Meteoritics & Planetary Science最新文献

Lunar dunite samples 72415–72417, collected by Apollo 17 astronauts from a South Massif boulder in the Taurus–Littrow valley, are crushed breccias composed of several types of olivine- and clinopyroxene-rich clasts, some of which are (or contain) intergrowths of Cr-spinel and pyroxenes or plagioclase. Among the clasts are ellipsoidal symplectites of Cr-spinel and pyroxene, up to 300 μm in diameter, which have bulk compositions consistent with those of olivine + garnet. These symplectites are inferred to originally have been olivine + Mg-Cr-rich garnet (pyrope–uvarovite) that formed deep in the lunar mantle and were subsequently transported closer to the lunar surface (spinel- or plagioclase-peridotite stability fields), perhaps during gravitationally driven overturn. Abundant microsymplectite (30 μm diameter) intergrowths of Cr-spinel and pyroxene inside olivine grains, many associated with inclusions of plagioclase and augite, formed during a later decompression event (perhaps excavation to the lunar surface). These inclusions have not previously been recorded in these samples and could be responsible for earlier reports of igneous zoning in olivine. Electron backscatter diffraction data show evidence of high shock pressures (>50 GPa), which are inferred to have occurred during the impact which excavated the dunites from the shallow anorthite-bearing lunar mantle. Apatite veinlets post-date the shock metamorphism and have been dated to 3983 ± 72 Ma and 3913 ± 118 Ma by the U–Pb method. This age is consistent with that inferred for the Imbrium impact basin, suggesting that the dunite was finally excavated from the mantle during formation of the Imbrium basin.

Basaltic shergottites are the most abundant rock type of Martian meteorites, and pyroxene grains within shergottites commonly show a zoned structure. Here, the detailed microscopic mineralogical characteristics of patchy zoned pyroxene in basaltic shergottite NWA 12522 were investigated by a combination of scanning electron microscopy, electron microprobe, Raman spectroscopy, and transmission electron microscopy. The results show that the cores of zoned pyroxene in NWA 12522 have a homogeneous Mg# value and consist mainly of augite and pigeonite. By contrast, the rim of zoned pyroxene is extremely ferroan and can be further divided into two regions based on quite distinct mineralogy and textures (i.e., far-core and near-core pyroxene rims). The near-core rim shows narrow exsolution lamellae (~35 nm) that were cross-cut by thin pigeonite veinlets and contain abundant nano-sized particles of metastable pyroxferroite and pigeonite. Only relatively coarse exsolution lamellae (~80 nm) were observed in the far-core pyroxene rim regions. The distinct mineralogical characteristics of the pyroxene rims and cores in NWA 12522 imply different crystallization conditions, and the homogeneous Mg-rich pyroxene cores should have slowly crystallized from magma within a deep-seated chamber, followed by an overgrown evolved melt on these pyroxene cores during their ascent to the Martian surface, and disequilibrium crystallization of nano-sized metastable phase (pyroxferroite) occurred in the near-core region. The abnormally low ΣREE contents and steep REE pattern (high Yb/La ratio) of the pyroxene rims in NWA 12522 imply that merrillite should have crystallized prior to the pyroxene rims, making the residual melt become REE-depleted and HREE-enriched.

The temperature and pressure conditions experienced by rocks during an impact event can be constrained using petrologic and microstructural analysis and is crucial to providing ground truth to the impact cratering process. Suevite is a polymict, impact melt-bearing breccia, specific to Ries crater in Germany. There are competing models for suevite formation and emplacement, such as clastic flows pushed out of the crater rim or ejecta plume fallback. Knowledge of the temperature and pressure pathways recorded by grains within the suevite can help distinguish between these and other models. The accessory phase zircon (ZrSiO₄) and its high-pressure polymorph reidite are particularly useful in such circumstances as they are highly refractory minerals that can record the high-temperature and/or high-pressure conditions of an impact event. Here, we present evidence for a wide array of temperature and pressure conditions recorded in zircon grains within a single thin section of suevite. Zircons in this study range from unshocked to highly shocked (>53 GPa), and record temperatures more than 1673°C. These findings confirm previous studies concluding that suevites contain material exposed to very diverse pressure and temperature conditions during initial shock compression and excavation but do not, as a whole, experience extreme temperatures (>1673°C) or pressures (>30 GPa).

Here we report on findings for four rock samples with melt texture found in a gravel pit within a glaciofluvial deposit near the small town of Kitkiöjärvi in northernmost Sweden. The samples are comprised of granitic clasts embedded in a brown fine-grained melt matrix. The samples all contain quartz grains and/or clasts exhibiting multiple sets of planar deformation features oriented parallel to crystallographic planes characteristic of shock metamorphism. The samples also contain Former Reidite In Granular Neoblastic (FRIGN) zircon. We therefore conclude that the investigated samples represent impact melt rock. We interpret a U-Pb concordia age of 658.9 ± 6.9 Ma (Cryogenian) derived using secondary-ion mass spectrometry analysis of shocked zircon, as the best estimate for the age of the impact event that formed the melt rocks. Zircon grains from two of the samples yield younger lower intercept ages, raising the possibility that the samples came from multiple impact events of different ages. Although we cannot exclude this possibility, we interpret the younger ages from the clast-rich melt rocks to reflect non-impact-related Pb loss events and suggest that all samples likely came from the same structure. Analysis of the glaciofluvial history of the region, along with the relatively high frequency of finds (five in total, as one similar melt rock was found in the pit in 2018), points to a short-distance glacial transportation of the samples from the southwest. Since there are no known impact structures in Sweden within that area and/or of similar age, we conclude that an old (the oldest known yet) impact structure in Sweden potentially is yet to be discovered somewhere in the vicinity of the gravel pit.

The 369 m deep M4 drill hole, located ~18 km NNW of the center of the 146 Ma Morokweng impact structure (MIS), intersects shocked Archean granitoid gneisses and subsidiary dolerite intrusions that are cut by faults, cataclasites and mm- to m-wide suevitic and pseudotachylitic breccia dikes. The shock features in quartz in the gneisses and breccia dikes include decorated planar deformation features (PDFs), planar fractures, feather features, and toasting. Other minerals show features that may be shock-related, such as multiple sets of planar features and alternate twin ladder structures in feldspars, kink bands in biotite, and planar features in titanite, apatite, and zircon; however, these are variably annealed and/or overprinted by hydrothermal alteration effects, and confirmation of their origin awaits further study. Universal Stage measurements of PDF sets in quartz from 12 gneissic target rocks and from lithic and mineral clasts in three suevitic and three pseudotachylitic breccia dikes reveal four dominant sets: (0001), {$��10�\overline{1}�3�$}, {$��10�\overline{1}�4�$} and {$��10�\overline{1}�2�$}. Based on these observations, the average peak shock pressure in these rocks is estimated at ≤16 GPa, which supports the original proximity (within one transient cavity radius) of these rocks to the point of impact. No discernible depth-dependent shock attenuation was noted in the core. These shock levels and the elevated structural position of the rocks in the M4 core relative to the impact melt sheet intersected in drill holes closer to the center of the MIS suggest that the M4 lithologies represent part of the parautochthonous peak ring volume that subsequently experienced 1.5–2 km of post-impact erosion before it was buried beneath younger sediments. Numerical modeling using the iSALE-2D code suggests that the original Morokweng crater had a rim-to-rim diameter of between 70 and 80 km, and that the rocks in the M4 core were originally located at a depth of 7–8 km and a radial distance of 8–9 km from the point of impact.