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1.
Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ18O and δ13C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ13C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3− (aq) reservoir during degassing of 13C-depleted CO2.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in 18O relative to equilibrium values. δ18O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ13C, δ18O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO2 hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO3− (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in 18O enrichment in the HCO3− (aq) reservoir and ultimately in the precipitated CaCO3.  相似文献   

2.
Stable isotopes of hydrogen and oxygen were determined in 45 samples of water (27 samples of oil-associated waters, 17 samples of mineral waters used by spas, 1 sample of surface river water) from the Central Carpathian Synclinorium, covering a stratigraphic range of flysch sediments from Upper Cretaceous to Oligocene. Moreover, oxygen isotope compositions of authigenic calcite (vein and cement) from core samples of four boreholes were made to evaluate isotopic equilibrium between waters and diagenetic carbonates as a function of temperature. The saline and brackish waters (TDS from1 g/l to 48.9 g/l) considered here, generally belong to four hydrogeochemical classes: Na-Cl, Cl-HCO3-Na, HCO3-Cl-Na and HCO3-Na. Their isotopic composition causes them to fall to the right of Global Meteoric Water Line (GMWL) showing enrichment in 18O and 2H. On the other hand, relative to Standard Mean Ocean Water (SMOW) they are depleted in 2H and both depleted and enriched in 18O. The observed isotopic composition can be explained by the three-component mixing of surface water, diagenetically modified sea water (kind of connate water) and metamorphic water. The mixing is accompanied by an exchange of oxygen isotopes between water and carbonate cements causes 18O enrichment of interstitial waters. The contribution of isotopic exchange between water and clay minerals in shales was evaluated only theoretically basing of the literature.  相似文献   

3.
Hercynian S-type granites from the southeastern Schwarzwald granite series represent cogenetic biotite-and two-mica granites. Oxygen- and hydrogen-isotope data show that hydrothermal alteration invoking isotopically light surface waters resulted in a drastic reduction in 18O and D and pronounced disequilibrium between the minerals. Effective water-rock ratios are calculated to be high, about 0.8 vol units. A shift in the18O/16O and the chemical composition of the fluid due to water-rock interaction is continuously traced from pure H2O with meteoric isotopic character in the deep-seated biotite granites to slightly saline water with rock-equilibrated isotopic composition in the two-mica granites at a shallower level. Substantial retrograde hydrometamorphism in the temperature range 500° to 200° C resulted mineralogically in high-temperature chloritization of biotite, and low-temperature muscovitization as well as feldspar alteration, respectively. Another result of the re-equilibration of cations is strong disturbance of the Rb–Sr system which affects measured ages and initial87Sr/86Sr values. Hydrothermal differentiation and alteration probably overlap to a very large extent magmatic differentiation processes.  相似文献   

4.
The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in δD and H2O+ from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between δD and H2O+ in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH4 and H2. A good correlation between δD values and H2O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having δD values as low as ?100.δD values vary with H2O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary δD values were similar to those of submarine lavas.Extrapolations to possible unaltered δD values and H2O contents indicate that the primary δD values of most thoteiite and alkali basalts are near ?80 ± 5: the weight percentages of water are variable, 0.15–0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary δD values of ?80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth.  相似文献   

5.
The major ion hydrochemistry, sodium absorption ratio (SAR), sodium percentage, and isotopic signatures of Hammamet-Nabeul groundwaters were used to identify the processes that control the mineralization, irrigation suitability, and origin of different water bodies. This investigation highlights that groundwater mineralization is mainly influenced by water-rock interaction and pollution by the return flow of irrigation water. The comparison of groundwater quality with irrigation suitability standards proves that most parts of groundwater are unacceptable for irrigation and this long-term practice may result in a significant increase of the salinity and alkalinity in the soils. Based on isotopic signatures, the shallow aquifer groundwater samples were classified into (i) waters with depleted δ18O and δ2H contents, highlighting recharge by modern precipitation, and (ii) waters with enriched stable isotope contents, reflecting the significance of recharge by contaminated water derived from the return flow of evaporated irrigation waters. The deep-aquifer groundwater samples were also classified into (i) waters with relatively enriched isotope contents derived from modern recharge and mixed with shallow-aquifer groundwater and (ii) waters with depleted stable isotope contents reflecting a paleoclimatic origin. Tritium data permit to identify three origins of recharge, i.e., contemporaneous, post-nuclear, and pre-nuclear. Carbon-14 activities demonstrate the existence of old paleoclimatic recharge related to the Holocene and Late Pleistocene humid periods.  相似文献   

6.
The isotopic composition of water and dissolved Sr as well as other geochemical parameters at the 2516 m deep Outokumpu Deep Drill Hole, Finland were determined. The drill hole is hosted by Palaeoproterozoic turbiditic metasediments, ophiolite-derived altered ultramafic rocks and pegmatitic granitoids. Sodium–Ca–Cl and Ca–Na–Cl-rich waters (total dissolved solids up to ca. 70 g L−1) containing significant amounts of gas, mainly CH4 (up to 32 mmol L−1), N2 (up to 10 mmol L−1), H2 (up to 3.1 mmol L−1) and He (up to 1.1 mmol L−1) discharge from fracture zones into the drill hole. This water is distinct from the shallow fresh groundwater of the area, and has an isotopic composition typical of shield brines that have been modified during long-term water–rock interaction. Based on water stable isotopes and geochemistry, the drill hole water profile can be divided into five water types, each discharging from separate fracture systems and affected by the surrounding rocks. The δ2H varies from −90‰ to −56‰ (VSMOW) and δ18O from −13.5‰ to −10.4‰ (VSMOW), plotting clearly above the Global and Local Meteoric Water Lines on a δ2H vs. δ18O diagram. The 87Sr/86Sr ratios range between 0.72423 and 0.73668. Simple two-component mixing between 2H and 18O rich end-member brine and meteoric water cannot explain the water stable isotopic composition and trends observed. Instead, hydration of silicates by ancient groundwaters recharged under different climatic conditions, warmer than at present, is the most likely mechanism to have caused the variation of the δ2H and δ18O values. Water types correlate with changes in microbial communities implying that different ecosystems occur at different depths. The different water types and microbial populations have remained isolated from each other and from the surface for long periods of time, probably tens of millions of years.  相似文献   

7.
The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ18O values of −5.4‰ (VSMOW) and are generally depleted in 18O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ18O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ18O values of +8.4‰ accounts for a permanent enrichment of porewater 18O and observed δ18O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO2 with δ18O values of +8.9‰ released oxygen enriched in 18O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO2 accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ18O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ18O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO2 reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.  相似文献   

8.
The O18/O16 and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in the Santa Rosa Range, Nevada, and the Eldora area, Colorado, with emphasis on pelitic rocks. A consistent order of O18/O16 and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are commonly observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a systematic decrease is observed in the oxygen isotopic fractionations of mineral pairs as one approaches the intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O18/O16 fractionation calibration curve, temperatures are estimated to be 525 to 625° C at the contacts of the granitic stocks studied.Small-scale oxygen isotope exchange effects between intrusive and country rock are observed over distances of 0.5 to 3 feet on both sides of the contacts; the isotopic gradients are typically 2 to 3 per mil per foot. The degree of oxygen isotopic exchange is essentially identical for different coexisting minerals. This presumably occurred through a diffusion-controlled recrystallization process. The size of the oxygen isotope equilibrium system in the small-scale exchanged zones varies from about 1.5 to 30 cm. A xenolith and a re-entrant of country rock projecting into an intrusive have both undergone much more extensive isotopic exchange (to hundreds of feet); they also show higher isotopic temperatures than the rocks in the aureole. The marginal portions of most plutons have unusually high O18/O16 ratios compared to normal igneous rocks, presumably due to large-scale isotopic exchange with metasedimentary country rocks when the igneous rocks were essentially in a molten state. The isotopic data suggest that outward horizontal movement of H2O into the contact metamorphic aureoles is very minor, but upward movement of H2O is important. Also, direct influx and absorption of H2O from the country rock appears to have occurred in certain intrusive stocks. The D/H ratios of biotites in the contact metamorphic rocks and their associated intrusions show a geographic correlation that is similar to that shown by the D/H ratios of meteoric surface waters, perhaps indicating that meteoric waters were present in the rocks during crystallization of the biotites.Except in the exchanged zones, the O18/O16 ratios of pelitic rocks do not change appreciably during contact metamorphism, even in the cordierite and sillimanite grades; this is in contrast to regional metamorphic rocks which commonly decrease in O18 with increasing grade. Thus, contact metamorphic rocks generally do not exchange with large quantities of igneous H2O, but regional metamorphic rocks appear to have done so.Publications of the Division of Geological Sciences, California Institute of Technology, Contribution No. 1565.  相似文献   

9.
Glendonites, calcite pseudomorphs after the metastable mineral ikaite (CaCO3 · 6H2O), occur in the Late Aptian interval of the Bulldog Shale in the Eromanga Basin, Australia and in other Early Cretaceous basins at high paleolatitudes. Ikaite precipitation in the marine environment requires near-freezing temperatures (not higher than 4°C), high alkalinity, increased levels of orthophosphate, and high PCO2. The rapid and complete transformation of ikaite to calcite at temperatures between 5 and 8°C provides an upper limit on the oxygen isotopic composition of the pore waters: −2.6 <δw <−3.4‰SMOW. If it is assumed that these pore waters are representative of the shallow Eromanga Basin, the calculated δw can be used to reassess belemnite fossil oxygen isotopic paleotemperatures—temperature recorded by fauna living in the basin at the time of ikaite precipitation. Data previously reported as 11 to 16°C (assuming δw = 0.0‰SMOW) yield paleotemperatures ranging from −1 to 5°C, squarely in the range of ikaite stability. The low δw indicates hyposaline conditions, most likely caused by mixing high latitude meteoric waters with seawater. The 18O depleted, low temperature waters suggest that the region was at least seasonally colder than previously accepted.  相似文献   

10.
As a result of the scarcity of isotopic reference waters for daily use, a new secondary isotopic reference material for international distribution has been prepared from ice‐core water from the Amundsen–Scott South Pole Station. This isotopic reference material, designated as USGS49, was filtered, homogenised, loaded into glass ampoules, sealed with a torch, autoclaved to eliminate biological activity and measured by dual‐inlet isotope‐ratio mass spectrometry. The δ2H and δ18O values of USGS49 are ?394.7 ± 0.4 and ?50.55 ± 0.04 mUr (where mUr = 0.001 = ‰), respectively, relative to VSMOW, on scales normalised such that the δ2H and δ18O values of SLAP reference water are, respectively, ?428 and ?55.5 mUr. Each uncertainty is an estimated expanded uncertainty (U = 2uc) about the reference value that provides an interval that has about a 95% probability of encompassing the true value. This isotopic reference material is intended as one of two isotopic reference waters for daily normalisation of stable hydrogen and oxygen isotopic analysis of water with an isotope‐ratio mass spectrometer or a laser absorption spectrometer. It is available by the case of 144 glass ampoules or as a set of sixteen glass ampoules containing 5 ml of water in each ampoule.  相似文献   

11.
应用氢氧同位素研究矿床成因的一些问题探讨   总被引：10，自引：1，他引：10       下载免费PDF全文
翟建平  胡凯 《地质科学》1996,31(3):229-237
成矿热液的氢、氧同位素组成与其水的类型、水/岩交换的岩石成分和同位素组成、水/岩交换时的温度及水/岩交换程度(W/R比值大小)等诸多因素有关,微生物和有机质也对其有一定的影响。因此,仅通过简单投影的方法将成矿热液的氢、氧同位素值与一些所谓的标准值进行类比,由此就推断出热液中水的来源,这种方法是不可取的;尤其当成矿热液的氢、氧同位素值介于大气降水和岩浆水的值之间时,切忌滥用两种水混合成矿模式,因为实际情况往往并不是这样。本文以胶东乳山金矿床为例,展开了这方面的讨论。  相似文献   

12.
Upper Visean limestones in the Campine Basin of northern Belgium are intensively fractured. The largest and most common fractures are cemented by non-ferroan, dull brown-orange luminescent blocky calcite. First melting temperatures of fluid inclusions in these calcites are around -57°C, suggesting that precipitation of the cements occurred from NaCl-CaCl2-MgCl2 fluids. The final melting temperatures (Tmice) are between -5 and -33°C. The broad range in the Tmice data can be explained by the mixing of high salinity fluids with meteoric waters, but other hypotheses may also be valid. Homogenization temperatures from blocky calcite cements in the shelf limestones are interpreted to have formed between 45 and 75°C. In carbonates which were deposited close to and at the shelf margin, precipitation temperatures were possibly in the range 70-85°C and 72-93°C, respectively. On the shelf, the calcites have a δ18O around -9.3‰ PDB and they are interpreted to have grown in a fluid with a δ18O between −3.5 and +1.0‰ SMOW. At the shelf margin, blocky calcites (δ18O∼ - 13.5‰ PDB) could have precipitated from a fluid with a δ18O betweenn -4.0 and -1.1‰ SMOW. The highest oxygen isotopic compositions are comparable to those of Late Carboniferous marine fluids (δ18O= - 1‰ SMOW). The lowest values are more positive than a previously reported composition for Carboniferous meteoric waters (δ18O= -7‰ SMOW). Precipitation is likely to have occurred in marine-derived fluids, which mixed with meteoric waters sourced from near the Brabant Massif. Fluids with a similar negative oxygen isotopic composition and high salinity are actually present in Palaeozoic formations. The higher temperature range in the limestones near the shelf margin is explained by the upward migration of fluids from the ‘basinal’ area along fractures and faults into the shelf.  相似文献   

13.
The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ18O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ18O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ18O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ18OH2O and δDH2O values range from 8 to 12%. and ?3 to +42%., respectively.δ18OH2O values calculated from δ18O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ18O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are 18O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ18OH2O and δDH2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. 18O-enrichment in andesites may be produced by isotopic exchange with high 18O waters at elevated temperatures and sufficiently high water/rock ratios.  相似文献   

14.
This work reports new hydrochemical data on the two types of cold high p CO2 groundwaters from the Mukhen deposit (Khabarovsk district). The first type is classed with HCO3-Ca-Mg waters with a relatively low TDS (up to 1.7 g/l) and high concentrations of Fe2+, Mn2+, Ba2+, and SiO2. The second type is of HCO3-Na composition with high TDS (up to 14 g/l) and elevated Li+, B, Sr2+, Br?, and I?. New oxygen (δ18O) and hydrogen (δD) isotopic data on the waters and carbon (δ13C) isotopic data on the gas phase, together with a detailed geological and hydrogeological analysis of the study area, allowed us to decipher the origin of both the mineral waters. Based on the tritium content (3H) in the ground and surface waters of the area, the duration of the mineral water circulation was estimated. It was established that the both types of groundwaters were formed during interaction of meteoric water with bedrock under active influence of CO2, however HCO3-Na groundwaters have longer residence time than HCO3-Ca-Mg groundwaters.  相似文献   

15.
Ice core from Greenland was melted, filtered, homogenised, loaded into glass ampoules, sealed, autoclaved to eliminate biological activity, and calibrated by dual‐inlet isotope‐ratio mass spectrometry. This isotopic reference material (RM), USGS46, is intended as one of two secondary isotopic reference waters for daily normalisation of stable hydrogen (δ2H) and stable oxygen (δ18O) isotopic analysis of water with a mass spectrometer or a laser absorption spectrometer. The measured δ2H and δ18O values of this reference water were ?235.8 ± 0.7‰ and ?29.80 ± 0.03‰, respectively, relative to VSMOW on scales normalised such that the δ2H and δ18O values of SLAP reference water are, respectively, ?428 and ?55.5‰. Each uncertainty is an estimated expanded uncertainty (U = 2uc) about the reference value that provides an interval that has about a 95‐percent probability of encompassing the true value. This reference water is available in cases containing 144 glass ampoules that are filled with either 4 ml or 5 ml of water per ampoule.  相似文献   

16.
《Applied Geochemistry》1988,3(3):317-332
The chemistry and stable isotopes (18O, D) of highly concentrated chloride brines and minerals from the Asse salt mine in the north of the Federal Republic of Germany were studied. Chemical data indicate the occurrence of three types of brines: (a) Mg-Cl type, of carnallitite origin with Li < 30 mg/kg; (b) Na-Cl type brines, of rock salt origin, with Li > 100 mg/kg; and (c) almost pure MgCl2-type brines with Li > 100 mg/kg. The first group may be subdivided into brines with Li < 4.0 mg/kg and brines with Li between 18 and 30 mg/kg. Lithium is shown to be an efficient complementary tool in tracing the origin of the brines. The complex evolution of carnallitite-type brines is discussed in detail. Isotopic data of brines that were sampled directly from seepages (presumably unaltered) indicate that these brines are not a mixture with relatively fresh ground water from the overburden sediments. The stable isotope composition (18O and D) of hydration water in carnallite, kieserite and polyhalite sampled from the Asse mine were also studied. It is shown that water extracted from the so-called primary carnallite is isotopically different from water extracted from secondary carnallite. The isotopic fractionation factors for 18O and D between carnallite hydration water and mother solution were studied in the laboratory. Assuming that crystallization water of the so-called primary carnallite samples is not altered, the isotopic composition of the mother solution is evaluated.  相似文献   

17.
The oxygen and carbon isotopic composition of surficial carbonates is a key technique for reconstructing past environments and climates. The understanding of modern isotopic systems is, however, a vital first step before applying these techniques to fossil examples. In this study the δ18O of aragonite shells from three different freshwater mollusc species and the δ18O of tufa stromatalites are analysed and compared to the modern temperature and isotopic regime of the river system in which they form (the river Gipping, Sproughton, southeast England). In all cases this range of carbonates appear to form in isotopic equilibrium with the δ18O of modern waters at water temperatures experienced during the summer months. It is, therefore, likely that in Quaternary interglacial deposits of the UK and western Europe the δ18O of fossil freshwater molluscs and tufa stromatolite carbonates will provide an indication of past summer temperature regimes and not mean annual temperatures, as has been previously suggested. The paper concludes by discussing the implications of this study for the isotopic analysis of interglacial deposits in Britain and western Europe.  相似文献   

18.
The West Coast belt, consisting of nearly 60 thermal springs, is one of the most diversified geothermal fields in India. The present work describes the multi-isotopic (O, H, C, S, B and Sr) characterization of thermal waters carried out in the Tural-Rajwadi geothermal field, situated in southern sector of the west coast geothermal area. The aim of this study is to delineate the origin of thermal water as well as to ascertain the sources of carbon, sulphur, boron and strontium dissolved in those thermal springs. The stable isotopes (δ2H and δ18O) and tritium data indicate that these thermal springs are not recently recharged rain water rather, it contains very old component of water. Oxygen-18 shift is observed due to rock-water interaction over a long period of time. Carbon isotopic composition of DIC points out to the silicate weathering with soil CO2 coming from C3 type of plants whereas δ34S of dissolved sulphate confirms the marine origin of sulphate. This marine signature is basically derived from paleo-seawater possibly entrapped within the flows. Boron isotopic data reveals that both the seawater and rock dissolution are the sources of boron in the thermal waters whereas high 87Sr/86Sr ratios (0.7220–0.7512) of the thermal waters conclusively establishes that archean granitic basement is the predominant rock source of strontium, not the Deccan flood basalts. In addition, like strontium, concentrations of lithium, rubidium and caesium are also governed by the rock-water interaction. Thus, the combined use of this multi-isotope technique coupled with trace element concentrations proves to be an effective tool to establish the sources of solutes in the thermal water.  相似文献   

19.
Sulphide-bearing Ca-carbonate, Na-carbonate, Na-hydroxide, Na-chloride and Ca-sulphate waters from Northern Apennines were investigated in order to determine their main chemical and isotopic composition and draw inferences on water-rock interaction. 2H and 18O values suggest an origin mostly meteoric for the analysed waters but a well drilled in Miocenic sediments. The Na-carbonate and the Ca-sulphate waters are the most interesting geochemically. Na-carbonate type, which sometimes reaches extreme composition (Na/Ca up to 228, equivalent ratio), may have been derived through prolonged interaction of Ca-carbonate waters with rocks containing feldspar, montmorillonite and illite under calcite saturation/oversaturation; the high F and pH and the very low PCO 2 agree with prograde dissolution of silicates and lasting water-rock interaction. However, Ca–Na ion exchange, involving clays of marine origin, cannot be excluded in addition. The Ca-sulphate waters, occurring in Messinian gypsum-bearing sediments, are saturated in gypsum and calcite and exhibit very high total H2S (up to 219 mg dm-3) and PCO 2 (up to 0.32 bar). Mass balance of sulphate sulphur, sulphide sulphur and delta34S suggests sulphate – derived from gypsum – as source for H2S; CH4 and organic matter generate the reducing conditions and sulphate reduction is mediated by bacteria. One Na-chloride water from a well in Miocenic sediments has unusual composition, containing about 700 mgdm-3 of potential CaCl2 and having 2H and 18O (-47.5 and -4.9 respectively) which plot far from the meteoric water lines; probably it is derived by mixing of meteoric and formation waters. The Na-hydroxide water, with very high pH (11.2), is generated through protracted interaction of meteoric waters with ultramafites.  相似文献   

20.
Major ion geochemistry and environmental isotopes were used to identify the origins and the mineralisation processes of groundwater flowing within the three aquifer levels of the multilayer system of the Gafsa-south mining district (Southwestern Tunisia). It has been demonstrated that groundwaters are characterised by a Ca–Mg–SO4 water type. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of dolomite. δ18O and δ2H values are much lower than the isotopic signature of regional precipitation and fall close to the meteoric water lines, indicating that groundwaters have not been significantly affected by evaporation or mineral–water reactions. The distribution of stable and radiogenic isotopes (δ18O, δ2H, δ13C and 14C) within the aquifer levels suggests that the deep confined aquifer receives a significant modern recharge at higher altitudes, while, the shallow unconfined aquifer has been mainly recharged under cooler paleoclimatic condition, likely during Late Pleistocene and Early Holocene humid periods. However, waters from the intermediate confined/unconfined aquifer have composite isotopic signatures, highlighting that they are derived from a mixture of the two first end-members.  相似文献   

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