Evolution of La Palma,Canary archipelago

La Palma is the northwestern island in the Canary archipelago. The island is volcanically active and in recent years there have been two eruptions (1949 and 1971). The oldest rocks that crop out on the island are altered spilitic lavas and they are intruded by numerous dykes and a number of mafic and ultramafic plutonic rocks. These rocks form part of the floor of the huge Caldera de Taburiente. In the walls of the Caldera one finds a 1000 metres thick unit that is composed of lavas and tephra. The rocks are called the El Time formation; and they together with the rocks of the Cumbre Vieja ridge are mainly composed of accumulative mafic rocks, basanites, hawaiites, benmoreites and phonolitic trachytes. These sodarich alkalic rocks are considered to have evolved by fractional crystallization of a basanitic magma in a near surface environment.It is proposed that La Palma developed on a relatively cold section of a plate of oceanic lithosphere that was over 100 m.y. old. During the Alpine-Atlas orogeny this section of the African plate was disrupted and a basanitic magma was extruded onto the sea floor. The volcanic focus of this eruption became established as a local centre for the degassing of the mantle and as a result this area has experienced a long series of eruptions.During the first phase in the evolution of La Palma the submarine edifice of the island was constructed and the rocks of the Caldera Floor formation were emplaced. After a period of marine erosion the huge old volcano Taburiente developed in the northern half of the island. During the final phase in the evolution of the island the southern section of the volcanic edifice subsided and formed the El Paso tectonic basin and the volcanic rift zone along which the many smaller volcanoes of the Cumbre Vieja ridge have developed.     

Evidence for sulfurization and the origin of some sudbury-type ores

The suggestion that Sudbury-type ores may be formed by the introduction of country rock sulfur into still hot intrusions (i.e., sulfurization) suffers from a reputed lack of field evidence. Permissive evidence for sulfurization includes the epigenetic nature of many Sudbury-type ores and that many Sudbury-type ores crystallized from sulfide melts. Visual evidence exists for sulfurization of a gabbro in Zambia. The lead isotopic composition of ore minerals at Sudbury implies that at least some of the metals were derived from the erruptive. Published sulfur isotopic data from several Sudbury-type ores differ from and do not exhibit a common pattern of isotopic enrichment with respect to sulfides within associated intrusions. Evidently the sulfur was derived from the country rocks. Sudbury-type ores exhibiting magmatic textures commonly occur within more siliceous dikes than the host intrusions. Inorganic reduction of sulfate occurs only above 600° C. Reduction of sulfate with resultant sulfurization of ferrous iron and traces of other metals originally present in the still hot parental intrusive rock would make the rock more siliceous. Above 1100° C the silicate-residue and newly formed sulfides would form immiscible magmas. Therefore, ore magmas within and near mafic intrusives can be epigenetic. The processes by which sulfur is introduced into intrusions are still speculative.

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Zusammenfassung Für die Annahme, daß Erze vom Sudbury-Typus sich durch Zuführung von Nebengesteinsschwefel in noch heiße Intrusionen bilden können (sulfurization), fehlt es angeblich an Feldunterlagen. Folgende Tatsachen lassen sich mit einer Schwefelung (sulfurization) vereinigen: der epigenetische Charakter vieler Erze vom Sudbury-Typ; auch sind viele Erze von Sudbury-Typus aus sulfidischen Schmelzen kristallisiert. In einem Gabbro in Zambia ist der Beweis für Schwefelung (sulfurization) direkt sichtbar. Die Isotopen-Zusammensetzung von Blei in Erzmineralien in Sudbury zeigt, daß mindestens einige der Metalle aus dem Eruptivgestein stammen. Schwefelisotop-Daten, die für mehrere Lagerstätten von Sudbury-Typen veröffentlicht worden sind, haben hinsichtlich der Isotop-Anreicherung keine gemeinsamen Züge. Offensichtlich stammt der Schwefel aus dem Nebengestein. Erze vom Sudbury-Typ mit magmatischem Gefüge finden sich oft in Gängen, die saurer sind als das Wirtsgestein. Anorganische Reduktion von Sulfat findet nur oberhalb 600°C statt. Reduktion von Sulfat und die entstehende Schwefelung (sulfurization) von zweiwertigem Eisen und Spuren anderer Metalle, die ursprünglich in dem noch heiß eruptiven Gestein anwesend sind, machen das Gestein noch saurer. Oberhalb 1100°C würden der Silicat-Rest und die neugeformten Sulfide nicht mischbare Magmen bilden. Deshalb können sich sulfidische Schmelzen innerhalb und in der Nähe von Mafic-Intrusionen später gebildet haben. Die Prozesse, durch die Schwefel in Intrusionen eingeführt wird, sind noch unbekannt.

     

Contributions from mafic alkaline magmas to the Bingham porphyry Cu–Au–Mo deposit, Utah, USA

The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These "mixed" lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic ore-related magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000-4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.     

Mobile Pb-isotopes in Proterozoic sedimentary basins as guides for exploration of uranium deposits

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (<250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of ²³⁸U/²⁰⁶Pb and ²⁰⁶Pb/²⁰⁴Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins. This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.     

Eutrophication and some European waters of restricted exchange

Regions of Restricted Exchange (RREs) are an important feature of the European coastline. They are historically preferred sites for human settlement and aquaculture and their ecosystems, and consequent human use, may be at risk from eutrophication. The OAERRE project (EVK3-CT1999-0002) concerns ‘Oceanographic Applications to Eutrophication in Regions of Restricted Exchange’. It began in July 2000, and studies six sites. Four of these sites are fjords: Kongsfjorden (west coast of Spitzbergen); Gullmaren (Skagerrak coast of Sweden); Himmerfjärden (Baltic coast of Sweden); and the Firth of Clyde (west coast of Scotland). Two are bays sheltered by sand bars: Golfe de Fos (French Mediterranean); and Ria Formosa (Portuguese Algarve). Together they exemplify a range of hydrographic and enrichment conditions. The project aims to understand the physical, biogeochemical and biological processes, and their interactions, that determine the trophic status of these coastal marine RRE through the development of simple screening models to define, predict and assess eutrophication. This paper introduces the sites and describes the component parts of a basic screening model and its application to each site using historical data. The model forms the starting point for the OAERRE project and views an RRE as a well-mixed box, exchanging with the sea at a daily rate E determined by physical processes, and converting nutrient to phytoplankton chlorophyll at a fixed yield q. It thus uses nutrient levels to estimate maximum biomass; these preliminary results are discussed in relation to objective criteria used to assess trophic status. The influence of factors such as grazing and vertical mixing on key parameters in the screening model are further studied using simulations of a complex ‘research’ model for the Firth of Clyde. The future development of screening models in general and within OAERRE in particular is discussed. In addition, the paper looks ahead with a broad discussion of progress in the scientific understanding of eutrophication and the legal and socio-economic issues that need to be taken into account in managing the trophic status of RREs.     

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

Causes of model-data discrepancies in european climate during oxygen isotope stage 3 with insights from the last glacial maximum

Oxygen isotope stage 3 (OIS 3), encompassing the long middle section of the last glacial interval, has been the focus of an intensive high-resolution climate modeling effort for Europe. These model simulations produce substantially colder climates than modern simulations; however, the temperatures appear warmer than many proxy indicators suggest. In order to evaluate the importance of the model boundary conditions, comparable simulations are completed for the last glacial maximum (LGM). The LGM simulation produces a much colder European continent than OIS 3, despite similarities in the specification of sea-surface temperatures (SSTs). Ice-sheet dimension is evidently a key factor in explaining the difference in European climates over the past 40,000 yr. However, underestimates in specified OIS 3 ice sheets cannot be invoked to explain the discrepancies, since data strongly indicate small ice-sheet extents at that time; this leaves errors in specified OIS 3 SSTs as the most likely cause.     

显微CT的锥束重建技术

显微CT技术一直是Iowa大学CT/显微CT实验室研究的焦点。这篇文章报告了我们在锥束显微CT方面最近取得的一些进展。我们利用Feklkamp类算法了多X光源和检测器偏置条件下的近拟图象重建；解释了Grangeat精确重构框架下的图象伪影。最后重点介绍了最近有关一般分块迭代Landweber格式的收敛性结果；给出了模拟和实验结果，探讨了进—步的研究方向。     

Nonlocal fluxes and Stokes drift effects in the K-profile parameterization

 The K-profile parameterization of upper-ocean mixing is tested and extended using observations and large eddy simulations of upper-ocean response to a westerly windburst. A nonlocal momentum flux term is added, and the amplitude of the nonlocal scalar flux is recalibrated. Parameterizations of Stokes drift effects are added following recent work by McWilliams and Sullivan (2001). These changes allow the parameterization to produce both realistic gradients of momentum and scalars in the nocturnal boundary layer and enhanced mixing during stable conditions. The revised parameterization is expected to produce improved representations of lateral advection and sea-surface temperature in large-scale models. Received: 31 August 2001 / Accepted: 15 December 2001     

Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

The Au-Ag (-Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6-3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H₂S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H₂S enrichment in the Vw-c inclusions observed at -200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.