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1.
The Platanares geothermal area in western Honduras consists of more than 100 hot springs that issue from numerous hot-spring groups along the banks or within the streambed of the Quebrada de Agua Caliente (brook of hot water). Evaluation of this geothermal area included drilling a 650-m deep PLTG-1 drill hole which penetrated a surface mantling of stream terrace deposits, about 550 m of Tertiary andesitic lava flows, and Cretaceous to lower Tertiary sedimentary rocks in the lower 90 m of the drill core.Fractures and cavities in the drill core are partly to completely filled by hydrothermal minerals that include quartz, kaolinite, mixed-layer illite-smectite, barite, fluorite, chlorite, calcite, laumontite, biotite, hematite, marcasite, pyrite, arsenopyrite, stibnite, and sphalerite; the most common open-space fillings are calcite and quartz. Biotite from 138.9-m depth, dated at 37.41 Ma by replicate 40Ar/39 Ar analyses using a continuous laser system, is the earliest hydrothermal mineral deposited in the PLTG-1 drill core. This mid-Tertiary age indicates that at least some of the hydrothermal alteration encountered in the PLTG-1 drill core occured in the distant past and is unrelated to the present geothermal system. Furthermore, homogenization temperatures (Th) and melting-point temperatures (Tm) for fluid inclusions in two of the later-formed hydrothermal minerals, calcite and barite, suggest that the temperatures and concentration of dissolved solids of the fluids present at the time these fluid inclusions formed were very different from the present temperatures and fluid chemistry measured in the drill hole.Liquid-rich secondary fluid inclusions in barite and caicite from drill hole PLTG-1 have Th values that range from about 20°C less than the present measured temperature curve at 590.1-m depth to as much as 90°C higher than the temperature curve at 46.75-m depth. Many of the barite Th measurements (ranging between 114° and 265°C) plot above the reference surface boiling-point curve for pure water assuming hydrostatic conditions; however, the absence of evidence for boiling in the fluid inclusions indicates that at the time the minerals formed, the ground surface must have been at least 80 m higher than at present and underwent stream erosion to the current elevation. Near-surface mixed-layer illite-smectite is closely associated with barite and appears to have formed at about the same temperature range (about 120° to 200°C) as the fluid-inclusion Thvalues for barite. Fluid-inclusion Th values for calcite range between about 136° and 213°C. Several of the calcite Th values are significantly lower than the present measured temperature curve. The melting-point temperatures (Tm) of fluid-inclusion ice yield calculated salinities, ranging from near zero to as much as 5.4 wt. % NaCl equivalent, which suggest that much of the barite and calcite precipitated from fluids of significantly greater salinity than the present low salinity Platanares hot-spring water or water produced from the drill hole.  相似文献   

2.
The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron.  相似文献   

3.
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   

4.
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   

5.
Philippine geothermal systems occur in the vicinity of large Holocene calc-alkaline volcanic complexes. Wells drilled in these areas encountered multiple intrusions; the latest dikes are the subsurface manifestations of the youngest heat source. Commonly, at least two hydrothermal regimes are juxtaposed in a single area, with the latest being in equilibrium with the present temperature and chemical regime.Alteration by neutral-pH water is pervasive and abundant. A contact-metamorphic aureole also occurs near intrusives. Alteration due to acid-sulfate fluids is generally confined to permeable structures. Neutral-pH alteration is divided into four zones on the basis of key clay minerals, and two subzones are defined by calc-silicates. These are the smectite (ambient to 180°C), transition (180–230°C), illite (230–320°C) and biotite (270–340°C) zones. Subzones are defined by epidote (250–340°C) and amphibole (280–340°C). The four main zones of acid alteration are: kaolinite (ambient to 120°C), dickite ± kaolinite (120–200°C), dickite ± pyrophyllite (200–250°C), and pyrophyllite ± illite (230–320°C). Where relict high-temperature alteration reaches the surface, the area being drilled is usually the outflow zone of the present system.These hydrothermal mineral assemblages are used: (1) as geothermometers; (2) to assist in determining the depth at which the production casing will be set during drilling; (3) to estimate fluid pH and other chemical parameters; (4) to predict possible corrosion and scaling tendencies of the fluids; (5) as a measure of permeability and possible cold water influx into wells; (6) as a guide to field hydrology; and (7) to estimate roughly the thickness of the eroded overburden.  相似文献   

6.
The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3–5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: porphyritic granodiorite, porphyritic granite, biotite-hornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by >70 wt% SiO2 at its inferred margin and <67 wt% SiO2 at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquid-dominated, convective hydrothermal system, with <380°C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. ‘Healed’ microfractures also trapped >600°C, hypersaline (35 wt% NaCleq) fluids of magmatic origin, with inferred minimum pressures of formation being 600–750 bar, which corresponds to fluid entrapment at 2.4–3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7–9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at 1.25 Ma) and shallower (6.5–7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1–5.9 mm/yr (although uplift was about 7.5 mm/yr prior to 1.2 Ma), which is faster than average uplift rates in the Japan Alps (3 mm/yr during the last 2 million years). A temperature–depth–time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500°C to 400°C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature–time–depth window is proposed: for 500°C at 1.54–1.57 Ma and 5.2±0.9 km (drilling) depth; and 400°C at 1.36–1.38 Ma and 3.3±0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature–depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan).  相似文献   

7.
Gabbro breccias were recovered from an anomalously shallow level of the ocean crust during DSDP Leg 82. The rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater-derived fluid under static conditions. Secondary phases consist of plagioclase, amphibole and minor clinopyroxene, ilmenite, sphene and chlorite. Petrological study indicates that deformation took place at high temperature, under anhydrous conditions, and was followed by hydrothermal circulation. The compositions of secondary minerals (i.e. strong zonations, presence of chlorine in amphiboles, varying compositions of secondary plagioclase) indicate that reactions of the gabbros with the fluids occurred at a low water/rock ratio. Relations between Cl, Na and K in amphiboles suggest penetration of at least two distinct fluids of different compositions. Metamorphic crystallization stopped when greenschist facies conditions were reached( 350°C), probably because hydrothermal circulation faded out.  相似文献   

8.
The Latera field (Vulsini volcanic complex, Latium, Italy) is one of the geothermal areas of the peri-Tyrrhenian belt along which a regional, high thermal anomaly has been detected. So far nine deep wells have been drilled within the Latera caldera and four of them have been productive. The geothermal reservoir is located within the fractured carbonatic rocks of the Tuscan nappe; the overlying volcanic units, sealed by hydrothermal minerals (mainly calcite and anhydrite), act as an impervious cover.The fluid produced by the wells comes from a deep aquifer (about 1000–1500 m depth) which at present is not connected with the shallow aquifer in the volcanoclastic units. Fluid temperatures range between 200 and 230°C; in-hole temperatures as high as 343°C at 2775 m depth have been measured in dry wells.The study of the newly formed mineral assemblages from both volcanic and sedimentary units as sampled from the geothermal wells can be used to reconstruct the thermal evolution of the geothermal field. The intrusion of a syenitic melt, up to a depth of about 2000 m, dated 0.86 Ma, represents the major thermal event for the units in the area and is assumed to represent the first step in the geothermal evolution of the Latera system.The above mentioned newly formed mineral assemblages can be divided into three groups: (a) “contact-metasomatic”: calcite, anhydrite, diopsidic pyroxene, grossularitic garnet, phlogopite, wollastonite or monticellite; (b) “high-temperature hydrothermal”: calcite, anhydrite, K-feldspar, vesuvianite, melanitic garnet, tourmaline, amphibole, epidote, sulphides; (c) “low-temperature hydrothermal”: calcite, anhydrite, K-feldspar, clay minerals, sulphides. Group (a) minerals are now relics. Part of (b) and all of (c) group are still in equilibrium with the existing conditions in different parts of the geothermal system.Thermodynamic calculations on the observed mineral assemblages permitted estimates of the P, T conditions and gas fugacities.  相似文献   

9.
The minerals of basic and acidic rocks from the volcano-sedimentary sequence in the Huelva area, Spain, Iberian Pyrite Belt, display an extendedδ18O enrichment. Quartzδ18O values from quartz-keratophyres vary from +10.5 to +17.0 and feldsparδ18O values from +14.4 to +16.0. For the spilite or spilitized doleritesδ18O values vary from +9.9 to +13.4 for feldspar, from +6.4 to +9.8 for chlorite, from +3.7 to +4.3 for ilmenite and from +13.6 to +14.0 for quartz, but pyroxene exhibits magmatic values, from +5.3 to +6.1 with an exception at +7.5. The chloriteδD values vary from −34 to −43‰.This is attributed to hydrothermal alteration with seawater enriched inδ18O by circulation through sediments.The temperatures of interaction determined from isotopic fractionations between minerals range from 400° to 520°C.CalculatedδD andδ18O values for water in equilibrium with the minerals at isotopic temperatures range from −16 to +5 and from +8.3 to +12.8, respectively.A model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high18O compositions of the rocks from the Huelva District. Water/rock mass ratios calculated from this model range between 0.3 and 0.7 for the determined range of temperatures.  相似文献   

10.
In order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization, we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite. Experiments consisted of either (1) isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered Os-bearing Fe-sulfide or (2) ‘relaxation’ of initially high near-surface osmium concentrations produced in the latter experiments (pyrite only). Osmium penetration into samples was characterized by depth profiling using Rutherford backscattering spectroscopy (RBS) (pyrite) or electron microprobe analyses across sectioned run products (pyrrhotite). Results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces, with the extent of penetration uncorrelated with run duration. Images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near-surface occurred by solute uptake during step growth and not by volume diffusion. Prolonged (1000+ h) ‘relaxation’ experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for RBS, a maximum diffusion coefficient of 2.5×10−23 m2/s at 500°C was calculated. Experiments involving pyrrhotite over the temperature range of 950–1100°C showed extensive osmium uptake and osmium concentration gradients that conform with Fickian diffusion behavior. We found that pyrrhotite Fe/S could be varied by changes in the composition of the starting material and osmium source and over the range of Fe/S produced in experiments (molar Fe/S=0.83–0.90), we observed no systematic variation in the osmium diffusion coefficient. Diffusion coefficients measured parallel to the a crystallographic axis were on average 1.4× higher than values measured parallel to c and regression of the c-axis data yielded the Arrhenius relation:
The application of these diffusion data to simple models of diffusive exchange during static or polythermal time–temperature histories is used to assess the conditions under which radiogenic osmium will be retained. During isothermal annealing, calculations indicate that the cores of millimeter-sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in ≤0.5 Ma at temperatures exceeding 400°C. Pyrite undergoing the same process at 500°C requires in excess of 10 Ma before crystal cores are affected. The relatively short ‘core retention’ time-scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events, thus possibly accounting for the scatter that commonly occurs in Re–Os isochrons generated from massive sulfide samples. Calculated closure temperatures (Tc) for osmium exchange in pyrrhotite yielded values of 300–400°C for grain sizes ranging from 10 to 1000 μm. These values of Tc are similar to those calculated for Ar retention in biotite, and considerably lower than for Sr in apatite and plagioclase, for example. Such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late-stage hydrothermal fluids. This latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage.  相似文献   

11.
A 3729-m-deep geothermal research well, WD-1a, provides us with a unique opportunity to study initial petrographic features of a high-temperature granite just after solidification of magma. The well succeeded in collecting three spot-cores of the Kakkonda Granite that is a pluton emplaced at a shallow depth and regarded as a heat source of the active Kakkonda geothermal system. The core samples were collected at the present formation temperatures of 370, 410 and over 500°C. These samples are granodiorite to tonalite consisting mainly of plagioclase, quartz, hornblende, biotite and K-feldspar. A sample collected at a formation temperature of over 500°C possesses the following remarkable petrographic features compared to the other two samples. Interstitial spaces are not completely sealed. K-feldspar exhibits no perthite by the exsolution of albite lamella. Quartz includes glassy melt inclusions without devitrification. Hornblende is less intensively altered to actinolite, and biotite is not altered. This study directly confirmed that perthite in K-feldspar is a recrystallization texture formed at 410–500°C based on a comparison of the in situ temperatures of the samples. Chemical compositions of minerals were analyzed to compare temperatures determined from geothermometers in several publications to the in situ temperatures of the samples.  相似文献   

12.
Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group.Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fo30)/quartz, iron-rich hypersthene(En55)/iron-poor augite(Wo43.5, En42.5, Fs14.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000°C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite ± hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in in which dacitic-andesitic magma had been fractionating.  相似文献   

13.
Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO2, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ18O) and 42°C (+27.8‰ δ18O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO2) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred.  相似文献   

14.
Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.  相似文献   

15.
We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km2 area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.  相似文献   

16.
Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO4–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and 18O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and 18O content. The large ΔSO4–S0 (23.8–26.4‰) measured in the lake suggest that dissolved SO4 forms during disproportionation of magmatic SO2 in the hydrothermal conduit at temperatures of 250280°C. The lake δ18OSO4 and δ18OH2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO2-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO4–Cl waters on the lakeshore and neutral pH HCO3–SO4–Cl–Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO4/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO4–Cl water and ground water.The lakeshore fumarole discharges (T=170245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is 260°C. The proportions of the oxidized, SO2-dominated magmatic vapor and of the reduced, H2S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO2-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H2S/SO2 ratio deduced for the gas producing the SO4–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H2S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H2S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ34S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.  相似文献   

17.
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   

18.
Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The δ15O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The δ18O values of quartz and calcite from the andesite and basalt flows (700–932 m) have isotopic values which require that the equilibrated water δ18O values increase slightly (− 11.3 to −9.2‰) with increasing measured temperatures (150–265°C). The lithic tuffs and brecciated lava flows (300–700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76–100°C). The δ18O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water (− 10.5 ‰) with meteoric water (− 15.5 ‰) recharged within the caldera. The δ13C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO2 of about −8 ‰.The δ18O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100°C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The δ18O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water.The calculated δ18O and δD values of bottom hole water determined from the fluid produced during the 20 hour flow test are −10.2 and −109‰, respectively. The δD value of the hydrothermal water indicates recharge from outside the caldera.  相似文献   

19.
Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events.The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1–2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C.Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000–1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapmentdepth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber.Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1–7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200–300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.  相似文献   

20.
Kokchetav ‘lamproite’ occurs in the east end of Kokchetav massif and consists of phenocryst (mainly clinopyroxene) and matrix (mainly feldspar). The compositions of clinopyroxene, magnetite and biotite phenocryst were determined using wavelength dispersive spectrometry on a JEOL Super-probe 8900 electron microprobe for the purpose of revealing the process of magma evolution. Analyses revealed a core–rim variation, which is consistent with three stages of magmatic evolution: Mg-rich clinopyroxene cores (diopside) and biotite cores (phlogopite) crystallized in a deep magma chamber (stage I); Fe-rich clinopyroxene rim (salite) and biotite rim crystallized at low pressure in a shallow magma chamber (stage II); Magnetite phenocryst core also crystallized in a shallow magma chamber, and co-exists with Fe-rich clinopyroxene rim and biotite rim. The magnetite rims probably formed during magma eruption at the same time when groundmass crystallized (stage III). The calculated temperatures for ilmenite–magnetite pair range from 679 to 887°C, log fO2 values range from −11.1 to −14.9 log units. These values represent the latest conditions of magma as ilmenite exsolution in magnetite probably occurred during magma eruption from the shallow chamber to surface.  相似文献   

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