Au-Sb association and fractionation in micro-disseminated gold deposits,southwestern Guizhou--geochemistry and thermodynamics

The dual geochemical character of paragenesis and fractionation between gold,arsenic and antimony in micro-disseminated gold deposits in southwestern Guizhou is discussed in terms of spatial distribution of independent deposits,lateral and vertical enrichment,mineralization stage and factor and correlation analyses.Thermodynamic calculations of solubility and speciation of gold and antimony minerals indicate that gold is transported in hydrothermal solution as Au(HS)-2 and antimony is mainly as Sb(OH)03 although HSb2S-4 may be of increasing importance in acid environment at low temperatures during the late stage of mineralization.Changes in physicochemical conditions hold the key to the association and fractionation between gold and antimony.Gold and antimony response differently to physicochemical variations because they are distinct from each other in soluble speciation and mineral solubility,leading to precipitation at different times and localities during hydrothermal evolution.     

The behaviors and sources of dissolved arsenic and antimony in Bohai Bay

In this paper, we determined the concentrations of antimony species (antimonite (Sb(III)), antimonate (Sb(V)) and dissolved inorganic antimony (DISb)) and arsenic, in Bohai Bay seawaters, as well as the relationships of the analytes with environmental factors such as seawater characteristics (e.g., suspended particulate material (SPM), salinity and total organic carbon (TOC)), heavy metals, nutrients and phytoplankton species, and evaluated the sources of arsenic and antimony. Dissolved arsenic and antimony concentrations in the surface waters were ranging spatially from 1.03 to 1.26 ng/ml and 0.386 to 1.075 ng/ml, with mean values of 1.18 and 0.562 ng/ml, respectively. Sb(V) as the prominent chemical species constituted about 89%. Regarding arsenic concentrations in the surface waters, there was a tendency for a small variation. However, antimony species concentrations were much variable than arsenic. The highest arsenic and antimony concentrations were found near the Haihe Estuary. These distribution patterns were controlled mainly by environmental factors, biological activities and sources. In this region, DISb and Sb(V) negatively correlated with salinity. Besides, arsenic and antimony correlated well with the nutrients, chlorophyll a and phytoplankton, implying that arsenic and antimony had been involved in biological cycling. In addition, according to our estimate, about 333.5×10⁸ mg/year of arsenic and 454.2×10⁸ mg/year of antimony reached Bohai Bay via rivers.     

The self-sealing geothermal field

A geothermal field producing dry steam or high temperature water is a trap for convection currents generated in an aquifer of high permeability and of sufficient thickness by a deep heat source. A basic implication of this concept is, that a geothermal field requires a cap-rock of more or less impermeable rocks above the producing aquifer. In Larderello, Monte Amiata, and Salton Sea geothermal fields, a clearly reconnaissable tight formation overlies the producing zone and limits the upward movements of the convection currents. In other fields,i.e. The Geysers (California), Wairakei and Waiotapu (New Zealand) we do not know a geologically well defined cap-rock formation, presenting a large difference in permeability in comparison with the reservoir formation. The hot water circulating in a hydrothermal system without a cap-rock can produce deposits and rock alteration in proper places along the flow paths. The tracture and pore filling and any other permeability reducing factors increase resistance to the water circulation: those processes can originate an effective cap-rock. By such processes a hydrothermal system can become a self-sealed geothermal field. The silica deposition is probably the main self-sealing process. In fact, 1) silica is very common. 2) it is available with almost no limitation, 3) its deposition is strictly related to temperature changes, and 4) it is likely to produce very effective patterns of deposition. Where an unlimited CO₂ supply is available at depth, the calcium carbonate deposition appears to be a noticeable sealing process, which is controlled by pressure, at relatively shallow depth. In other cases CaCO₃ precipitation should not be an important factor in the self-sealing of geothermal fields. Argillization appears to be an important shallow process. It is especially effective in the acid environment of many thermal shows, thus determining their migration and/or extinction. According to our analysis and to present evidence those three self-sealing processes are the most important ones. In The Geysers Field, the wells penetrated the same formation, the Franciscan graywackes, from top to bottom. The Franciscan Formation has a very low primary permeability; secondary or fissure permeability is at the contrary very high. It is evident that there is no recognizable cap-rock in the accepted sense of petroleum geology. The wells produce superheated steam; the producing zone begins at 300 m depth or so; the quantity of steam increases with the thickness of the producing zone penetrated by the holes. Beginning in 1964, the wells have been drilled with air as circulating medium. No steam or water has been observed in the top few hundreds meters drilled: we can safely conclude that the graywackes are impervious in the upper section of the holes. Cores and cuttings show frequent fissures filled with silica in different mineral forms and hydrothermally altered rocks are common. In the Geysers area, hot springs, steam vents, carbon dioxide and hydrogen sulphide fumaroles are numerous, and wide zones of rocks, altered by past hydrothermal activity, are prominent features. As usual in many hyperthermal areas, also in The Geysers the manifestations of surface heat change frequently in place, in size, and in fluids discharge. The filling of rock fissures by mineral deposition seems the simplest and most natural explanation of the place changes of the individual springs. The active faults continually generate new fissures, limit the sealing action, and account for the persistent surface thermal activity of the area. The composition of the waters from the hot springs at The Geysers has been re-considered, in comparison with both surface waters and natural steam. The hot springs mainly originate by natural steam condensation, as Allen and Day stated in 1927. This conclusion is now strenghtened and extended: the perched water table producing hot springs at The Geysers is purely condensed steam. Practically all its characteristics can be explained by this condition alone. Separation from other shallow water bodies is extremely sharp. Let us sumarize: the impermeability of the upper section of the holes is demonstrated by the lack of fluids in the Sulphur Bank area, whereas the geochemistry of the hot springs compared with shallow waters indicates that similar conditions occur in the Geysers and Little Geysers areas. Furthermore, silica and other fissure-filling processes occur all over the region, as well as argillization of graywackes. We conclude that:

1. a)
   a cap-rock exists in The Geysers Field; this fact readily explains the production of dry steam;     
   
   

Au-Sb association and fractionation in micro-disseminated gold deposits, southwestern Guizhou—geochemistry and thermodynamics

The dual geochemical character of paragenesis and fractionation between gold, arsenic and antimony in micro-disseminated gold deposits in southwestern Guizhou is discussed in terms of spatial distribution of independent deposits, lateral and vertical enrichment, mineralization stage and factor and correlation analyses. Thermodynamic calculations of solubility and speciation of gold and antimony minerals indicate that gold is transported in hydrothermal solution as Au(HS)^- ₂ and antimony is mainly as Sb (OH) ₃ ⁰ although HSb₂S₄ may be of increasing importance in acid environment at low temperatures during the late stage of mineralization. Changes in physicochemical conditions hold the key to the association and fractionation between gold and antimony. Gold and antimony response differently to physicochemical variations because they are distinct from each other in soluble speciation and mineral solubility, leading to precipitation at different times and localities during hydrothermal evolution.     

Trace elements in Acartia clausi from Elefsis bay of the Upper Saronikos Gulf,Greece

Twelve trace elements (antimony, arsenic, cadmium, chromium, cobalt, copper, iron, managenese, mercury, scandium, selenium and zinc) were determined by neutron activation analysis in a planktonic copepod (Acartia clausi) from Elefsis Bay of the Upper Saronikos Gulf, Greece. During the first cruise (January 1974) higher levels of most of these trace elements were found in this copepod collected from a polluted area of the bay. This trend however was not observed during the second cruise (February 1974) possibly due to the termination of the active vertical convection. Lower levels of cobalt, similar levels of iron and slightly increased levels of chromium were found in Elefsis Bay Acartia clausi from those reportedin Acartia clausi from the Bay of Roquebrune. Slightly increased concentrations of antimony and zinc as well as lower concentrations of arsenic and cadmium were found in Acartia clausi than those reported in other Copepoda. No significant differences were found for copper, manganese and mercury. More data are needed to confirm if higher levels of antimony, chromium and zinc found in Acartia clausi are due to the pollution of the bay.     

Volcanic steam output directly measured in fumaroles: the observed variations at Vulcano Island,Italy, between 1983 and 1987

The mass of steam emitted by the fumaroles of the crater of Vulcano (Aeolian Islands, Italy) was evaluated by using a direct measuring method. Measurements were effected with this method between 1983 and 1987 in nine field campaigns, the results of which are given in this paper. The steam flux measurements were performed by an instrument expressly designed for steam condensation. The estimates of the thermal energy transported daily by the steam gave values of up to 10E12 J. The sharp variations in the total mass outputs recorded in 1984 and 1985 are here interpreted as being a consequence of the probable accumulation of fluid pressure at depth, during the evolution of volcanic activity. The different results obtained from the various areas of the fumarolic field during the evolutionary phases have led to the hypothesis that different subsystems exist that feed the fumaroles, and that their activity is triggered by volcano-tectonics. This method can be applied in the evaluation of both fumarolic steam and thermal energy outputs, and therefore it is particularly useful in geothermal exploration, in the evaluation of the environmental impact of volcanic degassing, and in the forecasting of volcanic eruptions. The evaluation of the steam output is also useful for quantifying the release of other gaseous species and giving univocal interpretations regarding their variations in concentrations. Furthermore this direct method provides a new way of calibrating the flux estimates effected by other telemetric techniques (Lidar, Cospec, etc.).     

Modeling of the Removal of Arsenic Species from Simulated Groundwater Containing As,Fe, and Mn: A Neural Network Based Approach

The present paper deals with the modeling of the removal of total arsenic As(T), trivalent arsenic As(III), and pentavalent arsenic As(V) from synthetic solutions containing total arsenic (0.167–2.0 mg/L), Fe (0.9–2.7 mg/L), and Mn (0.2–0.6 mg/L) in a batch reactor using Fe impregnated granular activated charcoal (GAC‐Fe). Mass ratio of As(III) and As(V) in the solution was 1:1. Multi‐layer neural network (MLNN) has been used and full factorial design technique has been applied for the selection of input data set. The developed models are able to predict the adsorption of arsenic species with an error limit of ?0.3 to +1.7%. Combination of MLNN with design of experiment has been able to generalize the MLNN with less number of experimental points.     

The gas/steam ratio as indicator of heat transfer at the Solfatara fumaroles,Phlegraean Fields (Italy)

A simple geochemical model of Solfatara, Phlegraean Fields (Italy), is proposed on the basis of gas composition and temperature at the surface.Data on the Solfatara fumaroles have been collected since 1979 within the framework of a geochemical monitoring for the surveillance of the Phlegraean volcanic system.Surface manifestations of Solfatara are likely to be fed through isoenthalpic expansion of dry steam, which separates from a geothermal liquid in an intensively fractured zone at about 236°C. This value is consistent both with gas composition and surface temperature.The gas/steam ratio appears to be the most effective parameter to detect changes of heat flow at depth.Actually a remarkable decrease in the gas/steam ratio has been observed since 1981, while the gas composition and the temperature did not change significantly. These facts suggest increased heat flow at depth.     

Arsenic in glacial drift aquifers and the implication for drinking water--lower Illinois River Basin

The lower Illinois River Basin (LIRB) covers 47,000 km2 of central and western Illinois. In the LIRB, 90% of the ground water supplies are from the deep and shallow glacial drift aquifers. The deep glacial drift aquifer (DGDA) is below 152 m altitude, a sand and gravel deposit that fills the Mahomet Buried Bedrock Valley, and overlain by more than 30.5 m of clayey till. The LIRB is part of the USGS National Water Quality Assessment program, which has an objective to describe the status and trends of surface and ground water quality. In the DGDA, 55% of the wells used for public drinking-water supply and 43% of the wells used for domestic drinking water supply have arsenic concentrations above 10 micrograms/L (a new U.S. EPA drinking water standard). Arsenic concentrations greater than 25 micrograms/L in ground water are mostly in the form of arsenite (AsIII). The proportion of arsenate (AsV) to arsenite does not change along the flowpath of the DGDA. Because of the limited number of arsenic species analyses, no clear relations between species and other trace elements, major ions, or physical parameters could be established. Arsenic and barium concentrations increase from east to west in the DGDA and are positively correlated. Chloride and arsenic are positively correlated and provide evidence that arsenic may be derived locally from underlying bedrock. Solid phase geochemical analysis of the till, sand and gravel, and bedrock show the highest presence of arsenic in the underlying organic-rich carbonate bedrock. The black shale or coal within the organic-rich carbonate bedrock is a potential source of arsenic. Most high arsenic concentrations found in the DGDA are west and downgradient of the bedrock structural features. Geologic structures in the bedrock are potential pathways for recharge to the DGDA from surrounding bedrock.     

Physical and chemical characteristics of a metal-contaminated overbank deposit,west-central South Dakota,U.S.A.

A metal-contaminated overbank deposit in west-central South Dakota resulted from the discharge of a large volume of mine tailings into a river system between the late 1800s and 1977. The deposit along the Belle Fourche River is typically up to 2 m thick and extends about 90 m away from the channel along the insides of meander bends. The sediments contain above-background levels of copper, iron, manganese, zinc, and particularly arsenic, which is commonly two orders of magnitude above background level in the contaminated sediments. Carbonate minerals in the deposit limit the desorption of arsenic by preventing acid formation. Arsenic concentrations provide a measure of the dilution of mine tailings by uncontaminated sediment. The arsenic appears to have been transported and deposited as arsenopyrite, but is now at least partially associated with iron oxides and hydroxides. Within individual samples, arsenic concentration has an inverse relation with grain size that results from the more efficient accumulation of arsenic on the greater surface area of the smaller particles. Arsenic concentration is inversely related to the sample weight percent finer than 16 μm, however, as a consequence of the dilution of the contaminated sediments by uncontaminated sediment with a finer grain-size distribution. Dilution by uncontaminated sediment from tributaries cause arsenic concentrations to decrease by a factor of 3 along 100 km of floodplain. An influx at high streamflow of uncontaminated sediment from terraces and the premining floodplain as well as from tributaries causes arsenic concentrations in parts of the contaminated deposit that are farthest away from the channel to be two to three times less than arsenic concentrations in overbank sediment that is immediately adjacent to the channel.     

A positivity preserving method for simulation of steam injection for NAPL site remediation

Numerical modelling of steam injection methods for cleanup of non-aqueous phase liquid (NAPL) contamination of groundwater requires consideration of multiphase, multicomponent convective and dispersive transport. Standard techniques do not ensure that the solution of the discrete equations has positive mole fractions, for finite mesh sizes. Negative mole fractions may cause the simulation to abort due to failure of the Newton iteration. A method for alleviating this problem is described. This method ensures that the mole fractions are positive, and results in an error that is the same size as the usual finite element discretization error. Example computations are presented for cartesian and axisymmetric two-dimensional geometries.     

Numerical simulation of non-isothermal multiphase multicomponent processes in porous media.: 2. Applications for the injection of steam and air

In part 1 [Class et al., this issue], we presented a numerical simulator for non-isothermal multiphase systems. A module of this simulator that considers the phases water, an organic non-aqueous-phase liquid (NAPL), and the gas phase is used here for two applications. The aim of the numerical simulations is to show and to interpret the behavior of the non-isothermal multiphase processes. The first example deals with an experiment in which an air-dry, NAPL-contaminated column is remediated by a steam injection. The typical front behavior is investigated and the coupled physical processes are identified by a comparison with experimental temperature data. The second application is a two-dimensional case study, where the influence of different strategies for the remediation by injection of steam and/or air in a heterogeneous, NAPL-contaminated sand can be shown.     

Measurement of Natural Losses of LNAPL Using CO2 Traps

Efflux of CO₂ above releases of petroleum light nonaqueous phase liquids (LNAPLs) has emerged as a critical parameter for resolving natural losses of LNAPLs and managing LNAPL sites. Current approaches for resolving CO₂ efflux include gradient, flux chamber, and mass balance methods. Herein a new method for measuring CO₂ efflux above LNAPL bodies, referred to as CO₂ traps, is introduced. CO₂ traps involve an upper and a lower solid phase sorbent elements that convert CO₂ gas into solid phase carbonates. The sorbent is placed in an open vertical section of 10 cm ID polyvinyl chloride (PVC) pipe located at grade. The lower sorbent element captures CO₂ released from the subsurface via diffusion and advection. The upper sorbent element prevents atmospheric CO₂ from reaching the lower sorbent element. CO₂ traps provide integral measurement of CO₂ efflux based over the period of deployment, typically 2 to 4 weeks. Favorable attributes of CO₂ traps include simplicity, generation of integral (time averaged) measurement, and a simple means of capturing CO₂ for carbon isotope analysis. Results from open and closed laboratory experiments indicate that CO₂ traps quantitatively capture CO₂. Results from the deployment of 23 CO₂ traps at a former refinery indicate natural loss rates of LNAPL (measured in the fall, likely concurrent with high soil temperatures and consequently high degradation rates) ranging from 13,400 to 130,000 liters per hectare per year (L/Ha/year). A set of field triplicates indicates a coefficient of variation of 18% (resulting from local spatial variations and issues with measurement accuracy).     

The generation of a pressure profile in a geothermal reservoir: The Mutnovskii hydrothermal steam field

We consider the variation of pressure and temperature along the wellbores in the Mutnovskii hydrothermal steam field. We distinguish three types of barogram to characterize the state of the heat carrier in a well: the subvertical, the subvertical-inclined, and the inclined. The first two types are typical of the wellbore segment filled with the heat carrier in the state of steam and in the two-phase state, and the third is characteristic for the liquid state. A joint analysis of barograms and thermograms gave information on the phase state of the heat carrier along the wellbore and on the conditions peculiar to excitation of a well.     

Organic Contamination of Ground Water at Gas Works Park, Seattle, Washington

Gas Works Park, in Seattle, Washington, is located on the site of a coal and oil gasification plant that ceased operation in 1956. During operation, many types of wastes, including coal, tar, and oil, accumulated on-site. The park soil is currently (1986) contaminated with compounds such as polynuclear aromatic hydrocarbons, volatile organic compounds, trace metals, and cyanide. Analyses of water samples from a network of observation wells in the park indicate that these compounds are also present in the ground water.
Polynuclear aromatic hydrocarbons and volatile organic compounds were identified in ground water samples in concentrations as large as 200 mg/L. Concentrations of organic compounds were largest where ground water was in contact with a non-aqueous phase liquid in the soil. Where no non-aqueous phase liquid was present, concentrations were much smaller, even if the ground water was in contact with contaminated soils. This condition is attributed to weathering processes in which soluble, low-molecular-weight organic compounds are preferentially dissolved from the non-aqueous phase liquid into the ground water. Where no non-aqueous phase liquid is present, only stained soils containing relatively insoluble, high-molecular-weight compounds remain. Concentrations of organic contaminants in the soils may still remain large.     

Comparison of Trace Elements in Bottled and Desalinated Household Drinking Water in Kuwait

The levels of the trace elements aluminum (Al), arsenic (As), boron (B), barium (Ba), beryllium (Be), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), antimony (Sb), selenium (Se), strontium (Sr), titanium (Ti), vanadium (V), and zinc (Zn) were determined in 93 brands of non‐carbonated (BW) and carbonated (CBW) bottled water available in Kuwaiti markets and in 321 samples of desalinated household water (HW) collected from 99 sampling locations, which covered 95% of Kuwait's residential areas. The study yielded a large range of results for most of these elements, and the difference between the lowest and highest measured values exceeded three orders of magnitude for some elements in a number of the BW and CBW brands. With a few exceptions, the results for the HW and BW were found to comply with the United States‐Environmental Protection Agency and World Health Organization guideline values. The elements Fe, Pb, Hg, and Ni exceeded these GVs in 8.5, 0.3, 1.2, and 1% of the HW samples, respectively. One brand of BW exceeded the GV for B, while the levels of B, Ba, Be, Fe, and Mn exceeded the GVs in 35, 15, 5, 75, and 25% of the CBW brands, respectively. Correlations were found between the levels of some elements and either the desalination plant that produced the HW or the country of origin of the BW or CBW. The levels of the elements Al, Cu, Fe, and Zn in HW were higher in the summer than in the winter, with 14, 14, 42, and 18% increases, respectively. Nutritional evaluation revealed that significant percentages of the DRIs of Cr, Cu, Fe, and Se can be provided by drinking 2 L of HW, and significant percentages of the DRIs of B, Cr, Cu, and Mo can be provided by drinking 2 L of BW.     

Deep geothermal brine near Salton Sea,California

A well drilled for geothermal power near Salton Sea in Imperial Valley, Calif., is 5,232 feet deep; it is the deepest well in the world (1962) in a high-temperature hot spring area. In the lower half of the hole temperatures are too high to measure with available equipment, but are at loast 270°C, and may be as much as 370°C. For comparison, maximum temperature heretofore reported at depth (1962) for hot spring areas is 295°C. The well taps a very saline brine of Na-Ca-K-Cl type (about 185,000 ppm Cl) with exceptionally high potassium, and with the highest content of minor alkali elements known for natural waters; Fe, Mn, Zn, Pb, Cu, Ag, and some other metals are also exceptionally high. This brine may be connate, but present evidence favors a source in the magma chamber at depth that supplied late Quaternary rhyolite domes of the area. If the latter is correct, the brine is an undiluted magmatic water that is residual from the separation of a more volatile phase high in CO₂, H₂S, and with some alkali halides. Elsewhere, the hypothesized volatile phase may account for near-surface hot spring activity of most thermal areas of volcanic association. The residual brine of high salinity may ordinarily remain relatively deep in the volcanic systems because of high specific gravity and low content of volatiles, seldom appearing at the surface except in a greatly diluted form. The hot springs of Arima, Japan, may be a rare example of this type of magmatic water discharging at the surface in moderate concentration (Cl as much as 42,000 ppm). Independent evidence from fluid inclusions in minerals of high-temperature base-metal deposits also favors the existence of magmatic water high in Na, Ca, and Cl, and low in CO₂ and other volatile components. During a three-month production test several tons of material precipitated in the horizontal discharge pipe from the well. Amorphous silica with iron and manganese, and bornite are the dominant recognized components. This material contains the astonishingly high contents of about 20 percent copper, 2 percent silver, and notable sulfur, arsenic, bismuth, lead, antimony, and some other minor elements. Total quantities of all elements in the original whole brine are not yet known, but calculated amounts corresponding to 1 to 3 ppm of copper and 0.1 to 0.3 ppm of silver were precipitated from the brine to form the pipe deposits. The brine, therefore, may be man’s first sample of an « active » ore solution. Equally fascinating to many geologists is the possibility that in the lower part of the hole temperatures are so high and pressures are sufficient for young sedimentary rocks to be undergoing transformation into rocks with mineral assemblages of the greenschist facies of metamorphism. Drill cores from 4,400 to 5,000 feet in depth contain chlorite, albite, K-feldspar, epidote, mica, and quartz, with some indication of increase in metamorphic grade downward. Regional geological and geophysical studies favor a depth of about 20,000 feet to pre-Tertiary basement rocks in the general area. A shallow basement or local upfaulting of old metamorphic rocks are not likely possibilities for the thermal area.     

Concentrations and transport of trace metals in the St. Lawrence River

Samples of raw water were collected in the St. Lawrence River during six sampling trips from August 1990 to April 1992. Water samples were analyzed for both dissolved and particulate phases for five trace metals. Partition coefficients (Kd) and metal fluxes were calculated in order to determine metal transport. A mass balance equation was used for the determination of the major metal sources to the St. Lawrence River and an estimation of metal loadings to the estuary was made. Average dissolved metal concentrations were found to be Cd 10 ng/L, Co 74 ng/L, Cu 64 ng/L, Fe 69 µg/L and Mn 700 ng/L. Particulate concentrations were (in µg/g) 1.68 for Cd, 31 for Co, 73 for Cu, 25 mg/g for Fe and 1.69 mg/g for Mn. Co, Fe and Mn were transported essentially in the particulate phase while Cd and Cu were predominantly found in the dissolved phase at 56% and 48% of the total metal concentration respectively. Log Kd values varied from 5.1 (for Cu) to 6.8 (for Mn). In the dissolved phase the major sources were found to be the Great Lakes and the Ottawa River whereas in the particulate phase Québec tributaries appear to be the most important. Industrial inputs are quite important in both the dissolved and the particulate phases for Cd, whereas other sources are very variable, especially for the dissolved phase.     

Element transport by dehydration of subducted sediments: Implication for arc and ocean island magmatism

High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.     

上覆水pH值和总磷浓度对含铁盐的高砷沉积物中砷迁移转化的影响

含铁盐的高砷沉积物中,上覆水p H值和总磷浓度的变化对砷迁移的影响尚不明确.以含铁盐的高砷沉积物为研究对象,通过实验观测其暴露在不同p H值的上覆水及经过不同初始p H值、不同初始总磷浓度的溶液淋洗后,沉积物中砷的迁移规律.结果表明:当沉积物暴露在p H=10和11的上覆水时,分别在第11 d和第6 d后,沉积物中的砷溶出速度突然增大,说明当沉积物处于强碱性上覆水环境中,碱性越强,砷的溶出速度越快、溶出量越大.从室内实验条件的角度来看,上覆水的碱化有可能会增加沉积物中砷溶出的风险.而上覆水p H值在4~8之间时,上覆水的酸碱度对沉积物砷的迁移影响较小.此外,当上覆水总磷浓度不高于0.5 mg/L时,磷的竞争吸附对砷的迁移影响较小.