of the present invention.

Claims

1. A method for dissolving single-particle sphene, comprising the steps of: Mixing single-particle sphene, hydrofluoric acid and concentrated nitric acid, and then putting the mixture into a high-pressure cooker for thermal digestion to obtain an initial dissolution sample, the temperature of the thermal digestion is 180° C., the time is 24 hours, and the volume concentration of the concentrated nitric acid is 50%; evaporating the initially dissolved sample to obtain an evaporatively dried sample; The method for dissolving the sample includes mixing the evaporated and dried sample with concentrated hydrochloric acid, and then redissolving the mixture in a high pressure cooker to obtain a redissolved sample, the temperature for the redissolution being 180° C. and the time for the redissolution being 24 hours.

2. 2. The method according to claim 1, wherein the amount of hydrofluoric acid added is 350 μL, and the amount of concentrated nitric acid added is 25 μL.

3. 2. The method according to claim 1, wherein the amount of concentrated hydrochloric acid added is 300 μL.

4. 2. The method according to claim 1, wherein the metal impurity contents of the hydrofluoric acid, concentrated nitric acid and concentrated hydrochloric acid are all less than 0.01 ppb.

5. 2. The method according to claim 1, wherein the temperature of the heating, evaporating and drying is 60°C.

6. Step S1 of selecting a single particle sphene sample; Heating the single particle sphene sample ４ He is extracted and purified to obtain a purified gas, and the He in the purified gas is analyzed by isotope dilution using a helium isotope mass spectrometer. ４ The He content was measured in single particle sphene samples. ４ Step S2 of setting the He content; The single particle sphene sample is dissolved according to the dissolution method of any one of claims 1 to 5 to prepare a mixed solution to be measured, in which the step of heating and dissolving is replaced by the following: the single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, the resulting mixture is heated and dissolved, and the isotope dilution method is used to measure the isotope dilution method using an inductively coupled plasma mass spectrometer to detect the isotope dilution method in the single particle sphene sample. ２３８ U and ２３２ The content of Th is obtained, and the diluent solution is ２３５ U. ２３８ U. ２３２ Th, ２３０ Step S3, which is a concentrated nitric acid solution containing Th; Measured single particle sphene sample ４ He, ２３８ U and ２３２ The Th content is substituted into the age formula (1) and the sphene (uranium-thorium) / helium age is calculated. [0010] In formula (1), ４ He, ２３８ U and ２３２ Th is the measured atomic number, and t is the daughter isotope produced by radioactive decay. ４ is the time that He is accumulated, and λ ２３８ , λ ２３５ , λ ２３２ teeth ２３８ U. ２３５ U. ２３２ are the damping constants of Th, and are 1.55125×10 －１０ a －１ , 9.8485×10 －１０ a －１ , 4.9475×10 －１１ a －１ A method for (uranium-thorium) / helium dating of a single particle of sphene, comprising step S4:

7. The method for (uranium-thorium) / helium dating of single grain sphene according to claim 6, characterized in that the minimum width of the single grain sphene sample is >80 μm.

8. The above ４ Heating and extraction of He is performed with a 970 nm diode laser. ４ The method for (uranium-thorium) / helium dating of single-particle sphene according to claim 6, characterized in that the laser current for heating and extracting He is 15 A and the time is 10 minutes.

9. In the step S2, ４ To measure the He content, The purified gas and a diluent ３ He is mixed to obtain a sample mixed gas, and the sample mixed gas is analyzed using a helium isotope mass spectrometer. ４ He / ３ The He ratio was measured, and ４ He / ３ He) Ｓｐｉｋｅｄ Ｓａｍｐｌｅ and recording Known amount of ４ He standard gas and diluent ３ He is mixed to obtain a standard mixed gas, and the standard mixed gas is analyzed using a helium isotope mass spectrometer. ４ He / ３ The He ratio was measured, and ４ He / ３ He) Ｓｐｉｋｅ Ｑ ｓｔａｎｄａｒｄ and the diluent used to prepare the sample gas mixture and the standard gas mixture. ３ the volume of He is the same; Based on the formula (2), ４ The He content was calculated as follows: [0025] In formula (2), ４ He Ｓａｍｐｌｅ in the refined gas ４ is the content of He, ４ He Ｑ Ｓｔａｎｄａｒｄ teeth ４ He standard gas ４ or The minimum width of the single particle sphene sample is >80 μm, and in step S2, ４ To measure the He content, The purified gas and a diluent ３ He is mixed to obtain a sample mixed gas, and the sample mixed gas is analyzed using a helium isotope mass spectrometer. ４ He / ３ The He ratio was measured, and ４ He / ３ He) Ｓｐｉｋｅｄ Ｓａｍｐｌｅ and recording Known amount of ４ He standard gas and diluent ３ He is mixed to obtain a standard mixed gas, and the standard mixed gas is analyzed using a helium isotope mass spectrometer. ４ He / ３ The He ratio was measured, and ４ He / ３ He) Ｓｐｉｋｅ Ｑ ｓｔａｎｄａｒｄ and the diluent used to prepare the sample gas mixture and the standard gas mixture. ３ the volume of He is the same; Based on the formula (2), ４ The He content was calculated as follows: [0030] In formula (2), ４ He Ｓａｍｐｌｅ in the refined gas ４ is the content of He, ４ He Ｑ Ｓｔａｎｄａｒｄ teeth ４ He standard gas ４ The method for (uranium-thorium) / helium dating of single-particle sphene according to claim 6, further comprising a step of determining the content of He.

10. In step S3, ２３５ U. ２３８ U. ２３２ Th and ２３０ A concentrated nitric acid solution containing Th is provided as a diluent solution, ２３５ U / ２３８ U ratio and ２３０ Th / ２３２ The Th ratio has already been determined, ２３８ U and ２３２ The Th content is known, ２３０ A nitrate solution not containing Th was provided as a standard solution, and the ２３５ U / ２３８ The U ratio has already been determined, The single particle sphene sample is mixed with a diluent solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and re-dissolved in concentrated hydrochloric acid to obtain a mixed solution to be measured; The diluent solution is mixed with the standard solution and hydrofluoric acid, and the resulting mixture is successively heated and dissolved, heated and evaporated to dryness, and re-dissolved in concentrated hydrochloric acid to obtain a diluent-standard solution mixture, and the volume of the diluent solution used to prepare the mixture solution to be measured and the diluent-standard solution mixture is the same; Using an inductively coupled plasma mass spectrometer, the mixture solution to be measured and the diluent-standard solution mixture were ２３５ U / ２３８ U ratio and ２３０ Th / ２３２ Measuring the Th ratio; In diluent solution ２３８ The U content is calculated by formula (3): ２３８ U Ｓｐｉｋｅ was recorded, and then the ２３８ The U content is calculated by formula (4): ２３８ U Ｓａｍｐｌｅ It was recorded that [0045] In formula (3), ２３８ U Ｓｔａｎｄａｒｄ is the exact time when the standard solution is added to the diluent-standard mixture. ２３８ is the number of U atoms, ( ２３５ U / ２３８ U) Ｓｔａｎｄａｒｄ is the amount of the standard solution ２３５ U / ２３８ U ratio, ( ２３５ U / ２３８ U) Ｓｐｉｋｅ is the amount of the standardized diluent solution ２３５ U / ２３８ U ratio, ( ２３５ U / ２３８ U) ｍｉｘ in the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry ２３５ U / ２３８ U ratio, [0050] In formula (4), ２３８ U Ｓｐｉｋｅ is the exact concentration of the diluent solution calculated by equation (3) when it is added to the diluent-standard solution mixture. ２３８ is the number of atoms of U, i.e., the exact concentration of the diluent solution when it is added to the mixture to be measured. ２３８ It is also the number of U atoms, ( ２３５ U / ２３８ U) Ｓｐｉｋｅ is the standardized diluent solution ２３５ U / ２３８ U ratio, ( ２３５ U / ２３８ U) Ｓａｍｐｌｅ is the natural abundance of the sphene sample. ２３５ U / ２３８ U ratio, ( ２３５ U / ２３８ U) ｓｐｉｋｅ－ｓａｍｐｌｅ is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer. ２３５ U / ２３８ It is U ratio, in the diluent solution according to equation (5). ２３２ Calculate the Th content, ２３２ Th Ｓｐｉｋｅ and then use equation (6) to calculate the ２３２ Calculate the Th content, ２３２ Th Ｓａｍｐｌｅ And recorded, [006] In formula (5), in the standard solution ２３０ Th / ２３２ The Th ratio is 0, ２３２ Th Ｓｔａｎｄａｒｄ is the exact amount of standard solution added to the mixture. ２３２ is the number of Th atoms, ( ２３０ Th / ２３２ Th) Ｓｐｉｋｅ is the amount of the standardized diluent solution ２３０ Th / ２３２ Th ratio, ( ２３０ Th / ２３２ Th) ｍｉｘ in the diluent-standard solution mixture measured by inductively coupled plasma mass spectrometry ２３０ Th / ２３２ is the Th ratio, [0070] In formula (6), the sphene sample ２３０ Th / ２３２ The Th ratio is 0, ２３２ Th Ｓｐｉｋｅ is the exact concentration of the diluent solution added to the diluent-standard solution mixture, calculated based on equation (5). ２３２ is the number of Th atoms, i.e., the exact concentration of the diluent solution when it is added to the mixture to be measured. ２３２ This is also the number of Th atoms, ( ２３０ Th / ２３２ Th) Ｓｐｉｋｅ is the amount of the standardized diluent solution ２３０ Th / ２３２ Th ratio, ( ２３０ Th / ２３２ Th) ｓｐｉｋｅ－ｓａｍｐｌｅ is the amount of the mixture solution to be measured using an inductively coupled plasma mass spectrometer. ２３０ Th / ２３２ The method for (uranium-thorium) / helium dating of single-particle sphene according to claim 6, characterized in that the Th ratio is included.

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