Linking shock textures revealed by BSE,CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure,Brazil     

Natalia Hauser  Wolf Uwe Reimold  Aaron J. Cavosie  Alvaro P. Crsta  Winfried H. Schwarz  Mario Trieloff  Carolinna Da Silva Maia de Souza  Luciana A. Pereira  Eduardo N. Rodrigues  Matthews Brown 《Meteoritics & planetary science》2019,54(10):2286-2311

A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.  相似文献   

Late Quaternary vegetational and climatic change in the Popayán region,southern Colombian Andes     

Hermann Behling  Alvaro Jos Negret  Henry Hooghiemstra 《第四纪科学杂志》1998,13(1):43-53

Late Pleistocene and Holocene vegetational and climatic change have been studied palynologically at a site at 1750 m elevation in the subandean vegetation belt near Popayán, in the southern Colombian Andes. Time control on the pollen record is based on six AMS ¹⁴C ages, ranging from possibly Middle Pleniglacial time (around 50000 yr BP) to 1092 ± 44 yr BP. Because of the presence of two hiatuses only the Middle Pleniglacial and Late Holocene periods (the last 2300 yr BP) are represented. Pollen data indicate the presence of closed subandean forest during glacial time. Changes in the contribution of pollen originating from the uppermost and lowermost subandean forest belts, changes in the contribution of a number of other subandean forest taxa, and changes in species composition between the three pollen zones, suggest that the climate during the Middle Pleniglacial was markedly colder, and perhaps also wetter, than during the Late Holocene. Pollen assemblages from the Late Holocene indicate that the landscape has been affected by deforestation and agriculture since at least 2300 yr BP, but that human impact decreased in the last 780 yr BP. © 1998 John Wiley & Sons, Ltd.  相似文献   

Freezing-resistant liquid water in porous media, a possible mechanism to account for the fluidized transport of sediments on Mars: an example from East Gorgonum crater     

Roberto Oyarzun  Cristóbal Viedma  Alvaro Márquez  Javier Lillo 《地学学报》2003,15(4):238-242

ABSTRACT A mudflow-like deposit resting on the bottom of the East Gorgonum Crater (Mars; 37.4°S, 168.0°W) may provide new insight regarding the debate on the existence of water over the Martian surface. Because water in a mudflow is confined to a porous medium, we analyse this case from the perspective of non-equilibrium systems. Fluids confined to porous media behave in a special way, the system being ruled by kinetic restrictions, which alter the expected thermodynamic equilibrium. These non-equilibrium conditions allow the existence of pure liquid water to temperatures as low as − 40 °C, and even less if the system includes brines. Thus, application of the triple point diagram of water on the Martian surface may constitute a simplistic approach if we are dealing with confined, and yet moving, water in the form of a mudflow. We further suggest that the V-shaped channels excavated alongside the mudflow may have been caused by water rejected by syneresis from the moving sediment. We finally indicate that the series of deeply entrenched channels and debris aprons that occur only in the northern half of the crater might be related to the regional slope, which decreases in altitude to the south.  相似文献   

The Quebradagrande Complex: A Lower Cretaceous ensialic marginal basin in the Central Cordillera of the Colombian Andes     

Alvaro Nivia  Giselle F. Marriner  Andrew C. Kerr  John Tarney 《Journal of South American Earth Sciences》2006,21(4):423

The Quebradagrande Complex of Western Colombia consists of volcanic and Albian–Aptian sedimentary rocks of oceanic affinity and outcrops in a highly deformed zone where spatial relationships are difficult to unravel. Berriasian–Aptian sediments that display continental to shallow marine sedimentary facies and mafic and ultramafic plutonic rocks are associated with the Quebradagrande Complex. Geochemically, the basalts and andesites of the Quebradagrande Complex mostly display calc-alkaline affinities, are enriched in large-ion lithophile elements relative to high field strength elements, and thus are typical of volcanic rocks generated in supra-subduction zone mantle wedges. The Quebradagrande Complex parallels the western margin of the Colombian Andes’ Central Cordillera, forming a narrow, discontinuous strip fault-bounded on both sides by metamorphic rocks. The age of the metamorphic rocks east of the Quebradagrande Complex is well established as Neoproterozoic. However, the age of the metamorphics to the west – the Arquía Complex – is poorly constrained; they may have formed during either the Neoproterozoic or Lower Cretaceous. A Neoproterozoic age for the Arquía Complex is favored by both its close proximity to sedimentary rocks mapped as Paleozoic and its intrusion by Triassic plutons. Thus, the Quebradagrande Complex could represent an intracratonic marginal basin produced by spreading-subsidence, where the progressive thinning of the lithosphere generated gradually deeper sedimentary environments, eventually resulting in the generation of oceanic crust. This phenomenon was common in the Peruvian and Chilean Andes during the Uppermost Jurassic and Lower Cretaceous. The marginal basin was trapped during the collision of the Caribbean–Colombian Cretaceous oceanic plateau, which accreted west of the Arquía Complex in the Early Eocene. Differences in the geochemical characteristics of basalts of the oceanic plateau and those of the Quebradagrande Complex indicate these units were generated in very different tectonic settings.  相似文献   

Giant versus small porphyry copper deposits of Cenozoic age in northern Chile: adakitic versus normal calc-alkaline magmatism   总被引：63，自引：0，他引：63  

Roberto Oyarzun  Alvaro Márquez  Javier Lillo  Ivan López  Sergio Rivera 《Mineralium Deposita》2001,36(8):794-798

Cenozoic magmatic activity in northern Chile led to the formation of two contrasting porphyry copper belts: (1) a Paleocene-Early Eocene belt comprising small porphyry copper deposits (e.g., Lomas Bayas) of normal calc-alkaline affinity; and (2) a Late Eocene-Early Oligocene belt hosting huge porphyry copper deposits (e.g., Chuquicamata) of adakitic affinity. Although the first belt comprises both volcanic and plutonic rocks (andesitic-basaltic and rhyolitic lavas and tuffs, and associated sub-volcanic porphyries and felsic stocks), the latter only includes intrusions (mostly granodioritic types, including porphyry copper deposits). We suggest that the Late Eocene-Early Oligocene belt formed when fast and oblique convergence between the South America and Farallon plates led to flat subduction and direct melting of the subducting plate, hence giving rise to plutonic rocks of adakitic affinity. The absence of volcanism, under prevailing compressional conditions, prevented the escape of SO₂ from the adakitic, sulfur-rich, highly oxidized magmas ("closed porphyry system"), which allowed formation of huge mineral deposits. On the contrary, coeval volcanic activity during formation of the Paleocene-Early Eocene calc-alkaline porphyries allowed development of "open systems", hence to outgassing, and therefore, to small mineral deposits.  相似文献   

Climate change impacts on global agriculture   总被引：1，自引：0，他引：1  

Alvaro Calzadilla  Katrin Rehdanz  Richard Betts  Pete Falloon  Andy Wiltshire  Richard S. J. Tol 《Climatic change》2013,120(1-2):357-374

Based on predicted changes in the magnitude and distribution of global precipitation, temperature and river flow under the IPCC SRES A1B and A2 scenarios, this study assesses the potential impacts of climate change and CO₂ fertilization on global agriculture. The analysis uses the new version of the GTAP-W model, which distinguishes between rainfed and irrigated agriculture and implements water as an explicit factor of production for irrigated agriculture. Future climate change is likely to modify regional water endowments and soil moisture. As a consequence, the distribution of harvested land will change, modifying production and international trade patterns. The results suggest that a partial analysis of the main factors through which climate change will affect agricultural productivity provide a false appreciation of the nature of changes likely to occur. Our results show that global food production, welfare and GDP fall in the two time periods and SRES scenarios. Higher food prices are expected. No matter which SRES scenario is preferred, we find that the expected losses in welfare are significant. These losses are slightly larger under the SRES A2 scenario for the 2020s and under the SRES A1B scenario for the 2050s. The results show that national welfare is influenced both by regional climate change and climate-induced changes in competitiveness.  相似文献   

RECENT DEVELOPMENTS IN PROFESSIONAL GEOGRAPHY IN URUGUAY*     

Alvaro Lpez  Joseph L. Scarpaci 《The Professional geographer》1989,41(2):223-225

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A new method for the analysis of apsidal motions in eclipsing binaries     

Alvaro Gimenez  José M. Garcia-Pelayo 《Astrophysics and Space Science》1983,92(1):203-222

In this paper, a new method for the determination of apsidal motion parameters in eccentric eclipsing binaries is presented. A sequential procedure has been followed throughout allowing an automatized on-line computation of the elements and their accuracies as well as a more realistic fit to the observed times of minima. A determination of individual mean errors for the parameters of fit give us the possibility to compare the results with available theoretical models of stellar structure. In the procedure, we have introduced a determination of preliminary elements by means of Fourier transform and extended the practical equations to terms up to the fifth power of the eccentricity. An application to the well-known case of the eclipsing binary GL Car has been made. Our analysis shows an apsidal motion period of 25.3 years with a value of 0.152 for the orbital eccentricity.  相似文献   

Hydrodynamic approach to the evolution of cosmic structures – II. Study of N-body simulations at z= 0     

Alexander Knebe  Alvaro Domínguez  Rosa Domínguez-Tenreiro 《Monthly notices of the Royal Astronomical Society》2006,371(4):1959-1974

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Dynamics of mineral crystallization from precipitated slab-derived fluid phase: first in situ synchrotron X-ray measurements     

Nadia Malaspina  Matteo Alvaro  Marcello Campione  Heribert Wilhelm  Fabrizio Nestola 《Contributions to Mineralogy and Petrology》2015,169(3):1-12

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