Meteoritics & Planetary Science最新文献

CM chondrites have undergone varying degrees of aqueous alteration and thermal metamorphism on their parent bodies. Consequently, the petrologic grade of CM chondrites spans the entire type 2 scale (e.g., types 2.0–2.9). A 12236 is a very primitive petrologic type 2.9 carbonaceous chondrite that offers a unique window into the complex formation and evolution histories of CM chondrites. Based on its chemical composition, it is one of the least altered CM chondrites identified to date and one of the most primitive meteorites. Here, we present a comprehensive characterization of the organic and inorganic constituents of A 12236, determined through electron microscopy, micro-Raman, and s-SNOM nano-FT-IR spectroscopy. We identified FeNiS phases, including pentlandite, pyrrhotite, and troilite, within a fine-grained matrix composed predominantly of crystalline and amorphous silicates, including phyllosilicates. Raman spectroscopic results suggest that A 12236 experienced less thermal metamorphism than type 3 carbonaceous chondrites and contains polyaromatic organic matter with slightly differing structural order. Nano-FT-IR spectroscopy revealed chemically distinct aliphatic and aromatic organic phases, with observed compositional heterogeneity indicating variations in organic precursors and accreted materials. Correlation analysis highlights the complex associations between organic matter and phyllosilicates, along with evidence of differing aromatic compositions within the matrix. The varying abundances of nanoscale organics in different areas of A 12236 suggest that the organic matter is highly heterogeneously distributed within the matrix. Our findings demonstrate the effectiveness of nano-FT-IR spectroscopy for high-resolution, nondestructive analysis of extraterrestrial samples.

Ureilites are carbon-rich ultramafic achondrites that display unique textures, including strips of metal and carbon phases situated along grain boundaries and in fractures. Shock metamorphism observed in ureilites suggests an episode of brittle deformation caused by impact disruption of their parent body. The origin of carbon and metal has long been debated; in particular, whether either is endogenous or at least partly exogenous. We conducted experiments to simulate the metal-carbon textures and constrain their origin. Two model systems were investigated: (A) intrusion of FeS melt (analog for metal) into an olivine matrix containing dispersed graphite and (B) intrusion of graphite into a matrix containing dispersed FeS. After static annealing at 0.5–2 GPa and 1300°C, the samples were deformed at high strain rates to simulate an impact event. The microstructures of system A most closely resembled the textures observed in medium to low-shock main group ureilites, supporting an endogenous origin of carbon and a largely exogenous origin of metal. The grain boundary linings of ureilites were formed by impactor metal that intruded along grain boundaries and mixed with locally mobilized carbon. Hence, we establish a direct connection between the metal-carbon textures in ureilites and the collision history of their parent body.

Main group pallasite meteorites (PMG) are samples of an early, highly differentiated magmatic planetesimal dominated by olivine and metal-sulfide-phosphide assemblages with accessory chromite among other phases. This mineralogy reflects mantle- and core-related reservoirs, but the relative contributions of each and the overall petrogenesis are obscured by high degrees of protolith melting. Here, we present new data on the chemistry of chromite in these meteorites and review previous datasets. The purely lithophile elements Mg and Al partition into chromite via (Mg,Fe)(Al,Cr)₂O₄ and mainly reflect interactions with olivine and basaltic melt, respectively. Chromite cores are virtually always more aluminous than rims, and while MgO contents were likely reset during slow cooling, their Al₂O₃ contents are more robust and were largely set during the period of silicate magmatism. Main group pallasite chromites display bimodality in Al₂O₃ contents, with peak concentrations at ~7.7 wt% and below 6 wt%, which is unlike any other achondrite chromite population. Some chromites have very low Al₂O₃ contents (~0.01 wt%) due to formation in the absence of silicate melt, that is, via exsolution of Cr from cooling liquid metal. High-, low-, and very low-Al₂O₃ chromites in these meteorites broadly reflect relict, prograde, and retrograde periods of planetesimal heating followed by cooling. The Al₂O₃ contents of the chromites in many other achondrites and equilibrated chondrites are similar to the higher values in pallasites, with most greater than 3 wt%. This suggests that meteoritic chromite is a significant sink for ²⁶Al during its life as a heat source for planetesimal differentiation. To first order, it may be responsible for ~25%–50% (i.e., about one third) of heating in partially depleted mantles.

On October 24, 2024, an impressive fireball was visible over Austria. After the possible strewn field was calculated, the first sample of the Haag meteorite, with a mass of 8.76 g, was discovered on November 2, 2024, 8 days after the fireball event. Four more samples were found afterward putting the total sample mass at about 151 g. Short-lived radionuclides were measured shortly after recovery on a small sample, which was also used for almost all analyses presented here. Results confirm that the Haag meteorite derived from the bolide fireball event. Haag is a severely fragmented ordinary chondrite breccia and consists of typical equilibrated and recrystallized lithologies (LL4-6) as well as impact-related lithic clasts, such as dark, fine-grained impact breccias. Most fragments are highly recrystallized (type 6), but some show a well-preserved chondritic texture, which is of petrologic type 4 since the olivines are equilibrated. The olivines in the bulk rock have Fa contents of 29.5 ± 0.5 mol%, whereas the low-Ca pyroxenes have compositions of Fs_23.9±1.4Wo_1.6±0.7 with slightly variable Fs contents up to 28 mol%. However, the occurrence of type 3 fragments in other parts of the rock cannot completely be ruled out. Many clasts are moderately shocked (S4; C-S4). Using the fragment with the lowest degree of shock to determine the bulk rock's shock degree, Haag has an overall shock degree of S2 (C-S2). The LL chondrite classification is also supported by O isotope data, the results of bulk chemical analysis, and the physical properties of density and magnetic susceptibility. The nucleosynthetic Ti and Cr isotope data confirm that Haag is an ordinary chondrite, related to the noncarbonaceous (NC) meteorites. Haag does not contain detectable amounts of solar wind-implanted noble gases, and we rule out any substantial exposure at the direct surface of the parent body. Based on noble gases, Haag has an exposure age of 21–24 Ma and a pre-atmospheric meteoroid radius of 20–85 cm with a sample depth between 4 and 5 cm below the meteoroid surface, consistent with constraints from cosmogenic radionuclides. The soluble organic compositions of Haag are consistent with the profiles of the Stubenberg (LL6) breccia and show characteristics consistent with the complex shock, brecciation, and lithification history of the breccia. Haag and Stubenberg fell near each other (110 km away) within just 8 years. Since only 8.5% (about 110) of meteorite falls worldwide are LL chondrites, it is remarkable that two LL chondrites fell near each other in such a short time.

We conducted a scanning electron microscopy (SEM) and transmission electron microscopy (TEM) study of sulfide–metal assemblages (SMAs) in minimally to moderately altered CR2 chondrites. The assemblages occur on chondrule rims and consist of kamacite cores rimmed by pyrrhotite. The kamacite and pyrrhotite share orientation relationships, arguing for a genetic link. The SMAs contain secondary alteration products, including nanoscale magnetite at the sulfide–metal interface (minimally altered SMAs) and magnetite, serpentine, nanoscale Ni-rich metal at metal–magnetite interfaces, and Ni,S-bearing reaction fronts within magnetite (moderately altered SMAs). We argue the SMAs initially formed in the solar nebula from the separation of immiscible metal and silicate melts followed by sulfidization of the metal. Aqueous alteration on the asteroidal parent body caused the kamacite to transform into magnetite and the magnetite to transform into serpentine. Alteration of kamacite to magnetite occurred under oxidizing and alkaline conditions, whereas alteration of magnetite to serpentine occurred under reducing, alkaline, and higher aSiO₂ conditions. Serpentinization of magnetite appears to be a relatively common process in some carbonaceous chondrites. Additionally, theoretical and experimental studies are needed that simulate the oxidation of metal by H₂O gas and water and also serpentinization of magnetite to form serpentine with variable Mg-Fe contents.

Extraterrestrial breccia samples are formed through impact-related processes that combine the fragments of distinct lithologies. As such, they are valuable indicators of the complex formation and evolution history of planetesimals in our solar system. Samples from asteroid (162173) Ryugu, returned to the Earth by the Hayabusa2 mission in December 2020, were characterized as breccias. The boundaries of mineralogical assemblages are typically drawn manually based on interpreted results from specific techniques, mostly performed on artificially produced 2D surfaces. This process inherently introduces subjectivity. Here, we present a semi-automated analytical method using synchrotron radiation micro X-ray computed tomography (SR-μXCT) data, called the Local Histogram method. It enables an unsupervised detection and 3D visualization of a few tens to hundreds of micrometer-sized lithologies showing sub-micrometer heterogeneities. We developed the method on a millimeter-sized Ryugu sample (A0159) in combination with a more traditional global grayscale threshold segmentation. In A0159, we report five distinct lithologies. They were confirmed and further characterized by an additional scanning electron microscopy (SEM) analysis on Xenon plasma-focused ion beam (Xe-pFIB) produced sections. Some lithologies show specific relationships with large fractures, while one is particularly enriched in sub-micrometer sulfides. A0159 is rich in carbonates and hosts the largest millimeter-scale dolomite vein seen on Ryugu.

Phosphate minerals are significant carriers of volatiles (e.g., OH) and halogens in chondritic material; however, their origin in most groups of carbonaceous chondrites remains poorly characterized. We have determined the abundance, morphology, texture, and composition of phosphate grains in aqueously altered CI chondrites and in hydrated and thermally metamorphosed Antarctic CY chondrites using scanning electron microscopy and electron probe microanalysis. Phosphates include apatite (formula Ca5(PO₄)3X, where X = F-, Cl-, OH- or other anions) and sodium-bearing magnesium phosphate, both of which formed during episodes of aqueous alteration on the CI and CY parent bodies. Apatite grains in the CI chondrites range up to 40 μm in size with a modal abundance of ~0.10 area%, while in the CYs, the largest grains are ~50 μm in size and the modal abundance is ≤0.70 area%. Analysis by secondary ion mass spectrometry (SIMS) indicates that apatite in the CYs contains ~1.0–1.8 wt% H2O, with δD values of −84‰ to 393‰ likely reflecting aqueous and thermal processing. Apatite in both the CI and CY chondrites is rich in fluorine, with fluorine abundances that range from 20 to 80 mole% of the X (anion) site. This contrasts with apatite in other chondrite groups, which is predominantly Cl-rich. Estimated bulk chondrite F abundances based on F abundance in apatite are 12–21 ppm F for the CI chondrites and 61 ppm F for the CY chondrites. This is comparable to bulk CI chondrite F abundances in the literature, suggesting that most fluorine is hosted in apatite. However, the chlorine content of CI chondrite apatite (<0.05 wt%) is too low to account for the bulk chondrite Cl abundance, indicating that Cl is hosted in other phases. Mg,Na-phosphate, a rare extraterrestrial mineral, has a modal abundance of ~0.02 area% in both the CI and CY chondrites. Mg,Na-phosphates in the CI and CY chondrites are halogen-poor (<0.15 wt%) and are typically hydrated in the CIs (analytical totals as low as 67 wt%) and dehydrated in the CYs (analytical totals >96.0 wt%). The occurrence of Mg,Na-phosphates in the CI and Antarctic CY chondrites is indicative of brines on their respective parent bodies. Similarities between the two groups, as well as with the phosphate mineral assemblage in asteroids Ryugu and Bennu, indicate that comparable fluid compositions and environmental conditions were prevalent on numerous parent bodies in the early Solar System.

Mesosiderites are rare, differentiated meteorites, so-called stony-iron meteorites—they are impact breccias composed of an unusual mix of crustal basalt and pyroxenite, core-derived metal, but no mantle materials. This odd mixture makes their origin enigmatic and has inspired many different formation theories over the last several decades. Some of the outstanding questions have regarded the origin of the metal, whether it came from another celestial body or from within the main parent body, and the puzzlingly low abundance, or absence, of mantle material in mesosiderites. The role of impacts has been central to most of the suggested theories, but mesosiderites show little to no evidence of shock metamorphism. The mystery of the origin of mesosiderites is further compounded by the relatively limited amount of published data, as well as the restricted number of samples available for research. With the detailed investigation and reclassification of the mesosiderites Lamont, Acfer 265, Queen Alexandra Range 86900 (QUE 86900), and MacAlpine Hills 88102 (MAC 88102) presented herein, our new observations shine some much-needed light on this meteorite group. Based on their petrologic and metamorphic characteristics, Lamont is classified as a B3/4, Acfer 265 and QUE 86900 as A1, and MAC 88102 as an A4 mesosiderite. The observation of multiple sets of parallel thin lamellae in high-Ca plagioclase and cristobalite in Lamont, and a silicate emulsion in QUE 86900 is proposed to be shock-related features. In both Lamont and QUE 86900, these features are interpreted to be subsequent to the initial impact, which mixed crustal and core material, and prior to deep burial. No shock-related features were noted in Acfer 265 and MAC 88102.

Angrites and eucrites are among the oldest basaltic rocks in the solar system. However, the shock histories of these meteorite groups differ markedly, as most angrites show little to no evidence of shock metamorphism. While some angrites exhibit weak wavy extinction in olivine, indicative of low-level shock, only two—Northwest Africa (NWA) 1670 and NWA 7203—are known to preserve significant shock features such as shock melt veins. To better constrain the shock history of angrites, we performed noble gas analyses on the rare shock-metamorphosed angrite NWA 7203 to determine its cosmic ray exposure and gas retention ages. Neon in NWA 7203 is entirely cosmogenic, and combined neon and argon data yield a cosmic ray exposure age of 22.7 ± 3.1 Ma (2σ). This age nominally differs from that of the other shocked angrite, NWA 1670, but is comparable to that of the unshocked angrite NWA 7812. NWA 7203 may have been ejected from a rubble pile-like asteroid composed of both shocked and unshocked materials. Two distinct ⁴⁰Ar/³⁹Ar apparent ages, 3.38 ± 0.10 Ga and 1.41 ± 0.11 Ga, were obtained, likely reflecting variable argon loss during a single impact-induced thermal event that occurred no earlier than 1.41 ± 0.11 Ga (2σ). This is the first report for the shock metamorphic age of an angrite. Our results reinforce the view that even shocked angrites lack clear evidence of a catastrophic disruption of their parent body (>100 km) hypothesized to have occurred in the early solar system. To resolve this conundrum, we propose that angrites may have experienced extensive melting during such an event, which suppressed or erased conventional shock features. If this impact occurred near the time of their crystallization (>4564 Ma), it may have been a “hot shock” event driven by heat from short-lived radionuclides. Such an event could have generated large volumes of shock melt, from which quenched angrites subsequently formed. We suggest that differentiated planetary bodies may have commonly undergone such early-stage disruption events during the formative epoch of the solar system.