Meteoritics & Planetary Science最新文献

The Rochechouart impact structure in the northwestern part of the French Massif Central (FMC) has a great diversity of impactites, including monomict impact breccias, suevite, and impact melt rocks (IMRs). The structure is strongly eroded, which allows the study of impactites of the crater fill and the transition into the crater floor. The FMC has had a multistage geological evolution from the late Neoproterozoic to the Ordovician (600–450 Ma) until the later stages of the Variscan orogeny (~300 Ma). Previous geochronological work on Rochechouart has been focused mainly on the impactites and constraining the impact age, and scarce work has been done on the FMC-related target rocks. Here, U-Pb isotope analysis by LA-MC-ICP-MS has been conducted on zircon from two IMRs from the Recoudert and Montoume localities, and from a monzodiorite, a paragneiss, and two amphibolite samples of the basement to the impact structure. Zircon from the target rocks yielded mainly Neoproterozoic to Carboniferous ages (~924 to ~301 Ma) that can mostly be correlated to different stages of the geological evolution of the FMC. The monzodiorite also yielded a Permian age of 272 ± 12 Ma. Zircon from the IMRs, and especially from the Montoume sample, gave a comparatively higher diversity of Neoproterozoic to Jurassic ages (~552 to ~195 Ma). Provenance analysis for the zircon age populations of the impactites compared to those of the basement rocks shows overall poor correlation between the two age groups. This suggests that other target lithologies were involved in the formation of these impact melts as well. Post-Variscan and preimpact ages (281–226 Ma) obtained for both melt rocks probably reflect a previously unconstrained event in the evolution of the regional geological history. Ages similar to the currently most widely accepted impact age of ~204–206 Ma were obtained from both IMR samples. In addition, the Montoume melt rock yielded several post-204 Ma ages, which might reflect a to date unconstrained, about 194 Ma postimpact thermal/hydrothermal event.

Extensive, new outcrops along the MT-100 state road in the northern part of the central uplift of the 40-km diameter, 252–259 Ma old Araguainha impact structure, Central Brazil, have become available for investigation. They offer new insight into the contact relationships between the different lithologies and the genesis of different types of impact-related rocks, as well as the current level of erosion of the structure. Three types of impact melt rock (IMR) with different field relationships and compositions can now be distinguished: (1) Type-I of granitic composition and occurring mainly as veins and dikes, besides a few larger pods, in the central alkali granite core of the central uplift; (2) Type-II in the form of plastically deformed clasts of mainly highly silicious compositions in polymict impact breccia; and (3) Type-III, derived from partially melted conglomerate or sandstone precursors, and that occurs at selected sites in (meta)sedimentary strata of the basement in the immediate environs of the alkali granite core. Both polymict lithic and melt-bearing (suevitic) impact breccias are recognized in the 110-m thick integrated section through impact breccia directly overlying the crater floor. This crater floor is composed of (meta)-sedimentary basement strata with granite injections and, locally, sandstones of the Devonian sedimentary Furnas Formation of the Paraná Basin. Main breccia components are (meta)-pelites and (meta)sandstones of the basement that is currently favored to be related to the regional Paraguay Belt and to the lower sequence of the Paraná Basin sedimentary strata. Locally, breccia contains clasts of IMR Type-II, and only very rarely are granitic fragments observed. Clasts of IMR Type-I have never been observed in the breccia deposits. These new observations preclude significant involvement of alkali granite in the formation of the polymict breccia or in the production of shock melts. They also reveal the major role of the (meta)sedimentary precursors in the production of IMR by shock melting and provide essential information for better understanding the cratering processes involved in the formation of an impact structure in a sedimentary target, of the size of the Araguainha impact structure.

Precise measurements of Cr isotopic composition of terrestrial impactites have successfully provided evidence for the presence of extraterrestrial material and have, in some cases, allowed the identification of the type of impactor responsible for the formation of the impact structure. The high Cr abundance in most meteorite groups aids in detecting extraterrestrial contamination while their distinct isotopic compositions can help with the identification of the nature of the projectile. However, this common approach of detection and identification of extraterrestrial contamination using mass-independent ⁵³Cr and ⁵⁴Cr variations fails when the impactor type is an iron meteorite because of their low Cr abundances (which are in a similar range to terrestrial rocks). The present study demonstrates the viability of a spallogenic Cr contribution in iron meteorites (resulting from their long cosmic ray exposure times), which compensates for their low Cr abundances and facilitates the identification of iron-meteoritic contamination in terrestrial impactites. Thus, it broadens the scope of impactors (and impactites) that can be investigated using mass-independent Cr isotopes from solely chondrites and primitive achondrites to include iron meteorites. The Wabar impact craters are an optimal candidate for this study, characterized by low weathering, diverse impactites, partial meteorite survival, substantial impactor material contamination, and a felsic target lithology with low background Cr concentration. The Cr isotopic composition of the Wabar background sand, which represents the target lithology, is indistinguishable from the terrestrial Cr isotopic composition range, whereas the Wabar iron meteorites show coupled spallogenic excesses in ε⁵³Cr and ε⁵⁴Cr. The Cr isotopic compositions of Wabar impactites show resolved deviations from the terrestrial Cr isotopic composition, thereby indicating the presence of Wabar meteoritic contamination. Moreover, the study demonstrates that even an impactor with a non-carbonaceous chondritic origin, such as a IIIAB iron meteorite, can have a carbonaceous chondrite-like signature in ε⁵⁴Cr anomalies due to spallogenic Cr contamination. The study advocates for a comprehensive investigation combining platinum group elements and Cr (and/or Ni, Ru) isotopes to accurately characterize impactor types.

One of the most common types of allochthonous impactite produced in hypervelocity impact events is impact breccia that contains melt particles. In numerous terrestrial hypervelocity impact structures such melt-bearing breccias have been termed “suevite,” after the type locality at the Ries impact structure, Germany. Despite its widespread occurrence, the origin, emplacement, and classification of suevite remains debated. In this contribution, we re-examine the nature and origin of suevite at the Ries impact structure. The results of new field and laboratory investigations, when combined and synthesized with results from previous studies, lead to a multi-stage model for the origin and emplacement of allochthonous impactites during the Ries impact event. Following the creation of a transient cavity the so-called Bunte Breccia and “megablocks” were emplaced via ballistic sedimentation and subsequent radial flow during the excavation stage to form a continuous ejecta blanket. At the end of the excavation stage, a mixture of melt and lithic fragments formed a lining to the transient cavity and it is this material that later became the crater, dike, and outer suevite (OS) units. The crater suevite represents the material from the displaced zone of the transient cavity that was transported and mixed but never left the cavity. The emplacement of dike suevite occurred during the modification stage as the crater suevite was intruded into fractures in the underlying crater floor. The OS and rare impact melt rocks overlying the ballistic (Bunte Breccia) ejecta deposits were emplaced as outwards-directed ground-hugging flows largely during the modification stage of crater formation. The OS flows varied both spatially and temporally in terms of the flow characteristics, from being dominated by solid particles and gas (cf. pyroclastic density currents) to a mixture of solid particles, liquid (impact melt), and minor gases (i.e., particulate impact melt-rich flows). These particulate impact melt-rich flows dominated by far. Minor “fallback” of material from an ejecta plume is evidenced by accretionary lapilli in the Nördlingen 1973 core. In summary, allochthonous impactites at the Ries impact structure are not unusual but are consistent with observations from other terrestrial and planetary craters, where melt-rich impactites overly ballistic ejecta deposits both outside and inside crater rims and where melt-rich impactites occur in crater interiors.

Understanding the mineralogy of the Martian mantle is essential for constructing geochemical and geophysical models of Mars. This study employs the pMELTS program to determine the mineralogy at the solidus from 11 published bulk silicate Mars (BSM) compositions, within a pressure range of 2–5 GPa. The pMELTS results align with experimental data and calculations from another thermodynamic program (Perple_X/stx11). Mineral modes from compositional models based on Martian meteorite geochemistry show relatively consistent abundances modes (olivine: 48–56 wt%, orthopyroxene: 20–25 wt%, clinopyroxene: 15–17 wt%, garnet: 6–9 wt%). In contrast, mineral modes from compositional models that are not based on Martian meteorite geochemistry exhibit a wider range of olivine and garnet abundances. Additionally, we constrained the mineral modes of the Martian mantle using trace element partitioning and partial melting models. Our calculations indicate that melts derived from mantle sources with a hypothesized garnet content of 5–10 wt% closely match the analyzed compositions of shergottites, validating the garnet mode (6–9 wt%) constrained in our pMELTS calculations. Extracting low-degree (<4 wt%) melts from a BSM to form depleted Martian mantle (DMM) does not significantly alter the mineralogical modes of solid residues, but it does lead to substantial trace elemental depletion in the DMM. Therefore, enriched, intermediate, and depleted shergottite sources are likely characterized by similar mineral modes yet differ in incompatible element abundances.

Isolated corundum grains and corundum ± Mg-deltalumite [(Al,Mg)(Al,◻)₂O₄] ± hibonite assemblages were investigated in the CH3.0 metal-rich carbonaceous chondrite Sayh al Uhaymir (SaU) 290. Although very refractory inclusions containing abundant Zr- and Sc-rich oxides and silicates, hibonite, grossite, or perovskite have been previously described in CH chondrites, this is the first discovery of corundum and Mg-deltalumite in CHs and the first discovery of Mg-deltalumite in nature. Magnesium-deltalumite can be indexed by the Fd3m spinel-type structure and gives a perfect fit to the synthetic Al-rich spinel cells. Corundum-Mg-deltalumite grains, 5–20 μm in size, are occasionally rimmed by a thin layer of hibonite replacing corundum. Some corundum grains contain tiny inclusions of ultrarefractory Zr,Sc-rich minerals and platinum-group element (PGE) nuggets. All corundum, hibonite, and Mg-deltalumite grains studied have ¹⁶O-rich compositions (average Δ¹⁷O ± 2SD = −22 ± 3‰). Two corundum grains show evidence for significant mass-dependent fractionation of oxygen isotopes: Δ¹⁸O ~ +34‰ and ~ +19‰. We suggest that the SaU 290 corundum-rich objects were formed by evaporation and/or condensation in a hot nebular region close to the proto-sun where the ambient temperature was close to the condensation temperature of corundum. A corundum grain with tiny inclusions of Zr- and Sc-rich phases and PGE metal nuggets recorded formation temperatures higher than the condensation temperature of corundum. Two corundum-rich objects with highly fractionated oxygen isotopes must have crystallized from a melt that experienced evaporation. Corundum grains corroded by hibonite recorded gas–solid interaction in this region during its cooling. The Mg-deltalumite ± corundum ± hibonite objects were formed by rapid crystallization of high-temperature (>2000°C) refractory melts. The lack of minerals with condensation temperatures below those of corundum and hibonite in the SaU 290 corundum-rich objects suggests that after formation, these objects were rapidly removed from the hot nebular region by disk wind and/or by turbulent diffusion and disk spreading.

Northwest Africa (NWA) 14672, the most highly shocked Martian meteorite so far, has experienced >50% melting, compatible with peak pressure >~65 Gpa, at a transition stage 6/7. Despite these extreme shock conditions, the meteorite still preserves a population of “large” Fe sulfide blebs from the pre-shock igneous assemblage. These primary blebs preserve characteristics of basaltic shergottites in term of modal abundance, preferential occurrence in interstitial pores along with late-crystallized phases (ilmenite, merrillite), and Ni-free pyrrhotite compositions. Primary sulfides underwent widespread shock-induced remelting, as indicated by perfect spherical morphologies when embedded in fine-grained silicate melt zones and a wealth of mineral/glass/vesicle inclusions. Extensive melting of Fe-sulfides is consistent with the decompression path experienced by NWA 14672 after the peak shock pressure at ~70 GPa. Primary sulfides acted as preferential sites for nucleation of vesicles of all sizes which helped sulfur degassing during decompression, leading to partial resorption of Fe-sulfide blebs and reequilibration of pyrrhotite metal/sulfur ratios (0.96–0.98) toward the low oxygen fugacity conditions indicated by Fe-Ti oxides hosted in fine-grained materials. The extreme shock intensity also provided suitable conditions for widespread in situ redistribution of igneous sulfur as micrometric globules concentrated in glassy portions of fine-grained lithologies. These globules exsolved early on quenching, allowing dendritic skeletal Fe-Ti oxide overgrowths to nucleate on sulfides.