Fast neutron irradiation can transform 39K into 39Ar, so K-Ar age can be determined as a part of argon isotope analysis.
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Where: n is the captured neutron; P is the released proton.
In 1959, 39 Ar (t 1/2 =269a) and 4 1 Ar (t 1/2=2 hours) produced by neutron activation at 39 K were detected by counting technique. However, this method does not allow atmospheric argon correction because 36 Ar cannot be measured correctly.
The long half-life of 39Ar means that it can be regarded as a stable isotope in mass spectrometry. 1966 was first applied to 40 Ar-39 Ar dating.
39 Ar produced by 39 K during irradiation is expressed as
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Where: Δ t is the irradiation time; φ e is the neutron flux density with energy e; σe is the neutron capture cross section of 39K pairs with energy e, and the generated 39 Ar must be integrated in the whole neutron energy range. Actually, this calculation is very difficult. Therefore, the normal procedure is to use samples of known age as flow monitors.
Using the K-Ar attenuation equation (6-26) and dividing both sides of the equation by (6-30), we get:
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However, the items in brackets are the same for samples and standards. So it is customarily called a single quantity, and its reciprocal j can be used as a constant. Therefore, for the standard:
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Here t is known. For samples of unknown age, the rearrangement equation (6-3 1) gives:
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For each unknown sample, in order to obtain an accurate J value, several standard samples need to be placed to represent the known spatial position in the reactor relative to the unknown sample. Therefore, the j value of each sample can be interpolated.
During irradiation at 39 K, calcium and other potassium isotopes produce interfering Ar isotopes through neutron reaction (Figure 6- 10).
The detailed study of these influences and degrees shows that for minerals greater than 1Ma, if the K/Ca ratio is greater than 1, acceptable results can be obtained without interference correction. At this point, simple atmospheric correction is appropriate:
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Where: meas stands for measured value.
Changing appropriate irradiation parameters can reduce the interference to Ar. The main interferences that must be considered are:
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Other interferences will appear, but they can be ignored because they are not important.
The complete correction formula of these interferences is
Fig. 6-10 During the activation of 39ar-40ar, the formation reaction of nuclides in the potassium region (thick line) and the main interference reaction (solid line).
(According to Deakin 1995)
The dotted line reaction produces 37Ar interference monitor.
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In the formula: 37 Ar/39 Ar is unknown, so it is necessary to correct the monitoring volume interference ratio measured by the decay of 37 Ar (t 1/2=35 days) from irradiation to analysis; (36 A r/37 A r) Ca, (39 A r/37 A r) Ca and (40 A r/39 A r) K are the ar isotope generation ratios of the subscript elements. These yields are determined by irradiating pure Ca salt and K salt in the relevant reactors respectively, and reflect the characteristics of neutron flux in the reactors. The typical ranges of these yield ratios measured by different authors for different reactors are 2. 1 ~ 2.7, 6.3 ~ 30 and 0.006 ~ 0.03 1 (Dickin, 1995) respectively.
2. Step heating
Because the characteristics of potassium in the sample are converted into argon in situ by 40 Ar-39 Ar technology, argon can be released from different domains of the sample in stages, and all age information of each step can be recovered. Compared with the conventional "full melting" technology, the advantage of staged heating is that the abnormal system in the sample can be identified by gradually degassing, and the abnormality can be ideally excluded from the analysis of the "normal behavior" part of the sample. This method is suitable for single mineral and whole rock. It is most commonly used to understand samples that suffer from argon loss, but it is also helpful to explain samples that contain genetic argon.
In the case of partial disturbance of the system, the region in the sample that is most likely to lose argon due to diffusion (such as crystal edge) should be outgassed at a relatively low temperature, while the tightly bound argon (the most disturbance-resistant) should be outgassed at a higher temperature. In order to understand the history of disturbed samples, the results of stage heating analysis are generally given in one of two ways: Ar-Ar isochron diagram, which is similar to analyzing a group of samples in conventional K-Ar; Or age spectrum, usually called platform age.
The stage heating results (Dickin, 1995) of Bjurbole meteorite are shown in Figure 6- 1 1 isochron. The linear arrangement shows that this meteorite is a simple first-class closed system history. The initial 40 Ar/36 Ar ratio may only be of partial significance, because it is the initial mixture of argon and air pollution.
When hereditary argon is suspected, isochronous diagram is useful, but age spectrum is more helpful to evaluate argon loss. In order to construct the age spectrum, the size of each gas released at continuous high temperature was measured by measuring the intensity of the generated 39 Ar ion beam. After each gas release, a bar graph can be made, the length of which represents the fraction of the total 39 Ar released in the sample, and the value on the Y axis is the 40Ar/39Ar ratio corrected by Equation (6-35). The latter is proportional to age, sometimes plotted on a logarithmic graph and sometimes expressed as linear. Determining a reliable age from the age spectrum depends on the recognition of the "flat" age. The strict criteria for plateau age are (Ludwig, 1997):
Fig. 6- 1 1 Ar-Ar isochron diagram shows the stage heating data of Beijing Urboer meteorite.
(According to Deakin 1995)
The data points in the figure represent the temperature of each degassing stage (× 100℃).
1) There are three or more 39 Ar released in the next gas release stage, accounting for more than 60% of the total;
2) The fitting probability of the weighted average age of these stages is greater than 5%;
3) The slope of the error-weighted line passing through the plateau age is equal to 0 with 5% confidence;
4) There should be no significant difference between the outermost two stages on both sides of the platform and the weighted average age (gas release needs 6 or more stages);
5) The outermost two steps on both sides of the platform shall not have the same non-zero slope (1.8 times error) (only applicable to gas release of Grade 9 or above).
Fig. 6- 12 shows the ideal properties of the age spectrum of the Ar-Ar system in a glass meteorite, which is a complete melt of continental crust material and rapidly quenched when flying into the atmosphere. The 40 Ar-39 Ar method is most suitable for samples with complicated geological history of argon loss in the later period.
Fig. 6- 12 age spectrum of 40- Ar 39 Ar glass meteorite in Texas, USA
(According to Deakin 1995)
3. Argon loss
The amphiboles of three minerals in the surrounding rocks of Eldora rock in the United States (Figure 6- 13a, b) show a thermal diffusion degassing model. The farthest sample (not shown) obtained the best plateau age of 1400Ma. The samples at about 350, 290, 75 and 10m show serious Ar loss outside the particles, but are close to the "true" age at the highest temperature. However, this model may reflect the alteration of biotite, rather than the Ar diffusion loss of amphibole. This explanation is supported by the dating experiment of synthetic amphibole-biotite mixture. Another phenomenon is that the sample 35 meters away from the contact zone presents a high-quality middle "false" plateau. Finally, the sample 0.6m away from the contact zone is saddle-shaped, and its younger part is close to metamorphic age.
The behavior of coarse biotite (Figure 6- 13c) is slightly different. Its maximum infinite age (1250Ma) is lower than that of amphibole. The age spectrum of middle distance is irregular, but it shows that age generally decreases with approaching the "flatness" of rocks. Therefore, biotite seems to be partially but unevenly degassed, which may be due to enhanced diffusion parallel to cleavage. Finally, potash feldspar suffered irregular and catastrophic ar loss, as known by conventional K-Ar analysis (Figure 6- 13d).
Fig. 6- 13 40 Ar-39 Ar age spectrum of minerals with different distances from the contact zone of Eldora rock series in Colorado, USA.
(According to Deakin 1995)
The number in the figure indicates the distance between the rock plant and the contact zone, in meters.
Hornblende can produce high-quality but meaningless plateau. This may make it dangerous to use amphibole as the basis of geological age interpretation without independent and clear evidence. The partially restarted biotite can always be identified by its irregular pattern, which makes biotite a reliable basis for age explanation. The exact meaning of biotite and amphibole samples far away from rocks is not clear, because the surrounding rock is accessory gneiss with a long thermal history.
4.39 caliber recoil
It is found that the Ar-Ar dating technique is especially suitable for the whole rock samples of small moon materials, especially the fine-grained moon sea basalt. The dotted line in Figure 6- 14 shows the typical gas release mode, because the Ar retention capacity of potassium-rich sites is low, so the radiation-induced Ar loss is 8%. However, the surface age of other samples either decreased significantly at high temperature, or decreased gradually in most gas evolution stages, especially in fine-grained rocks, which may be due to the redistribution of Ar during irradiation.
Fig. 6-14 Effect of 39ar irradiation recoil on 40 Ar-39 Ar age spectrum.
(According to Deakin 1995)
The dotted line is the analyzed moon sea basalt fragment; The solid line is the same sample used for activation analysis after fine grinding.
The recoil of 39 Ar in 39 K (n, p) reaction can cause small-scale redistribution of nuclides. This effect can lead to argon deficiency from the surface of potassium-bearing minerals to an average depth of 0.08 μ m.
Later research found that when trying to apply 40 Ar-39 Ar dating to authigenic sedimentary mineral glauconite, the recoil problem of 39 Ar was more serious. This may be because the glauconite crystals that make up spherical particles are very small. This problem can be solved by packaging glauconite particles in small glass ampoules before irradiation and opening the glass ampoules during heating analysis to analyze the recoil products together.