Grain growth and inclusion formation in partially molten carbonate rocks

To learn more about the kinetics and mechanisms of coarsening and melt inclusion formation, we investigated the effects of melt content, viscosity, and topology on the microstructural evolution of partially molten and melt-free calcite aggregates. Synthetic marbles with eutectic melts were produced by annealing mixtures of calcite and either calcium hydroxide or lithium carbonate for up to 80 h at a confining pressure of 300 MPa and temperatures of 973-1,023 K. The melts produced in the two systems are expected to differ significantly in viscosity. Generally, coarsening rates decrease with increasing melt fraction, probably because the diffusion length across melt pockets increases. Analysis of grain shapes in the samples with about 40% melt indicated that coarsening was accommodated by agglomeration in the samples of the calcium/lithium carbonate system. In the calcium carbonate/hydroxide system, classical Ostwald ripening occurred. For melt contents about 10% and below, melt-filled pores are either dropped from or dragged along with migrating grain boundaries, depending on the pore size and the grain boundary curvature. These data can be used to constrain the conditions where fluid or melt inclusions form under natural conditions. Combining our results and previous studies illustrates a systematic relation between the grain boundary mobility in calcite aggregates and the diffusion kinetics associated with second phases residing on the grain boundaries. In particular, boundaries with no porosity are most mobile, those boundaries dragging melt-filled pores are slower, those with gas-filled pores are slower yet, and those containing solid phases are slowest or may even be motionless.     

Use of rhodamine WT with XAD-7 resin for determining groundwater flow paths

A passive sampling system for use with rhodamine WT (RWT) in groundwater tracing experiments was developed to assist in the characterisation of groundwater flow paths. Amberlite XAD-7 resin was found to be suitable for adsorption of RWT, which can then be extracted using an ethanol/water mix and analysed fluorometrically. Batch and column experiments showed that XAD-7 resin has a high RWT capacity. The adsorption was slightly dependent on pH, but was always above 75% under batch conditions. The resin had a high percentage mass recovery at flow velocities around 1.5 m/day, but this decreased with increasing flow velocities. Desorption of RWT off the resin in water is dependent on the flow velocity of water and the time after the peak RWT has passed. The mass of RWT extracted from the resin bags correlated very well with both the RWT mass flux in the water and the peak concentrations observed in the monitoring wells in a field experiment. The results of resin bags were reproducible in the field with a mean coefficient of variation equal to 16%. This method has been successfully applied to two field situations with different flow velocities to indicate groundwater flow paths. Electronic Publication     

The origin of the acidic and basic rocks of the Tichka Massif,Morocco, based on rare earth elements

The Tichka Massif is one of many late-Paleozoic massifs emplaced in northwestern Africa. It consists of granitic pods surrounded by a complex assemblage of dioritic and gabbroic rocks. Previous workers have suggested an independent origin for the granitic and gabbroic rocks and this study evaluates this suggestion using REE data.The Ce/Yb ratio versus Ce plot for the gabbro shows a large Ce/Yb variation for small changes in Ce, whereas the granites show a linear increase of Ce/Yb with Ce. In the granites, the samples with the highest Ce/Yb ratio have the lowest SiO₂, whereas in the gabbros the Ce/Yb ratio is lowest in the low SiO₂ rocks. The diorites show a tightly clustered pattern.Based on the REE data the granite-diorite-gabbro sequence cannot be related by fractional crystallization. Partial melting of a garnet bearing mantle is consistent with the gabbroic trends. Partial melting of a crustal source with small amounts of apatite and abundant hornblende can produce the granitic rocks.     

Effect of iron and carbonation on the diffusion of iodine and rhenium in waste encasement concrete and soil fill material under hydraulically unsaturated conditions

Assessing long-term performance of Category 3 cement wasteforms and accurate prediction for radionuclide encasement requires knowledge of the radionuclide–cement interactions and mechanisms of retention (i.e. sorption or precipitation). A set of sediment-concrete half-cell diffusion experiments was conducted under unsaturated conditions (4% and 7% by weight moisture content) using carbonated and non-carbonated concrete–soil half cells. Results indicate the behavior of Re and I release was comparable within a given half-cell test. Diffusivity in soil is a function of moisture content; a 3% increase in moisture content affords a one to two order of magnitude increase in diffusivity. Release of I and Re was 1–3 orders of magnitude less from non-carbonated, relative to carbonated, concrete monoliths. Inclusion of Fe in non-carbonate monoliths resulted in the lowest concrete diffusivity values for both I and Re. This suggests that in the presence of Fe, I and Re are converted to reduced species, which are less soluble and better retained within the concrete monolith. The release of I and Re was greatest from Fe-bearing, carbonated concrete monoliths, suggesting carbonation negates the effect of Fe on the retention of I and Re within concrete monoliths. This is likely due to enhanced formation of microcracks in the presence of Fe, which provide preferential paths for contaminant migration. Although the release of I and Re were greatest from carbonated concrete monoliths containing Fe, the migration of I and Re within a given half cell is dependent on the moisture content, soil diffusivity, and diffusing species.     

An upper‐ocean general circulation model for the north pacific: Preliminary experiments

Abstract

A model with two active layers, a mixed layer and a pycnocline layer, over a semipassive deep ocean is described. The model is used to simulate a climatological seasonal cycle in the upper North Pacific. The formulation is similar to that in Cherniawsky et al. (1990). The model resolution is 1° latitude by 1.5° longitude, extending from 62°N to the equator. It is driven with monthly wind stress (Hellerman and Rosenstein, 1983) and with Newtonian heat and freshwater fluxes, which were inferred from climatological (Levitus, 1982) sea‐surface monthly temperatures and annual mean salinities. The monthly temperature anomalies (without the annual mean) are multiplied by a prescribed gain factor and advanced in time, compensating for time delay in the response of the mixed layer. No‐slip and no‐flux constraints are applied on north, east, west and land boundaries, while the following open boundary conditions are used at the equator: (a) free‐slip on zonal velocities in the two layers; (b) a prescribed meridional transport, due to local curl of the wind stress, in the mixed layer; (c) an antisymmetric meridional velocity plus a small flux‐balancing term in the second layer; and (d) across‐equator symmetry for layer depths, temperatures and salinities. Sensitivity to two aspects of parametrization is investigated: (1) the change to horizontal diffusion/viscosity coefficients that depend on the velocity deformation field (as in Smagorinsky, 1963), and (2) the use of idealized piecewise‐linear profiles for second‐layer temperatures and salinities for calculating mixed layer entrainment fluxes.     

Interaction between meander dynamics and floodplain heterogeneity in a large tropical sand‐bed river: the Rio Beni,Bolivian Amazon

The evolution of meandering river floodplains is predominantly controlled by the interplay between overbank sedimentation and channel migration. The resulting spatial heterogeneity in floodplain deposits leads to variability in bank erodibility, which in turn influences channel migration and planform development. Despite the potential significance of these feedbacks, few studies have quantified their impact upon channel evolution and floodplain construction in dynamic settings (e.g. locations characterized by rapid channel migration and high rates of overbank sedimentation). This study employs a combination of field observations, geographic information system (GIS) analysis of satellite imagery and numerical modelling to investigate these issues along a 375 km reach of the Rio Beni in the Bolivian Amazon. Results demonstrate that the occurrence of clay‐rich floodplain deposits promotes a significant reduction in channel migration rates and distinctive styles of channel evolution, including channel straightening and immobilization of bend apices leading to channel narrowing. Clay bodies act as stable locations limiting the propagation of planform disturbances in both upstream and downstream directions, and operate as ‘hinge’ points, around which the channel migrates. Spatial variations in the erodibility of clay‐rich floodplain material also promote large‐scale (10–50 km) differences in channel sinuosity and migration, although these variables are also likely to be influenced by channel gradient and tectonic effects that are difficult to quantify. Numerical model results suggest that spatial heterogeneity in bank erodibility, driven by variable bank composition, may force a substantial (c. 30%) reduction in average channel sinuosity, compared to situations in which bank strength is spatially homogeneous. Copyright © 2015 John Wiley & Sons, Ltd.     

Earth Fissures in Su–Xi–Chang Region,Jiangsu, China

The earth fissures in the Su-Xi-Chang area are caused by differential land subsidence due to long-term excessive groundwater withdrawal and controlled by the bedrock ridge or cliff underlying. There have been more than 15 earth fissures in the area since 1989. The field investigations have lasted for more than 20 years. The earth fissures generally have a main fissure and a number of secondary ones parallel to the main one. The main fissure (crack) has a scarp, is steeply dipping, and can be more than 2000 m long. Geophysical surveys (2D or 3D seismic investigation, controlled source audio frequency magnetotelluric sounding, and electric sounding) combined with geological drilling are effective for the investigation of earth fissures. Geodetic leveling is effective to monitor the ground deformation across the earth fissure, so is the extensometer for the opening of the fissure. The activities of earth fissures are directly related to different stages of land subsidence and controlled by geological abnormalities. Most earth fissures in the area are still active.     

Modelling soil erosion with a downscaled landscape evolution model

The measurement and prediction of soil erosion is important for understanding both natural and disturbed landscape systems. In particular numerical models of soil erosion are important tools for managing landscapes as well as understanding how they have evolved over time. Over the last 40 years a variety of methods have been used to determine rates of soil loss from a landscape and these can be loosely categorized into empirical and physically based models. Alternatively, physically based landscape evolution models (LEMs) have been developed that provide information on soil erosion rates at much longer decadal or centennial scales, over large spatial scales and examine how they may respond to environmental and climatic changes. Both soil erosion LEMs are interested in similar outcomes (landscape development and sediment delivery) yet have quite different methodologies and parameterizations. This paper applies a LEM (the CAESAR model) for the first time at time and space scales where soil erosion models have largely been used. It tests the ability of the LEM to predict soil erosion on a 30 m experimental plot on a trial rehabilitated landform in the Northern Territory, Australia. It then continues to discuss the synergies and differences between soil erosion and LEMs. The results demonstrate that once calibrated for the site hydrology, predicted suspended sediment and bedload yields from CAESAR show a close correspondence in both volume and timing of field measured data. The model also predicts, at decadal scales, sediment loads close to that of field measured data. Findings indicate that the small‐scale drainage network that forms within these erosion plots is an important control on the timing and magnitude of sediment delivery. Therefore, it is important to use models that can alter the DEM to reflect changing topography and drainage network as well as having a greater emphasis on channel processes. Copyright © 2012 John Wiley & Sons, Ltd. and Commonwealth of Australia