Three caldera-shaped accidents: Volcanic calderas,meteoric scars and lunar cirques

A caldera is a large volcanic depression, more or less circular, the diameter of which is many times greater than those of the included volcanic vents. Calderas must be separated from feetono-volcanic depressions, which have an irregular shape. Volcanic calderas are produced by engulfment. The scars, produced by the impact of meteorites on the earth, are circular or elliptical depressions. Lunar cirques are nearly all circular; some of them have a polygonal, and then sometimes an hexagonal shape. On the surface of the moon elliptical depressions are wholly absent. Moreover, on the moon it is a rigid law that when intersections of ring plains do occur, the smaller cirque is entire, the rim of the middlemost being interrupted by the smallest, whereas the biggest is interrupted by the middle one. This phenomenon would be in accordance with a volcanic origin and a decreasing volcanic activity whereas it is incompatible with an impact of meteorites.     

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (??_in) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in ??_in signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the ??_in pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al₂O₃, TiO₂) and ??_in for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations ??_in may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.     

Prediction and analysis of GBAS integrity monitoring availability at LinZhi airport

Prediction and analysis of GBAS integrity monitoring is important, especially at the airports where a GBAS station it to be installed. Based on existing standard documents and published research, we present a software tool for GBAS availability prediction. Simulations have been conducted that include single point, single approach path, and multiple repetitions of a same approach path in order to analyze the availability of GNSS signal integrity monitoring with a GBAS at LinZhi airport. The results show that the long-term 24-h service availability figure at two typical single points along the approach path for GBAS Approach Service Type C (GAST C) is above 99.999 % for each point, and for GBAS Approach Service Type D (GAST D) at three typical single points, it is lower than 99.8 % for each point. The unavailability percentage over a 24-h period is 0.76 and 2.40 % for GAST C and GAST D, respectively. The results of sensitivity tests show that the impact of the mask angle and the latitude on the GBAS availability at LinZhi airport are more important than that of the constellation. Our conclusions could also be of interest for the implementation of GBAS stations at other plateau airports.     

Multi-isotope (Ba,C, O) partitioning during experimental carbonatization of a hyper-alkaline solution

Carbonates formed from hyperalkaline aqueous solutions at the Earth?s surface are known to bear the most extreme disequilibrium isotope signatures reported so far in nature. We present here the results for stable carbon (C), oxygen (O), and barium (Ba) isotope fractionation during the precipitation of witherite (BaCO₃) induced by the chemical absorption of atmospheric carbon dioxide (CO₂) into an aqueous hyper-alkaline solution (at 4° and 21?°C; 1?atm total pressure). Independent from temperature, the barium carbonate formation was associated with a substantial enrichment of the lighter C and O isotopes in the solid compared to the atmosphere (C, O), close to previous results found in experiments and nature. A new approach is introduced to explain oxygen isotope fractionation upon hydroxylation of CO₂. With Ba isotope enrichment factors between ?0.45 and ?0.53‰ (^138/134ε) or ?0.34 and ?0.40‰ (^137/134ε), respectively, the synthesized BaCO₃ displays the highest kinetic enrichment of the light Ba isotope in the carbonate solid reported so far.     

Insight into the development of a carbonate platform through a multi-disciplinary approach: a case study from the Upper Devonian slope deposits of Mount Freikofel (Carnic Alps,Austria/Italy)

The development and behavior of million year-scaled depositional sequences recorded within Palaeozoic carbonate platform has remained poorly examined. Therefore, the understanding of palaeoenvironmental changes that occur in geological past is still limited. We herein undertake a multi-disciplinary approach (sedimentology, conodont biostratigraphy, magnetic susceptibility (MS), and geochemistry) of a long-term succession in the Carnic Alps, which offers new insights into the peculiar evolution of one of the best example of Palaeozoic carbonate platform in Europe. The Freikofel section, located in the central part of the Carnic Alps, represents an outstanding succession in a fore-reef setting, extending from the Latest Givetian (indet. falsiovalis conodont zones) to the Early Famennian (Lower crepida conodont zone). Sedimentological analysis allowed to propose a sedimentary model dominated by distal slope and fore-reef-slope deposits. The most distal setting is characterized by an autochthonous pelagic sedimentation showing local occurrence of thin-bedded turbiditic deposits. In the fore-reef slope, in a more proximal setting, there is an accumulation of various autochthonous and allochthonous fine- to coarse-grained sediments originated from the interplay of gravity-flow currents derived from the shallow-water and deepwater area. The temporal evolution of microfacies in the Freikofel section evolves in two main steps corresponding to the Freikofel (Unit 1) and the Pal (Unit 2) limestones. Distal slope to fore-reef lithologies and associate changes are from base to top of the section: (U1) thick bedded litho- and bioclastic breccia beds with local fining upward sequence and fine-grained mudstone intercalations corresponding, in the fore-reef setting, to the dismantlement of the Eifelian–Frasnian carbonate platform during the Early to Late Frasnian time (falsiovalis to rhenana superzones) with one of the causes being the Late Givetian major rift pulse; (U2) occurrence of thin-bedded red nodular and cephalopod-bearing limestones with local lithoclastic grainstone intercalations corresponding to a significant deepening of the area and the progressive withdrawal of sedimentary influxes toward the basin, in relation with Late Frasnian sea-level rise. MS and geochemical analyses were also performed along the Freikofel section and demonstrate the inherent parallel link existing between variation in MS values and proxy for terrestrial input. Interpretation of MS in terms of palaeoenvironmental processes reflects that even though distality remains the major parameter influencing MS values, carbonate production and water agitation also play an important role.     

Über die Entstehung des Reliefs auf den sogenannten „Rillensteinen“

Ohne Zusammenfassung     

Near-isothermal decompression within a clockwise P–T evolution recorded in migmatitic mafic granulites from Clavering Ø, NE Greenland: implications for the evolution of the Caledonides

The high grade rocks (metapelites and metabasites) of Clavering Ø represent the easternmost exposures of granulites in the Palaeozoic Caledonian Orogen of East Greenland. Mafic granulites which occur as sheet‐like bodies and lenses within metapelitic migmatites and orthogneiss complexes have experienced migmatisation and mineral equilibria which define a clockwise P–T path incorporating a near‐isothermal decompression segment. Textures demonstrate the existence of early garnet‐clinopyroxene‐melt assemblages which equilibrated at >8–11 kbar and 850–915 °C. Subsequently, decompression melting led to formation of orthopyroxene‐plagioclase‐melt assemblages at conditions below >8–11 kbar. Continued syn‐deformational decompression is indicated by a combination of both static and syn‐deformational recrystallization textures which generated finer grained orthopyroxene‐plagioclase assemblages. P–T constraints indicate these assemblages equilibrated at c. 5.0–6.5 kbar at 850–915 °C. These data are consistent with the rocks undergoing a stage of rapid tectonic‐induced exhumation involving some 3.0–4.5 kbar (c.10–12 km) uplift as part of a clockwise P–T path in a collisional setting.