海水入侵的实验研究

在海岸含水层中,含较多盐分的海水向陆地方向入侵形成淡-咸水交界面,通常称为海水入侵界面。该界面的动态变化受气象、水文地质和人类活动等因素的影响,是海水入侵问题的主要研究内容。本文介绍了海水向含水层中入侵的室内实验研究,以着色海水和图像处理方法获得入侵界面的形态及其动态变化。实验方法先进,所得数据整体性强,精度高,而且观测方法对流场没有干扰,比传统的提取水样进行化验的方法更有效。实验中模拟了海洋潮汐条件,揭示了入侵界面在不同实验条件下的动态特征。应用数值模拟方法揭示了流场的特征。计算表明,流场分两个区———淡水区和咸水区,由入侵界面分开。淡水区的水流相对活跃,淡水由入侵界面以上区域渗出边界,在界面以上形成淡水通道。     

Clues to chondrule precursors: An investigation of vesicle formation in experimental chondrules

The absence of vesicles in chondrules and their presence in synthetic analogs yields information about the origin of chondrules. A variety of melting-crystallization experiments demonstrate the cause of vesicles in synthetic chondrules. Experiments involving the use of binding agents in sample preparation, samples with residual adsorbed moisture, incompletely melted samples, and the use of fine-grained sizesorted starting powder all generated more vesicles than experiments on control samples. Volatiles such as Na were not responsible for vesicles in our experiments because Na was not lost under our flashheating conditions. Because Wdowiak (1983) assumed chondrule precursors contained volatiles, and his electrical discharge melting generated vesicles, he suggested chondrules were not formed by flashmelting events. However, vesicle-free chondrules are to be expected with flash melting provided that the precursors were poor in highly volatile material. Flash-melting experiments with serpentine in the precursor powder developed extremely porous “popcorn” spherules, as in some meteorite ablation spherules. Chondrule precursors must have consisted of anhydrous phases assembled at low ambient gas pressure above the condensation temperature of ice. The absence of vesicles in all chondrules, including those unlikely to have been heated multiple times, e.g., ¹⁶O-rich and granular chondrules, demonstrates that their original precursors, whether interstellar dust or nebular condensates, cannot have consisted of hydrous silicates.     

An experimental investigation into the metastable formation of phosphoran olivine and pyroxene

The formation of phosphoran olivine by crystallization from a melt was investigated experimentally using a one atmosphere furnace, using San Carlos olivine [(Mg,Fe)₂SiO₄] mixed with either iron phosphide (FeP) or magnesium pyrophosphate (Mg₂P₂O₇). Both dynamic crystallization and isothermal experiments produced phosphoran olivine as zoned single crystals and as overgrowths surrounding normal, phosphorus-free olivine grains. The crystallization pathways that form phosphoran olivine were traced and confirm that it is a metastable phase that can crystallize from a phosphorus-rich melt over timescales of hours to days. Removal of the P and equilibration of the olivine however requires weeks to months in the presence of silicate melt. Phosphoran olivine with up to 27 wt% P₂O₅ was generated and up to 69% of the Si tetrahedral sites were replaced by P. The substitution of Si by P into olivine was confirmed as 4^VIM⁺² + 2^IVSi⁺⁴ ↔ 3^VIM⁺² + 2^IVP⁺⁵ + ^VI[]. Phosphoran olivine compositions that vary from (Mg,Fe)₂SiO₄ to (Mg,Fe)_1.65[]_0.35Si_0.3P_0.7O₄ have been produced in these experiments.Phosphoran pyroxene was also generated in a few experiments and forms when phosphoran olivine reacts with either tridymite or melt. It has compositions compatible with protopyroxene, orthopyroxene, pigeonite and sub-calcic augite, and can contain up to 31.5 wt% P₂O₅. Like phosphoran olivine, it is also a metastable phase. Phosphorus replaces Si in pyroxene by the following substitution methods: 8^IVSi⁺⁴ ↔ 3^IVSi⁺⁴ + 4^IVP⁺⁵ + ^IV[] with Al entering the structure by the exchange 2^IVSi⁺⁴ ↔ ^IVAl⁺³ + ^IVP⁺⁵. Phosphoran pyroxene compositions vary from (Mg,Fe)₈Si₈O₂₄ to (Mg,Fe)₈Si₃P₄[]O₂₄.     

An experimental investigation of high-temperature interactions between seawater and rhyolite,andesite, basalt and peridotite

Natural seawater was allowed to react with rhyolite, andesite, basalt, and peridotite at 200°–500° C, and 1,000 bars at water/rock mass ratios of 5 and 50 in order to investigate the effects of rock type, water/rock ratio, and temperature on solution chemistry and alteration mineralogy. The results indicate that interactions of seawater with various igneous rocks are similar in the production of a hydrous Mg-silicate and anhydrite as major alteration products. Fluids involved in the interactions lose Mg to alteration phases while leaching Fe, Mn, and Si from the rocks. The pH of the solutions is primarily controlled by Mg-OH-silicate formation and therefore varies with Mg and Si concentration of the system. Other reactions which involve Mg (such as Mg-Ca exchange) or which produce free H⁺, cause major differences in fluid chemistry between different seawater/ rock systems. High water/rock ratio systems (50/1) are generally more acidic and more efficient in leaching than low ratio systems (5/1), due to relatively more seawater Mg available for Mgsilicate production. The experiments show that large-scale seawater/rock interaction could exert considerable control on the chemistry of seawater, as well as producing large bodies of altered rock with associated ore-deposits.Active plate margins of convergence or divergence are suitable environments for hydrothermal systems due to the concurrence of igneous activity, tectonism, and a nearby water reservoir (seawater or connate water). The experimental data indicate that seawater interactions with igneous host rocks could generate many of the features of ore-deposits such as the Kuroko deposits of Japan, the Raul Mine of Peru, the Bleida deposit of Morocco, and deposits associated with ophiolites. Serpentinization of peridotite and alteration of igneous complexes associated with plate margins can also be explained by seawater interaction with the cooling rock. Geothermal energy production could benefit from experimental investigations of hot water/rock systems by development of chemical, temperature, and pressure control systems to maximize the lifetime of hydrothermal flow.     

An experimental investigation of olivine morphology

Olivine crystals can adopt ten types of shape. Experimental crystallization of eight rock melts shows that there is a systematic change from polyhedral or granular olivines hopper olivines branching olivines randomly oriented chain olivines parallel-growth chain olivines chain+lattice olivines plate or feather olivines, with increase in cooling rate and with increase in degree of supercooling. This sequence involves changes from complete to progressively less complete crystals and from equant habit to elongate bladed habit (c>ab) to tabular habit (ac b). The sequence is not affected by the phase relations of the melt. The larger the olivine content of a melt the slower the cooling rate at which a particular olivine shape grows, whereas the lower the melt viscosity, the greater the cooling rate. Irrespective of the melt composition, comparable crystal shapes grow at the same degrees of supercooling. By comparison of the shapes of olivine crystals in experiments with those in rocks of similar composition, it is possible to deduce the cooling rate through the olivine crystallization interval and the approximate degree of supercooling at which the olivine crystals nucleated and grew in the rocks. The various shapes of skeletal olivines in many picrites, olivine-rich basalts and the Archaean spinifex rocks are not due to rapid cooling, but to rapid olivine growth caused by the high normative olivine content of the magma.     

Xenon in Archean barite: Weak decay of Ba, mass-dependent isotopic fractionation and implication for barite formation

In order to investigate radioactive decay of ¹³⁰Ba and ¹³²Ba which have half-lives on the order 10²⁰-10²¹ a, the isotopic composition of xenon has been measured in 3.5 Ga barite of the Dresser Formation, Pilbara, Western Australia. The analyzed samples were collected at about 86 m depth from a diamond drill core (Pilbara Drilling Project). The fact that the sample has been shielded from modern cosmic ray exposure reduces the number of potentially interfering production pathways, simplifying interpretation of the Xe isotope spectrum. This spectrum is clearly distinct from that of either modern or ancient atmospheric Xe. A strong excess of ¹³⁰Xe is identified, as well as other isotopic excursions which are attributed to mass-dependent isotopic fractionation and contributions from products of uranium fission. The mass-dependent fractionation, estimated at 2.1 ± 0.3% amu⁻¹, can be accounted for by mutual diffusion and Rayleigh distillation during barite formation that is consistent with geological constraints. After correction for mass-dependent fractionation, the concentrations of fissiogenic Xe isotopes demonstrate that the U-Xe isotope system has remained closed over 3.5 Ga. From the excess of ¹³⁰Xe, the two successive electron capture half life of this isotope is estimated at 6.0 ± 1.1 × 10²⁰ a, which is 3.4 times faster than previously estimated (Meshik et al., 2001). We could not find evidence of ¹³²Ba decay within our Xe isotope spectra.     

An experimental investigation of density-stratified inertial gravity currents

Turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation. Analogue, density‐stratified laboratory currents are described, using layers of salt solutions with different concentrations and depths to create the initial vertical stratification. The evolving structure of the flow depends on the distribution of the driving buoyancy between the layers, B* (proportional to the layer volumes and densities), and their density ratio, ρ*. When the lower layer contains more salt than the upper layer, and so has a greater proportion of the driving buoyancy (B* < 0·5), this layer can run ahead leading to streamwise or longitudinal stratification (ρ*→0), or the layers can mix to produce a homogeneous current (ρ*→1). If the upper layer contains more salt and thus buoyancy (B* > 0·5), this layer travels to the nose of the current by mixing into the back of the head along the body/wake density interface to produce a homogeneous flow (ρ*→1) or overtaking, leading to streamwise stratification (ρ*→0). Timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations. Distance–time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but, during their later stages, they travel more slowly. In natural flows that are stratified in concentration and grain size, internal features, such as stepwise grading, gradual upward fining and reverse grading, could be produced depending on B* and ρ*. Stratification may also be expected to affect interactions with topography and overall fan architecture.     

An experimental investigation of antigorite dehydration in natural silica-enriched serpentinite

Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, southern Spain). At 630–660°C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680°C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al₂O₃) and chromium contents (0.59 wt% Cr₂O₃) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70°C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680–710°C and 1.6–1.9 GPa.     

An experimental investigation of gravity-driven shale tectonics in progradational delta

Many deltas exhibit gravitational deformation of their sedimentary cover. In these systems, the décollement layers do not always consist of rock salt but sometimes of overpressured shale. Unlike salt, the efficiency of detachment in shale depends on the magnitude of fluid overpressures and it varies through time and space, as rapid sedimentary burial progrades into deeper water. As a result, the gravity deformational domains are progressively translated seaward. Sandbox models involving high air pore pressures were used to simulate such gravity-driven shale tectonics in prograding deltas. Models were built with sand of various permeabilities and air was injected to simulate the mechanical effects of fluid overpressure. Our apparatus for the injection of air allowed us to control subsurface pressures in space and time during the experiments, and it was used to simulate the advance of the front of the overpressured domain during the sedimentary progradation. In our models, sand kept obeying a frictional behavior, for medium to high pore pressures, and the detachment appeared as very thin shear bands. Compressional belts that formed during the experiment were dominated by asymmetric basinward-verging fore-thrusts, as is often observed in deep-water, shale-detached foldbelts. Where the value of fluid pressures approached that of the lithostatic stress, sand was fluidized, resulting in ductile strains analogous to what occurs in highly overpressured mobile shale. During progradation, ancient buried thrustbelts were reactivated, thereby controlling later extension. During the experiments, sand volcanoes, analogous to mud volcanoes, formed in relation with tectonic structures. Some of them developed near normal faults but many of them formed directly above old buried thrusts.     

An experimental investigation on diffusion of water in haplogranitic melts

  The diffusivity of water has been investigated for a haplogranitic melt of anhydrous composition Qz₂₈Ab₃₈Or₃₄ (in wt %) at temperatures of 800–1200°C and at pressures of 0.5–5.0 kbar using the diffusion couple technique. Water contents of the starting glass pairs varied between 0 and 9 wt %. Concentration-distance profiles for the different water species (molecular water and hydroxyl groups) were determined by near-infrared microspectroscopy. Because the water speciation of the melt is not quenchable (Nowak 1995; Nowak and Behrens 1995; Shen and Keppler 1995), the diffusivities of the individual species can not be evaluated directly from these profiles. Therefore, apparent chemical diffusion coefficients of water (D _water) were determined from the total water profiles using a modified Boltzmann-Matano analysis. The diffusivity of water increases linearly with water content <3 wt % but exponentially at higher water contents. The activation energy decreases from 64 ± 10 kJ/mole for 0.5 wt % water to 46 ± 5 kJ/mole for 4 wt % water but remains constant at higher water contents. A small but systematic decrease of D _water with pressure indicates an average activation volume of about 9 cm³/mole. The diffusivity (in cm²/s) can be calculated for given water content (in wt %), T (in K) and P (in kbar) by

An experimental investigation of hydroxyl solubility in jadeite and Na-rich clinopyroxenes

The solubility and incorporation mechanisms of water in synthetic, water-saturated jadeite and Na-rich clinopyroxenes have been experimentally investigated. Infrared spectra for water-saturated jadeite synthesised from 2.0 to 10 GPa show two prominent sharp peaks at 3,373 and 3,613 cm^–1 together with several weaker features in the OH-stretching region, indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure. Water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 GPa and slowly decreases with increasing pressure to about 100 ppm at 10 GPa. Solubility can be described by the function c_OH=A f_H2O^0.5 exp (–PV_Solid/RT), where c_OH is water solubility in ppm H₂O by weight, A is 7.144 ppm/bar^0.5, f_H2O is water fugacity, and V_Solid=8.019 cm³/mol is the volume change of the clinopyroxene upon incorporation of OH. Jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions. Assignment of absorption bands in IR spectra verifies the importance of cation vacancies on the M2 site in providing mechanisms for hydrogen incorporation. However, results also suggest that substitution of lower valency cations onto the M1 site may also be important. Solid solution of jadeite with diopside and in particular, with Ca-Eskola component leads to a drastic increase of water solubility, and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature. Omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals.Editorial responsibility: W. Schreyer     

An experimental study on felsic rock–artificial seawater interaction: implications for hydrothermal alteration and sulfate formation in the Kuroko mining area of Japan

Experimental studies on the interactions between artificial seawater (ASW) and fresh rhyolite, perlite and weakly altered dacitic tuff containing a small amount of smectite suggest changing cation transfer during smectite-forming processes. Initially, dissolution of K from the rocks accompanies incorporation of Mg and Ca from ASW during both earlier (devitrification stage) and later smectite formation, whereas Ca incorporated with early smectite formation redissolves with progressive reaction. Barium mobility increases toward the later smectite-forming reactions. Therefore, the large amounts of barite, anhydrite and gypsum in Kuroko ore deposits are considered to have precipitated from hydrothermal solutions derived from the interaction with previously altered felsic rocks during late smectite formation, rather than by the reaction with fresh felsic rocks.Editorial handling: D. Lentz     

An experimental study on the process of zeolite formation

Results are reported of an experimental study which examined the effect of solution composition on the composition, structure, and crystallization path of phillipsite and merlinoite in the system Na₂OK₂O-Al₂O₃-SiO₂H₂O at 80°C and $\text{pH = 13.34–13.71}$.At a fixed 3.5 M total dissolved silica concentration, zeolite Si/Al ratio was found to be a linear function of pH within the pH range of the experiments.²⁹Si NMR spectra of the initial solutions show that pH determines the distribution of aqueous aluminosilicate species and, as a result, the precipitated zeolite Si/Al ratio. SEM observations reveal that zeolites may precipitate with or without the presence of an intermediary gel phase, depending on solution composition. The growth rate of the zeolites was found to be dependent upon solution pH and total dissolved aluminum concentration. These observations are discussed in terms of their possible applications to natural zeolite paragenesis and serve to delineate the framework of a comprehensive theory for the mechanism of zeolite crystallization from highly alkaline solutions.     

Hydrothermal processes along mid-ocean ridges: An experimental investigation

An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200 ° C and 500 ° C at 500–800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na⁺, Mg⁺⁺, SO₄ ⁼, Cl^? solution to a reducing, acidic, Na⁺, Ca⁺⁺, Cl^?, solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid reach concentrations of 200–600 ppm; however, Al abundances remain very low (～0.5 ppm). Gray and green smectites, anhydrite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and could be important in the genesis of certain Fe-Cu sulfide ore deposists. The process could also affect the geochemical budgets of certain elements and exert substantial control of the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si, and H to the oceans.     

An experimental investigation of LNAPL migration in an unsaturated/saturated sand

Accidental spills of hydrocarbons, such as Light Non-Aqueous Phase Liquids (LNAPLs), are one of the most common sources of subsurface contamination. Migration of LNAPL in a porous medium is influenced by various factors such as the number of fluids present in the unsaturated/saturated zones and the proportion of pores occupied by each fluid. The results for relationship between matric suction and degree of saturation are presented in this paper for water–air, water–LNAPL and LNAPL–air systems in a sand. A simple and reliable setup using Buchner funnel was designed to obtain these relations. It was found that the relationship between matric suction head and degree of saturation is hysteretic for all the fluid systems (water–air, water–LNAPL and LNAPL–air). Furthermore, the amount of hysteresis depended upon the fluid system, with the maximum hysteresis occurring for water–air system. The results suggest that the amount of trapped air depends upon the reversal degree of saturation from drying to wetting.     

An experimental investigation of the delay in nucleation of olivine in Mafic Magmas

The delay in nucleation of olivine in basaltic melts increases systematically with decreasing degree of supercooling, cooling rate, superheat, olivine content of the melt and with increasing melt viscosity. These findings imply homogeneous nucleation of olivine in melts run in the Pt-wire loop sample container used. Unlike plagioclase nucleation, the appearance of the first olivine crystal in a melt is predictable (to within 0.1–1 h of the event). The metastable region (i.e., the minimum degree of supercooling necessary for nucleation) is less than 13 ° C. The cause of the delay in nucleation is discussed in terms of the finite growth rate of embryos and the progressive polymerization of the melt with decreasing temperature and increasing time. At degrees of superheat <18 ° the melts are inferred to be more highly disordered than expected. Some implications of the results for petrology and experimental petrology are discussed, including the possibility that continuous zoning may develop under isothermal conditions due to the sluggish attainment of equilibrium of melt structure following a sudden change in temperature.     

组构对自然休止角影响的试验研究

利用自制的模型试验装置,通过在不同方位沉积散体颗粒生成堆积体,研究不同沉积组构特征对自然休止角的影响,以及通过在母堆积体底部设置漏孔以生成子堆积体,研究堆积体内部结构对自然休止角的影响。研究发现,母堆积体休止角? 和子堆积体休止角? 随着沉积面倾角? 的增大（0° → 90°）呈现先减小后增大的趋势,子堆积体休止角? 则先增大后减小,且最小和最大值的分布区间正好互余,分别为? = 30°～45°和? = 45°～60°。当漏孔位置从左至右靠近母堆坡脚时,? 先增大后减小,? 则单调增大。基于模型的分析和推导,发现当沉积面与潜在滑动面越接近平行,休止角越小,越偏离,休止角则越大,并从细观上合理解释了?、? 和? 在特殊角度区间出现最大或最小值的原因。最后,提出了散粒堆积体的扰动分区模型,并以此为基础阐释了漏孔位置（堆积体内部结构）对休止角? 和? 的影响机制。     

非饱和带中非均质条件下LNAPL运移与分布特性实验研究

本文通过室内实验建立物理模型，研究非饱和带中层状非均质条件下轻非水相液体LNAPL（Light Non-Aqueous Phase Liquid）运移机制和分布特性。实验结果发现，介质的结构突变面改变了LNAPL的迁移模式。在LNAPL到达干湿界面以后，由于毛细水压力的作用，其入渗速度明显加快。当LNAPL迁移到夹层上界面时，纵向迁移受到阻碍。随着入渗过程的继续，污染物沿着界面产生向右横向迁移；在夹层上界面下部出现“优势流”现象，污染物由集中走向分散，随着分散块的体积不断增大，最终污染物又连接到一起，由分散走向集中；在夹层下界面附近锋面线集中在一起，说明污染物向下的纵向迁移基本停止。另外，介质的局部密实“透镜体”对LNAPL的局部迁移起到“阻滞”作用，并迫使其改变迁移路径。地下水位的变动将使污染物的产生重新分布。当地下水位降低以后，污染物产生明显的向下迁移。     

Stratabound barite deposits in Nevada

The barite deposits in the Shoshone Range contain rhythmic layering of several types. One type includes interlayered barite and clay seams with iron oxide grains and stains, where the barite beds gradually change in thickness upwards. Top-bottom (geopetal) features are present but differ from those observed in the Arkansas Stanley Shale deposits. Chert and very fine-grained quartz are interlayered with the barite. The thickest barite beds lie where the chert and shaly layers are thinnest. These are features one would normally find in a sediment. These and other observations suggest a formation by syn-diagenetic processes.

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Zusammenfassung Die Schwerspatlagerstätten der Shoshone Range in Nevada zeigen rhythmische Lagen verschiedener Arten. So gibt es eine Wechsellagerung von Schwerspat mit Tonhäutchen, die Eisenoxidkörner und -filme enthalten. Bei diesem Typ schwankt die Schichtmächtigkeit von unten nach oben. Die Lagen sind sehr reich an Geopetalgefügen aber unterscheiden sich von denjenigen in den Schwerspatlagerstätten von Stanley Shale, Arkansas. Die rhythmischen Lagen bestehen aus Flint (Chert), feinkörnigem Quarz und Schwerspat. Dichte Schwerspat-Lagen treten da auf, wo die Flint-Lagen sehr dünn sind oder fehlen. Solche Erscheinungen treten normalerweise sehr häufig in Sedimenten auf. Diese und andere erwähnte Beobachtungen geben Anlaß zur Annahme, daß es sich hier um synsedimentäre Lagerstätten handelt.

     

Microbial mobilization of barite

Bacteria from lake mud grown under anaerobic conditions with powdered barite present significantly increase the amount of barium that dissolves in the culture medium. Innocula of lake mud were added to a growth medium specified for sulfate-reducing bacteria. Powdered barite was added to each culture flask and flasks were purged with nitrogen, sealed and maintained at 20°C for 17 days. When the cultures were opened for analysis, hydrogen sulfide and iron sulfide had formed in all growing cultures. In cultures containing barite as the only sulfate, the barium content of the culture solutions had risen to an average of 3.3 ppm (maximum = 3.8 ppm, minimum = 1.8 ppm). Cultures containing ferrous sulfate and barite showed less than 0.1 ppm barium in solution. Cultures that had been sterilized by poisoning or autoclaving showed approximately 0.5 ppm barium in solution. Sulfate-reducing bacteria may be responsible for the high barium concentrations recorded in streams draining barite mining areas.