A method for accurately determining the magnesium isotope micro-region of olivine by a nano-ion probe

Abstract

The present invention discloses a method for accurately determining the Mg isotope microarea of olivine by a nanoion probe and its application to lunar samples, specifically relating to the application of nanoion probe (SIMS) technology in magnesium isotope analysis, especially a method for high-resolution and high-precision magnesium isotope analysis and on-line correction of matrix effects using a nanoion probe (NanoSIMS). The steps include: (1) synthesizing olivine reference materials with a Fo value (magnesium-iron ratio) range of 0 to 90 by a high-temperature and high-pressure experimental method, and verifying their compositional homogeneity and isotope stability through an electron microprobe (EMPA) and a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS); (2) optimizing the NanoSIMS instrument parameters to achieve synchronous detection of 24Mg+, 26Mg+, 28Si+ and 56Fe+ through a static multi-collection mode; (3) proposing a two-phase Hill equation to correct matrix effects based on the 24Mg / (24Mg + 56Fe) ratio to achieve on-line correction; (4) applying this method to analyze geological samples to reveal the chemical and isotope variation characteristics at the micron scale. This method has a high resolution (1 μm) and high precision (0.4‰ - 0.6‰), and is suitable for microarea isotope analysis in the fields of planetary science and geochemistry.

Description

Technical Field

[0001] The present invention relates to a method for accurately determining the magnesium isotope micro-region of olivine by a nano-ion probe. Background Art

[0002] Olivine is one of the most common rock-forming minerals in nature and is widely present 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 compositions in olivine are widely used in the study of geological processes, magma evolution, and deep-earth material cycling. Olivine usually contains some trace and ultra-trace elements such as Fe, Ni, Cr, Ca, etc. The combination of the contents of these elements and the magnesium isotope composition can help study the conditions at the time of olivine formation, the diagenetic environment, and the evolution process of magma, and thus provide an important basis for understanding the composition and evolution of the crust and mantle.

[0003] The analysis methods for trace elements and magnesium isotopes in olivine are mainly divided into two categories: one is in-situ analysis of natural olivine and its associated host rock to measure the content of trace elements and the magnesium isotope composition in olivine; the other is the analysis of laboratory-synthesized olivine, and the isotope and trace element compositions are obtained by precisely controlling conditions for calibrating and standardizing the analysis of natural samples. Traditional measurement methods usually use secondary ion mass spectrometry (SIMS) or laser ablation inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for in-situ analysis on olivine and its associated rocks or melt glasses. These methods ablate the sample surface by a primary ion source or a laser beam, the detection device receives the ionized products, and the trace element and isotope compositions of olivine are obtained by calibration with a standard sample.

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

[0005] (1) Interference from inclusions: The crystal structure of olivine is complex and usually contains fine inclusions. The concentrations of some trace elements (such as Cr, Ni, etc.) in these inclusions may be several orders of magnitude higher than those in the olivine matrix. Traditional SIMS or LA-MC-ICP-MS measurement methods are difficult to avoid eroding the 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 the zoning structure: Obvious zoning structures often exist in olivine. These zonings are formed due to changes in physical and chemical conditions during crystal growth. These zoning structures can be observed through backscattered electron images (BSE) or cathodoluminescence (CL) imaging techniques. On zonings with different brightness levels, the distribution of trace elements usually shows significant differences. However, the spatial resolution of traditional measurement methods is limited, making it difficult to accurately locate specific zoning regions for analysis, which also introduces uncertainties into the measurement results.

[0007] (3) Limitations of spatial resolution: The current spatial resolution of SIMS and LA-MC-ICP-MS is usually greater than 10 micrometers, which is insufficient for the analysis of olivine in some extraterrestrial materials (such as lunar samples and meteorite samples). In these samples, the variation of the Fo value of olivine crystals often concentrates on the micrometer scale. Traditional analysis methods cannot effectively resolve such small-scale isotope and element variations, limiting 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, accurate determination of magnesium isotopes and trace elements in olivine at the micrometer scale has been achieved. This method uses an upgraded radio frequency oxygen source and combines 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 isotope and chemical zoning characteristics of olivine crystals at the micrometer scale have been successfully revealed, providing new scientific basis for understanding the lunar magmatic evolution process and reconstructing the thermal history of lunar volcanism. Summary of the Invention

[0009] The present invention relates to a method for accurately determining micro-area magnesium isotopes in olivine by a nano ion probe, especially applied to micrometer-scale chemical and isotope analysis of terrestrial and lunar samples. It has the characteristics of high resolution, high precision, and low matrix effect. The method includes the following steps:

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

[0011] Olive reference materials with Fo values (Mg / Fe ratios) ranging from 0 to 90 were prepared through high-temperature synthesis experiments to simulate the compositional variations of natural olivine. Fo0 (fayalite) and Fo100 (forsterite) were mixed in different proportions and synthesized under controlled oxygen fugacity conditions. Subsequently, hot-pressing sintering and annealing treatments under high temperature and high pressure were carried out to improve crystal uniformity. These reference materials were verified by electron microprobe (EMPA) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to ensure their compositional homogeneity and isotopic stability, providing standardized samples for subsequent NanoSIMS analysis.

[0012] (2) Optimization of the 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 approximately 10 pA) 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 enhanced the signal-to-noise ratio and spatial resolution of the analysis, reaching a sub-micron resolution ability and enabling the acquisition of accurate isotope ratio data at the micron scale.

[0014] (3) Matrix effect correction:

[0015] The matrix effect is an important issue in the analysis of magnesium isotopes in olivine. Due to the diversity of Fo values, traditional correction methods often struggle to obtain consistent correction results. Based on the 24Mg / (24Mg + 56Fe) ratio measured by NanoSIMS, the present invention uses the two-phase Hill equation to correct the matrix effect for 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 achieving higher calibration accuracy and data consistency.

[0016] The method of the present invention can not only be used for olivine samples synthesized in the laboratory but also applies to the analysis of natural geological samples. In the lunar samples returned by Chang'e-5, this method was used to in-situ measure the magnesium isotopes of olivine, and significant isotopic zoning characteristics were found inside the crystals. At the micron scale, the δ26Mg value varies from +3‰ to +4‰ at the crystal edge to -2‰ to -1‰ inside the crystal. This significant isotopic variation far exceeds the detection range of current whole-rock analysis. The results indicate that the chemical zoning within olivine crystals is formed by the diffusion process of magnesium and iron caused by the chemical gradient between the crystal and the melt, providing important information for understanding the formation and evolution of lunar magmas.

[0017] Through the nano-ion probe olivine magnesium isotope micro-area analysis method of the present invention, high-resolution and high-precision analysis of natural olivine and synthetic olivine samples can be achieved, effectively overcoming the matrix effect and spatial resolution limitations of traditional methods such as SIMS and laser ablation. This method is applicable to a wide range of application scenarios including terrestrial magmatic 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 celestial bodies. Brief Description of the Drawings

[0018] Figure 1 : Flow chart of the method for accurately determining the olivine magnesium isotope in the nano-ion probe micro-area;

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

[0020] Figure 3 . (a) Backscattered electron (BSE) image of basaltic clasts in the Chang'e-5 lunar soil sample. The analysis area (filled yellow box) and analysis line (white arrow line) are marked in the figure. (b) Fo profile measured by NanoSIMS, (c) δ26Mg profile measured by NanoSIMS. Detailed Embodiment

[0021] The invention relates to a method for accurately determining the olivine magnesium isotope in a micro-area by 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 present invention.

[0022] (1) Synthesize olivine reference materials covering the Fo value range from 0 to 90. First, prepare Fayalite (Fo0, fayalite) and Forsterite (Fo100, forsterite) through high-temperature synthesis experiments, synthesized from Fe2O3 and SiO22 and Mg2SiO4 respectively. Then mix Fayalite and Forsterite in different proportions to prepare samples covering the Fo value range from 0 to 90. Next, carry out the synthesis of olivine in a high-temperature environment with controlled oxygen fugacity. The synthesis process is carried out in a one-atmosphere furnace, and the oxygen fugacity is controlled by a CO-CO2 mixture, equivalent to the Ni / NiO buffer zone, to ensure the consistency of the composition of the synthesized material. After synthesizing the olivine, put it into a high-temperature and high-pressure device for hot pressing sintering and annealing treatment to improve the uniformity of the crystals.

[0023] (2) Verify the compositional homogeneity of the reference material using an electron microprobe (EMPA). The specific operation is to analyze synthetic or natural olivine samples using an electron microprobe, and determine the contents 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 major elements in the sample is controlled within 1.5% (when the content is greater than 1.0 wt.%), and the typical error of minor elements is controlled within 5.0% (when the content is less than 1.0 wt.%). Calculate the elemental contents of each region in the sample, and evaluate its compositional homogeneity based on the standard deviation. If the homogeneity meets the requirements, the sample can be used for further analysis or calibration.

[0024] (3) Verify the isotopic homogeneity by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Dissolve the sample powder (0.5 - 2 mg) in concentrated HF-HNO3, and reflux successively with concentrated HCl-HNO3 and HNO3, and finally dissolve it in 1M HNO3 for magnesium separation. Cation exchange chromatography using Bio-Rad AG50W-X8 resin is carried out twice for separation to obtain a pure magnesium solution. Measure magnesium isotopes on a Neptune Plus MC-ICP-MS using the critical mixture double spike technique, and represent the results by the δ26Mg value, and the results are calibrated relative to the DSM3 standard. Each sample solution is analyzed 4 times, and the results are expressed as the mean and twice the standard error, and the long-term repeatability is better than ±0.03‰. Use the reference material BHVO-2 for calibration, and the measurement results are consistent with the literature. In addition, measure the δ26Mg of two olivine samples with the same Fo value but different annealing times, and the results show that their isotopic compositions are consistent within the uncertainty range, indicating the homogeneity of the reference material.

[0025] (4) Optimize the NanoSIMS instrument parameters and achieve the simultaneous detection of 24Mg+, 26Mg+, 28Si+ and 56Fe+ through the static multi-collection mode. Use a CAMECA NanoSIMS 50L instrument to generate an O- primary ion beam with a current of about 10 pA and a beam spot diameter of about 250 nm through a radio frequency plasma ion source. In the static multi-collection mode, use 4 electron multipliers (EMs) to simultaneously detect 24Mg+, 26Mg+, 28Si+ and 56Fe+, and the mass resolution is about 5000. Initially obtain the secondary ion image of a 5×5 μm 2 area, select the region of interest according to the ion image, and focus the primary ion beam to 1×1 μm 2Point analysis is performed on small regions, and the point analysis includes approximately 250 cycles, each cycle lasting 0.541 seconds, to obtain a stable isotope signal and achieve high-precision isotope ratio measurement.

[0026] (5) Based on the 24Mg / (24Mg + 56Fe) ratio, the matrix effect is corrected using the biphasic Hill equation to achieve online correction. The 26Mg / 24Mg ratio in the sample is measured by NanoSIMS. According to the definition of the international standard DSM3, 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 is calculated based on the difference between the δ26Mg value measured by NanoSIMS and the value measured by MC-ICP-MS. The formula is:

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

[0030] The IMF value is normalized to the Fo30 reference material, and the biphasic Hill equation is used to fit and correct the IMF and the Fo value measured by 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 biphasic Hill equation is used to correct the NanoSIMS measurement data, analyze the deviation of the corrected data, and verify its consistency within the Fo value range of 30.6 to 90.8. The corrected data has a smaller deviation compared to the recommended value, and the analysis uncertainty is approximately 0.7 to 0.8‰ (1SD). This uncertainty includes internal error, inter-point repeatability error, and fitting residuals.

[0033] (6) Apply the above method to analyze olivine in the Chang'e-5 lunar samples, revealing its chemical and isotopic variation characteristics at the micron scale. By performing NanoSIMS analysis on olivine in the Chang'e-5 lunar samples and measuring the microzone distribution of its magnesium isotopes, the results show that the δ26Mg values vary significantly at the crystal edge and inside, ranging from +3‰ to +4‰ to -2‰ to -1‰. This significant isotopic variation far exceeds the detection range of current whole-rock analysis. Through analysis, it is determined that these chemical zoning characteristics are caused by the chemical gradient resulting from the diffusion of magnesium and iron between the crystal and the melt. This result provides important information for studying the formation and evolution process of lunar magmas.

Claims

1. A method for accurately determining the magnesium isotope micro-region of olivine by a nano-ion probe, characterized in that, Including the following steps: (1)Synthesis of olivine reference materials with Fo values ranging from 0 to 90: First, prepare Fo0 and Fo100 separately, mix them in different proportions, and synthesize them in a high-temperature environment with controlled oxygen fugacity; then, hot-press sinter and anneal them under high-temperature and high-pressure conditions to improve crystal uniformity; analyze the contents of Mg, Fe, Si, Ti, Al, and Ca by electron probe, and use natural and synthetic standards to correct based on the ZAF program to ensure that the typical error of major elements is ≤1.5% and that of minor elements is ≤5.0%; dissolve the sample in concentrated HF-HNO3, reflux with concentrated HCl-HNO3 and HNO3 in sequence, and obtain a pure Mg solution by separating it twice with AG50W-X8 resin. Determine δ 26 Mg by the double-labeling method of MC-ICP-MS to verify isotope stability; (2) Using CAMECA NanoSIMS 50L, a radio frequency plasma O-ion source generates a primary beam with a current of 10 pA and a beam spot of 250 nm. Four-channel electron multipliers are synchronously detected in the static multi-collection mode. 24 Mg + 、 26 Mg + 、 28 Si + 、 56 Fe + ; First, obtain a 5×5 μm 2 secondary ion image, and then focus to 1×1 μm 2 for 250 cycles of point analysis; (3) Based on the 26 Mg / 24 Mg ratio, calculate δ 26 Mg, calibrated against the DSM-3 standard, and calculate the instrumental mass fractionation correction factor IMF based on the difference between the δ 26 Mg measured by NanoSIMS and the MC-ICP-MS results for the reference material; (4) Fitting the Fo values measured by IMF and NanoSIMS according to the biphasic Hill equation to obtain the IMF after biphasic online correction, so as to achieve high-precision correction of the matrix effect of the sample; (5) Applying the above process to conduct in-situ micro-area analysis of olivine in geological and Chang'e-5 lunar return samples, and revealing its chemical and magnesium isotope distribution characteristics at the micron scale.

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