Method for accurately measuring nano ion probe olivine magnesium isotope microcell

Abstract

The invention discloses a method for accurately measuring a nano ion probe olivine Mg isotope micro-region and applying a moon sample, particularly relates to application of a nano ion probe (SIMS) technology in magnesium isotope analysis, and particularly relates to a method for carrying out high-resolution and high-precision magnesium isotope analysis and on-line matrix effect correction by utilizing a nano ion probe (NanoSIMS). The method comprises the following steps: (1) synthesizing an olivine reference substance with a coverage Fo value (magnesium-iron ratio) range of 0-90 by using a high-temperature and high-pressure experimental method, and verifying composition uniformity and isotope stability of the olivine reference substance through an electron probe (EMPA) and a multi-receiving inductively coupled plasma mass spectrometry (MC-ICP-MS); (2) optimizing NanoSIMS instrument parameters, and realizing synchronous detection of 24Mg < + >, 26Mg < + >, 28Si < + > and 56Fe < + > through a static multi-collection mode; (3) based on the ratio of 24Mg / (24Mg + 56Fe), proposing a biphase Hill equation to correct the matrix effect, and realizing online correction; and (4) analyzing the geological sample by using the method, and revealing the chemical and isotope change characteristics of the micron scale. The method has high resolution (1 [mu] m) and high precision (0.4 per thousand to 0.6 per thousand), and is suitable for micro-area isotope analysis in the fields of planetary science and geochemistry.

Description

Technical Field

[0001] The invention relates to a method for accurately measuring magnesium isotopes in micro-areas of olivine using a nano-ion probe. Background Art

[0002] Olivine is one of the most common rock-forming minerals in nature, and is widely found in igneous rocks, metamorphic rocks, and some sedimentary rocks. Due to its sensitivity to changes in temperature and pressure conditions, the magnesium isotope and trace element composition in olivine is widely used to study geological processes, magma evolution, and material circulation in the deep earth. Olivine usually contains some trace and trace elements, such as Fe, Ni, Cr, Ca, etc. The content of these elements combined with the composition of magnesium isotopes can help study the conditions of olivine formation, the diagenetic environment, and the evolution of magma, and thus provide important basis for understanding the composition and evolution of the crust and mantle.

[0003] The trace element and magnesium isotope analysis methods of olivine are mainly divided into two categories: one is to conduct in-situ analysis of natural olivine and its associated host rock to measure the content of trace elements and magnesium isotope composition in olivine; the other is to analyze laboratory-synthesized olivine, obtain its isotope and trace element composition by precisely controlling the conditions, and use it to calibrate and standardize the analysis of natural samples. Traditional measurement methods usually use secondary ion mass spectrometry (SIMS) or laser ablation plasma mass spectrometry (LA-MC-ICP-MS) to perform in-situ analysis on olivine and its associated rocks or melt glass. These methods use a primary ion source or laser beam to ablate the sample surface, and the detection device receives the ionized product, and calibrates with the standard sample to obtain the trace element and isotope composition of olivine.

[0004] However, traditional methods have significant limitations in analyzing trace element and isotope composition of olivine, mainly in the following aspects:

[0005] (1) Interference from inclusions: Olivine has a complex crystal structure and usually contains tiny inclusions. The concentrations of certain trace elements (such as Cr, Ni, etc.) in these inclusions may be several orders of magnitude higher than that in the olivine matrix. Traditional SIMS or LA-MC-ICP-MS measurement methods cannot avoid the erosion of inclusions during the analysis process, resulting in a large deviation between the measurement results and the actual values, thus affecting the accuracy and repeatability of the data.

[0006] (2) Influence of zoning structure: Obvious zoning structures are often present in olivine, which are formed by changes in physical and chemical conditions during the growth of the crystal. These zoning structures can be observed using backscattered electron imaging (BSE) or cathode luminescence (CL) imaging techniques. The distribution of trace elements in zonings of different brightness usually differs significantly. However, the spatial resolution of traditional measurement methods is limited, making it difficult to accurately locate a specific zoning area for analysis, which also introduces uncertainty into the measurement results.

[0007] (3) Limitation of spatial resolution: The spatial resolution of current SIMS and LA-MC-ICP-MS is usually greater than 10 microns, which is insufficient for the analysis of olivine in some extraterrestrial materials (such as lunar samples and meteorite samples). In these samples, the Fo value changes of olivine crystals are often concentrated on the micron scale. Traditional analysis methods cannot effectively resolve such small-scale isotope and element changes, which limits the study of magmatic processes and thermal history.

[0008] In view of the above problems, the present invention has developed a method for analyzing magnesium isotopes in olivine based on a high-resolution nano-ion probe (NanoSIMS). By establishing a new matrix matching standard and an online calibration procedure, the accurate determination of magnesium isotopes and trace elements in olivine at the micrometer scale is achieved. The method uses an upgraded radio frequency oxygen source, combined with a static multi-collection mode, to simultaneously detect multiple isotopes and elements, achieving high spatial resolution and analysis accuracy. By applying this method to the Chang'e 5 lunar samples, the isotopic and chemical zoning characteristics of olivine crystals at the micrometer scale were successfully revealed, providing a new scientific basis for understanding the lunar magma evolution process and reconstructing the thermal history of lunar volcanism. Summary of the invention

[0009] The present invention relates to a method for accurately measuring magnesium isotopes in olivine in micro areas by using a nano-ion probe, and is particularly applicable to micrometer-scale chemical and isotopic analysis of earth and lunar samples. The method has the characteristics of high resolution, high precision and low matrix effect. The method comprises the following steps:

[0010] (1) Synthesis of olivine reference material:

[0011] Olivine reference materials covering the Fo value (magnesium-iron ratio) range of 0 to 90 were prepared through high-temperature synthesis experiments to simulate the compositional variation of natural olivine. Fo0 (fayalite) and Fo100 (forsterite) were mixed in different proportions and synthesized under controlled oxygen fugacity conditions, followed by high-temperature and high-pressure hot pressing sintering and annealing to improve crystal uniformity. These reference materials were verified by electron probe (EMPA) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to ensure their uniform composition and isotopic stability, providing standardized samples for subsequent NanoSIMS analysis.

[0012] (2) Optimization of NanoSIMS instrument:

[0013] During the NanoSIMS analysis, by optimizing the instrument parameters, including using a radio frequency plasma ion source to generate a primary ion beam (current of about 10pA) and adopting a static multi-collection mode, the simultaneous detection of 24Mg+, 26Mg+, 28Si+ and 56Fe+ was achieved, with a mass resolution of 5000. This improvement effectively improved the signal-to-noise ratio and spatial resolution of the analysis, achieving sub-micron resolution, and enabling accurate isotope ratio data to be obtained at the micron scale.

[0014] (3) Matrix effect correction:

[0015] Matrix effect is an important issue in magnesium isotope analysis of olivine. Due to the diversity of Fo values, traditional correction methods often find it difficult to obtain consistent correction results. Based on the 24Mg / (24Mg+56Fe) ratio measured by NanoSIMS, the present invention uses a two-phase Hill equation to correct the matrix effect to achieve online correction. The fitting method of the two-phase Hill equation can handle the non-monotonic trend of magnesium isotopes between different Fo values, thereby obtaining higher calibration accuracy and data consistency.

[0016] The method of the present invention can be used not only for laboratory synthesized olivine samples, but also for the analysis of natural geological samples. This method was used to perform in-situ measurements of magnesium isotopes in olivine in the lunar samples returned by Chang'e 5, and significant isotopic zoning characteristics were found inside the crystals. On a micrometer scale, the δ26Mg value changes from +3‰ to +4‰ at the edge of the crystal to -2‰ to -1‰ inside the crystal. This significant isotopic change far exceeds the range of changes that can be detected by current whole-rock analysis. The results show that the chemical zoning within the olivine crystal is formed by the diffusion process of magnesium and iron caused by the chemical gradient between the crystal and the melt, which provides important information for understanding the formation and evolution of lunar magma.

[0017] The nano-ion probe olivine magnesium isotope micro-analysis method of the present invention can achieve high-resolution and high-precision analysis of natural olivine and synthetic olivine samples, effectively overcoming the matrix effect and spatial resolution limitations of traditional SIMS and laser ablation methods. The method is suitable for a wide range of application scenarios including terrestrial igneous rocks, metamorphic rocks, and extraterrestrial materials (such as lunar and Martian samples), and can provide a scientific basis for studying geological processes, magma evolution, and the thermal history of extraterrestrial bodies. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 :Flow chart of the method for accurate determination of magnesium isotopes in micro-areas of olivine using nano-ion probe;

[0019] Figure 2 : Relationship between IMF*(RM-Fo30)(‰) and the Fo value after NanoSIMS correction (using two-phase Hill fitting) (a), and the residual diagram of the two-phase Hill equation fitting of IMF* (b);

[0020] Figure 3 (a) Backscattered electron (BSE) image of basaltic debris from the Chang'e-5 lunar soil sample. The analysis area (filled with yellow boxes) and analysis lines (white arrow lines) are marked in the figure. (b) Fo profile measured by NanoSIMS, (c) δ26Mg profile measured by NanoSIMS. DETAILED DESCRIPTION

[0021] The invention relates to a method for accurately measuring magnesium isotopes in micro-areas of olivine using a nano-ion probe (NanoSIMS), which is particularly suitable for the analysis of geological samples. The following are the specific implementation steps of the invention.

[0022] (1) Synthesis of olivine reference materials covering the Fo value range of 0 to 90. First, Fayalite (Fo0, fayalite) and Forsterite (Fo100, forsterite) were prepared by high-temperature synthesis experiments, synthesized from Fe2O3 and SiO22 and Mg2SiO4, respectively. Fayalite and Forsterite were then mixed in different proportions to prepare samples covering the Fo value range of 0 to 90. Next, olivine was synthesized in a high-temperature environment with controlled oxygen fugacity. The synthesis process was carried out in an atmospheric pressure furnace. The oxygen fugacity was controlled by a CO-CO2 mixture, which was equivalent to a Ni / NiO buffer to ensure the consistency of the composition of the synthesized material. After the olivine was synthesized, it was placed in a high-temperature and high-pressure equipment for hot pressing and sintering and annealing to improve the uniformity of the crystal.

[0023] (2) Use electron probe (EMPA) to verify the composition uniformity of the reference material. The specific operation is to use an electron probe to analyze synthetic or natural olivine samples and determine the content of major elements (such as Mg, Fe, Si) and minor elements (such as Ti, Al, Ca). During the analysis, natural and synthetic standard samples are used for calibration, and the ZAF program is used for matrix correction. The typical error of the major elements in the sample is controlled within 1.5% (when the content is greater than 1.0wt.%), and the typical error of the minor elements is controlled within 5.0% (when the content is less than 1.0wt.%). Calculate the element content of each area in the sample, and evaluate its composition uniformity based on the standard deviation. If the uniformity meets the requirements, the sample can be used for further analysis or correction.

[0024] (3) Isotopic homogeneity was verified by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The sample powder (0.5-2 mg) was dissolved in concentrated HF-HNO3, refluxed with concentrated HCl-HNO3 and HNO3 in sequence, and finally dissolved in 1M HNO3 for magnesium separation. The cation exchange chromatography was performed twice using Bio-Rad AG50W-X8 resin to obtain a pure magnesium solution. The magnesium isotopes were measured using the critical mixture double spike technique on the Neptune Plus MC-ICP-MS and expressed by the δ26Mg value. The results were calibrated relative to the DSM3 standard. Each sample solution was analyzed 4 times, and the results were expressed as the mean value and two times the standard error. The long-term repeatability was better than ±0.03‰. The reference material BHVO-2 was used for calibration, and the measurement results were consistent with those in the literature. In addition, the δ26Mg of two olivine samples with the same Fo value and different annealing times were measured. The results showed that their isotopic compositions were consistent within the uncertainty range, indicating that the standard material is homogeneous.

[0025] (4) Optimize the NanoSIMS instrument parameters and achieve simultaneous detection of 24Mg+, 26Mg+, 28Si+ and 56Fe+ through the static multi-collection mode. Using the CAMECA NanoSIMS 50L instrument, an O-primary ion beam was generated by a radio frequency plasma ion source with a current of approximately 10pA and a beam spot diameter of approximately 250nm. In the static multi-collection mode, 4 electron multipliers (EMs) were used to simultaneously detect 24Mg+, 26Mg+, 28Si+ and 56Fe+ with a mass resolution of approximately 5000. A preliminary 5×5μm 2 Secondary ion images of the area were obtained by selecting the region of interest based on the ion image and focusing the primary ion beam to 1 × 1 μm 2The point analysis consists of about 250 cycles, each lasting 0.541 seconds, to obtain stable isotope signals and achieve high-precision isotope ratio measurements.

[0026] (5) Based on the 24Mg / (24Mg+56Fe) ratio, the two-phase Hill equation is used to correct the matrix effect and achieve online correction. The 26Mg / 24Mg ratio in the sample is measured by NanoSIMS, and according to the international standard DSM3 definition, the formula is:

[0027] δ 26 Mg m (‰)=[( 26 Mg / 24 Mg) measured / ( 26 Mg / 24 Mg) DSM3 -1]×1000

[0028] For each reference material, the instrumental mass fractionation correction (IMF) value was calculated based on the difference between the δ26Mg value measured by NanoSIMS and the value measured by MC-ICP-MS, using the formula:

[0029] IMF(‰)=[(1+δ 26 Mg m / 1000) / (1+δ 26 Mg MC-ICP-MS / 1000)-1]×1000

[0030] The IMF value was normalized to the Fo30 standard substance, and the biphasic Hill equation was used to fit and correct the Fo value determined by IMF and NanoSIMS. The formula is:

[0031]

[0032] , where y represents IMF*(RM-Fo30), x is the Fo value measured by NanoSIMS, and the fitting correlation coefficient is R 2 =0.97. The NanoSIMS measurement data were corrected using the two-phase Hill equation, the deviation of the corrected data was analyzed, and its consistency was verified within the Fo value range of 30.6 to 90.8. The corrected data had a smaller deviation than the recommended value, and the analytical uncertainty was about 0.7 to 0.8‰ (1SD), which included internal error, point-to-point repeatability error, and fitting residual.

[0033] (6) The above method was applied to analyze the olivine in the Chang'e-5 lunar sample, revealing its chemical and isotopic variation characteristics at the micrometer scale. NanoSIMS analysis of the olivine in the Chang'e-5 lunar sample was performed to measure the micro-area distribution of its magnesium isotopes. The results showed that the δ26Mg value varied significantly at the edge and inside of the crystal, from +3‰ to +4‰ to -2‰ to -1‰. This significant isotopic variation far exceeds the range of variation that can be detected by current whole-rock analysis. Through analysis, it was determined that these chemical zoning characteristics are due to the chemical gradient caused by the diffusion of magnesium and iron between the crystal and the melt. This result provides important information for studying the formation and evolution of lunar magma.

Claims

1. A method for accurately measuring magnesium isotopes in micro-areas of olivine using a nano-ion probe, characterized in that: The method comprises the following steps: (1) Synthesize olivine reference materials covering the Fo value range of 0 to 90 and verify their compositional homogeneity and isotopic stability; (2) Optimize the NanoSIMS instrument parameters to achieve simultaneous detection of 24Mg+, 26Mg+, 28Si+, and 56Fe+ through static multi-collection mode; (3) Based on the 24Mg / (24Mg+56Fe) ratio, the two-phase Hill equation is used to correct the matrix effect and achieve online correction; (4) Apply the above method to analyze geological samples to reveal the chemical and isotopic variation characteristics at the micrometer scale.

2. The method according to claim 1, characterized in that: The synthesis of the olivine reference material comprises: (1) Preparation of Fayalite (Fo0) and Forsterite (Fo100); (2) Mixing Fayalite and Forsterite in different proportions to generate samples covering different Fo values; (3) Synthesizing olivine under high temperature conditions with controlled oxygen fugacity; (4) Improve the uniformity of the crystal through high temperature and high pressure hot pressing sintering and annealing treatment.

3. The method according to claim 1, characterized in that: The electron probe (EMPA) verification of the homogeneity of the reference material comprises the following steps: (1) Analyze the content of major elements (such as Mg, Fe, Si) and minor elements (such as Ti, Al, Ca); (2) Calibration was performed using natural and synthetic standard samples and matrix correction was performed using the ZAF program; (3) Evaluate the uniformity of element content in each area to ensure that the typical error of major elements is within 1.5% and that of minor elements is within 5.0%.

4. The method according to claim 1, characterized in that: The matrix effect was corrected using the two-phase Hill equation, which includes the following steps: (1) Calculate the 26Mg / 24Mg ratio in the NanoSIMS sample, expressed as δ26Mg value and calibrated based on the international standard DSM3; (2) Calculate the instrumental mass fractionation correction (IMF) based on the NanoSIMS and MC-ICP-MS measurements of the reference material; (3) The IMF value was fitted with the Fo value measured by NanoSIMS using the two-phase Hill equation to obtain the corrected IMF value.

5. The method according to claim 1, characterized in that The NanoSIMS instrument parameter optimization includes: (1) Using a CAMECA NanoSIMS 50L instrument, a primary ion beam was generated using a radio frequency plasma ion source with a current of approximately 10 pA; (2) Using static multi-collection mode, 24Mg+, 26Mg+, 28Si+ and 56Fe+ were detected simultaneously with a mass resolution of approximately 5000; (3) Get 5×5μm 2 A secondary ion image of the area is taken and a specific area is selected for point analysis based on the image.

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