Clastic rock low-content hematite oxygen isotope determination method based on chemical-physical combined treatment

Abstract

The invention relates to the technical field of geochemical analysis, and particularly provides a method for determining low-content hematite oxygen isotope in clastic rock, which is characterized by comprising the following steps: S1, grinding and acid-treating a clastic rock sample to remove carbonate minerals in the clastic rock sample; s2, performing H2O2 solution oxidation and deionized water washing on the sample treated in the S1, and removing organic matters in the clastic rock sample; s3, adding deionized water into the sample treated in the step S2, fully stirring, calculating the standing time according to a Stokes sedimentation formula, and after sedimentation, sucking an upper suspension by using a siphon so as to enrich hematite; s4, performing alkali treatment on the sample treated in S3 to dissolve and remove most silicate minerals; s5, carrying out acid treatment on part of the sample treated in S4 to remove hematite, and carrying out centrifugal separation to obtain acidified silicate; s6, calculating the mass fraction of silicate and hematite in S4, and respectively converting the mass fraction of silicate and hematite into mole numbers A1 and A2 of oxygen; and S7, respectively measuring oxygen isotope values of the sample in S4 and the silicate after acidification in S5, and calculating the delta < 18 > O value of the hematite.

Description

Technical Field

[0001] The present invention belongs to the technical field of geochemical analysis, and particularly relates to an efficient and high-purity oxygen isotope pretreatment method for low-content hematite in clastic rocks, which is applicable to the research on paleoclimate, paleohydrology and reconstruction of atmospheric CO2 partial pressure. Background Art

[0002] As an important carrier for paleoenvironmental records, the oxygen isotope (δ 18 O) composition of hematite can quantitatively invert paleotemperature, the intensity of paleohydrological cycles, and the evolution path of ore-forming fluids, providing key geochemical evidence for reconstructing continental weathering mechanisms and surficial oxidation processes. However, in sedimentary systems, hematite often forms symbiotic structures with submicron-sized particles and phyllosilicate minerals such as illite and kaolinite, and is generally wrapped inside framework silicates such as quartz and feldspar, resulting in the effective separation of the target mineral phase becoming a technical bottleneck restricting data reliability.

[0003] Traditional separation methods are difficult to effectively extract low-content hematite, leading to insufficient isotope test accuracy, mainly reflected in three aspects: First, chemical extraction methods (such as the sodium dithionite-sodium citrate buffer system) can achieve selective dissolution of iron oxides, but it is difficult to avoid the loss of hematite caused by edge etching of silicate minerals. Especially for low-content samples (<5 wt.%), the extraction efficiency is significantly lower than the mass balance theoretical value; Second, physical separation methods (such as high-gradient magnetic separation and density centrifugation) are limited by the overlapping range of mineral dissemination particle size and magnetic susceptibility (for example, the magnetic susceptibility difference between hematite and goethite is only 0.5 - 1.5×10 -3 m 3 / kg), and the silicate residue rate in the separated products is generally higher than 30%; Third, multi-step purification processes (such as the combination of pickling-ultrasonic dispersion-sedimentation separation) not only increase the risk of exogenous pollution (such as the leaching of centrifugal tube polymers resulting in a negative drift of δ 18 O reaching 0.8‰), but also cause recrystallization of amorphous iron oxides in an alternating acid-base environment, resulting in distortion of the isotope fractionation effect.

[0004] The above technical defects lead to significant systematic errors in the current hematite δ 18 O test data: Statistics from the international geoscience database (GEOROC) show that the deviation of hematite δ 18 O values in different laboratories for the same geological unit can reach ±2.5‰, seriously restricting the establishment of quantitative models for paleoenvironmental parameters. For example, in the study of Precambrian banded iron formations (BIFs), due to the interference of 12O released from silicate inclusions, the estimated value of paleo-seawater temperature shows an abnormal fluctuation of ±7°C. Therefore, developing a hematite separation technology with both mineral selectivity and process stability has become an urgent need in the field of high-resolution paleoenvironmental reconstruction.

[0005] The present invention significantly improves the extraction purity and test reliability of hematite by optimizing the chemical treatment and physical separation processes and combining mass balance calculations. Summary of the Invention

[0006] The present invention significantly improves the extraction purity of hematite, simplifies the operation steps, and reduces pollution to improve the test reliability by optimizing the extraction method and parameters of hematite and combining the method of mass balance calculation.

[0007] To achieve the above invention purpose, the technical solution provided by the present invention is as follows:

[0008] The present invention provides a method for determining the oxygen isotope of low-content hematite in clastic rocks, which is characterized by including the following steps:

[0009] S1: Grind and acid-treat the clastic rock sample to remove carbonate minerals in the clastic rock sample;

[0010] S2: Perform H2O2 solution oxidation and deionized water washing on the sample treated in S1 to remove organic matter in the clastic rock sample;

[0011] S3: Add deionized water to the sample treated in S2 and stir well. Calculate the standing time according to Stokes sedimentation formula. After sedimentation, suck the upper suspension with a siphon to enrich hematite;

[0012] S4: Perform alkali treatment on the sample treated in S3 to dissolve and remove most of the silicate minerals;

[0013] S5: Perform acid treatment on the sample treated in S4 to remove hematite, and centrifuge to separate the post-acid silicate;

[0014] S6: Calculate the mass fractions of silicate and hematite in S5, and convert them into the number of moles of oxygen A1 and A2 respectively;

[0015] S7: Measure the oxygen isotope values (δ 18 O 前 and δ 18 O 后 ) of the sample in S4 and the post-acid silicate in S5 respectively, and calculate the δ18O value of hematite.

[0016] In the present invention, the δ18O value of hematite in S7 can be calculated by the following formula: δ 18 OFe2O3 = [A1 * (δ 18 O 前 - δ 18 O 后 ) + A2 * δ 18 O 前 / A2.

[0017] In the present invention, the grinding and acid treatment in S1 are specifically as follows:

[0018] (1) Grind the clastic rock sample with an agate mortar until the particle size is less than 63 μm;

[0019] (3) Perform diffuse reflectance spectroscopy analysis on the sample to ensure that the sample contains hematite;

[0020] (3) Add 0.5 M acetic acid solution until no bubbles are generated, let it stand for 2 hours, and then wash it 2 - 3 times with deionized water.

[0021] In the present invention, S3 is specifically as follows: Transfer the sample after S2 treatment to a high beaker, add deionized water, stir in a clockwise and counterclockwise alternating direction for 3 - 5 minutes, calculate the standing time according to Stokes sedimentation formula, and after sedimentation, suck the suspension at the upper 10 cm depth into a 2000 - 2500 mL beaker with a siphon.

[0022] In the present invention, the enrichment step of S3 is repeated at least 2 times.

[0023] In the present invention, the alkali treatment of S4 is specifically as follows: Add the enriched sample after S3 treatment to 5 M NaOH solution, heat and stir at 90 - 100 °C for 3 hours to dissolve and remove silicate minerals, then wash it with deionized water until neutral, and dry it in a vacuum oven.

[0024] In the present invention, the acid treatment of S5 is specifically as follows: Take 0.1 g of the sample after S4 treatment and add 6 M HCl solution, react at 80 °C for 2 hours to dissolve hematite, and obtain post - acid silicate after centrifugation.

[0025] In the present invention, S5 further includes verifying the mineral composition of the post - acid silicate by XRD, and entering S6 after detecting no hematite residue.

[0026] In the present invention, S5 further includes using SEM to assist in verifying whether there is hematite residue.

[0027] In the present invention, A1 in S6 is calculated as SiO2, and A2 is calculated as Fe2O3.

[0028] Through experimental verification, the technical solution combining chemistry and physics of the present invention can effectively improve the purity of hematite and the test reliability. Description of the Drawings

[0029] Figure 1 It is the diffuse reflectance spectrum of the original sample of Well AH1 according to the embodiment of the present invention.

[0030] Figure 2XRD pattern of the sample from Well AH1 after NaOH treatment according to an embodiment of the present invention.

[0031] Figure 3 XRD pattern of the sample from Well AH1 after hydrochloric acid treatment. Detailed implementation manners

[0032] In the following, the present invention will be described in detail according to exemplary embodiments, but the present invention is not limited to these embodiments. The present invention is embodied in the following various forms, but should not be construed as limited to the exemplary embodiments set forth herein. Therefore, the detailed description and embodiments of the present invention will convey the scope of the present invention to those of ordinary skill in the art and be construed as being within the scope of the present invention.

[0033] According to the present invention, pretreatment of hematite oxygen isotope and calculation of hematite oxygen isotope are required.

[0034] In some embodiments, the method for pretreatment of hematite oxygen isotope includes:

[0035] Step S1: Removal of carbonate minerals

[0036] Take 5 g of clastic rock sample, grind it in an agate mortar until the particle size is less than 63 microns, and the sample sieve used is greater than or equal to 200 mesh. Further, perform diffuse reflectance spectroscopy analysis on the sample to ensure that the sample contains hematite, then add 0.5 M acetic acid solution until no bubbles are generated, let it stand for 2 hours, and wash it 2 - 3 times with deionized water to thoroughly remove carbonate minerals in the sample.

[0037] Step S2: Oxidative decomposition of organic matter

[0038] Slowly add 150 mL of 30% H2O2 solution to the residual sample after treatment in Step S1, let it stand for 0.5 hour, fully oxidize and remove the organic matter in the rock, and then wash it 2 - 3 times with deionized water.

[0039] Step S3: Enrich hematite by particle size

[0040] Transfer the sample after treatment in Step S2 to a high beaker, add deionized water, and stir in an alternating clockwise and counterclockwise direction. Preferably, the stirring duration is 3 - 5 minutes. Further, calculate the standing time according to Stokes' sedimentation formula. For example, when extracting the suspended particles with a particle size < 3.9 μm in the upper part, let it stand for 42 minutes and 7 seconds at 25°C. After sedimentation, use a siphon to suck the upper 10 cm depth of the suspension into a 2000 - 2500 mL beaker, and repeat this process 2 times or more to ensure sufficient enrichment. Preferably, this step is repeated 3 times.

[0041] Step S4: Alkali treatment of silicate minerals

[0042] Add the enriched sample to 5M NaOH solution, heat and stir at 90 - 100 °C for 3 hours to dissolve and remove silicate minerals, then wash with deionized water until neutral and dry in a vacuum oven.

[0043] In some embodiments, the method for calculating the oxygen isotope of hematite includes:

[0044] Step S5: Acid dissolution of hematite

[0045] Generally, quartz and a small amount of insoluble silicate still remain after the treatment in step S4. Take 0.1 g of the sample and add it to 6M HCl solution, react at 80 °C for 2 hours to dissolve hematite, and obtain the acid - treated silicate residue after centrifugation.

[0046] Verify the composition of silicate minerals by XRD to ensure that there is no hematite residue, and use SEM for auxiliary verification if necessary.

[0047] Step S6: Mass balance calculation of hematite

[0048] Weigh the mass of each part again, calculate the mass fractions of hematite and silicate, and convert them into the number of moles of oxygen. Among them, the number of moles of oxygen in silicate calculated as SiO2 is A1, and the number of moles of oxygen in hematite calculated as Fe2O3 is A2.

[0049] Step S7: Oxygen isotope calculation of hematite

[0050] Measure the oxygen isotope values (δ 18 O 前 and δ 18 O 后 ) of the total sample and the acid - treated silicate respectively, and calculate the δ 18 O value of hematite using the following formula: δ 18 O Fe2O3 =[A1*(δ 18 O 前 -δ 18 O 后 )+A2*δ 18 O 前 / A2.

[0051] Core samples of the Triassic Baikouquan Formation conglomerate from Well AH1 in the Mahu Sag, Junggar Basin

[0052] Select the core samples of the Triassic Baikouquan Formation conglomerate from Well AH1 in the Mahu Sag, Junggar Basin. First, clean the mud or contaminants on the surface of the core with clean water and then air - dry it naturally. Wrap the dried sample with aluminum foil and break it into small pieces with a hammer to prevent direct contact between the hammer and the sample, thus avoiding contamination.

[0053] Put the sample into an agate mortar and grind it until there are no obvious particles, then screen it with a 200-mesh sample sieve. The coarse particles screened out are ground continuously until all the samples can pass through the 200-mesh sieve. At this time, the particle size of the sample is less than 63 microns, that is, greater than 200 meshes. Finally, mix all the samples evenly.

[0054] Perform diffuse reflectance spectroscopy (DRS) analysis on the sample. Hematite has a characteristic absorption peak at 565 - 575 nm in the first derivative curve of the diffuse reflectance spectrum, as shown in the appendix Figure 1 As shown, the spectrum indicates the presence of hematite in the sample.

[0055] Take about 5 g of the ground sample and put it into a 500-ml beaker. Add sufficient 0.5 M acetic acid solution and stir until there are no bubbles. After standing for 2 hours, wash it 2 - 3 times with deionized water to thoroughly remove carbonate minerals in the sample.

[0056] Slowly add 150 mL of 30% H2O2 solution to the treated sample, stir and then stand for 0.5 hour to fully oxidize and remove the organic matter in the rock. Subsequently, wash it 2 - 3 times with deionized water.

[0057] Transfer the treated sample to a large beaker with an inner diameter of 20 cm and a height of 40 cm. Add deionized water to the large beaker to a height of 30 cm. Measure the temperature in the beaker as 25 °C. Stir for 3 - 5 minutes in a clockwise and counterclockwise alternating direction and then start timing. After 42 minutes and 7 seconds, use a siphon to suck the suspension at the upper 10-cm depth into a 2000 - 2500-ml beaker. According to Stokes' sedimentation formula, particles with a particle size < 3.9 μm suspended in the upper part can be extracted for physical enrichment of hematite. Repeat this process 3 times to ensure sufficient enrichment.

[0058] Add 5 M NaOH solution to the enriched sample, heat and stir at 90 - 100 °C for 3 hours to dissolve and remove silicate minerals. Subsequently, wash it with deionized water until neutral and dry it in a vacuum oven.

[0059] Perform X-ray diffraction (XRD) mineral composition analysis on the residual sample. The analysis spectrum is as shown in the appendix Figure 2 As shown, the results show that the main minerals of the residual sample are: quartz, illite, chlorite and hematite, indicating that the NaOH solution dissolved and removed some silicate minerals. Now, mainly quartz and a small amount of insoluble silicate minerals remain, and hematite is enriched.

[0060] Weigh more than 93.1 mg of the residual sample, then add 6M HCl solution, react at 80 °C for 2 hours to dissolve hematite, and obtain 65.6 mg of acid-treated silicate residue after washing with deionized water and centrifuging. Analyze the mineral composition again by XRD, as shown in the appendix Figure 3 As shown, the residue after hydrochloric acid treatment is mainly quartz, and there is no residual hematite. By calculation, the mass fraction of hematite before adding acid is 30%, and the mass fraction of silicate minerals is 70%. Assuming that all oxygen atoms (O) are in quartz SiO2 and hematite Fe2O3, and other silicates can be ignored, then the number of moles of oxygen A1 in quartz = 0.093 * 70% / (28 + 32) * 2 = 0.00217, and the number of moles of oxygen A2 in hematite = 0.093 * 30% / (112 + 48) * 3 = 0.00052.

[0061] Measure the oxygen isotope values of the samples before and after hydrochloric acid treatment, δ 18 O 前 = 9.28‰, δ 18 O 后 = 10.20‰, and use the following formula to calculate the δ 18 O value of hematite:

[0062] δ 18 O Fe2O3 = [A1 * (δ 18 O 前 - δ 18 O 后 ) + A2 * δ 18 O 前 / A2 = 5.47‰.

[0063] The above experimental results show that through the present invention, high-purity hematite can be effectively extracted and can be used for accurate determination of the oxygen isotope of hematite.

[0064] It should be understood that the exemplary embodiments described herein should be considered only in a descriptive sense and not as a limitation on the scope of the present invention. The description of the technical solutions and embodiments of the present invention is intended to provide a comprehensive understanding of the present invention, but should not be construed as limiting the present invention to these specific embodiments. Those skilled in the art can make appropriate modifications or adjustments to the described embodiments without departing from the spirit and scope of the present invention.

[0065] The description of the features or aspects in each embodiment should generally be considered applicable to similar features or aspects in other embodiments as well, unless it is explicitly stated that they are not feasible. The specific technical means, parameter selections, experimental conditions, etc. mentioned in the present invention are only for illustrative purposes and should not be regarded as the sole limitation of the technical solution of the present invention. Without departing from the basic principles of the present invention, those skilled in the art can replace, combine or optimize these features or aspects to achieve other specific embodiments of the present invention.

[0066] In addition, some technical features or operation steps mentioned in the present invention may not be described in detail in some embodiments, but this does not mean that these features or steps are unavailable or inapplicable in other embodiments. On the contrary, those skilled in the art can flexibly select and apply these technical features or operation steps according to specific requirements and technical backgrounds to achieve the intended purpose of the present invention.

Claims

1. A method for determining the oxygen isotope of low-content hematite in clastic rocks, characterized in that, It includes the following steps: S1: Grind and acid-treat the clastic rock sample to remove carbonate minerals in the clastic rock sample; S2: For the sample treated in S1, conduct H2O2 solution oxidation and deionized water washing to remove organic matter in the clastic rock sample; S3: For the sample treated in S2, add deionized water and stir well. Calculate the standing time according to Stokes' sedimentation formula. After sedimentation, suck the upper suspension with a siphon to enrich hematite; S4: For the sample treated in S3, conduct alkali treatment to dissolve and remove most of the silicate minerals; S5: For the sample treated in S4, take a part and conduct acid treatment to remove hematite, and centrifuge to obtain post-acid silicate; S6: Calculate the mass fractions of silicate and hematite in S4, and convert them into the molar amounts of oxygen A1 and A2 respectively; S7: Measure the oxygen isotope values (δ 18 O 前 and δ 18 O 后 ) of the sample described in S4 and the silicate after acid treatment in S5 respectively, and calculate the δ18O value of hematite.

2. The method according to claim 1, characterized in that, The δ18O value of hematite in S7 is calculated using the following formula: δ 18 O Fe2O3 = [A1 * (δ 18 O 前 - δ 18 O 后 ) + A2 * δ 18 O 前 / A2.

3. The method according to claim 1, wherein The grinding and acid treatment in S1 are specifically as follows: (1) Grind the clastic rock sample with an agate mortar to a particle size less than 63 μm; (2) Conduct diffuse reflectance spectroscopy analysis on the sample to ensure that the sample contains hematite; (3) Add 0.5M acetic acid solution until no bubbles are generated, stand for 2 hours, and then wash with deionized water 2-3 times.

4. The method according to claim 1, wherein S3 is specifically as follows: Transfer the sample treated in S2 to a high beaker, add deionized water, stir in a clockwise and counterclockwise alternating direction for 3-5 minutes, calculate the standing time according to Stokes' sedimentation formula. After sedimentation, suck the suspension at a depth of 10 cm from the upper part with a siphon into a 2000-2500 mL beaker.

5. The method according to claim 4, characterized in that The enrichment step in S3 is repeated at least 2 times.

6. The method according to claim 1, characterized in that, The alkali treatment in S4 is specifically as follows: Add the enriched sample treated in S3 to 5M NaOH solution, heat and stir at 90-100 °C for 3 hours to dissolve and remove silicate minerals, then wash with deionized water until neutral, and dry in a vacuum oven.

7. The method according to claim 1, characterized in that The acid treatment in S5 is specifically as follows: Take 0.1 g of the sample treated in S4 and add 6M HCl solution, react at 80 °C for 2 hours to dissolve hematite, and centrifuge to obtain post-acid silicate.

8. The method according to claim 7, wherein S5 further includes verifying the mineral composition of the post-acid silicate by XRD, and entering S6 after detecting no residual hematite.

9. The method according to claim 8, wherein S5 further includes using SEM to assist in verifying whether there is residual hematite.

10. The method according to claim 1, wherein In S6, A1 is calculated based on SiO2, and A2 is calculated based on Fe2O3.

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