首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km3 Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures.  相似文献   

2.
The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ?Dclay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C.  相似文献   

3.
The oxygen minimum zones (OMZs) are recognized as intense sources of N2O greenhouse gas (GHG) and could also be potential sources of CO2, the most important GHG for the present climate change. This study evaluates, for one of the most intense and shallow OMZ, the Chilean East South Pacific OMZ, the simultaneous N2O and CO2 fluxes at the air–sea interface. Four cruises (2000–2002) and 1 year of monitoring (21°–30°–36°S) off Chile allowed the determination of the CO2 and N2O concentrations at the sea surface and the analysis of fluxes variations associated with different OMZ configurations. The Chilean OMZ area can be an intense GHG oceanic local source of both N2O and CO2. The mean N2O fluxes are 5–10 times higher than the maximal previous historical source in an OMZ open area as in the Pacific and Indian Oceans. For CO2, the mean fluxes are also positive and correspond to very high oceanic sources. Even if different coupling and decoupling between N2O and CO2 are observed along the Chilean OMZ, 65% of the situations represent high CO2 and/or N2O sources. The high GHG sources are associated with coastal upwelling transport of OMZ waters rich in N2O and probably also in CO2, located at a shallow depth. The integrated OMZ role on GHG should be better considered to improve our understanding of the past and future atmospheric CO2 and N2O evolutions.  相似文献   

4.
Fracturing and hydrothermal alteration in normal fault zones   总被引：9，自引：0，他引：9  
Large normal fault zones are characterized by intense fracturing and hydrothermal alteration. Displacement is localized in a slip zone of cataclasite, breccia and phyllonite surrounding corrugated and striated fault surfaces. Slip zone rock grades into fractured, but less comminuted and hydrothermally altered rock in the transition zone, which in turn grades abruptly into the wall rock. Fracturing and fluid flow is episodic, because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes.Fracture networks are described by a fracture fabric tensor (F). The permeability tensor (k) is used to estimate fluid transport properties if the trace of F is sufficiently large. Variations in elastic moduli and seismic velocities between fault zone and wall rock are estimated as a function of fracture density (). Fracturing decreases elastic moduli in the transition zone by 50–100% relative to the country rock, and similar or even greater changes presumably occur in the slip zone.P-andS-wave velocity decrease, andV p /V s increases in the fault zone relative to the wall rock. Fracture permeability is highly variable, ranging between 10–13 m2 and 10–19 m2 at depths near 10 km. Changes in permeability arise from variations in effective stress and fracture sealing and healing.Hydrothermal alteration of quartzo-feldspathic rock atT>300°C creates mica, chlorite, epidote and alters the quartz content. Alteration changes elastic moduli, but the changes are much less than those caused by fracturing.P-andS-wave velocities also decrease in the hydrothermally altered fault rock relative to the country rock, and there is a slight decrease inV p /V s , which partially offsets the increase inV p /V s caused by fracturing.Fracturing and hydrothermal alteration affect fault mechanics. Low modulus rock surrounding fault surfaces increases the probability of exceeding the critical slip distance required for the onset of unstable slip during rupture initiation. Boundaries between low modulus fault rock and higher modulus wall rock also act as rupture guides and enhance rupture acceleration to dynamic velocity. Hydrothermal alteration at temperatures in excess of 300°C weakens the deeper parts of the fault zone by producingphyllitic mineral assemblages. Sealing of fracture in time periods between large earthquakes generates pods of abnormally pressured fluid which may play a fundamental role in the initiation of large earthquakes.  相似文献   

5.
The influence of humic substances (HS) on the biodegradation of naphthalene, phenanthrene, and pyrene was studied. As a source of HS, water samples of a bog lake (Hohlohsee) were used. PAH degradation experiments, both in the presence and absence of HS were carried out. All investigated PAHs were degradable by the used bacterial mixed culture. A correlation between the number of aromatic rings of the PAHs and the influence of HS on biodegradation has been shown. Adding of HS led to a decrease in degradation rate in the case of naphthalene. By way of contrast, the presence of HS gave rise to an increase of degradation rate in the case of phenanthrene. The degradation processes of pyrene revealed a marked alteration in the presence of HS which could be deduced from the biochemical oxygen demand (t(1/2BODmax) values). With regard to the total turnover of the PAHs, there was no noticeable difference between degradation experiments with and without HS. Analyses of the HS after degradation experiments using liquid chromatography coupled with DOC detection (LC/OCD) verify that there was no formation of stable associations between HS and PAHs or their metabolites. The determination of the toxicity of the degradation media as luminescence inhibition against Vibrio fischeri (Photobacterium phosphoreum) showed no detoxification as a result of the presence of HS.  相似文献   

6.
Hydrothermal systems in regions of high relief may not have obvious thermal features indicative of their central upflow zone at depth. Investigation of such areas for geothermal energy production in the Philippines has, however, encountered relatively large ( > 10,000 m2) areas of volumetrically significant but diffuse, cold-gas emission and associated intense argillic alteration. These features are likely to be best developed above, or close to, hydrothermal upflow zones. The identification of such features is important for interpreting the hydrology of active geothermal systems in the course of exploration for power development, or when interpreting the paleohydrology of fossil geothermal systems which host epithermal mineral deposits. Such zones of argillic alteration are likely to be barren of precious metals or other elements (except mercury) which are commonly used as pathfinders for precious-metal deposits, but are indicative of subsurface processes significant in ore genesis.It is proposed that the term “kaipohan” be used for such features. Their existence depends on the presence of either a low-permeability geologic formation or where a suitable relationship exists between the hydrology of the deep geothermal reservoir and that of the shallow groundwater system. Such a situation occurs in steep terrain, where much of the area supplying meteoric recharge to the geothermal system is at a lower elevation than the area overlying the centre of the system, and where there is a high annual rainfall. These conditions are typical of andesitic stratovolcanoes in tropical or subtropical island arcs.  相似文献   

7.
A thick sequence of alkaline intrusions and volcanic rocks underlies a Quaternary caldera on Lihir Island. The sequence is host to a still-active subaerial hydrothermal system and associated epithermal gold mineralization. Chondrite-normalized (La/Lu)cn and (La/Sm)cn ratios progressively increase up the alteration sequence from the potassic alteration zone, to the argillic zone to the advanced argillic zone. (Tb/Lu)cn ratios only significantly increase in argillic and advanced argillic assemblages. Surface oxide alteration lithologies and acid sulphate water precipitates possess distinctly lower (La/Lu)cn and (Tb/Lu)cn ratios than the underlying subsurface alteration units. The changes in the REE, LREE and HREE fractionation trends from subsurface to surface alteration zones reflect the transition from a magmatic-hydrothermal, neutral chloride fluid regime at depth to acid sulphate meteoric waters in the upper portion of the alteration profile.Boiling of the LREE-Eu-enriched magmatic fluids occurred at a depth of at least 750 m. It is proposed that pronounced differential flow rates of the vapour and liquid phases and solution chemistry changes approximately above 300–350 m caused the incorporation of LREE and Eu into anhydrite-calcite veins and the deposition of LREE and HREE into wallrocks of upper parts of the potassic alteration unit. Condensation of the vapour phases into meteoric waters gave rise to low-temperature acid fluids that deposited large amounts of LREE within argillic and advanced argillic alteration units. This was also accompanied by HREE mobility due to large fluid volumes, acid fluid conditions and abundant sulphate complexes within the solutions. The HREE were either lost from the hydrothermal system or deposited in oxide assemblages and acid sulphate water precipitates.  相似文献   

8.
A combination of field mapping, geochemistry, and remote sensing methods has been employed to determine the extent of hydrothermal alteration and assess the potential for failure at the Santiaguito lava dome complex, Guatemala. The 90-year-old complex of four lava domes has only experienced relatively small and infrequent dome collapses in the past, which were associated with lava extrusion. However, existing evidence of an active hydrothermal system coupled with intense seasonal precipitation also presents ideal conditions for instability related to weakened clay-rich edifice rocks. Mapping of the Santiaguito dome complex identified structural features related to dome growth dynamics, potential areas of weakness related to erosion, and locations of fumarole fields. X-ray diffraction and backscattered electron images taken with scanning electron microscopy of dacite and ash samples collected from around fumaroles revealed only minor clay films, and little evidence of alteration. Mineral mapping using ASTER and Hyperion satellite images, however, suggest low-temperature (<150 °C) silicic alteration on erosional surfaces of the domes, but not the type of pervasive acid-sulfate alteration implicated in collapses of other altered edifices. To evaluate the possibility of internal alteration, we re-examined existing aqueous geochemical data from dome-fed hot springs. The data indicate significant water–rock interaction, but the Na–Mg–K geoindicator suggests only a short water residence time, and δ18O/δD ratios show only minor shifts from the meteoric water line with little precipitation of secondary (alteration) minerals. Based on available data, hydrothermal alteration on the dome complex appears to be restricted to surficial deposits of hydrous silica, but the study has highlighted, importantly, that the 1902 eruption crater headwall of Santa María does show more advanced argillic alteration. We also cannot rule out the possibility of advanced alteration within the dome complex interior that is not accessible to the methods used here. It may therefore be prudent to employ geophysical methods to make further assessments in the future.  相似文献   

9.
Field, hand specimen, and microscopic investigations alongside X-ray diffraction analyses revealed four types of hydrothermal alteration (Type-A, -B, -C, and -D) based on the mode of occurrence of altered rocks and alteration mineral assemblage at Hakusui-kyo and Horai-kyo along the Arima-Takatsuki Tectonic Line (ATTL) in western Japan. Type-A alteration locally occurred as gray alteration halos with sulfide minerals. Type-B and -C alterations were confined to fault gouge veins and occurred as greenish-gray veins and brown veins, respectively. Type-C alteration crosscut Type-B alteration. These alterations were associated with a number of granitic fragments including cohesive breccia and micrographic facies. Type-D alteration occurred locally in brown sediments. Different mineralogical features in the four alterations are summarized as (Type-A) illite; (Type-B) chlorite; (Type-C) limonite (Fe3+ hydroxides and goethite) and calcite; and (Type-D) limonite. We propose that the alterations can be broadly divided into Paleocene hydrothermal alteration (Type-A) and post-Late Miocene hydrothermal alteration (Type-B, -C, and -D): Type-A alteration occurred at approximately 200 °C during hydrothermal activity after a granitic intrusion in Late Cretaceous; Type-B, -C and -D alterations occurred under hydrothermal activity accompanying deep fluids with repeated ascents invoked by the seismicity of the ATTL after the Late Miocene. The fluids may have been the “Arima-type thermal waters” (i.e., mixtures of convective groundwater and Na-Ca-Cl-HCO3-type fluids). Type-B alteration occurred in fractures at depths where the temperature was ≥150 °C. Type-C alteration overprinted Type-B alteration as a result of mixing of new deep fluids and descending oxidized meteoric water near the surface. Fe3+ hydroxides and calcite precipitated from the fluids due to the oxidation of Fe2+ and the degassing of CO2, respectively, at ambient to near-boiling temperatures. When the ascending fluids gushed out from the fractures, they generated Type-D alteration at the surface under similar temperature conditions due to the oxidation of Fe2+.  相似文献   

10.
A two-year chemical monitoring program of Ruapehu Crater Lake shows that it has evolved considerably since the volcano's more active eruptive periods in the early 1970s. The present pH (20°C) of 0.6 is about one half unit more acid than the baseline values in the 1970s, whereas S/Cl ratios have increased markedly owing in part to declining HCl inputs into the lake, but also to absolute increases in SO4 levels which now stand at the highest values ever recorded. Increases in K/Mg and Na/Mg ratios over the 20-year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite. These observations, combined with results of a recent heat budget analysis of the lake, have led to the development of hydrothermal convection model for the upper portion of the vent. Possible vent/lake chemical reaction processes between end member reactants have been modelled with the computer code CHILLER. The results are consistent with the view that variations in lake chemistry, which are initiated by the introduction of fresh magmatic material into the vent, reflect the extent of dissolution reaction progress on the magmatic material and/or its alteration products. The results also provide insights into the role of such vent processes in the formation of high sulfidation-type ore deposits.  相似文献   

11.
This work reports a study on the rare-earth element (REE) behaviour in the high-sulfidation-type superficial alteration and in the phyllic and propylitic alteration in the subsurface of the island of Vulcano. The results of this study demonstrate notable differences in the REE behaviour in the different alteration facies. In silicic alteration, all REE are equally strongly depleted; in advanced argillic alteration, LREE are quite immobile whereas HREE are strongly depleted; in intermediate argillic, phyllic and propylitic alteration, REE patterns are almost unchanged compared with their fresh rock equivalents. These features indicate that the behaviour of REE in hydrothermal alteration facies at La Fossa is mainly controlled by pH, availability of complexing ions in the fluid (mainly SO42− and, in the silicic facies, F−) and the presence of secondary minerals able to host REE in their structures. The origin of the acid fluids is related to the hypogenic introduction of prevalently magmatic acid gases produced by the degassing of the shallow magmatic system of La Fossa volcano. In the active high-temperature fumarolic field of La Fossa, REE behaviour illustrates the transition from a dry vapor system, corresponding to the high-temperature fumaroles, to a wet condensing system in the surrounding area. The results of this research, carried out on an active high-sulfidation system, provide a useful tool for the study of high-sulfidation epithermal ore deposits and particularly help to understand the alteration processes during the main stage of hypogene wall rock alteration.  相似文献   

12.
Hundreds of small (diameter 2–50 m) hydrothermal explosion craters are dispersed across the top plateau of a hyaloclastite ridge in central Iceland. The craters are undisturbed by erosion and must be of Recent age. The ridge, Dyngjufjöll Ytri, is a tectonic horst, separated from the Dyngjufjöll volcanic center and the Askja caldera by a narrow graben. The ridge is 20 km by 6 km with a flat top partly covered with glacial sediments and air-fall tephra. It is composed of two major volcanic units, a lower hyaloclastite flow and an upper complex series containing pillow lavas, air-fall and surge tephra deposits and water lain sediments. Large (max. 9 m diameter) cylindrical gas pipes, coated with calcite, are locally exposed in the hyaloclastite flow. Evidence suggests that volcanism contributing to the formation of the ridge was long extinct when its surface was broken by the hydrothermal explosions. The elevated position of the ridge and its narrow form make unlikely the existence of a long-lived hydrothermal system. This is also borne out by the absence of intense thermal alteration of the material blown out in the explosions. The ridge is surrounded by postglacial lavas. One of these lavas issued from a fissure with the same strike as the ridge that terminates in a small crater at the base of its southern slope. It is suggested that the fissure continues as an intrusion into the basal hyaloclastite flow where water in gas pipes of the hyaloclastite was converted into high-pressure steam that exploded through the overlying unconsolidated formations.  相似文献   

13.
Chemical interaction of thermal fluids with reservoir rock in the Roosevelt Hot Springs thermal area, Utah, has resulted in the development of characteristic trace-element dispersion patterns. Multielement analyses of surface rock samples, soil samples and drill cuttings from deep exploration wells provide a three-dimensional perspective of chemical redistribution within this structurally-controlled hot-water geothermal system.Five distinctive elemental suites of chemical enrichment are recognized, each characteristic of a particular combination of physical and chemical conditions within the geothermal system. These are: (1) concentrations of As, Sb, Be, and Hg associated with siliceous material at locations of liquid discharge, fluid mixing or boiling; (2) concentrations of Mn, Ba, W, Be, Cu, Co, As, Sb and Hg in manganese and iron oxide deposits; (3) high concentrations of Hg in argillized rock near fumaroles and lower concentrations in a broad diffuse halo surrounding the thermal center; (4) concentrations of As in sulfides and Li in silicate alteration minerals immediately surrounding high-temperature fluid flow-controlling fractures; (5) deposits of CaCO3 at depth where flashing of brine to steam has occurred due to pressure release. The geochemical enrichments are not, in general, widespread, pervasively developed zones of regular form and dimension as are typical in many ore-forming hydrothermal systems.As the geothermal system develops, changes and eventually declines through time, the chemical deposits are developed, remobilized or superimposed upon each other, thus preserving within the rocks a record of the history of the geothermal system. Recognition of trace-element distribution patterns during the exploration of a geothermal system may aid definition of the present geometry and interpretation of the history of the system.  相似文献   

14.
Mount Erebus, Antarctica, is a large (3794 m) alkaline open-conduit stratovolcano that hosts a vigorously convecting and persistently degassing lake of anorthoclase phonolite magma. The composition of the lake was investigated by analyzing glass and mineral compositions in lava bombs erupted between 1972 and 2004. Matrix glass, titanomagnetite, olivine, clinopyroxene, and fluor-apatite compositions are invariant and show that the magmatic temperature (∼ 1000°C) and oxygen fugacity (ΔlogFMQ = − 0.9) have been stable. Large temperature variations at the lake surface (~ 400–500°C) are not reflected in mineral compositions. Anorthoclase phenocrysts up to 10 cm in length feature a restricted compositional range (An10.3–22.9Ab62.8–68.1Or11.4–27.2) with complex textural and compositional zoning. Anorthoclase textures and compositions indicate crystallization occurs at low degrees of effective undercooling. We propose shallow water exsolution causes crystallization and shallow convection cycles the anorthoclase crystals through many episodes of growth resulting in their exceptional size. Minor variations in eruptive activity from 1972 to 2004 are decoupled from magma compositions. The variations probably relate to changes in conduit geometry within the volcano and/or variable input of CO2-rich volatiles into the upper-level magma chamber from deeper in the system.  相似文献   

15.
Epidote and/or chlorite are common minerals in the roots of the fossil geothermal system of Saint Martin (Lesser Antilles). They appear in four distinct assemblages: (1) epidote+actinolite+quartz±magnetite near the contact between the tuffaceous host rocks (andesitic modal composition) and the quartz-diorite intrusion of Philipsburg; (2) epidote+chlorite+quartz in host rocks as far as a lateral distance of about 3 km from the intrusion; (3) epidote+chlorite+haematite+quartz locally in iron and manganese rich host rocks; (4) chlorite±phengite±magnetite appearing as late sealing of porosity in fracture-controlled quartz veins with strongly phengitized wall rocks. All these assemblages constitute a large alteration grading from propylitic alteration to thermal metamorphism (actinolite-bearing assemblage).Detailed microprobe studies of epidotes replacing plagioclases and of chlorites replacing glass and mafic minerals reveal notable compositional variations which have been studied with respect to temperature paleogradients (estimated from fluid inclusions study), distance from the thermal source and fo2 conditions. The mean Ps+Pm [100 × (Fe3+ + Mn3+)/(Al3+ + Fe3+ + Mn3+)] of epidotes vary from 21 in the presence of magnetite near the intrusion to 32 in haematite-bearing iron and/or manganese volcanic and sedimentary formations. The intra-grain chemical scattering of epidotes increases with increasing distance of the pluton and decreasing temperature of crystallization. All the chlorites coexisting with epidote are Mg-rich (XFe<0.50). Their main compositional variation consists in a significant enrichment in magnesium (toward the chlinochlore end member) in presence of haematite. The intra-grain chemical scattering of chlorite (expressed by the aluminium content in the structural formula) increases with increasing distance of the pluton and decreasing temperature of crystallization. Chlorites associated with phengite and magnetite in vein alteration are Fe- and Al-rich. The Mössbauer spectra indicate that the Fe3+ content of chlorite varies between 25 and 32% of total Fe in the presence of epidote; the higher content being attained in the presence of haematite. The Fe3+ content of chlorite associated with magnetite and phengite is 16% of total Fe. The compositional variations of epidote and/or chlorite of the four distinct assemblages observed at Saint Martin result from the combined effects of fO2, temperature, and time of heating. The effect of fO2 is particularly perceptible in the control of the epidote Ps content, of the chlorite XFe ratio of Fe3+ distribution between coexisting epidotes and chlorites. Despite the fact that it may be partially canceled out by the effect of fO2, the variation of compositional ranges of both epidotes and chlorites, which increases toward the outer part of the geothermal system in response to the combination of decreasing temperatures and decreasing time of heating of the rocks, suggests that chemical equilibrium has not been attained in the assemblages bearing epidotes and chlorites.  相似文献   

16.
The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO4 and Cl–HCO3 hydrothermal waters, with peaks in SO4 and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO4 and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO4 geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km2) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).Editorial responsibility: H. Shinohara  相似文献   

17.
Hydrothermal alteration zones have been investigated by X-ray diffraction, mineralogical–petrographical techniques, and geochemical analysis. Examination of cores and cuttings from two drill sites, obtained from a depth of about 814–1020 m, show that the hydrothermal minerals occuring in the rock include: K-feldspar, albite, chlorite, alunite, kaolinite, smectite, illite, and opaque minerals.In the studied area, silicified, smectite, illite, alunite, and opal zones have been recognized. These alteration mineral assemblages indicate that there are geothermal fluids, which have temperatures of 150–220°C in the reservoir.The distribution of the hydrothermal minerals shows changes in the chemical composition of the hydrothermal fluid, which are probably due not only to interaction with host rock, but also to dilution of the Na–K–Cl-rich hydrothermal fluid of the deep reservoir by cold sea water at shallow levels. Geochemical analyses of the solid and liquid phases indicate that the hydrothermal fluids of the Tuzla geothermal system are in equilibrium with alteration products.The tectonic structure of the studied area is caused by NW–SE and NE–SW directional forces. The volcanic rocks where hydrothermal zones are observed in the studied area are of Lower–Middle Miocene age comprise latite, andesite, dacite, rhyolite-type lavas, tuff, and ignimbrites.  相似文献   

18.
Chlorine- and sulphur-bearing compounds in fumarole discharges of the La Fossa crater at Vulcano Island (Italy) can be modelled by a mixing process between magmatic gases and vapour from a boiling hydrothermal system. This allows estimating the compounds in both endmembers. Magma degassing cannot explain the time variation of sulphur and HCl concentrations in the deep endmember, which are more probably linked to reactions of solid phases at depth, before mixing with the hydrothermal vapours. Based on the P–T conditions and speciation of the boiling hydrothermal system below La Fossa, the HCl and Stot contents in the hydrothermal vapours were used to compute the redox conditions and pH of the aqueous solution. The results suggest that the haematite–magnetite buffer controls the hydrothermal fO2 values, while the pH has increased since the end of the 1970s. The main processes affecting pH values may be linked to Na–Ca exchanges between evolved seawater, feeding the boiling hydrothermal system, and local rocks. While Na is removed from water, calcium enters the solution, undergoes hydrolysis and produces HCl, lowering the pH of the water. The increasing water–rock ratio within the hydrothermal system lowers the Ca availability, so the aqueous solution becomes less acidic. Seawater flowing towards the boiling hydrothermal brine dissolves a large quantity of pyrite along its path. In the boiling hydrothermal system, dissolved sulphur precipitates as pyrite and anhydrite, and becomes partitioned in vapour phase as H2S and SO2. These results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at Vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles.  相似文献   

19.
Four 40Ar/39Ar dates on mineral separates from fresh and hydrothermally altered volcanic and plutonic rocks from the Ngatamariki geothermal field indicate that andesitic volcanism took place in the eastern portion of the Taupo Volcanic Zone (TVZ) prior to 1.2 Ma and probably considerably earlier. These data significantly extend the onset and duration of andesitic volcanism in the east-central TVZ over previous estimates. Intrusive activity is represented at Ngatamariki by a dioritic pluton, the only such pluton yet recognized in the entire TVZ. Hornblende from the pluton yields a crystallization age of near 550 ka. Hydrothermal alteration spatially associated with the pluton produced sericite of a similar age. Overlying and postdating the most intense hydrothermal alteration zone is the Whakamaru Ignimbrite (or its equivalent) which was emplaced at 330 ka. Two distinct geothermal systems may have been active at nearly the same site from 550 ka to present. The most intense activity occurred before 330 ka and was associated with emplacement of the Ngatamariki diorite. This was followed by the less intense system that is currently active. The geothermal regime at Ngatamariki has, therefore, probably been active intermittently for at least 550 ka.  相似文献   

20.
A historical data set is used to describe the coastal transition zone off Northwest Africa during spring 1973 and fall 1975, from 17° to 26°N, with special emphasis on the interaction between subtropical (North Atlantic Central Waters) and tropical (South Atlantic Central Waters) gyres. The near-surface geostrophic circulation, relative to 300 m, is quite complex. Major features are a large cyclonic pattern north of Cape Blanc (21°N) and offshore flow at the Cape Verde front. The large cyclone occurs in the region of most intense winds, and resembles a large meander of the baroclinic southward upwelling jet. The Cape Verde frontal system displays substantial interleaving that may partly originate as mesoscale features at the coastal upwelling front. Property–property diagrams show that the front is an effective barrier to all properties except temperature. The analysis of the Turner angle suggests that the frontal system is characterized by large heat horizontal diffusion as a result of intense double diffusion, which results in the smoothing of the temperature horizontal gradients. Nine cross-shore sections are used to calculate along-shore geostrophic water-mass and nutrient transports and to infer exchanges between the coastal transition zone and the deep ocean (import: deep ocean to transition zone; export: transition zone to deep ocean). These exchanges compare well with mean wind-induced transports and actual geostrophic cross-shore transport estimates. The region is divided into three areas: southern (18–21°N), central (21–23.5°N), and northern (23.5–26°N). In the northern area geostrophic import is roughly compensated with wind-induced export during both seasons. In the central area geostrophic import is greater than wind-induced export during spring, resulting in net import of both water (0.8 Sv) and nitrate (14 kmol s−1), but during fall both factors again roughly cancel. In the southern area geostrophy and wind join to export water and nutrients during both seasons, they increase from 0.6 Sv and 3 kmol s−1 during fall to 2.9 Sv and 53 kmol s−1 during spring.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号