Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a numerical simulation

The evolution of a subduction channel and orogenic wedge is simulated in 2D for an active continental margin, with P-T paths being displayed for selected markers. In our simulation, subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years, with four zones from the trench into the forearc: (i) an accretionary complex of low grade metamorphic sedimentary material, (ii) a wedge of nappes with alternating upper and lower crustal provenance, and minor interleaving of oceanic or hydrated mantle material, (iii) a megascale melange composed of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks extruded from the subduction channel, and (iv) the upward tilted frontal part of the remaining lid. The P–T paths and time scales correspond to those typically recorded in orogenic belts. The simulation shows that HP/UHP metamorphism of continental crust does not necessarily indicate collision, but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction.     

Kampelite,Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O,a new very complex Ba-Sc phosphate mineral from the Kovdor phoscorite-carbonatite complex (Kola Peninsula,Russia)

Mineralogy and Petrology - Kampelite, Ba3Mg1.5Sc4(PO4)6(OH)3·4H2O, is a new Ba-Sc phosphate from the Kovdor phoscorite-carbonatite complex (Kola Peninsula, Russia). It is orthorhombic, Pnma,...     

Analyzing the commercial activities of a street network by ranking their nodes: a case study in Murcia,Spain

Urban researchers and planners are often interested in understanding how economic activities are distributed in urban regions, what forces influence their special pattern and how urban structure and functions are mutually dependent. In this paper, we want to show how an algorithm for ranking the nodes in a network can be used to understand and visualize certain commercial activities of a city. The first part of the method consists of collecting real information about different types of commercial activities at each location in the urban network of the city of Murcia, Spain. Four clearly differentiated commercial activities are studied, such as restaurants and bars, shops, banks and supermarkets or department stores, but obviously we can study other. The information collected is then quantified by means of a data matrix, which is used as the basis for the implementation of a PageRank algorithm which produces a ranking of all the nodes in the network, according to their significance within it. Finally, we visualize the resulting classification using a colour scale that helps us to represent the business network.     

Numerical models of the thermomechanical evolution of planetesimals: Application to the acapulcoite‐lodranite parent body

The acapulcoite‐lodranite meteorites are members of the primitive achondrite class. The observation of partial melting and resulting partial removal of Fe‐FeS indicates that this meteorite group could be an important link between achondrite and iron meteorites, on the one hand, and chondrite meteorites, on the other. Thus, a better understanding of the thermomechanical evolution of the parent body of this meteorite group can help improve our understanding of the evolution of early planetesimals. Here, we use 2‐D and 3‐D finite‐difference numerical models to determine the formation time, initial radius of the parent body of the acapulcoite‐lodranite meteorites, and their formation depth inside the body by applying available geochronological, thermal, and textural constraints to our numerical data. Our results indicate that the best fit to the data can be obtained for a parent body with 25–65 km radius, which formed around 1.3 Ma after calcium‐aluminum‐rich inclusions. The 2‐D and 3‐D results considering various initial temperatures and the effect of porosity indicate possible formation depths of the acapulcoite‐lodranite meteorites of 9–19 and 14–25 km, respectively. Our data also suggest that other meteorite classes could form at different depths inside the same parent body, supporting recently proposed models (Elkins‐Tanton et al. 2011 ; Weiss and Elkins‐Tanton 2013 ).     

Protocore destabilization in planetary embryos formed by cold accretion: Feedbacks from non-Newtonian rheology and energy dissipation

It is suggested that a distinct population of Mars-sized planetary embryos can form by cold accretion in the outer part of the terrestrial zone. A possible radial structure for such embryos consists of an outer silicate-rich layer and an undifferentiated solid protocore. The protocore is divided in a cold undifferentiated central region, a layer of iron, and an outer shell of undifferentiated material. This structure is gravitationally unstable. We investigate the influence of the rheology of the silicate-rich material and of the thermal effects on the protocore destabilization and on the iron core formation. We use a 2D thermomechanical numerical model of a self-gravitating planetary body. The planetary surface is treated as a free surface by imposing a massless weak medium. Our calculations include a non-Newtonian temperature-, pressure-, and strain rate-dependent viscoplastic rheology and thermal contributions from shear heating. We explored the influence of rheology by varying the activation volume, friction angle, and the maximum yield strength of silicate-rich layers. We distinguished three different core formation regimes: exposure mode, fragmentation mode, and transition mode. Like in previous models with Newtonian rheology, the core experiences large deviations from the spherical shape and is temporarily exposed at the surface in the exposure mode. In contrast to the Newtonian models the destruction of the protocores in the fragmentation modes is driven by (i) the spontaneous strain localization along planetary-scale shear zones forming inside the protocore, and/or (ii) descending localized iron diapirs/sheets penetrating the protocore. Feedback from energy dissipation notably modifies the thermal structure of the deforming planetary body. In particular it causes a temperature increase of up to several hundred Kelvin (i) around the moving and deforming protocore and (ii) along planetary scale rupture zones that form inside the protocore. If the protocore is large and has a high viscosity, a large fraction of the dissipated heat is used to increase the temperature of iron.     

Alumovesuvianite,Ca19Al(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group member from the Jeffrey mine,asbestos, Estrie region,Québec,Canada

Mineralogy and Petrology - Alumovesuvianite (IMA 2016–014), ideally Ca19Al(Al,Mg)12Si18O69(OH)9, is a new vesuvianite-group member found in the rodingite zone at the contact of a gabbroid...     

Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation

Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m⁻³ than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (T_Moho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (T_Moho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.     

Cl/Br ratios and stable chlorine isotope analysis of magmatic–hydrothermal fluid inclusions from Butte,Montana and Bingham Canyon,Utah

A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ³⁷Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10⁻³ M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to 1.08 × 10⁻³ and from 0.92 to 1.88 × 10⁻³ M, respectively, clustering below seawater (1.54 × 10⁻³ M) and overlapping mantle values (~1–2 × 10⁻³ M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and 0.64 × 10⁻³ M. Butte δ³⁷Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10⁻³ M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte, δ³⁷Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures.