Meteoritics & Planetary Science最新文献

A medium-grade, poorly weathered CO3-type meteorite was subjected to artificial space weathering by 1 keV protons in three subsequent steps, with gradually increasing doses from 10¹¹ to 10¹⁷ protons per cm². The resulting mineral modifications were identified by Raman spectroscopy, with specific emphasis on main minerals such as olivine (bands: 817, 845 cm⁻¹), pyroxene (1007 cm⁻¹), and partly amorphous feldspar (509 cm⁻¹), considering variation in band shift and bandwidth (full width at half maximum, FWHM). After the first and second irradiations, variable band position changes were observed, probably from metastable alterations by Mg loss of the minerals, while the third stronger irradiation showed band shift dominated by amorphization. The olivine and pyroxene show weak increase in FWHM after the first irradiation, while more changes happened after the second and third irradiations. The flux after the third irradiation was higher than in other works, caused stronger damage in crystal lattice, partly resembling to dimerization as described by shock metamorphism. The glassy feldspar was characterized by high FWHM values already at the beginning, indicating weak crystallinity already that become even less crystallized, thus their bands disappeared after the third irradiation. Bands of hydrous minerals (goethite clay, chlorite) were not visible after the third irradiation, confirming some earlier results in the literature. Based on our results, moderately fresh surfaces could show stochastic but small spectral differences compared to the fresh most meteorites by metastable mineral alterations. The interpretation of Raman spectra of heavily space-weathered surfaces could further benefit from the joint evaluation of alteration induced by both shock impact alteration and space weathering.

The Kwajalein micrometeorite collection utilized high volume air samplers fitted with polycarbonate membrane filters to capture particles directly from the atmosphere at the Earth's surface. This initial study focused on identifying cosmic spherule-like particles, conservatively categorizing them into four groups based on bulk compositional data: Group I exhibit a range of compositions designated terrestrial in origin; group II are Fe-rich and contain only additional O, S, and/or Ni; group III are silicate spherules with Mg-to-Si At% ratios less than 0.4; group IV are silicate spherules with Mg-to-Si At% ratios greater than 0.4. Spherules in groups I, II, and III have compositions that are also consistent with particles that are produced in great numbers by natural and/or anthropogenic terrestrial activities (e.g., volcanic microspherules, fly ash from coal fired power plants, etc.) and thus are assumed terrestrial in origin. Group IV spherules exhibit compositions closest to those of cosmic spherules identified in other collections and are, therefore, designated cosmic spherule candidates. Detailed analysis of seven group IV spherules found that whilst five exhibited morphology and compositions consistent with S-type cosmic spherules, two appear unique to this collection and could not be matched to either terrestrial or extraterrestrial spherules studied to date.

Reckling Peak (RKP) 17085 is a newly classified Antarctic CM chondrite that preserves a complex alteration history characterized by mild aqueous alteration (CM2.7), overprinted by a short-lived thermal metamorphic event (heating stage III [<750°C]), and affected by low-grade terrestrial weathering. This meteorite contains abundant Fe-rich bands within the fine-grained matrix, composed of micron-scale Fe-oxyhydroxide minerals. They are interpreted as “alteration fronts” arising due to the dissolution and transport of Fe (typically <500 μm) before being abruptly deposited. This alteration texture is relatively rare among hydrated carbonaceous chondrites, with only five reported instances to date (Murchison, Murray, Allan Hills 81002, Miller Range 07687, and Northwest Africa 5958). Evidence from RKP 17085 suggests that early aqueous alteration operated as multiple geochemically isolated microenvironments, which moved outwards from local point sources within the matrix. Low permeability fine-grained rims on chondrules appear to have acted as barriers to fluid flow, controlling the migration of fluid across the parent body. Furthermore, the higher porosity regions within the altered fine-grained matrix represent either void space generated by the dehydration of hydrated minerals during post-hydration metamorphism and/or sites of ice accretion (water-ice or C-bearing ices) preserved within a mildly altered primitive matrix.

Northwest Africa (NWA) 12384 is a lunar polymict breccia composed almost entirely of basaltic components. The clast content includes low- to very-low-Ti volcanic picritic glass, basaltic vitrophyre, and crystalline pigeonite basalt—an assemblage of volcanic materials that can be tested for petrogenetic relationships. We present the inferred history of select mare components of NWA 12384 as suggested by texture, mineralogy, and petrography, and compare them to Apollo samples and other lunar meteorites. In addition, we used the volcanic glasses in the breccia as a primary composition for crystallization modeling and comparison to the lithic clast compositions. We find that the mafic clasts in NWA 12384 cannot be derived from the picritic glass through a common liquid line of descent because of higher Ti content, though they may have crystallized from a separate, common liquid line of descent. These clasts could represent local source-region heterogeneity or differential assimilation of more Ti-rich material. Pb-Pb SIMS analyses of a large basalt clast in NWA 12384 reveal an age of 3044 ± 41 Ma (2σ), which is used together with the chemical data and 4π cosmic ray exposure age of less than 20 kyr and terrestrial age of between 3.1 and 17.3 kyr to constrain the possible locations of provenance for this meteorite.

Many of the CM carbonaceous chondrites are regolith breccias and so should have abundant evidence for collisional processing. The constituent clasts of these fragmental rocks frequently display compactional petrofabrics; yet, olivine microstructures show that most CMs are unshocked. To better understand the reasons for this contradiction, we have sought other evidence for hypervelocity impact processing of CM chondrites using the Cold Bokkeveld meteorite. We find that this regolith breccia contains rare particles of vesicular shock melt that are close in chemical composition to bulk CM chondrite. Transmission electron microscopy of a melt bead shows that it is composed of silicate glass with inclusions of pentlandite, pyrrhotite, and wüstite. Characterization of shards of another bead by atom probe tomography reveals nanoscale clusters of sulfur that represent sulfide inclusions arrested at an early stage of growth. These glass particles are mineralogically comparable to micrometeoroid impact melt described from the Cb-type asteroid Ryugu and melt that has been experimentally produced by pulsed laser irradiation of CM targets. The glass could have formed by in situ shock-melting, but petrographic evidence is more consistent with an origin as ballistic ejecta from a distal impact. The scarcity of melt in this meteorite, and CM chondrites more broadly, is consistent with the explosive fragmentation of hydrous asteroids following energetic collisions. Cold Bokkeveld's parent body is likely to be a second-generation asteroid that was constructed from the debris of one or more earlier bodies, and only a small proportion of the reaccreted material had been highly shocked and melted.

Samples of B-type asteroid (101955) Bennu returned by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft will provide unique insight into the nature of carbonaceous asteroidal matter without the atmospheric entry heating or terrestrial weathering effects associated with meteoritic samples. Some of the Bennu samples will undergo characterization by X-ray computed tomography (XCT). To protect the pristine nature of the samples, it is important to understand any adverse effects that could result from irradiation during XCT analysis. We analyzed acid-insoluble residues produced from two powdered samples of the Murchison carbonaceous chondrite, one control and one XCT-scanned, to assess the impact on insoluble organic matter (IOM) and presolar grains. Using a suite of in situ analytical techniques (field-emission scanning electron microscopy, optical and ultraviolet fluorescence microscopy, microprobe two-step laser mass spectrometry, and nanoscale secondary ion mass spectrometry), we found that the two residues had indistinguishable chemical, molecular, and isotopic signatures on the micron to submicron scale, indicating that an X-ray dosage of 180 Gy (the maximum dose to be used during preliminary examination of Bennu materials) did not damage the IOM and presolar grains. To explore the use of acid-insoluble residues to infer parent body processes in preparation for Bennu sample analysis, we also analyzed a residue produced from the Sutter's Mill carbonaceous chondrite. Multiple lines of evidence, including severely degraded UV fluorescence signatures and D-rich hotspots, indicate that the parent body of Sutter's Mill was heated to >400°C. This heating event was likely short lived because the abundance of presolar SiC grains, which are destroyed by thermal metamorphism and prolonged oxidation, was consistent with those in Murchison and other unheated chondrites. The results of these in situ analyses of acid-insoluble residues from Murchison and Sutter's Mill provide complementary detail to bulk analyses.

Lunar basaltic meteorite Asuka-881757 (A-881757), a member of the source crater paired YAMM meteorites (Yamato-793169, A-881757, Miller Range 05035 and Meteorite Hills 01210), provides information on potassium-rare earth element-phosphorous (KREEP)-free magmatic sources within the Moon. Asuka-881757 is an unbrecciated and Fe-rich (Mg# 36) gabbro with coarse pyroxene (2–8 mm) and plagioclase (1–3 mm). The coarse pyroxene preserves mm-scale, near-complete hour-glass sector zoning with strong Ca and Fe partitioning, similar to some Fe-rich Apollo basalts. In contrast to the most Mg-rich Apollo basalts, A-881757 contains various types of symplectites (~8 vol%) formed by the breakdown of pyroxferroite due to slow cooling, resembling a few extreme Fe-rich (Mg# $��\le �$40) Apollo basalts. Petrographic observations and thermodynamic modeling suggest crystallizing in the order: Fe-poor pyroxenes (Mg# 58–55) → co-crystallized plagioclase and Fe-rich pyroxenes (Mg# 49–20) → late-stage assemblage including Fe-augite, Fayalite, and Fe-Ti oxides. Combining phase stability at variable P–T with petrographic observations, the minimum depth of formation of the A-881757 parent magma can be constrained to between 60 and 100 km. KREEP-free basalts (such as A-881757 and the YAMM meteorites) originated from a relatively shallow mantle source and later underwent polybaric crystallization that occurred prior to eruption at the lunar surface. In contrast, the Apollo mare basalts mostly crystallized within lava flows from relatively deeper-seated mantle sources. The crystallization of A-881757 and other YAMM meteorites is unlike most Apollo basalts from the Procellarum KREEP terrane, and likely represent hidden cryptomare basalts close to lunar surface.

CI, CM, and CR carbonaceous chondrites contain hydrous minerals, indicating that their parent bodies underwent aqueous alteration at low temperatures. Some of these chondrites, such as heated CM, CI, and CY chondrites, experienced thermal dehydration by impacts or solar radiation after aqueous alteration. This study conducted heating experiments on carbonaceous chondrites and evaluated their dehydration/dehydroxylation kinetics in an effort to explain the thermal history of the parent asteroids of heated carbonaceous chondrites using their degrees of dehydration/dehydroxylation of hydrous minerals. Murchison (CM2.5) and Ivuna (CI1), relatively primitive (having not undergone thermal alteration) carbonaceous chondrites, were used as starting materials. Weakening in the OH band at ~3680 cm⁻¹ (2.72 μm) with isothermal heating at 350–500°C (Murchison) and 450–525°C (Ivuna) were observed under in situ infrared spectroscopy (FT-IR) equipped with a heating stage. To determine the rate constants, the decrease in the OH band was fitted using kinetic models such as first-order reactions, two-dimensional diffusion, and three-dimensional diffusion. The apparent activation energies and frequency factors were determined using the Arrhenius equation. Time–temperature transformation diagrams were drawn to represent the decrease in the OH-band intensity as a function of temperature and heating duration. Such kinetic approaches can provide constraints on the temperature and time of the dehydration/dehydroxylation processes and enable us to estimate long-term effects from experiments in the laboratory within a short time.