The mineralogy and the isotopic composition of sulfur in hydrothermal sulfide/sulfate deposits on the East Pacific Rise, 21°N latitude

The mineralogy of five groups of hydrothermal chimneys from the East Pacific Rise has been examined. Three of the chimneys, where the exit temperature of the hydrothermal fluids was close to 350°C, are rich in copper sulfides. Exit temperatures from the other two chimneys were less than 300°C; in these, the chimney walls are rich in zinc sulfide. The major sulfides in the chimneys as a whole were found to be wurtzite, chalcopyrite, pyrite, and cubanite. Anhydrite is always the dominant sulfate, and is present in all the deposits. Silicates are also present but in relatively minor amounts. There are considerable differences in the mineralogy of sulfides, sulfates, and silicates between the active and inactive vent deposits.The isotopic composition of sulfur in anhydrites from active vents is close to that of seawater; the δ³⁴S values of the sulfides range from +1.3 to +4.1‰. The isotopic composition of sulfur in the anhydrites is consistent with a derivation predominantly from seawater sulfate. The sulfur in the sulfides must have a complex origin including contributions from both sulfur in basalts and sulfide produced by reduction of sulfate in seawater. Mixing of H₂S-dominated hydrothermal fluids with cold seawater near the seafloor resulted in the precipitation of non-equilibrium assemblages of sulfides and sulfates.     

S, O and Sr isotope systematics of active vent materials from the Mariana backarc basin spreading axis at 18°N

Stable isotope ratios of S, O and Sr have been measured for active vent materials which were first found and sampled in April 1987 from the Mariana backarc spreading axis at 18°N. Chimneys consisted mostly of barite with a lesser proportion of sulfide minerals such as sphalerite, galena, chalcopyrite and pyrite. Theδ³⁴S values of sphalerite and galena taken from several chimneys and various parts of a chimney showed a narrow range from 2.1 to 3.1‰, suggesting uniform conditions of fluid chemistry during chimney growth. The sulfur isotopic results imply a contribution of hydrogen sulfide reduced from seawater sulfate in the deep hydrothermal reaction zone, considering that fresh glasses of the Mariana Trough basalts haveδ³⁴S= −0.6 ± 0.3‰. Sulfur isotopic compositions of hydrogen sulfide in the high temperature vent fluids (δ³⁴S= 3.6–4.8‰) which are higher than those of the sulfide minerals suggest the secondary addition of hydrogen sulfide partially reduced from entrained seawater SO₄²⁻ at a basal part of the chimneys. This interpretation is consistent with theδ³⁴S values of barite (21–22‰) that are higher than those of seawater sulfate. The residence time of the entrained SO₄²⁻ was an order of an hour on a basis of oxygen isotopic disequilibrium of barite. Strontium isotopic variations of barite and vent waters indicated that Sr in barite was mostly derived from the Mariana Trough basalts with a slight contribution from Sr in circulating sea-water, and that 10–20% mixing of seawater with ascending hydrothermal fluids induced precipitation of barite at the sea-floor.     

Soil-structure interaction effects on the seismic response of tall chimneys

The effects of soil-structure interaction on the seismic response of tall (>100m) steel and reinforced concrete chimneys are described. Detailed models of a 130m high steel chimney and a 150m high reinforced concrete chimney are used as structural models. The foundations are represented as rigid blocks resting on a uniform viscoelastic soil model. Perfect bonding between the foundation and the soil is assumed. Parametric studies of the interaction effects on the magnitude and distribution of bending moments and shear forces include four soil rigidities and two seismic excitations characterized by very different frequency contents. The results obtained indicate strong interaction effects for intermediate and soft soils (V_s500 m/sec). The extent of the interaction effects are highly dependent on the characteristics of the seismic excitation.     

Temperature, density and buoyancy fluxes in “black smoker” plumes, and the criterion for buoyancy reversal

Measurements of the temperature and composition of effluent from vents on the sea floor can be used to deduce the in-situ density of this fluid, which is required for calculations of flow in the chimneys and through their porous walls. This density is, however, not directly relevant when calculating the buoyancy flux in the plume above a smoker. It is the asymptotic buoyancy flux, following extensive dilution with seawater, which is required when estimating the height of rise of plumes in a stably stratified ocean, and when calculating the criterion for reversal of buoyancy due to non-linear mixing effects. The results of mixing calculations show that the effluent from hydrothermal vents on the sea floor will exhibit reversing buoyancy if the ejected fluid has a temperature of 300°C and a salinity greater than 8 wt.% NaCl. If the temperature of the effluent is 200°C the salinity required for reversing buoyancy falls to 5.5 wt.% NaCl. Measurements of temperature and salinities of sea-floor hydrothermal fluid suggest that fluids with the characteristics required to form reversing plumes are ejected at the sea floor. The possibility that reversing plumes may be found has important implications for the formation of massive sulfide deposits.     

The dynamical and thermal structure of deep mantle plumes

If the interpretation of the D″ layer at the base of the mantle as a thermal boundary layer, with a temperature increment in the order of 800 K, is correct, then the formation of deep-mantle plumes to vent material from it appears inevitable. We demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long-lived chimneys that are ～ 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material. The speed of flow up the axis of the plume is correspondingly fast; 1.6 m y^?1 at the base and 4.8 m y^?1 at 670 km depth. Thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there. This convergence, which contributes only a small increment to the flux of material up the plume, also serves to throttle the flow in the chimney. The global plume mass flux necessary to transport 1.6 × 10¹² W of core heat upward through the mantle is 1.8 × 10⁶ kg s^?1. At its base, plume material is probably still significantly below its solidus or eutectic temperature, but substantial partial melting is very likely as it rises. We speculate that a small fraction of this fluid component eventually emerges at the surface in “hot spots”, with the fate of the remainder being unknown. The behaviour and properties of D″ and of plumes are closely coupled. Not only are plumes a necessary consequence of a thermal boundary layer, but their existence is impossible without that layer.     

论烟囱的地震力

在文献[1]中我们根据砖烟囱的震害证明了水平地震力起决定性作用的传统观点是错误的,实际上是竖向地震力在起主要作用。同时还阐述了砖烟囱出现多条裂缝的破坏机理。本文在文献[1]的基础上对烟囱出现斜缝、错位及破坏截面接近顶端等现象,计算其最大水平地震力及最小竖向地震力以证明上述观点.对出现的多条裂缝,也用计算数字来说明其破坏机理.      

Statistical characteristics of flow as indicators of channeling in heterogeneous porous and fractured media

We introduce two new channeling indicators D_ic and D_cc based on the Lagrangian distribution of flow rates. On the basis of the participation ratio, these indicators characterize the extremes of both the flow-tube width distribution and the flow rate variation along flow lines. The participation ratio is an indicator biased toward the larger values of a distribution and is equal to the normalized ratio of the square of the first-order moment to the second-order moment. Compared with other existing indicators, they advantageously provide additional information on the flow channel geometry, are consistently applicable to both porous and fractured media, and are generally less variable for media generated using the same parameters than other indicators. Based on their computation for a broad range of porous and fracture permeability fields, we show that they consistently characterize two different geometric properties of channels. D_ic gives a characteristic scale of low-flow zones in porous media and a characteristic distance between effectively flowing structures in fractured cases. D_cc gives a characteristic scale of the extension of high-flow zones in porous media and a characteristic channel length in fractured media. D_ic is mostly determined by channel density and permeability variability. D_cc is, however, more affected by the nature of the correlation structure like the presence of permeability channels or fractures in porous media and the length distribution in fracture networks.     

Dimensional analysis of two-phase flow including a rate-dependent capillary pressure–saturation relationship

The macroscopic modelling of two-phase flow processes in subsurface hydrosystems or industrial applications on the Darcy scale usually requires a constitutive relationship between capillary pressure and saturation, the P_c(S_w) relationship. Traditionally, it is assumed that a unique relation between P_c and S_w exists independently of the flow conditions as long as hysteretic effects can be neglected. Recently, this assumption has been questioned and alternative formulations have been suggested. For example, the extended P_c(S_w) relationship by Hassanizadeh and Gray [Hassanizadeh SM, Gray WG. Mechanics and thermodynamics of multiphase flow in porous media including interphase boundaries. Adv Water Resources 1990;13(4):169–86] proposes that the difference between the phase pressures to the equilibrium capillary pressure is a linear function of the rate of change of saturation, thereby introducing a constant of proportionality, the coefficient τ. It is desirable to identify cases where the extended relationship needs to be considered. Consequently, a dimensional analysis is performed on the basis of the two-phase balance equations. In addition to the well-known capillary and gravitational number, the dimensional analysis yields a new dimensionless number. The dynamic number Dy quantifies the ratio of dynamic capillary to viscous forces. Relating the dynamic to the capillary as well as the gravitational number gives the new numbers DyC and DyG, respectively. For given sets of fluid and porous medium parameters, the dimensionless numbers Dy and DyC are interpreted as functions of the characteristic length and flow velocity. The simulation of an imbibition process provides insight into the interpretation of the characteristic length scale. The most promising choice for this length scale seems to be the front width. We conclude that consideration of the extended P_c(S_w) relationship may be important for porous media with high permeability, small entry pressure and high coefficient τ when systems with a small characteristic length (e.g. steep front) and small characteristic time scale are under investigation.     

Growth history of hydrothermal chimneys at EPR 9-10°N: A structural and mineralogical study

Based on structural and mineralogical characteristics of four hydrothermal chimney samples collected by submersible Alvin, growth history and formation environment of hydrothermal chimney at EPR 9-10°N are established. It is shown that there occur two types of hydrothermal chimney with different deposition environments at EPR 9-10°N according to differences in their shape, structure and mineral assemblage: type I chimney forms in an environment with high temperature, low pH and strong reducing hydrothermal focus flow and type II chimney forms in a relatively low temperature, high pH and rich Zn hydrothermal environment. Growth of type I chimney begins with the formation of anhydrite. Subsequently deposition of Cu-Fe-Zn sulphide in various directions of chimneys decides the final structure of this type of chimney. According to observation and analysis of mineral assemblages, the formation process of type I chimney could be divided into three stages from early, middle to late. Changes of temperature and major chemical reaction type in the process of hydrothermal chimney formation are also deduced. Different from type I chimney, quenching crystalline of pyrite and/or crystalline of sphalerite provide the growth foundation of type II chimney in the early stage of chimney forma-tion.     

The numerical and empirical evaluation of chimneys considering soil structure interaction and high-temperature effects

During the past strong ground motions, chimneys constructed according to international standards are representative of similar structures at industrial areas throughout the world, including those collapsed or moderately damaged in earthquake-prone regions. This is due to the specialty of structural characteristics and the special loads acting on the structure such as earthquakes, wind and differences in the level of temperature, etc. In this context, the researchers and designers should focus on the dynamic behavior of chimneys especially under high temperature and seismic effects. For this purpose, the main focus of this study is to evaluate the dynamic response of a chimney under the above-mentioned effects considering soil-structure interaction (SSI). A 52 m steel chimney in Yeşilyurt township of Samsun City in Turkey was studied. The in-situ model testing and numerical models were compared. Before the commissioning of the chimney, a series of tests was realized to define its dynamic characteristics in case of no-heat and after the fabric got to work, the same tests were repeated for the same sensor locations to understand the heat effect on the dynamic response of the chimney. The ambient vibration tests are proven to be fast and practical procedures to identify the dynamic characteristics of those structures. The dynamic testing of the towers promises a widespread use, as the identification of seismic vulnerability of such structures becomes increasingly important. The data presented in this study are considered to be useful for the researchers and engineers, for whom the temperature and SSI effects on steel chimneys are a concern. Using the modal analysis techniques, presented finite element simulation for the soil/pile foundation-chimney interaction system is verified. The results of modal analyses using numerical solutions are shown to have acceptable accuracy compared with results obtained by in-situ test. The present study also aims to provide designers with material examples about the influence of these on the seismic performance of steel chimneys by means of reflecting the changes in the dynamic behavior.     

Influence of soil flexibility on the seismic behaviour of chimneys

Various methodologies and modelling criteria for analysing seismically reinforced concrete chimneys are now available. All require consideration of the soil-structure interaction (SSI) effects especially in designing tall chimneys. This becomes more critical when the soil deposit, on which the chimney is founded, is softer. Two examples are considered in this paper: a chimney of moderate height (110 meters) founded directly on a sedimentary rock layered half-space and another very tall chimney (230 meters), founded by means of short prefabricated piles on a deep, very soft, sand deposit. The method of analysis used is a conventional response spectrum modal analysis of a chimney lumped mass model, which includes shear deformation of the chimney shaft. To ignore this effect can lead to an underestimation of the shear force at chimney base level. The suggestions of the specification ACI STANDARD 307-88 are also considered. Different hypotheses to account for the SSI effects are assumed in the analyses which provide a set of conclusions of interest to the point of view of the designer.     

A laboratory model of a replenished magma chamber

It has recently been suggested that periodic influxes of hot but heavy magma into the base of a basaltic magma chamber can remain isolated from the rest of the chamber while the new magma cools and crystallization proceeds. When thermal equilibrium is almost complete, the suspended crystals settle out and the residual, less dense liquid can then mix with the fluid above. In the present paper the basic fluid-dynamical processes underlying this model have been investigated in laboratory experiments using aqueous solutions. The lower layer was hot KNO₃ solution, for which saturated solutions become less dense as the temperature decreases. With a cold, deeper layer of less dense NaNO₃ or K₂CO₃ above the lower layer, there was strong convective transfer of heat through a sharp interface separating the layers, at a rate which is predicted here drawing on previous studies carried out with oceanographic applications in mind. Once crystallization began, non-equilibrium effects became important and the observed temperatures differ somewhat from those predicted. In the experiments crystals grew mainly from the bottom rather than while in suspension, but this is not an essential aspect of the model. The important fact is that the density of the residual liquid in the lower layer decreased until it became equal to that of the upper layer, and then the interface broke down so that the two layers mixed thoroughly together, leaving a layer of KNO₃ crystals at the base. No crystallization at all occurred when the hot input liquid was forced to mix initially with the cold solution already in the chamber.     

Hydrothermal silica chimney fields in the Galapagos Spreading Center at 86°W

Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO₂, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ¹⁸O) and 42°C (+27.8‰ δ¹⁸O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO₂) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred.     

竖向地震作用对高柔结构的影响

对烟囱的震害规律进行了分析.认为地震时烟囱的破坏,竖向地震力起了很重要的作用.提出了用冲量法来计算烟囱的竖向地震作用,并以190 m 高的钢筋砼烟囱为例进行了计算.对计算结果进行了分析,并指出了烟囱设计中应注意的问题.进一步阐明了计算高柔结构和特定条件下某些结构的竖向地震作用的必要性.     

Growth history of hydrothermal chimneys at EPR 9―10°N: A structural and mineralogical study

Based on structural and mineralogical characteristics of four hydrothermal chimney samples collected by submersible Alvin, growth history and formation environment of hydrothermal chimney at EPR 9―10°N are established. It is shown that there occur two types of hydrothermal chimney with different deposition environments at EPR 9―10°N according to differences in their shape, structure and mineral assemblage: type I chimney forms in an environment with high temperature, low pH and strong reducing hydrothermal focus flow and type II chimney forms in a relatively low temperature, high pH and rich Zn hydrothermal environment. Growth of type I chimney begins with the formation of anhydrite. Subsequently deposition of Cu-Fe-Zn sulphide in various directions of chimneys decides the final structure of this type of chimney. According to observation and analysis of mineral assemblages, the formation process of type I chimney could be divided into three stages from early, middle to late. Changes of temperature and major chemical reaction type in the process of hydrothermal chimney formation are also deduced. Different from type I chimney, quenching crystalline of pyrite and/or crystalline of sphalerite provide the growth foundation of type II chimney in the early stage of chimney formation.     

Groundwater recharge in the Akaki catchment,central Ethiopia: evidence from environmental isotopes (δ18O, δ2H and 3H) and chloride mass balance

Recharge patterns, possible flow paths and the relative age of groundwater in the Akaki catchment in central Ethiopia have been investigated using stable environmental isotopes δ¹⁸O and δ²H and radioactive tritium (³H) coupled with conservative chloride measurements. Stable isotopic signatures are encoded in the groundwater solely from summer rainfall. Thus, groundwater recharge occurs predominantly in the summer months from late June to early September during the major Ethiopian rainy season. Winter recharge is lost through high evaporation–evapotranspiration within the unsaturated zone after relatively long dry periods of high accumulated soil moisture deficits. Chloride mass balance coupled with the isotope results demonstrates the presence of both preferential and piston flow groundwater recharge mechanisms. The stable and radioactive isotope measurements further revealed that groundwater in the Akaki catchment is found to be compartmentalized into zones. Groundwater mixing following the flow paths and topography is complicated by the lithologic complexity. An uncommon, highly depleted stable isotope and zero‐³H groundwater, observed in a nearly east–west stretch through the central sector of the catchment, is coincident with the Filwoha Fault zone. Here, deep circulating meteoric water has lost its isotopic content through exchange reactions with CO₂ originating at deeper sources or it has been recharged with precipitation from a different rainfall regime with a depleted isotopic content. Copyright © 2006 John Wiley & Sons, Ltd.     

Laboratory investigations of viscous effects in replenished magma chambers

Some laboratory experiments are described which investigate the dynamical effects of replenishment of a magma chamber containing high viscosity magma by hotter, denser and much more fluid magma. In the experiments a layer of hot KNO₃ solution is emplaced beneath cold glycerine, which has a viscosity 3000 times greater. Less dense fluid is released immediately and continuously from the interface as a result of crystallization in the lower layer and rises as plumes through the overlying glycerine. Further crystallization occurs in the plumes, and the crystals fall out; but there is little mixing between the two fluids and a layer of depleted KNO₃ solution forms at the top. The experiments demonstrate that interfacial processes begin to dominate where there are large viscosity differences between adjacent fluid layers as would be the case in a rhyolitic magma chamber replenished by basaltic magma.     

Comparison of continuum and pore-scale models of nutrient biodegradation under transverse mixing conditions

Recent studies indicate that during in situ bioremediation of contaminated groundwater, degradation occurs primarily along transverse mixing zones. Classical reactive-transport models overpredict the amount of degradation because solute spreading and mixing are not distinguished. Efforts to correct this have focused on modifying both dispersion and reaction terms, but no consensus on the best approach has emerged. In this work, a pore-scale model was used to simulate degradation along a transverse mixing zone between two required nutrients, and a continuum model with fitted parameters was used to match degradation rates from the pore-scale model. The pore-scale model solves for the flow field, concentration field, and biomass development within pore spaces of porous medium. For the continuum model, the flow field and biomass distributions are assumed to be homogeneous, and the fitting parameters are the transverse dispersion coefficient (D_T) and maximum substrate utilization rate (k_S,c). Results from the pore-scale model show that degradation rates near the system inlet are limited by the reaction rate, while degradation rates downgradient are limited by transverse mixing. For the continuum model, the value of D_T may be adjusted so that the degradation rate with distance matches that from the pore-scale model in the mixing-limited region. However, adjusting the value of k_S only improves the fit to pore-scale results within the reaction-limited region. Comparison with field and laboratory experiments suggests that the length of the reaction rate-limited region is small compared to the length scale over which degradation occurs. This indicates that along transverse mixing zones in the field, values of k_S are unimportant and only the value of D_T must be accurately fit.     

Bench‐Scaled Nano‐Fe0 Permeable Reactive Barrier for Nitrate Removal

There are many fundamental problems with the injection of nano‐zero‐valent iron (NZVI) particles to create permeable reactive barrier (PRB) treatment zone. Among them the loss of medium porosity or pore blocking over time can be considered which leads to reduction of permeability and bypass of the flow and contaminant plume up‐gradient of the PRB. Present study provides a solution for such problems by confining the target zone for injection to the gate in a funnel‐and‐gate configuration. A laboratory‐scale experimental setup is used in this work. In the designed PRB gate, no additional material from porous media exists. NZVI (d₅₀ = 52 ± 5 nm) particles are synthesized in water mixed with ethanol solvent system. A steady‐state condition is considered for the design of PRB size based on the concept of required contact time to obtain optimum width of PRB gate. Batch experiment is carried out and the results are used in the design of PRB gate width (~50 mm). Effect of high initial NO₃^–‐N concentration, NZVI concentration, and pore velocity of water in the range of laminar groundwater flow through porous media are evaluated on nitrate‐N reduction in PRB system. Results of PRB indicate that increasing the initial NO₃^–‐N concentration and pore velocity has inhibitor effect—against the effect of NZVI concentration—on the process of NO₃^–‐N removal. Settlement velocity (S.V.) of injected NZVI with different concentrations in the PRB is also investigated. Results indicate that the proposed PRB can solve the low permeability of medium in down‐gradient but increasing of the S.V. especially at higher concentration is one of the problems with this system that needs further investigations.     

DESIGN OF SLIT DAMS FOR CONTROLLING STONY DEBRIS FLOWS

1 INTRODUCTION Stony debris flows are natural, highly concentrated water-sediment mixture, which forms wherever the simultaneous availability of water, debris material and an adequate slope, steeper than o10 are satisfied (Gregoretti, 2000). In mountainous regions of Taiwan, due to vast development and utilization of hills, stony debris flows are important from the point of disaster prevention, since they occur frequently and often bring about heavy loss of lives and properties. Therefo…