Meteoritics & Planetary Science最新文献

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.

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.