Late Quaternary development of the southern sector of the Greenland Ice Sheet, with particular reference to the Qassimiut lobe

The evolution of the southern Greenland Ice Sheet is interpreted from a synthesis of geological data and palaeoclimatic information provided by the ice-sheet cores. At the Last Glacial Maximum the ice margin would have been at the shelf break and the ice sheet was fringed by shelf ice. Virtually all of the present ice-free land was glaciated. The initial ice retreat was controlled by eustatic sea level rise and was mainly by calving. When temperatures increased, melt ablation led to further ice-margin retreat and areas at the outer coast and mountain tops were deglaciated. Retreat was interrupted by a readvance during the Neria stade that may correlate with the Younger Dryas cooling. The abrupt temperature rise at the Younger Dryas-Holocene transition led to a fast retreat of the ice margin, and after ∼9 ka BP the ice sheet was smaller than at present. Expansion of the ice cover began in the Late Holocene, with a maximum generally during the Little Ice Age. The greatest changes in ice cover occurred in lowland areas, i.e. in the region of the Qassimiut lobe. The date of the historical maximum advance shows considerable spatial variability and varies between AD 1600 and the present. Local anomalous readvances are seen at possibly 7-8 ka and at c. 2 ka BP. A marked relative sea level rise is seen in the Late Holocene; this is believed to reflect a direct glacio-isostatic response to increasing ice load.     

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suiteof calc-alkaline and high-alumina-olivine tholeiitic lavas fromthe Lassen region of California and are used, in conjunctionwith previously published mineralogical, geochemical, and isotopicdata, to constrain their petrogenesis. Positive correlationbetween     

THE INSENSITIVITY OF REFLECTED SH WAVES TO ANISOTROPY IN AN UNDERLYING LAYERED MEDIUM1

Propagation in the plane of mirror symmetry of a monoclinic medium, with displacement normal to the plane, is the most general circumstance in anisotropic media for which pure shear-wave propagation can occur at all angles. Because the pure shear mode is uncoupled from the other two modes, its slowness surface in the plane is an ellipse. When the mirror symmetry plane is vertical the pure shear waves in this plane are SH waves and the elliptical SH sheet of the slowness surface is, in general, tilted with respect to the vertical axis. Consider a half-space of such a monoclinic medium, called medium M, overlain by a half-space of isotropic medium I with plane SH waves incident on medium M propagating in the vertical symmetry plane of M. Contrary to the appearance of a lack of symmetry about the vertical axis due to the tilt of the SH-wave slowness ellipse, the reflection and transmission coefficients are symmetrical functions of the angle of incidence, and further, there exists an isotropic medium E with uniquely determined density and shear speed which gives exactly the same reflection and transmission coefficients underlying medium J as does monoclinic medium M. This means that the underlying monoclinic medium M can be replaced by isotropic medium E without changing the reflection and transmission coefficients for all values of the angle of incidence. Thus no set of SH seismic experiments performed in the isotropic medium in the symmetry plane of the underlying half-space can reveal anything about the monoclinic anisotropy of that underlying half-space. Moreover, even when the underlying monoclinic half-space is stratified, there exists a stratified isotropic half-space that gives the identical reflection coefficient as the stratified monoclinic half-space for all angles of incidence and all frequencies.     

AUTOMATED FIRST ARRIVAL PICKING: A NEURAL NETWORK APPROACH1

A back-propagation neural network is successfully applied to pick first arrivals (first breaks) in a background of noise. Network output is a decision whether each half-cycle on the trace is a first or not. 3D plots of the input attributes allow evaluation of the attributes for use in a neural network. Clustering and separation of first break from non-break data on the plots indicate that a neural network solution is possible, and therefore the attributes are suitable as network input. Application of the trained network to actual seismic data (Vibroseis and Poulter sources) demonstrates successful automated first-break selection for the following four attributes used as neural network input: (1) peak amplitude of a half-cycle; (2) amplitude difference between the peak value of the half-cycle and the previous (or following) half-cycle; (3) rms amplitude ratio for a data window (0.3 s) before and after the half-cycle; (4) rms amplitude ratio for a data window (0.06 s) on adjacent traces. The contribution of the attributes based on adjacent traces (4) was considered significant and future work will emphasize this aspect.     

How round is round? A new approach to the topic ‘roundness’by Fourier grain shape analysis

Numerous methods for roundness measurement have been developed. None, however, has been generally accepted, because of conceptual and practical deficiencies. Modern image processing and Fourier grain shape analysis have eliminated the practical shortcomings, but the conceptual ones remained. Single, higher harmonics of the Fourier series, for example, fail to serve as reliable equivalents for roundness evaluation. The concept outlined in this paper recognizes three criteria for the evaluation of roundness. (1) All curvatures, convex as well as concave or plane elements, must be considered. (2) The positions of morphological elements are significant because salient parts of a particle are more easily abraded than protected ones. Consequently, the curvatures have to be weighted by their relative position on the particle. (3) Positions and curvatures of morphological elements have to be compared with the particle's ultimate abraded shape, which is assumed to be an ellipsoid. The ellipsoid reflects the aspect of form or sphericity. The distinction between sphericity and roundness is retained because there is no evidence that sphericity changes significantly during transport. The measurement is based on the outline of a particle's maximum projection plane, which is transformed to a Fourier series. Roundness data are derived from the complete amplitude spectrum. The aspect of sphericity is eliminated by subtracting the amplitude spectrum of the best approximating ellipse from the spectrum of the empirical shape. The residual amplitudes are normalized and summed. In a final step the resulting values are rescaled. This guarantees reasonable boundaries and a normal distribution of roundness values. The procedure is automated and its efficiency permits the calculation of large samples. Tests on fluvial and coastal gravel populations demonstrate that the method is sensitive to abrasional wear during all stages of roundness.     

The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas

Hydrothermal experiments on natural samples of trachyandesiteand dacite bulk composition show that anhydrite (CaSO₄) mayoccur as a stable phenocryst phase at oxygen fugacities greaterthan or equal to 1.0 to 1.5 log f_O2 units above the Ni–NiOequilibrium. The dissolved sulfur concentration in anhydritesaturated melts from MnO–Mn₃O₄ buffered experiments decreaseswith decreasing temperature, from approximately 2300 p.p.m.Sat 1025?C to 250 p.p.m.S at 850?C (all at 2 kb P_fluid = P_total).In FeS-saturated melts equilibrated at the Ni–NiO bufferand 2 kb pressure, the concentration of dissolved sulfur alsodecreases with decreasing temperature, varying from approximately400 p.p.m. S at 1025?C to less than 100 p.p.m. S at 850?C. AtNNO or lowerf_O2s, decreasing melt FeO content due to crystalfractionation may explain the observed decrease in sulfur solubilitywith decreasing temperature. Sulfur solubility values equivalent to the approximately 0.6wt. per cent S present in fresh bulk pumice samples from the1982 eruptions of El Chichon volcano are not readily achievedunder any reasonable combinations of pressure, temperature,and oxidation state. Dissolved sulfur contents approaching 0.6wt. per cent might occur if the source regions of melts parentalto the El Chichon trachyandesite were at an f_O2 several logunits above the Ni–NiO equilibrium. Because such elevatedoxidation states are far greater than the generally acceptedvalues for mantle-derived partial melts we believe the highsulfur content of the El Chichon pumices is not a primary feature;it reflects reaction with sulfur enriched material at some unknowndepth beneath the volcano. Published sulfur isotopic and petrologicdata suggest that hydrothermally altered rocks similar to thepyrite- and anydritebearing lithic fragments found in the 1982pumices could have provided a source of sulfur for crystallizationof magmatic anhydrite. The anhydrite was an important sourceof sulfur for evolution of a sulfur-rich vapor phase duringeruption of the magma. Although many calc-alkaline dacites and rhyolites appear toattain oxidation states high enough to stabilize anhydrite,the characteristically low bulk sulfur contents of these rockswill limit anhydrite abundances to less than approximately 0.1wt. per cent, assuming sufficient sulfur is present to achievesaturation. Such small amounts of a water soluble mineral couldbe easily removed by subaerial weathering processes, dissolvedduring vapor exsolution from a magma, or simply overlooked duringroutine petrographic examination.     

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: II. The Genesis of Cryptic Hybrids

Tholeiitic lavas of the Servilleta Basalt exhibit only subtletextural and mineralogical evidence for a hybrid origin, butelemental and isotopic analyses of these basalts are best modeledin terms of mixing Servilleta parent magma with a range of contemporaneousandesite and dacite magmas. Cryptic compositional heterogeneitiesin some flows interpreted as hybrids apparently reflect incompletehomogenization following pre-emptive magma mingling. The generalscarcity of mixing-related textural disequilibrium is ascribedin part to mixing of mineralogically similar end-members. Eradicationof some phenocrysts during post-mixing residence and evolutionin a convecting magma body may be an even more important factor. Eruptions of hybrid magmas may frequently be triggered by magmamixing events (i.e. injection or replenishment), and minglingof compositionally diverse magmas may ensue as a consequenceof tapping a compositionally graded or layered magma chamber.These hybrids are instantly recognizable by the preservationof disequilibrium textures and mineral assemblages, and by discontinuouscompositional heterogeneities. Cryptic hybrids, which have notpreserved this record, will be recognizable as mixed magmasprimarily by geochemical evidence for open system evolution.     

Polybaric Geochemical Evolution of Two Shield Volcanoes from Santorini, Aegean Sea, Greece: Evidence for Zoned Magma Chambers from Cyclic Compositional Variations

Petrographic, mineralogic, and geochemical data are reportedfor lavas from two of the major shield volcanoes of the Santorinivolcanic complex (Skaros and Micro Profitis Ilias), both ofwhich were active prior to the well-known Minoan eruption withassociated caldera collapse. Field work and whole-rock chemicalanalyses indicate four cycles of eruptive activity within theSkaros sequence and three within the Micro Profitis Ilias (MP1) sequence. SiO₂ and LIL-element contents decrease from thebase to the top of all cycles except for the uppermost cycleof Skaros. Chemical variations within cycles are interpretedto result from eruption from compositionally and thermally zonedmagma chambers. Major oxide data and the results of least-squares, mass balancemodeling indicate that fractional crystallization played animportant role in the development of the observed chemical variations.However, observed systematic variations in groundmass compositionswithin each cycle, observed irregular variations in total phenocrystcontent and the results of density calculations require thatgeneration of the chemical zonation did not involve crystalsettling but reflects unstable density stratification, probablyresulting from sidewall crystallization. Some of the primitivelavas erupted on Santorini preserve phenocryst and xenocrystevidence for a stage of high-pressure fractional crystallization(involving removal of olivine, clinopyroxene, orthopyroxene,and Cr-spinel). Trace element data combined with petrographic data (i.e. theoccurrence of abundant phenocrysts with resorption textures)is taken as evidence that magma mixing was also important inthe development of cyclic variations. Most basaltic andesitesfrom Skaros appear to be hybrids derived by mixing of basaltand andesite/dacite. Mineralogic data demonstrate that mixingwas also important in the development of zonation in the chambersbeneath MPI, but trace element data cannot be explained by combinedfractionation and mixing alone. Specifically, incompatible,and compatible element abundances are lower than predicted iffractionation and mixing occurred and it is suggested that theanomalous trace element behaviour of especially LIL elementsreflects the simultaneous operation of assimilation, for whichthere is support from isotopic studies. It is concluded thatinter- cyclic chemical variations are explicable in terms offractionation, mixing and assimilation. The LIL element andhighly compatible element concentrations in the most primitivelavas erupted in each cycle of Skaros and MPI increase withtime, indicating that mixing became more important with time.     

Experimental Phase Relations in the System Tonalite-Peridotite-H2O at 15 kb; Implications for Assimilation and Differentiation Processes near the Crust-Mantle Boundary

Experiments at 15 kb in the tonalite-peridotite-H₂O system provideinformation on some of the phase equilibrium factors that mayinfluence reaction and assimilation processes between quartznormativemagmas and ultramafic rocks in the deep crust and upper mantle.Experiments were done with 5 or 10 wt.% H₂O added to powderednatural samples of tonalite, and mixtures of tonalite with 5or 10 wt.% peridotite added (TP5 and TP10, respectively). Theliquidus phase relations of these starting compositions wereinvestigated between 850 and 1100?C at 15 kb, using gold capsulesso that iron loss to the sample containers was not a problemand meaningful glass and mineral analyses could be obtained.Experiments on the tonalite alone show either liquidus garnet,for samples with 5% H₂O added, or liquidus hornblende, for sampleswith 10% H₂O. In contrast, orthopyroxene is the sole liquidusphase, irrespective of water content, in experiments using startingmixtures of 5 or 10 wt.% peridotite added to tonalite. Glassanalyses of partially crystallized tonalite define a crystallizationpath diverging significantly from the calc-alkaline trend towardshigher Ca/(Mg + Fe) in the CaO–(MgO + FeO)–?SiO₂triangle. In contrast, glasses from partially crystallized mixturesof tonalite with 5 or 10 wt.% peridotite added define a liquidtrend close to natural calc-alkaline compositions in terms ofCa/(Mg + Fe). Of more general significance, the proximity ofa field ofliquidus orthopyroxene on the high (Mg + Fe) sideof compositions along the calc-alkaline trend serves to limitthe Mgenrichment of such melts by interaction with ultramaficrocks. Unless heat is added to the system, reaction of tonaliticcomposition melts with ultramafic rocks will produce only slightlyMg-enriched melts: increasing degree of reaction simply resultsin further precipitation of orthopyroxene + garnet ? clinopyroxeneonce melt compositions reach the orthopyroxene field boundary.