Method for measuring chromite oxygen isotope through laser fluorination-gas mass spectrometry

Abstract

The invention relates to a method for measuring oxygen isotope of chromite by laser fluorination-gas mass spectrometry, which comprises the following steps: 1) sample screening: analyzing major elements and oxygen isotope of chromite to obtain a homogeneous sample: 2) mixing the chromite sample with oxygen-containing minerals, and analyzing oxygen isotope by laser fluorination-gas mass spectrometry; and 3) establishing a correction equation of the Cr2O3 content and the oxygen isotope delta < 18 > O difference value, and correcting the oxygen isotope delta < 18 > O value. According to the method, a certain amount of zircon is mixed into the chromite sample, so that the oxygen yield is remarkably improved, and the data of the test method are effectively corrected by utilizing a correction calculation equation between the delta < 18 > O difference value and the Cr2O3 content, so that the accuracy of oxygen isotope detection is improved.

Description

Technical Field

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

[0002] Chromite is the chromium-rich end member of the solid solution (Mg, Fe)(Al, Cr, Fe)2O4 spinel group and can be divided into three types: aluminum spinel, chromium spinel and iron spinel. As one of the main minerals in the earliest crystallization of ultramafic-mafic magma, chromite can record important information about the original magma. Its oxygen isotope composition has important scientific significance in revealing the characteristics of the upper mantle, studying the migration process of melts and fluids, and analyzing the recycling of crustal materials in subduction zones.

[0003] The oxygen isotope test method of laser fluorination-gas mass spectrometry (LF-IRMS) is a method of extracting oxygen from the reactants using a CO2 laser probe under the action of a strong oxidant, BrF5. Compared with the traditional method, the sample volume 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 using the oxygen isotope test benchmark method (laser fluorination-gas mass spectrometry) for analysis and testing, chromite has the problem of low oxygen yield. Heck et al. (2010) analyzed chromite in the Bushveld Complex in South Africa, the world's largest platinum group element deposit, and found that the oxygen yield during the test was low and the repeatability of the oxygen isotope value was poor. By increasing the amount of BrF5 reagent, the oxygen yield can be increased only 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 BrF5 reagent cannot completely solve the problem of low oxygen yield in the chromite oxygen isotope test. Moreover, O2 generated by the reaction of BrF5 with chromite as an oxidant participates in the partial oxidation of the variable valence element Cr, resulting in a decrease in oxygen yield and oxygen isotope fractionation. 18 There is a certain uncertainty in the O value. In addition, the existing technology shows 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 mass spectrometer Faraday cup, which will lead to δ 18 The O value test is not accurate.

[0004] In order to solve the problems of low oxygen yield, oxygen isotope fractionation, and chromite δ 18In order to solve the technical problems such as uncertain O value, the present invention develops an improved method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry (LF-IRMS), which effectively improves the oxygen yield and the accuracy of oxygen isotope values. Summary of the invention

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

[0006] In a first aspect of the present invention, there is provided a method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry, comprising the following steps: 1) Sample screening: Analyze the major elements and oxygen isotopes of chromite to obtain homogeneous samples.

[0007] The specific steps include: 1.1 Chromite ore samples with high purity, few cracks and inclusions were selected by scanning electron microscopy (SEM) analysis, and the homogeneity of their major elements (MgO, Al2O3, Cr2O3, FeO) was checked by electron probe microscopy (EPMA).

[0008] 1.2 The oxygen isotopes of chromite samples were analyzed by laser fluorination-gas mass spectrometry.

[0009] Furthermore, 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 peak counting time is 10-20 s, and the counting time of the front and back background values ​​is 5-10 s.

[0010] Furthermore, the X-ray intensity was corrected using the ZAF correction method.

[0011] Furthermore, SPI standard mineral samples were used: santheminite (K), pyrope (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), and rutile (Ti).

[0012] Furthermore, the Cr# value (Cr / (Cr+Al)) of the chromite is 16-84.

[0013] The present invention selects chromite samples with relatively high homogeneity of major elements (MgO, Al2O3, Cr2O3, FeO).

[0014] In one 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, respectively. The MgO content of all chromite samples ranges from 13.9 to 19.3 wt%, with an RSD of 0.75 to 1.16 %; the Al2O3 content ranges from 8.1 to 51.5 wt%, with an RSD of 0.47 to 2.08%; the Cr2O3 content ranges from 14.9 to 62.6 wt%, with an RSD of 0.55-2.71 %; and the FeO content ranges from 12.8 to 16.6 wt%, with an RSD of 1.41 to 3.08 %. The results show that the major elements (MgO, Al2O3, Cr2O3, and FeO) of all chromite samples are uniformly distributed.

[0015] Furthermore, in step 1.2, in the laser fluorination-gas mass spectrometry, the weight of the sample to be tested is 1-4 mg, and it reacts with the purified BrF5 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 O2 is tested on a mass spectrometer.

[0016] The BrF5 (in gas form) is used in an amount of 300-500 mbar.

[0017] The low temperature cold trap uses liquid nitrogen, and the temperature of the liquid nitrogen is -190~-200°C.

[0018] The laser fluorination-gas mass spectrometry method uses standard substances zircon and garnet to monitor data quality.

[0019] In one embodiment of the present invention, the oxygen isotopes of four chromite samples DZ15-93B, DZ15-102, ZD17-57, and 531-6 have good uniformity.

[0020] Optionally, the residue after the reaction is analyzed by TESCAN Intergrated Mineral Analyzer (TIMA), 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 the automatic procedure of the platinum Faraday cup, and the EDS signal is calibrated using the Mn standard. The dissociation mode is used in the test, and the BSE image and EDS data are obtained simultaneously, with an X-ray count of 1000 at each point. The pixel size is 2-3 μm, and the energy spectrum step is 5-6 μm.

[0021] In one embodiment of the present invention, it is verified that the residue contains CrO3, and the reaction process of the strong oxidant BrF5 and chromite is deduced, thereby determining the reason for the reduction in oxygen yield: O2 generated by the oxidation reaction acts as an oxidant and participates in the partial oxidation process of the variable valence element Cr, converting it from Cr 3+ Oxidized to Cr 6+ The product is CrO3, which consumes part of O2, resulting in a decrease in the yield of O2.

[0022] 2) The chromite samples were mixed with oxygen-containing minerals and analyzed for oxygen isotopes by laser fluorination-gas mass spectrometry.

[0023] The oxygen-containing mineral is zircon (ZrSiO4), and the mass ratio of zircon to chromite is (1-3):1.

[0024] Zircon (ZrSiO4) is often used as a standard material for isotope dating (U-Pb) and isotope tracing (Zr, Si, O) tests because of its stable silicon-oxygen tetrahedral structure, stable physical and chemical properties, and high closure temperature. The inventors found that the oxygen yield of chromite samples can be increased by adding oxygen-containing mineral zircon, which is beneficial to improve the detection of oxygen isotope δ 18 The accuracy of O value.

[0025] Furthermore, the chromite samples were mixed with oxygen-containing minerals and oxygen isotopes were analyzed by laser fluorination-gas mass spectrometry.

[0026] In the laser fluorination-gas mass spectrometry, the sample to be tested weighs 1 to 4 mg and reacts with a purified BrF5 reagent in the sample chamber to release oxygen. The gas produced in the laser chamber is purified by a cryogenic cold trap, and the collected purified O2 is tested on a mass spectrometer.

[0027] The BrF5 dosage is 300-500 mbar.

[0028] The low temperature cold trap uses liquid nitrogen, and the temperature of the liquid nitrogen is -190~-200°C.

[0029] The laser fluorination-gas mass spectrometry method uses standard substances zircon and garnet to monitor data quality.

[0030] In one embodiment of the present invention, after adding zircon, 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%.

[0031] 3) Establish a correction equation for data correction 3.1 By testing the chromium oxide content and δ 18O 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.

[0032] 3.2 Perform δ 18 O value data correction Since the oxygen yield after adding zircon is 89%, which still does not reach the theoretical value of 100%, it is necessary to make another correction based on the difference between the actual oxygen yield and the theoretical oxygen yield to obtain the oxygen yield difference and δ 18 The regression equation of O difference and δ 18 The deviation of O value.

[0033] The oxygen yield difference and δ 18 The regression equation of the O difference is: y=-15.421x 2 +8.2966x+0.0251(II), where y is δ 18 O difference, x is the oxygen yield difference.

[0034] Combining equation (I) with equation (II), we can get the relationship between chromium oxide Cr2O3 content and δ 18 Correction equation for O difference.

[0035] In one embodiment of the present invention, according to equation (II), δ 18 The deviation of O value is 0.75‰, so the content of chromium oxide Cr2O3 is related to δ 18 Correction equation for O difference y = -0.0004x 2 + 0.051x - 0.6933 + 0.75 (III), where y is δ 18 O difference, x is the Cr2O3 content in chromite.

[0036] In one embodiment of the present invention, after the oxygen isotopes of samples 531-6, ZD17-57, and DZ15-102 are calculated by the calibration equation, δ 18 The O values ​​are 4.15‰, 4.00‰ and 4.50‰ respectively.

[0037] The second aspect of the present invention provides the method for determining oxygen isotopes in chromite by laser fluorination-gas mass spectrometry as described in the first aspect, which is used for the detection and analysis of oxygen isotopes in chromite.

[0038] Beneficial effects of the present invention: 1. The present invention adopts the laser fluorination-gas mass spectrometry (LF-IRMS) oxygen isotope test method, and by mixing an appropriate proportion of zircon into the improved test method, the oxygen yield is increased to 89%, and the higher the Cr2O3 content, the more obvious the increase. At the same time, δ 18 O value also increases accordingly, showing the same trend that the higher the Cr2O3 content, the more obvious the increase. 18 The correction calculation equation between O difference and Cr2O3 content was used to effectively correct the data of traditional test methods.

[0039] 2. The present invention investigates the relationship between the Cr2O3 content of the sample and the oxygen yield, and investigates the appropriate range of the amount of the sample to be tested, so as to avoid the situation where the sample amount is too small to reach the Faraday cup detection limit of the mass spectrometer, thereby causing δ 18 To solve the problem of inaccurate O value test, the mass spectrometer can achieve sufficient detection limit while reducing the sample amount to avoid inaccurate detection.

[0040] 3. The present invention verifies that the O2 generated by the reaction of BrF5 and chromite in the traditional test method acts as an oxidant and participates in the partial oxidation of the variable valence element Cr. 3+ Oxidized to Cr 6+ , the oxide CrO3 is generated, which leads to a decrease in oxygen yield and causes oxygen isotope fractionation, thus causing data errors, laying the foundation for the role and dosage selection of the oxidant BrF5 in the process of oxygen isotope detection.

[0041] 4. The detection method of the present invention is suitable for oxygen isotope testing of chromite ore samples (high Al to high Cr) of different composition types, with high oxygen yield, good accuracy of oxygen isotope value and strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The scanning electron microscope (SEM) images of the chromite samples of the present invention, wherein a, b, c, d are reflected light images of samples 531-6, ZD17-57, DZ15-102, and DZ15-93B, respectively; e, f, g, h are the corresponding BSE images, and the white scale is 100 μm; Figure 2 The major element distribution of chromite sample; Figure 3 It is the relationship diagram between oxygen yield and Cr2O3 content; Figure 4 a is the sample plate after the test analysis; b is the TIMA image of the residue sticking to the target after the test reaction, in which the red is the oxide CrO3; Figure 5 To establish the correction equation, a is the Cr2O3 content and δ 18O difference regression equation; b is the oxygen yield difference and δ 18 Regression equation for O difference. DETAILED DESCRIPTION

[0043] The chromite samples of the present invention mainly come from: Zetang, Dazhuqudi in Tibet, and Kempirsai area in Kazakhstan.

[0044] The electron probe composition analysis was completed in the JEOL JXA-8230 electron probe laboratory equipped with a 4-channel spectrometer at the Global Tectonic Center of the School of Earth Sciences, China University of Geosciences (Wuhan).

[0045] TIMA (TESCAN Intergrated Mineral Analyzer) testing was performed at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. using a Mira-3 scanning electron microscope equipped with four energy spectrum probes (EDAX Element 30).

[0046] The oxygen isotope test analysis by laser fluorination-gas mass spectrometry (LF-IRMS) was completed in the Stable Isotope Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. The equipment used was the New Wave Research MIR10-30 laser, and the collected purified O2 was tested on the MAT 252 mass spectrometer.

[0047] Example 1 A method for determining oxygen isotopes of chromite by laser fluorination-gas mass spectrometry comprises the following steps: 1) Sample screening: Analyze the major elements and oxygen isotopes of chromite to obtain homogeneous samples.

[0048] 1.1 After the chromite samples were made into thin sections, 29 chromite ore samples with high purity and few cracks and inclusions were selected by scanning electron microscopy (SEM) analysis. The SEM images of chromite samples DZ15-93B, DZ15-102, ZD17-57 and 531-6 were as follows: Figure 1 As shown, a, b, c, and d are the reflected light images of samples 531-6, ZD17-57, DZ15-102, and DZ15-93B, respectively; e, f, g, and h are the corresponding BSE images, and the white scale is 100 μm.

[0049] Further, the chromite was crushed and sieved, and hand-selected 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 microanalysis. The homogeneity of the major elements (MgO, Al2O3, Cr2O3, FeO) was checked by electron probe microanalysis (EPMA). During the experiment, the voltage was set to 15 kV, the current was 20 nA, and the beam spot diameter was 1 μm. The counting time of the peak position was 10 s, and the counting time of the front and back background values ​​was 5 s. The X-ray intensity was corrected using the ZAF correction method. The laboratory standard used was the SPI standard mineral standard. The standards used were as follows: santhemidite (K), pyrope (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), and rutile (Ti). The test results of the main elements MgO, Al2O3, Cr2O3 and FeO of chromite samples DZ15-93B, DZ15-102, ZD17-57 and 531-6 are as follows Figure 2 And as shown in Table 1.

[0050] 1.2 The oxygen isotopes of chromite samples were analyzed by laser fluorination-gas mass spectrometry.

[0051] Four chromite samples DZ15-93B, DZ15-102, ZD17-57 and 531-6 with different composition types and uniform major elements were selected, each weighing about 2 mg. In the laser fluorination-gas mass spectrometry oxygen isotope test analysis, oxygen was released by reacting with purified BrF5 reagent (300mbar) in the sample chamber. The gas produced in the laser chamber was purified by a series of low-temperature cold traps, and the cold trap temperature was liquid nitrogen temperature (-196 ℃). The collected purified O2 was tested on a MAT 252 mass spectrometer. The standard material in the test was 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‰) was used to monitor data quality, and the test results are shown in Table 1.

[0052] It can be seen from Table 1 that the RSD of MgO, Al2O3 and Cr2O3 of all samples is basically less than 3%, and the RSD of FeO is slightly higher than that of other major elements, but it is basically within 3%, which may be related to the valence factor of Fe element itself. In general, the distribution of major elements (MgO, Al2O3, Cr2O3 and FeO) of all samples is uniform. The δ 18The O values ​​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.

[0053] In addition, after evaluating the oxygen yield of all chromite samples, it was found that only the high Al sample DZ15-93B (Cr# value 16) had an oxygen yield of 99%. The yields of other samples were lower, especially the high Cr sample 531-6 (Cr# value 84), which had an oxygen yield of only 57%. In addition, the oxygen yield and the Cr2O3 content of the chromite sample had a good linear negative correlation (R 2 = 0.9965), indicating that when the Cr2O3 content in the chromite is higher, the oxygen yield is lower, that is, the higher the Cr value, the lower the yield ( Figure 3 ).

[0054] 2) The chromite samples were mixed with oxygen-containing minerals and analyzed for oxygen isotopes by laser fluorination-gas mass spectrometry.

[0055] The chromite sample and zircon were mixed in a mass ratio of 1:2 to obtain 2 mg of a mixed mineral sample, and oxygen isotopes were analyzed by laser fluorination-gas mass spectrometry. The detection method was the same as in step 1.2. The detection results are shown in Table 1.

[0056] It can be seen that when zircon is not added, the oxygen yields of chromite samples 531-6, ZD17-57 and DZ15-102 are low; after adding zircon, the oxygen yields of each chromite mixed mineral sample are greatly improved. The oxygen yield of the 531-6 sample increased from 57% to 89%, the oxygen yield of the ZD17-57 sample increased from 67% to 89%, and the oxygen yield of the DZ15-102 sample increased from 82% to 89%.

[0057] 3) Establish a correction equation for data correction 3.1 Through step 1), the content of chromium oxide Cr2O3 and oxygen isotope δ 18 O difference, and the regression equation is: y = -0.0004x 2 + 0.051x -0.6933 , where y is δ 18 O difference, x is the Cr2O3 content in chromite ( Figure 5 a).

[0058] After adding zircon, the oxygen yield of the mixed mineral is 89%, which still does not reach the theoretical value of 100%. Therefore, based on the difference between the actual oxygen yield and the theoretical oxygen yield, a correction is needed to obtain the oxygen yield difference and δ 18The regression equation of O difference is y=-15.421x 2 +8.2966x+0.0251( Figure 5 b), where y is δ 18 O difference, x is the oxygen yield difference. According to this equation, we can get the difference before and after correction, δ 18 The O difference is 0.75‰.

[0059] Therefore, the content of chromium oxide Cr2O3 is related to δ 18 The correction equation for the O difference is: y = -0.0004x 2 + 0.051x -0.6933 + 0.75, where y is δ 18 O difference, x is the Cr2O3 content in chromite.

[0060] After the oxygen isotope values ​​of chromite samples 531-6, ZD17-57, and DZ15-102 were calculated using the calibration equation, the actual δ 18 The O values ​​are 4.15‰, 4.00‰ and 4.50‰ respectively.

[0061] Table 1 Quality of chromite sample monitoring data ; *The oxygen production rate has reached 99%, close to the theoretical value, so no correction is required.

[0062] Example 2 Investigating the Effect of Changing Chromite Quality on Oxygen Yield Weigh 1, 2, 3, and 4 mg of chromite 531-6 sample 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 sample, the yield increases accordingly, and the two have a good linear correlation. However, even when the sample amount has increased to 4 mg, the yield is still only 65%. At this time, the sample amount is too large, which will increase the laser melting sample time and lead to problems such as isotope fractionation. Therefore, simply increasing the mass of chromite cannot effectively solve the problem of low oxygen yield.

[0063] Table 2 Chromite δ 18 O and oxygen production rate .

[0064] At the same time, during the experiment, it was observed that the color of the chromite sample changed from black to dark red after the laser fluorination-gas mass spectrometry test reaction ( Figure 4a). According to the literature, the color of CrO3 is dark red. In order to further identify the post-reaction substances, the residue after the test reaction was collected, made into a resin target, and TIMA analysis was performed. During the TIMA detection process, the accelerating voltage was 25 kV, the current was 9 nA, the working distance was 15 mm, the current and BSE signal intensity were calibrated using the automatic program of the platinum Faraday cup, and the EDS signal was calibrated using the Mn standard. The dissociation mode was used in the test, and the BSE image and EDS data were obtained simultaneously, with an X-ray count of 1000 at each point. The pixel size was 2 μm, and the energy spectrum step was 6 μm.

[0065] TIMA analysis results show the presence of CrO3 in the residue ( Figure 4 b), which is consistent with the conclusion drawn from the above observation of color change. Therefore, it can be determined that after the strong oxidant BrF5 reacts with chromite to generate O2 (reaction formula 1 is the oxidation process of Cr2O3), O2, as an oxidant, participates in the partial oxidation process of the variable valence element Cr, converting it from Cr 3+ Oxidized to Cr 6+ The product is CrO3 (reaction formula 2), which consumes part of O2, resulting in a decrease in the yield of O2.

[0066] 6BrF5+ 2Cr2O3 → 4CrF6+ 2BrF3+ 2Br2+ 3O2 (1) 3O2 + 2Cr2O3 → 4CrO3 (2) During this reaction, BrF5 will preferentially oxidize Cr because its oxidizing property is stronger than O2. 3+ Cr 6+ (Reaction 1). Therefore, when the Cr2O3 content in chromite is low, the proportion of O2 participating in oxidation will decrease; conversely, when the Cr2O3 content in chromite is high, the proportion of O2 participating in oxidation will also increase, consuming more O2. Therefore, the phenomenon mentioned in Example 1 will occur: the oxygen yield of the high Al sample (Cr2O3: 14.9 wt%) is 99%, and the oxygen yield of the high Cr sample (Cr2O3: 62.6 wt%) is 57%, and the two have a good linear negative correlation ( Figure 3 ). Therefore, when the sample amount is increased, due to the relationship between BrF5 and chromite, the results and rules of the test in Example 1 appear, verifying the accuracy of the data in Example 1.

[0067] In summary, the present invention adopts the laser fluorination-gas mass spectrometry (LF-IRMS) oxygen isotope testing method, and by mixing a certain amount of zircon into the chromite sample, the oxygen yield can be increased, and at the same time, δ 18 O value also increases accordingly, further establishing the δ 18The correction calculation equation between the O difference and the Cr2O3 content effectively corrected the test method data and improved 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.

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 The oxygen-containing mineral is zircon (ZrSiO4), and the mass ratio of zircon to chromite is (1-3):

1.

7. The method according to claim 6, 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.

8. The method according to claim 7, 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.

9. 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.

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

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