Hydrous Basaltic Andesites Associated with Minette and Related Lavas in Western Mexico

The convergent margin of western Mexico is uniquely characterizedby a volcanic front of lamprophyric and related lavas located{small tilde}70 km closer to the Middle America trench thanthe main axis of andesitic volcanism. This front, defined bysmall volcanic centers ranging in age from {small tilde}1 kato 3 Ma, contains several lava types: minette, absarokite, leucitite,spessartite, and kersantite, all without feldspar phenocrysts.Many of the lavas contain hydrous phenocrysts; they are enrichedin potassium and other incompatible elements, and they are moreoxidized relative to the andesitic suite of the main axis. Intimatelyassociated are flows of basaltic andesite of comparable volume.They range in composition from 53 to 58 wt.%SiO₂, have 5–9wt.%MgO and contain phenocrysts of olivine, sparse augite, andvarying amounts of plagioclase. Their alkali contents are typicalof calc-alkaline varieties, with average Na₂O and K₂O concentrationsof 4?2 and 1?1 wt.% respectively. The basaltic andesites oftencontain olivine of unusually high forsterite content, reflectingcrystallization under oxidizing conditions, and they have oxygenfugacities up to 3?3 log units above the Ni-NiO buffer. Manifestationsof high water contents are (1) the ubiquitous occurrence ofgroundmass olivine rather than orthopyroxene, and (2) the suppressionof plagioclase as an early crystallizing phase. Both featuresreflect the role of water in reducing the activity of silicain the melt. The progressive influence of water during crystallizationis also seen in the continuum between the two intermediate lavatypes, basaltic andesite and kersantite, as plagioclase is suppressedand hornblende is stabilized in the phenocryst assemblage. Thus,despite the absence of hydrous minerals in the basaltic andesites,their phenocryst assemblages reveal the influence of substantialamounts of water, and thereby show a genetic link to the variouslamprophyric lavas.     

Evolution and sedimentology of a channel fill in the sandy braided South Saskatchewan River and its comparison to the deposits of an adjacent compound bar

The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture.     

Fountains in Magma Chambers

Cyclic layering is a common feature of the ultramafic zone oflayered intrusions and is usually attributed to the entry ofnew pulses of dense magma into the chamber. Since the crystallizationof olivine and bronzite lowers the density of the magma, a newpulse of the parent magma will be denser than the fractionatedmagma in the chamber. If the new pulse enters with excess momentumit will initially rise up into the host magma to form a fountain,then fall back around the feeder when negative buoyancy forcesovercome the initial momentum of the pulse. Laboratory experimentsusing aqueous solutions with both point and line sources havebeen conducted to obtain a quantitative understanding of thefluid-dynamical processes that are important in fountains. Itis observed that convection within the fountain is highly turbulent,resulting in appreciable entrainment of the host magma. A gravity-stratifiedhybrid layer develops at the floor and this breaks up into aseries of double-diffusive convecting layers if the new pulseis hotter than the host magma. The number of layers that formdepends on a number of factors, especially R, the ratio of thecontributions of composition and heat to the total density differencebetween the host magma and the new pulse. Raising the valueof R, results in the formation of more, thinner layers. The thickness of the hybrid layer at any time t is given byH = h₀+(V₀/A)t where V₀ is the volume flux through the feederand A is the horizontal area of the chamber. h₀ is related tothe initial steady-state height of the fountain and, for a linesource, is given by h₀=CU₀^4/3 d^–1(g/)^–2/3 whereU₀ is the volume flux per unit length, g is the accelerationdue to gravity, d is the width of the feeder, is the densityof the host magma, is the density difference between the magmasand C is a constant. Calculations based on these results and the consideration ofthe flow in the feeder dykes below the chamber indicate thata fountain will rise at least 350 m in a continental magma chamberif the feeder width is greater than 10 m. This will lead toextensive mixing between the new pulse and the fractionatedmagma in the chamber, producing a zoned hybrid layer at thefloor that is commonly over 1000 m thick. If the chamber receivesmany pulses of dense magma, the resulting zoning may persistthroughout much of the life of the chamber, especially if thefirst pulse to enter becomes contaminated by light magma releasedby melting at the margins. The highest Mg/Fe ratio for olivineand pyroxenes from cyclic units from the ultramafic zones oflayered intrusions is often well below the value expected forminerals crystallizing from a melt derived directly from themantle, supporting the hypothesis that new pulses of dense magmacan mix extensively with the fractionated magma in the chamber. The feeder dykes to some oceanic magma chambers, such as theBay of Islands Ophiolite, are believed to be narrower, so thatfountains do not rise more than a few metres above the floorof the chamber. This restricts mixing between the input magmaand the host magma and can result in the formation of a hybridzone that is only a few metres thick.     

Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla,Switzerland – a consequence of resetting at the glacier bed?

Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques.     

Provenance and Magmatic-Metamorphic Evolution of a Variscan Island-Arc Complex: Constraints from U-Pb Dating, Petrology, and Geospeedometry of the Kyffhauser Crystalline Complex, Central Germany

The Kyffhäuser Crystalline Complex, Central Germany, formspart of the Mid-German Crystalline Rise, which is assumed torepresent the Variscan collision zone between the East Avalonianterrane and the Armorican terrane assemblage. High-precisionU–Pb zircon and monazite dating indicates that sedimentaryrocks of the Kyffhäuser Crystalline Complex are youngerthan c. 470 Ma and were intruded by gabbros and diorites between345 ± 4 and 340 ± 1 Ma. These intrusions had magmatictemperatures between 850 and 900°C, and caused a contactmetamorphic overprint of the sediments at P–T conditionsof 690–750°C and 5–7 kbar, corresponding toan intrusion depth of 19–25 km. At 337 ± 1 Ma themagmatic–metamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 7–11 km.This is inferred from a diorite, and conforms to P–T pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 5–7 to 2–4 kbar at 690–750°C.Subsequently, the metamorphic–magmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published K–Ar muscoviteage of 333 ± 7 Ma. With respect to P–T–D–tdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; U–Pb dating; hornblende; garnet; Mid-German Crystalline Rise; P–T pseudosection     

Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe

Santonian-Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment-ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high- and low-productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well-oxygenated, current-swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate-fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment-seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post-oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo-Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.     

Environmental impacts of the Norse settlement: palaeoenvironmental data from Myvatnssveit, northern Iceland

Lawson, I. T., Gathorne‐Hardy, F. J., Church, M. J., Newton, A. J., Edwards, K. J., Dugmore, A. J. & Einarsson, Á. 2007 (January): Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland. Boreas, Vol. 36, pp. 1–19. Oslo. ISSN 0300–9483. The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically‐rich Mvatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvastjörn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landnám, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post‐landnám pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landnám, probably because of anthropogenic disturbance. An increase in sedimentation rate after landnám appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change.     

Observations of walruses along the Norwegian coast 1967–1992

Extralimital observations of walruses are known to be quite common in Norway. The present review covers observations of walruses along the Norwegian coast between 1967 and 1992. A total of 34 different walruses have been recorded observed since 1967. These observations indicate a significant increase in the number of walrus observations in recent years, most likely due to an increase in the walrus population in the Barents Sea area. Most of the walruses observed are assumed to be subadult males.     

The Chemistry and Petrogenesis of a Suite of Pantellerites from the Ethiopian Rift

Data on the major and trace element chemistry of a suite oftwenty pantelleritic pitchstone lavas from the Quaternary Ethiopianvolcano Fantale is presented. This reveals a contrast betweenthe composition of the pre-caldera flows and the more siliceous,less peralkaline post-caldera lavas. Comparison with experimental and theoretical studies suggeststhat nearly all of the major and trace element variation withinthe two suites can be explained by assuming fractional crystallizationof alkali-feldspar, the most abundant phenocryst phase. Fractionationof the mafic phases appears to have been less significant. The trace element data strongly indicate that the lavas allbelong to a single suite. However, it is suggested that themajor element chemistry of the post-caldera flows was modifiedby the loss of volatiles at the time of the formation of thecaldera, an event which coincided with the eruption of a 2 km³welded ash-flow tuff.