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1.
New data are reported relating to the presence and relative concentrations of organic compounds of medium volatility in samples from the water of hot springs and in the condensate of a steam–water mixture from wells drilled in the Mutnovskii geothermal area and from a well and springs in the Uzon caldera. The Mutnovskii area was found to contain 95 compounds belonging to 16 homologous series, with 71 compounds (12 series) in the Uzon caldera. Among these we found homologous series of biologically important compounds: carbonic acids, alcohols, ethers, aldehydes, ketones, and others. Evidence was obtained for a contribution of the abiogenic component in organic matter from sterile condensates of an overheated steam–water mixture from wells (alkanes, aromatic hydrocarbons, ketones, alcohols, and others) and in thermal water from natural discharges (in particular, alkanes and chlorine-containing hydrocarbons). The results of this study can be used for the preparation and conduction of laboratory experiments in prebiological chemistry under conditions that simulate a hydrothermal environment.  相似文献   

2.
Continental Scientific Drilling Program (CSDP) drill hole VC-2B [total depth 1761.7 m (5780 ft); maximum temperature 295 °C] was continuously cored through the Sulphur Springs hydrothermal system in the western ring-fracture zone of the 1.14 Ma Valles caldera. Among other units, the hole penetrated 760.2 m (2494.1 ft) of Paleozoic carbonate and siliciclastic strata underlying caldera fill and precaldera volcanic and epiclastic rocks. Comparison of the VC-2B Paleozoic rocks with corresponding lithologies within and around the 32.1 Ma Socorro caldera, 192 km ( 119 miles) to the south-southwest, provides insight into the variability of alteration responses to similar caldera-related hydrothermal regimes.The Pennsylvanian Madera Limestone and Sandia Formation from VC-2B preserve many of the sedimentological and diagenetic features observed in these units on a regional basis and where unaffected by high temperatures or hydrothermal activity. Micrites in these formations in VC-2B are generally altered and mineralized only where fractured or brecciated, that is, where hydrothermal solutions could invade carbonate rocks which were otherwise essentially impermeable. Alteration intensity (and correspondingly inferred paleopermeability) is only slightly higher in carbonate packstones and grainstones, low to intermediate in siltstones and claystones, and high in poorly cemented sandstones. Hydrothermal fracture-filling phases in these rocks comprise sericite (and phengite), chlorite, allanite, apatite, an unidentified zeolite and sphene in various combinations, locally with sphalerite, galena, pyrite and chalcopyrite. Terrigenous feldspars and clays are commonly altered to chlorite and seriate, and euhedral anhydrite “porphyroblasts” with minor chlorite occur in Sandia Formation siltstone. Fossils are typically unaltered, but the walls of some colonial bryozoans in the Madera Limestone are altered to the assemblage chlorite-sericite-epidote-allanite. La, Ce and Nd are present in an unidentified hydrothermal mineral occurring throughout much of the VC-2B Pennsylvanian sequence.Carboniferous carbonate and siliciclastic formations within and around the Socorro caldera show a similar style of alteration and mineralization to their Valles caldera counterparts, but by contrast locally host commercial, caldera-related, base-metal sulfide deposits. As in the Valles rocks, mineralization and alteration in those of the Socorro caldera were strongly controlled by porosity. Unless disrupted by fractures, breccias, or karst cavities ( not identified in Valles caldera drill holes), the rocks remained relatively unaltered. Where these features allowed ingress of mineralizing hydrothermal solutions, base-metal sulfides and rare-earth-element-bearing minerals were precipitated.  相似文献   

3.
This paper reports the results of magnetotelluric sounding (MTS) in the area where the Bol’she-Bannyi hydrothermal springs are discharged. The MTS curves were inverted on the assumption of a twodimensional inhomogeneous model using longitudinal and transverse curves of apparent resistivity. It was found that the geoelectric section contains a nearly vertical anomaly of high electrical conductivity at depths of 5.5–8 km, which is the signature of a deep-seated fault. The resulting geoelectric section for the upper crust and data from regional magnetotelluric soundings were used as a basis for developing a conceptual deep model of the Bol’she-Bannyi hydrothermal system. We quote an approximate estimate of rock porosity. According to the model, deep fluids come from a crustal layer into the subvertical deep-seated fault then penetrate via fissures into the sedimentary–volcanogenic cover, and finally arrive at the ground surface in zones of high rock permeability. We provide a recommendation for drilling a deep well in order to determine the potential of the Bol’she-Bannyi hydrothermal field.  相似文献   

4.
The loci and abundance of U and Th were examined in tuffaceous rocks encompassing hydrothermal systems at the Long Valley caldera, California and the Valles caldera, New Mexico. Aspects of these systems may be analogous to conditions expected in a potential site for a high-level waste repository in welded tuff. Examination of radioelements in core from scientific drill holes at these sites was accomplished by gamma-ray spectrometry and fission-track radiography. In the lateral-flowing hydrothermal system at the Long Valley caldera, where temperatures range from 140 to 200 °C, U is concentrated to 20 ppm in Fe-rich zones of varved tuff and to 50 ppm with Fe-rich mineral phases in tuff fragments of a calcite-cemented breccia. U-series disequilibrium in some of these samples suggests mobilization/deposition of parent U and/or its daughters. In the vapor zone of the Valles caldera's hydrothermal system (temperature ˜ 100 °C), the concordance of high U, low Th/U and decreasing whole-rock O-isotope ratios suggests that U was concentrated in response to hydrothermal circulation when the system was formerly liquid-dominated. In the underlying present-day liquid-dominated zone (temperature to 210 °C), U, up to several tens of parts per million, occurs with pyrite and Fe-oxide minerals, and in concentrations to several percents with a Ti-Nb-Y-rare earth mineral. In the Valles system's outflow zone, U is also concentrated in Fe-rich zones as well as in carbonaceous-rich zones in the Paleozoic sedimentary rocks that underlie the Quaternary tuff. Th, associated with accessory minerals, predominates in breccia zones and in a mineralized fault zone near the base of the Paleozoic sedimentary sequence. Relatively high concentrations of U occur in springs representative of water recharging the Valles caldera's hydrothermal system. In contrast, considerably lower U concentrations occur in hot waters (> 220 °C) and in the system's outflow plume, suggesting that U is concentrating in the hotter part of the system. The Long Valley and Valles observations indicate that U and Ra are locally mobile under hydrothermal conditions, and that reducing conditions associated with Fe-rich minerals and carbonaceous material are important factors in the adsorption of U, and thus can retard its transport in water at elevated temperature.  相似文献   

5.
Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.  相似文献   

6.
Plio-Quaternary volcanism played an important role in the present physical state of Eastern Anatolia. Mount Nemrut, situated to the west of Lake Van is one of the main volcanic centers in the region, with a spectacular summit caldera 8.5 × 7 km in diameter. The most recent eruptions of the volcano were in 1441, 1597 and 1692. Nemrut Lake covers the western half of the caldera; it is a deep, half-bowl-shaped lake with a maximum depth of 176 m. Numerous eruption centers are exposed within the caldera as a consequence of magma–water interaction. Current activity of Nemrut caldera is revealed as hot springs, fumaroles and a small, hot lake.Self-potential and bathymetric surveys carried out in the caldera were used to characterize the structure of the caldera and the associated hydrothermal fluid circulation. In addition, analyses based on digital elevation models and satellite imagery were used to improve our knowledge about the structure of the caldera. According to SP results, the flanks of the volcano represent “the hydrogeologic zone”, whereas the intra-caldera region is an “active hydrothermal area” where the fluid circulation is controlled by structural discontinuities. There is also a northern fissure zone which exhibits hydrothermal signatures. Nemrut caldera collapsed piecemeal, with three main blocks. Stress controlling the collapse mechanism seems to be highly affected by the regional neotectonic regime. In addition to the historical activity, current hydrothermal and hydrogeologic conditions in the caldera, in which there is a large lake and shallow water table, increase the risk of the quiescent volcano.  相似文献   

7.
Magnetotelluric and differential geomagnetic sounding surveys, consisting of nine soundings, were performed in 1984 along a 200-km profile across the southern Senegal basin. They were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents. Magnetotelluric data indicate that two-dimensional resistivity models are appropriate for the region. The zone above the basement is highly inhomogeneous in geoelectrical structure. Very conductive sediments (0.6-3 ohm m) appear in the Mesozoic-Cenozoic Senegal basin. These sediments lie at depths of up to 4500 m on the west end of the profile. Below this, a modest resistivity material (10–30 ohm m) extends to a maximum depth of about 3000 m. The material at depth on the cast part of the traverse line is thought to be Palaeozoic sediments of the Bove basin. The depth of the magnetotelluric basement lies between about 250 m (in the east) and 4800 m (in the west). The crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile. Considering the total section, we observe a general trend towards lower resistivities at depths in excess of 100 km, the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km. An analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin. It appears that the additional currents flowing in the striking direction of the Senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km. Model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well-conducting sediments.  相似文献   

8.
The Hakata hot springs area is located in Fukuoka City, which is in the southwestern part of Japan. Gamma-ray and gravity surveys were conducted to understand the relationship between the low-temperature hydrothermal systems and geophysical data of the area. The depth of the reservoir basement, which was derived from gravity data, gradually deepens toward the east; it includes some steep depth gradients in the Hakata hot springs area. High intensities of gamma-rays were detected around these gradients. In addition, higher hot spring temperatures and flow rates can be observed in this area. These results indicate that some part of the level of the basement where the hot springs are concentrated is a part of the Kego Fault and is similar to the fracture zone created by past activities of the fault. Moreover, these steep depth gradients act as a path for hot spring water from the deeper side of the granitic body to the surface.  相似文献   

9.
Active thermal springs associated with the late Pleistocene Calabozos caldera complex occur in two groups: the Colorado group which issues along structures related to caldera collapse and resurgence, and the Puesto Calabozos group, a nearby cluster that is chemically distinct and probably unrelated to the Colorado springs. Most of the Colorado group can be related to a hypothetical parent water containing ∼400 ppm Cl at ∼250°C by dilution with ≥50% of cold meteoric water. The thermal springs in the most deeply eroded part of the caldera were derived from the same parent water by boiling.The hydrothermal system has probably been active for at least as long as 300,000 years, based on geologic evidence and calculations of paleo-heat flow. There is no evidence for economic mineralization at shallow depth. The Calabozos hydrothermal system would be an attractive geothermal prospect were its location not so remote.  相似文献   

10.
The results of the magnetotelluric (MT) soundings before and after an earthquake are analyzed. The interpretation is based on the longitudinal and transverse MT curves (along and across the strike of the main tectonic elements, respectively). The MT curves are distorted by the ρ- and coast effect. The distortions due to the coast effect are estimated by the testing three-dimensional (3D) model. It is established that the coast effect distortion at the periods up to 1000 s is small and can be disregarded. The divergence of the longitudinal and transverse MT curves, which points to the presence of the deep faults, is thoroughly studied. The inversion of the MTsounding curves is carried out by the REBOCC program of the numerical two-dimensional modeling. This program implements the procedures of elimination of the ρ-effect and the joint inversion of the longitudinal and transverse MT sounding curves. The obtained geoelectrical cross sections provide an insight into the structure of electrical conductivity of the lithosphere before and after the earthquake. The more intense variations in the electric conductivity are observed in the zone of the deep faults. These variations are related to the changes in the porosity and saturation of the rocks by the highly mineralized fluids.  相似文献   

11.
One of the seven potentially active andesite stratovolcanoes in southern Peru, Misti (5822 m), located 17 km northeast and 3.5 km above Arequipa, represents a major threat to the population (900,000 inhabitants). Our recent geophysical and geochemical research comprises an extensive self-potential (SP) data set, an audio–magnetotelluric (AMT) profile across the volcano and CO2 concentrations in the soil along a radial profile. The SP survey is the first of its kind in providing a complete mapping of a large andesitic stratovolcano 20 km in diameter. The SP mapping enables us to analyze the SP signature associated with a subduction-related active volcano.The general SP pattern of Misti is similar to that of most volcanoes with a hydrogeologic zone in the lower flanks and a hydrothermal zone in the upper central area. A quasi-systematic relationship exists between SP and elevation. Zones with constant SP/altitude gradients (Ce) are observed in both hydrogeologic (negative Ce) and hydrothermal (positive Ce) zones. Transition zones between the different Ce zones, which form a concentric pattern around the summit, have been interpreted in terms of lateral heterogeneities in the lithology. The highest amplitudes of SP anomalies seem to coincide with highly resistive zones. The hydrothermal system 6 km in diameter, which extends over an area much larger than the summit caldera, may be constrained by an older, concealed collapse caldera. A sealed zone has apparently developed through alteration in the hydrothermal system, blocking the migration of CO2 upward. Significant CO2 emanations are thus observed on the lower flanks but are absent above the hydrothermal zone.  相似文献   

12.
Uzon caldera, located in the eastern volcanic belt of the Kamchatka peninsula, is a complicated structure of Middle Pleistocene age. The composition of the co-existing solid and fluid phases, temperature and pH were determined with the aim of establishing the distribution of sulphur species, As, Sb and the main ore-forming metals. In the solid samples, the following sulphur-bearing minerals were identified: pyrite, realgar, orpiment, alacranite (As8S9), uzonite (As4S5), amorphous As-sulphide, stibnite, cinnabar and native sulphur. The following sulphur-bearing species H2S, H2S2+S52−(aq)(aqueous polysulphanes), S0(aq), SO32−(aq), S2O32−, SO42− and total concentration of sulphur were determined in solutions. Eh, pH and H2S concentration were measured potentiometrically in situ. Zero-valent sulphur (S0(aq)+H2S2+S52−(aq)) predominates in Uzon solutions. The pair H2S–Scolloidal is Eh-determining in Uzon solutions up to 75–85°C. A quantitative thermodynamic model of the mineral deposition process at Uzon was constructed using the collected data. It was obtained that the composition of the hydrothermal solution and the precipitation of Sb–As–Hg species can be described using two only main factors: the initial composition of fluid and the temperature variation.  相似文献   

13.
The structure of Cerdanya Basin (north-east of Iberian Peninsula) is partly known from geological cross sections, geological maps and vintage geophysical data. However, these data do not have the necessary resolution to characterize some parts of Cerdanya Basin such as the thickness of soft soil, geometry of bedrock or geometry of geological units and associated faults. For all these reasons, the main objective of this work is to improve this deficiency carrying out a detailed study in this Neogene basin applying jointly the combination of passive seismic methods (H/V spectral ratio and seismic array) and electromagnetic methods (audio-magnetotelluric and magnetotelluric method). The passive seismic techniques provide valuable information of geometry of basement along the profile. The maximum depth is located near Alp village with a bedrock depth of 500 m. The bedrock is located in surface at both sites of profile. The Neogene sediments present a shear-wave velocity between 400 and 1000 m/s, and the bedrock basement presents a shear-wave velocity values between 1700 and 2200 m/s. These results are used as a priori information to create a 2D resistivity initial model which constraints the inversion process of electromagnetic data. We have obtained a 2D resistivity model which is characterized by (1) a heterogeneous conductivity zone (<40 Ohm m) that corresponds to shallow part of the model up to 500 m depth in the centre of the profile. These values have been associated with Quaternary and Neogene sediments formed by silts, clays, conglomerates, sandstones and gravels, and (2) a deeper resistive zone (1000–3000 Ohm m) interpreted as Palaeozoic basement (sandstones, limestones and slates at NW and conglomerates and microconglomerates at SE). The resistive zone is truncated by a discontinuity at the south-east of the profile which is interpreted as the Alp-La Tet Fault. This discontinuity is represented by a more conductive zone (600 Ohm m approx.) and is explained as a combination of fractured rock and a fluid network. The result highlights that the support between different geophysical methods is essential in producing geophysical meaningful models.  相似文献   

14.
The article discusses the results of the interpretation of magnetotelluric sounding (MTS) data generated in the vicinity of the Mutnov steam hydrothermal deposit during the period from 2004 to 2006. Approaches to an interpretation of magnetotelluric (MT) data in a situation characterized by significant 3D distortions and the coastal effect phenomenon are discussed. The resulting 3D geoelectric model is presented. Based on analysis of the created geoelectric model, the principal geoelectric horizons are characterized, which includes the parameters of the integral conductivity and the morphology of the geoelectric boundaries. Zones of the highest values of the electrical conductivity of the watered tuffaceous sediment horizon have been identified, as well as zones of the maximum lateral electrical conductivity contrasts that are interpreted as discharge zones of hydrotherms. The results of the drilling carried out based on MTS data are presented.  相似文献   

15.
The results of ten tensor magnetotelluric soundings recorded within the Charlevoix crater are presented. The geological complexity of the region (the crater is located at the junction of two major geological provinces, i.e. the crystalline basement of the Canadian shield and the more conductive sediments of the Appalachian system) and the amount of cultural noise present in our data prevent a highly definitive interpretation of the shallow structure. A subhorizontal conductive fault related to Rondot's (1971) structural sketch of the original impact structure may exist within the upper 5 km, but it is not completely constrained by the data. Pseudo 3D modelling using stacked thin sheets indicates that superficial patches of high conductivity may explain the large anisotropy observed at the long periods and suggests that our deeper structures fit well with the 2D model previously proposed by Kurtz (1982) for the region.  相似文献   

16.
Recent seismic and tectonic activity in Rabaul Caldera, Papua New Guinea, suggests that magma is accumulating at a shallow depth beneath this partially submerged structure and that a new volcano may be developing. Changes in onshore elevation since 1971 (as much as 2 m on south Matupit Island) indicate that rapid and large-scale uplifts have occurred on the seafloor near the center of the caldera. The frequency of seismic events within the caldera has also increased during this period. Earthquake locations define an elliptical ring surrounding the center of this uplift within the caldera.A marine geophysical survey in 1982 by the U.S. Geological Survey's R/V “S.P. Lee” in Rabaul Caldera shows the development of a bulge in the seafloor near the center of the caldera. High-resolution seismic reflection profiles show that this bulge consists of two domal uplifts bounded and separated by two major north-south-trending fault zones. Deformed sediments overlie these zones; a prominent slump flanks the area of the bulge.Five major acoustic units were identified in the seismic reflection profiles: an acoustic basement and four sedimentary units consisting of irregularly layered, cross-layered, contorted, and well-layered sequences. The acoustic basement is probably composed of crystalline volcanic rocks, and the layered acoustic units are probably sediments, primarily ash deposited in different environments. The cross-layered, irregularly layered, and contorted units appear to have been deposited in a dynamic environment subjected to strong currents, seismicity, and/or mass wasting, while the well-layered units were deposited in a low-energy environment. Locally, well-layered sequences interfinger with the other sedimentary units, indicating a transitional environment that alternated between high-energy and low-energy depositional processes.A submarine channel cuts most of the acoustic units and appears to be the conduit for sediment transport out of the caldera; it occupies an older buried channel north of the caldera that is presently being exhumed. In the south, active erosion of well-layered sediments is taking place. What are believed to be several young volcanic cones also disrupt the depositional layers.We conclude that the bulge in the seafloor and the associated fault zones are a result of emplacement of magma at a shallow depth. Contorted sediment and slumps adjacent to the bulge are probably the result of uplift and seismic activity. The pattern of seismicity appears to reflect increased magma pressure at depth beneath the caldera floor. This activity may eventually lead to an eruption.  相似文献   

17.
This paper considers the geoecological impacts of eruptions on Karymskii Volcano and the Tokareva crater for the 1996–2008 period, which resulted in changes in (a) the relief around these edifices, (b) the discharge and composition of water in the Karymskii River and other streams in the area, and (c) the discharge and composition of gases in thermal springs. It was found that the concentration of CH4 previously had been abnormally high in free gases that emanate from the new Piipovskii Springs and an explanation is provided of the decrease in their concentration over time. We detected variations in the radon activity, OARn (Bq/m3), in free gases that are released in the Karymskii caldera hydrothermal occurrences; the variations are consistent with those in the eruptive activity of Karymskii Volcano in 2005–2006. We describe permafrost rocks in the Karymskii caldera that favor the generation of a cryolithic zone.  相似文献   

18.
Detailed total-intensity aeromagnetic surveys of the Kuttyaro and Aso caldera regions, eastern Hokkaido and central Kyushu, were made during early 1964 under the auspices of the U.S.-Japan Co-operative Science Program in conjunction with a project for geophysical studies of calderas in Japan. Each caldera has a maximum diameter of about 22 km; the flights cover a 60 × 60 km rectangular area in each region. The Kuttyaro survey also encompasses the older caldera Akan, south-west of Kuttyaro, and the younger caldera Mashu to the east. All three lie within the Chīshīma (Kurile) volcanic zone. The isomagnetic contour map shows this zone as a belt of short wave-length anomaies which trends east-northeast across the region. Broad wavelength anomalies with trends intersecting the Chīshīma belt at an acute angle probably reflect structural relief on the Neogene volcanic basement concealed beneath Kuttyaro pyroclastic flows. The centre of Kuttyaro caldera coincides with the sharp southern termination of a strong basement high, whereas caldera faults and post-caldera domes have little magnetic expression. Mashu caldera is marked by a minimum in the position of the caldera lake; a symmetrical positive anomaly centering southeast of the caldera suggests either a buried older volcanic edifice or an intrusion. Akan caldera is represented by a magnetic depression encompassing a positive anomaly produced by its central post-caldera cone. The depression extends north of the geologically-deduced boundary of the caldera and may include an earlier collapse structure. Several volcanoes and lava sequences in the region produce negative anomalies due to inverse polarization. The most significant feature of the Aso isomagnetic map is a large, elongate positive anomaly that occupies the southern half of the caldera and extends about one caldera diameter to the south-west along the trend of the Median Tectonic Line of south-west Japan. Whether the anomaly represents the pre-Tertiary basement complex or a younger intrusion perhaps associated with Aso eruptive activity is uncertain. However, the causative body is abruptly truncated within the caldera by a major east-south-east structure passing through the eastern rim and coincident with the approximate locus of resurgent central vent eruptions. The structure may be a fault system that provided egress for the Aso pyroclastic flows. Superimposed on the basement anomaly are the effects of the topography of the caldera, the superficial caldera structure, and the post-caldera cones. An area of intense solfataric activity in the Kuju group of young volcanoes north of Aso has a pronounced negative anomaly. These two surveys illustrate the utility of the magnetic method for investigations of basement structure in caldera regions. They have served as a guide in interpreting reconnaissance aeromagnetic profiles flown concurrently for this project across some 14 other calderas or caldera-like structures in the Japanese islands.  相似文献   

19.
 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO4/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO4/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998  相似文献   

20.
We carried out a magnetotelluric field campaign in the South–East Lower Saxony Basin, Germany, with the main goal of testing this method for imaging regional Posidonia black shale sediments. Two‐dimensional inversion results of the magnetotelluric data show a series of conductive structures correlating with brine‐saturated sediments but also with deeper, anthracitic Westphalian/Namurian coals. None of these structures can be directly related with the Posidonia black shale, which appears to be generally resistive and therefore difficult to resolve with the magnetotelluric method. This assumption is supported by measurements of electrical resistivity on a set of Posidonia shale samples from the Hils syncline in the Lower Saxony basin. These rock samples were collected in shallow boreholes and show immature (0.53% Ro), oil (0.88% Ro), and gas (1.45% Ro) window thermal maturities. None of the black shale samples showed low electrical resistivity, particularly those with oil window maturity show resistivity exceeding 104 Ωm. Moreover, we could not observe a direct correlation between maturity and electrical resistivity; the Harderode samples showed the highest resistivity, whereas the Haddessen samples showed the lowest. A similar trend has been seen for coals in different states of thermal maturation. Saturation of the samples with distilled and saline water solutions led to decreasing electrical resistivity. Moreover, a positive correlation of electrical resistivity with porosity is observed for the Wickensen and Harderode samples, which suggests that the electrical resistivity of the Posidonia black shale is mainly controlled by porosity.  相似文献   

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