In-situ microcell iron and manganese oxide Sr isotope dating method

Abstract

The invention relates to an in-situ microcell iron and manganese oxide Sr isotope dating method. The in-situ microcell iron and manganese oxide Sr isotope dating method comprises the following steps: cutting iron and manganese oxide to be detected to obtain an optical sheet sample; the method comprises the following steps: performing point denudation on an optical sheet sample, loading denudated aerosol into an MC-ICP-MS plasma source by using carrier gas for ionization, and collecting ion signal intensity data including 82Kr < + >, 84Sr < + >, 86Sr < + >, 87Sr < + >, 88Sr < + >, 85Rb < + >, 87Rb < + >, 167Er < 2 + >, 170Er < 2 + > and 173Yb < 2 + >; processing the obtained ion signal intensity data to obtain a corresponding isotope ratio, and comparing the isotope ratio with an ocean Sr isotope evolution curve to obtain the formation time of the to-be-detected iron and manganese oxide. The method solves the problems of low spatial resolution, low accuracy and complicated operation of the existing iron and manganese oxide dating method.

Description

Technical Field

[0001] The invention relates to the technical field of geological exploration, and in particular to an in-situ micro-area Sr isotope dating method for iron-manganese oxides. Background Art

[0002] Iron-manganese oxides, a general term for iron-manganese nodules and crusts, are mainly composed of minerals such as barium-magnesium manganese ore, birnessite, and manganite. They are an extremely important type of deep-sea mineral resources. For a long time, due to the lack of suitable dating minerals, the exact formation age of deep-sea iron-manganese oxides has been difficult to accurately determine. These oxides are usually rich in cobalt (Co) and a small amount of osmium (Os). Currently, Co chemical dating methods and Os isotope dating methods have been developed and applied to the study of deep-sea iron-manganese oxides. However, the growth zoning of iron-manganese oxides is extremely complex. The above two methods are both overall analysis techniques, which require the samples to be crushed and ground into powder for analysis, resulting in the obtained age data often being an average value, which cannot accurately reflect the specific era of its formation, thus limiting the wide application of the above methods.

[0003] Iron-manganese oxides usually contain a certain amount of strontium (Sr, >1000 μg g -1 ) and a very small amount of rubidium (Rb), with an Sr / Rb ratio greater than 100, which makes it possible to use Sr isotope dating. However, the current Sr isotope analysis method requires sample drilling, strong acid digestion, chemical purification, and machine testing to obtain data. Not only is the operation process cumbersome and time-consuming, it is also easily contaminated during the processing, and the amount of data obtained is also limited. At the same time, as an overall analysis method, the current Sr isotope analysis method also has the defect of low spatial resolution, which makes it very limited when analyzing iron-manganese oxides with complex growth rings. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide an in-situ micro-area iron-manganese oxide Sr isotope dating method, which adopts the sampling method of laser micro-area point ablation to perform Sr isotope determination on the iron-manganese oxide at the micrometer scale. This is to solve the problems of low spatial resolution and low accuracy, as well as cumbersome operation, of the existing iron-manganese oxide dating methods, thereby providing more favorable technical support for related geochronological applications.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows: An in-situ micro-area Sr isotope dating method for iron-manganese oxides comprises the following steps: Cutting the iron-manganese oxide to be tested to obtain an optical thin slice sample; The optical thin-section sample was point-ablated, and the aerosol produced by the ablation was carried into the MC-ICP-MS plasma source by carrier gas for ionization, and the data including 82 K + , 84 Sr + , 86 Sr + , 87 Sr + , 88 Sr + , 85 R + , 87 R + , 167 Er 2+ , 170 Er 2+ and 173 Yb 2+ Ion signal intensity data; The obtained ion signal intensity data were processed using Iolite 4.0 software to obtain the corresponding isotope ratio. The isotope ratio was compared with the oceanic Sr isotope evolution curve to obtain the formation age of the iron-manganese oxide to be tested.

[0006] According to the above technical means, through the point stripping technology, isotope analysis can be performed in a micro area, which is particularly suitable for studying samples with complex structures or small-scale changes, and effectively solves the problems of low spatial resolution and low efficiency of existing iron-manganese oxide dating methods. By using the MC-ICP-MS plasma source for ionization, ion signal intensity data can be quickly obtained, thereby improving the analysis efficiency. By accurately measuring the Sr isotope ratio in iron-manganese oxides and combining the oceanic Sr isotope evolution curve, high-precision dating results can be provided. This method is less destructive to the sample during the analysis process, which is conducive to protecting precious or non-renewable sample resources. This method can simultaneously determine the ion signal intensity of multiple elements, including Sr, Rb, Er and Yb, which is helpful for more comprehensive geological or environmental analysis. Since iron-manganese oxides are widely present in marine sediments, soils and rocks, this method can be used in a variety of geological and environmental research fields and has broad application prospects.

[0007] Preferably, the acquisition process also includes simultaneously acquiring ion signal intensities with mass numbers of 82, 83.5, 85.5 and 86.5.

[0008] Preferably, by collecting the signal with a mass number of 82 (corresponding to 42 Ca 40 Ar and 42 Ca 40 The ion signal intensity of Ca84 Sr. 85 Rb, 86 Sr. 87 Sr. 87 Rb and 88 The interfering ion signal of Sr is subtracted. 42 Ca 40 Ar and 42 Ca 40 Ca 84 Sr. 85 Rb, 86 Sr. 87Sr，87 Rb and 88 The influence of Sr. Preferably, by collecting the mass number of 83.5 (corresponding to 167 Er 2+ ion signal intensity), 85.5 (corresponding to 170 Er 2+ ion signal intensity) and 86.5 (corresponding to 173 Yb 2+ The signal intensity of the ion signal is used to 84 Sr. 85 Rb, 86 Sr. 87 Sr. 87 Rb and 88 The Sr ion signal is subtracted. 167 Er 2+ , 170 Er 2+ and 173 Yb 2+ right 84 Sr. 85 Rb, 86 Sr. 87 Sr. 87 R 和88 The influence of Sr.

[0009] Preferably, in the acquisition process, it also includes acquiring a blank signal of a preset time as a rare gas 84 Kr and 86 The background value of Kr is 84 Sr and 86 The ion signal of Sr is subtracted. Thus, the noble gas is subtracted. 84 Kr and 86 Kr 84 Sr and 86 The influence of Sr.

[0010] Preferably, the preset time is 10 seconds.

[0011] Preferably, during the test, after every 10 optical thin slice samples corresponding to the iron-manganese oxide to be tested are tested, at least two NIST SRM 610 and two iron-manganese oxide standard materials GSMC-1S are tested repeatedly, so as to ensure that the test conditions of the standard materials and the iron-manganese oxide samples to be tested are the same.

[0012] Preferably, during the test, after every 10 optical thin film samples corresponding to the iron-manganese oxide to be tested are tested, at least two NIST SRM 610, two iron-manganese oxide standard materials GSMC-1S and two iron-manganese oxide standard materials GSPN-2S are tested repeatedly.

[0013] Preferably, according to the collected 84 Sr + , 85 Rb+, 86 Sr + , 87 Sr + , 88 Sr +和87 R + The ion signal intensity data is calculated 87 Sr / 86 Sr and 87 Rb / 86 The ratio of Sr and NIST SRM 610 was used to 87 Sr / 86 Sr and 87 Rb / 86 The Sr ratio was used to correct the instrument drift.

[0014] Among them, by using the matrix-matched iron-manganese oxide standard material GSMC-1S for calibration and subtracting various interference signals, the accurate determination of the Sr isotope and age of the iron-manganese oxide was achieved.

[0015] Preferably, by the GSMC-1S standard substance 87 Sr / 86 Sr and 87 Rb / 86 The measured value of Sr and the standard value are used to obtain the corresponding mass fractionation coefficient to determine the mass fractionation coefficient of the iron-manganese oxide sample to be tested. 87 Sr / 86 Sr and 87 Rb / 86 The Sr ratio was corrected.

[0016] The calculation formula is: in,( 87 Sr / 86 Sr)c is the measured value after mass fractionation correction and interference correction; ( 87 Sr / 86 Sr) m is the measured value for Rb interference correction; β is the mass fractionation factor; I x is the signal value of isotope x; 88 Sr / 86 The natural abundance ratio of Sr in nature is 8.375209; M x is the relative atomic mass of the isotope corresponding to x; isotope x represents 85 Rb, 86 Sr or 87 Sr; RM represents the Sr isotopic composition of the reference material; 87 R + / 85 R + is the signal intensity ratio of the two isotopes of Rb; 87 Rb / 85 The natural abundance ratio of Rb in nature is 0.3856.

[0017] Preferably, when the Sr / Rb ratio in the iron-manganese oxide sample to be tested is greater than 100, 87 Sr / 86 The Sr ratio is the target value for comparison with the Sr isotope evolution curve to obtain the formation age of the iron-manganese oxide to be tested.

[0018] Preferably, the optical thin film sample is point-ablated by laser, the laser beam spot diameter is 90 μm, the ablation frequency is 6 Hz, and the energy density is 3 J cm -2 .

[0019] Beneficial effects of the present invention: The in-situ micro-area iron-manganese oxide Sr isotope dating method of the present invention can perform isotope analysis in a micro-area through point-stripping technology, and is particularly suitable for studying samples with complex structures or small-scale changes. By using the MC-ICP-MS plasma source for ionization, ion signal intensity data can be quickly obtained, thereby improving the analysis efficiency. By accurately measuring the Sr isotope ratio in the iron-manganese oxide, combined with the oceanic Sr isotope evolution curve, a high-precision dating result can be provided. The method is less destructive to the sample during the analysis process, which is conducive to protecting precious or non-renewable sample resources. The method can simultaneously measure the ion signal intensity of multiple elements, including Sr, Rb, Er and Yb, which is helpful for more comprehensive geological or environmental analysis, thereby effectively solving the shortcomings of the existing in-situ micro-area analysis method, such as low spatial resolution, poor accuracy, complex chemical pretreatment, and time-consuming and labor-intensive, and has promotion and application value in the field of geological exploration technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart of the in situ micro-area Sr isotope dating method for iron-manganese oxides; Figure 2 This is the backscattering image of the actual iron-manganese oxide sample Crust01 to be tested. DETAILED DESCRIPTION

[0021] The following will describe the embodiments of the present invention with reference to the preferred embodiments, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention, not for limiting the scope of protection of the present invention.

[0022] The present invention aims to disclose an in-situ micro-area Sr isotope dating method for iron-manganese oxides. By adopting a laser micro-area point ablation sampling method, Sr isotope determination of iron-manganese oxides is carried out on a micron scale to solve the problems of low spatial resolution, cumbersome operation and low accuracy in existing iron-manganese oxide dating methods, thereby providing more favorable technical support for related geochronological applications.

[0023] Among them, an in-situ micro-area Sr isotope dating method for iron-manganese oxides comprises: The test samples include a glass standard material NIST SRM 610, two iron-manganese oxide standard materials GSMC-1S and GSPN-2S, and an actual iron-manganese oxide sample Crust-01.

[0024] The two iron-manganese oxide standard materials GSMC-1S and GSPN-2S have Sr contents similar to those of the actual iron-manganese oxide samples (1000 μg g -1 ), the Sr isotopes of these standard substances are known and can be used as the test standards in this method to test the accuracy and precision of this method.

[0025] like Figure 1 As shown, the specific steps are: S1. The glass standard material NIST SRM 610 and two iron-manganese oxide standard materials GSMC-1S and GSPN-2S were cast into sample targets (2.5 cm in diameter and about 6 mm in thickness) using epoxy resin, and then polished to expose a flat polished surface.

[0026] S2. Cut the actual iron-manganese oxide sample Crust-01 perpendicularly to the growth layer into thick slices (2 cm in width, 5 cm in length, and about 3 mm in thickness), and roughly grind the thick slices to remove the rough parts and broken particles to make the surface smooth; apply thin slice glue on one side of the roughly ground sample and stick it to a glass slide; then use fine sandpaper for fine grinding and polishing, gradually grinding the sample to a thickness of about 50 microns to obtain a smooth thin slice.

[0027] S3, put the sample target obtained in S1 and the optical slice obtained in S2 into the laser ablation cell, and purge the sample chamber with helium to remove the air and fill the sample cell with helium. MC-ICP-MS was optimized using NIST SRM 610. 88 The Sr signal intensity is optimal, and 4 mL / min of nitrogen is introduced as an auxiliary gas to increase the sensitivity of the instrument and reduce the oxide yield. The laser spot diameter for point ablation of the optical thin film sample is 90 μm, the ablation frequency is 6 Hz, and the energy density is 3 J cm -2 .

[0028] S4. The target sampling point was analyzed using the point ablation mode. The laser beam spot diameter was 90 μm, the ablation frequency was 6 Hz, and the energy density was 3 J cm -2 ; The aerosol produced by the ablation is carried into the MC-ICP-MS plasma source by carrier gas for ionization.

[0029] The acquisition sequence is as follows: instrument blank for 20 seconds and laser ablation signal acquisition for 30 seconds. During the measurement process, every 10 unknown actual sample points are analyzed, and 3 NIST SRM 610, 3 GSMC-1S and 3 GSPN-2S are tested repeatedly to ensure that the measurement conditions of the standard material and the unknown sample are the same during the test.

[0030] In this embodiment, it is necessary to collect83.5 M, 84 Sr. 85 Rb, 86 Sr. 86.5 M, 87 Sr. 87 Rb and 88 The ion signal intensity data of Sr was measured with an integration time of 0.262 seconds.

[0031] S5. Process the obtained ion signal intensity data using Iolite 4.0 software, including: S51. First, blank subtraction is performed on the instrument. The plasma generating gas (argon) and the laser carrier gas (helium) inevitably contain impurity gas Kr, and its isotopic 84 Kr, 86 Kr will affect the accurate determination of Sr isotopes, so a 10-second blank signal is collected as a rare gas. 84 Kr and 86 The background value of Kr is then deducted 84 Sr, 86 Sr, 88 The influence of Sr; S52, calculated based on the measured ion signal intensity 87 Sr / 86 Sr and 87 Rb / 86 The ratio of Sr is based on NISTSRM 610. 87 Sr / 86 Sr and 87 Rb / 86 Sr ratio for instrument drift correction; S53, by collecting the signal with a mass number of 82 (corresponding to 42 Ca 40 Ar, 42 Ca 40 Ca and 82 Kr ion signal intensity) to deduct their 84 Sr. 86 Sr and 88 Since iron-manganese oxides contain a certain amount of Ca, they may polymerize with argon gas during the analysis, thus affecting the determination of Sr isotopes. 86 Sr / 88 Sr=0.1194 to calibrate 84 Sr / 86 Sr and 84 Sr / 88The ratio of Sr makes the ratio of the two theoretical values ​​(0.0565 and 0.00675) respectively; by collecting the mass number of 83.5 (corresponding to 167 Er 2+ ion signal intensity), 85.5 (corresponding to 170 Er 2+ ion signal intensity), 86.5 (corresponding to 173 Yb 2+ The ion signal intensity of 84 Sr, 85 Rb, 86 Sr, 87 Sr and 88 As for the influence of Sr, since the ratios of Er / Sr and Yb / Sr of iron-manganese oxides are much smaller than 0.1, their influence on Sr isotopes is almost negligible. In the actual test process, it is only necessary to monitor the signal values ​​of 83.5 and 85.5 and discard the abnormal points where Er / Sr and Yb / Sr are greater than 0.1.

[0032] S54, through GSMC-1S standard material 87 Sr / 86 Sr and 87 Rb / 86 The measured value of Sr and the standard value are used to obtain the corresponding mass fractionation coefficient to determine the mass fractionation coefficient of the iron-manganese oxide sample to be tested. 87 Sr / 86 Sr and 87 Rb / 86 The Sr ratio is corrected, and the specific calculation formula is: in,( 87 Sr / 86 Sr) c is the measured value after mass fractionation correction and interference correction; ( 87 Sr / 86 Sr) m is the measured value for Rb interference correction; β is the mass fractionation factor; I x is the signal value of isotope x; 88 Sr / 86 The natural abundance ratio of Sr in nature is 8.375209; M x is the relative atomic mass of isotope x; isotope x represents 85 Rb, 86 Sr or 87 Sr; RM represents the Sr isotopic composition of the reference material; 87 R + / 85 R +is the signal intensity ratio of the two isotopes of Rb; 87 Rb / 85 The natural abundance ratio of Rb in nature is 0.3856.

[0033] S55, calculate 84 Sr / 86 Sr. 84 Sr / 88 Sr. 87 Sr / 86 Sr and 87 Rb / 86 Since the Sr / Rb ratio of iron-manganese oxide is relatively low, usually more than 100 times, the iron-manganese oxide obtained by testing 87 Rb / 86 The Sr ratio is less than 0.01, which is important for the final 87 Sr / 86 The influence of Sr ratio can be neglected.

[0034] S6, the obtained iron-manganese oxide 87 Rb / 86 By comparing the Sr ratio with the global ocean Sr isotope evolution curve (McArthur et al., 2020), the formation age of the growth lamination corresponding to the iron-manganese oxide to be tested, that is, the Sr isotope age, can be obtained.

[0035] The Sr isotope results of iron-manganese oxides are shown in Table 1.

[0036] Table 1 Sr isotope results of iron-manganese oxides Real-time monitoring 84 Sr / 86 Sr and 84 Sr / 88 The ratio of Sr is ensured to be maintained at about 0.0565±0.0005 and 0.00675±0.00005 respectively.

[0037] From Table 1, we can see that the measured values ​​of the two iron-manganese oxide standard materials GSMC-1S and GSPN-2S are 87 Rb / 86 The values ​​of Sr are 0.70911±0.00003 and 0.70908±0.00001, respectively, which proves that the measurement results have high precision.

[0038] The actual sample of iron-manganese oxide to be tested, Crust01 (see Figure 2(shown) is a Co-rich crust sample from a seamount area in the western Pacific Ocean, with an Sr content of about 1500±78ppm and an Sr / Rb ratio of 112. During the experiment, the Sr isotope composition of the actual iron-manganese oxide sample Crust-01 was measured perpendicular to the growth layer, and the measurement was performed every 40-50 mm (i.e., Crust01-01, Crust01-02, Crust01-03, Crust01-04, Crust01-05, Crust01-06, Crust01-07, Crust01-08, Crust01-09, Crust01-10 in Table 1), and the Sr isotope composition of the test point was obtained, and then compared with the global ocean Sr isotope evolution curve (McArthur et al., 2020), and the formation age of the sample was obtained. This technology can provide more favorable technical support for the application of Sr isotope chronology of iron-manganese oxides.

[0039] The data in the above examples were all completed on a Geolas Pro excimer laser in series with a Thermo Fisher NeptunePlus MC-ICP-MS. The examples cited are only used to illustrate the use of the present invention, rather than to limit the present invention. Those skilled in the art can obtain the same results on a similar LA-MC-ICP-MS according to this method.

[0040] In summary, the in-situ micro-area iron-manganese oxide Sr isotope dating method of the present invention can perform isotope analysis in a micro-area through point-stripping technology, and is particularly suitable for studying samples with complex structures or small-scale changes. By using the MC-ICP-MS plasma source for ionization, ion signal intensity data can be quickly obtained, thereby improving the analysis efficiency. By accurately measuring the Sr isotope ratio in the iron-manganese oxide, combined with the oceanic Sr isotope evolution curve, a high-precision dating result can be provided. The method is less destructive to the sample during the analysis process, which is conducive to protecting precious or non-renewable sample resources. The method can simultaneously determine the ion signal intensity of multiple elements, including Sr, Rb, Er and Yb, which helps to conduct a more comprehensive geological or environmental analysis, thereby effectively solving the shortcomings of the existing in-situ micro-area analysis method, such as low spatial resolution, poor accuracy, complex chemical pretreatment, and time-consuming and labor-intensive, and has a value for promotion and application in the field of geological exploration technology.

[0041] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or change made by a person skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. An in-situ micro-area Sr isotope dating method for iron-manganese oxides, characterized in that: The following steps are involved: Cutting the iron-manganese oxide to be tested to obtain an optical thin slice sample; The optical thin-section sample was point-ablated, and the aerosol produced by the ablation was carried into the MC-ICP-MS plasma source by carrier gas for ionization, and the data including 82 K + , 84 Sr + , 86 Sr + , 87 Sr + , 88 Sr + , 85 R + , 87 R + , 167 Er 2+ , 170 Er 2+ and 173 Yb 2+ Ion signal intensity data; The obtained ion signal intensity data are processed to obtain the corresponding isotope ratio, and the isotope ratio is compared with the oceanic Sr isotope evolution curve to obtain the formation age of the iron-manganese oxide to be tested.

2. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 1, characterized in that: During the acquisition process, the ion signal intensities of mass numbers 82, 83.5, 85.5 and 86.5 were also acquired simultaneously.

3. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 2, characterized in that: By collecting the signal with mass number of 82, 84 Sr. 85 Rb, 86 Sr. 87 Sr. 87 Rb and 88 The interfering ion signal of Sr was subtracted.

4. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 2, characterized in that: By collecting the signals with mass numbers of 83.5, 85.5 and 86.5, 84 Sr. 85 Rb, 86 Sr. 87 Sr. 87 Rb and 88 The interfering ion signal of Sr was subtracted.

5. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 1, characterized in that: During the acquisition process, it also includes collecting blank signals of preset time as rare gas 84 Kr and 86 The background value of Kr is 84 Sr and 86 The interfering ion signal of Sr was subtracted.

6. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 1, characterized in that: During the test, after every 10 optical thin slice samples corresponding to the iron-manganese oxide to be tested are tested, at least two NISTSRM 610 and two iron-manganese oxide standard materials GSMC-1S are tested repeatedly.

7. The in-situ micro-area Sr isotope dating method of iron-manganese oxides according to claim 6, characterized in that: According to the collected 84 Sr + , 85 R + , 86 Sr + , 87 Sr + , 87 R + and 88 Sr + The ion signal intensity data is calculated 87 Sr / 86 Sr and 87 Rb / 86 The ratio of Sr and NIST SRM 610 was used to 87 Sr / 86 Sr and 87 Rb / 86 The Sr ratio was used to correct the instrument drift.

8. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 7, characterized in that: The standard substance GSMC-1S 87 Sr / 86 Sr and 87 Rb / 86 The measured value of Sr and the standard value are used to obtain the corresponding mass fractionation coefficient to determine the mass fractionation coefficient of the iron-manganese oxide sample to be tested. 87 Sr / 86 Sr and 87 Rb / 86 The Sr ratio was corrected.

9. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 8, characterized in that: When the Sr / Rb ratio in the iron-manganese oxide sample to be tested is greater than 100, the corrected iron-manganese oxide sample to be tested 87 Sr / 86 The Sr ratio is the target value for comparison with the Sr isotope evolution curve to obtain the formation age of the iron-manganese oxide to be tested.

10. The in-situ micro-area Sr isotope dating method for iron-manganese oxides according to claim 1, characterized in that: The optical thin film sample was point-ablated by laser, with a laser beam spot diameter of 90 μm, an ablation frequency of 6 Hz, and an energy density of 3 J cm -2 .

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