Sulphide Segregation in Ferropicrites from the Pechenga Complex, Kola Peninsula, Russia

The Palaeoproterozoic Ni–Cu sulphide deposits of the PechengaComplex, Kola Peninsula, occur in the lower parts of ferropicriticintrusions emplaced into the phyllitic and tuffaceous sedimentaryunit of the Pilgujärvi Zone. The intrusive rocks are comagmaticwith extrusive ferropicrites of the overlying volcanic formation.Massive lavas and chilled margins from layered flows and intrusionscontain <3–7 ng/g Pd and Pt and <0·02–2·0ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. Theabundances of platinum group elements (PGE) correlate with eachother and with chalcophile elements such as Cu and Ni, and indicatea compatible behaviour during crystallization of the parentalmagma. Compared with the PGE-depleted central zones of differentiatedflows (spinifex and clinopyroxene cumulate zones) the olivinecumulate zones at the base contain elevated PGE abundances upto 10 ng/g Pd and Pt. A similar pattern is displayed in intrusivebodies, such as the Kammikivi sill and the Pilgujärvi intrusion.The olivine cumulates at the base of these bodies contain massiveand disseminated Ni–Cu-sulphides with up to 2 µg/gPd and Pt, but the PGE concentrations in the overlying clinopyroxenitesand gabbroic rocks are in many cases below the detection limits.The metal distribution observed in samples closely representingliquid compositions suggests that the parental magma becamesulphide saturated during the emplacement and depleted in chalcophileand siderophile metals as a result of fractional segregationof sulphide liquids. Relative sulphide liquid–silicatemelt partition coefficients decrease in the order of Ir >Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphidemass ratio) are high and of the order of 10⁴–10⁵, andthey indicate the segregation of only small amounts of sulphideliquid in the parental ferropicritic magma. In differentiatedflows and intrusions the sulphide liquids segregated and accumulatedat the base of these bodies, but because of a low silicate–sulphidemass ratio the sulphide liquids had a low PGE tenor and Pt/Irand Cu/Ir ratios similar to the parental silicate melts. Duringcooling the sulphide liquid crystallized 40–50% of monosulphidesolid solution (mss) and the residual sulphide liquid becameenriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. TheCu-rich sulphide liquid locally assimilated components of thesurrounding S-rich sediments as suggested by the radiogenicOs isotopic composition of some sulphide ores (     

Oldest Atlantic seafloor

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode.Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated mantle plume activity. Projection of leg 11 sites back along accepted spreading flow lines to their presumed points of origin shows that these origins are also outside the influence of modern plume activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these plumes. The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of plume-related basalt on the Mid-Atlantic Ridge (20 °–40 °N). This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. Plumes related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may alos have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr⁸⁷/Sr⁸⁶ in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.Contribution Number 3953 from the Woods Hole Oceanographic Institution     

La Géologie et la Tectonique de la Plateforme Amazonienne

Résumé Le travail met au point pour la première fois dans la Géologie brésilienne, une synthèse tectonique et géologique de la Plateforme amazonienne. Trois phases géologiques et tectoniques principales ont été considérées: le socle cristallin (d'âge précambrien); la couverture sédimentaire plissée ou phase géologique intermédiaire (developpée du Précambrien supérieur au Silurien) et la converture sédimentaire non-plissée (du Dévoniem inférieur au Tertiaire).Dans le socle cristallin, deux noyaux cratoniques ont été individualisés: le noyau cratonique Brésil-Central et le noyau cratonique guyannais.Dans le socle cristallin les zones tectoniques ou phases orogéniques suivantes ont été décrites: pour le noyau cratonique Brésil-Central: le Rondonide, et le Sud-Amazonide (ou phase orogénique Curua-Xingu); pour le noyau cratonique guyannais, du coté brésilien: l'Amapaïde, le Paruïde et le Negroïde (ou phase orogénique Rio Negro).La couverture sédimentaire plissée a été divisée en trois régions, chacune avec ses propres caractéristiques: Cachimbo-Cubencranquém (sur le noyau cratonique Brésil-Central), Uatumã-Trombetas (dans la partie moyenne et basse de la depression amazonique — la plus jeune) et Roraima (sur le noyau cratonique guyannais — la plus ancienne).La couverture sédimentaire non-plissée a été divisée dans deux sous-phases: péricratonique et intracratonique. Le Bassin amazonique a été divisé à son tour en 5 (cinq) petits bassins internes dénommés: Acre, Haut-Amazone, Moyen Amazone, Bas-Amazone et Marajó.

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In this article a synthesis of the geologic and tectonic history of the Amazonian Platform was made for the first time. Three principal geologic and tectonic phases were considered: the crystalline basement (of Precambrian age); the folded sedimentary cover (of late Precambrian to Silurian age), and the non-folded sedimentary cover (of Devonian to Tertiary age).In the crystalline basement, there are two cratonic nuclei separated by the Amazonian Depression: the Central-Brazil cratonic nucleus and the Guyannian cratonic nucleus.Also, in this crystalline basement the following tectonic zones or orogenic phases were described and separated: Rondonides and South-Amazonides in the cratonic nucleus of Central-Brazil; Amapaides, Paruides and Negroïdes, in the Guyannian cratonic nucleus.The folded sedimentary cover was characterized by different regions: Cachimbo-Cubencranquém (on the Central-Brazil nucleus); Uatumã-Trombetas (in the middle and low Amazonic Depression — the younger region considered) and Roraima, (on the Guyannian cratonic nucleus the older region).The non-folded sedimentary cover was divided into two (2) principal subphases: pericratonic and intracratonic. The Amazonian Basin was studied separately, and five (5) small tectonic basins were established: Acre, High-Amazonas, Middle Amazonas Low-Amazonas and Marajó basins.

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Zusammenfassung In dieser Arbeit wird zum ersten Mal eine Synthese der geologischen und tektonischen Geschichte der Amazonas-Plattform vorgelegt. Drei geologische und tektonische Hauptphasen werden untersucht: das kristalline Basement (Präkambrium), die gefaltete Sedimentdecke (jüngeres Präkambrium bis Silur) und die ungefaltete Sedimentdecke (Devon bis Tertiär).Im Basement werden zwei kratonische Kerne unterschieden, zwischen denen die Amazonas-Depression liegt: der zentralbrasilianische Kern und der guyannische Kern.Weiterhin werden im Basement die folgenden tektonischen Zonen oder orogenen Phasen beschrieben und unterschieden: Rondonides und Süd-Amazonides (im zentralbrasilianischen Kern); Amapaides, Paruides und Negroides (im guyannischen Kern).Die gefaltete Sedimentdecke ist in folgenden Gebieten anzutreffen: Cachimbo-Cubencranquém (auf dem zentralbrasilianischen Kratonkern); Uatumã-Trombetas (in der mittleren und unteren Amazonas-Depression: die jüngeren Schichten) und Roraima (auf dem guyannischen Kratonkern: die älteren Schichten).Die ungefaltete Sedimentdecke ist in zwei Phasen unterteilt: eine perikratonische und eine intrakratonische. Das Amazonasbecken wurde für sich untersucht; es ist in fünf kleinere tektonische Becken unterteilt: Acre-, Hochamazonas-, Mittelamazonas-, Unteramazonas- und Marajó-Becken.

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Resumo O presente artigo mostra pela primeira vez na geologia brasileira, uma síntese da tectônica e da geologia, do que se resolveu denominar Plataforma Amazônica. Três fases Geológicas e Tectônicas principais foram consideradas: a do Embasamento Cristalino (do Pré-Cambriano); a da cobertura Sedimentar Dobrada — ou Fase Geológica Intermediária — (do Pré-Cambriano Superior ao Siluriano) e a da Cobertura Sedimentar Não Dobrada (do Devoniano ao Terciário).No Embasamento Cristalino, dois Núcleos cratonicos foram individualizados, separados que estão, pela grande Depressão, Amazônica, de direção geral ENEOSO: o Núcleo cratónico Brasil-Central e o Núcleo cratônico Guianês. Neste Embasamento Cristalino as seguintes Zonas Tectônicas ou Fases orogênicas foram descritas e saparadas. Para o Núcleo Brasil-Central: o Rondonides e o Sul-Amazonides (ou Fase orogênica Curua-Xingu); para o Núcleo Guianês: o Amapaides, o Paruides e o Negroïdes (ou Fase orogênica Rio Negro).Quanto à cobertura Sedimentar Dobrada, às seguintes regiões foram caracterizadas: Cachimbo-Cubencranquém (sôbre o Núcleo Brasil-Central); Uatumã-Trombetas (no média e baixa Depressão Amazônica), a mais jovem, e o Roraima (sôbre o Núcleo Guianês), a mais antiga.A cobertura Sedimentar Não-Dobrada foi dividida em 2 (duas) Sub-Fases principais: a Pericratônica e a Intracratônica. A Bacia Amazônica, estudada também em separado, foi dividida em 5 (cinco) pequenas bacias internas: a do Acre, a do Alto-Amazônas a do Médio-Amazônas, a do Baixo-Amazônas e a. do Marajó.

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( , , ). , , - . - - , — , . Cachimbo-Cubencranquem, Uatuma-Trombetas Roraima . : perikratonische intrakratonische. : Acre, , , Marajó.

     

Development of groundwater modeling for the Azraq Basin, Jordan

 The three-dimensional groundwater flow model MODFLOW was applied to simulate water level change in the complex multi-aquifer systems (the Upper and Middle Aquifers) of the Azraq basin. The model was calibrated by matching observed and simulated drawdown for steady and transient states over the period 1970–1992. Drawdown data for the period 1993–1997 were used to test the model's ability to predict the response of the aquifers. The model performed well in representing the water level contours of the Upper and Middle Aquifers for steady state calibration. Agreement between the observed and simulated drawdowns was obtained for transient state calibration. To predict the aquifer system responses for the period of 1997–2025, four different pumping schemes (scenarios) have been investigated. The first scenario (present pumping rate) reveals that there will be approximately a 25 m drop in the water level at the well-field area in 2025. However, the worst scenario (pumping rate at 1.5 times the present rate) reveals an approximate 39 m drop in the water level at the well-field area in 2025. The safe yield for the Upper Aquifer System was found to be about 25 million cubic meters (MCM) yearly. Received: 24 June 1999 · Accepted: 30 November 1999     

Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia

Composite dykes consisting of leucominette and dacite as wellas discrete dykes and flows of minette and lamproite composition,occur in the Veliki Majdan area, western Serbia. This area ispart of the Serbian Tertiary magmatic province, which consistsof numerous small occurrences of ultrapotassic igneous rocks.The composite dykes have leucominette margins (up to 150 cmthick) enclosing a central part of dacite up to 100 m in width.Between these two lithologies, a decimetre-sized transitionzone may occur. Petrography, mineral chemistry and bulk-rockgeochemistry, including Sr, Nd and Pb isotopes, provide evidencethat the minettes and leucominettes formed by hybridizationbetween a felsic magma similar in composition to dacite anda mantle-derived lamproitic magma. The leucominettes and minettescontain all phenocryst types (biotite, plagioclase, quartz)present in the dacites, but in partly resorbed and reacted form.The mica displays a great diversity of resorption textures asa result of partial dissolution, incipient melting and phlogopitization,suggesting superheating of the felsic melt during hybridization;the mineral modes and mineral compositions of the leucominettesand minettes resemble those in the lamproites. A model for themodification of lamproite melt towards minette is presentedin which minette is formed by mixing of lamproite and <30%felsic magma. The lack of any significant correlation betweenPb isotopic ratios and some of the ‘mixing-indices’(SiO₂, Zr, Zr/Nb, ¹⁴³Nd/¹⁴⁴Nd_i) recognized in the hybridizationmodel for the Veliki Majdan dykes may be a result of similarityof the Pb-isotopic signature in the two end-members. Highlyphlogopitized biotite xenocrysts in the minettes are ascribedto the retention of volatile components after magma mixing andcrystallization of a new generation of phlogopite from the hybridizedmagma. The magma-mixing model explains the reverse zoning andresorption features of phlogopite macrocrysts commonly recognizedin calcalkaline lamprophyres elsewhere. Therefore, this mixingmechanism may be globally applicable for the origin of minettesassociated with calcalkaline granitic plutonism in post-orogenicsettings. KEY WORDS: Serbia; lamproites; micas; phlogopitization; calcalkaline lamprophyres; superheating; magma mixing     

Simulated decadal oscillations of the Atlantic meridional overturning circulation in a cold climate state

The significance of the Atlantic meridional overturning circulation (MOC) for regional and hemispheric climate change requires a complete understanding using fully coupled climate models. Here we present a persistent, decadal oscillation in a coupled atmosphere–ocean general circulation model. While the present study is limited by the lack of comparisons with paleo-proxy records, the purpose is to reveal a new theoretically interesting solution found in the fully-coupled climate model. The model exhibits two multi-century-long stable states with one dominated by decadal MOC oscillations. The oscillations involve an interaction between anomalous advective transport of salt and surface density in the North Atlantic subpolar gyre. Their time scale is fundamentally determined by the advection. In addition, there is a link between the MOC oscillations and North Atlantic Oscillation (NAO)-like sea level pressure anomalies. The analysis suggests an interaction between the NAO and an anomalous subpolar gyre circulation in which sea ice near and south of the Labrador Sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient. The latter induces a positive feedback via synoptic eddy activity in the atmosphere. In addition, the oscillation only appears when the Nordic Sea is completely covered by sea ice in winter, and deep convection is active only near the Irminger Sea. Such conditions are provided by a substantially colder North Atlantic climate than today.     

Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska

The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Q_p) attained during failure of the debris-avalanche dam in the range 10⁴<Q_p<10⁶ m³ s⁻¹ for plausible breach erosion rates of 10–100 m h⁻¹. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 10³<Q_p<10⁴ m³ s⁻¹ for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.     

Observations of the convective plume of a lake under cold-air advective conditions

Moderating effects of Lake Apopka, Florida on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 °C under conditions of moderate winds (~4 m s^–1). No substantial temperature effects were observed with surface wind speed less than 1 m s^–1. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for non-advective conditions.IFAS Journal Series No. 1006.     

Evaluation of the North Atlantic Oscillation as simulated by a coupled climate model

 The realism of the Hadley Centre’s coupled climate model (HadCM2) is evaluated in terms of its simulation of the winter North Atlantic Oscillation (NAO), a major natural mode of the Northern Hemisphere atmosphere that is currently the subject of considerable scientific interest. During 1400 y of a control integration with present-day radiative forcing levels, HadCM2 exhibits a realistic NAO associated with spatial patterns of sea level pressure, synoptic activity, temperature and precipitation anomalies that are very similar to those observed. Spatially, the main model deficiency is that the simulated NAO has a teleconnection with the North Pacific that is stronger than observed. In a temporal sense the simulation is compatible with the observations if the recent observed trend (from low values in the 1960s to high values in the early 1990s) in the winter NAO index (the pressure difference between Gibraltar and Iceland) is ignored. This recent trend is, however, outside the range of variability simulated by the control integration of HadCM2, implying that either the model is deficient or that external forcing is responsible for the variation. It is shown, by analysing two ensembles, each of four HadCM2 integrations that were forced with historic and possible future changes in greenhouse gas and sulphate aerosol concentrations, that a small part of the recent observed variation may be a result of anthropogenic forcing. If so, then the HadCM2 experiments indicate that the anthropogenic effect should reverse early next century, weakening the winter pressure gradient between Gibraltar and Iceland. Even combining this anthropogenic forcing and internal variability cannot explain all of the recent observed variations, indicating either some model deficiency or that some other external forcing is partly responsible. Received: 20 August 1998 / Accepted: 12 May 1999     

Local mesoscale circulation over Venice as a result of the mountain-sea interaction

The local sea-breeze circulation in Venice is found to have typical characteristics but the nocturnal flow is found to be quite complex. This latter result is due to unequal cooling of the Alps and adjacent low-lying regions. The horizontal and vertical structure of the flow has been investigated using data obtained from surface anemometers and pilot balloon observations. Results show that the circulation is about 500 m in depth and achieves a maximum speed of 6 m s^–1