Dispersion of the salt-marsh periwinkle Littoraria irrorata: Effects of water level,size, and season

This paper documents horizontal and vertical dispersion patterns of a Texas population of the saltmarsh periwinkle, Littoraria irrorata, over a 15-month period. The study was conducted within a tidal marsh on the Anahuac National Wildlife Refuge in Galveston Bay. Two mark-recapture experiments demonstrated that L. irrorata rarely move more than 2 m from their release point over long periods of time and do not home to individual Spartina plants. Adult L. irrorata forage farther away from the base of Spartina stalks at low tide than do juvenile snails. Remaining near the plant base may decrease both temperature and desiccation stress on juveniles. During warm months, L. irrorata climb grass stalks with tidal inundation and forage on the substratum at low tide. Snails are inactive and aggregate in detrital debris at the base of Spartina clumps during the winter. *** DIRECT SUPPORT *** A01BY058 00016     

Stratiform and vein U,Mo and Cu mineralization in the Novoveská Huta area,CSFR

U-Mo and Cu mineralization occurs in horizons as well as in veins in the Permian formations near Novoveská Huta. Ore mineralization is represented by uraninite, U-Ti oxides, coffinite, molybdenite, chalcopyrite, tennantite and pyrite. The isotopic composition of S and C displays a larger variability in the stratiform ores (³⁴S from –32.7 to +2.7 and ¹³C from –27.1 to — 0.5) These data suggest mixing of meteoric solutions with fluids of volcanic origin and a complex history. There is a narrower range of ³⁴S from –18.8 to –4.6 and ¹³C from –6.3 to –2.5% in quartz-carbonate veins with Cu mineralization suggesting a deep source of ore-bearing solutions. The Permian volcanics were a significant source of ore elements.Their contents of U, Mo, Cu and Y are from two to eight times higher than in sedimentary rocks. Accumulations of ore elements in the horizons were formed by the reduction and adsorption processes 240 ± 30 Ma ago according to U-Pb isotopic dating. Due to Alpine tectonism, these low-grade ores (U<0.1 wt%) were remobilized and higher-grade U-Mo ores (U>0.1 wt%) were formed 130 ± 20 Ma ago at temperatures ranging from 110 to 120° C, according to fluid inclusions. Younger veins with Cu mineralization were formed 115 ± 10 Ma according to the model age of Pb at temperatures ranging from 95 to 190°C.     

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al₂O₃], low [TiO₂]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO₂ content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir.We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.     

Structure of the αFexAl1:xOOH solid solution

The three-dimensional local organisation around Fe atoms in a natural diaspore (Al_0.9955Fe_0.0045OOH) has been investigated by angular measurements of X-ray absorption spectra. It is demonstrated that in a single crystal of diaspore, the absorption cross-section exhibits the special case of trichroism where three independent measurements are needed to determine the absorption cross-section for any direction of polarization. Extended X-ray absorption fine structure (EXAFS) spectra were thus recorded at the following orientations of the polarization vector: , , and . The incoming white beam was monochromatized using two Si(331) crystals, which deliver at the Fe K-edge a completely linearly polarized X-ray beam. The reliability of our measurements was checked by comparing the isotropic EXAFS spectrum calculated from the three orthogonal measurements to the one of the diaspore powder recorded at the magic angle. It is shown that Fe³⁺ ions are not randomly distributed within the diaspore framework. Furthermore, only part of the Fe³⁺ ions substitutes Al, the others being located in the channels of the structure. The 3D local structure of the Fe domains has been modeled assuming hematite-like clusters of three Fe octahedra topotactically grafted to aluminous chains. These Fe clusters are thought to represent ancient multinuclear Fe surface complexes having formed at the time of the diaspore growth, and being sealed in its bulk structure. The whole crystal of diaspore is then thought to have kept the memory of the heterogeneous nucleation mechanism of these hematite nuclei. In addition to the well-known examples of atom segregations, ion vacancies, and intergrowths of discrete phases, this new type of crystal defect represents another evidence of non-equilibrium crystallization process under the thermodynamic and kinetic conditions that prevail at the earth's surface.     

Review of Carboniferous-Permian volcanicity in Scotland

The main Carboniferous outcrop in Scotland is in the Midland Valley rift, though rocks of that age also occur farther south along the English Border. The cyclic sedimentary sequence comprises up to 3,000 m. of Dinantian (mainly Viséan) and 2,000 m. of Silesian beds which at one locality or another include lavas or tuffs almost at every level. The distribution of these volcanic rocks is known in exceptional detail because of mining and exploratory borings in the Viséan oil shale field and in the various Namurian and Westphalian coalfields.The lavas and tuffs form part of an alkaline (sodic) magma series. Their outpourings were greatest in volume during the Dinantian, with the formation of the extensive lava piles which now form the Clyde Plateau (maximum thickness c. 900 m.) in the west, the Garleton Hills (c. 600 m.) and Burntisland Anticline (c. 400 m.) in the east, and the Kelso Traps of the Tweed Basin (c. 120 m.), the Birrenswark Lavas of Dumfriesshire (c. 90 m.), the Kershopefoot Basalt of Liddesdale (c. 60 m.) and the Glencartholm Volcanic Beds (mostly tuffs) of Eskdale (c. 180 m.) to the south. Most of the lavas are varieties of olivine-basalt, but subordinate trachybasalts, trachytes and rhyolites are included in the upper parts of the Clyde Plateau and Garleton Hills successions. The areas covered by individual lava fields are difficult to assess because they varied from time to time and occasionally overlapped.By the end of the Dinantian this form of activity had ceased everywhere except in West Lothian, where it continued into early Namurian (E₁). Thereafter volcanicity continued periodically from scattered centres and gave rise to relatively short-lived ash-cones. They formed in Ayrshire (E₁ age) but were most abundant in Fife, occurring there almost throughout the Namurian succession. The highly explosive nature of this phase of volcanicity is apparent from the rarity of associated lavas (though minor basaltic intrusions occur in the necks) and from the presence within the surrounding sediments of thin layers of tuff representing fine ash carried for distances of up to 32 km from source. These layers, some of which are kaolinized and are a variety of tonstein, are of local use in correlation.The explosive phase is now known to have continued throughout Westphalian A and possibly into Westphalian B in Fife, but in Ayrshire volcanicity was more intermittent and is represented by two outpourings of basalt lavas, one late Namurian to early Westphalian in age and up to 150 m. thick, the other Stephanian in age and 90 to 237 m. thick. The Stephanian lavas and associated necks in Ayrshire, like some of the necks exposed on the classic Fife coastal section on the Firth of Forth, were until recently believed to be Permian, but there is now no positive evidence of Permian eruption in Scotland.Two groups of intrusions cut the Carboniferous rocks. The most extensive is of alkaline dolerites which are co-magmatic with the basaltic lavas and tuffs, and are of similar geographic distribution. They form sills up to 120 m. thick and are probably of various ages up to Stephanian. It is possible that some of them were high-level reservoirs from which diatremes issued. The other group is of quartz-dolerite or tholeiite which forms a series of east-west dykes and a sill-complex extending over an area of about 1500 km² and having a maximum thickness of 127 m. Their age is late Carboniferous.The distribution of the Carboniferous volcanic rocks is not apparently related to the rift-valley, but is clearly linked in places to the earlier Caledonoid structures which determined the pattern of sedimentation. The lavas and, to a lesser extent, the tuffs, influenced the sedimentary processes by partially enclosing basins, by causing local anomalies in cyclic sequences and by providing sources of atypical sediment.

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Zusammenfassung Im Midland Valley (Schottland) war der permokarbonische Alkali-(Na)-Vulkanismus besonders im Dinantium aktiv, wobei bis 900 m mächtige Laven (Olivinbasalte, untergeordnet Trachytandesite, Trachyte und Rhyolite) gefördert wurden. Gegen Ende des Dinantium hörte diese Art der Aktivität fast ganz auf, dafür setzte eine explosive, periodische Tätigkeit ein, die zu kurzlebigen Aschenkegeln führte. Ein Beweis für das früher angenommene permische Alter dieser Necks läßt sich nicht erbringen.Zwei Gruppen von Intrusiva sind in die karbonischen Sedimente eingedrungen. Die größere der beiden wird durch Alkali-Dolerite (Sills) gebildet, die co-magmatisch zu den Laven und Tuffen sind. Die andere besteht aus Quarz-Doleriten und Tholeyiten (vorwiegend Dykes), die spätkarbonisch bis frühpermischen Alters sind.Die Verbreitung der karbonischen Vulkanite ist nicht an das Midland Valley, sondern an frühere caledonische Strukturen gebunden und für den Sedimentationsprozeß von Bedeutung.

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Résumé Le principal affleurement carbonifère en Ecosse se trouve dans le graben du Midland Valley, quoiqu'on rencontre les roches de cet âge le long de la frontière anglaise. La succession du cycle sédimentaire comprend jusqu'à 3,000 m. de roches de l'âge Dinantien (principalement Viséen) et à 2,000 m. des couches Silésiennes qui dans une localité quelconque comprennent des laves ou des tufs à presque tous les niveaux. La distribution de ces roches volcaniques est bien connu dans tous les détails à cause des mines et des forages.Les laves et les tufs font partie d'une série alkaline (sodique) magma. Leurs éruptions atteignirent leur maximum en volume pendant Ie Dinantien, avec la formation du Plateau de Clyde (épaisseur maximum 900 m. env.) à l'ouest, les Collines de Garleton (600 m. env.) et l'anticlinal de Burntisland (400 m. env.) a l'est, et les Kelso Traps du bassin du Tweed (120 m. env.), les Birrenswark Lavas de Dumfriesshire (90 m. env.), le Kershopefoot Basait de Liddesdale (60 m. env.) et les Glencartholm Volcanic Beds (des tufs pour la plupart) d'Eskdale (180 m. env.) au sud. La plupart des laves sont des variétés de basalte-olivine, mais des quantié peu considérables de trachybasalte, trachytes et rhyolites sont comprises dans les parties supérieures dans les successions du Plateau de Clyde et des Collines de Garleton.A la fin du Dinantien, cette forme d'activité avait cessé partout excepté dans l'Ouest Lothian, où elle continuait jusqu'au commencement du Namurien. Après cela le volcanisme continua périodiquement des centres dispersés, et donna naissance à des cônes de cendre de relativement petite durée. Ceux-ci se formèrent en Ayrshire (age E₁), mais on les trouvait beaucoup plus souvent en Fife, et on les y rencontrait presque pendant toute la succession namurienne. La nature explosive de cette phase de volcanisme est apparente par la rareté des laves et par la présence dans le sédiment qui se trouve dans les couches étroites de tuf, résultat du cendre fin porté jusqu'à 32 km. de sa source. Ces couches-ci dont quelques-unes furent transformées en kaolin, sont une variété de tonstein qui s'emploie beaucoup en corrélation. La distribution des quelques cheminées volcaniques coïncide aux structures nord—est d'un âge précarbonifère.La phase explosive est maintenant connue d'avoir continué à travers toute la Westphalie «Äs» et possiblement d'avoir pénétré dans la Westphalie «B» en Fife, mais en Ayrshire le volcanisme était plus intermittent et est représenté par deux éruptions de lave basaltique, l'une pendant la fin du Namurien jusqu'au début du Westphalien et d'une épaisseur de 150 m., l'autre du Stéphanien et d'une épaisseur de 90 à 237 m. Les laves du Stéphanien et les cheminées associées étaient classées auparavant comme Permien, mais maintenant il n'y a aucune évidence positive d'éruption permianique en Ecosse.Deux groupes d'intrusions scindèrent les roches carbonifères. Le groupe le plus étendu est fait de dolérites alkalines qui sont co-magmatiques avec les laves basaltiques et les tufs, et sont de pareille distribution géographique. Elles forment des silles d'une épaisseur jusqu; à 120 m. et elles sont probablement d'âges variés jusqu'au Stéphanien. Il est possible que quelques-une d'entre elles furent des réservoirs de haut-niveau desquels des diatrèmes sortirent. L'autre groupe est fait de quartz-dolérite ou tholéiite qui forme une série de dikes est—ouest et une sille-complexe couvrant une étendue de 1,500 km² et ayant une épaisseur maximale de 127 km. Leur áge est de la fin du Carbonifère ou du début du Permien.

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Published by permission of the Director, Institute of Geological Sciences.     

A Preliminary Study of the Self-Directed Photography of Middle-Class,Homeless, and Mobility-Impaired Children*

Understanding the ways in which children with different life experiences come to terms with day-to-day contexts and constraints has become an important topic of social science research. This study applies the technique of auto-photography to the study of children-environment transactions. How children apprehend their environments is described through a leitmotif analysis and an interpretation of photographs taken by children from middle-class families, homeless children, and children whose mobility is impaired by cerebral palsy. We speculate upon the social and physical contexts of these children based upon the images that they selected to photograph. Although impressionistic, our findings suggest the importance of auto-photography as a method for uncovering children-environment transactions.     

Large-Scale Structures in Comet Hale–Bopp

The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt and Snow (2000), Icarus 148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare. In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly near the head, but images taken with a CO⁺ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed.