Meteoritics & Planetary Science最新文献

Over 20,000 meteorites have been recovered from hot deserts. The effects of hot desert weathering upon highly siderophile elements (HSE) have been little studied. We have investigated the effects of neutral to mildly acidic leaching of three L6-type ordinary chondrites of different weathering grades on HSE concentrations and Re-Os isotopic systematics. We have characterized the bulk sample HSE patterns of these meteorites and conducted leaching experiments with progressively longer leaching times to determine the possible effects of long-term residence in a desert. The most weathered sample (NWA 14239) displayed greater HSE concentration homogeneity than the other samples and released lower quantities of HSEs during leaching. Water leaching was milder than acetic acid and did not significantly modify the Re-Os isotopic systematics of the residue relative to the bulk sample of NWA 869. Short-term leachates of the less weathered samples (Viñales and NWA 869) were characterized by low ¹⁸⁷Os/¹⁸⁸Os ratios, indicating the preferential dissolution of early solar system–formed phases such as non-magnetic chondrules and matrix with low Re/Os that are no longer intact in the most weathered sample. Of the HSE, Pd is most resistant to both water and acetic acid leaching, with a maximum removal of ~5% Pd, while Re, Os, and Ir are most mobile with up to 40% removal.

The processes of planetary accretion and differentiation, whereby an unsorted mass of primitive solar system material evolves into a body composed of a silicate mantle and metallic core, remain poorly understood. Mass-dependent variations of the isotope ratios of non-traditional stable isotope systems in meteorites are known to record events in the nebula and planetary evolution processes. Partial melting and melt separation, evaporation and condensation, diffusion, and thermal equilibration between minerals at the parent body (PB) scale can be recorded in the isotopic signatures of meteorites. In this context, the acapulcoite–lodranite meteorite clan (ALC), which represents the products of thermal metamorphism and low-degree partial melting of a primitive asteroid, is an attractive target to study the processes of early planetary differentiation. Here, we present a comprehensive data set of mass-dependent Fe, Zn, and Mg isotope ratio variations in bulk ALC species, their separated silicate and metal phases, and in handpicked mineral fractions. These non-traditional stable isotope ratios are governed by mass-dependent isotope fractionation and provide a state-of-the-art perspective on the evolution of the ALC PB, which is complementary to interpretations based on the petrology, trace element composition, and isotope geochemistry of the ALC. None of the isotopic signatures of ALC species show convincing co-variation with the oxygen isotope ratios, which are considered to record nebular processes occurring prior to the PB formation. Iron isotopic compositions of ALC metal and silicate phases broadly fall on the isotherms within the temperature ranges predicted by pyroxene thermometry. The isotope ratios of Mg in ALC meteorites and their silicate minerals are within the range of chondritic meteorites, with only accessory spinel group minerals having significantly different compositions. Overall, the Mg and Fe isotopic signatures of the ALC species analyzed are in line with their formation as products of high-degree thermal metamorphism and low-degree partial melting of primitive precursors. The δ^66/64Zn values of the ALC meteorites demonstrate a range of ~3.5‰ and the Zn is overall isotopically heavier than in chondrites. The superchondritic Zn isotopic signatures have possibly resulted from evaporative Zn losses, as observed for other meteorite parent bodies. This is unlikely to be the result of PB differentiation processes, as the Zn isotope ratio data show no covariation with the proxies of partial melting, such as the mass fractions of the platinum group and rare earth elements.

Orbital observations of Bennu revealed a surface covered in boulders that are most similar among meteorites in our collections to aqueously altered carbonaceous chondrites, and initial analyses of the returned Bennu sample have begun to reveal insights into Bennu's origins. We identified a suite of paired CM2 chondrite meteorites that have a finely layered texture and bear a striking similarity, although at a different scale, to rugged, layered boulders on Bennu. We investigated the nature and potential origin of this layered texture by performing a petrofabric analysis on samples MET 00431, 00434, and 00435. We developed a micro-geospatial mapping framework that is more commonly used for landscape-scale investigations. Our results reveal a pervasive fracture network that exhibits a similar orientation to flattened particles dominated by tochilinite–cronstedtite intergrowths (TCI). We propose that their petrofabrics originated from a low-energy impact on the parent body that occurred after the main period of aqueous alteration halted. The impactdeformed TCI (which formed during earlier aqueous alteration) and generated the fractures. We propose that the sample from Bennu may contain particles with similar layered textures to these meteorites which, if present, would likewise indicate the dominant role of impacts and aqueous alteration on Bennu's parent body.

The US Antarctic Search for Meteorites (ANSMET) discovered a dense cluster of 88 ordinary chondrites with a total mass of more than 100 kg on a blue ice area (BIA) of 1.6 × 0.3 km² near the Otway Massif, Grosvenor Mountains, Antarctica. The larger masses (weighing up to 29 kg) were found at one end of an oval-shaped pattern and the smaller masses (50–200 g) at the other end. We measured concentrations of the cosmogenic radionuclides ¹⁰Be (half-life—1.36 × 10⁶ year) and ³⁶Cl (3.01 × 10⁵ year) in the metal fraction of 17 H chondrites, including 14 fragments of this cluster, to verify the hypothesis that this meteorite cluster on the Otway Massif BIA represents a meteorite strewn field produced by the atmospheric breakup of a single meteoroid. The ¹⁰Be and ³⁶Cl concentrations confirm that 10 out of 14 H chondrites from different locations within this small area are paired fragments of the same meteorite fall, while the four other H chondrites represent two additional—smaller—falls. The radionuclides suggest a pre-atmospheric mass of 200–400 kg for the large pairing group, suggesting that 25%–50% of the meteoroid survived atmospheric entry. Based on the distribution of the paired H chondrites and evidence of their common cosmic-ray exposure history in space, we conclude that most of the 88 meteorites within this small area represent a meteorite strewn field. The small size of the strewn field suggests that the meteoroid entered at a steep angle (>60°), while the low amount of fusion crust on most meteorite surfaces most likely indicates atmospheric break up at low altitude, while additional fragmentation of a large surviving fragment may have occurred during impact on the ice. This well-documented strewn field provides a good opportunity to apply model simulations of the atmospheric fragmentation of this object as a function of entry angle, velocity, and meteoroid strength. Cosmogenic ¹⁴C analyses in two members of the Otway Massif pairing group yield a terrestrial age of 15.5 ± 1.5 kyr, which represents the time elapsed since this meteorite fell on Earth. The excellent preservation of an Antarctic meteorite strewn field suggests that the Otway Massif BIA represents a relatively stagnant blue ice field.

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.