Endogenous and nonimpact origin of the Arkenu circular structures (al‐Kufrah basin—SE Libya)

Abstract– The twin Arkenu circular structures (ACS), located in the al‐Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica‐impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind‐erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper‐gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic‐phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.     

Steinheim suevite—A first report of melt‐bearing impactites from the Steinheim Basin (SW Germany)

Abstract– The 3.8 km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well‐preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B‐26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO₂ (～50 wt%) and Al₂O₃ (～28 wt%), and contain particles of Fe‐Ni‐Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and droplet‐shaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and/or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt‐bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones (“Eisensandstein” Formation) that crop out at the center of the central uplift as the source for the melt fragments.     

Environmental dependence of the presence of active galactic nuclei (AGNs) in the volume‐limited Main galaxy samples of SDSS DR6

Using two volume‐limited Main galaxy samples of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6) above and below the value of M^∗︁_r, we have explored the environmental dependence of the AGN fraction. It is found that the fraction of AGNs declines substantially with increasing local density in the luminous volume‐limited sample, but in the faint volume‐limited sample this change is very weak. We also note that the presence of AGNs is not correlated with the stellar mass (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The nondetection of continuum radiation from comet IRAS-Araki-Alcock (1983d) at 2- to 6-cm wavelengths and its implication on the icy-grain halo theory

Observations of Comet IRAS-Araki-Alcock have been made with the VLA (Very Large Array) at 6 and 2 cm, when the comet was at geocentric distances of 0.08 and 0.035 AU, respectively. The 3σ upper limits are 90 and 750 μJy at 6 and 2 cm, respectively. We show that the “conventional” icy-grain halo theory is not adequate to explain the data. If there is such a halo, it is either very thin, or does not contain grains with sizes larger than 10–100 μm. Comparison of our limits with a reported detection at 1.3-cm wavelengths shows that if the centimeter-wavelength radiation all arises in the halo, the halo should have an extent of the order of 300–400 km, but an effective area of 100 km². If only thermal emission from the nucleus is significant, the temperature decreases from about 200°K at the layers probed at 1.3 cm to about 50°K or less at depths probed at 2 cm (assuming unit emissivity at all wavelengths and depths). This can be due to a combination of a lower emissivity and lower physical temperature at larger depths in the comet; both effects are expected when considering theories on microwave emission from glaciers on Earth.     

In situ Pb‐Pb dating of silica‐rich Northwest Africa (NWA) 6594 basaltic eucrite and its constraint on thermal history of the Vestan crust

Eucrites represent one of the major lithologies of the Vestan upper crust, which had experienced pervasive and intense thermal metamorphism. To better constrain the timing and mechanism of thermal metamorphism, we carried out in situ Pb‐isotope analysis of an unbrecciated basaltic eucrite NWA 6594 on the basis of detailed mineralogical and petrographic investigations. Zircon Pb‐Pb dating reveals that NWA 6594 emplaced before or at 4547 ± 11 Ma (95% confidence, MSWD = 1.3). Studies of silica minerals indicate that NWA 6594 had experienced intense thermal metamorphism after emplacement, followed by a late impact reheating and rapid cooling. Apatite grains yield a weighted mean Pb‐Pb age of 4523 ± 2 Ma (95% confidence, MSWD = 0.76). This age could not be attributed to slow cooling after the initial crystallization, but most likely related to an independent thermal event that induced thermal metamorphism. The protracted time lag (~24 ± 13 Myr) between zircon and apatite closure ages indicates that this thermal event is most probably induced by an intense impact event that was synchronous with the metal–silicate mixing event recorded by mesosiderites. HEDs may have experienced multiple stages of thermal metamorphism after emplacement. The late impact reheating occurred after thermal metamorphism, which caused crystallization of tridymite.     

Tuite, γ‐Ca3(PO4)2, formed by chlorapatite decomposition in a shock vein of the Suizhou L6 chondrite

Tuite, γ‐Ca₃(PO₄)₂, was first discovered as the high‐pressure phase of whitlockite in shock veins of the Suizhou L6 meteorite. This study reports the finding of tuite in a shock vein of the same Suizhou chondrite as a product of decomposition of chlorapatite, where it coexists with coarse‐grained ringwoodite, majorite, lingunite, fine‐grained majorite‐pyrope solid solution, and magnesiowüstite. Moreover, we also successfully synthesized tuite with a multianvil apparatus from chlorapatite at 15 GPa and 1573 K over 24 h. Both natural and synthetic tuite crystals were examined by means of optical microscopy, scanning electron microscope, electron microprobe analysis, X‐ray diffraction, and Raman spectroscopy. Our results suggest that the Na₂O, MgO, and Cl contents in natural tuite may serve as good indicators for distinguishing the precursor phosphate mineral, chlorapatite or whitlockite.