Formation, Crystallization, and Migration of Melt in the Mid-orogenic Crust: Muskoka Domain Migmatites, Grenville Province, Ontario

Migmatitic orthogneisses in the Muskoka domain, southwesternGrenville Province, Ontario, formed during the Ottawan stage(c. 1080–1050 Ma) of the Grenvillian orogeny. Stromaticmigmatites are volumetrically dominant, comprising granodioriticgneisses with 2–5 cm thick granitic leucosomes, locallyrimmed by thin melanosomes, that constitute 20–30 vol.%, and locally 40–50 vol. %, of the outcrops. Patch migmatitesin dioritic gneisses form large (>10 m) pinch-and-swell structureswithin the stromatic migmatites, and consist of decimetre-scale,irregular patches of granitic leucosome, surrounded by medium-grainedhornblende–plagioclase melanosomes interpreted as restite.The patches connect to larger networks of zoned pegmatite dykes.Petrographic and geochemical evidence suggests that the patchleucosomes formed by 20–40% fluid-present, equilibriummelting of the dioritic gneiss, followed by feldspar-dominatedcrystallization. The dyke networks may have resulted from hydraulicfracturing, probably when the melts reached water saturationduring crystallization. Field and geochemical data from thestromatic migmatites suggest a similar petrogenesis to the patchmigmatites, but with significant additions of externally derivedmelts, indicating that they acted as conduits for melts derivedfrom deeper structural levels within the orogen. We hypothesizethat the Muskoka domain represents a transfer zone for meltsmigrating to higher structural levels during Grenvillian deformation. KEY WORDS: migmatite geochemistry; partial melting; melt crystallization; melt transport; Grenville orogen     

Whither Critical Inquiry?*

In his article, “Whither Geography?,” Jay R. Harman raised a number of important issues regarding the nature and direction of geographic scholarship. In this response I critically address some of those issues and offer an alternative perspective. In doing so, I support Harman's desire for the discipline to “help answer deep human questions” but argue that rather than reinforce the vitality and legitimacy of the discipline, a market‐based approach of the sort he advocates could instead potentially undermine such objectives and compromise disciplinary integrity by discouraging critical inquiry. While acknowledging the influence of “the market,” I defend a place for critical inquiry within the discipline.     

Analytical solutions for transport of decaying solutes in rivers with transient storage

Analytical solutions are presented for solute transport in rivers including the effects of transient storage and first order decay. The solute transport model considers an advection–dispersion equation for transport in the main channel linked to a first order mass exchange between the main channel and the transient storage zones. In case of a conservative tracer, it is shown that different analytical solutions presented in the literature are mathematically identical. For non-conservative solutes, first order decay reactions are considered with different reaction rate coefficients in the main river channel and in the dead zones. New analytical solutions are presented for different boundary conditions, i.e. instantaneous injection in an infinite river reach, and variable concentration time series input in a semi-infinite river reach. The correctness and accuracy of the analytical solutions is verified by comparison with the OTIS numerical model. The results of analytical and numerical approaches compare favourably and small differences can be attributed to the influence of boundary conditions. It is concluded that the presented analytical solutions for solute transport in rivers with transient storage and solute decay are accurate and correct, and can be usefully applied for analyses of tracer experiments and transport characteristics in rivers with mass exchange in dead zones.     

Sequestration of organic contaminants in soil/sediment and its impact on contaminant fate

Adsorption and desorption are critical processes controlling the fate and transport of organic contaminants in natural environment. Numerous studies have shown that only a fraction of contaminants in soil or sediment can readily be desorbed to the aqueous phase, while desorption of the remaining fraction is very difficult and does not follow the conventional desorption models-a phenomenon typically referred to as ＂resistant desorption＂ or ＂sequestration＂ and is argued by many as the cause of reduced availability of contaminants in soil and sediment. While this reduced availability often reduces the effectiveness of soil and sediment remediation, the same effect can also greatly reduce risk of contaminated soil and sediment. The authors have conducted extensive lab and field studies to characterize the unique properties of sequestration of organic contaminants and to understand the mechanism（s） controlling sequestration. Thus far, over 50 different contaminant-soil/sediment combinations have been evaluated, using contaminants of varied physical-chemical properties （e.g., chlorinated benzenes, PAHs, PCBs, DDT, atrazine, hexachlorocyclohexane isomers, etc.）. Several unique characteristics of sequestration have been observed, and the most striking one is that despite the significant differences in chemical and soil properties, desorption of sequestered contaminants for all the contaminant-soil/sediment combinations exhibited very similar KOC （organic-carbon normalized partition coefficient） and very similar desorption kinetics.     

Gaseous transport and deposition of gold in magmatic fluid: evidence from the active Kudryavy volcano, Kurile Islands

The distribution of gold in high-temperature fumarole gases of the Kudryavy volcano (Kurile Islands) was measured for gas, gas condensate, natural fumarolic sublimates, and precipitates in silica tubes from vents with outlet temperatures ranging from 380 to 870°C. Gold abundance in condensates ranges from 0.3 to 2.4 ppb, which is significantly lower than the abundances of transition metals. Gold contents in zoned precipitates from silica tubes increase gradually with a decrease in temperature to a maximum of 8 ppm in the oxychloride zone at a temperature of approximately 300°C. Total Au content in moderate-temperature sulfide and oxychloride zones is mainly a result of Au inclusions in the abundant Fe–Cu and Zn sulfide minerals as determined by instrumental neutron activation analysis. Most Au occurs as a Cu–Au–Ag triple alloy. Single grains of native gold and binary Au–Ag alloys were also identified among sublimates, but aggregates and crystals of Cu–Au–Ag alloy were found in all fumarolic fields, both in silica tube precipitates and in natural fumarolic crusts. Although the Au triple alloy is homogeneous on the scale of microns and has a composition close to (Cu,Ni,Zn)₃(Au,Ag)₂, transmission electron microscopy (TEM) shows that these alloy solid solutions consist of monocrystal domains of Au–Ag, Au–Cu, and possibly Cu₂O. Gold occurs in oxide assemblages due to the decomposition of its halogenide complexes under high-temperature conditions (650–870°C). In lower temperature zones (<650°C), Au behavior is related to sulfur compounds whose evolution is strongly controlled by redox state. Other minerals that formed from gas transport and precipitation at Kudryavy volcano include garnet, aegirine, diopside, magnetite, anhydrite, molybdenite, multivalent molybdenum oxides (molybdite, tugarinovite, and ilsemannite), powellite, scheelite, wolframite, Na–K chlorides, pyrrhotite, wurtzite, greenockite, pyrite, galena, cubanite, rare native metals (including Fe, Cr, Mo, Sn, Ag, and Al), Cu–Zn–Fe–In sulfides, In-bearing Pb–Bi sulfosalts, cannizzarite, rheniite, cadmoindite, and kudriavite. Although most of these minerals are fine-grained, they are strongly idiomorphic with textures such as gas channels and lamellar, banded, skeletal, and dendrite-like crystals, characteristic of precipitation from a gas phase. The identified textures and mineral assemblages at Kudryavy volcano can be used to interpret geochemical origins of both ancient and modern ore deposits, particularly gold-rich porphyry and related epithermal systems.     

Hydrodynamic response of subduction zones to seismic activity: A case study for the Costa Rica margin

Thermal anomalies in tectonically active areas are often attributed to sub-seafloor fluid circulation and faulting mechanisms, particularly in subduction zones where the largest thrust earthquakes occur. Postseismic fluid flow is enabled by the poroelastic response of the fault system to the earthquake's strain field, as well as by the rupturing of permeability barriers in the vicinity of the fault zone. We investigated the relative importance of these mechanisms on postseismic pore-pressure diffusion and advective heat transport in the subduction zone setting. A two-dimensional numerical fluid flow and heat transport model was developed for the Costa Rica subduction zone offshore of the Nicoya Peninsula. The flow and transport model was coupled with an earthquake strain model to quantify the effects of coseismic strain and permeability enhancement on fluid pressures and temperatures within the Costa Rica margin. Coseismic changes in pore pressure and postseismic pore-pressure diffusion were found to be sensitive to the compressibility of the porous medium, and patterns of pore-pressure recovery were more complex than that predicted by theoretical faulting models. Coseismic contraction and extension of the crust produced high fluid pressures close to the fault, while the inflow of fluid from depth increased fluid pressures several years following the simulated fault slip. Crustal deformation alone was not observed to perturb the temperature field. Laterally extensive permeability increases of two orders of magnitude along the décollement were required to produce small changes in heat flow. Local permeability changes in the upper slope region of least five orders of magnitude were necessary to noticeably affect heat flow. The results of the numerical simulations may help to refine conceptual faulting models and provide guidance for locating long-term hydrologic monitoring sites at Costa Rica and other subduction zones.     

Observed thermal pollution and post-development simulations of low-temperature geothermal systems in Winnipeg, Canada

Two issues arise in the long-term use of groundwater for thermal purposes: (1) the sustainability of an individual system; and (2) the effect of neighbouring systems on each other. Both of these effects are observed in an area of the Carbonate Rock Aquifer beneath Winnipeg in Manitoba, Canada, where groundwater has been exploited in thermal applications since 1965. In this area, there are four systems that utilize groundwater for cooling purposes that are closely spaced. The current temperatures observed in this area of the Carbonate Rock Aquifer and the results of the numerical modeling conducted in this study confirm that in each system, temperatures at the production well have risen as a result of breakthrough of injected water. The results of numerical modeling also indicate that interference effects are present in three of the four systems examined in this study. The influence of these systems on each other implies that these systems have a spacing that is smaller than the optimum spacing for such systems, and indicates that there is a limit to the density of development that can occur in a given aquifer.Contaminación termal y simulaciones post-desarrollo de sistemas geotermales de baja temperatura en Winnipeg, Canadá     

Modelling Transport of Linearly Sorbing Solutes in a Single Fracture: Asymptotic Behavior of Solute Velocity and Dispersivity

The asymptotic behavior of the solute velocity and dispersivity for a system of parallel fractures with matrix diffusion is made using numerical modeling and theoretical analyses. The study is limited to linearly sorbing solutes with a constant continuous source boundary condition. Expressions are provided for solute velocity and effective dispersivity in terms of fracture porosity during asymptotic stage using spatial moment analyses. The importance of matrix porosity and fracture porosity on solute velocity as well as the relationship governing effective dispersivity and fracture porosity is discussed for both non-reactive and linearly sorbing solutes. By using a dimensionless effective dispersivity parameter it is shown that the relationship between the fracture porosity and dimensionless effective dispersivity is linear for non-reactive solutes. It is also shown that this holds true for the linearly sorbing solutes with the same proportionality constant.     

Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters are usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study.