Analysis for restricting silicon isotope composition of ancient ocean shell based on stable isotope

Abstract

The invention discloses analysis for restricting silicon isotope composition of an ancient crust based on stable isotope, and the silicon isotope composition of the ancient crust is restricted by analyzing the silicon isotope composition of rock in a green rock zone. Rock in green rock zones of different ages can record the characteristics of the ancient ocean crust, represents the composition of the ancient ocean crust at different time and different places, plays an important role in restricting the silicon isotope composition of the ancient ocean crust, and also has important significance in understanding early mainland evolution. The method comprises the following steps: (1) determining research areas which are six different green rock zones with different ages; (2) determining the silicon content in the green rock zone rock sample; (3) determining the stable silicon isotope composition of the green rock zone rock sample; and (4) comparing Si isotope compositions of the green rock zone rock samples at different age stages, and analyzing the influence of the green rock zone rock samples on the silicon isotope composition of the ancient ocean shell.

Description

Technical Field

[0001] The present invention belongs to the field of earth science research, and particularly relates to an analysis for constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes Background Art

[0002] At present, the research on the evolution law of ancient crust is not comprehensive enough. By analyzing the silicon isotope composition of greenstone belt rock samples and constraining the silicon isotope composition of ancient oceanic crust, the geochemical characteristics of the Archean oceanic crust at different times and locations can be revealed, providing key data support for the study of the evolution of early continental crust and having important significance for understanding the evolution law of ancient crust. Summary of the Invention

[0003] To solve the above problems, the present invention adopts the following technical solutions.

[0004] An analysis for constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes, comprising the following steps:

[0005] (1) Determine the research area and collect rock samples of greenstone belts with different ages;

[0006] (2) Measure the silicon content in the greenstone belt rock samples;

[0007] (3) Measure the silicon stable isotope composition in the greenstone belt rocks;

[0008] (4) By comparing the silicon isotope compositions of greenstone belt rock samples in different age stages, analyze the influence of greenstone belt rock samples on the silicon isotopes of ancient oceanic crust.

[0009] For the above analysis for constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes,

[0010] In step (1), select the greenstone belt rock samples required for the experiment according to different ages and different regions.

[0011] For the above analysis for constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes,

[0012] In step (2), the water used should be high-purity water, the acid used should be GR or higher-purity acid purified by double sub-boiling distillation, the sodium hydroxide (NaOH) used should be a powder reagent with a purity of 99.5%, and the containers used should be a 30 mL PFA container made of polytetrafluoroethylene with a screw cap, a 10 mL silver crucible with a lid, and a 50 mL PFA standard sample dissolution tank;

[0013] In step (2), operate in a clean laboratory of class 1000 or higher

[0014] The above analysis of constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes

[0015] In step (2), the sample digestion process is as follows:

[0016] First, weigh an appropriate amount of the sample and NaOH powder reagent in a silver crucible. Then, place the silver crucible containing the weighed sample and NaOH reagent in a muffle furnace, heat it from room temperature to 730 °C and heat at this temperature for 10 minutes. After cooling, place it on an asbestos board to cool. After rinsing the outer wall of the cooled silver crucible with secondary water, place it in a 50 ml PFA sample dissolution tank, add an appropriate amount of water, shake it slightly, and then let it stand overnight. The next day, add an appropriate amount of concentrated nitric acid to the sample to keep the pH of the sample between 1 and 2. After the sample is completely dissolved, take out the silver crucible.

[0017] The above analysis of constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes

[0018] After digestion in step (2), the content of silicon element is determined by inductively coupled plasma optical emission spectrometer (ICP-OES).

[0019] The above analysis of constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes

[0020] In step (3), weigh an appropriate amount of the sample so that the sample contains 8 - 10 micrograms of silicon, and digest it according to the steps to ensure that the sample is completely dissolved.

[0021] The above analysis of constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes

[0022] The silicon separation and purification process in step (3) is as follows:

[0023] Separation and purification are carried out using cation exchange resin AG50W-X12. First, clean the resin with 6 mol / L concentrated hydrochloric acid and secondary water and then load the sample. After loading the sample solution, obtain the purified Si solution by rinsing with secondary water, and the collected solution is used to determine the Si isotope composition by multi-collector inductively coupled plasma mass spectrometer.

[0024] The above analysis of constraining the silicon isotope composition of ancient oceanic crust based on stable isotopes

[0025] In step (3), plot the measured stable isotope composition and analyze the change of isotope composition. Description of the Drawings

[0026] Figure 1 It is a relationship diagram between the Si isotope composition of greenstone belt rocks and the SiO 2 content.

[0027] Figure 2Relationship diagram between Si isotope composition and age of greenstone belt rocks. Detailed implementation manners

[0028] The present invention will be further described below in conjunction with specific embodiments.

[0029] Si has three stable isotopes: 28 Si, 29 Si and 30 Si, and their average abundances are 92.23%, 4.68% and 3.09% respectively. The Si isotope composition is usually expressed as δ 30 Si, which is defined as the permille deviation of the 30 Si / 28 Si ratio of the sample relative to the NBS28 standard sample:

[0030] δ 30 Si = [( 30 Si / 28 Si) sample / ( 30 Si / 28 Si) NBS28 - 1] × 1000.

[0031] The implementation of this method is divided into the following steps:

[0032] Step 1: Determine the research area and collect rock samples of greenstone belts with different ages.

[0033] In this step, the samples are selected according to different ages and different regions.

[0034] The purpose of this step is to obtain as much information as possible about the research area and the sampling locations, and collect the required samples according to the research object. This is not the innovative content of this method.

[0035] Step 2: Determine the silicon content in the rock samples of the greenstone belt.

[0036] The water used in this step should be high-purity water, the acid used should be GR grade or higher purity acid purified by double sub-boiling distillation, the sodium hydroxide (NaOH) used should be a powder reagent with a purity of 99.5%, and the containers used should be 30 mL PFA containers with screw caps made of polytetrafluoroethylene, 10 mL silver crucibles with lids, and 50 mL PFA standard sample dissolution tanks. This step should be operated in a clean laboratory of class 1000 or higher.

[0037] The sample digestion process recommended by this method is as follows:

[0038] 1) Weigh the sample and the NaOH powder reagent on an analytical balance in a silver crucible;

[0039] 2) Place the silver crucible containing the weighed sample and NaOH reagent into a muffle furnace, heat it from room temperature to 730 °C and heat at this temperature for 10 minutes. After cooling, place it on an asbestos board to cool.

[0040] 3) After rinsing the outer wall of the cooled silver crucible with secondary water, place the crucible without the lid into a 50 ml sample dissolution tank, ensuring the opening is upward.

[0041] 4) Add 15 mL of secondary water and gently shake it horizontally, then place it in a fume hood and let it stand overnight.

[0042] 5) Observe whether the sample is completely dissolved the next day. If there is still undissolved sample at the bottom of the silver crucible, ultrasonicate the sample dissolution tank for 15 minutes to fully dissolve it. Then add an appropriate amount of concentrated nitric acid of secondary water to keep the pH of the sample between 1 and 2. After the sample is completely dissolved, take out the silver crucible.

[0043] After the sample digestion is completed, the content of Si element is determined by inductively coupled plasma optical emission spectrometer (ICP-OES).

[0044] The purpose of this step is to obtain the content of silicon element in the greenstone belt rock sample to determine the sample loading amount for the determination of silicon isotope composition. The advantages of reagent selection, purification and operation in a clean laboratory in this step are to reduce the background value of the whole process and reduce experimental errors. The advantage of using inductively coupled plasma optical emission spectrometer to determine the element content in this step is to improve the test accuracy.

[0045] Step three, determine the silicon isotope composition of the greenstone belt rock sample.

[0046] The containers, reagents and clean experimental conditions used in this step are the same as those in step two. Weigh an appropriate amount of sample so that the sample contains 8 micrograms of silicon, and digest it using the same steps as above. After digestion, place it in a sample dissolution tank and wait for column passing.

[0047] The Si separation and purification process recommended by this method is as follows:

[0048] The process requires 1 cation exchange resin to ensure the complete removal of matrix elements

[0049]

[0050] Use a multi-collector inductively coupled plasma mass spectrometer to determine the stable isotope composition of Si in the sample after separation and purification. The instrument parameters used in this method are as follows:

[0051] Instrument parameters Recommended parameter values RF power ～1200W Cooling gas flow rate ~16 L / min Auxiliary gas flow rate ~0.8 L / min Sample gas flow rate ~1.0 L / min Resolution High resolution (>6000) <![CDATA 28 Si sensitivity]]> ~5 V / (μg / g) Nebulizer PFA nebulizer (~50 μL / min) Cone assembly H-intercept cone and NiJet sample cone Spray chamber Quartz dual-path cyclone spray chamber

[0052] The purpose of this step is to obtain the silicon isotope composition in the sample, providing the necessary data for analyzing the Si composition in the greenstone belt rock samples. The benefits of reagent selection, purification, and operation in a clean laboratory in this step are to reduce the background of the entire process and minimize experimental errors. The advantage of using a multi-collector inductively coupled plasma mass spectrometer to determine Si isotopes in this step is to improve the measurement accuracy and make the results reliable.

[0053] According to the foregoing implementation, the silicon element content and silicon isotope composition of the greenstone belt samples were determined ( Figure 1 ). It can be found from the figure that the silicon isotope composition of the greenstone belt samples ranges from -0.13±0.05‰ to -0.26±0.03‰, and the average silicon isotope composition is approximately -0.19±0.07‰. Moreover, by plotting the silicon isotope composition against age, it is found that the silicon isotope composition is independent of age ( Figure 2 ). The results show that the slightly heavier Si isotope composition of the greenstone belt rocks represents the common Si isotope characteristics of the ancient oceanic crust at that time.

Claims

1. An analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints, characterized in that: The following steps are involved: (1) Determine the research area and collect rock samples from greenstone belts of different ages; (2) Determine the silicon content of greenstone belt rock samples; (3) Determine the silicon isotope composition of greenstone belt rocks; (4) By comparing the silicon isotope compositions of greenstone belt rock samples of different ages, the influence of greenstone belt rock samples on the silicon isotopes of ancient oceanic crust is analyzed.

2. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 1, characterized in that: In step (1), the greenstone belt rock samples required for the experiment are selected according to different ages and different regions.

3. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 1, characterized in that: The water used in step (2) should be high-purity water, the acid used should be high-purity GR or higher purity acid purified by two sub-boiling distillations, the sodium hydroxide (NaOH) used should be a powder reagent with a purity of 99.5%, and the containers used should be a 30 mL polytetrafluoroethylene PFA container with a screw cap, a 10 mL silver crucible with a lid, and a 50 mL PFA standard sample dissolving tank; Step (2) is performed in a clean laboratory of Class 1000 or higher.

4. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 3, characterized in that: The sample digestion process in step (2) is: First, weigh an appropriate amount of sample and NaOH powder reagent in a silver crucible, then put the silver crucible with the sample and NaOH reagent into a muffle furnace, heat from room temperature to 730°C and heat at this temperature for 10 minutes, cool down and place on an asbestos board to cool. Rinse the outer wall of the cooled silver crucible with secondary water and place it in a 50ml PFA sample dissolving tank, add an appropriate amount of water and shake it slightly, then let it stand overnight. On the second day, add an appropriate amount of concentrated nitric acid to the sample to keep the pH of the sample between 1-2, and remove the silver crucible after the sample is completely dissolved.

5. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 4, characterized in that: After the digestion in step (2) is completed, the silicon content is determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

6. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 1, characterized in that: In step (3), an appropriate amount of sample is weighed so that the sample contains 8-10 μg of silicon, and digested according to the steps to ensure that the sample is completely dissolved.

7. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 6, characterized in that: The silicon separation and purification process in step (3) is as follows: Cation exchange resin AG50W-X12 was used for separation and purification. The resin was first washed with 6 mol / L concentrated hydrochloric acid and secondary water before loading the sample. After loading the sample solution, the purified silicon solution was obtained by secondary water elution. The collected solution was determined for silicon isotope composition by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).

8. The analysis of the silicon isotope composition of ancient oceanic crust based on stable isotope constraints according to claim 1, characterized in that: In step (3), the measured stable isotope composition is plotted and the change in isotope composition is analyzed.

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