Meteoritics & Planetary Science最新文献

At the Dhala impact structure, the monomict breccia and the impact melt rock outcrops are present in proximity. Generally, these impactite lithologies are formed by different mechanisms and in different parts of the crater. The emplacement setting of impact melt rocks at Dhala has been well studied. Therefore, we studied the emplacement of monomict breccia using field, microscopic, and magnetic fabric investigations. Our results show that the intensities of the rock magnetic parameters in monomict breccia are comparable with the unshocked target granitoid at Dhala. Thus, the magnetic fabrics developed during pre-impact processes and were not altered due to impact. The absence of the reorientation of magnetic fabrics indicates that the peak shock pressures were below 0.5 GPa. Such shock pressures typically exist near the crater wall/floor or outside the crater. Moreover, there is no local variation in the orientations of magnetic fabrics at different locations in the same outcrop. Thus, the monomict breccia was not displaced from their pre-impact position. Based on the shock barometry and absence of displacement, we propose that the present-day annular outcrops of monomict breccia are located just outside the final crater. Furthermore, the monomict breccia annular outcrop ring has an internal diameter of ~4.5 km and is juxtaposed with impact melt rocks, which formed within the crater (previous studies). We, thus, suggest that the present-day crater diameter is ~4.5 km.

CI chondrites are poorly lithified and highly friable regolith breccias. To examine their physical properties and the nature of their breccation, we investigated nine samples of the Ivuna and Orgueil CI chondrites ranging in size from 1 mm to 4 cm in approximate diameter. The combined mass of unique material investigated in this work is 113 g. For our investigations, we use ideal gas pycnometry, 3-D laser scanning, x-ray computed microtomography (μCT), and accompanying digital data extraction techniques. We found that the bulk density of the samples ranged from 1.61 to 2.10 g cm⁻³. Larger samples tend to have a lower bulk density. Grain density (ranging from 2.44 to 2.55 g cm⁻³) is significantly less variable than the bulk density in our samples and the quantity of porosity (ranging from 14.6% to 33.8%) is the dominant factor in determining the bulk density of CI chondrite material. Our μCT results show that the visible porosity across all sizes of our CI chondrite samples is in the form of cracks, but these cracks can account for less than two-thirds of the porosity in the CI chondrites. Other porosity is not visible, even at μCT resolutions of 2.7 μm voxel edge⁻¹ and we conclude that it is sub-micron in nature. It is not clear if the cracks seen in our samples are indigenous to the chondrites or are a result of terrestrial processes. We also find that the CI chondrites are excellent examples of the fractal-like nature of brecciation, where clasts can be observed at all scales we imaged. The breccias are composed of sub-equant-shaped and sub-rounded-textured clasts like melt-free impact breccias on other solar system bodies. From our μCT volume and digital data extraction, we determine that the Ivuna CI chondrite breccia is organized: the mostly sub-equant clasts within our ~2 cm chunk of Ivuna have a mean diameter of 1.33 mm and their aligned longest axes define a lineation structure. We speculate that the lineation was imparted after fragmentation of the clasts by slight shear on the parent asteroid which could be the result of seismic-related granular flow or mild non-axial impact-related compaction. These data will help to place returned asteroidal material from asteroids 162173 Ryugu and 101955 Bennu and the CI chondrites into a mutual geological context.

Results of microanalysis study of NWA 869 meteorite, an ordinary chondrite, where silicates, Fe-Ni alloys, and troilite are major constituents, are reported. The presented study of microtextures in metallic and sulfide grains provides information on processes occurring from the asteroid's accretion, through the impacts until cooling. The presence of metal–silicate emulsion, swiss-cheese texture, and polycrystalline kamacite–troilite aggregates observed indicates very rapid increase in temperature due to impact. Fizzed troilite, slightly homogenized taenite domains, and intergrowth of native copper in plessite imply rapid cooling in isolated regions in space. Agrell effect on the interface of kamacite–taenite grains and non-corroded tetrataenite rims indicates that the sample contains rock fragments from a region unaffected directly by the impact or rapid heating. Diversity of petrological types, lithic clasts with shock grade higher than S3, shock-darkened clasts or impact melting rocks, and variation of microtextures suggest that the parent body of L-type chondrites after accretion, radiogenic metamorphism, and consequent formation of the onion–shell model broke up into debris after the impact of eucrite body.

Samples of impactite from the small (~350 m diameter) Monturaqui crater in northern Chile contain Fe-Ni metallic spherules sourced from the iron meteorite impactor. Textural characterization and quantification were done using SEM and μCT data. Two textural types are distinguished, with different size distributions. The smaller spherical objects (mostly <100 μm in diameter) follow a power law size distribution, while larger objects are mostly irregular-shaped patches. These are analogous to the small (nm to 50 μm) immiscible spherical metal droplets and large (150–500 μm) irregular partly fused pieces of the iron meteorite projectile observed in highly shocked ejecta fragments during hypervelocity impact experiments. Compositions of both spherule types were determined using in situ methods (electron microprobe, LA-ICP-MS), as well as solution ICP-MS on individual spherules separated from impact melt glass using electric pulse disintegration. Spherules are enriched in Ni and Co relative to Fe and W and relative to the inferred iron meteorite impactor composition, and PGEs show similar enrichments with limited fractionation between different PGEs, all consistent with selective oxidation processes. All spherules have similar chondrite-normalized patterns that are also broadly similar to weathered fragments of the iron meteorite impactor. Ni-Ge and Ni-Ir data on large (>300 μm) spherules and weathered meteorite fragments suggest that the Monturaqui impactor was a group IAB iron meteorite.

Xenoliths in carbonaceous chondrites include lithologies that are unrepresented in the meteorite record and so are a rich source of information on asteroid diversity. Cold Bokkeveld is a CM2 regolith breccia that contains both hydrous and anhydrous lithic clasts. Here, we describe a hydrous clast with a fine-grained rim. This rim shows that the clast is a xenolith that interacted with dust in the protoplanetary disk between liberation from its protolith and incorporation into Cold Bokkeveld's parent body. Prior to its fragmentation, the xenolith's protolith had undergone brittle deformation, with the fractures produced being cemented by carbonates to make veins. After being incorporated into Cold Bokkeveld's parent body, the veined xenolith experienced a second phase of aqueous alteration leading to hydration of its fine-grained rim, replacement of carbonate by tochilinite–cronstedtite intergrowths, and formation of magnetite within its fine-grained matrix. The veined xenolith's protolith underwent its entire geological evolution (accretion–aqueous alteration–fracturing–fragmentation) before Cold Bokkeveld's parent body had accreted. Such a short lifespan may be explained by explosive breakup of the protolith due to overpressure from gases produced internally during water–rock interaction. Early fragmentation effectively acted as a thermostat to limit runaway heating that may have otherwise resulted from the body's high concentrations of ²⁶Al. Many other hydrous lithic clasts in CM carbonaceous chondrite meteorites could be the remains of such ephemeral early asteroids, but they are hard to identify without evidence that they were accreted as hydrous lithologies and contemporaneously with chondrules.

Refractory inclusions (RIs) in chondrites are widely used as tracers of early solar system formation conditions. In the context of sample-return missions, a non-destructive and non-invasive analytical tool that can rapidly detect and characterize RIs in space samples during their early phase of study is highly needed. Here, we performed mid-infrared (MIR) fine-scale hyperspectral imaging over large fields of view to detect RIs in CM and CO chondrites. A database of MIR spectra of typical RIs minerals was built (1) to support future remote sensing observations in astronomical environments and (2) to develop a detection method based on machine-learning algorithms and spectral distance between sample and reference minerals. With this method, up to 96.5% of the RI content is detected in a meteorite section. Further comparison between scanning electron microscopy and spot analysis acquired in reflectance in the full MIR range shows that RIs can be classified following their mineralogy based on infrared (IR) properties. Finally, we show that the relative OH content of several RIs in CM chondrites determined from IR spectroscopy can be used to infer the extent of modification caused by aqueous alteration on the asteroidal parent body.

Solar wind Fe and Mg fluences (atoms/cm²) were measured from Genesis collectors. Fe and Mg have similar first ionization potentials and solar wind Fe/Mg should equal the solar ratio. Solar wind Fe/Mg is a more valid measure of solar composition than CI chondrites and can be measured more accurately than spectroscopic photospheric abundances. Mg and Fe fluences analyzed in four laboratories give satisfactory agreement. Si and diamond-like carbon collector fluences agree for both elements. The Mg and Fe fluences are 1.731 ± 0.073 × 10¹² and 1.366 ± 0.058 × 10¹² atoms/cm². All plausible sources of errors down to the 1% level are documented. Our value for the solar system Fe/Mg, 0.789 ± 0.048 agrees within 1 sigma errors with CI chondrites, spectroscopic photospheric abundances, and with the solar wind data from the ACE spacecraft. CI samples from asteroid Ryugu give Fe/Mg in agreement with Genesis and meteoritic CI samples despite very small sample sizes. The higher accuracy of the Genesis solar Fe/Mg permits a comparison with chondritic Fe/Mg at the 10% level. Intermeteorite Fe/Mg averages differ among the main C chondrite groups but are within, or very close to, the ±1 sigma Genesis solar Fe/Mg.