Meteoritics & Planetary Science最新文献

Brachinites are a group of ultramafic achondritic meteorites thought to sample a planetesimal from the early inner solar system. They yield predominately ancient crystallization ages within 4 Ma of CAI formation, and while the formation mechanism for these samples is debated, they are widely thought to be partial melt residues from a differentiated planetesimal(s). Here, we conduct a correlated microstructural (electron backscatter diffraction; EBSD), trace element, and U–Pb age (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) study of a unique, large phosphate mineral assemblage in brachinite Northwest Africa (NWA) 7828 to constrain the origin and evolution of this sample and its parent body. Oxygen isotope analysis of NWA 7828 yields values in agreement with other brachinites and supportive of origin from the brachinite parent body. The phosphate assemblage is >90% chlorapatite, with merrillite occurring around grain boundaries and within fractures that crosscut the larger crystal. All calcium phosphate grains are highly crystalline, with domains of chlorapatite displaying <16° of internal misorientation, with merrillite displaying a range of unique orientations. When all concordant apatite and merrillite U-Th-Pb analyses are considered together, they yield a precise weighted average ²⁰⁷Pb-²⁰⁶Pb date of 4431 ± 5 Ma suggestive of a single population recording their crystallization age. Textural, chemical, and isotopic measurements of NWA 7828 are hard to reconcile with the formation of the phosphate assemblage in an igneous environment, instead supporting a metasomatic origin. The relatively younger age of the assemblage (4431 Ma) places it outside the estimated prolonged heating period on the brachinite parent body, instead requiring a later source of energy such as through impact-induced heating. This event coincides with the timing of impacts recorded by other brachinite (and brachinite-like) meteorites, as well as impact ages recorded by some Apollo melt breccias, and suggests a widespread, significant bombardment event around 4430 Ma.

Spectroscopy-based approach for remote exploration of any planetary body is significant in providing detailed understanding of surface composition, vital to any scientific exploration. Imaging Infrared Spectrometer (IIRS) is a hyperspectral imaging sensor flown over ISRO's Chandrayaan-2 (Ch-2) orbiter for mapping mineral composition and complete characterization of hydration feature on the lunar surface. With the extended spectral range (0.8–5 μm), high-spatial resolution (80 m) and high signal-to-noise ratio, IIRS data are capable of measuring surface composition based on diagnostic spectral absorption features of known/unknown characteristic minerals present on the lunar surface. The present paper discusses for the first time the methodology to process Ch-2 IIRS data to generate photometrically corrected reflectance images after thermal correction. Spectrally and radiometrically calibrated Level-1 IIRS spectral radiance data were subjected to various data processing techniques including thermal emission correction beyond 2.5 μm, conversion to apparent reflectance, and empirical line correction for smoothing the observed reflectance spectra. The thermally corrected IIRS reflectance data in the 0.8–3.3 μm range after correction for standard geometry were calibrated with ground-based observations of the lunar surface from the Apollo 16 site to generate Level-2 product. The results generated for the selected study regions representing the dominant landforms of the Moon (Mare, Highland and Polar region) were analyzed based on overall spectral reflectance variation and prominent absorption features at particular wavelengths corresponding to their surface properties. Finally, the results were compared with observations from Chandrayaan-1 Moon Mineralogy Mapper (M³) data within the overlapping spectral range from the same region to validate the absolute reflectance of the IIRS. Overall, slight differences in reflectance have been observed in the spectral profile from both the sensors in the lower wavelength range attributed mainly due to differences in resolution and observation geometry. However, beyond 2 μm, the spectral slope variation could be clearly visible, possibly because of thermal contributions that have been removed efficiently in the case of Ch-2 IIRS spectra.

Secondary ion mass spectrometry U-Pb geochronology has been performed on zircon grains separated from impact melt rock from the 2.7 km-in-diameter Ritland impact structure, southwestern Norway. Scanning electron microscope-based imaging techniques, including electron backscatter diffraction analysis, reveal various zircon grain microtextures, including shock-recrystallization and high-temperature zircon decomposition. Analyses from unshocked zircon grains yield two distinct concordant age populations at 1.5 and ~2.5 Ga, interpreted to represent igneous crystallization ages. The former aligns with Telemarkian magmatism (1.52–1.48 Ga) which dominates the local area of the Sveconorwegian orogeny and the target sequence at Ritland. The latter indicates a more ancient zircon population in Southern Norway, representing detrital grains in cover sediments present at the time of impact in the Cambrian. Collectively, the U-Pb data form two distinct discordant arrays with poorly resolved lower intercept ages spanning the Cambro-Ordovician boundary. The melt rock at Ritland is highly altered, and significant postimpact Pb loss is observed throughout the U-Pb data, likely in response to burial-induced thermal overprinting during the Caledonian orogeny. Post-filtering and selection of the data to minimize the effects of nonimpact-specific Pb loss, the two discordia produce indistinguishable lower intercept ages of 586 ± 73 Ma (MSWD 1.6, n = 15) and 545 ± 48 Ma (MSWD = 11, n = 9) which coincide in the Cambrian–Late Ediacaran. We therefore provide radioisotopic support for previous stratigraphic age constraints for the formation of the structure (500–542 Ma).

Planetary scientists heavily depend on meteorite curation facilities for the preparation and allocation of protected (e.g., Antarctic), highly valuable extraterrestrial specimens. In this work, a fragment of the Dyalpur ureilite obtained from a museum is discussed. The sample is found to contain microstructural, geochemical, and isotopic features inconsistent with any meteorite. The fragment consists of pargasitic amphibole, Ni-sulfides, and chromite grains in Fo₉₂ olivine groundmass, cut by serpentine veins. Amphibole geothermobarometry yields equilibrium conditions that are not compatible with the assumed ureilite parent body. Assuming the fragment represented a rare clast in an ureilite, further analyses were performed. Both the oxygen isotopic composition and the extremely low level of cosmogenic radionuclides confirm the terrestrial origin of the fragment; it is a partially serpentinized peridotite. This work stresses the importance of petrographic characterization of samples used for (isotope) geochemical analyses, of a well-documented sample curation, and of cosmogenic nuclide measurements for the unequivocal identification of extraterrestrial material. Finally, caution is recommended before making sensational claims in cases of anomalous results.

Multiple generations of calcite and dolomite precipitated in CM chondrites during ice melting events that led to episodes of liquid water. Models and laboratory analysis have suggested a long-term transition from oxidizing to reducing conditions during aqueous alteration on the CM parent body. We found that synchrotron X-ray absorption near edge spectroscopy (XANES) can detect relative differences in the oxidation state of trace iron within these carbonates. In CM chondrites, previous work interpreted Mn abundance in calcite as an indicator of relatively early or late formation, and dolomite is understood to form relatively late. In the CM1 chondrite Meteorite Hills 01070, XANES maps reveal that Mn-poor calcite contains more oxidized iron relative to Mn-rich calcite. While these measurements of carbonates support increasing iron reduction with progressive aqueous alteration in MET 01070, comparison among different CM chondrites suggests a complex picture of redox evolution. In addition to carbonates, we performed XANES measurements of the phyllosilicate-rich matrix of Allan Hills 83,100. Pre-edge centroid analysis indicates that this CM1/2 has an oxidation state similar to typical CM2 chondrites. While additional measurements are warranted to confirm the full span of redox trends in CM carbonates, our data do not support a correlation between redox state and petrologic type.

Dating rocks with a 2σ precision of 200 Ma is required to understand the history of Martian habitability and volcanic activity since ~4000 Ma. In situ K-Ar dating using a spot-by-spot laser ablation technique has been developed for isochron dating on Mars. The precision of isochron ages is determined mainly by the relationship between the laser spot diameter and the grain size of the sample. However, the achievable precision of age estimates using a realistic mineralogy of Martian rocks has yet to be investigated. We simulated isochrons under various conditions, including different laser spot sizes, K and Ar measurement errors, and numbers of analyses based on the mineral abundances of representative Martian meteorites (NWA 817, Zagami, and NWA 1068) analyzed using an electron probe microanalyzer. We found that attaining a precision of 200 Ma necessitates an isochron data range, defined as the ratio of the maximum to minimum K concentrations, of >6, a laser spot diameter of 250 μm, and measurement errors of <10% for both K and Ar. Reducing the laser spot size and selecting a sample with a large grain size are effective in obtaining a large K range. Furthermore, minimizing the variance in measurement errors between K and Ar is essential to increase the accuracy of the age estimates. We demonstrate that the precision required for in situ dating on Mars is achievable with realistic instrument settings, thus demonstrating the feasibility of establishing an in situ K-Ar geochronology for Mars.

We present isotope concentrations of the light noble gases He and Ne for samples from five well-documented strewnfields and two individual meteorites from the Omani desert. Cosmogenic (²²Ne/²¹Ne)_cos for the strewnfield samples are low, as expected considering the total known masses. A (²²Ne/²¹Ne)_cos of 1.210 for the LL6 chondrite RaS 267 from Oman indicates a small pre-atmospheric size of less than 10 cm. The CRE ages for the Omani meteorites calculated using ²¹Ne_cos range from 1 to 20 Ma. Using the (²²Ne/²¹Ne)_cos and previously established correlations, new shielding-corrected ¹⁴C and ¹⁴C-¹⁰Be terrestrial ages are calculated. For the strewnfield samples, the new ages are similar to the earlier ages but are more consistent. The new terrestrial age for RaS 267 is more than 20% lower than the previous age. Motivated by this success, we reinvestigated meteorites from other hot deserts (Acfer, Adrar, and Nullarbor regions) and Antarctica using literature data for ¹⁴C and (²²Ne/²¹Ne)_cos, along with the newly established correlations between ¹⁴C production rates and (²²Ne/²¹Ne)_cos. For these meteorites, the new terrestrial ages are systematically younger than the ages calculated earlier using a shielding-independent approach. Using shielding-corrected ¹⁴C terrestrial ages, the long-term puzzling problem that there is a lack of meteorites with short terrestrial ages disappears. The new histogram, though with only a limited number of data, shows the expected decrease in the number of meteorites with increasing terrestrial age. Therefore, the unexpected shape in the terrestrial age histogram was most likely due to a bias in the ¹⁴C dating system, that is, ages of small meteorites are overestimated.

Cliftonites, polycrystalline aggregates of graphite with unusual cuboid morphology, are important carbon components of certain iron meteorites. Although they consist predominantly of sp²-bonded carbon, recent studies suggest that those from the Canyon Diablo (IAB) meteorite also include composite sp²- and sp³-bonded structures, named diaphites. Here, we investigate the nanostructure of cliftonites in a Campo del Cielo specimen and demonstrate that these cliftonites also contain a nanocomposite mixture of well-ordered 3R graphite regions interfingered with type 1 diaphite structure, consisting of <01–10> projected graphite and <011> projected diamond domains. This finding suggests that certain pieces of the Campo del Cielo meteorite experienced moderate shock pressures (>~10 GPa), which exceed the 4–10 GPa pressure range previously reported for the main meteorite. We propose that a portion of Campo del Cielo cliftonites provides evidence for the shock-induced diamondization of graphite and the “projectile decapitation” process during terrestrial impact. The complexity of the initial carbonaceous material, combined with the wide range of pressures encountered during terrestrial impact events, may explain the diversity of nanostructures in the Campo del Cielo and Canyon Diablo cliftonites. Our findings could assist in the development of a pressure/shock classification system for characterizing impact events in graphite-bearing meteorites.

On the evening of February 14, 2023, at 17:58 UT, a fireball was detected by three cameras of the Italian PRISMA network (FRIPON network). The first samples of the Matera meteorite, collected 3 days after the fall, lay on the balcony of a private home. Meanwhile, four samples weighing more than 10 g (including the main mass of 46.21 g) and many minor samples (less than 10 g each) were recovered, with a total mass of 117.5 g. The analyses show that Matera is a monomict chondrite breccia, exhibiting no weathering (W0) and shock (S1). Based on the mineral compositions of olivine and low-Ca pyroxene (Fa_18.0±0.3 and Fs_17.0±0.3, respectively), the rock is an H-group ordinary chondrite. Since all low-Ca pyroxene is orthoenstatite, an H5-type classification is appropriate; although texturally, a type 4 classification could be assigned to distinct portions of the rock with well-defined chondrules. The analyzed oxygen isotopes also align with an H chondrite (δ¹⁷O‰ = 2.750 ± 0.051; δ¹⁸O‰ = 4.036 ± 0.103; Δ¹⁷O‰ = 0.650 ± 0.004). X-ray tomography and a structured light 3D scanner yielded a mean bulk density of 2.87 ± 0.04 g cm⁻³, whereas ideal gas pycnometry yielded grain densities of 3.47 ± 0.05 g cm⁻³, resulting in a porosity of 17.2 ± 1.2 vol%. The magnetic susceptibility of this meteorite is log χ = 5.46 ± 0.05. The radionuclides and fireball observations suggest that the Matera meteoroid was relatively small (with a maximum radius of 20 cm, though more likely around 15 cm). This datum is also consistent with (²¹Ne/²²Ne)_cos, which suggests the origin of Matera samples from the uppermost cm of a small meteoroid, ≤10 cm radius. Different from many other H chondrites, the transfer time in space for Matera, based on 3 He alone, is 10–12 Ma. Moreover, the Matera meteorite does not contain solar wind gases. In conclusion, the Matera meteorite is not a fairly typical ordinary chondrite, due to its low bulk density and high total porosity. The presence of ordinary chondrites with these physical characteristics must be taken into account during the asteroid modeling process, as in the case of the Didymos–Dimorphos binary system.

Hydrous Mg-phosphate was first described from astromaterials in particles returned from the C-type asteroid Ryugu, and has subsequently been found in samples of the B-type asteroid Bennu and CI1 carbonaceous chondrites. This phase may have been highly significant as a source of bioessential compounds for early Earth. Here, we describe Mg-phosphate from a petrologic type 1 clast (called “C1MP”) in the Cold Bokkeveld CM2 carbonaceous chondrite. This clast has a fine-grained serpentine–saponite matrix that in addition to the Mg-phosphate contains magnetite, Mg-Fe carbonate, calcite, pentlandite, transjordanite, eskolite, and daubréelite/zolenskyite. The Mg-phosphate grains are 7–36 μm in size and together constitute 0.27% of the clast by area. They have a “cracked” texture in scanning electron microscope images, and scanning transmission electron microscopy (STEM) shows that they are highly porous suggesting alteration of originally hydrous grains. The Mg-phosphate has Mg/P and Na/P ratios (atom%) of 1.02 and 0.25, respectively, along with minor concentrations of C, S, Cl, K, Ca, and Fe. Nitrogen was sought because ammonia has been reported from Ryugu Mg-phosphate, but none was detected by X-ray or electron spectroscopy. 4D-STEM shows that the C1MP clast's Mg-phosphate is amorphous, and radial distribution function analysis of electron diffraction patterns reveals that its P-O and Mg-P bonding distances are comparable to newberyite (MgHPO₄.3H₂O). The C1MP clast's Mg-phosphate formed from late-stage alkaline brines and subsequently underwent dehydration, amorphization, and partial loss of Na in response to heating in its parent body and/or during laboratory analysis.