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1.
The K?rka borate deposit was deposited in a Miocene lacustrine basin which is closely associated with volcanic activity which lasted from Paleogene to the beginning of Quaternary. Borate mineralization alternates with claystone, mudstone, tuff and fine-layered limestone and mostly shows a lenticular structure. The mineral paragenesis is composed of borax, tincalconite, ulexite, kurnakovite, probertite, tunellite, colemanite, dolomite, smectite group minerals, illite and some firstly reported minerals for the K?rka deposit including hydrochloroborite, brianroulstonite, hilgardite-4M and searlesite minerals. In comparison to average values of earth crust, concentrations of Cs, Sr, Li, As and Se were significantly enriched with respective rates of 21, 15, 14, 3 and 188 folds. Regarding KY, KS1 and KS2 locations, there are differences in both element abundances and their geochemical tendencies which are attributed to variations in discharge regime and physico-chemical conditions of the depositional environment. Independent behaviour of B2O3 might indicate that boron is not associated with clays and carbonates and, therefore, most part of boron must be derived from volcanic activity (hydrothermal solutions, gases). REE data indicate that the K?rka borate deposit was formed in a sedimentary environment where highly alkaline (high pH) hydrothermal solutions also took part in borate precipitation process.  相似文献   

2.
Fluids from the ultramafic-hosted Lost City hydrothermal field were analyzed for total dissolved organic carbon and dissolved organic acids. Formate (36-158 μmol/kg) and acetate (1-35 μmol/kg) concentrations are higher than in other fluids from unsedimented hydrothermal vents, and are a higher ratio of the total dissolved organic carbon than has been found in most marine geothermal systems. Isotopic evidence is consistent with an abiotic formation mechanism for formate, perhaps during serpentinization processes in the sub-surface. Further support comes from previous studies where the abiological formation of low molecular weight organic acids has been shown to be thermodynamically favorable during hydrothermal alteration of olivine, and laboratory studies in which the reduction of carbon dioxide to formate has been confirmed. As the second most prevalent carbon species after methane, formate may be an important substrate to microbial communities in an environment where dissolved inorganic carbon is limited. Acetate is found in locations where sulfate reduction is believed to be important and is likely to be a microbial by-product, formed either directly by autotrophic metabolic activity or indirectly during the fermentative degradation of larger organic molecules. Given the common occurrence of exposed ultramafic rocks and active serpentinization within the worlds ocean basins, the abiotic formation of formate may be an important process supporting life in these high pH environments and may have critical implications to understanding the organic precursors from which life evolved.  相似文献   

3.
The Miocene borate deposits of western Turkey are associated with extensive medium- to high-K calc-alkali ignimbritic volcanism and a differentiated comagmatic alkaline trachybasalt–trachydacite lava suite. Ignimbritic air-fall and reworked pumiceous clastic materials are intimately associated with lake sediments that host the borate deposits. Local ignimbritic volcanism is considered the primary source of the B for the Kirka borate deposit in this area. Comparison of the geochemical composition of Turkish ignimbrites associated with borates (`fertile' ignimbrites) with those that do not (`barren' ignimbrites), exhibit a number of features that might prove useful in the exploration for borates in similar volcanic domains. In particular, `fertile' ignimbrites are (a) generally a high-K calc-alkali suite, well-evolved and fractionated (K/Rb is low, <200) with a high-silica rhyolitic bulk composition, (b) exhibit a combined high content of B, As, F, Li and Pb, with high B/La (>1) and B/K (>0.001) ratios, and (c) a mildly fractionated REE pattern (LaN/YbN∼2) and large positive Eu anomaly (Eu/Eu*∼0.1). Other apparent discriminants involving both compatible and incompatible elements (relative to major silicate phases) are largely a function of different degrees of partial melting and fractionation. It is suggested that the initial source of the B (and other associated elements) was from LIL-rich fluids released by the progressive dehydration of altered oceanic crust and pelagic sediments in a subduction zone. The absence or presence of sediments in a segmented subduction zone may influence the variable lateral distribution of borates in active margins on a global scale. Once the crust has become enriched in B via previous or contemporary subduction-related calc-alkali magmatism, the effect of tectonic environment, climate and hydrothermal activity influence the local development of the deposits.  相似文献   

4.
A combination of high-energy X-ray scattering (HEXS) and X-ray diffraction (XRD) is used to structurally and chemically characterize uranyl-silicate solutions and precipitates. Starting with a U to Si ratio of 1:2, solutions prepared at room temperature from pH 2.2 to 9.0 and at 150 °C from pH 5.1 to 9.1 showed U-U correlations out to distances of 10 Å or longer in both final solutions and precipitates. With one exception, all of the precipitates were amorphous, with no evidence of Bragg diffraction in the XRD data. The room temperature samples above pH 3.1 all had similar Fourier transforms of their HEXS data, which were obtained from suspended slurries or precipitates. In contrast, the hydrothermal sample precipitates showed considerable variation in their HEXS correlations at longer distances. The XRD pattern of the hydrothermal sample with a pH of 5.1 exhibited Bragg reflections indexable as soddyite. While showing no evidence of crystallinity using XRD, the hydrothermal sample at pH 6 showed similar HEXS correlations, which evolved in samples at increasing pH into correlations more consistent with sodium boltwoodite. These findings are supported by thermodynamic modeling. The structural correlations seen in the HEXS data to distances of about 4 Å are similar in all samples prepared at pH 4 or higher. This similarity of structure is used to propose a model for solid formation that includes a uranyl silicate building block, or synthon, which preorganizes in solution. Varying the pH changes how these synthons link into larger structures.  相似文献   

5.
The Rainbow hydrothermal field is located at 36°13.8′N-33°54.15′W at 2300 m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10-15 active chimneys that emit high-temperature (∼365 °C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5-2 mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H2, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH(T,P) (400 °C, 500 bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH(T,P) and dissolved H2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H2 defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite + magnetite + talc ± tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.  相似文献   

6.
Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350 °C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO2 condition, and predict a generation of SiO2-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life.  相似文献   

7.
Understanding the partitioning of aqueous boron species into marine carbonates is critical for constraining the boron isotope system for use as a marine pH proxy. Previous studies have assumed that boron was incorporated into carbonate through the preferential uptake of tetrahedral borate B(OH)4−. In this study we revisit this assumption through a detailed solid state 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of boron speciation in biogenic and hydrothermal carbonates. Our new results contrast with those of the only previous NMR study of carbonates insofar as we observe both trigonal and tetrahedral coordinated boron in almost equal abundances in our biogenic calcite and aragonite samples. In addition, we observe no strict dependency of boron coordination on carbonate crystal structure. These NMR observations coupled with our earlier re-evaluation of the magnitude of boron isotope fractionation between aqueous species suggest that controls on boron isotope composition in marine carbonates, and hence the pH proxy, are more complex that previously suggested.  相似文献   

8.
Origin of garnet in skarn (magmatic vs. hydrothermal) and the prograde skarn fluid evolution are still controversial. Two generations of garnet (Grt1, Grt2) were identified at the Tongshankou deposit: Grt1 is anisotropic with oscillatory zoning and resorbed boundary, whilst Grt2 grew around Grt1 and formed oscillatory rims. In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept 206Pb/238U age of 142.4 ± 2.8 Ma (n = 57; MSWD = 1.16) and 142.3 ± 9.6 Ma (n = 60; MSWD = 1.06), respectively, coeval with the ore formation and ore-related granodiorite emplacement. Positive Eu anomaly, non-CHARAC Y/Ho value and low TiO2 content, together with the mineral assemblages indicate that both Grt1 and Grt2 have a hydrothermal origin. The existence of melt and melt-fluid inclusions in Grt1, together with similar LREE-enriched patterns to the granodiorite, further indicate that Grt1 may have formed in the magmatic-hydrothermal transition. Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO2. The inner gray Grt2 rims (Grt2A) is HREE-enriched with low U contents, indicating that fluid II has nearly neutral pH and high fO2. The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims (Grt2B) show that the evolved magmatic fluid II had mixed with an external fluid, characterized by being mildly acidic pH and with high fO2. Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou, and the mixing of meteoric water may have been prominent in the prograde skarn stage.  相似文献   

9.
A series of hydrothermal experiments covering a range of temperatures from 175 to 260°C examined the decomposition of formic acid and formate and also investigated the production of formate from reduction of CO2. Decomposition rates measured in this study, which were conducted in gold-TiO2 reactors, were several orders of magnitude slower than those reported in previous studies conducted in steel and Ti-metal reactors, indicating the previous studies substantially overestimated the rate of the reaction owing to reactor catalysis. Although experiments were conducted with several different minerals present (hematite, magnetite, serpentinized olivine, NiFe-alloy), the decomposition rates were similar in each experiment once the effects of fluid pH were accounted for, suggesting that the minerals had no effect on the stability of formic acid or formate. At higher temperatures (>225°C), the rates of both the decomposition of formate and the reduction of CO2 to formate were sufficiently rapid that reactions between dissolved CO2 and formate rapidly attained a state of metastable thermodynamic equilibrium. The results suggest that the amount of formate in many subsurface and hydrothermal fluids is likely to be controlled by equilibrium with dissolved CO2 at the prevailing oxidation state and pH of the fluid. This may account for the high concentrations of formate observed in strongly reducing environments such as serpentinites, as well as the low concentrations relative to other organic acid anions in mildly reducing environments such as oil-filed brines and formation waters in sedimentary basins. Although formate has been suggested to be a reaction intermediate in the formation of abiotic hydrocarbons from reduction of aqueous CO2, production of hydrocarbons was not observed in any of the experiments, except for trace amounts of methane, despite high concentrations of formate and strongly reducing conditions.  相似文献   

10.
Numerical computer simulations have been used to gain insight into the evolution of marine hydrothermal systems and the formation conditions of massive sulfide deposits in ancient and modern submarine volcanic terrains. Simulation results have been used to gain a better understanding of the formation of massive sulfide ore deposits, their location, zonation, size, and occurrence in various geotectonic settings.Most hydrothermal fluid discharging at the seafloor exhibits temperatures ranging from 200 °C to about 410 °C and average fluid discharge velocities of 1 to 2 m/s in agreement with seafloor observations. Mass calculations imply that average massive sulfide deposits may form in ~ 5000 years while giant deposits take longer than 5000 years to accumulate; supergiant deposits either need much longer time to form (> 35,000 years) or at least 100 ppm of metal in solution. Results indicate that supergiant deposits may only form in certain geotectonic environments where longevity and preservation potential of the hydrothermal system are high. An additional process (mineral precipitate cap) is proposed here to explain the zinc content of massive sulfide deposits. This cap would prevent the widespread dissolution of anhydrite and the ‘wash-out’ of zinc by subsequent hydrothermal fluid discharge.  相似文献   

11.
Titanite (sphene, CaTiSiO5) is sensitive to changes in temperature, oxygen and water fugacity, and fluid composition. In order to understand formation processes and the nature of hydrothermal fluids, various types of titanite from Cu ores at the Baogutu reduced porphyry Cu deposit were chosen for detailed study. Magmatic titanite is associated with biotite, plagioclase and K-feldspar, whereas hydrothermal titanite occurs with K-feldspar, chlorite, actinolite and calcite. The formation of hydrothermal titanite was related to hydration of igneous minerals under high fH2O, whereas the widespread replacement of ilmenite by titanite (without magnetite) indicates a relatively low oxygen fugacity. Magmatic titanite has low Al, high Fe, Y, Sn, Zr, Nb and REE contents, relative to hydrothermal titanite. On the basis of the Zr-in-titanite and Al-in-chlorite geothermometers, formation temperatures for magmatic and hydrothermal titanite are estimated to be 687–739 °C and 250–670 °C, respectively. The gradual decrease in REE, Y and Sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of REE-bearing minerals. Magmatic and hydrothermal titanites have similar chondrite-normalized REE patterns with negative Eu anomalies and relatively flat HREE. Randomly selected titanites have Nd isotopic compositions similar to the host rocks. Thus, both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source.  相似文献   

12.
硼的地球化学性质及其在俯冲带的循环与成矿初探   总被引：2，自引：1，他引：1  
硼是广泛应用于化工、农业、材料科学及核工业领域的重要元素。硼与氢的核聚变反应是未来具备运用潜力的清洁能源。硼作为典型的亲石元素,是高度不相容元素。硼元素容易富集于蚀变洋壳及蛇纹石化地幔橄榄岩中。而在板块俯冲过程中,由于硼具有强的流体活动性,会优先赋存于流体中。因此,当蛇纹石化的大洋岩石圈及覆于其上的沉积物在俯冲过程中发生脱水,这使得弧前地幔楔发生大规模的蛇纹石化。此时大量硼元素很可能随俯冲流体释放并封存于弧前地幔楔中。目前已发现的超大型硼矿床主要位于聚合型板块边缘,尤其土耳其拥有世界上最大的硼酸盐储量。我们推测这些矿床的形成基础条件之一可能与弧前高度蛇纹石化的地幔楔有关。尤其是在洋 陆俯冲环境,弧前蛇纹岩或蛇绿混杂岩首先通过俯冲侵蚀再循环到火山弧岩浆中,使得岩浆更富集硼。随后弧火山喷发大量富硼的火山岩、岩浆热液及水气。在岩浆冷却过程中,硼元素析出、沉淀于火山表面,并伴随风化、侵蚀过程汇聚至碰撞造山带的封闭湖盆之中。此外,干冷的气候条件下也进一步促进了硼的成矿。我国具有形成大型、超大型硼矿的地质条件,应加大研究及探勘力度,并适当购买硼作为战略储备。  相似文献   

13.
A series of samples from the Hine Hina hydrothermal field (HHF) and the Mariner hydrothermal field (MHF) in the Central and Southern Valu Fa Ridge (VFR), Lau Basin were examined to explain the source origin and formation of the hydrothermal Fe-Si-Mn oxide deposits. The mineralogy was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, and energy-dispersive spectroscopy (EDS). For the Fe-Mn oxide crusts in the HHF, varying amounts of volcanic fragments and some seawater contributions were recognized, along with higher concentrations of Mn, Al, Co, Ni, Zn, Sr, Mo, elevated ∑REE and negative Ce anomalies. In contrast, the Si-rich oxide samples of the MHF were enriched in Cu, Pb and Ba, indicative of proximity to a hydrothermal jet. Moreover, conductive cooling of hydrothermal fluid evoked the Si-rich deposit formation in the MHF. The Sr, Nd and Pb isotope data provided further constraints regarding the source and formation of the Fe-Si-Mn deposits in the VFR by showing that the samples of the HHF are a mixture of three components, namely, hydrothermal fluid, seawater and volcanic materials, whereas the samples of the MHF were dominated by hydrothermal fluids. The seawater had a minor influence on the Nd isotope data, and the Pb isotope data exhibited a close association with the substrate rock and preformed volcaniclastic layers in this area. The occurrence of relatively high Mn/Fe ratios in the hydrothermal deposits of this area may be a good indicator of the propagating activities of the VFR over geological time.  相似文献   

14.
The behaviour of tungsten in granitic melt-vapour systems   总被引：9，自引：0，他引：9  
An experimental study has been carried out to determine the effect of solution composition on the partitioning behaviour of tungsten in granitic melt-vapour systems at 800° C and 1 kbar. With chloride and phosphate solutions, tungsten partitions strongly into the aqueous phase, whereas with fluoride, carbonate and borate solutions, and water alone, tungsten partitions in favour of the melt. With chloride solutions, the fluid/melt partition coefficients (K D) for W show a marked positive correlation with chloride concentration, and suggest that at low chloride concentrations W-Cl complexes with low Cl∶W ratios (such as associated equivalents of (WO3)2C1−) may be present. In contrast, at higher chloride concentrations complexes with high Cl∶W ratios (such as WOCl4, WCl6 and associated ionic equivalents) may predominate. With phosphate solutions, K D shows little variation with phosphate concentration, and phosphorus heteropolytungstates (such as H3[PW12O40]) may be present. There is no evidence to suggest that fluoride, carbonate or borate complexes of tungsten are important under the experimental conditions: the data for these compositions can be interpreted assuming that isopolytungstates (such as H6[H2W12O40]) are present. Within high temperature hydrothermal solutions tungsten may be transported principally as isopolytungstates and heteropolytungstates in addition to chloride complexes, and this may, in part, account for the common association of apatite and arsenopyrite with scheelite and wolframite in tungsten deposits.  相似文献   

15.
Stability and solubility of arsenopyrite, FeAsS, in crustal fluids   总被引：3，自引：0，他引：3  
The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)30(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite.Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H2 and H2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH)30(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed.Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>∼300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits.  相似文献   

16.
Jochen Kolb   《Tectonophysics》2008,446(1-4):1-15
The fabric, mineralogy, geochemistry, and stable isotope systematics of auriferous shear zones in various hydrothermal gold deposits were studied in order to discuss the role of fluids in rock deformation at temperatures between 500 °C and 700 °C. The strong hydrothermal alteration and gold mineralization indicates that effective permeability development goes ahead with high-temperature rock deformation. The economic gold enrichment is often hosted by breccias and quartz veins in the ductile shear zones, which either formed at fast strain rates or by low strain continuous deformation at slow strain rates. Both processes require (1) a close-to lithostatic to supralithostatic fluid pressure and/or (2) a strong rheology contrast of the deformed lithologies that is often developed during progressive hydrothermal alteration. Compartments of high fluid pressure are sealed from the rest of the shear zones by high-temperature deformation mechanisms, e.g. intracrystalline plasticity and diffusion creep, and compaction. In contrast, in mylonites with heterogeneous crystal plastic and brittle deformation mechanisms for the various minerals, an interconnected network of a grain-scale porosity forms an effective fluid conduit, which hampers fluid pressure build-up and the formation of veins.The auriferous shear zones of the various gold mines represent fluid conduits in the deeper crust, 100 m along strike and up to 1000 m down-dip. The hydrothermal fluids infiltrated may be responsible for low magnitude earthquakes in the Earth's lower crust, which otherwise deforms viscously.  相似文献   

17.
The chemical equilibrium model of et al. (1984) has been extended to include borate species. The model is based upon the semi-empirical equations of (1973) and coworkers and is valid to high ionic strength (≈14 m) and high borate concentration. Excellent agreement with the existing emf, isopiestic and solubility data in the system (Na-K-Ca-Mg-H-Cl-SO4-CO2-B(OH)4-H2O) is obtained. Calculated mineral solubilities are in general within 10% of their experimental values, even at high ionic strengths.

The model was applied to the multicomponent, high ionic strength (I ˜ 10) and high borate concentration (BT ˜ 0.5 m) Searles Lake evaporite deposit. Utilizing the chemical composition of the interstitial brine, the model predicts equilibrium between the brine and only those minerals which are known to be in contact with the brine. These calculations clearly demonstrate the applicability of the model to high ionic strength, high borate concentration natural waters.

The model was also utilized to calculate the mineral sequences which should result from evaporation of the major source of water for Searles Lake, the Owens River. The geochemical conditions necessary for the formation of the most recent mud and saline units are examined. The final results indicate that the mineral sequences found in the most recent saline unit in Searles Lake can be produced by evaporation of a water close in composition to present Owens River water, provided primary dolomite formation is delayed and back reaction between the Parting Mud and the Upper Salt is inhibited.  相似文献   


18.
ODP drilling of the active TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20,000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50-m-high pyritic mound and a silicified stockwork complex containing approximately 3.9 million tonnes of sulfide-bearing material with an average of 2.1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1–125 m below the seafloor. The periodic release of high-temperature hydrothermal fluids at the same location for several thousand years with intermittent periods of hydrothermal quiesence is the dominating process in the formation of the TAG hydrothermal mound. Distinct geochemical, mineralogical and isotopic zonation as well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extremely low base and trace element concentrations for the interior of the mound, which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwelling, hot (>350 °C) hydrothermal fluids. Mixing of these fluids with infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of bulk sulfides (+4.4 to +8.2‰δ34S; average +6.5‰) is unusually heavy when compared to other sediment-free mid-ocean ridge deposits and implies the introduction of heavy seawater sulfur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides related to wallrock alteration, and massive as well as late vein sulfides indicates widespread entrainment of seawater deep into the system. Fluid inclusion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 °C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into the mound. Formation temperatures for a central anhydrite-bearing zone range from 340–360 °C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360–369 °C). Fluid inclusions in quartz and anhydrite from the stockwork zone are characterized by formation temperatures higher than 375 °C, indicating conductive cooling of the hydrothermal fluids or mixing with ambient seawater prior to venting. Formation temperatures for quartz from an area of extremely low heat flow at the western side of the mound reach up to 390 °C, implying that this area was once part of a high-temperature hydrothermal upflow zone. The low heat flow and the absence of anhydrite within this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provides evidence for significant changes in the fluid flow regime underneath the deposit between hydrothermal cycles. Received: 16 November 1998 / Accepted: 19 August 1999  相似文献   

19.
The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in δ11B (11-16‰) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of δ11B values (9-10‰), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.  相似文献   

20.
A dark green authigenic nontronite is the major component of the Galapagos hydrothermal mounds field sediments. Oxygen isotopic compositions of the chemically purified, <0.2-μm fraction of the nontronitic clays indicate formation temperatures of 25° to 47°C, in contrast with measured in situ mounds temperatures of up to 15°C. Assuming an authigenic origin, the Fe-rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27° to 39°C, compared with measured in situ temperatures of ca. 3.5° to 6.5°C. The higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the Holocene-Pleistocene. The apparent high isotopic temperature of the Fe-rich montmorillonite may reflect: (1) formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents, (2) a detrital origin of the mineral, or (3) a mixture of authigenic Fe-montmorillonite and detrital Al-montmorillonite in this region.  相似文献   

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