In argon precipitation system, the sample is completely melted under ultra-high vacuum and evenly mixed with a known amount of 38Ar diluent. The purified mixed gas enters the mass spectrometer for argon isotope analysis, and the radioactive argon content of the sample is obtained. Determine the potassium content of the sample and substitute it into the formula (86. 1 1 1) to get the potassium argon age.
This method requires the peak intensity measurement accuracy of isotopes 40Ar, 39Ar and 36Ar to be 0.05% ~ 0.5%, 0.05% ~ 0.5% and 1% ~ 10% respectively. When the potassium content is ≥ 1%, the relative error of potassium is required to be less than1%; 0. 1% & lt； When the potassium content is less than 65438 0%, the measurement error is 2% ~ 4%; When the potassium content is
Instruments, equipment and appliances
The analytical accuracy of gas isotope mass spectrometer is better than 0.005%.
Argon precipitation system.
The sensitivity of analytical balance is 0.0001g.
The power of the oven is more than 600W, and the inner cavity is 250 mm× 300 mm× 250 mm ..
Ultrasonic cleaner.
Glass tube.
The inner diameter of quartz tube is 1 1mm and 5 mm.
Glass timely transition joint
Ultra-high vacuum valve for all-metal baking φ= 19mm, φ=38mm.
Reagents and materials
Deionized water.
muriatic acid
Sulfuric acid.
Polyphenylene ether.
Acetone.
Activated carbon is granular and analytically pure.
Sponge titanium.
Indium block.
The argon diluent used to enrich 38Ar needs 38ar/36ar >: 1× 105, 38ar/40ar >; 1× 104。
Black mica standard sample ZBH-25.
Molecular sieve.
Gold or silver wire φ = 0.5mm
Tungsten wire φ = 0.25mm
The thickness of molybdenum sheet is 0. 1 ~ 0.2 mm.
Aluminum foil or oxygen-free copper sheet.
Liquid nitrogen.
Analytical method
1) sample handling, weighing and loading. According to the identification results of thin slices, fresh, weathered and unchanged rocks are selected, crushed to 0.2 ~ 0.5 mm, washed with deionized water for dust removal, and treated at low temperature (
Wash the selected samples with deionized water and acetone in turn, use ultrasonic cleaner if necessary, and dry at low temperature (
2) Ultra-high vacuum and baking degassing of argon precipitation system. Weld the quartz tube containing the sample on the argon precipitation system, and make the system obtain high vacuum according to the following steps:
A. First, start the low vacuum of the mechanical air extraction system, and at the same time, detect whether the system leaks air and eliminate it;
B, then start the diffusion pump to vacuum the system to10-4 ~10-5pa;
C, according to the actual situation, heating the oven on the cover of the whole pipeline system at 200-250 DEG C for several hours to dozens of hours for degassing;
D, every time the titanium wire current is raised to 40A by the sublimation titanium pump, 65438±0min, the titanium sponge furnace is heated at 800-900℃ for 65,438 0 h, and if necessary, it is kept at 600℃ for 4-8 h or repeatedly heated for several times until the vacuum of the system reaches 65,438 00-6 Pa.
3) sample melting. When the vacuum degree of the system meets the requirements (10-6Pa), separate the molten sample from the vacuum, put the sample package into the furnace, and slowly raise the temperature to the melting point of the sample within 20-60 minutes (generally 1000 ~ 1400℃ depending on the sample), and keep it at the highest temperature of 60℃.
4) Adding diluent and gas purification. While raising the temperature, mix a known amount of argon diluent rich in 38Ar with the gas precipitated from the sample, introduce the mixed gas into a sponge titanium furnace, purify it at 800℃ for 30-60 minutes, then naturally cool it to 400℃ (if necessary, keep it at 65438+/-00 minutes), introduce the mixed gas into a sublimation titanium pump, heat the titanium wire at 40A current for 65438+-0 minutes, and then The argon mixed with liquid nitrogen is sucked into the sample receiving tube filled with activated carbon for analysis, or directly introduced into the mass spectrometer for argon isotope analysis immediately.
5) argon isotope mass spectrometry analysis. When the vacuum degree of the mass spectrometer reaches 10-6Pa and the instrument is stable, argon gas is discharged from the sample receiving tube and introduced into the ion source of the mass spectrometer. Assuming that the access time is zero, the intensities with mass numbers of 40.5, 40, 39.5, 38.5, 38, 37.5, 36.5, 36 and 35.5 and the time t corresponding to each mass number are recorded in sequence, and 5-8 groups of data are measured circularly. Subtract the average value of the intensities of the masses 40.5 and 39.5 from the intensity of the mass 40 to obtain the peak intensity of 40Ar. Calculate the peak intensities of 38Ar and 36Ar in the same way. The peak intensity of 40Ar is taken as the ordinate, and the corresponding time t is taken as the abscissa for least square fitting. The intercept of a straight line on the ordinate is the peak intensity of 40Ar injection. Using the same method, the peak intensities of 38Ar and 36Ar samples were calculated, and the isotopic ratios of (40Ar/36Ar)m and (38Ar/36Ar)m after the samples were mixed with diluent were calculated.
6) Determination of potassium content. Concentrate 0.5g from the same sample used for argon isotope analysis, grind it to 200 meshes, and determine the content of potassium (mass fraction wK) by atomic absorption spectrometry or inductively coupled plasma spectrometry.
Determination result calculation
1) potassium concentration:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Wherein c40K is 40K molar concentration, mol/g; WK is the mass fraction of k; 1.167×10-4 is the atomic ratio of 40K/K; 39. 102 is the molar mass of potassium, g/mol.
2) Calculation of radioactive argon:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Where: 40Arγ is the molar concentration of 40Ar caused by radiation, mol/g; T, A and M in the lower right corner respectively represent diluent, atmospheric argon and mixed gas of sample and diluent; 38Art is the amount of diluent 38Ar added, and the isotope ratio of mol atmospheric argon is constant: (36Ar/38Ar)A=5.349, (40ar/38ar) a =1581; MS is the sample mass, g.
3) Calculation of the percentage of total 40Ar (40ar) and radioactivity 40ar(40Arγ) in 40ar:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
4) Age and error calculation. Substituting the contents of 40K and radioactive 40Ar(40Arγ) in the formula (86.11) to get the K-Ar age, and its relative standard deviation is:
Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis
Where: (EK)2 is the determination of potassium variance; (Etr)2 is the calibration variance of 38Ar diluent concentration; (E4038)2 is the variance of the ratio of (40Ar/38Ar)m; (E3638)2 is the variance of the ratio of (36ar/38ar) m.
laboratory report
Potassium content (%), radioactive argon content (mol/g), percentage of radioactive 40Ar in total 40Ar, age value and its error.