A method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry

Abstract

The present invention relates to a method for determining oxygen isotopes of chromite by laser fluorination - gas mass spectrometry, which comprises the following steps: 1) Sample screening: analyzing the major elements and oxygen isotopes of chromite to obtain a homogeneous sample; 2) Mixing the chromite sample with an oxygen-containing mineral and performing laser fluorination - gas mass spectrometry to analyze oxygen isotopes; 3) Establishing a calibration equation for the difference between the Cr2O3 content and the oxygen isotope δ 18 O and performing calibration of the oxygen isotope δ 18 O value. By mixing a certain amount of zircon into the chromite sample, the present invention significantly improves the oxygen production rate, and uses the calibration calculation equation between the δ 18 O difference and the Cr2O3 content to effectively calibrate the data of the test method, thereby improving the accuracy of oxygen isotope detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical analysis, and particularly relates to a method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry (LF-IRMS). Background Art

[0002] Chromite is a chromium-rich end member in the spinel group solid solution (Mg, Fe)(Al, Cr, Fe) 2 O 4 and can be divided into three major types: aluminospinel, chromite, and ferrospinel. As one of the main minerals crystallized in the earliest stage of ultramafic-mafic magma, chromite can record important information of the original magma. Its oxygen isotope composition has important scientific significance in revealing the characteristics of the upper mantle, studying the migration process of melt and fluid, and analyzing the recycling of crustal materials in subduction zones.

[0003] The laser fluorination-gas mass spectrometry (LF-IRMS) oxygen isotope test method is a method for extracting oxygen in reactants using a CO 2 laser probe under the action of BrF 5 strong oxidant. Compared with traditional methods, the sample amount required for LF-IRMS is reduced by 70%, and the analysis efficiency is increased by 2 times; the high temperature (>2000 K) generated by the laser can analyze refractory minerals such as zircon, olivine, and garnet. However, when analyzing chromite using the oxygen isotope test reference method (laser fluorination-gas mass spectrometry), there is a problem of low oxygen yield in chromite. When Heck et al. (2010) analyzed chromite in the Bushveld Complex in South Africa, the world's largest platinum group element deposit, they found that the oxygen yield was low during the test and the repeatability of oxygen isotope values was poor. By increasing the amount of BrF 5 reagent, the oxygen yield can only be increased for some samples, and better oxygen isotope values can be obtained. However, for some samples (such as UWCr-3), the oxygen yield is only 63% - 85%, and the oxygen yield has not been improved. Therefore, simply increasing the amount of BrF 5 reagent cannot completely solve the problem of low oxygen yield in the chromite oxygen isotope test process. Moreover, O 5 generated by the reaction of BrF 2 with chromite participates in the partial oxidation of the variable valence element Cr as an oxidant, resulting in a decrease in oxygen yield and causing oxygen isotope fractionation. Therefore, there is a certain uncertainty in the δ 18 O value. In addition, existing technologies show that reducing the sample amount during the test can also improve the oxygen yield. However, reducing the sample amount may not reach the detection limit of the Faraday cup of the mass spectrometer, resulting in inaccurate measurement of the δ 18 O value.

[0004] To solve the technical problems such as low oxygen yield, oxygen isotope fractionation, and uncertainty of the δ 18 O value of chromite during the oxygen isotope test of the traditional laser fluorination-gas mass spectrometry method, the present invention develops an improved method for determining the oxygen isotope of chromite by laser fluorination-gas mass spectrometry (LF-IRMS), effectively improving the oxygen yield and the accuracy of the oxygen isotope value. SUMMARY OF THE INVENTION

[0005] To solve the problem of low oxygen yield during the oxygen isotope test of chromite by laser fluorination-gas mass spectrometry method, the present invention provides a method for testing oxygen isotope by laser fluorination-gas mass spectrometry (LF-IRMS), which can effectively improve the oxygen yield, reduce oxygen isotope fractionation, and establish a calibration equation to correct the data, so as to obtain the accurate δ 18 O value in chromite.

[0006] In the first aspect of the present invention, a method for determining the oxygen isotope of chromite by laser fluorination-gas mass spectrometry is provided, including the following steps:

[0007] 1) Sample screening: Analyze the major elements and oxygen isotope of chromite to obtain a homogeneous sample.

[0008] Specifically, it includes the following steps:

[0009] 1.1 Select chromite ore samples with high purity, few cracks and inclusions through scanning electron microscope (SEM) analysis, and check the homogeneity of its major elements (MgO, Al 2 O 3 、Cr 2 O 3 、FeO) by electron probe microanalyzer (EPMA).

[0010] 1.2 Analyze the oxygen isotope of chromite samples by laser fluorination-gas mass spectrometry.

[0011] Further, in step 1.1, during the electron probe detection process, the voltage is 10-20 kV, the current is 10-30 nA, the beam spot diameter is 0.5-1.5 μm, the counting time at the peak position is 10-20 s, and the counting times of the front and back background values are both 5-10 s.

[0012] Further, the X-ray intensity is corrected by the ZAF correction method.

[0013] Further, the SPI standard mineral reference samples are used: sanidine (K), pyrope (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), rutile (Ti).

[0014] Further, the Cr# value (Cr / (Cr + Al)) of the chromite is 16 - 84.

[0015] The present invention selects chromite samples with relatively high homogeneity of major elements (MgO, Al 2 O 3 、Cr 2 O 3 、FeO).

[0016] In an embodiment of the present invention, the Cr# values (Cr / (Cr + Al)) of four chromite samples DZ15 - 93B, DZ15 - 102, ZD17 - 57, and 531 - 6 are 16, 38, 66, and 84 in sequence. The MgO content of all chromite samples varies in the range of 13.9 - 19.3 wt%, and the RSD is 0.75 - 1.16%; the content of Al 2 O 3 ranges from 8.1 to 51.5 wt%, and the RSD is 0.47 - 2.08%; the content of Cr 2 O 3 ranges from 14.9 to 62.6 wt%, and the RSD is 0.55 - 2.71%; the FeO content ranges from 12.8 to 16.6 wt%, and the RSD is 1.41 - 3.08%. The results show that the major elements (MgO, Al 2 O 3 、Cr 2 O 3 and FeO) of all chromite samples are evenly distributed.

[0017] Further, in step 1.2, in the laser fluorination - gas mass spectrometry, the weight of the sample to be measured is 1 - 4 mg, and it reacts with purified BrF 5 reagent in the sample chamber to release oxygen. The gas generated in the laser chamber is purified through a cryogenic cold trap, and the collected purified O 2 is tested on a mass spectrometer.

[0018] The dosage of the BrF 5 in the form of gas is 300 - 500 mbar.

[0019] The cryogenic cold trap uses liquid nitrogen, and the temperature of the liquid nitrogen is - 190~ - 200 °C.

[0020] In the laser fluorination - gas mass spectrometry, standard substances zircon and garnet are used to monitor the data quality.

[0021] In an embodiment of the present invention, the oxygen isotopes of four chromite samples DZ15 - 93B, DZ15 - 102, ZD17 - 57, and 531 - 6 have good homogeneity.

[0022] Optionally, the reaction residue is detected and analyzed by a TESCAN Integrated Mineral Analyzer (TIMA). The accelerating voltage is 20 - 30 kV, the current is 8 - 12 nA, the working distance is 10 - 20 mm. The current and the BSE signal intensity are automatically calibrated using a platinum Faraday cup program, and the EDS signal is calibrated using a Mn standard sample. The dissociation mode is used during the test to simultaneously obtain BSE images and EDS data. The X-ray count for each point is 1000. The pixel size is 2 - 3 μm, and the energy spectrum step size is 5 - 6 μm.

[0023] In one embodiment of the present invention, it is verified that the residue contains CrO 3 , and the reaction process of the strong oxidant BrF 5 with chromite is deduced, thereby determining the reason for the decrease in oxygen yield: The O 2 generated by the oxidation reaction also acts as an oxidant and participates in the partial oxidation process of the variable valence element Cr, oxidizing it from Cr 3+ to Cr 6+ , and the product is CrO 3 , consuming part of the O 2 , thus resulting in a decrease in the yield of O 2 .

[0024] 2) The chromite sample is mixed with an oxygen-containing mineral, and laser fluorination - gas mass spectrometry is used to analyze oxygen isotopes.

[0025] The oxygen-containing mineral is zircon (ZrSiO 4 ), and the mass ratio of the zircon to the chromite is (1 - 3):1.

[0026] Zircon (ZrSiO 4 ) is often used as a reference material for isotope dating (U - Pb) and isotope tracing (Zr, Si, O) tests due to its stable silicon-oxygen tetrahedron structure, stable physical and chemical properties, and high closure temperature. The inventors found that by adding the oxygen-containing mineral zircon, the oxygen yield of the chromite sample can be increased, which is beneficial to improving the accuracy of the measured δ 18 O value of oxygen isotopes in chromite.

[0027] Further, the chromite sample and the oxygen-containing mineral are mixed, and laser fluorination - gas mass spectrometry is used to analyze oxygen isotopes.

[0028] In the laser fluorination - gas mass spectrometry, the weight of the sample to be measured is 1 - 4 mg, and it reacts with purified BrF 5 reagent in the sample chamber to release oxygen. The gas generated in the laser chamber is purified by a cryogenic cold trap, and the collected purified O 2 is tested on a mass spectrometer.

[0029] The BrF 5 dosage is 300 - 500 mbar.

[0030] The low - temperature cold trap uses liquid nitrogen, and the temperature of the liquid nitrogen is - 190~ - 200 °C.

[0031] In the laser fluorination - gas mass spectrometry method, reference materials zircon and garnet are used to monitor the data quality.

[0032] In one embodiment of the present invention, after adding zircon, the oxygen yield of the 531 - 6 sample is increased from 57% to 89%, the oxygen yield of the ZD17 - 57 sample is increased from 67% to 89%, and the oxygen yield of the DZ15 - 102 sample is increased from 82% to 89%.

[0033] 3) Establish a calibration equation for data calibration

[0034] 3.1 By detecting the difference between the chromium oxide content and δ 18 O difference, establish a regression equation between the chromium oxide Cr 2 O 3 content and the δ 18 O difference: y = - 0.0004x 2 + 0.051x - 0.6933 (I), where y is the δ 18 O difference and x is the Cr 2 O 3 content in chromite.

[0035] 3.2 Conduct δ 18 O value data calibration

[0036] Since the oxygen yield is 89% after adding zircon and still does not reach the theoretical value of 100%, based on the difference between the actual oxygen yield and the theoretical oxygen yield, another calibration is required to obtain the regression equation between the oxygen yield difference and the δ 18 O difference and the deviation of the δ 18 O value.

[0037] The regression equation between the oxygen yield difference and the δ 18 O difference is: y = - 15.421x 2 + 8.2966x + 0.0251 (II), where y is the δ 18 O difference and x is the oxygen yield difference.

[0038] Combining equation (I) and equation (II), obtain the calibration equation between the chromium oxide Cr 2 O 3 content and the δ 18 O difference.

[0039] In one embodiment of the present invention, δ is obtained according to Equation (II). 18 The deviation of the δO value is 0.75‰. Therefore, for chromium oxide Cr 2 O 3 content and the correction equation for the difference with δ 18 O is y = -0.0004x 2 + 0.051x - 0.6933 + 0.75 (III), where y is the difference in δ 18 O, and x is the Cr 2 O 3 content in chromite.

[0040] In one embodiment of the present invention, after the oxygen isotopes of samples 531-6, ZD17-57, and DZ15-102 are calculated by the correction equation, the δ 18 O values are 4.15‰, 4.00‰, and 4.50‰ respectively.

[0041] In the second aspect of the present invention, there is provided the method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry described in the first aspect, which is used for the detection and analysis of oxygen isotopes of chromite.

[0042] Advantages of the present invention:

[0043] 1. The present invention adopts the oxygen isotope test method of laser fluorination-gas mass spectrometry (LF-IRMS). By mixing an appropriate proportion of zircon to improve the test method, the oxygen yield is increased to 89%, and it shows a trend that the higher the Cr 2 O 3 content, the more obvious the increase. At the same time, the δ 18 O value also increases correspondingly, and also shows a trend that the higher the Cr 2 O 3 content, the more obvious the increase. Based on this, a correction calculation equation between the difference in δ 18 O and the Cr 2 O 3 content is established, effectively correcting the data of the traditional test method.

[0044] 2. The present invention examines the relationship between the Cr 2 O 3 content of the sample and the oxygen yield, and examines the appropriate range of the amount of the sample to be measured, avoiding the problem that the δ 18 O value test is inaccurate due to the sample amount being too small to reach the detection limit of the Faraday cup of the mass spectrometer. While ensuring a reduction in the sample amount, the mass spectrometer reaches a sufficient detection limit to avoid inaccurate detection.

[0045] 3. The present invention verifies that the O 5 generated by the reaction of BrF2 As an oxidant, it participated in the partial oxidation of the variable-valence element Cr, oxidizing Cr 3+ to Cr 6+ , generating the oxide CrO 3 , resulting in a decrease in the oxygen yield and causing oxygen isotope fractionation, thus being the reason for data errors. This laid the foundation for the selection of the role and dosage of the oxidant BrF 5 in the oxygen isotope detection process.

[0046] 4. The detection method of the present invention is suitable for oxygen isotope testing of chromite ore samples with different component types (from high Al to high Cr), with a high oxygen yield, good accuracy of oxygen isotope values, and strong practicability. Brief Description of the Drawings

[0047] Figure 1 This is an electron microscope (SEM) image of the chromite sample of the present invention, where a, b, c, and d are respectively reflected light images of samples 531-6, ZD17-57, DZ15-102, and DZ15-93B; e, f, g, and h are the corresponding BSE images, and the white scale bar is 100 μm;

[0048] Figure 2 shows the distribution of major elements in the chromite sample;

[0049] Figure 3 is a relationship diagram of oxygen yield and Cr 2 O 3 content;

[0050] Figure 4 In it, a is the sample disk after test analysis; b is the TIMA image of the residue adhering to the target after the test reaction, where the red color is the oxide CrO 3 ;

[0051] Figure 5 is for establishing a calibration equation, where a is the regression equation of the Cr 2 O 3 content and the difference in δ 18 O; b is the regression equation of the difference in oxygen yield and the difference in δ 18 O. Detailed Embodiments

[0052] The chromite samples of the present invention mainly come from: Zedang, Dazhuqudi in Tibet, and the Kempirsai region in Kazakhstan.

[0053] The electron probe composition analysis was completed in the JEOL JXA-8230 electron probe laboratory equipped with 4 spectrometers in the Global Tectonics Center of the School of Earth Sciences, China University of Geosciences (Wuhan).

[0054] The TIMA (TESCAN Intergrated Mineral Analyzer) test was completed at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. using a Mira-3 scanning electron microscope equipped with 4 energy-dispersive spectroscopy probes (EDAX Element 30).

[0055] The oxygen isotope test and analysis of laser fluorination-gas mass spectrometry (LF-IRMS) were completed in the Stable Isotope Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. The equipment used was a New Wave Research MIR10-30 laser, and the purified O 2 was tested on a MAT 252 mass spectrometer.

[0056] Example 1

[0057] A method for determining the oxygen isotope of chromite by laser fluorination-gas mass spectrometry includes the following steps:

[0058] 1) Sample screening: Analyze the major elements and oxygen isotopes of chromite to obtain homogeneous samples.

[0059] 1.1 After making the chromite samples into thin slices, 29 chromite ore samples with high purity, fewer cracks and inclusions were selected by scanning electron microscope (SEM) analysis. The scanning electron microscopes of chromite samples DZ15-93B, DZ15-102, ZD17-57 and 531-6 are as Figure 1 shown. a, b, c, d are the reflected light images of samples 531-6, ZD17-57, DZ15-102, DZ15-93B respectively; e, f, g, h are the corresponding BSE images, and the white scale bar is 100 μm.

[0060] Furthermore, the chromite was crushed and screened, and handpicked under a binocular microscope to obtain about 5 grams of pure chromite particles (20-80 mesh). The chromite particles were fixed in epoxy resin and polished for in-situ micro-area analysis. The major elements (MgO, Al 2 O 3 、Cr 2 O 3, homogeneity of FeO). During the experiment, the voltage was set at 15 kV, the current was 20 nA, and the beam spot diameter was 1 μm. The counting time for the peak position was 10 s, and the counting times for the front and back background values were both 5 s. The X-ray intensity was corrected using the ZAF correction method. The laboratory standard samples used were SPI standard mineral samples. The standard samples used were as follows: sanidine (K), pyrope (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), rutile (Ti). The main elements MgO, Al 2 O 3 , Cr 2 O 3 , FeO test results are as Figure 2 shown in and Table 1.

[0061] 1.2 Analyze the oxygen isotopes of chromite samples by laser fluorination-gas mass spectrometry.

[0062] Select about 2 mg of each of the chromite samples DZ15-93B, DZ15-102, ZD17-57, and 531-6 with 4 different composition types and homogeneous main elements. In the oxygen isotope test analysis by laser fluorination-gas mass spectrometry, react with purified BrF 5 reagent (300 mbar) in the sample chamber to release oxygen. The gas generated in the laser chamber is purified through a series of low-temperature cold traps. The cold trap temperature is the liquid nitrogen temperature (-196 °C). The collected purified O 2 is tested on a MAT 252 mass spectrometer. During the test, the reference materials zircon PL (Institute of Geology and Geophysics, Chinese Academy of Sciences; δ 18 O = 5.17 ± 0.08‰) and garnet 04BXL07 (University of Science and Technology of China; δ 18 O = 3.70 ± 0.11‰) are used to monitor the data quality. The test results are shown in Table 1.

[0063] As can be seen from Table 1, the RSD of all samples for MgO, Al 2 O 3 and Cr 2 O 3 is basically less than 3%. The RSD of FeO is slightly higher than that of other main elements, but it is basically within the range of 3%. This may be related to the variable valence factor of the Fe element itself. Generally speaking, the main elements (MgO, Al 2 O 3 , Cr 2 O 3The distributions of (and FeO) are uniform. The δ 18 O values of chromite samples DZ15-93B, DZ15-102, ZD17-57, and 531-6 are 4.15 ± 0.15‰, 3.17 ± 0.27‰, 2.20 ± 0.02‰, and 2.29 ± 0.08‰, respectively. Therefore, the laser fluorination-gas mass spectrometry test further shows that the oxygen isotopes of all samples have good uniformity.

[0064] In addition, after evaluating the oxygen yields of all chromite samples, it is found that only the high-Al sample DZ15-93B (Cr# value 16) has an oxygen yield of 99%. The yields of other samples are relatively low. In particular, the oxygen yield of the high-Cr sample 531-6 (Cr# value 84) is only 57%. Moreover, there is a good linear negative correlation between the oxygen yield and the Cr 2 O 3 content of the chromite samples (R 2 = 0.9965), indicating that when the Cr 2 O 3 content in the chromite is higher, the oxygen yield is lower, that is, the higher the Cr value, the lower the yield ( Figure 3 ).

[0065] 2) Mix the chromite samples with oxygen-containing minerals and analyze the oxygen isotopes by laser fluorination-gas mass spectrometry.

[0066] Mix the chromite samples and zircon in a mass ratio of 1:2 to obtain 2 mg of the mixed mineral sample, and analyze the oxygen isotopes by laser fluorination-gas mass spectrometry. The detection method and steps are the same as those in 1.2, and the detection results are shown in Table 1.

[0067] It can be seen that when no zircon is added, the oxygen yields of chromite samples 531-6, ZD17-57, and DZ15-102 are relatively low; while after adding zircon, the oxygen yields of each chromite mixed mineral sample are greatly improved. The oxygen yield of the sample containing 531-6 is increased from 57% to 89%, the oxygen yield of the sample containing ZD17-57 is increased from 67% to 89%, and the oxygen yield of the sample containing DZ15-102 is increased from 82% to 89%.

[0068] 3) Establish a calibration equation for data correction

[0069] 3.1 By detecting the relationship between the content of chromium oxide Cr 2 O 3 in the chromite sample and the difference in oxygen isotope δ 18 O in step 1), the regression equation is obtained: y = -0.0004x 2 + 0.051x - 0.6933, where y is δ 18O difference, where x is the Cr content in chromite 2 O 3 content ( Figure 5 a).

[0070] Since the oxygen yield of the mixed minerals is 89% after adding zircon, which still does not reach the theoretical value of 100%, correction is needed based on the difference between the actual oxygen yield and the theoretical oxygen yield to obtain the regression equation of the oxygen yield difference and δ 18 O difference: y = -15.421x 2 + 8.2966x + 0.0251 ( Figure 5 b), where y is the δ 18 O difference and x is the oxygen yield difference. According to this equation, the δ 18 O difference before and after correction is 0.75‰.

[0071] Therefore, the correction equation for the Cr 2 O 3 content and the δ 18 O difference is: y = -0.0004x 2 + 0.051x - 0.6933 + 0.75, where y is the δ 18 O difference and x is the Cr 2 O 3 content in chromite.

[0072] After calculating the oxygen isotope values of chromite samples 531-6, ZD17-57, and DZ15-102 using the correction equation, the actual δ 18 O values are 4.15‰, 4.00‰, and 4.50‰ respectively.

[0073] Table 1 Quality of Monitoring Data for Chromite Samples

[0074] ;

[0075] * The oxygen yield has reached 99%, approaching the theoretical value, so no correction is needed.

[0076] Example 2 examines the effect of changing the mass of chromite alone on the oxygen yield

[0077] Weigh 1, 2, 3, and 4 mg of chromite 531-6 samples respectively and conduct experiments according to step 1) of Example 1. The data results are shown in Table 2. It can be seen that as the amount of chromite sample increases, the oxygen production also increases. For every 1 mg increase in the sample, the production rate increases correspondingly, and there is a good linear correlation between the two. However, even when the sample amount has increased to 4 mg, the production rate is still only 65%. At this time, the excessive sample amount will increase the laser melting sample time, resulting in problems such as isotope fractionation. Therefore, simply increasing the mass of chromite cannot effectively solve the problem of low oxygen production rate.

[0078] Table 2 Chromite δ 18 O and oxygen production rate

[0079] 。

[0080] At the same time, during the experiment, it was observed that after the chromite sample underwent a laser fluorination-gas mass spectrometry test reaction, the color changed from black to dark red ( Figure 4 of a). Referring to the literature, it is known that the color of CrO 3 is dark red. In order to further identify the substance after the reaction, collect the residue after the test reaction, make a resin target, and conduct TIMA analysis. During the TIMA detection process, the acceleration voltage in the experiment is 25 kV, the current is 9 nA, the working distance is 15 mm, the current and BSE signal intensity are automatically calibrated using a platinum Faraday cup program, and the EDS signal is calibrated using a Mn standard sample. The dissociation mode is used in the test, and BSE images and EDS data are obtained simultaneously. The X-ray count for each point is 1000. The pixel size is 2 μm, and the energy spectrum step size is 6 μm.

[0081] The TIMA analysis results show the presence of CrO 3 in the residue ( Figure 4 of b), which is consistent with the conclusion obtained from the above observed color change. Therefore, it can be determined that when the strong oxidant BrF 5 reacts with chromite to generate O 2 (Reaction formula 1 is the oxidation process of Cr 2 O 3 ), O 2 acts as an oxidant and participates in the partial oxidation process of the variable valence element Cr, oxidizing it from Cr 3+ to Cr 6+ , and the product is CrO 3 (Reaction formula 2), consuming part of the O 2 , thus resulting in a decrease in the production rate of O 2 .

[0082] 6BrF 5 + 2Cr 2 O 3 → 4CrF6 + 2BrF 3 + 2Br 2 + 3O 2 (1)

[0083] 3O 2 + 2Cr 2 O 3 → 4CrO 3 (2)

[0084] During this reaction process, since the oxidizing property of BrF 5 is stronger than that of O 2 , it will preferentially oxidize Cr 3+ to Cr 6+ (Reaction formula 1). Therefore, when the content of Cr 2 O 3 in chromite is low, the proportion of O 2 participating in oxidation will decrease; conversely, when the content of Cr 2 O 3 in chromite is very high, the proportion of O 2 participating in oxidation will also increase, consuming more O 2 . Therefore, it will lead to the phenomenon mentioned in Example 1: the oxygen production rate of the high-Al sample (Cr 2 O 3 : 14.9 wt%) is 99%, and the oxygen production rate of the high-Cr sample (Cr 2 O 3 : 62.6 wt%) is 57%, and there is a good linear negative correlation between the two ( Figure 3 ). Therefore, when increasing the sample amount, due to the relationship of mutual growth and decline between BrF 5 and chromite, the results and rules during the test in Example 1 appear, verifying the data accuracy of Example 1.

[0085] In summary, the present invention adopts the laser fluorination-gas mass spectrometry (LF-IRMS) oxygen isotope test method. By mixing a certain amount of zircon in the chromite sample, the oxygen production rate can be increased, and at the same time, the δ 18 O value also increases correspondingly. Further, a calibration calculation equation between the δ 18 O difference and the content of Cr 2 O 3 is established, effectively calibrating the test method data and improving the accuracy of oxygen isotope detection.

Claims

1. A method for determining oxygen isotopes in chromite by laser fluorination-gas mass spectrometry, characterized in that: The steps include: 1) Sample screening: Analyze the major elements and oxygen isotopes of chromite to obtain homogeneous samples: 1.1 Select chromite ore samples with high purity, few cracks and inclusions by scanning electron microscopy analysis, and check the homogeneity of major elements by electron probe; 1.2 Analyze oxygen isotopes of chromite samples by laser fluorination-gas mass spectrometry; 2) Chromite samples were mixed with oxygen-containing minerals and analyzed for oxygen isotopes using laser fluorination-gas mass spectrometry; 3) Establish the relationship between Cr2O3 content and oxygen isotope δ 18 O difference correction equation, oxygen isotope δ 18 Correction of O value; Among them, the oxygen-containing mineral is zircon (ZrSiO4), and the mass ratio of zircon to chromite is (1~3):

1.

2. The method according to claim 1, characterized in that The main elements are MgO, Al2O3, Cr2O3, and FeO; and the Cr / (Cr+Al) ratio of the chromite is 16-84.

3. The method according to claim 1, characterized in that During the electron probe detection process, the voltage is 10-20 kV, the current is 10-30 nA, the beam spot diameter is 0.5-1.5 μm, the counting time of the peak position is 10-20 s, and the counting time of the front and back background values ​​is 5-10 s.

4. The method according to claim 3, characterized in that The standard mineral samples are: santheminite (K), pyrope (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), and rutile (Ti).

5. The method according to claim 1, characterized in that: In the laser fluorination-gas mass spectrometry, the weight of the sample to be tested is 1-4 mg, and the amount of BrF5 reagent used is 300-500 mbar.

6. The method according to claim 1, characterized in that Step 3) includes: 3.1 By testing the content of chromium oxide Cr2O3 and δ 18 O difference, establish the relationship between chromium oxide Cr2O3 content and δ 18 Regression equation for O difference: y = -0.0004x 2 + 0.051x -0.6933 (I), where y is δ 18 O difference, x is the Cr2O3 content in chromite; 3.2 Perform δ 18 O value data correction: Establish the oxygen yield difference and δ 18 Regression equation of O difference: y=-15.421x 2 +8.2966x+0.0251(II), where y is δ 18 O difference, x is the oxygen yield difference; Combining equation (I) and equation (II), we can get the relationship between chromium oxide Cr2O3 content and δ 18 Correction equation for O difference.

7. The method according to claim 6, characterized in that The Cr2O3 content and δ 18 The correction equation for the O difference is y = -0.0004x 2 + 0.051x - 0.6933 + 0.75 (III), where y is δ 18 O difference, x is the oxygen yield difference.

8. The method according to claim 1, characterized in that Step 1.2 also includes the use of the TESCAN automatic mineral analysis system to detect and analyze the residue, with an accelerating voltage of 20-30 kV, a current of 8-12 nA, a working distance of 10-20 mm, and the current and BSE signal intensity are calibrated using a platinum Faraday cup automatic program, and the EDS signal is calibrated using a Mn standard.

9. The method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry according to any one of claims 1 to 8 is used for the detection and analysis of oxygen isotopes of chromite.

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