Meteoritics & Planetary Science最新文献

The alignment of non-spherical “flattened” chondrules into a petrofabric is a common feature of hydrated carbonaceous chondrite meteorites. This texture can form as a result of impacts at peak shock pressures exceeding 10 GPa. However, many carbonaceous chondrites with petrofabrics are unshocked. While several processes have been proposed to explain this incongruency, including erasure of shock effects by alteration (both aqueous and thermal), none have yet been confirmed. Kolang is a brecciated Mighei-like carbonaceous chondrite wherein analysis of chondrule shape and orientation shows that it has a pronounced petrofabric defined by elongate chondrules that is shared between clasts with differing aqueous and thermal alteration histories. Its petrofabric, therefore, must have developed after the altered clasts had been juxtaposed; any sign of shock associated with impact-driven deformation cannot have been erased. We have investigated the shock experienced by Kolang with a combination of traditional optical methods and electron backscatter diffraction. We find that the peak shock pressure experienced by Kolang was likely ~4–5 GPa, too low to generate an impact-induced petrofabric. Kolang has not experienced sufficient shock, whether by a single or multiple impacts, to deform its chondrules from spheres into elongate chondrules. The most likely explanation, therefore, is that Kolang accreted elongate chondrules that were aligned under relatively low pressure.

We studied Caleta el Cobre 022, a nakhlite showing a high abundance of aqueous alteration products, commonly called “iddingsite” and compared it to eight other nakhlites, in order to constrain the composition and the history of the aqueous alteration of nakhlites. Olivine grains in nakhlites display planes of secondary fluid inclusions, composed of pyroxene, magnetite, and a void potentially filled by a fluid. They were formed by a first fluid alteration event, previous to the iddingsite alteration event, probably from a late magmatic fluid circulation. We observed magnetite–pyroxene symplectites in olivine grains in most nakhlites, related to the same fluid-assisted tardi-magmatic event as the crystallization of the secondary inclusion planes. Those secondary inclusions and symplectites can be observed at the center of iddingsite veins, inside the most altered nakhlites, and are thus interpreted as being weakness planes, easing the circulation of the fluid forming the iddingsite inside the olivine grains. In every nakhlite, the alteration veins show at least two types of iddingsite: a coarse iddingsite with crystals around 50 nm, up to 200 nm, and a fine iddingsite with a nanocrystalline to amorphous texture with crystalline domains <10 nm. Both iddingsite types are composed mainly of Si, Mg, and Fe, with anticorrelated Si and Fe contents. The coarse iddingsite is composed of a mixture of phyllosilicates, with Fe-oxyhydroxides and minor siderite, and the fine iddingsite has a composition close to saponite. Organic matter located in coarse iddingsite is detected by Raman spectroscopy in the iddingsite of many nakhlites and was confirmed by the TEM study of NWA 10153. In addition, the TEM study of NWA 10153 displays complex chemical zoning in the fine iddingsite of Mg, Ca, Mn, S, P, and Al, suggesting at least two stages of circulations. Both the compositions and textures of the two types of iddingsite are suggestive of a progressive evolution of the alteration fluid, enriched in elements from basaltic mineral dissolution, with crystallization mainly by filling of existing fractures, and selective dissolution of host olivine. We also observe pyrrhotite–magnetite veinlets at the center of iddingsite veins and cross-cutting iddingsite veins and silicates, which are interpreted as the result of another later fluid circulation.

Pink spinel anorthosite (PSA), a distinctive plagioclase and spinel-rich lithology (spinel >20%) observed on the lunar surface by the Moon Mineralogy Mapper (M³) imaging spectrometer, has sparked considerable interest in understanding magmatic processes on the Moon that cannot be explained by the well-established lunar magma ocean paradigm. Competing ideas on the PSA-forming mechanisms have invoked either (1) impact melting of troctolitic source rocks on the lunar surface or (2) magma–wallrock interactions between anorthositic crust and Mg-suite parental melts, but have been difficult to evaluate given the lack of ground truth samples. Here, we investigate the textures and mineral compositions of seven PSA clasts in lunar meteorite Northwest Africa (NWA) 15500, and the bulk trace element compositions of a PSA clast separate and NWA 15500 host lithologies A and B. Our findings suggest derivation of PSA from an incompatible-element-poor source and are consistent with PSA representing an Mg-suite lithology genetically related to pink spinel troctolites that reflects increased degrees of crustal assimilation during magma–wallrock interactions, and a sourcing of PSA far from the Procellarum KREEP Terrane. Excavation of PSA material was followed by multiple, subsequent localized impact events, resulting in the formation of Lithologies A and B.

MetBase has been the world's largest database for meteorite compositions, but has now passed this torch on to the Astromaterials Data System (Astromat), into which MetBase has recently been merged. This merger had been planned for some time and took almost 1 year to complete. Not only differences in the structure of the databases, in the content and organization of data and metadata, and in the terminology used but also incorporation of new data needed to be resolved to combine the data holdings of MetBase with the Astromat synthesis database. Astromat is NASA's primary archive for laboratory analyses of astromaterial samples and funded by NASA to provide services for the preservation and open access of data from astromaterials, including meteorites, in alignment with the FAIR principles. After merging MetBase into Astromat's synthesis database, this now provides the cosmochemical community the largest compilation of cosmochemical analytical data by far: over 2 million analytical data points. Astromat is also part of a bigger ecosystem of geo- and cosmochemcial databases, as its foundation is aligned with other large geochemical databases such as EarthChem and GEOROC. The visualization tools and the teaching tool from MetBase will be further developed and now exist as independent tools. We provide a brief history of the two databases and their journeys, an outlook toward the future, as well as lessons learned from this merger. We recommend that other cosmochemical databases try whenever possible to adopt the Astromat database schema as early as possible, or get in contact for alternative options. We believe MetBase now being a part of Astromat is a match made in heaven and hope Astromat will become a reliable and trusted service within the community.

The Shanghai Astronomy Museum (SAM) has opened its meteorite collections to the planetary science community. Inaugurated in July 2021, SAM is recognized as the world's largest astronomical museum and currently houses a collection of 97 meteorites weighing a total of 469 kg. These meteorites come from over 40 nations and encompass a diverse array of 37 different groups. Among them, 70 meteorites are displayed in the museum. The museum also features a series of interactive exhibition areas showcasing the internal structure of meteorites, engaging games introducing meteorite identification, and simulating the formation process of asteroid impact craters. This comprehensive range of offerings enables public access to extensive scientific knowledge about meteorites, making the museum a pivotal platform for disseminating meteoritics to the public.

Petrogenetic models for the howardite–eucrite–diogenite (HED) clan of achondrites have been challenged by the lack of substantial plagioclase in the HED record, which is at odds with the chemical composition of diogenites. Northwest Africa (NWA) 15118, an anorthositic achondrite, displays strong isotopic affinities with HEDs and has been proposed as a lunar-style primary flotation crust of the Vestan magma ocean. Nevertheless, a geochemical link with known HEDs, particularly diogenites, remains to be demonstrated. We present major, minor, and trace element data for plagioclase and orthopyroxene in NWA 15118. Despite textural evidence for post-crystallization shock and thermal metamorphism, transect major and minor element data reveal that igneous crystallization trends are preserved. Normalized trace element data reveal depletion in Ti, Nb, Hf, Zr in plagioclase and corresponding enrichment in orthopyroxene. Orthopyroxene in NWA 15118 does not plot on the Y versus Ti array formed by diogenite orthopyroxenes, which have a higher Ti/Y ratio. The calculated melt composition in equilibrium with NWA 15118 plagioclase has lower Ti/Y, Ti/Yb, and La/Sm ratios than melts in equilibrium with diogenite orthopyroxenes; differences in the melt compositions cannot be accounted for by the choice of partition coefficients or by single-stage magmatic processes. Therefore, we argue that NWA 15118 and diogenites are not complementary cumulates that crystallized simultaneously from a global Vestan magma ocean. Furthermore, the modeled evolution curve of such a magma ocean does not produce the composition of NWA 15118 plagioclase equilibrium melts in Ti-Y-Yb space, indicating that NWA 15118 is unlikely to have been a primary flotation crust of a global magma ocean. Our findings suggest that the incompatible trace element composition of NWA 15118 likely reflects more complex, multistage magmatic processes and/or source heterogeneities than envisioned in geochemistry-based HED petrogenetic models proposed to date.

The central uplift area of Araguainha impact structure (Brazil) includes a quartzite pebble- and cobble-bearing stratigraphic facies that have been profoundly affected by impact processes. These quartzite clasts have been studied previously for their planar deformation features (PDFs), but not with regard to their noteworthy transverse fractures. Petrographic study of transverse fractures within seven selected impact-affected cobbles from the conglomeratic fluvial facies of the target Devonian Furnas Formation (near the central uplift of Araguainha impact structure) has revealed that there is a micro-breccia within these transverse fractures, and this micro-breccia originated by comminution of the host cobble. Further, the transverse fractures in these cobbles have different styles (well-defined, poorly defined, complex, and diffuse) and are evidently post-shock, brittle deformation features. We suggest that a late compression-stage process, perhaps collapse of the central peak, may be responsible for the development of these transverse fractures.

Nucleosynthetic isotope variations are powerful tracers to determine genetic relationships between meteorites and planetary bodies. They can help to link material collected by space missions to known meteorite groups. The Hayabusa 2 mission returned samples from the Cb-type asteroid (162173) Ryugu. The mineralogical, chemical, and isotopic characteristics of these samples show strong similarities to carbonaceous chondrites and in particular CI chondrites. The nucleosynthetic isotope compositions of Ryugu overlap with CI chondrites for several elements (e.g., Cr, Ti, Fe, and Zn). In contrast to these isotopes, which are of predominately supernovae origin, s-process variations in Mo isotope data are similar to those of carbonaceous chondrites, but even more s-process depleted. To further constrain the origin of this depletion and test whether this signature is also present for other s-process elements, we report Zr isotope compositions for three bulk Ryugu samples (A0106, A0106-A0107, C0108) collected from the Hayabusa 2 mission. The data are complemented with that of terrestrial rock reference materials, eucrites, and carbonaceous chondrites. The Ryugu samples are characterized by distinct ⁹⁶Zr enrichment relative to Earth, indicative of a s-process depletion. Such depletion is also observed for carbonaceous chondrites and eucrites, in line with previous Zr isotope work, but it is more extreme in Ryugu, as observed for Mo isotopes. Since s-process Zr and Mo are coupled in mainstream SiC grains, these distinct s-process variations might be due to SiC grain depletion in the analyzed materials, potentially caused by incomplete sample digestion, because the Ryugu samples were dissolved on a hotplate only to avoid high blank levels for other elements (e.g., Cr). However, local depletion of SiC grains cannot be excluded. An alternative, equally possible scenario is that aqueous alteration redistributed anomalous, s-process-depleted, Zr on a local scale, for example, into Ca-phosphates or phyllosilicates.