Meteoritics & Planetary Science最新文献

Estimation of the composition of planetary rocks and minerals is crucial for understanding their formation processes. In this study, we present the application of X-ray nano-computed tomography (nano-XCT) for non-destructive three-dimensional (3-D) phase analysis and estimation of phase abundances in rare Martian meteorite samples, specifically chassignite Northwest Africa (NWA) 2737. We determined the most suitable laser power for minimizing artifacts and maximizing phase contrast. By utilizing nano-XCT, we successfully identified and segmented primary phases in the bulk meteorite sample. Additionally, we were able to locate and segment crystallized silicate melt inclusions within the meteorite. The phase abundances in bulk NWA 2737 and within melt inclusions calculated using nano-XCT were in good agreement with previous studies that used thin section calculations, demonstrating the reliability of nano-XCT as a non-destructive alternative for estimating bulk phase abundances in rare samples. This study develops a benchmarking protocol and demonstrates the efficacy of nano-XCT as a non-destructive technique for generating an overview of phase distribution and assemblages of melt inclusions within rare samples. Future research can benefit from combining non-destructive 3-D phase assemblage estimations with non-destructive 3-D chemical analysis techniques to achieve a fully non-destructive parental magma composition estimation of rare cumulate samples.

Brachinites and brachinite-like achondrites are olivine-rich meteorites that represent materials after partial metal–silicate differentiation on multiple early Solar System bodies. Both meteorite types show macroscopic textures of olivine crystals, which make up >70 modal percent of their mineralogy. We investigated the orientations of olivine using electron backscatter diffraction (EBSD) and elemental compositions from paired brachinite-like achondrites and one brachinite. The olivine orientations are characterized by a strong concentration of [010] axes with maxima perpendicular to the foliation/layering and a concentration of [001] axes distributed in a girdle or, in a few samples, as point maxima. Trace element abundances of the olivine in these meteorites determined using laser ablation inductively coupled plasma–mass spectrometry have uniformly low concentrations of sodium (<300 μg g⁻¹), aluminum (<70 μg g⁻¹), and titanium (<40 μg g⁻¹) that are distinct from olivine in chondrites or within terrestrial lavas. Instead, brachinite and brachinite-like olivine compositions broadly overlap those of olivine from melt-depleted mantle lithologies on Earth. Evidence from olivine trace element geochemistry, in conjunction with mineral fabrics, supports that these meteorites formed as melt residues on their host planetary body(ies).

Charge disproportionation of ferrous iron has been considered as one of the mechanisms for the formation of metallic iron on the lunar surface. However, the detailed mechanism of the disproportionation reaction on the Moon is yet to be elucidated. We provide direct evidence for the ferrous disproportionation reaction that produces nano phase metallic iron (npFe⁰) during a rapid cooling process after splash melting from a lunar sample returned by China's Chang'e-5 mission. Space weathering processes have resulted in the formation of three distinct zones at the rim of a pyroxene fragment, as observed through transmission electron microscopy. These zones, made up of splashed melts, newly formed melts from the substrate, and the mineral, are distinguished as I, II, and III. Quantitative analyses of the iron valence state by electron energy loss spectroscopy show that disproportionation reactions occurred in zone II at a low temperature of <570°C during a rapid cooling process. The reaction led to the production of α-structure npFe⁰ and Fe³⁺ reserve in the glass phase. The npFe⁰ produced by the disproportionation reaction has a larger grain size than those formed from solar wind irradiation, implying that micrometeoroid impacts mainly contribute to the darkening of visible and near-infrared reflectance. These findings reveal a novel rim structure by repeated space weathering and a universal formation mechanism of npFe⁰ during micrometeoroid impacts, suggesting that the disproportionation reaction could be widespread on airless bodies with impact-induced splash processes.

Apatite is present as an accessory phase in many meteorites and is often formed as a secondary product of aqueous alteration. Its propensity to incorporate rare earth elements (REE) results in apatite usually being the main REE-bearing phase in hydrously altered meteorites. Asteroid Ryugu is thought to have experienced pervasive aqueous alteration and material collected from the surface of Ryugu is expected to provide insight into asteroidal aqueous alteration processes without influence by terrestrial weathering. Morphologies and mineral associations of apatite grains from five rock fragments collected from the asteroid Ryugu by the Hayabusa2 spacecraft were examined and their REE concentrations were measured by synchrotron X-ray fluorescence (SXRF) spectroscopy. The main minerals associated with apatite are dolomite, magnetite, and pyrrhotite. Grain boundary corrosion of the interfaces between apatite assemblages and the surrounding matrix suggest that paragenetic formation on the asteroid was followed by a later episode of hydrous alteration. Light REE (LREE) concentration levels recorded at 20–150 times those of bulk CI levels together with a steady increase from LREE toward enrichment of medium REE (MREE, up to Er) at 50–400 times bulk CI levels may suggest postgenetic removal of LREE from Ryugu apatite grains by late-stage circulation of a hydrothermal fluid.

Meteoritical Bulletin 112 contains the 2487 meteorites approved by the Nomenclature Committee of the Meteoritical Society in 2023. It includes 14 falls, 1926 ordinary chondrites, 141 HED, 127 carbonaceous chondrites (including 6 ungrouped), 71 lunar meteorites, 37 ureilites, 36 mesosiderites, 27 Martian meteorites, 23 iron meteorites (6 ungrouped), 20 Rumuruti chondrites, 19 ungrouped achondrites, 19 enstatite chondrites, 18 primitive achondrites (2 ungrouped), 12 pallasites, 6 angrites, 4 enstatite achondrites, and 1 ungrouped chondrite. Of the meteorites approved in 2023, 968 were from Africa, 836 from South America, 572 from Antarctica, 66 from Asia, 35 from North America, 9 from Europe, and 1 from Oceania.

Millimeter-to-nanometer-sized iron- and nickel-rich metals are ubiquitous on the lunar surface. The proposed origin of these metals falls into two broad classes which should have distinct geochemical signatures—(1) the reduction of iron-bearing minerals or (2) the addition of metals from meteoritic sources. The metals measured here from the Apollo 16 regolith possess low Ni (2–6 wt%) and elevated Ge (80–350 ppm) suggesting a meteoritic origin. However, the measured Ni is lower, and the Ge higher than currently known iron meteorites. In comparison to the low Ni iron meteorites, the even lower Ni and higher Ge contents exhibited by these lunar metals are best explained by impact-driven volatilization and condensation of Ni-poor meteoritic metal during their impact and addition to the lunar surface. The presence of similar, low Ni-bearing metals in Apollo return samples from geographically distant sites suggests that this geochemical signature might not be restricted to just the Apollo 16 locality and that volatility-driven modification of meteoritic metals are a common feature of lunar regolith formation. The possibility of a significant low Ni/high Ge meteoritic component in the lunar regolith, and the observation of chemical fractionation during emplacement, has implications for the interpretation of both lunar remote-sensing data and bulk geochemical data derived from sample return material. Additionally, our observation of predominantly meteoritic sourced metals has implications for the prevalence of meteoritic addition on airless planetary bodies.

Although CI chondrites are susceptible to terrestrial weathering on Earth, the specific processes are unknown. To elucidate the weathering mechanism, we conduct a laboratory experiment using pristine particles from asteroid Ryugu. Air-exposed particles predominantly develop small-sized euhedral Ca-S-rich grains (0.5–1 μm) on the particle surface and along open cracks. Both transmission electron microscopy and synchrotron-based computed tomography combined with XRD reveal that the grains are hydrous Ca-sulfate. Notably, this phase does not form in vacuum- or nitrogen-stored particles, suggesting this result is due to laboratory weathering. We also compare the Orgueil CI chondrite with the altered Ryugu particles. Due to the weathering of pyrrhotite and dolomite, Orgueil contains a significant amount of gypsum and ferrihydrite. We suggest that mineralogical changes due to terrestrial weathering of particles returned directly from asteroid occur even after a short-time air exposure. Consequently, conducting prompt analyses and ensuring proper storage conditions are crucial, especially to preserve the primordial features of organics and volatiles.

The nitrogen isotope compositions of two samples returned from the asteroid Ryugu were determined using a stepwise combustion method, along with Ivuna (CI) and Y-980115, a CI-like Antarctic meteorite, as references. The two Ryugu samples A0105-07 and C0106-07 showed bulk δ¹⁵N values of +1.7 ± 0.5‰ and +0.2 ± 0.6‰, respectively, significantly lower than Ivuna with +36.4 ± 0.4‰, but close to Y-980115 with +4.0 ± 0.3‰. The Ryugu samples are further characterized by C/N and ³⁶Ar/N ratios up to 3.4× and 4.9× the value of Ivuna, respectively. Among all Ryugu samples and CI chondrites, a positive correlation was observed between nitrogen concentrations and δ¹⁵N values, with samples with lower nitrogen concentrations exhibiting lower δ¹⁵N. This trend is explained by a two-component mixing model. One component is present at a constant abundance among all CI-related samples, with a δ¹⁵N value around 0‰ or lower. The other varies in abundance between different samples, and exhibits a δ¹⁵N value of +56 ± 4‰. The first ¹⁵N-poor endmember is seemingly tightly incorporated into a carbonaceous host phase, whereas the ¹⁵N-rich endmember can be mobilized and decoupled from carbon, potentially because it is in the form of ammonia. Asteroid materials with volatile compositions that are similar to those reported here for the Ryugu samples are attractive candidates for the volatile sources among Earth's building blocks.