Meteoritics & Planetary Science最新文献

This paper presents the results of proton irradiation actions of three meteorites which were studied by LV-SEM, Raman spectroscopy, and FTIR spectroscopy methods, both before and after the artificial irradiations. The three samples are the Dhofar (Dho) 007 eucrite, the Northwest Africa (NWA) 4560 LL3.2, and the NWA 5838 H6 chondrite meteorites, which were irradiated by 1 keV average solar wind protons using the ECR ion source at ATOMKI with 10¹⁷ and 10¹⁹ ions cm⁻² fluence values. According to FTIR spectra, the first irradiation induced metastable alteration, and after the second irradiation, crystals organized into more stable phases. In the Dho 007 sample, the pyroxene shows a positive peak shift and FWHM change after the first irradiation, with decreased intensity of spectra. After the second irradiation, the peak position and FWHM decreased but showed an increase in comparison with the state before the irradiation in the FTIR spectra. The minor band near 620 cm⁻¹ disappeared after the irradiations in the FTIR spectra; however, the Raman spectra do not show the disappearance of minor bands. The olivine (in NWA 4560 and NWA 5838) and pyroxene (in Dho 007) showed negative peak shifts indicating escape of Mg²⁺ ions from the crystal lattice, together with positive peak shifts and increase of FWHM indicating amorphization of the crystal structure. Considering band shapes and intensities, both FTIR and Raman spectra showed decreasing intensity after the first irradiation, with possible metastable alteration. However, the spectra after the second irradiation show a moderate increase in FWHM change, which indicates a change in the crystal lattice. In the FTIR spectra, the minor band at 620 cm⁻¹ disappeared in the case of pyroxene.

Triple oxygen isotope analyses of meteorites are a fundamental tool for classifying meteorites and investigating early solar system processes. However, its utility can be significantly compromised by terrestrial oxygen contamination during weathering processes on Earth's surface. Aiming to restore the original bulk oxygen isotope composition of meteorites through the removal of terrestrial weathering products, leaching procedures with hydrochloric acid (HCl) or ethanolamine thioglycollate (EATG) are often employed, but their effects remain poorly understood. Therefore, here we obtained high-precision triple oxygen isotope data for a comprehensive set of meteorites to systematically evaluate the efficacy and consequences of these leaching methods as a function of meteorite group, weathering grade, petrologic type, and find/fall location and status. Our data for untreated and leached bulk meteorite powders show that leaching can cause shifts of several permil in ¹⁸O/¹⁶O and ¹⁷O/¹⁶O in aqueously altered and pristine chondrites, and lower magnitude shifts in thermally metamorphosed chondrites and achondrites. Though some shifts can be explained by removal of terrestrial weathering products, many suggest the inadvertent removal of indigenous phases. As such, this study highlights the benefits and disadvantages of leaching methods for meteorites, which can be best assessed by analyses of both untreated and HCl/EATG-leached aliquots.

While polar regions on Mars have long been recognized as primary reservoirs of ice, recent studies suggest that ice-rich deposits may also exist at lower latitudes due to cyclic variations in Martian climate. This study presents findings from geomorphological research conducted in the deepest region of Xanthe Terra, an unnamed impact crater. The objectives were to investigate the morphology and topography of the area to assess the occurrence of glacial features and to establish their potential age and geological context. We identified compelling evidence for fluvial and glacial activities within the crater by carefully analyzing various landforms, including theater-head valleys, layered terrains, fans, sinuous ridges, and viscous flows. The findings suggest a dynamic environment shaped by water and ice processes, likely influenced by an impact event approximately 3.5 billion years ago. The presence of Amazonian fan deposits dating back to approximately 750 million years ago further highlights the continued activity of fluvial processes in the region. Our study contributes to a deeper understanding of Mars' geological evolution and underscores the importance of further research to unravel the complex history of the planet's low-latitudinal regions and its changing environmental conditions over time.

The Nakhla Martian meteorite is known to contain secondary minerals, in particular phyllosilicates, that have recorded the conditions of aqueous alteration of the parent rock. A section of this meteorite was analyzed by transmission electron microscopy to characterize the phyllosilicates in veins and mesostasis. High resolution and electron diffraction, combined with chemical data, suggest the presence of veins in olivine filled by carbonates and hisingerite or hisingerite alone. In the mesostasis, phyllosilicates with composition close to that of ferripyrophyllite were observed in rounded pores within feldspars—these phyllosilicates are associated with areas rich in Si likely due to the presence of amorphous silica. Iron oxides/hydroxides were not found in this study. In addition, for the first time, wadsleyite was observed within the vein margins in olivine. Wadsleyite is evidence of shock metamorphism in Nakhla, whereas the veins result from the decompression after the shock wave passed through due to impact(s). The identification of these secondary minerals constrains the temperature, pH, and redox conditions during the aqueous alteration, underlying that these conditions changed over time. For example, hisingerite forms at T = 120–140°C and ferripyrophyllite at 55–65°C, confirming a progressive temperature decrease when the alteration went forward. The occurrence of these Fe-rich phyllosilicates has also implications on possible past life on Mars: H₂-fueled life cannot survive at T > 122°C; thus, it is incompatible with the formation of hisingerite. Life could have been possible only during the last step of aqueous alteration, that is, when the temperature decreased and ferripyrophyllite formed.

Olivine is a major constituent in ureilites and commonly defines macroscopic fabric via shape-preferred orientation of elongate grains. In this study, we examined olivine fabric (crystallographic preferred orientation, or CPO) and microstructures in the unbrecciated olivine-pigeonite ureilites Northwest Africa (NWA) 7059 and Nova 018 using electron backscatter diffraction (EBSD) analysis. Point-per-grain orientation data of NWA 7059 indicate a <010> lineation subparallel to grain elongation. Misorientation data of two grains in NWA 7059 indicate dominant activity of (010) [100], (001) [100], (100) [001], and {hk0} [001] slip systems. Nova 018 displays an axial-[010] fabric, with misorientations indicating (010) [001], {hk0} [001] slips, and formation of (010) twist boundaries. Axial-[010] fabric in Nova 018 is consistent with compaction of residual olivine during melt extraction. The <010> lineation in NWA 7059 is unlike typical ureilite fabric and requires a [010] Burgers vector, uncommon in terrestrial samples. Rotational axis analysis of 2°–10° misorientations in olivine shows that the relative proportion of [001] slips and [100] slips in both ureilites are similar to warm-shocked ordinary chondrites, which were deformed at subsolidus temperatures. However, subsolidus deformation temperatures for both studied ureilites are inconsistent with a “hot disruption” model for the ureilite parent body (UPB). The further lack of correlation between 2°–10° misorientation metrics and olivine core Fo content argues against deformation temperature as the main control on olivine slip systems in ureilites. Our findings highlight the use of olivine petrofabric to gain insights into ureilite deformation, as well as complexities in interpreting olivine deformation data with respect to the history of the UPB.

We report on the mineralogy, petrology, oxygen, and aluminum–magnesium isotopic systematics of the secondary corundum-bearing assemblages in type B CAIs 3529Z and 3529G and fluffy type A (FTA) CAI ALH-2 from Allende (CV > 3.6). In 3529Z and 3529G, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary alumoåkermanite [(Ca,Na)₂AlSi₂O₇], grossular, spinel, grossite, celsian, kushiroite, and wadalite. In ALH-2, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary grossular, nepheline, spinel, and kushiroite. In 3529Z and 3529G, corundum and associated secondary grossite, spinel, alumoåkermanite, grossular, and kushiroite have similar ¹⁶O-poor compositions (Δ¹⁷O = −2.2 ± 1.5‰); primary spinel is ¹⁶O-rich (Δ¹⁷O ~ −23‰); Al,Ti-diopside shows a range of Δ¹⁷O (from ~ −24‰ to ~ −15‰); anorthite and melilite are ¹⁶O-depleted to various degrees (−6.5‰ ≤ Δ¹⁷O ≤ −4.5‰ and Δ¹⁷O = −2.7 ± 0.8‰, respectively). In ALH-2, corundum shows a range of Δ¹⁷O, from ~ −9‰ to ~ −1‰; primary hibonite and spinel are ¹⁶O-rich (Δ¹⁷O ~ −23‰); melilite and perovskite are ¹⁶O-poor (Δ¹⁷O = −2.6 ± 1.5‰ and −3.1 ± 1.3‰, respectively). On the Al-Mg isotope diagram (²⁶Mg* versus ²⁷Al/²⁴Mg), primary Al,Ti-diopside, hibonite, melilite, and spinel in the Allende CAIs studied along the canonical isochron with inferred initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] of ~5 × 10⁻⁵. All secondary minerals have resolved excesses of ²⁶Mg*: alumoåkermanite, corundum, and grossite plot below the canonical isochron, whereas most spinel analyses plot above it. An internal isochron defined by the coexisting secondary corundum and alumoåkermanite in 3529Z has (²⁶Al/²⁷Al)₀ = (7.5 ± 2.6) × 10⁻⁷. We conclude that the corundum-bearing assemblages in Allende CAIs resulted from metasomatic alteration of primary melilite and anorthite, ~4–5 Ma after their crystallization. Metasomatic alteration of CAIs in the Allende parent asteroid by an aqueous fluid having Δ¹⁷O of ~ −3 ± 2‰ modified the O-isotope composition of their primary melilite, anorthite, and Ti-rich pyroxene; O-isotope compositions of primary hibonite, spinel, and low-Ti pyroxene escaped this modification.

Samples of observed meteorite falls provide important constraints on alteration histories of Solar System materials. Due to its rapid collection, terrestrial alteration in the observed Mighei-type (CM) carbonaceous chondrite fall Winchcombe was minimal. In this work, the petrography and mineralogy of three Winchcombe lamellae, two from the matrix and one from a lithological clast, were analyzed by transmission electron microscopy. Our results demonstrate that the matrix of Winchcombe is dominated by Mg-Fe-rich serpentine-type phyllosilicates and tochilinite-cronstedtite intergrowth (TCI)-like phases with variable, but generally high (petrologic type 2.0–2.3) alteration degrees that agree with petrologic types acquired on TCIs on larger scales in other work. However, we also located pristine areas in investigated lamellae such as homogeneous amorphous silicates and glassy particles with sulfide and metal inclusions that resemble altered cometary GEMS (glass with embedded metal and sulfides). One distinct GEMS-like domain shows Fe-rich metal and sulfide grains with oxygen-enriched rims in a Mg-rich amorphous groundmass embedded in organic matter, which likely shielded it from more severe alteration. Fe-Ni-sulfides are mainly pentlandite and concentrated in matrix lamellae. In addition to the sub-μm scale brecciated texture, the three lamellae show different alteration extents, further demonstrating the complex alteration nature of this CM2 meteorite.