MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement,cooling history,and trace element contents

MgAl₂O₄ spinels from Allende and NWA 763 carbonaceous chondrites were studied by X‐ray single crystal diffraction, SEM, electron microprobe, LA‐ICP‐MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeO_tot zonation. Spinels from NWA 763 show Mg‐Fe²⁺ substitutions. Almost pure MgAl₂O₄ spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (T_c) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a T_c of about 720 °C. X‐ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr‐spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO.     

On the formation of cold fronts in massive mergers

Using adiabatic hydrodynamical simulations, we follow the evolution of two symmetric cold fronts forming in the remnant of a violent   z = 0.3  massive cluster merger. Because the fronts develop after the first passage of the two gas cores of the merging subclusters, and because they soon move ahead of their associated dark matter cores, both the structure and the location of our simulated cold fronts may correspond to a stage that is later than that of most cold fronts observed so far. The cold fronts are preceded by a roughly spherical shock that originates in the centre of the cluster and disappears in the outer regions after 1.6 Gyr. The cold fronts last longer, until   z ∼ 0  . We follow the spatial evolution of the gas of the subcluster cores, and find that a fraction of this gas is liberated in the intracluster medium after core passage, but mainly at apocentre, and that it does not fall back onto the cluster centre. Conversely, we trace back the low-temperature gas constituting the fronts and find that it is initially associated with the two dense cores of the merging clusters. In addition, we find some evidence for discontinuity of the gas velocity field across the edge of the forming cold fronts, suggesting the presence of a contact discontinuity there. In the light of other recent work, we then speculate on the physical mechanism resulting in the cold fronts. We suggest that sloshing induced by strongly varying ram pressure along the subcluster's orbit and/or spatial segregation between the dark matter and gas components of the cores of the subclusters results in strong tidal forces on the gas, and that these forces could be responsible for the deposition of part of the cold dense gas in the surrounding hot intracluster medium. This deposited gas then expands, cools down further, and constitutes the cold fronts.     

The densest meteorite collection area in hot deserts: The San Juan meteorite field (Atacama Desert,Chile)

Abstract– We describe the geological, morphological, and climatic setting of the San Juan meteorite collection area in the Central Depression of the Atacama Desert (Chile). Our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3.88 km² area. The recovery density is in the range 9–12 falls km^?2 depending on pairing, making it the densest among meteorite collection areas in hot deserts. This high meteorite concentration is linked to the long‐standing hyperaridity of the area, the stability of the surface pebbles (> Ma), and very low erosion rates of surface pebbles (approximately 30 cm Ma^?1 maximum). The San Juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka, and limited weathering compared with other dense collection areas in hot desert. Chemical weathering in San Juan is slow and mainly controlled by the initial porosity of meteorites. As in the Antarctic and other hot deserts, there is an overabundance of H chondrites and a shortage of LL chondrites compared with the modern falls population, suggesting a recent (< few ka) change in the composition of the meteorite flux to Earth.     

Relativistic positioning: four-dimensional numerical approach in Minkowski space-time

We simulate the satellite constellations of two Global Navigation Satellite Systems: Galileo (EU) and GPS (USA). Satellite motions are described in the Schwarzschild space-time produced by an idealized spherically symmetric non rotating Earth. The trajectories are then circumferences centered at the same point as Earth. Photon motions are described in Minkowski space-time, where there is a well known relation, (Coll et al. in Class. Quantum Gravit. 27:065013, 2010a), between the emission and inertial coordinates of any event. Here, this relation is implemented in a numerical code, which is tested and applied. The first application is a detailed numerical four-dimensional analysis of the so-called emission coordinate region and co-region. In a second application, a GPS (Galileo) satellite is considered as the receiver and its emission coordinates are given by four Galileo (GPS) satellites. The bifurcation problem (double localization) in the positioning of the receiver satellite is then pointed out and discussed in detail.