Abstract:Abstract

Abstract:As the Chinese lunar exploration project prepares for future exploration activities on the Moon, there is a growing need to develop high-fidelity lunar soil simulants. The morphological analysis of lunar soil and its simulant is important for matching the unique properties of the agglutinates. To date, several techniques, including scanning electron, X-ray, and optical microscopies, have been extensively applied to analyze the three-dimensional (3D) morphology of lunar samples. However, none of these tools can acquire the natural color fine 3D microstructure of the samples, which is necessary to analyze components of the lunar meteorite and soil particles. In this letter, we present a high-resolution, natural color 3D tomographic system for the initial analysis of lunar samples. The superior performance of the system is demonstrated by the fine details and color 3D tomography of a lunar meteorite and lunar soil simulant. This method is expected to provide an essential tool for visually presenting the geological evolution of the Moon.

Abstract:The adhesion properties of lunar dust grains are a key to determine its motion state. Atomic force microscopy (AFM) is conducted to evaluate the adhesion properties of such ultrasmall grains. However, an efficient method to measure the adhesion properties of lunar grains has not yet been developed because of the difficulties in eliminating the effect of adsorption of water and gases on the grain surface. In this study, an improved method was proposed to measure the adhesion force of grains while effectively eliminating the gas molecule adsorption effect. In the proposed method, using a focused ion beam, a small grain was mounted onto the tip of an AFM probe then used to measure the adhesion force of the grain. To determine the effects of environmental pressure and temperature, the adhesion force between a silica ball and a silica wafer was measured under different conditions. Based on the results, the gas molecule adsorption effect can be effectively eliminated during adhesion force measurement through AFM at a temperature of 200 ℃ and an environmental pressure of <2.4 ×10?4 Pa, at which strong adhesion of the grain is achieved. The proposed method is suitable for the measurement of adhesion force in lunar grain samples.

Abstract:Glass or amorphous state materials are vital components of lunar regolith and have attracted considerable attention. The analytical data on the texture and structure of glassy and crystallized materials can be used to reconstruct the geological history of the Moon. However, it is often challenging to distinguish glass from crystals based on morphology and elemental composition, especially in complex extraterrestrial samples that have been subjected to significant impact and metamorphism. In this study, micro-X-ray diffraction (μXRD) techniques were used to identify crystalline minerals and glassy phases in lunar breccia samples extracted in the Chang'e-5 mission. The samples were processed using different operating methods to identify a technique that would minimize damage to their original appearance and structure and yield optimal results. The diffraction results, which possessed two-dimensional patterns were classified into dominant dispersion halos, concentric diffraction rings, independent diffraction spots, and coexisting rings and spots. The results correspond to four types of sample states, namely, the amorphous material, polycrystal, single crystal, and a mixture of polycrystals and single crystals. By identifying the crystallization state and phases of various samples in a non-destructive and intuitive manner, μXRD can facilitate in situ analysis of special samples generated during important geological events, thereby promoting the understanding of complex origins and evolution of extraterrestrial bodies.

Abstract:Lunar glass can provide critical information on the genesis of lunar rocks and the evolution of the Moon's interior. Several minor and trace elements in lunar glass, such as Na, K, P, S, Cr, and Ni, can be measured by electron probe microanalysis (EPMA) and are informative for understanding glass type, impact volatilization, and magmatic evolution processes. However, the analytical accuracy and precision of these trace-level elements in glass can be impeded by beam sensitivity, peak shift of X-ray and secondary fluorescence effects. Using EPMA, we constructed an optimized analytical method with high accuracy and precision to analyze trace elements simultaneously with major elements in lunar glass. The method was developed using the CAMECA SXFive EPMA at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The optimal analytical conditions for Na, K, P, S, Cr, and Ni in lunar glass were as follows: 20 kV accelerating voltage, 100 nA beam current, 10 μm beam diameter, linear background mode, using large-area analytical crystals and aggregate counting strategy and a 10-min total counting time. Replicate analyses on basalt and komatiite glass standards show that the analytical results are consistent with the reference values. Variations in the levels of the analyzed trace elements fall within ± 10%. The detection limits (3σ) for Na, K, P, S, Cr, and Ni can be lowered to 17-96 ppm. Our method can prohibit sample damage of lunar glass even after a 10-min analysis time. Therefore, this optimized method can provide precise trace and major element analysis of lunar glass and help to trace its origin.

Abstract:Spinel, an important mineral in basalts and ultramafic rocks on Earth, Mars, and the Moon, is sensitive to petrologic and geochemical processes, and redox evolution. Due to the small grain size of extraterrestrial samples, investigations on the composition of spinel samples including presence of trace elements and ferric iron have been hindered by the lack of appropriate in-situ analytical techniques with high spatial resolution and the shortage of reference materials. This paper presents a combined method of simultaneously measuring the major and trace elements, and Fe3+/∑Fe ratio in spinel samples using electron probe microanalysis (EPMA). Our new EPMA method is performed under double beam condition at a beam current of 200 nA for trace elements (Ti, V, Mn, Co, Ni, and Zn) and 60 nA for major elements (Mg, Fe, Al, and Cr) with an acceleration voltage of 25 kV. In addition, large analyzing crystals and peak overlap corrections were applied to reduce the detection limits and improve the analytical precision. The detection limits of 16–55 μg/g (3σ) for trace elements were achieved, and the estimated accuracies for the major elements and trace elements were within ± 2 and ± 6% (1σ), respectively. We selected seven spinel samples from the Luobusha and Stillwater intrusions to evaluate the validity of our method. They were sufficiently homogeneous with a relative standard deviation (RSD) of ± 2.0% (1σ) for the major elements (except MgO in 16SW3-9) and ± 7.0% (1σ) for the trace elements. The EPMA results obtained for the major and trace elements of the most homogeneous spinel LBS13-04 were compared with those measured using X-ray fluorescence and laser ablation inductively coupled plasma mass spectrometry. These values were in good agreement with the uncertainty of the methods. Thus, this spinel is highly suitable as a reference material for in situ microanalysis. The Fe3+/ΣFe ratios of high Cr# (57.7-79.1) spinel standards determined using M?ssbauer spectroscopy varied from 0.07 to 0.27, which were used for secondary standard calibration method to determine the spinel Fe3+/ΣFe ratio with an accuracy of < ± 0.04 (2σ). Our results offer a high-precision EPMA method that can simultaneously determine the major and trace elements together with the Fe3+/ΣFe ratio in spinel. This method provides robust and precise data on spinel for small, precious, and rare terrestrial or extraterrestrial samples, which can be used to understand the formation and evolution of rocky planets.

Abstract:Nanophase iron (np-Fe0) is a major product of space weathering and its presence significantly alters the reflectance spectral characteristics of lunar soil. Previous studies have established that the np-Fe0 particles originate from the reduction of ferrous ions in the plasma, in-situ decomposition of olivine and pyroxene, and disproportionation of ferrous ions in solid ferrosilicates. In this study, sample charging effects were eliminated and in situ nanoscale valence state analysis of iron-bearing phases in Chang’E-5 lunar soil was conducted by combining focused ion beam (FIB) microscopy, Auger electron spectroscopy (AES), and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS) techniques. The results indicate that the contents and valence states of iron in the np-Fe0 particles, amorphous matrix, and ferrosilicates differ. The np-Fe0 particles were found to be composed of pure metallic iron, whereas ferrous and ferric iron ions were present in olivine crystals and the amorphous matrix, respectively. The discovery of both metallic and ferric iron in the amorphous matrix of Chang’E-5 lunar soil offers new insights regarding the disproportionation reaction of Fe2+ on the lunar surface. This study demonstrates that the combination of FIB, AES, and TEM-EELS is an effective and precise approach for analyzing the valence states of iron-bearing phases in lunar soil, which can be extended to other extraterrestrial samples and other multivalent elements.

Abstract:Plagioclase, pyroxene and glass are the main component phases of different planetary materials. In situ Rb-Sr dating of these common phases thus would represent the timing of magmatic differentiation, effectively complementary to the rare, tiny U-Pb bearing phases that only form at a late stage. In this study, we selected lunar meteorites as examples to establish an in situ Rb-Sr dating method where plagioclase, pyroxene, ilmenite, and glasses were the laser-ablation (LA) targets. The accuracy of 87Sr/86Sr and 87Rb/86Sr measured by LA-MC-ICP-MS was better than 0.2 ‰ and 3 %, respectively, for samples with an 87Rb/86Sr ratio lower than 1. However, we found that the distributions of Rb and Sr in the natural materials were heterogeneous at the micrometer scale, leading to inaccurate 87Rb/86Sr ratio correction when calculated by normal data reduction methods. A new data reduction strategy of the smallest unit of isochron age (SUIA) was developed. Using the SUIA, the Rb-Sr isochron age of 2984 ± 43 Ma and 3149 ± 20 Ma was obtained for two lunar meteorites (NWA 10597 and NWA 6950, respectively). These results are identical within 1-2% deviation relative to the U-Pb dating ages for baddeleyite and apatite using SIMS. The present method may have broad applicability for determining the Rb-Sr isochron ages of other planetary samples.

Abstract:Water is perhaps the most important molecule in the solar system, and determining its origin and distribution in planetary interiors has significant implications for understanding the formation and evolution of planetary bodies. Zircon (present in both extraterrestrial and terrestrial samples) is a resistant and versatile accessory mineral, and its water content has the potential to characterize the hydrous status of zircon-crystallizing magma. However, the hydrous status can be altered by magma fractionation, mixing, or degassing. Although hydrogen isotopes of zircon can help to trace these processes, the lack of relevant microanalytical technology and reference materials has hindered significant breakthroughs in this area. In this study, we employed an analytical technique of secondary ion mass spectrometry (SIMS) to simultaneously measure the hydrogen isotopes of zircon and their H2O content. The homogeneity test of hydrogen isotope and the H2O content of the potential reference materials (D15395, D15814, and Temora 2) were conducted by measuring the 16OD and 16O1H signals (using SIMS) in a conventional peak-hopping mono-collector mode with electron-multiplier (EM) detectors. SIMS results show that the apparent external precision (1SD) of hydrogen isotope of large grains D15395 and D15814 are 16‰ and 22‰, which is comparable with their internal error and theoretical precision according to counting statistics, indicating that they are sufficiently homogenous at the micrometer sampling level in terms of hydrogen isotopes. However, zircon Temora 2 shows a much worse external precision (83‰, 1SD) and an obvious negative correlation between hydrogen isotope and H2O content, hence it is not suitable to be used as reference material for hydrogen isotope. Both hydrogen isotope and H2O content of three samples were also measured using a continuous-flow thermal conversion elemental analyzer operating online with a mass spectrometer (TC/EA-MS). The homogeneous samples zircon D15395 and D15814 yielded recommended δD values of -87 ± 9‰ and -68 ± 2‰ (1SD) and H2O content of ~478 ppm and ~332 ppm, respectively. Additionally, H2O contents acquired by TC/EA-MS and SIMS (calibrated by FTIR determined standards) were consistent within a 10% error range, demonstrating the reliability of the SIMS H2O content calibration curve for zircon and the FTIR absorption coefficient used before.

Abstract:Olivine is the most abundant mineral in the planetary mantle. Its water content provides critical constraints on the processes and dynamics of the planetary interior. Olivine usually develops zonings with typical widths of 5–20 μm, which requires high spatial resolution. For secondary ion mass spectrometry (SIMS) measurements, primary beams with low currents were utilized to achieve high spatial resolution. However, this strategy also resulted in high background, which cannot be applied to nominally anhydrous minerals, e.g., olivine. Therefore, achieving high spatial resolution with low background is essential but challenging. For example, even though the NanoSIMS is designed for high-spatial-resolution measurements, the spatial resolution remained at > 10 μm for water content analysis in order to maintain a low water background of < 10 ppm. In this study, we optimized the primary beam settings and raster size for water content analysis of olivine using a CAMECA NanoSIMS 50L to improve the spatial resolution and the background simultaneously. Olivine standard samples (KLB-1, ICH-30, Mongok) with a water content ranging from 11.2 ppm to 70.6 ppm were measured for water content calibration with 1H-/16O- ratio. San Carlos olivine with a water content of 1.42 ppm was used for background monitoring. The results showed that a spatial resolution of ~6 μm (primary beam size + raster size) with a background of 6 ± 2 ppm could be achieved by applying a Cs+ beam current of 2 nA with a diameter of ~2 μm, rastering an area of 4 × 4 μm2. The analytical reproducibility of this method is better than 13% for standard samples with a water content of > 10 ppm. Overall, this method improved the spatial resolution for measuring water content by a factor of ~2 (in comparison to previous studies) and could be applied to olivine grains with complex zoning.

Abstract:Synchrotron-based scanning transmission X-ray microscopy (STXM) efficiently integrates X-ray microscopy and X-ray absorption spectroscopy (XAS) to provide quantitative, chemically specific imaging of elements, functional groups, bonding, and oxidation states in 2D and 3D modes at high spatial resolution (sub-10 to 30 nm), high energy resolution, and low radiation doses. STXM has been increasingly used to study various materials and samples for life, earth, planetary, and environmental sciences. In this progress report and minireview, the STXM principle and instrumentation of conventional STXM and the latest STXM-ptychography at the Canadian Light Source are first discussed. Then, two representative applications of STXM on geoscience-related samples, including magnetotactic bacteria, soil microaggregates, and related systems, are presented to illustrate the strong capabilities and suitability of STXM to elucidate complex systems, processes, and associations in the natural sciences. Finally, the potential applications and prospects of the STXM-related techniques in characterizing precious extraterrestrial samples (e.g., lunar samples returned by China’s Chang’e-5 mission) are briefly discussed.