Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
In addition, according to the need of the best line fitting, the sample points that constitute an isochron should have a reasonable distribution, that is, the Rb/Sr ratio between samples should change to some extent. Generally speaking, samples with low Rb/Sr ratio are easy to obtain, and the key is to choose samples with high Rb/Sr ratio. The following formula can help you choose:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
The formula depends on the determination accuracy of Sr isotope at present, δ (Rb/Sr) represents the maximum difference of Rb/Sr ratio between samples, and the age t is in Ma.
Overview of methods
According to the requirements of isochron, a group of rock or single mineral samples were selected, dissolved by hydrofluoric acid and perchloric acid, and rubidium and strontium were separated by hydrochloric acid with different concentrations on cation exchange resin column. The contents of rubidium and strontium were determined by isotope dilution method on Thermoelectric Ionization Mass Spectrometer (TIMS), and the ratio of 87Rb/86Sr was obtained. At the same time, the ratio of 87Sr/86Sr of samples was calculated, and the isochron age was calculated by least square method. In addition to the accuracy requirements of isotope ratio determination, the key to the success of the determination results is to select suitable samples and meet the requirements of optimal dilution in dilution method.
The accuracy requirements of this method are: the relative error of 87Rb/86Sr ratio is 1% ~ 2%, the relative error of 87Sr/86Sr ratio is less than 1× 10-4, and the isochron age is100 ~1000.
Instruments, equipment and appliances
Thermoelectric ionization mass spectrometers MAT260, MAT26 1, MAT262, VG354, TRITON, etc.
Matching equipment of spot welder mass spectrometer.
Mass spectrometer filament preheating device, mass spectrometer supporting equipment.
Teflon beaker 10mL and 30mL.
Fluoropolymer (F46) reagent bottle 500mL, 1000mL and 2000mL.
Polyethylene plastic bottle washing 500 ~ 1000ml.
Dropping bottle of fluoropolymer (F46) 30ml.
500 ml of fluoropolymer (F46), used in a double-bottle sub-boiling still.
Timely reagent bottle 2000mL.
Timely Asia is still boiling.
Timely vacuum subboiling distiller.
The internal diameter of the timely exchange column is 6 mm and the height is 300 mm. The upper part is connected with an open container with an inner diameter of 20mm and a height of110mm, and the end is embedded with a timely sieve plate. It is required that the above resin does not leak and the dropping speed of the solution is appropriate. The resin bed is 6mm in diameter and 100mm in height, and 13 or 16 branches form a group.
Fluoroplastics (PFA) sealed sample dissolver 15mL.
Platinum disc 30mL, flat bottom.
Timely dropper.
Timely measuring cylinder (cup) 10mL, 50mL.
Stearic acid glass measuring cylinder 1000mL.
Triangular glass bottle 250mL.
Glass beaker 3000mL.
Water purification system.
The sensitivity of analytical balance is 0.0000 1mg.
Electric heating plate (temperature controllable).
Ultrasonic cleaner.
Stainless steel constant temperature oven
High speed centrifuge.
Polyethylene or timely centrifugal tube.
Micro-sampler10μ l and 50μ l
Ware cleaning
All used fluoroplastics and seasonable utensils are boiled with (1+ 1) high-purity hydrochloric acid and high-purity nitric acid in sub-boiling state 1h, cleaned with deionized water, boiled with deionized water 1h, rinsed with ultra-pure water one by one, and dried on the electric heating plate in the ultra-clean working cabinet. The new appliances used for the first time need to be scrubbed with detergent and then boiled with acid. Special beaker and (1+ 1) high-purity hydrochloric acid are used to clean the platinum disk.
Reagents and materials
The secondary distilled water of deionized water is purified by Milli-Q water purification system.
Ultra-pure deionized water is distilled by a timely distiller.
Ultra-pure hydrochloric acid is made by sub-boiling distillation of high-grade pure (1+ 1) hydrochloric acid in a timely distiller. The actual concentration is calibrated with sodium hydroxide standard solution and prepared with ultrapure water as required.
Subboiling distillation was carried out on superior ultra-pure nitric acid (1+ 1) in a timely still. The actual concentration is calibrated with sodium hydroxide standard solution, and the required concentration is prepared with ultrapure water as required.
Ultra-pure hydrofluoric acid is prepared by two bottles of sub-boiling distillers corresponding to fluoroplastics (F46).
Ultra-pure perchloric acid is prepared by sub-boiling distillation of ultra-pure perchloric acid under reduced pressure in a timely still.
Sodium hydroxide standard solution c(NaOH)≈0.3mol/L was prepared with analytically pure solid sodium hydroxide and deionized water, and calibrated with potassium hydrogen phthalate;
The purity of acetone is very high.
Analytical purity of anhydrous ethanol. Solid strontium nitrate [Sr (NO3) 2] with 84Sr isotope is enriched with 84Sr diluent. Solid rubidium chloride (RbCl) was enriched with 87Rb or 85Rb isotope as diluent. See Appendix 86.2A for solution preparation and concentration calibration of 84Sr+87Rb (or 85Rb) mixed diluted solution.
Solid strontium nitrate [Sr (NO3) 2] is spectrally pure and is a reference substance, which is stored in a dryer.
Solid rubidium chloride (RbCl) is a spectrally pure reference substance, which is stored in a dryer.
NBS987 strontium carbonate (SrCO3) international isotope reference material.
NBS607 (or NBS70a) potassium feldspar international standard material.
GBW044 1 1 potassium feldspar national first-class reference material.
Paraffin film for laboratory use.
Strong acid cation exchange resin Bio Rada 50×8 or Dowex50×8, or other resins with similar or better performance, 200 ~ 400 mesh.
Cationic resin exchange column: put about 200g of AG50×8 or Dowex50×8 cationic resin with 200 ~ 400 meshes used for the first time in a timely beaker, soak it in absolute ethanol for 24h, pour out the ethanol, rinse it with deionized water, soak it in (1+ 1) high-purity hydrochloric acid for 24h, and rinse it with deionized water after pouring out the hydrochloric acid. Finally, transfer to the prepared reaction column, so that the resin bed has a diameter of 6mm and a height of100 mm. When draining water, add 30mL( 1+ 1) excellent pure hydrochloric acid and 15mL ultra-pure water in turn for elution, and finally use 1.0 mol/L ultra-pure HCl 10ml for balance. Continue to use in the future, backwash with 30mL( 1+ 1) excellent pure hydrochloric acid, rinse with 15mL ultra-pure water, and balance with 10mL 1.0mol/L ultra-pure HCl.
The specification of rhenium tape is18mm× 0.03mm× 0.8mm. ..
sample preparation
A group of fresh unaltered rock samples are collected from the same igneous rock body or the same volcanic rock horizon. According to the sample size, the weathered surface or other pollution on the surface is removed and crushed to 200 meshes, which are divided into about 10g according to the rules. The contents of Rb and Sr are roughly determined by general chemical analysis methods (such as atomic absorption spectrometry). According to formula (86.44) or experience, 5 ~ 6 samples with large change of Rb/Sr ratio were selected for age determination.
Sample decomposition
Weigh 30 ~ 50mg (accurate to 0. 1mg) of rock or single mineral powder sample, put it in PFA fluoroplastics sealed sample dissolver or platinum dish, add 84Sr+87Rb (or 85Rb) mixed diluent (accurate to 0. 1mg) according to the best dilution requirement, gently shake it to fully disperse the block sample, and add 3mL of ultrapure. After the sample is completely decomposed, it is evaporated to dryness, then the wall is washed with a small amount of 6mol/L ultrapure hydrochloric acid and evaporated to dryness, and the temperature is raised to 180℃ to drive off fluorine and excess perchloric acid. Dissolve dry matter with 1mL 1.0mol/LHCl, and pour the solution into the exchange column. If it is found that the sample solution is turbid or has obvious residue, it means that the sample decomposition is incomplete and the centrifugal separation step needs to be added. If the iron content of the sample is high, it is also necessary to transfer the sample solution to a platinum dish and put it on an electric furnace at 500℃ for several minutes. Cooling, dissolving in water, centrifuging to extract clear liquid, and loading on column.
Separation of rubidium and strontium;
After loading the sample solution into the column, clean the wall of the sample dissolver (or platinum dish) with 1mL 1.0mol/L ultra-pure HCl, and then transfer it to the exchange column. After the solution is dried, add 14mL 1.0mol/L ultrapure HCl to elute the impurity elements Li, Na, K and Fe, and discard the eluent. Add 6 ml 1.0 mol/l ultrapure hydrochloric acid to analyze rubidium, and collect it in a 10 ml PTFE beaker. Then magnesium, calcium, aluminum, iron, etc. were eluted with 6 ml of 2.5 mol/L ultrapure hydrochloric acid. And discard the eluent. Strontium was continuously analyzed with 6 ml of 2.5 mol/L ultrapure hydrochloric acid, collected in a 10 ml PTFE beaker, and evaporated to dryness.
Dissolve the evaporated rubidium and strontium samples with 1mL 1.0mol/LHCl, regenerate and balance them with 1mL 1.0mol/LHCl, then pour them into a cation resin column, and further purify rubidium and strontium according to the above procedures. Evaporate the thin film of the analytical solution and cover it, waiting for mass spectrometry analysis.
Rubidium and strontium isotope analysis;
1) sample loading. Rubidium and strontium isotopes were analyzed by dual-band thermal ionization mass spectrometer. The following operations take MAT26 1 as an example, and other mass spectrometers are similar.
Pretreatment of filament rhenium belt: clean the rhenium belt with absolute ethanol, spot weld the rhenium belt on the filament bracket with spot welder, insert the bracket with rhenium belt into the ion source turntable in turn, and put it into the filament preheating device as a whole. After vacuumizing to n× 10-5Pa, electrify the rhenium belts according to the preset procedure, and each rhenium belt is pre-sintered at 1800℃ for 4 ~ 6a.
The burned rhenium belt on the ion source turntable is initially formed, and the ionization belt is taken out in turn. Dissolve the purified sample with a drop of ultrapure water, drop the solution in the center of the evaporation belt with a micro-sampler, apply a current to the evaporation belt with the current intensity of about 1A, and then gradually increase the current until the white smoke on the belt disappears, further increase the temperature until the rhenium belt turns dark red, then quickly adjust the current to zero and switch to adding the next sample. When all samples are installed, the ionization zone is inserted into the original position to further reshape the rhenium zone. It is required that the evaporation zone and ionization zone should be parallel to each other, but they should not be connected together. The distance between the two zones should be 0.7 mm Install a shielding cover and send it to the ion source of mass spectrometer for vacuum pumping.
2) Isotopic determination of rubidium and strontium. The measured objects are metal ion currents Sr+ and Rb+. When the vacuum degree of the ion source reaches 5× 10-6Pa, the isolation valve in the analysis room is opened, and the filaments in the ionization zone and evaporation zone are energized respectively, and the temperature is slowly raised. In the process of increasing the current, pay attention to the exhaust and vacuum drop of the sample to avoid the vacuum drop too fast. When the vacuum degree reaches 2× 10-6Pa, the ionization band current reaches more than 2A, the evaporation band current is about 1.5A, and the filament temperature reaches 1000 ~ 1200℃, the measuring system will be in a manual state, and strontium ions will be found in the range of mass number 88 ~ 84. According to the different types of mass spectrometer, it is analyzed whether the strontium isotope ion current is received by multiple receivers at the same time or by a single receiver in turn. Start the automatic measurement program, and the system collects the strontium isotope ratio data of 84Sr/86Sr, 87Sr/86Sr and 88Sr/86Sr. When the monitoring ratio is greater than 10-4, the 87Rb comparative value 87Sr/86Sr has obvious interference, so the belt temperature should be reduced appropriately. 4 ~ 6 pieces of data were collected from each sample, and each group of data consisted of 8 ~ 10 scans. The average and standard deviation of strontium isotopes in the diluted samples were calculated respectively.
The isotopic analysis of rubidium is similar to that of strontium, but the temperature when collecting 85Rb/87Rb data is relatively low, about 1000℃ (the ionization band current is above 1.5A, and the lower the evaporation band current, the better.
3) directly measuring 3) strontium isotope ratio. The age determination of young marine carbonate only needs to determine the strontium isotope ratio, while other young magmatic rocks only need to determine the strontium isotope ratio when they are used in geochemical research, without determining the concentration of rubidium and strontium. In this case, samples of the same order of magnitude are roughly weighed, strontium is separated and purified by the same chemical separation procedure, and isotope analysis is carried out by the same method without adding diluent, and the 87Sr/86Sr ratio of the samples is directly obtained after correcting the mass fractionation effect.