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1.
The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between “normal-marine” (δ13C ca. 0‰) and −14.6‰ which suggests that biogenic CO2 contributed to the carbonate formation. The δ13O values of dolomite and coexisting calcite are very similar but variable down-core.Dolomite and calcite precipitated early from pore water where SO42− was not reduced. However, during (and after?) SO42− reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.  相似文献   

2.
δ87Sr values and Ca/Sr ratios were employed to quantify solute inputs from atmospheric and lithogenic sources to a catchment in NW Germany. The aquifer consists primarily of unconsolidated Pleistocene eolian and fluviatile deposits predominated by >90% quartz sand. Accessory minerals include feldspar, glauconite, and mica, as well as disperse calcium carbonate in deeper levels. Decalcification of near-surface sediment induces groundwater pH values up to 4.4 that lead to enhanced silicate weathering. Consequently, low mineralized Ca–Na–Cl- and Ca–Cl-groundwater types are common in shallow depths, while in deeper located calcareous sediment Ca–HCO3-type groundwater prevails. δ87Sr values and Ca/Sr ratios of the dissolved pool range from 7.3 to −2.6 and 88 to 493, respectively. Positive δ87Sr values and low Ca/Sr ratios indicate enhanced feldspar dissolution in shallow depths of less than 20 m below soil surface (BSS), while equilibrium with calcite governs negative δ87Sr values and elevated Ca/Sr ratios in deep groundwater (>30 m BSS). Both positive and negative δ87Sr values are evolved in intermediate depths (20–30 m BSS). For groundwater that is undersaturated with respect to calcite, atmospheric supplies range from 4% to 20%, while feldspar-weathering accounts for 8–26% and calcium carbonate for 62–90% of dissolved Sr2+. In contrast, more than 95% of Sr2+ is derived by calcium carbonate and less than 5% by feldspar dissolution in Ca–HCO3-type groundwater. The surprisingly high content of carbonate-derived Sr2+ in groundwater of the decalcified portion of the aquifer may account for considerable contributions from Ca-containing fertilizers. Complementary tritium analyses show that equilibrium with calcite is restricted to old groundwater sources.  相似文献   

3.
Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolution, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios(δ13C = 2.14‰, δ18O = -5.77‰), and was precipitated early. It was precipitated directly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid(δ18O = -3‰), which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios(δ13C = -2.36‰, δ18O = -15.68‰). This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2+, Fe3+ and Mg2+ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the burial process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite cements in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone(δ13C = 1.93‰, δ18O = -6.11‰), demonstrating that the two are from the same source that had a heavier oxygen isotope ratio(δ18O of about 2.2‰). The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different water-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles(lithic quartz sandstone) has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.  相似文献   

4.
Cave air PCO2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO2 production, resulting in high summer PCO2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO2 degassing rates from drip water, by considering both the seasonal and high-frequency cave air PCO2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h− 1) and minima in late June/early July (0.033 μm h− 1). For well-mixed ‘diffuse flow’ cave drips such as those that feed CC-Bil, high summer cave air PCO2 depresses summer calcite deposition, while low winter PCO2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ18O, integrated annual stalagmite calcite δ18O is unaffected; however, seasonality in cave air PCO2 may influence non-conservative geochemical climate proxies (e.g., δ13C, Sr/Ca). Stalagmites fed by ‘seasonal’ drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air PCO2 may increase relative to cave air PCO2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ18O records towards isotopically heavier summer drip water δ18O values, resulting in elevated calcite δ18O values compared to the ‘equilibrium’ values predicted by calcite–water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air PCO2 variability and the resulting intra-annual variability in calcite deposition rates.  相似文献   

5.
Stable isotope compositions (δD, δ18O and δ34S) of volcanic lake waters, gas condensates and spring waters from Indonesia, Italy, Japan, and Russia were measured. The spring fluids and gas samples plot in a broad array between meteoric waters and local high-temperature volcanic gas compositions. The δD and δ18O data from volcanic lakes in East Indonesia plot in a concave band ranging from local meteoric waters to evaporated fluids to waters heavier than local high-temperature volcanic gases. We investigated isotopic fractionation processes in volcanic lakes at elevated temperatures with simultaneous mixing of meteoric waters and volcanic gases. An elevated lake water temperature gives enhanced kinetic isotope fractionation and changes in equilibrium fractionation factors, providing relatively flat isotope evolution curves in δ18O–δD diagrams. A numerical simulation model is used to derive the timescales of isotopic evolution of crater lakes as a function of atmospheric parameters, lake water temperature and fluxes of meteoric water, volcanic gas input, evaporation, and seepage losses. The same model is used to derive the flux magnitude of the Keli Mutu lakes in Indonesia. The calculated volcanic gas fluxes are of the same order as those derived from energy budget models or direct gas flux measurements in open craters (several 100 m3 volcanic water/day) and indicate a water residence time of 1–2 decades. The δ34S data from the Keli Mutu lakes show a much wider range than those from gases and springs, which is probably related to the precipitation of sulfur in these acid brine lakes. The isotopic mass balance and S/Cl values suggest that about half of the sulfur input in the hottest Keli Mutu lake is converted into native sulfur.  相似文献   

6.
The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ44/40Ca and δ18O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ44/40Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater (δ44/40CaSW), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite (αCC–SW) of ∼ 0.9986. The inferred δ44/40CaSW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ44/40CaSW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ44/40CaSW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ44/40CaSW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates.  相似文献   

7.
Porcellanite has been found in Antarctic deep-sea sediments of shallow burial depth at four different sites in host sediments younger than 4 Ma. Oxygen isotope analysis shows that the opal-CT samples are extremely rich in18O (δ18O= 41.2to44.7‰ rel. SMOW). According to the quartz/water fractionation the calculated isotopic opal-CT formation temperature is in the range of 0 to 4°C. This agrees well with sediment temperature measurements. The low opal-CT precipitation temperature contrasts with current ideas about later diagenetic formation of opal-CT at higher temperatures of 18 to 56°C.  相似文献   

8.
Hydrogen and oxygen isotopes of water are common environmental tracers used to investigate hydrological processes, such as evaporation, vegetation water use, surface water–groundwater interaction, and groundwater recharge. The water isotope signature in surface water and groundwater evolves from the initial rain signature. In mountain terrain, rain water stable isotope composition spatially varies due to complex orographic precipitation processes. Many studies have examined the isotope–elevation relationships, while few have quantitatively investigate the terrain aspect and slope effect on rain isotope distribution. In this paper, we examine the orographic effects more completely, including elevation, terrain slope and aspect, on stable isotope distribution in the Mount Lofty Ranges (MLR) of South Australia, using a multivariate regression model. The regression of precipitation isotope composition suggests that orographic effects are the dominant controls on isotope spatial variability. About 75% of spatial variability in δ18O and deuterium excess is represented by the regression using solely orography-related variables (elevation, terrain aspect and slope), with about 25% of δ18O spatial variability attributed to the terrain aspect and slope effect. The lapse rate is about −0.25‰ for every 100 m at both windward and leeward slopes. However, at the same elevation, δ18O at the leeward slope (eastern MLR) is 0.5‰ larger than that at the windward slope. The difference can be explained by different mechanisms – continuous rain-out processes on the windward side and sub-cloud evaporation on the leeward side. Both δ18O and deuterium excess maps (1 km resolution) are constructed based on the regression results for the MLR. Both maps are consistent with groundwater of local precipitation origin, and useful to examine groundwater recharge.  相似文献   

9.
The Long Valley Exploratory Well, at the center of the Resurgent Dome of Long Valley caldera, penetrated pre-caldera basement rocks at a depth of 2101.72–2313.0 m, beneath the caldera-forming Bishop Tuff and post-caldera Early Rhyolite. The basement rocks contain prominent quartzites, with ubiquitous milky white quartz veins (with minor calcite and pyrite) and fractures of varied orientation and geometry. The other members of the basement sequence are very fine-grained quartz-rich graphitic pelites with calcite veins, spotted hornfels, and shallow intrusive rocks. Previous studies established the presence of a post-caldera, paleohydrothermal system (500–100 ka) to a depth of 2000 m that affected the Bishop Tuff and a recent (40 ka to present) hydrothermal system at shallow depth (<1 km). The deeper extent of these hydrothermal activities is established in this paper by a detailed oxygen isotope analysis of the drill core samples. 238 analyses of δ18O in 50 quartz veins within the 163.57 m depth interval of basement rocks reveal extreme heterogeneity in δ18O values (8–19.5‰). Majorities of the 84 bulk analyses of quartzites show variation of δ18O within a narrow range of 14–16‰. However, certain samples of these quartzites near the contacts with veins and fractures exhibit sharp drops in δ18O. The interbedded pelitic rocks and spotted hornfels have whole-rock δ18O ranging from 2.2 to 11.8‰. Clear, euhedral vuggy quartz that partially fills earlier open fractures in both the quartzites and quartz veins, has distinctive δ18O, ranging between −3.2 and +8.4‰. Low values of δ18O are also found in the hydrothermal minerals and whole rocks adjacent to the thin veins, clearly indicating infiltration of meteoric water. Three distinct observed patterns of fractionation in δ18O between veins and host quartzites are analyzed with the principles of mass balance, equilibrium oxygen isotope fractionation in closed system, and kinetically controlled oxygen isotope exchange in an open system. This analysis suggests that the early quartz veins formed due to a magmatic-hydrothermal activity with no influx of external water once the system comprising the sedimentary envelope and a magmatic-hydrothermal fluid phase became closed. Two-stage isotopic exchange processes caused fractionation in the δ values that originally formed arrays with slope 1 in a δvein quartz–δhost quartzite space. Another array in the same space, with near zero slope was also formed due to variation in temperature, initial isotopic compositions of the quartzite sequence and the fluid phase. Variation in temperature was mostly in the range of 300–400°C giving Δ (=δvein quartz–δhost quartzite)≈−2.8 to +2.8. The δ18O of the fluid could range from −5 to +10; however a narrower range of +5 to +10 can explain the data. This episode of hydrothermal activity could take place either as a single pulse or in multiple pulses but each as a closed system. A later, fracture-controlled, meteoric water (δ18O−0.46 to −12.13) flow and interaction (at 250°C) is interpreted from the analysis of δ18O values of the coexisting quartz and calcite pairs and existence of markedly 18O-depleted pelitic horizons interbedded with 18O-enriched quartzite layers. Thus, the interpreted earlier magmatic-hydrothermal activity was overprinted by a later meteoric-hydrothermal activity that resulted in steep arrays of δ18O values in the δvein quartz–δhost quartzite space. Calculations show that the likely life span of the post-caldera, hydrothermal activity in the depth range of 2.1–2.3 km beneath Long Valley was 0.08–0.12 Ma. Diffusive ±advective transport of oxygen isotopes from fracture-channelized meteoric water to nearly impermeable wall rocks caused a lowering of δ18O values in the quartz over short distances and in calcites over greater distances. Thus, the hydrothermal activity appears pervasive even though the meteoric water flow was primarily controlled by fractures.  相似文献   

10.
Irrigation, urbanization, and drought pose challenges for the sustainable use of ground water in the central Couloir sud rifain, a major agricultural region in north-central Morocco, which includes the cities of Fès and Meknès. The central Couloir is underlain by unconfined and confined carbonate aquifers that have suffered declines in hydraulic head and reductions in spring flow in recent decades. Previous studies have surveyed ground water flow and water quality in wells and springs but have not comprehensively addressed the chemistry of the regional aquifer system. Using graphical techniques and saturation index calculations, we infer that major ion chemistry is controlled (1) in the surficial aquifer by cation exchange, calcite dissolution, mixing with deep ground water, and possibly calcite precipitation and (2) in the confined aquifer and warm springs by calcite dissolution, dolomite dissolution, mixing with water that has dissolved gypsum and halite, and calcite precipitation. Analyses of 2H and 18O indicate that shallow ground water is affected by evaporation during recharge (either of infiltrating precipitation or return flow), whereas deep ground water is sustained by meteoric recharge with little evaporation. Mechanisms of recharge and hydrochemical evolution are broadly consistent with those delineated for similar regional aquifer systems elsewhere in Morocco and in southern Spain.  相似文献   

11.
The precipitation of freshwater carbonates (tufa) along karstic rivers is enhanced by degassing of carbon dioxide (CO2) downstream of karstic springs. However, in most karstic springs CO2 degassing is not enough to force the precipitation of tufa sediments. Little is known about the role of dissolution of gypsum or dolomite in the hydrochemistry of these systems and how this affects the formation of tufa deposits. Here we present a monitoring study conducted over a year in Trabaque River (Spain). The river has typical karst hydrological dynamics with water sinking upstream and re‐emerging downstream of the canyon. Mixing of calcium–magnesium bicarbonate and calcium sulphate waters downstream of the sink enhances the dissolution of carbonates and potentially plays a positive role in the formation of tufa sediments. However, due to the common‐ion effect, dissolution of dolomite and/or gypsum causes precipitation of underground calcite cements as part of the incongruent dissolution of dolomite/dedolomitization process, which limits the precipitation of tufa sediments. Current precipitation of tufa is scant compared to previous Holocene tufa deposits, which likely precipitated from solutions with higher saturation indexes of calcite (SIcc values) than nowadays. Limited incongruent dissolution of dolomite/dedolomitization favours higher SIcc values. This circumstance occurs when waters with relatively high supersaturation of dolomite and low SO42? composition sink in the upper sector of the canyon. In such a scenario, the process of mixing waters enhances the exclusive dissolution of limestones, preventing the precipitation of calcite within the aquifer and favouring the increase of SIcc values downstream of the springs. Such conditions were recorded during periods of high water level of the aquifers and during floods. This research shows that the common‐ion effect caused by the dissolution of gypsum and/or dolomite rocks can limit [or favour] the precipitation of tufa sediments depending on the occurrence [or not] of incongruent dissolution of dolomite/dedolomitization. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

12.
Whole-rock oxygen isotope compositions of cores and cuttings from Long Valley exploration wells show that the Bishop Tuff has been an important reservoir for both fossil and active geothermal systems within the caldera. The deep Clay Pit-1 and Mammoth-1 wells on the resurgent dome penetrate mildly to strongly altered Bishop Tuff with δ18OWR values as low as −2.6% (vs V-SMOW). The idfu 44-16 well intercepts a thinner Bishop Tuff section with δ18OWR values of +0.4 to +2.3%. in the western caldera moat, where milder and more sporadic 18O depletions occur in Tertiary volcanic rocks of the western caldera floor (δ18OWR = +2.2 to +6.4‰). Bishop Tuff samples from deeper parts of the 715 m rdo-8 (Shady Rest) well in the SW moat are also strongly depleted in 18O (δ18OWR = −1.5 to +0.6‰). Four shallow thermal gradient wells (469–715 m td drilled in the western moat did not penetrate Bishop Tuff, but Early Rhyolites from two of these holes are depleted in 18O (δ18OWR = −1.2 to +6.0‰ inplv-1 +4.6 to +5.3%. inmlgrap-1), compared to lithologic equivalents from the other two holes (δ18OWR = +6.3 to +8.0‰ inplv-2 andmlgrap-2).Whole-rock oxygen isotope profiles for the resurgent dome wells are unlike profiles calculated assuming alkali feldspar-H2O fractionation behavior and total O-isotopic equilibration with −14.3‰ fluids at measured temperatures. The sense of this divergence implies an earlier hydrothermal episode within the central caldera driven by one or more shallow intrusions. Geochemical similarities between an intrusive granophyre at the bottom of the Clay Pit-1 well and a nearby Moat Rhyolite dome with a K/Ar cooling age of 0.5 Ma suggest that vigorous hydrothermal activity beneath the central resurgent dome may have occurred as much as 0.5 m.y. ago. Calculated and measured O-isotope profiles are similar for deep wells that penetrate the western moat of the caldera, where steep temperature gradients and low δ18OWR values in Early Rhyolites from plv-1 are attributed to an active hydrothermal aquifer that has descended slightly from earlier, shallower elevations. Similarly, severe 18O depletions in Bishop Tuff samples from the idfu 44-16 and rdo-8 wells reflect active convection beneath the western moat, whereas milder 18O depletions in Early Rhyolites from mlgrap-1 were apparently caused by hydrothermal alteration at lower temperatures. The O-isotope profiles imply that surface discharge within and around the resurgent dome results from shallow, eastward-directed outflow from a zone of higher enthalpy hydrothermal upflow beneath the western caldera moat. Intrusive magmatic heat source(s) are inferred to exist beneath the western moat, perhaps beneath Mammoth Mountain.  相似文献   

13.
Oxygen isotope exchange between anhydrite and water was studied from 100 to 550°C, using the partial equilibrium method. The exchange rate was extremely low in NaCl solution. In the lower-temperature range, acid solutions were used to produce sufficient reaction to determine the oxygen isotope fractionation factors. The fractionation factors obtained in the present study are definitely different from those given by Lloyd [8]. They are similar to those for the HSO4?-water system studied by Mizutani and Rafter [19], and are consistently 2‰ higher than those of the barite-water system by Kusakabe and Robinson [5]. The temperature dependence of the oxygen isotope fractionation factors was calculated by the least squares method in which the weight was taken to be inversely proportional to the experimental error. The fractionation is given by:103lnαanhydrite-water=3.21×(103/T)2?4.72Available δ18O values of natural anhydrite were used to test the validity of this expression. It is shown that this newly revised geothermometer can be successfully applied to natural hydrothermal anhydrite.  相似文献   

14.
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   

15.
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   

16.
Recent drilling and sampling of hydrothermal fluids from Long Valley permit an accurate characterization of chemical concentrations and equilibrium conditions in the hydrothermal reservoir. Hydrothermal fluids are thermodynamically saturated with secondary quartz, calcite, and pyrite but are in disequilibrium with respect to aqueous sulfide-sulfate speciation. Hydrothermal fluids are enriched in 18O by approximately 1‰ relative to recharge waters. 18O and Cl concentrations in well cuttings and core from high-temperature zones of the reservoir are extensively depleted relative to fresh rhyolitic tuff compositions. Approximately 80% of the Li and 50% of the B are retained in the altered reservoir rock. Cl mass balance and open-system 18O fractionation models produce similar water-rock ratios of between 1.0 and 2.5 kg kg−1. These water-rock ratios coupled with estimates of reservoir porosity and density produce a minimum fluid residence time of 1.3 ka. The low fluid Cl concentrations in Long Valley correlate with corresponding low rock concentrations. Mass balance calculations indicate that leaching of these reservoir rocks accounts for Cl losses during hydrothermal activity over the last 40 ka.  相似文献   

17.
The paper presents oxygen and hydrogen isotopes of 284 precipitation event samples systematically collected in Irkutsk, in the Baikal region (southeast Siberia), between June 2011 and April 2017. This is the first high-resolution dataset of stable isotopes of precipitation from this poorly studied region of continental Asia, which has a high potential for isotope-based palaeoclimate research. The dataset revealed distinct seasonal variations: relatively high δ18O (up to −4‰) and δD (up to −40‰) values characterize summer air masses, and lighter isotope composition (−41‰ for δ18O and −322‰ for δD) is characteristic of winter precipitation. Our results show that air temperature mainly affects the isotope composition of precipitation, and no significant correlations were obtained for precipitation amount and relative humidity. A new temperature dependence was established for weighted mean monthly precipitation: +0.50‰/°C (r2 = 0.83; p <.01; n = 55) for δ18O and +3.8‰/°C (r2 = 0.83, p < 0.01; n = 55) for δD. Secondary fractionation processes (e.g., contribution of recycled moisture) were identified mainly in summer from low d excess. Backward trajectories assessed with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicate that precipitation with the lowest mean δ18O and δD values reaches Irkutsk in winter related to moisture transport from the Arctic. Precipitation originating from the west/southwest with the heaviest mean isotope composition reaches Irkutsk in summer, thus representing moisture transport across Eurasia. Generally, moisture transport from the west, that is, the Atlantic Ocean predominates throughout the year. A comparison of our new isotope dataset with simulation results using the European Centre/Hamburg version 5 (ECHAM5)-wiso climate model reveals a good agreement of variations in δ18O (r2 = 0.87; p <.01; n = 55) and air temperature (r2 = 0.99; p <.01; n = 71). However, the ECHAM5-wiso model fails to capture observed variations in d excess (r2 = 0.14; p < 0.01; n = 55). This disagreement can be partly explained by a model deficit of capturing regional hydrological processes associated with secondary moisture supply in summer.  相似文献   

18.
This paper presents 19 months of stable isotope (δ2H and δ18O) data to enhance understanding of water and solute transport at two spatial scales (2.3 km2 and 122 km2) in the agricultural Lunan catchment, Scotland. Daily precipitation and stream isotope data, weekly lake and spring isotope data and monthly groundwater isotope data revealed important insights into flow pathways and mixing of water at both scales. In particular, a deeper groundwater flow path significantly contributes to total streamflow (25-50%). Upstream lake isotope dynamics, susceptible to evaporative fractionation, also appeared to have an important influence on the downstream isotope composition. This unique tracer data set facilitated the conceptualization of a lumped catchment-scale flow-tracer model. The incorporation of hydrological, mixing and fractionation processes based on these data improved simulations of the stream δ2H isotope response at the catchment outlet from 0.37 to 0.56 for the Nash-Sutcliffe statistic. The stable isotope data successfully aided model conceptualization and calibration in the quest for a simple water and solute transport model with improved representation of process dynamics.  相似文献   

19.
We used hydrochemistry and environmental isotope data (δ18O, δD, tritium, and 14C) to investigate the characteristics of river water, groundwater, and groundwater recharge in China's Heihe River basin. The river water and groundwater could be characterized as Ca2+? Mg2+? HCO3?? SO42? and Na+? Mg2+? SO42?? Cl? types, respectively. Hydrogeochemical modelling using PHREEQC software revealed that the main hydrogeochemical processes are dissolution (except for gypsum and anhydrite) along groundwater flow paths from the upper to middle Heihe reaches. Towards the lower reaches, dolomite and calcite tend to precipitate. The isotopic data for most of the river water and groundwater lie on the global meteoric water line (GMWL) or between the GMWL and the meteoric water line in northwestern China, indicating weak evaporation. No direct relationship existed between recharge and discharge of groundwater in the middle and lower reaches based on the isotope ratios, d‐excess, and 14C values. On the basis of tritium in precipitation and by adopting an exponential piston‐flow model, we evaluated the mean residence time of shallow groundwater with high tritium activities, which was around 50 years (a). Furthermore, based on the several popular models, it is calculated that the deep groundwaters in piedmont alluvial fan zone of the middle reaches and in southern part of the lower reaches are modern water, whereas the deep groundwaters in the edge of the middle reaches and around Juyan Lake in the lower reaches of Heihe river basin are old water. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

20.
Based on summer observations of stable isotope of precipitation at Muztagata, western China, during 2002-2003, this paper presents the relationship between δ 18O in precipitation and air temperature, and discusses the effect of moisture transport on δ 18O in precipitation. Results show that air temperature correlates positively with δ 18O in precipitation, and the temperature effect controls the δ 18O of precipitation in this area. The Muztagata region exhibits high δ 18O values in summer precipitation, similar to those shown at stations in adjacent regions. According to the results of our model set up to trace the moisture trajectories, the westerlies and local moisture circulation contribute to variations of oxygen isotopes in precipitation. In addition, the impacts of the moisture transport distance, the moisture transport level, and the incursion of the polar air mass also influence the variations of δ 18O in precipitation. The moisture origins and transport mechanisms also contribute to the variation of δ 18O in precipitation at Muztagata.  相似文献   

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