Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland I. Relationship Between the Magma Types

The Torfaj?kull volcano, Iceland's largest silicic centre, issituated close to the junction of the active, southerly propagatingEastern Rift Zone and the South Eastern Zone, an older crustalsegment. This paper provides major, trace, and some Sr isotopedata on postglacial (<10000 y) rocks, i.e., tholeiitic magmasof the Eastern Rift Zone and transitional basalts, icelandites,and rhyolites of the Torfaj?kull centre, and assesses the relationshipsbetween the magma types in terms of the development of the Icelandiccrust. Tholeiitic magmas from the Eastern Rift Zone are LILE-enrichedrelative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxenefractionation at low pressures. Compared with the tholeiites,Torfaj?kull transitional basalts show LILE/HFS enrichment andhigher (⁸⁷Sr/⁸⁶Sr)₁ ratios. They define several magmatic lineagesand have equilibrated over a wide range of pressures. Both basalttypes were derived by very small degrees of partial meltingof compositionally similar mantle sources, the main differencebeing that the tholeiites were generated in the spinel Iherzolite,and the transitional basalts in the garnet lherzolite, stabilityfields, a conclusion previously reached by Meyer et al. (1985).The mantle sources may have contained LILE-enriched streaks. Low-pressure differentiation of Torfaj?kull transitional basaltproduced an iceiandite to sub-alkaline rhyolite sequence bycrystal fractionation, the rhyolites representing >90% crystallizationof parental basalts. The rhyolites were emplaced as nine separatelava fields, formed during 11 eruptive episodes. The compositionalrange within each field is limited, and, although similar, theranges define several magmatic lineages. Continued fractionationof plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zirconassemblages generated peralkaline rhyolites in certain post-glacialeruptions. Chemical variations in the deposits from the Hrafntinnuskerperalkaline eruption were related predominantly to alkali feldsparfractionation, and the melts were erupted from a zoned magmachamber. All postglacial volcanic rocks at Torfajokull havebeen mantle derived and thus represent new additions to theIcelandic crust.     

Crustal Influences in the Petrogenesis of the Naivasha Basalt--Comendite Complex: Combined Trace Element and Sr-Nd-Pb Isotope Constraints

The bimodal Naivasha complex (central Kenya) comprises 2 suitesof transitional basalts and 7 chemostratigraphic groups of comendites.The early basalt series (EBS) predates the Group 1 comenditeswith the later series (LBS) erupted between Groups 5 and 6.Basalts from both suites are notable for their relatively radiogenic²⁰⁷Pb/²⁰⁴Pb isotope ratios which are higher than in the majorityof ocean island basalt (OIB, Zindler & Hart, 1986), and⁸⁷Sr/⁸⁶Sr ratios more radiogenic than basalts from northernKenya. Both basalt suites exhibit systematic trace element andisotopic variations which appear related to greater assimilationof Proterozoic amphibolite facies crust by the chemically moreevolved rock types. Their mantle source regions show evidenceof residual plagioclase and have a ‘Dupal’-likeOIB trace element and Pb-Sr-Nd isotope signature (Hart, 1984).A contribution from the sub-continental lithosphere is proposedin basalt genesis. The seven comendite groups have distinct trace element and isotopesystematics. Hydration of comendite glass causes significantchanges in Sr and Pb isotope ratios. In terms of their Sr-Ndisotope relationships the unaltered comendites could be derivedfrom the basalts by an assimilation-fractional crystallization(AFC) process dominated by the fractional crystallization offeldspars. However, the Pb systematics clearly demonstrate thatthe basalts and comendites are not part of a cogenetic suite.Chemical variations within individual comendite groups are predominantlythe result of fractional crystallization of the observed phenocrystassemblages (i.e. alkali feldspar dominated) and minor crustalinteraction. The majority of the chemical and isotopic differencesbetween Groups 1–7 cannot be explained by fractional crystallizationand appear to represent crustal melts derived from close tothe interface between Pan African basement and the overlyingMiocene-Holocene volcanoclastic rocks, at approximately 6 kmdepth (KRISP working group, 1987). Halogens play a fundamental role in the petrogenesis of thecomendites (Cl+F<1?7 per cent) permitting small degree meltsof low viscosity to be extracted from the crust and causingthe breakdown of minor phases e.g, zircon. These factors explainthe extreme enrichment of certain incompatible trace elements(Zr<2500, Nb<700) in the comendites and coupled with theretention of zircon in the source of the halogen poor comendites(Group 1<0?6 per cent Cl+F) result in notable fractionationamong the HFSE (Zr/Nb 1?5–5?5). Halogens may be concentratedin the source region from the surrounding crust by the presentlyactive hydrothermal system. Each of the chemostratigraphic comenditegroups is chemically distinct, implying that partial meltingof the heterogeneous crust is on a limited scale and that noextensive magma chambers exist beneath Naivasha.     

The potential of perennial cave ice in isotope palaeoclimatology

Perennial ice from caves on and to the east of the Canadian Great Divide yield δ¹⁸O and δD values, which are unusually high measurements when compared with the average precipitation for the region. Furthermore, these ice data fall below and along lines of lower slope than the Global Meteoric Water Line. To explain the observed relationships, we propose the following process. A vapour-ice isotopic fractionation mechanism operates on warm-season vapour when it precipitates as hoar ice on entering the caves. The subsequent fall of hoar to the cave floor through mechanical overloading, along with ice derived from ground-water seepage (with a mean annual isotopic composition), results in massive ice formation of a mixed composition. This mixed composition is what is observed in the characteristic relationships found here. Such findings suggest that a warm versus cold climate interpretation for ancient cave ice may be the opposite of that found in the more familiar polar and glacial ice cores.     

Preparing For Hyperseed MAC: An Observing Campaign To Monitor The Entry Of The Genesis Sample Return Capsule

The imminent return of the Genesis Sample Return Capsule (SRC) from the Earth’s L1 point on September 8, 2004, represents the first opportunity since the Apollo era to study the atmospheric entry of a meter-sized body at or above the Earth’s escape speed. Until now, reentry heating models are based on only one successful reentry with an instrumented vehicle at higher than escape speed, the 22 May 1965 NASA “FIRE 2” experiment. In preparation of an instrumented airborne and ground-based observing campaign, we examined the expected bolide radiation for the reentry of the Genesis SRC. We find that the expected emission spectrum consists mostly of blackbody emission from the SRC surface (T∼ ∼2630 K@peak heating), slightly skewed in shape because of a range of surface temperatures. At high enough spectral resolution, shock emission from nitrogen and oxygen atoms, as well as the first positive and first negative bands of N₂⁺, will stand out above this continuum. Carbon atom lines and the 389-nm CN band emission may also be detected, as well as the mid-IR 4.6-μm CO band. The ablation rate can be studied from the signature of trace sodium in the heat shield material, calibrated by the total amount of matter lost from the recovered shield. A pristine collection of the heat shield would also permit the sampling of products of ablation.     

THEME 1A DERIVATION OF EROSION AND SEDIMENT YIELD PARAMETERS IN AREAS WITH DEFICIENT DATA-RECONNAISSANCE MEASUREMENTS

Abstract

After defining the several terms that have come to be used to describe basins of various types this paper deals with some of the problems of basin studies. The relative sizes of different basins is one such problem, overland flow and interflow are others. Simulation of basins is considered, particularly the use of the Stanford Watershed Model and how this model can be applied to investigate changes due to urbanization and deforestation. The future role of representative and experimental basins is discussed.     

Postglacial climate and vegetation history, north-central Kola Peninsula, Russia: pollen and diatom records from Lake Yarnyshnoe-3

A sediment core from Lake Yarnyshnoe-3 (69°04'N; 36°04'E), an emerged coastal lake from the tundra of the north-central Kola Peninsula, has been analyzed for fossil pollen and diatoms. The pollen record shows the Younger Dryas event marked by increasing Artemisia coupled with decreases in Poaceae, Cyperaceae and Salix at c. 10 700 to 10 000 BP. This core provides the first well-defined palynological record of the Younger Dryas event on the Kola Peninsula. Stomates from Pinus were recovered from the core interval between 8000 and 6000 BP. The stomates, coupled with elevated values of pine pollen, indicate that Pinus sylvestris grew near the arctic coastline of the central Kola Peninsula in the middle Holocene. However, the small number of stomates suggests that pines were not plentiful. The diatom record from the core reflects basin isolation from the sea and indicates additional limnological changes during the climate transition between c. 5000 and 4000 BP. The broadly similar climate and vegetation history on the north-central Kola Peninsula and in Fennoscandia demonstrates the propagation of late glacial and Holocene climate events from the North Atlantic region into the Eurasian Arctic.     

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.