Influence of landscape morphology and vegetation cover on the sampling of mixed plutonic bodies

Summary A plethora of evidence indicates that magma mixing processes can take place at any evolutionary stage of magmatic systems and that they are extremely common in both plutonic and volcanic environments. Furthermore, recent studies have shown that the magma mixing process is governed by chaotic dynamics whose evolution in space and time generates complex compositional patterns. The fact that magma mixing processes can produce igneous bodies exhibiting a large compositional complexity brings up the key question about the potential pitfalls that may be associated with the sampling of these systems for petrological studies. In particular, since commonly only exiguous portions of the whole magmatic system are available as outcrops for sampling, it is important to address the point whether the sampling may be considered representative of the complexity of the magmatic system. Here, we attempt to address this crucial point by performing numerical simulations of magma mixing processes in 3D, and by evaluating the best conditions for sampling by considering different landscape morphologies and percentages of vegetation cover. It is shown that the goodness of sampling is strongly dependant on the roughness of the landscape, with highly irregular morphologies being the best candidates to give the most complete information on the whole magma body. Vegetation cover, on the contrary, does not appear to significantly influence the representativeness of sampling.     

U-Pb SHRIMP zircon dating of andesite from the Dolomite area (NE Italy): geochronological evidence for the early onset of Permian Volcanism in the eastern part of the southern Alps

The Athesian Volcanic District (AVD), a thick sequence of andesitic to rhyolitic lava and ignimbrite, overlies both the Variscan basement of the Dolomites and, where present, the continental basal conglomerate of Upper Carboniferous(?) to Early Permian age. This volcanic activity is known to mark the margin of the intra-Pangea megashear system between Gondwana and Laurasia, the onset age of which is determined in this study.SHRIMP U-Pb dating on zircon from Ponte Gardena/Waidbruck (Isarco/Eisack valley) basaltic andesite yields an age of 290.7 ± 3 Ma, providing the oldest record of andesite volcanic activity yet documented in the AVD. Two younger dates (279.9 ± 3.3 and 278.6 ± 3.1 Ma) obtained for the andesitic necks of M. dei Ginepri (Eores/Aferer valley) and Col Quaternà (western Comelico), respectively, probably represent a second pulse of andesite magmatic activity.Near Chiusa/Klausen, the volcanoclastic deposits at the bottom of the Funes/Villnöss valley volcano-sedimentary complex only contain detrital zircons, dated at 469 ± 6 Ma; these probably derive from erosion of Paleozoic porphyroids. Other zircons from the same sediments and inherited cores of magmatic andesite crystals give Paleoproterozoic (1953.6 ± 22.1, 1834.6 ± 69.3, 1773.6 ± 25.1 Ma), Early Neoproterozoic (1015 ± 14 Ma) and Late Neoproterozoic (728.4 ± 9.6, 687.6 ± 7.6 Ma) ages. These ancient detrital and inherited zircon ages fit the model that envisages the Dolomite region as being tectonically coherent with Africa, at least until the Lower Permian.     

Landslide Representation Strategies in Susceptibility Studies using Weights-of-Evidence Modeling Technique

This paper is focused primarily on how to represent landslide scarp areas, how to analyze results achieved by the application of specific strategies of representation and how to compare the outcomes derived by different tests, within a general framework related to landslide susceptibility assessment. These topics are analyzed taking into account the scale of data survey (1:10,000) and the role of a landslide susceptibility map into projects targeted toward the definition of prediction, prevention, and mitigation measures, in a wider context of civil protection planning. These aims are achieved by using ArcSDM (Arc Spatial Data Modeler), a software extension to ArcView GIS useful for developing spatial prediction models using regional datasets. This extension requires a representation by points of the investigated problems (landslide susceptibility, aquifer vulnerability, detection of mineral deposits, identification of natural habitats of animals, and plants, etc.). Maps of spatial evidence from regional geological and geomorphological datasets were used to generate maps showing susceptibility to slope failures in two different study areas, located in the northern Apennines and in the central Alps (Italy), respectively. The final susceptibility maps for both study areas were derived by the application of the weights-of-evidence (WofE) modeling technique. By this method a series of subjective decisions were required, strongly dependent on an understanding of the natural processes under study, supported by statistical analysis of the spatial associations between known landslides and evidential themes. Except for maps of attitude, permeability, and structure, that were not available for both study areas, the other data were the same and comprised geological, land use, slope, and internal relief maps. The paper illustrates how different representations of scarp areas by points (in terms of different number of points) did not greatly influence the final response map, considering the scale of this work. On the contrary, some differences were observed in the capability of the model to describe the relations between predictor variables and landslides. In effect, a representation of the scarp areas using one point every 50 m led to a more efficient model able to better define relationships of this type. It avoided both problems of redundancy of information, deriving by the use of too many points, and problems related to a random positioning of the centroid. Moreover, it permitted to minimize the uncertainty related with identification and mapping of landslides.     

Microbial growth and air pollutants in the corrosion of carbonate building stone: results of laboratory and outdoor experimental tests

The combined effect of microbial colonization by fungal growth and atmospheric pollutants in the sulfation of carbonate rocks was investigated by means of laboratory experiments of dry deposition of sulfur and nitrogen dioxides, in vitro microbiological assays and outdoor exposure tests. All the experiments were performed on Scaglia limestone, which is a very common building material in ancient monuments in Central Italy. Results point to a combined action of particulate matter deposition and sulfation in the formation of gypsum on the samples exposed outdoor, and to a significant influence of fungal growth in the conversion of metal sulfide particulate matter to sulfate thus promoting subsequent formation of gypsum also in the absence of pollution. The chemical and textural features of stone tend to enhance chemical weathering and fungal colonization in the short term, and to preserve the material in the long term.     

Time-scales of recent Phlegrean Fields eruptions inferred from the application of a ‘diffusive fractionation’ model of trace elements

The variation of chemical element compositions in two pyroclastic sequences (Astroni 6 and Averno 2, Phlegrean Fields, Italy) is studied. Both sequences are compositionally zoned indicating a variability of melt compositions in the magma chamber prior to eruption. A clear dichotomy between the behaviour of major vs. trace elements is also observed in both sequences, with major elements displaying nearly linear inter-elemental trends and trace elements showing a variable scattered behaviour. Together with previous petrological investigations these observations are consistent with the hypothesis that magma mixing processes played a key role in the evolution of these two magmatic systems. Recently it has been suggested that mixing processes in igneous systems may strongly influence the mobility of trace elements inducing a ‘diffusive fractionation’ phenomenon, whose extent depends on the mixing time-scale. Here we merge information from 1) numerical simulations of magma mixing, and 2) magma mixing experiments (using as end-members natural compositions from Phlegrean Fields) to derive a relationship relating the degree of ‘diffusive fractionation’ to the mixing time-scales. Application of the ‘diffusive fractionation’ model to the two studied pyroclastic sequences allowed us to apply the relationship derived by numerical simulations and experiments to estimate the mixing time-scales for these two magmatic systems. Results indicate that mixing processes in Astroni 6 and Averno 2 systems lasted for approximately 2 and 9 days, respectively, prior to eruption.     

Tertiary anorogenic granites of the western border of the Yemen Plateau

North Yemen (Y.A.R.) was affected, from 30 to 20 Ma ago, by intense magmatic activity (Trap Series), contemporary with the main tensional tectonic phases of the Afro-Arabian Plate. In the Trap activity, two main phases have been recognized; the former, mainly basaltic, between 30 and 26 Ma; and the second, mainly acidic, between 23 and 20 Ma. During the latter period of activity, a basaltic dyke swarm and stock-like bodies and dykes (granophyres) were emplaced along the escarpment and coastal plain. Some granitic bodies intruded the Trap Series towards the end of the first phase and during the second one.

On the basis of geochemical and petrological data, the Tertiary granites of Yemen belong to alkaline or peralkaline associations of A-type. The textural characteristics indicate that the granites are either hypersolvus or subsolvus. The mafic mineral assemblages allow three main types of granites to be distinguished by their different environments of crystallization.

As regards the origin of these kinds of granites, several hypotheses account for the peculiarities observed. The lack of crustal xenoliths, the preliminary data on the Sr isotopic ratio, the data on major and trace elements point to a direct origin by fractional crystallization, without substantial crustal interaction. This agrees well with the strong tensile tectonics, linked with the uplift of the Afro-Arabian dome and with the lithosphere thinning, which provide a minimal opportunity for interaction of the magmas with the crust.     

Ternary Ca–Fe–Mg carbonates: subsolidus phase relations at 3.5 GPa and a thermodynamic solid solution model including order/disorder

Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO₃–MgCO₃–FeCO₃ at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite, which closes at X _MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO₃–Ca_0.5Mg_0.5CO₃ is reached at 1,100°C and between CaCO₃–FeCO₃ at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO₃–MgCO₃–FeCO₃ in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.     

The first description and confirmation of the Vista Alegre impact structure in the Paraná flood basalts of southern Brazil

Abstract– The Vista Alegre structure, centered at 25°57′S and 52°41′W, has been recently proposed as a meteorite impact structure. The 9.5 km‐diameter structure is located in the Paraná state of southern Brazil, within the Paraná Basin, which contains one of the largest and most extensive flood basalt provinces on Earth. The Paraná flood basalts belong to the Serra Geral Formation and are temporally related to the opening of the South Atlantic Ocean, having been dated at about 133–132 Ma. Tholeiitic basalts dominate the western portion of Paraná state, with some minor rhyodacites. Morphologically, Vista Alegre has a prominent circular outline, in the form of an incomplete ring of escarpments, and an inner depression. The presence of a central uplift is not obvious, but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure. These sandstones are possibly related to the Triassic Pirambóia Formation and/or to the Cretaceous Botucatu Formation. These units are normally at stratigraphic depths of about 700–800 m below the present surface in this portion of the Paraná Basin. The structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick, extensively used for agriculture. As a result there are limited outcrops in the interior of the structure, all of polymict breccias, some of them melt‐bearing. We report the extensive occurrence of shatter cones, in the form of fine‐grained rock clasts within the polymict breccias. The shatter cone‐bearing breccias occur at different locations within the structure, separated by several kilometers. The nested shatter cones range in size from about 0.5 to 20 cm for individual cones, and up to half a meter for complete assemblages. The shatter cones formed in fine‐grained Parana flood basalt and might be the first examples of shatter cones in such a rock type. In addition, planar deformation features (PDFs) were found in quartz grains within sedimentary rock clasts of the polymict breccia. These findings confirm the impact origin of the Vista Alegre structure.     

Assimilation of satellite observations of the atmosphere

Measurements of the atmosphere by satellite were first collected in the 1960s. However, it was not until the mid-1990s that these observations were translated into systematic improvements of numerical weather forecasts. We present here the data and methodology of data assimilation that enabled this achievement. Data assimilation is essentially a filtering processing that exploits the (assumed) known error statistical properties of the observations and of the underlying numerical model in which data are assimilated. It is also a process which corrects the state of the numerical model with physical observations of the real world. This part relies on (assumed) known physical laws to relate meteorological quantities (such as temperature, humidity, pressure, and wind) to observables. Atmospheric data collected by satellite all come from the interaction of electromagnetic waves with the atmosphere. Satellite data assimilation has greatly supported the progress in numerical weather prediction and holds promises for climate studies, for example via reanalysis.