A Re-Os isotope analysis method for organic-rich sedimentary rocks

Abstract

The present invention provides a Re-Os isotope analysis method for organic-rich sedimentary rocks, wherein a powder sample of organic-rich sedimentary rocks is weighed into a Carius tube, a Re-Os diluent is added, the material in the Carius tube is frozen, and then pure nitric acid is added, the freezing is continued, the mouth of the Carius tube is sealed, the sample is dissolved, and then extraction, separation and purification of the Re element, purification of the Os element, and testing are performed in sequence. The present invention selectively dissolves the Re and Os elements in the organic component, and the Re and Os elements in the debris component are not dissolved as much as possible: it has a lower oxidation capacity, and has a lower dissolution capacity for the Re and Os elements in the debris component; under the condition of ensuring that the temperature and duration are appropriate, there is sufficient oxidation capacity, and the Re and Os elements in the organic component can be fully dissolved, ensuring the reliability of the Re-Os isochron.

Description

Technical Field

[0001] The invention relates to a Re-Os isotope analysis method for organic-rich sedimentary rocks, belonging to the technical field of geological rock analysis. Background Art

[0002] Since organic-rich sedimentary rocks (ORS) are widely found in sedimentary strata, the determination of their depositional age is very important for determining the age of sedimentary strata. During the deposition of ORS, due to the affinity of Re and Os elements for organics, Re and Os elements are relatively enriched in their organic components and exist as a closed system afterwards. Therefore, the Re-Os isotope system in ORS is an ideal carrier for ORS dating (the isotope system can be applied to three conditions for dating: one is the same source, that is, the same material source; the second is simultaneous; the third is that the system remains closed after formation). There are organic components and detrital (non-organic) components in ORS. Among them, the Re-Os isotope system in the organic component is an ideal object for dating, while the sources of Re and Os elements in the detrital component are different from those of Re and Os elements in the organic component, and cannot represent the sediment source characteristics of ORS. Therefore, when using the Re-Os isotope system in organic-rich sedimentary rocks for dating, it is necessary to ensure that the Re-Os elements in the organic matter component are fully dissolved and that the Re and Os elements in the debris component are not dissolved.

[0003] The difficulty of Re-Os isotope analysis lies in the Os element. Since the valence state behavior of the Os element is relatively complex, there are four types: +3, +4, +6 and +8. The highest valence state is easy to volatilize (easily soluble in water and organic solvents). Isotopes of different valence states cannot be exchanged. Therefore, in order to achieve a balance in the isotope exchange between the diluent and the sample during the experiment, an oxidizing acid is generally used to oxidize the Os in the sample and the diluent to the highest valence state, so that the isotope exchange between the two can reach a balance.

[0004] The experimental analysis of the Re-Os isotope system mainly includes the following five steps: (1) sample digestion (dissolution); (2) separation; (3) purification of the Re element; (4) purification of the Os element; and (5) on-machine testing.

[0005] Among them, the first step, sample digestion, mainly has the following existing methods:

[0006] 1. Aqua regia solution: Aqua regia solution has a strong oxidizing property and will dissolve Re and Os in the non-organic components, increase the dispersion of the isochrons, and affect the accuracy of the isochron ages.

[0007] 2. Disadvantages of the cadmium anhydride solution dissolution method:

[0008] (1) It will dissolve the non-organic components in ORS;

[0009] (2) CrO 3 -H 2 SO 4 Even after purification, the solution still contains relatively more Re, which increases the blank of the whole process to a certain extent;

[0010] (3)CrO 3 -H 2 SO 4 The solution is toxic;

[0011] (4)CrO 3 -H 2 SO 4 The presence of a large amount of +6 valent Cr in the solution will affect the separation of Re by anion exchange resin; and if +6 valent Cr is reduced to +3 valent Cr, the steps are relatively complicated.

[0012] 3. Dissolve the sample in a fixed ratio of hydrogen peroxide-nitric acid mixed solution, H 2 O 2 -HNO 3 When the mixed solution (optimal mixing ratio is 5:1) dissolves ORS, due to the presence of a large amount of hydrogen peroxide, a large amount of gas will be generated during the reaction, which can easily cause the Caius tube used in the process to burst, resulting in low safety and a relatively low success rate of sample digestion. Summary of the invention

[0013] In view of the above technical problems, the present invention provides a Re-Os isotope analysis method for organic-rich sedimentary rocks to solve the following technical problems:

[0014] (1) Reduce the background of the experimental process (including Re and Os elements);

[0015] (2) Only dissolve the Re and Os elements in the organic components and minimize the dissolution of Re and Os elements in the non-organic components;

[0016] (3) Ensure the success rate of sample dissolution and improve the success rate of sample digestion

[0017] The specific technical solutions are:

[0018] A Re-Os isotope analysis method for organic-rich sedimentary rocks comprises the following steps:

[0019] (1) Sample digestion: weigh the organic-rich sedimentary rock powder sample into a Carius tube, add an appropriate amount of Re-Os diluent, then place the Carius tube in a thermos cup with a refrigerant, freeze the contents of the Carius tube, then add pure nitric acid, add liquid nitrogen to the thermos cup, continue freezing for 5-10 minutes, then use a welding torch to melt the middle part of the Carius tube neck and seal the Carius tube mouth. Then place the Carius tube in a steel sleeve, put it in an oven, and dissolve the sample. After returning to room temperature, wait for chemical separation.

[0020] In step (1), the refrigerant is a mixture of ethanol and liquid nitrogen.

[0021] 0.1g powder sample was added with 6ml HNO 3 ; The dissolution temperature is 140-230°C and the duration is 24-72 hours.

[0022] The pure nitric acid is purified before dissolving the sample; the purification steps are as follows: the high-grade pure nitric acid is heated at 120° C. on a hot plate, and clean air is blown into it by an air pump until the nitric acid evaporates to a ratio of 650 ml per 1000 ml; and then the sub-boiling distillation system is used for distillation and purification.

[0023] (2) Extraction: Using Os element and organic reagent CCl 4 Heavier than pure nitric acid solution, with CCl 4 The Re and Os elements in the nitric acid solution obtained in the solution separation step (1) are separated into the nitric acid solution containing the Re element and the CCl containing the Os element. 4 The solution was ready for the next step of purification.

[0024] (3) Separation and purification of Re element: The separation and purification of Re element was carried out using 200 mesh anion resin AG1X-8;

[0025] (4) Purification of Os element: CCl 4 The Os element in the solution is extracted into HBr and purified by micro-distillation.

[0026] (5) Test: The Re element test was carried out on an inductively coupled plasma mass spectrometer, and the Os element test was carried out on a thermal ionization mass spectrometer.

[0027] Beneficial effects brought by the technical solution of the present invention:

[0028] (1) Selectively dissolve the Re and Os elements in the organic matter component, and try not to dissolve the Re and Os elements in the debris component: HNO 3Compared with reverse aqua regia, chromic anhydride solution and the mixed solution of hydrogen peroxide and nitric acid, it has lower oxidation capacity and lower dissolution capacity for Re and Os elements in the debris components; under the condition of ensuring the appropriate temperature and duration, it has sufficient oxidation capacity to fully dissolve the Re and Os elements in the organic matter components, ensuring the reliability of the Re-Os isochron;

[0029] (2) Lower background in the whole process: Pure nitric acid is used as the acid for dissolving the sample. Theoretically, the background of the whole process is relatively low, and this is also true from the experimental data.

[0030] (3) High success rate of sample digestion: Compared with the sample dissolution method using a mixed solution of hydrogen peroxide and nitric acid, which also has a low background, pure nitric acid will produce less gas during the sample dissolution process, making the pressure inside the CT relatively small, the tube burst rate relatively low, and the success rate of sample digestion relatively high. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a simplified flow chart of Re-Os isotope analysis of the present invention;

[0032] Figure 2 This is a comparison chart of Re and Os element contents and Os isotope ratios of the lacustrine shale standard sample SGR-1b under different dissolution temperatures and dissolution times in the example;

[0033] Figure 3 This is a trend chart of the content changes of Re and Os elements in the basalt standard sample BIR-1a obtained by dissolving the sample in pure nitric acid under different dissolution conditions in the example. DETAILED DESCRIPTION

[0034] The specific technical solution of the present invention is explained in conjunction with embodiments.

[0035] In this example, pure nitric acid was used to dissolve the oil shale standard sample SGR-1b and the basalt standard sample BIR-1a (both purchased from the United States Geological Survey, USGS). 0.1 g of the oil shale standard sample SGR-1b was added with 6 ml of HNO 3 ; The dissolution temperature (140, 160, 180, 200, 220 and 230°C respectively) and the duration (24, 48 and 72h respectively) were changed to compare the experimental results in order to explore the appropriate dissolution conditions.

[0036] 1 Preparation of pure nitric acid

[0037] The purity of the pure nitric acid used in the experiment is high-grade pure. Pure nitric acid is an oxidizing acid, and the background of Os element is relatively high. Therefore, before using pure nitric acid for sample dissolution, the high-grade pure nitric acid needs to be purified. Purification steps: Place 1000ml of high-grade pure nitric acid on a hot plate and heat it at 120°C, while using an air pump to blow clean air into it (the Os element in pure nitric acid is volatile) until the nitric acid evaporates to 650ml. Then use a right-angled sub-boiling distillation system produced by Savillex with only one exhaust hole for distillation and purification.

[0038] 2 Experimental Procedure

[0039] The overall experimental process is as follows Figure 1 As shown:

[0040] (1) Sample digestion: weigh the OSR powder sample into a special Carius tube (CT), add an appropriate amount of Re-Os diluent, and then put the CT into a thermos cup with a refrigerant (ethanol and liquid nitrogen mixed in a certain proportion, in a smoothie or viscous state), freeze the material in the CT, and then add 6mL of pure nitric acid. Add liquid nitrogen to the thermos cup and continue freezing for 5-10 minutes. Then use a welding torch to melt the middle part of the CT neck and seal the CT tube mouth. Then put the CT into a matching steel sleeve, put it in an oven, set the temperature and time, and dissolve the sample. After returning to room temperature, wait for chemical separation.

[0041] (2) Extraction: Using Os element and organic reagent CCl 4 Heavier than pure nitric acid solution, with CCl 4 The solution separates the Re and Os elements in the nitric acid solution. Separate the nitric acid solution containing the Re element and the CCl containing the Os element 4 The solution was ready for the next step of purification.

[0042] (3) Separation and purification of Re element: The separation and purification of Re element was carried out using anionic resin AG1X-8 (200 mesh). The specific separation steps refer to Li et al., 2010.

[0043] (4) Purification of Os element: CCl 4 The Os element in the solution was extracted back into HB and purified by micro-distillation in a conical flask with a pointed bottom (the flask was turned upside down to form a micro-distillation system). For specific operation steps, please refer to Paul et al., 2009 and Pearsan et al., 2000.

[0044] (5) Test: The Re element was tested on an inductively coupled plasma mass spectrometer (ICP-MS), and the Os element was tested on a thermal ionization mass spectrometer (TIMS).

[0045] In order to determine the feasibility of pure nitric acid for Re-Os analysis of ORS, the results of pure nitric acid dissolution were compared with the experimental data obtained by the existing method (Yin L., Li J., Liu J., et al. Precise and Accurate Re–Os Isotope Dating of Organic-rich Sedimentary Rocks by Thermal Ionization Mass Spectrometry with an Improved H 2 O 2 -HNO 3 Digestion Procedure. International Journal of Mass Spectrometry, 2017: 267-270. http: / / dx.doi.org / 10.1016 / j.ijms.2017.07.013). Different dissolution temperatures and dissolution times were used to determine the appropriate dissolution conditions for pure nitric acid dissolution. The experimental results of the international standard SGR-1b (shale, organic matter content of 24.8%) and BIR-1a (basalt standard, no organic matter) under different dissolution temperatures and time conditions are shown in Tables 1 and 2, respectively.

[0046] Table 1 Re-Os isotope analysis results of standard sample SGR-1b at different dissolution temperatures and dissolution times

[0047]

[0048] Table 2 Re-Os isotope analysis results of standard sample SGR-1b at different dissolution temperatures and dissolution times

[0049]

[0050]

[0051] 3 Experimental results analysis

[0052] (1) Blankness of the entire experimental process

[0053] When pure nitric acid was used for sample dissolution, the whole process of the experiment was blank, Re was 0.013±0.020ng / g, Os was 0.394±0.002pg / g, 187 Os / 188Os is 0.252±7, which is much lower than the background of the whole process of dissolving samples with cadmium anhydride solution. Compared with the content of standard samples SGR-1b and BIR-1a, it is negligible.

[0054] (2) Re-Os isotope analysis results of oil shale SGR-1b standard sample under different dissolution temperatures and durations

[0055] Under different dissolution temperatures and dissolution times, the Re-Os isotope analysis of the lacustrine oil shale standard sample SGR-1b was performed. The data results are shown in Tables 1 and Figure 2 From the data in the table, we can see that when pure nitric acid is used to dissolve the oil shale standard sample SGR-1b, the Os content and Os isotope ratio obtained are different from those obtained by other dissolution methods (reverse aqua regia, CrO 3 -H 2 SO 4 and H 2 O 2 -HNO 3 ) are very consistent, both at about 0.44ng / g and 1.78, respectively. Therefore, there is no Re-Os in the clastic components of the oil shale standard SGR-1b or the content is extremely low, so that the influence on the seawater-derived Re-Os isotope system in the standard sample is almost negligible. This may be because the clastic components of the oil shale standard SGR-1b are mainly carbonates, which are different from most sedimentary rock samples with high organic matter content (the clastic components are usually siliceous). The results of pure nitric acid dissolution show that the contents of Re and Os can reach up to 35.3ng / g and 0.457ng / g, respectively. Even if there is a small amount of Re-Os in the terrigenous clastic components (mainly from the upper crust, in which the Re-Os content is very low), it will not affect the application of the seawater-derived Re-Os isotope system in the standard sample SGR-1b.

[0056] When the dissolution temperature and dissolution time changed, the Re-Os isotope analysis results showed obvious changes. When the dissolution temperature changed from 140℃ to 230℃ and the dissolution time was 24h, 48h and 72h respectively, the experimental results showed a large change in Os content (mean range of 0.238-0.457ng / g) and 187 Os / 188The Os value (mean value range is 1.712-1.848), and the reproducibility of the experimental results is poor under the conditions of short duration and relatively low temperature (temperature not higher than 220℃, 24h and 48h). Pure nitric acid is one of the three major acids, with strong acidity and oxidizing properties, while aqua regia is a recognized extremely acidic acid (stronger than pure nitric acid), which can almost completely dissolve the insoluble minerals in rock samples, and the acidity of pure nitric acid is relatively lower than that of aqua regia, so pure nitric acid cannot dissolve more Os elements than aqua regia; the Re element content obtained when using pure nitric acid to dissolve the sample is consistent with the results obtained by previous researchers using aqua regia to dissolve the sample, and the reproducibility is also very good, which also shows that pure nitric acid does not dissolve excess substances under certain temperature and duration conditions. Therefore, the high Os content, the large variation of Os isotope ratio, and the poor reproducibility in the experimental results may be caused by the fact that the Os elements in the sample and the diluent were not completely oxidized to the same valence state during the dissolution process, so the Os isotope exchange between the sample and the diluent did not reach equilibrium. The premise for the application of the isotope dilution method is that the isotope exchange between the sample and the diluent can reach equilibrium (the premise is that the Os elements between the two are in the same valence state, and there is no isotope exchange between elements of different valence states). If the isotope exchange between the diluent and the sample is not completely balanced, there will be a large error between the calculated Os isotope value of the mixed solution and the true value, and when the diluent is subtracted later, a completely wrong result may be obtained. Therefore, when dissolving the sample, both Re and Os elements should be oxidized to the highest valence state to ensure that the isotope exchange can proceed. The +7 valence compound of Re is the most stable form of Re, so the isotope exchange equilibrium of Re is very easy to achieve; however, Os has two stable valence states (+6 and +8), so in order to ensure that the Os isotope exchange reaches equilibrium, the oxidizing ability of the dissolving acid needs to be enhanced. According to the experimental results of the present invention, it can be found that with the increase of temperature and the increase of dissolution time, the dissolution amount of elements and the stability of data increase, which shows that increasing the dissolution temperature or dissolution time can improve the oxidizing ability of pure nitric acid. Through the analysis of the experimental results of the oil shale standard sample SGR-1b, it can be found that pure nitric acid dissolution can be compared with the previous methods of reverse aqua regia, CrO 3 -H 2 SO 4 and H 2 O 2 -HNO 3The solution dissolution obtained similar Re element content, which indicates that pure nitric acid dissolution can fully oxidize the organic matter in the acid-hydrolyzed oil shale standard sample, and fully oxidize the Re element in the sample organic matter and diluent. However, for the dissolution conditions of different dissolution temperatures and dissolution times, the results of Os element content and Os isotope composition showed great changes. This may be because when the dissolution temperature and dissolution time are not suitable, the Os element in the sample and diluent may have a lower valence state, so the isotope exchange between them is not fully balanced. Therefore, it is necessary to control the dissolution temperature and dissolution time to ensure the oxidation ability of pure nitric acid.

[0057] Depend on Figure 2 It can be seen that with the increase of temperature, even if the dissolution time is different, the reproducibility of the Re element, Os element, and Os isotope composition of the sample becomes better; when the dissolution temperature reaches above 220°C, the Re-Os isotope data in the sample is consistent with the experimental results obtained by previous researchers using other dissolution methods. Relatively speaking, when the dissolution temperature is the same, the dissolution time has little effect on the experimental results. That is to say, for pure nitric acid dissolution, the increase in dissolution temperature has a significant effect on improving its oxidation capacity, and the dissolution time does not have a great effect on pure nitric acid while ensuring that all elements can be dissolved. Under the condition of a dissolution temperature of 230°C, the Re-Os isotope data of the oil shale standard SGR-1b using pure nitric acid dissolution has the best reproducibility, and the experimental results are consistent with the previous results using reverse aqua regia, CrO 3 -H 2 SO 4 and H 2 O 2 -HNO 3 The results obtained from solution samples were consistent.

[0058] (3) Re-Os isotope analysis results of basalt BIR-1a standard sample under different dissolution temperatures and durations

[0059] When conducting Re-Os isotope analysis on ORS, avoiding the dissolution of Re and Os in detrital components (non-seawater sources) is a current concern. There are a small amount of non-seawater sources in organic-rich sedimentary rocks, which may contain a small amount of Re and Os. Although the proportion of the total is extremely small, it may still have a certain impact on the Re-Os isochron age. As mentioned earlier, the basalt standard sample BIR-1a with a high silica content (47.9wt.%) and no organic matter can be used to simulate the siliceous detrital part, and the dissolution ability of pure nitric acid on Re and Os in silicate rocks under different dissolution conditions can be investigated. The results of Re-Os analysis of the basalt standard sample BIR-1a by pure nitric acid solution under different dissolution temperatures and dissolution times are shown in Tables 2 and 3. Figure 3 shown.

[0060] From the results of Re-Os isotope analysis, it can be seen that the contents of Re (average ~0.602ng / g) and Os (average ~0.176ng / g) obtained by pure nitric acid dissolution are generally lower than the results obtained by previous researchers using reverse aqua regia dissolution (Re and Os contents are 0.69ng / g and 0.36ng / g, respectively). This indicates that the dissolution capacity of pure nitric acid for Re and Os in siliceous rocks is lower than that of reverse aqua regia. Comparing the dissolution temperatures of 200℃, 220℃ and 230℃, and the dissolution times of 24h, 48h and 72h, the Re-Os isotope analysis results of the basalt standard BIR-1a show that: when the dissolution time is the same, as the dissolution temperature increases, the Re and Os content obtained by analysis is also higher; when the dissolution temperature is the same, as the dissolution time increases, the Re and Os content obtained by analysis is also higher. Therefore, it can be shown that under the same conditions, the increase of dissolution temperature and the increase of dissolution time can promote the dissolution of Re and Os in siliceous rocks to a certain extent. Therefore, in order to obtain accurate isochron ages when conducting Re-Os isotope analysis on organic-rich sedimentary rocks, the dissolution temperature should be reduced as much as possible and the dissolution time should be reduced. In addition, through the analysis of the oil shale standard sample SGR-1b, it was found that only at a temperature of 230°C can the Re and Os elements in the sample organic matter and diluent be fully oxidized to the highest valence state, and the isotope exchange between them be completely balanced. Through the Re-Os isotope analysis of the basalt standard sample BIR-1a, it is believed that 24 hours is the best dissolution time for pure nitric acid as the dissolution acid in the Re-Os isotope analysis experiment of organic-rich sedimentary rocks. This analysis method can minimize the dissolution of Re-Os elements in the debris components of organic-rich sedimentary rocks.

[0061] 4 Conclusion

[0062] (1) Pure HNO 3 It is completely feasible to dissolve samples for Re-Os isotope analysis of ORS;

[0063] (2) Pure HNO 3 The best dissolution conditions are: 230℃ and 24h.

Claims

1. A Re-Os isotope analysis method for organic-rich sedimentary rocks, characterized in that: The following steps are involved: (1) Sample digestion: weigh the organic-rich sedimentary rock sample powder into a Carius tube, add an appropriate amount of Re-Os diluent, then place the Carius tube in a thermos cup with a refrigerant, freeze the contents of the Carius tube, then add pure nitric acid, add liquid nitrogen to the thermos cup, continue freezing for 5-10 minutes, then use a welding torch to melt the middle part of the Carius tube neck and seal the Carius tube mouth; then place the Carius tube in a steel sleeve, put it in an oven, and dissolve the sample; after returning to room temperature, wait for chemical separation; The pure nitric acid is purified before dissolving the sample; the purification step is as follows: the high-grade pure nitric acid is heated on a hot plate at 120°C, and clean air is blown into it by an air pump until the nitric acid evaporates to a ratio of 650 ml per 1000 ml; and then the sub-boiling distillation system is used for distillation and purification; (2) Extraction: utilizing the fact that Os is organic-philic and the organic reagent CCl4 is heavier than pure nitric acid solution, using CCl4 solution to separate the Re and Os elements in the nitric acid solution obtained in step (1); separating the nitric acid solution containing the Re element and the CCl4 solution containing the Os element to wait for the next step of purification; (3) Separation and purification of Re element: The separation and purification of Re element was carried out using 200 mesh anion resin AG1X-8; (4) Purification of Os element: extract Os element from CCl4 into HBr and purify it by micro-distillation; (5) Test: The Re element test was carried out on an inductively coupled plasma mass spectrometer, and the Os element test was carried out on a thermal ionization mass spectrometer.

2. The method for Re-Os isotope analysis of organic-rich sedimentary rocks according to claim 1, characterized in that: For every 0.1 g of powder sample, 6 ml of HNO3 was added.

3. The method for Re-Os isotope analysis of organic-rich sedimentary rocks according to claim 1, characterized in that: In step (1), the dissolution temperature is 140-230°C and the dissolution time is 24-72 hours.

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