Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland

The Krafla volcanic system consists of a central volcano andassociated fissure swarm in the NE axial rift zone of Iceland.Lavas spanning the whole of Krafla's exposed volcanic history(estimated to be 0-> 300 ka) have been analysed and rangein composition from olivine tholeiite to rhyolite. Major-elementcompositions suggest that fractional crystallization exertsthe main control over the differentiation process. However,K₂O and the very incompatible trace elements, Rb, Th, and U,are all enriched beyond the extent expected by closed-systemfractional crystallization. Fractionation coupled with periodicreplenishment and tapping of the reservoir is unlikely to beresponsible for this enrichment, despite the geophysical evidencesuggesting a large number of inflations and deflations of ashallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authorsthat crustal assimilation is operating on a local scale beneathKrafla. A model is suggested, fitting both the Th- and O-isotopicdata, which involves the partial melting and incorporation ofa hydrothermally altered wall-rock contaminant during fractionalcrystallization (i. e., AFC processes). This process of partialmelting is likely to enhance the most highly incompatible elementconcentrations (e. g., increasing Rb/Zr) more than expectedby closed-system fractional crystallization.     

Palaeo-ice streams and the subglacial landscape mosaic of the North American mid-continental prairies

The analysis of the glacial landscape of southern Saskatchewan (Canada) through multiple data sets (e.g. digital elevation model, till compositional data) has revealed previously unrecognized subglacial sediment–landform assemblages. A southwest-trending corridor of mega-scale till lineations (Maskwa corridor) bounded on each side by hummocky terrain extends from the Canadian Shield to southwestern Saskatchewan. This landform assemblage is clearly cross-cut by a broad south to southeast trending corridor (Buffalo corridor) consisting of subparallel curvilinear till ridges. The carbonate content of the surface till is spatially consistent within these assemblages, suggesting a strong sediment–landform relationship. The two corridors are interpreted as the product of palaeo-ice streams. The Maskwa palaeo-ice stream flowed up the regional slope and across preglacial valleys, indicating it was thick and stable. Narrow dispersal trains extending across as well as down-glacier from the Athabasca Basin suggest that the Maskwa palaeo-ice stream extended far into the ice sheet across contrasting shield and platform terrains. In comparison, the Buffalo palaeo-ice stream was thinner and largely controlled by subglacial geology and topography. Its catchments were located at the Canadian Shield boundary and the system was oriented along-slope. It experienced lateral shifts and it was fed by a network of tributaries. The glacial dynamics shift from the Maskwa to the Buffalo system occurred at about 13.5 ¹⁴C kyr BP. The Buffalo system later evolved into thin outlet lobes until final deglaciation of the area. The proposed model has implications for ice-sheet reconstruction and the assessment of till properties in the prairies and in similar terrains.     

Rapid Change of Lava Composition from 1998 to 2002 at Piton de la Fournaise (Reunion) Inferred from Pb Isotopes and Trace Elements: Evidence for Variable Crustal Contamination

After an unusually long quiet period of nearly 6 years, in 1998the Piton de la Fournaise volcano started a new cycle of intensevolcanic activity. We report geochemical data on the first nineevents (53 samples), from the long-lived initial eruption (sixand a half months) of 1998 to the high-flux picritic eruptionof January 2002. Pb isotopes and trace elements display systematic,coupled variations, which are mostly confined to the beginningand the end of the period. Two well-defined binary mixing trendsare shown by Pb–Pb and Pb–trace element relationships.These trends indicate a change of end-member components betweenMarch and June 2001 that coincides with the transition fromsteady-state basalts to picrites. A three-component mixing modelinvolving a homogeneous plume and two contaminants successfullyexplains the data. The Pb–Pb relationship requires thattwo mixing processes occur successively: plume-derived magmainteracts first with altered oceanic crust, and the resultinghybrid product then interacts at shallower level with the oldlavas constituting the base of the volcanic edifice. Assimilationof edifice material decreased continuously from 1998 to 2002,whereas assimilation of oceanic crust drastically increasedduring the late-stage picritic eruption. These results suggestthat picrites may have resided for an unusually long time atan oceanic crustal level before ascending rapidly through thevolcanic edifice with little interaction with channel walls. KEY WORDS: assimilation; lead isotopes; picrites; Piton de la Fournaise; trace elements     

MIGRATION TO ZERO OFFSET (DMO) FOR A CONSTANT VELOCITY GRADIENT: AN ANALYTICAL FORMULATION1

Migration to zero offset (MZO) is a prestack partial migration process that transforms finite-offset seismic data into a close approximation to zero-offset data, regardless of the reflector dips that are present in the data. MZO is an important step in the standard processing sequence of seismic data, but is usually restricted to constant velocity media. Thus, most MZO algorithms are unable to correct for the reflection point dispersal caused by ray bending in inhomogeneous media. We present an analytical formulation of the MZO operator for the simple possible variation of velocity within the earth, i.e. a constant gradient in the vertical direction. The derivation of the MZO operator is carried out in two steps. We first derive the equation of the constant traveltime surface for linear V(z) velocity functions and show that the isochron can be represented by a fourth-degree polynomial in x, y and z. This surface reduces to the well-known ellipsoid in the constant-velocity case, and to the spherical wavefront obtained by Slotnick in the coincident source-receiver case. We then derive the kinematic and dynamic zero-offset corrections in parametric form by using the equation of the isochron. The weighting factors are obtained in the high-frequency limit by means of a simple geometric spreading correction. Our analytical results show that the MZO operator is a multivalued, saddle-shaped operator with marked dip moveout effects in the cross-line direction. However, the amplitude analysis and the distribution of dips along the MZO impulse response show that the most important contributions of the MZO operator are concentrated in a narrow zone along the in-line direction. In practice, MZO processing requires approximately the same trace spacing in the in-line and cross-line directions to avoid spatial aliasing effects.     

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RESUME

La méthode classique de simulation d'une nappe captive consiste à résoudre le problème en régime transitoire à partir de la situation originelle. Dans cet article est présentée une méthode qui permet de déterminer directement l'état de la nappe à un instant donné si l'on peut connaître la chute piézométrique à cet instant.     

The origin and glaciodynamic significance of sandstone ridge networks from the Hirnantian glaciation of the Djado Basin (Niger)

The Djado Basin (Niger) was located beneath the inner part of the Late Ordovician ice sheet. The Felar‐Felar Formation consists mainly of glaciomarine deposits, associated with the major ice sheet recession within the glaciation, and is bounded by two glacial unconformities. Structures corresponding to sandstone ridges are found within the Felar‐Felar Formation. Sandstone ridges are several metres high, about 10 m wide and hundreds of metres long. These structures are organized in extensive anastomosed to sub‐polygonal networks. The association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub‐glacial conditions for their origin. Sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the Felar‐Felar Formation by sub‐glacial, turbulent and pressurized meltwater; then sub‐glacial cavities were infilled with sand derived from glacial abrasion. Sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo‐ice sheet.     

Reversal of Fe-Mg Partitioning Between Garnet and Staurolite in Eclogite-facies Metapelites from the Champtoceaux Nappe (Brittany, France)

This paper concentrates on the petrology of eclogite-faciesmetapelites and, particularly, the significance of staurolitein these rocks. A natural example of staurolite-bearing eclogitic micaschistsfrom the Champtoceaux nappe (Brittany, France) is first described.The Champtoceaux metapelites present, in addition to quartz,phengite, and rutile, two successive parageneses: (1) chloritoid+staurolite+garnetcores, and (2) garnet rims+kyanite?chloritoid. Detailed microprobe analyses show that garnet and chloritoidevolve towards more magnesian compositions and that stauroliteis more Fe-rich than coexisting garnet. A comparison of thestudied rocks with other known occurrences of eclogitic metapelitesshows that whereas staurolite is always more Fe-rich than garnetin high-pressure eclogites, the reverse is true in low- to medium-pressuremicaschists. Phase relations between garnet, staurolite, chloritoid, biotite,and chlorite are analysed in the KFMASH system (with excessquartz, phengite, rutile, and H₂O). The topology of univariantreactions is depicted for a normal and a reverse Fe-Mg partitioningbetween garnet and staurolite. Mineral compositional changesare also predicted for varying bulk-rock chemistries. In the studied micaschists, the zonal arrangement of garnetinclusions and the progressive compositional changes of ferromagnesianphases record part of the prograde P–T path, before theattainment of ‘peak’ metamorphic conditions (atabout 65O–7OO?C, 18–20 kb). The retrograde path,which records the uplift of the Champtoceaux nappe, occurs underdecreasing temperatures.     

Position of the palaeontological site Aven I des Abîmes de La Fage, at Noailles (Corrèze, France), in the European Pleistocene chronology

Mourer-Chauviré, C., Philippe, M., Quinif, Y., Chaline, J., Debard, E., Guérin, C. & Hugueney, M. 2003 (September): Position of the palaeontological site Aven I des Abîmes de La Fage, at Noailles (Corrèze, France), in the European Pleistocene chronology. Boreas , Vol. 32, pp. 521–531. Oslo. ISSN 0300–9483.
The palaeontological locality of Aven I des Abîmes de La Fage, at Noailles, Corrèze, France, is a limestone swallow hole filling which includes a rich fauna of micro- and macromammals, and one of the most complete Pleistocene avifaunas in the Palearctic. According to the mammal fauna and to the sedimentological study, the filling was previously assigned to the penultimate, Saalian, glaciation. U/Th datings made on a stalagmitic floor deposited in a lateral gallery on top of the filling confirm this previous attribution. Revised lists of the mammal and bird faunas are provided. The comparison of the La Fage fauna with that of other European localities, radiochronologically or biostratigraphically dated, leads to the conclusion that most of the fossiliferous layers were deposited during the Marine Isotope Stage (MIS) 8 (242 to 301 ka).     

Morphology and facies architecture of a falling sea level strandplain, Umiujaq, Hudson Bay, Canada

Coastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground-probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse-sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing-upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp-based bar-and-beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion.