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"First lossless, then micro-loss", "first single particles, then micro-nano scale, and finally atomic level", "first focus on the surface, then develop the internal structure".
As one of the first scientific research units to obtain lunar soil samples, the Institute of Geology and Geophysics of the Chinese Academy of Sciences set up a scientific research team at the first time, united with the National Astronomical Observatory of the Chinese Academy of Sciences, the Institute of High Energy Physics and the Institute of Physics, etc., to coordinate the research and development of related technologies, and established a set of effective workflows for the comprehensive analysis of single particles of lunar soil return samples (Figure 1), which has important reference significance for the subsequent Chang'e 6 lunar soil return and asteroid sample return.
The research results were published in Geoscience Frontiers, sponsored by China University of Geosciences (Beijing) and Peking University (Li Jinhua, Li Qiuli, Zhao Liang, Zhang Jinhai, Tang Xu, Gu Lixin, Guo Qian, Ma Hongxia, Zhou Qin, Liu Yan, Liu Peiyu, Qiu Hao, Li Gang, Gu Lin, Guo Shun, Li Chunlai, Li Xianhua, Wu Fuyuan, Pan Yongxin. Rapid screening of Zr-containing particles fromChang’e-5 lunar soil samples for isotope geochronology: Technical roadmap forfuture study. Geoscience Frontiers, 2022, 13(3): 101367. https://doi.org/10.1016/j.gsf.2022.101367）。
Figure 1. Chang'e-5 lunar soil and future planets return samples of single particles
Comprehensive analysis workflow diagram
A "six-step" workflow for the "six-step" workflow for the integrated analysis of single-particle integrated analysis of samples returned from Chang'e-5 lunar soil and future planets (brief version).
Step-1: After microscopic sampling, use XRF to quickly select the target particle and prepare the sample on demand.
Step-2: 3D-XRM/FIB-SEM combined analysis to obtain 3D topography, structure and composition information.
Step-3: SEM comprehensive analysis to obtain surface morphology, structure and chemical composition information.
Step-4: Comprehensive micro-region analysis to obtain information such as morphology, structure, mineral facies, and chemical composition.
Step-5: FIB-SEM 3D reconstruction analysis and precision micro-cutting to prepare micro-nano size "flake" or "needle tip" samples.
Step-6: Comprehensively use STXM, TEM and APT technologies to perform comprehensive analysis of morphology, structure, mineral phase, chemical composition, elemental valence, elemental isotopes and micromagnetism on "flake" or "needle tip" samples.
On December 17, 2020, China's Chang'e-5 brought back 1731 g of lunar soil samples from the moon. This is the first time That China has collected samples from extraterrestrial objects, and it is also the second time that humans have brought lunar soil back to Earth after a gap of 44 years. Chang'e-5 successfully completed the final step of the three-step strategy of "orbiting, falling and returning" of the mainland lunar exploration project, enabling Chinese scientists to have their own extraterrestrial object return samples for the first time, which is a milestone event in the history of planetary science development.
On July 12, 2021, the first batch of lunar soil samples were officially distributed to 13 domestic scientific research institutions. Up to now, 3 batches of lunar soil samples have been issued, and more than 30 domestic scientific research units have obtained a total of 44.8577g samples, and a series of scientific research work has been carried out successively. Lunar soil samples are extremely valuable, mostly submillimeter and micron-sized particles. How to use limited precious samples to obtain as much basic data as possible, while carrying out efficient and high-quality scientific research, poses a huge challenge to mainland researchers, which is also the premise of obtaining major original results. In the next decade, China has laid out a series of major tasks such as Chang'e 6 lunar south pole sampling, asteroid sampling and Mars sampling. There is no doubt that with the continuous deepening of the study of chang'e-5 lunar soil samples and the continuous return of more types of extraterrestrial object samples, China's planetary science will usher in a new era. Formulating reasonable scientific goals, establishing efficient work processes, giving priority to small sample research without loss or small loss, and focusing on multi-scale and multi-parameter comprehensive research of single particle samples are the objective needs of carrying out research on precious extraterrestrial object samples, and also the general trend of future development of planetary science.
The Chang'e-5 lunar soil sample comes from the northeastern part of the lunar storm ocean and belongs to the mid-latitudes where humans have never "set foot" on it. Determining age is the basis for the use of Chang'e 5 lunar soil samples to carry out research on major scientific issues such as "lunar magma activity and evolution, lunar generator and magnetic field evolution, and space weathering", and can also provide a key anchor for the correction of the statistical dating curve of impact craters, so it is the primary scientific issue. Precisely selecting target samples suitable for dating from thousands of monthly soil particles and accurately exposing minerals suitable for in situ dating in microregions from target minerals is the key to solving this primary problem.
Rapid screening of zirconium-rich particles and precise localization of zirconium-containing minerals
In view of the above problems, after the joint research of multiple research groups, the researchers finally established two sets of feasible schemes for the rapid screening of zirconium-rich (Zr) particles in the Chang'e-5 lunar soil sample and the accurate positioning of zirconium-containing minerals in the target particles.
Solution 1: Fast positioning of multiple particles
Using single-particle micromanipulation techniques, micron-sized lunar soil samples are prepared into an array of single particles. The sample array was then analyzed by a "non-destructive, contact-free, and contamination-free" rapid scan using micro-X-ray fluorescence spectroscopy (XRF) to obtain images of chemical elements and their distributions of all particles, combined with spectroscopic semi-quantitative analysis techniques, to select the zirconium-rich target particles (Figure 2, such as P6 and P17). After the selected zirconium-rich particles are resin-embedded, single-particle mechanical polishing is combined with cross-section scanning electron microscopy observation (SEM) to finally expose zirconium-containing minerals to the flat cross-section of the sample, and subsequent secondary ion mass spectrometry probe (SIMS) micro-region in situ isotope analysis is performed to obtain the high-precision age of the sample. Using this scheme, the researchers selected 47 single-particle target samples from the first lunar soil samples within 24 hours after obtaining them, and accurately located 51 zirconium-containing minerals to obtain the age of 2030±4 Ma (Li QL et al., 2021, Nature 600, 54-58) formed in chang'e 5 lunar soil.
Figure 2. Micro X-ray fluorescence spectroscopy to quickly screen zirconium-rich particles
Figure 3. The combined analysis of the target particle "mechanical polishing-scanning electron microscopy" identifies zirconium-containing minerals
Scheme 2: Precise positioning of single particles
Although the first scheme works, the mechanical polishing process cannot be visualized, and there is a risk of premature grinding or damage to the target mineral (such as oblique zircon, etc.). So, in the second protocol, the researchers combined XRF's imaging of chemical elements with three-dimensional X-ray microscopy (3D-XRM) structural imaging analysis to pinpoint zirconium-rich minerals from the selected zirconium-rich target particles (Figure 4). Then, using virtual digitization and 3D-XRM and focused ion beam-scanning electron microscopy (FIB-SEM) 3D slicing techniques, target particles in zirconium-rich minerals are then exposed to a flat cross-section (Figure 5) for subsequent SIMS micro-region in situ isotope analysis. The FIB cutting process of the entire sample can also be recorded in stages by SEM technology, thereby partially retaining the three-dimensional structure and composition information of the sample.
Figure 4. XRF/3D-XRM are used in conjunction to precisely locate zirconium-containing minerals in the target sample
Fig. 5.3D-XRM/FIB-SEM combined with microcutting and precise preparation of a level section of zirconium-containing minerals
Single-particle comprehensive analysis of samples returned from the Chang'e-5 lunar soil and future planets
Although the Chang'e-5 lunar soil sample is small, each small particle is equivalent to an independent small rock block to some extent, and its mineral composition, surface morphology, internal structure and chemical composition are rich in information about "lunar evolution and space weathering". Just as zirconium-rich particles are selected for isotope chronology, the classification and selection of lunar soil particles can also purposefully carry out other scientific studies. Compared with microbeam analysis methods such as electron beam and ion beam, XRF relies on X-ray analysis technology, which has strong penetration, high chemical sensitivity, and does not require complex pretreatment of the sample and its surface, so it can quickly obtain the chemical element composition of lunar soil particles and the distribution characteristics of various elements at the micron scale, which can be used to select various types of target particles. Whether mechanical polishing is combined with SEM, or XRM is combined with FIB-SEM, it can expose the mineral of interest to a flat cross-section, suitable for SIMS (elemental and isotopic analysis at micron resolution) and for microanalysis at different scales and different types, such as microman (raman, micron resolution compound and mineral phase identification), SEM (morphology, structure and composition analysis of nano resolution), electronic probe (EPMA, nano resolution master, Quantitative analysis of trace elements), nano-secondary ion mass spectrometry (NanoSIMS, elemental and isotope analysis at sub-micron to nano resolution), can also be used directly for precise micro-cutting of FIB-SEM, preparation of micro-nano-sized "flakes" or "pinpoint" samples, for more elaborate synchrotron radiation scanning transmission X-ray microscopy (STXM, chemical composition, elemental valence and magnetic analysis of nano-resolution), transmission electron microscopy (TEM, morphology, structure, composition, composition of sub-nanometer to atomic resolution, Mineral phase and micromagnetic analysis) and atomic probes (APT, elemental and isotopic analysis of atomic resolution) studies.
Based on the commonality of sample selection and subsequent analysis, the researchers proposed a "six-step" workflow chart for the single-particle comprehensive analysis of samples returned by Chang'e-5 lunar soil and future planets (Figure 1).
Step-1: Micro-operate a single-particle sample, prepare it into a sample array, use XRF technology to quickly scan and analyze the target particles, and prepare different types of single-particle samples (e.g., resin embedding, mechanical polishing, surface conductive treatment, etc.) according to subsequent analytical tests.
Step-2: 3D-XRM/FIB-SEM combined analysis of the sample of interest to obtain three-dimensional morphology, structure and composition information of the sample at the micro and nanoscale.
Step-3: SEM comprehensive analysis of the target particle sample to obtain surface morphology, structure and chemical composition information of the sample at the micro and nano scale.
Step-4: Comprehensive micro-region analysis of the target particle cross-section sample (such as SEM, Raman, EPMA, SIMS and NanoSIMS) to obtain information such as the morphology, structure, mineral phase, and chemical composition (including principal quantities, trace elements and their isotopes) of the sample cross-section at the micro and nanoscale.
Step-5: Utilize advanced FIB-SEM technology to perform 3D reconstruction analysis of the micro-regions of interest in the sample of the target particle, as well as precise micro-cutting of it, to prepare micro-nano-sized "flake" or "needle tip" samples.
Step-6: Comprehensively utilize synchrotron radiation STXM, advanced TEM and APT technologies to perform comprehensive analysis of morphology, structure, mineral phase, chemical composition, elemental valence, elemental isotopes and micromagnetism on "flake" or "pinpoint" samples at the nano-to-atom level.
It should be pointed out that the "six-step" workflow proposed in this study does not cover all the technologies required for Chang'e-5 and future planets to return samples, nor is it set in stone and standardized, and it needs to be adjusted and improved according to the characteristics of the sample or specific scientific goals in actual work. For example, as demonstrated by this institute, steps-1, step-2/step-4 can be combined to quickly find zirconium-rich particles and accurately locate zirconium-containing minerals, and carry out research work such as microisotope chronology and geochemistry of samples. Combine step-1, step-3, step-5 and step-6 to select specific types of single-particle samples for research in space weathering, planetary mineralogy and micromagnetism. In addition, the "six-step" workflow proposed in this study, in accordance with the analysis ideas of "first lossless, then micro-loss", "first single particle, then micro-nano scale, and finally atomic level", "first focus on the surface, then carry out the internal structure", the existing various microscopy and microscopy techniques are arranged and combined at the time node of the analysis, which can obtain a variety of information at different scales for the same sample, so it is also suitable for a variety of precious samples on the earth (such as from the early days of the earth, deep or deep sea, etc., which are not easy to come by. a comprehensive study of tiny samples that cannot be reproduced.
The author of this article, Li Jinhua, Ph.D., is currently a researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences, and a doctoral supervisor/director of the Electron Microscopy Laboratory. He graduated from the School of Life Sciences of Northwest University with a bachelor's degree in biology in 2001, a master's degree in microbiology from the State Key Laboratory of Microbiology Technology of Shandong University in 2006, a doctorate degree in solid state geophysics from the Institute of Geology and Geophysics of the Chinese Academy of Sciences in 2010, and a postdoctoral research in geology from 2010 to 2013. From 2012 to 2014, he was a visiting scholar at the Institute of Mineralogy, Materials Physics and Cosmic Chemistry (IMPMC, CNRS-UPMC) of the University of Paris VI in France. He has presided over major projects of national natural science and key international (regional) cooperative research projects. At present, more than 90 SCI articles have been published. Among them, the first author/ newsletter more than 30 articles. He was invited to write review articles in Journals such as Chemical Geology, Atomic Spectroscopy, Science in China, and others. He was awarded the title of "Fu Chengyi Youth Science and Technology Award" of the National Natural Science Foundation of China in 2015 and the "Fu Chengyi Youth Science and Technology Award" of the Chinese Geophysical Society in 2015. His research interests are in biomagnetism. In recent years, it has carried out fruitful work in the field of interdisciplinary and technological research and development, promoted the large-scale interdisciplinary intersection in the fields of paleomagnetism, microbiology, paleontology and geology, with microbial mineralization and microfossil identification as the core, and deeply explored the evolution process of the earth and life with microbial participation and microfossil recording, thus handing over the two most important scientific lines of life evolution and geological evolution, and making pioneering work in the field of biomagnetism and geobiology. On the other hand, it is committed to promoting the discipline development and technology research and development of nano-earth and planetary science, organizing the first symposium on "Microscopy and Microscopy and Its Application in Nano-Earth Science" in China, building an interdisciplinary and interdisciplinary microscopy and microscopy research platform, developing the "fluorescence-electron microscope combination" technology, and realizing microbial species identification and magnetosome biominalization research at the single-cell level for the first time.
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