Structure and viscosity of rhyolitic composition melts

The average local structure of a rhyolitic composition glass has been determined at 25°C using X-ray radial distribution analysis (RDA) and quasi-crystalline modelling and is best described as similar to that in a stuffed framework composed principally of six-membered rings of Si and Al tetrahedra (basically a stuffed tridymite-like model). Using this model it is possible to calculate a density (2.41 g/cm³) which compares well with the measured density (2.40 g/cm³); a structural model based on four-membered rings (an albite-like model) results in a substantially higher calculated density (2.60 g/cm³). We suggest that the rhyolite glass structural model is appropriate for rhyolitic melts, based on evidence from the recent literature. New viscosity data for an anhydrous rhyolite composition measured between 1200°C and 1500°C are presented and interpreted in terms of our proposed structural model and previous melt structure models for the major normative components of rhyolite. A mechanism for diffusion and viscous flow in framework silicate melts (including rhyolite composition) is proposed on the basis of recent molecular orbital calculations and molecular dynamics simulations of silicate and fluoride melts.     

Continual long-term monitoring of methane in wells above the Utica Shale using total dissolved gas pressure probes

Hydrogeology Journal - Monitoring of dissolved methane concentrations in groundwater is required to identify impacts from oil and gas development and to understand temporal variability under...     

Rheology and geodynamic modelling: the next step forward

The application of continuum mechanics and microstructural analysis to geological studies over the past 30 years has spurred earth scientists to reassess fundamental tectonic processes such as subduction, collision and rifting in terms of dynamics. Armed with new analytical methods, geologists have returned to the field to look at rock structures with more mechanistic eyes. The advent of sophisticated computers, programs, and laboratory deformation equipment has facilitated the simulation of geodynamic processes that range in scale from the grain to the lithosphere. The result has been specialization, with the concomitant opening of communication gaps between geodynamicists, field geologists and rock mechanicists. Partly, these gaps reflect differences of perception and approach. In order to bridge these gaps, a workshop was organized after the DRM conference to debate how field and laboratory studies of deformed rocks can improve our understanding of lithospheric rheology, and in turn, how this understanding can be used to refine dynamic models of orogenesis. The workshop hosted participants with backgrounds in structural geology, experimental rock mechanics, metamorphic petrology and both numerical and analogue modelling. This paper summarizes the main controversies and conclusions reached during the workshop. For the sake of brevity, referencing in this summary is restricted to literature referred to during the oral presentations and to comments made by speakers themselves (names italicized).     

Matching classical boundary-layer solutions toward a geostrophic drag coefficient relation

The relationship between geostrophic, Ekman-layer, and surface-layer flow is explored. Formalized limit solutions for each layer are developed; together they form a continuous solution for the semi-infinite flow over a surface. These classical solutions are re-derived and related, to prepare for a two-layer composite boundarylayer solution with patching criteria. The patching process yields a resistance law which indicates a correspondence between the stress and the geostrophic flow deviation involving a similarity relation with two arbitrary parameters.     

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

The surface reactivity of biogenic, nanoparticulate UO₂ with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO₂. Synthesis of biogenic UO₂ was accomplished by reduction of aqueous uranyl ions, by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO₂ as a function of particle size and surface-associated organic matter. The N₂-BET surface areas of the two biogenic UO₂ samples following the washing procedures are 128.63 m² g⁻¹ (NAUO2) and 92.56 m² g⁻¹ (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO₂ (both NAUO2 and BIUO2) over the pH range 3-9 is the same as that of bulk. The U L_III-edge EXAFS spectra for biogenic UO₂ (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO₂. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO₂ is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m⁻² UO₂ (bulk uraninite), 2.34 ± 0.12 μmol m⁻² UO₂ (NAUO2), and 2.57 ± 0.10 μmol m⁻² UO₂ (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO₂ indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH)₆ sorbs as bidentate, edge-sharing complexes to UO₈ polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO₂ surface. Surface-associated organic matter on the biogenic UO₂ particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO₂ surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.     

The First Hop: Use of Beaufort Sea Deltas by Hatch-Year Semipalmated Sandpipers

River deltas along Alaska’s Beaufort Sea coast are used by hatch-year semipalmated sandpipers (Calidris pusilla) after leaving their terrestrial natal sites, but the drivers of their use of these stopover sites on the first “hop” of fall migration are unknown. We quantified sandpiper temporal distribution and abundance as related to food resources at three river deltas during the beginning of their fall migration (post-breeding period) to compare the habitat quality among these deltas. We conducted population counts, sampled invertebrates, and captured birds to collect blood samples from individuals for triglyceride and stable isotope analyses to determine fattening rates and diet. Patterns of sandpiper and invertebrate abundance were complex and varied among deltas and within seasons. River deltas were used by sandpipers from late July to late August, and peak sandpiper counts ranged from 1000 to 4000 individuals, of which 98% were hatch-year semipalmated sandpipers. Isotopic signatures from blood plasma samples indicated that birds switched from a diet of upland tundra to delta invertebrate taxa as the migration season progressed, suggesting a dependence on delta invertebrates. Despite differences in diet among deltas, we found no differences in fattening rates of juvenile sandpipers as indicated by triglyceride levels. The number of sandpipers was positively associated with abundance of Amphipoda and Oligochaeta at the Jago and Okpilak-Hulahula deltas; an isotopic mixing model indicated that sandpipers consumed Amphipoda and Oligochaeta at Jago, mostly Chironomidae at Okpilak-Hulahula and Spionidae at Canning. Regardless of the difference in sandpiper diets at the Beaufort Sea deltas, their similar fattening rates throughout the season indicate that all of these stopover sites provide a critical food resource for hatch-year sandpipers beginning their first migration.     

Probability Density Function Integrals And Small-Scale Mixing In Turbulent Plumes

We analyse cross-wind-integrated statistics of theconcentration field of a conserved scalar for pointand line sources in grid turbulence. In particular,using wind-tunnel measurements we calculate thecross-wind integrated probability density function(pdf) for the scalar concentration. We then use thatquantity in the exact evolution equation for the pdfto calculate the cross-wind integrated mean of therate of dissipation of scalar variance, conditional onthe scalar concentration. Much of the variation ofthese statistics with distance downstream is accountedfor by scaling with concentration, length and timescales based on the development of the mean plume.This scaling thus suggests some simple practicalparameterisations of these statistics in terms ofmean-field quantities. One of the motivations for thiswork is to find a simple parameterisation for thescalar dissipation that can be used for modellingchemical reactions in plumes.We also consider the cross-wind integral of the firstfew moments of the concentration field and show thatthe integration greatly simplifies the budgets forthese moments. Thus the first moment is just thedownstream flux of the scalar, which is constant. Thesecond moment budget provides a check on the meandissipation estimated directly from the pdf evolutionequation.     

Lithologic and stratigraphic setting of zinc mineralization in Grenville metasediments,Quebec

For many years, high-grade metamorphism and multiphase deformation have discouraged mineral exploration in the Grenville terrain of the Canadian Shield. However, detailed studies of known zinc occurrences in the Grenville Supergroup metasediments of Quebec now reveal that mineralization is stratigraphically controlled and consistently limited to mapable lithologic units. Within the vast regions composed predominantly of calcitic marbles, zinc mineralization is found to be confined to the much more limited areas of dolomitic marble, and more precisely, to the lithologic transition from metaclastic beds to meta-dolomites. Massive sphalerite mineralization of the Balmat-type is very closely restricted to this contact zone, while disseminated sphalerite with sparse amounts of galena are commonly observed in the adjacent dolomite. With these guidelines, and with evidence that mineralization is fundamentally stratiform in nature, it has now been demonstrated that mineralized stratigraphic horizons can be located beyond the areas of previously known sphalerite showings, and consequently the potential for important discoveries of zinc mineralization in the Grenville Supergroup has become very attractive to the present-day mining industry.     

Stratiform copper deposits and interactions with co-existing atmospheres, hydrospheres, biospheres and lithospheres

Fundamental evidence from world-class stratiform copper deposits repeatedly indicates that they originated by a two-stage mineralization process: a syn-diagenetic enrichment of the host sediment in iron sulfide, followed by a sub-surface addition of copper during early diagenesis of the sediment. In contrast to the previously favored syngenetic hypothesis, which suggested an open equilibration between atmosphere, hydrosphere, biosphere and upper lithosphere during mineralization, the two-stage model implies that only the initial deposition of iron sulfides should take place with open exchanges between the upper “spheres”. The sub-surface emplacement of copper should occur in relative isolation from surface environments and, therefore, the occurrence and distribution of stratiform copper deposits through time and space should not be used to make direct interpretations of contemporaneous conditions in the upper “spheres”.