Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid‐ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite‐pumpellyite facies or the lower‐greenschist facies at high‐X_CO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca‐Al silicates including Ca‐zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high‐X_CO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca‐amphibole + epidote/clinozoisite + chlorite + Ca‐Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at X_CO2 of 0.012–0.140, such values are higher than present‐day vent fluids collected near mid‐ocean ridges with low‐X_CO2 values, up to 0.005. The Archaean seawater depth at the mid‐ocean ridge was estimated to be 1600 m at X_CO2 = 0.06 using a depth‐to‐boiling point curve for a fluid. The carbonation due to high‐X_CO2 hydrothermal fluids occurred near the ridge‐axis before or was coincident with ridge metamorphism.     

Analysis of tsunami impact scenarios at an oil refinery

This work analyses the potential consequences of two tsunami scenarios and their impacts on an oil refinery located in Sicily. Two credible tsunamis originating in the Tyrrhenian Sea were selected based on historical data. The potential for damage and hazardous materials releases resulting from the tsunami impacts to a refinery was assessed. The results obtained by the JRC tsunami propagation and inundation code HyFlux2 indicate that in both scenarios there would be eighteen storage tanks (of 43 located within 400 m from the shoreline) at the refinery subject to flooding. Water flow velocities were found to be generally low, <1 m/s, except for a central section of the refinery near the shoreline where the water flow velocities reach 3?C4 m/s. These results indicate that any damage would most likely occur due to buoyancy loads particularly in the western part of the facility where inundation levels are higher and storage tanks are less protected. Potential damage caused by impact of floating debris may be a problem in the central area of the refinery near the shoreline due to high flow velocities (3?C4 m/s) in both tsunami scenarios. Small hazardous materials releases could occur due to breakage of connected pipes and flanges caused by floating off of almost empty storage tanks or other equipment. Salt water intrusion could affect electrical equipment, such as control panels, pumps, and motors that are not raised above the inundation level. We conclude that in the two tsunami scenarios analysed, the risk to nearby residents and neighbouring facilities from potential hazardous materials releases, fires or explosions triggered by the tsunamis is likely to be small. Nonetheless, recommendations are made on prevention measures to reduce the risk of tsunami-triggered accidents and to mitigate their consequences if they do occur. The results of this study are limited by the uncertainty in the input data and most importantly by the accuracy of the elevation data and the model resolution.     

Evaluation of an air bubbler to mitigate fish impingement at an electric generating plant

To determine whether fish impingement at an electric generating plant was significantly reduced during the operation of an air bubbler, impingement monitoring data was subjected to statistical analysis. Daily impingement rates were calculated for total fish collections and for three impinged species, and associated with water chemistry conditions and air curtain operation during impingement periods. Analysis for each species by season indicated that the air curtain was not an effective fish deterrent.     

Investigation of desaturation in an old tunnel and new galleries at an argillaceous site

A numerical investigation of the desaturation process at the argillaceous Tournemire site has been carried out. This desaturation is initialized by the contact of the saturated rock with the ambient air in excavated openings. The used hydraulical model is based on the Richards’ approximation for unsaturated one phase flow coupled with the deformation of a porous medium with anisotropic linearly elastic behavior. In relation to the extent of the desaturated zone around an excavated opening, the intrinsic permeability and the relative permeability have been identified to be the most important model parameters. The mechanical deformation process itself, the seasonal influences and the tunnel lining are less important for the formation of the desaturated zone. The comparison with measured saturation values bares some difficulties but indicates the principle capacity of the applied finite element codes to simulate the desaturation process. The consideration of seasonal changes in humidity in the ambient air results in a constantly recurring desaturation–resaturation cycle in the near field of the openings. This seasonally influenced zone amount 1–2 m and is nearly independent from time and from a variation of model parameters within a reasonable range. The possibility of material weakening in this zone is of special interest, since claystone is a potential host rock for the disposal of radioactive waste.     

Heterogeneous modelling behaviour at an interface in porous media

The fluid flow induced by an incident wave at a discontinuity separating two porous media is governed by the hydraulic permeabilities of both media and that of the interface. In the context of Biot’s theory, we derive the time-harmonic Green’s function for the two half-space problem allowing incident fast and slow dilatational waves to assess the heterogeneous modelling behaviour for diverse hydraulic conditions. It is found that when at least one of the media is permeated with inviscid fluids, heterogeneous modelling simulates open boundary conditions. On the other hand, when the model is saturated with viscous fluids, the modelling reproduces restrained fluid flow whose values correspond to sealed pore interface conditions, in agreement with the theoretical results. Therefore the numerical technique models correctly the wave diffusion and propagation phenomena attendant at the boundary.     

Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (upto 0·56 wt%), and TiO₂ (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO₂ contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H₂O contentof the produced melt decreases with increasing temperature.The K₂O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa₂O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K₂O/H₂O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K₂O/H₂O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE     

Migmatite formation at subsolidus conditions–an alternative to anatexis

Abstract This contribution discusses the formation of stromatic high-grade migmatites. Volume considerations require that most of the leucosome material is not added from outside the system. A segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure. Although partial melting is most often advocated to provide the segregation mechanism, several arguments can be raised against high degrees of melting: mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes; sufficient degrees of melting at reasonable temperatures require more than the available amounts of water; the leucosomes do not always approximate to a minimum melt composition; high degrees of melting cannot occur without an appreciable volume increase; etc. Diffusion works as a segregation mechanism at low temperatures. As diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures. A model is suggested based on the diffusion of components in response to the gradient δσ/δx, where σ= 1/3∑³_i=1 σ_i is the mean pressure. In homogeneously strained rocks, σ₃ is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases. Accordingly, mechanically competent but chemically incompetent high-volume phases like quartz and feldspars stressed in micadominated parts of a rock (high σσ) migrate to parts of the rock that are depleted in mica (low σ¯). It is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn-migmatitic and due to its mechanical competency it initiates the segregation. Diffusion occurs along grain boundaries and is enhanced by small amounts of ‘intergranular fluid’;. At the best, semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times.     

Modeling of an MTBE plume at Pascoag, Rhode Island

A numerical groundwater flow and mass transport model was developed to predict the extent of impact from methyl tertiary butyl ether (MTBE) release on a down-gradient drinking water well field. An MTBE incident in Pascoag, Rhode Island, was used as a case study and the plume’s past and future development was simulated using scenario analysis. The numerical code used was GeoSys/Rockflow, which permits a coupled flow and transport simulation as one object, thus alleviating the need for simulating the MTBE fate with separate flow and transport codes. The numerical model was built on available hydrogeological and chemical data as well as on GIS information of the site. By comparing the simulated results with observed field data, it was found that the model could provide reliable results even when the simulated aquifer was simplified to a two-dimensional flow and transport domain. Finally, the calibrated model was used for exploring a location that may be suitable for a new well field. Despite the model limitations associated with uncertainties of data and simplifying assumptions, numerical modeling of this MTBE contaminated site proved a useful tool and provided guidance for future municipal well field operation strategies and aquifer remediation alternatives.     

Glacitectonic sedimentary and hydraulic processes at an oscillating ice margin

An assemblage of subglacial, ice-terminal and proglacial landforms and sediments provides evidence for the relationship between ice-marginal glacitectonics, sedimentary processes and subglacial and proglacial hydraulic processes at a retreating late Devensian ice margin in north-central Ireland. Deltas were deposited in glacial lakes impounded between the retreating ice margin and the southern Sperrin Mountains, followed by outwash and end moraine formation as the ice margin retreated south. Sediments within the moraines show evidence for ice margin oscillation from two opposing ice margins, including subglacial bedrock rafts and breccias which are separated by glacitectonic shears with silty partings. In adjacent outwash, vertically-disturbed proglacial sands, gravels and silts located in front of moraine positions attest to high hydraulic pressure and subsurface water flow during ice oscillation. The relationship between sedimentary and hydraulic processes in the ice margin region is described by a depositional model which links glacitectonic thrusting and subsurface water flow during ice oscillation to formation of subglacial, ice-terminal and proglacial sediments. The evidence presented in this paper shows that subglacial and proglacial morphosedimentary processes and patterns of sediment deposition are mediated by the presence of proglacial permafrost, which helps direct processes and patterns of groundwater flow.     

Groundwater flow and contaminant transport modelling at an air weapons range

Numerical modelling was done at the Cold Lake Air Weapons Range, Canada, to test whether the dissolved RDX and nitrate detected in groundwater come from the same sources, and to predict whether contamination poses a threat to the surface water receptors near the site. Military live fire training activities may indeed pose a risk of contamination to groundwater resources, however field investigations on military bases are quite recent, and little information is available on the long-term behaviour of munition residues related contaminants. Very limited information was available about the contaminant source zones, which were assigned based on our knowledge of current training activities. The RDX plume was well represented with the model, but the heterogeneous distribution of nitrate concentrations was more difficult to reproduce. It was nonetheless determined that both contaminants originate from the same areas. According to the model, both contaminants should reach the nearby river, but concentrations in the river should remain very low if the source zone concentration does not change. Finally, the model allowed the recommendation of a new location for the main bombing target, which would offer added protection to the river and the lake into which it flows.     

Rock mass mechanical characteristics in an opencut at bor copper mine

SummaryRock Mass Mechanical Characteristics in an Opencut at Bor Copper Mine Preliminary testing for the design of the new opencut, which will go to a depth of about 300 m, included testing rock mechanical characteristics. The character of the preliminary testing demanded by an engineering undertaking of this scale and the fundamental structural and physical properties of rock masses (fissuring, anisotropy, heterogeneity, natural stresses) are discussed and it is emphatically concluded that priority should go to in situ testing. Results are presented of determinations of the velocity of propagation of longitudinal elastic waves, elasticity modulus, coefficient of damage and shear strength parameters.With 7 Figures     

多年冻土区路基分层变形现场观测研究

在人类工程活动和气候变暖的双重作用下,多年冻土区的道路路基普遍产生以路基下沉为主的病害．多年冻土地区的路基沉降涉及多个物理力学过程：未冻土的蠕变,冻土上限下降所产生的融沉,由于多年冻土层升温而产生的高温冻土的蠕变,以及活动层中冻融循环改变了土的工程性质而导致的附加沉降变形．为了进一步探索各个物理力学过程对总沉降量贡献,...     

Rapidly spreading mangroves at Port Gawler,South Australia: an update

The previously reported rapid spread of mangroves at Port Gawler on the northeastern shore of Gulf St Vincent, South Australia has continued. This is a tidal-dominated coastline where reduced incoming tidal currents have led to increased sedimentation on the tidal flat and the development of an area of samphire and mangroves that have split the tidal flat in two. We predict that the remaining sections of the tidal flat will eventually become a mature mangrove forest, although human activity may delay this process.     

Flash flood at Slheimajkull heralds the reawakening of an Icelandic subglacial volcano

An unpredicted sudden outburst flood from Sólheimajökull, Southern Iceland, in July 1999 may herald a major subglacial volcanic eruption beneath Mýrdalsjökull ice cap.     

Reducing local scour at bridge piers using an upstream subsidiary triangular pillar

The objective of this experimental study was to examine the effect of an upstream subsidiary triangular pillar on the development of local scour at a single circular bridge pier. The triangular pillar was placed such that its apex faced the approach flow and its base length was equal to the pier diameter (D) in all experiments. The experiments were conducted to investigate the effect of the spacing between the base of the subsidiary pillar and the bridge pier (S) and the apex angle (α) of the triangular pillar on the dimensions of the scour hole and its temporal development at different Froude numbers. The experiments were conducted in a rectangular flume under clear-water scour conditions at Froude number ranging between 0.1 and 0.3. Relative spacing (S/D) of 0, 0.5, 1, 1.5, 2, and 3, as well as apex angle (α) of 60°, 90° and 120° were tested. The results show that the maximum scour depth at the pier could be minimized using three different combinations of spacing and apex angle. The highest reduction in the maximum scour depth (d _sm) of approximately 28 % was possible using S/D of 3 and α of 90° compared to the pier-alone case. Overall the performance of the 90° apex angle pillar surpassed the other two angles.     

Geochemical assessment of an analogue site for an engineered landform at Ranger Uranium Mine, Northern Territory, Australia

 An essential element in identifying sites as analogues for the long-term development of soils and vegetation on engineered landforms of the rehabilitated Ranger Uranium Mine, Northern Territory, Australia, is the need to match rocks. Comparison of the geochemistry of rocks from the waste rock dump of Ranger Uranium Mine and the potential analogue site of Tin Camp Creek area, Western Arnhem Land, indicates that there are several sites along Tin Camp Creek that may be used as analogue sites. Detailed comparisons between the Ranger and Tin Camp Creek lithologies have been undertaken using a variety of techniques, including cluster analysis. It is demonstrated that at least 70% of the rocks being mined at Ranger have analogues in the Tin Camp Creek area. Received: 2 October 1996 · Accepted: 4 November 1997     

Geoecology of ecosystem recovery at an inactive coal mine site,New Zealand

Ecological restoration of the Wangaloa coal mine in southern New Zealand is hindered by a range of geoecological factors. The site has some substrate acidification (down to pH 1) and acid rock drainage with discharge waters initially down to pH 4, although this has since risen to ca. pH 6. Surface and ground waters develop elevated sulfate (up to 700 mg/kg) during oxidation of pyrite in coal and quartz in waste rock. Coal has elevated boron content (up to 450 mg/kg) and surface waters on coal-rich waste rock have up to 6 mg/L dissolved boron. Evaporation causes formation of salt encrustations dominated by gypsum with minor boron salts. Boron is bioavailable and may be at toxic levels (>200 mg/kg) in some plants. Quartz-rich waste rock is readily eroded, and develops a cm-scale low-nutrient quartz pebble armouring layer with low water retention properties. All waste rocks including loess siltstone have low nutrient contents, and low moisture retention properties, that are barely sufficient for plant establishment. Native plants introduced to the site during rehabilitation have grown on loess substrate (up to fivefold increase in height over 3 years), with poor or no growth on coal-rich and quartz-rich substrates. In contrast, natural colonisation of manuka (Leptospermum scoparium) has been most effective at revegetation on even the most hostile substrates. This natural revegetation has been facilitated by islands of manuka established accidentally during 60 years of mining history. Manuka from local genetic stock is most viable for this revegetation, and introduced manuka seedlings have had a 70% mortality rate. Natural plant colonisation is the key step in overall ecosystem recovery, and invertebrates rapidly colonise beneath new shrubs irrespective of the nature of the substrate from vegetation islands that have high invertebrate numbers and species richness.