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1.
The sedimentary succession of Gallocanta lake, a closed saline lake located in the Iberian Range (NE Spain), documents two successive lacustrine stages: (1) brackish lake stage and (2) shallow saline lake stage. The saline stage corresponds to the present-day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance. The carbonates of the brackish lake stage have relatively constant δ18O values, however, they are rather high (δ18ODo = 2.4‰ and δ18OCc = 4.5‰ mean values) suggesting a hydrologically closed lake with a long residence time of the waters. δ18O values of carbonates from the saline stage vary greatly, and are lighter than in the previous stage (δ18ODo = 0.5‰, δ18OCc = −0.7‰, δ18OMgs = −2.3‰ mean values). These carbonates also precipitated in a hydrologically closed lake, but in equilibrium with a lake water of more variable isotopic composition. The δ13C values for carbonates of both stages reflect a mixing of different pools of carbon, but during saline stage δ13C values have been more controlled by the equilibrium of the lake waters with atmospheric CO2. During the current stage, calcite and dolomite precipitate in Gallocanta lake mainly during spring and summer, although dolomite precipitation is more favoured towards the summer. Magnesite precipitates at the beginning of autumn, when the first rainfall re-dissolves the saline surface crust, producing saline waters with a high Mg2+ content. The isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates. It is considered that this could be due to two factors: either because there have not been many extremely dry years (like the year 2005) during the development of the lake, or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals.  相似文献   

2.
Late Pleistocene paleoclimatic variability on the northeastern Qinghai–Tibetan Plateau (NE QTP) was reconstructed using a chronology based on AMS 14C and 230Th dating results and a stable oxygen isotopic record. These are derived from lake carbonates in a 102-m-long Qarhan sediment core (ISL1A) collected from the eastern Qaidam Basin. Previous research indicates that the δ18O values of lacustrine carbonates are mainly controlled by the isotopic composition of lake water, which in turn is a function of regional P/E balance and the proportion of precipitation that is monsoon-derived on the NE QTP. Modern isotopic observations indicate that the δ18O values of lake carbonates in hyper-arid Qaidam Basin are more positive during the warm and wet period. Due to strong evaporation and continental effect in this basin, the positive δ18O values in the arid region indicate drier climatic conditions. Based on this interpretation and the δ18O record of fine-grained lake carbonates and dating results in ISL1A, the results imply that drier climatic conditions in the Qarhan region occurred in three intervals, around 90–80 ka, 52–38 ka and 10–9 ka, which could correspond to late MIS 5, middle MIS 3 and early Holocene, respectively. These three phases were almost coincided with low lake level periods of Gahai, Toson and Qinghai Lakes (to the east of Qarhan Lake) influenced by ASM on the orbital timescales. Meanwhile, there was an episode of relatively high δ18O value during late MIS 3, suggesting that relatively dry climatic condition in this period, rather than “a uniform Qarhan mega-paleolake” spanning the ∼44 to 22 ka period. These results insight into the understanding of “the Greatest Lake Period” on the QTP.  相似文献   

3.
 Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions that can be divided into two groups, one with positive δ13C values from+3.1 to +1.4 ‰ (PDB) and high δ18O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ13C values from –3.3 to –3.6‰ and low δ18O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent, clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ18O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite marbles typically have low δ18O values from +2.0 to +9.3‰. The δ18O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and +14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ18O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and quartz overgrowths in dolomite marbles, low δD values of micas, negative δ13C values and low δ18O values of calcite and dolomite, and depleted δ18O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the Tertiary. Partial oxygen isotopic exchange between calcite and low 18O meteoric fluids lowered the δ18O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration. Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in 18O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the quartz as a result of its dissolution by meteoric fluids. The negative δ13C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western Nevada in Tertiary time. Received: 1 February 1994/Accepted: 12 September 1995  相似文献   

4.
An intensive investigation of the spatial and temporal variations of δD and δ 18O in precipitation was conducted during 2002–2004 in six sites in the Heihe River Basin, Northwestern China. The δD and δ 18O values for 301 precipitation samples ranged from +59 to −254 and +6.5 to −33.4‰, respectively. The relationship between δD and δ 18O defines a well-constrained line given by dD = 7.82d18\textO + 7.63 \delta D = 7.82\delta {}^{18}{\text{O}} + 7.63 , which is nearly identical to the meteoric water line in the Northern China. This wide range indicates that stable isotopes in precipitation were primarily controlled by different condensation mechanisms as a function of air temperature and varying sources of moisture. The results of backward trajectory of each precipitation day at Xishui show that the moisture of the precipitation in cold season (October–March) mainly originated from the west while the moisture source was more complicated in warm season (April–September). The simulation of seasonal δ 18O variation shows that the stable isotope composition of precipitation tended to a clear sine-wave seasonal variation. The spatial variation of δ 18O shows that the weighted average δ 18O values decreases with the increasing altitude of sampling sites. The great difference of air temperature which led to the differences of condensation mechanisms and local recycled continental moisture may have influence upon the isotopic composition of rain events in different sites.  相似文献   

5.
The non-marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre-thick carbonate- and mudstone-dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate. The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ13C and δ18O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ13C and δ18O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend. The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.  相似文献   

6.
The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ13C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ18O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.  相似文献   

7.
The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered (“best”) samples of the Burzyan carbonates, which retain the 87Sr/86Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480 and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ18O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7 to 0‰, which is consistent with the assumption that δ18O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago. C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least altered carbonates of the Early Riphean, the δ18O variation range corresponds to 4.0–4.5‰.  相似文献   

8.
Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in Palenyi Island in the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10–11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineralogical and chemical composition. The closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to the values obtained with mineralogical geothermometers (Grt-Opx and Grt-Bt) and correspond to the high-temperature granulite facies (860–900°C). Identified systematic variations in the δ18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in δ18O cannot be explained by the primary isotopic heterogeneity of the protolith. The model calculations of the extent and trend of the δ18O variations in minerals suggest that the only mechanism able to generate the zoning was fluid-rock interaction at various integral fluid/rock ratios in discrete zones. This demonstrates that a focused fluid flux could occur in lower crustal shear zones. The preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.  相似文献   

9.
Five soil samples collected from shooting ranges in 1998 were stored under a closed condition for 5 years. In addition to SEM and XRD investigations, the total organic carbon content (TOC), inorganic carbon content (TIC), δ13C and δ18O of carbonates and δ13C of plant remains were analyzed. The concentrations of total carbon contents, the sum of TOC and TIC, remained the same within the 5 years storage, whereas the TOC and TIC contents decreased and increased, respectively. The TIC increase is associated with a decreased TOC, suggesting a conversion of organic carbon into inorganic carbon. Such a conversion is evidenced by the presence of primary lead carbonates and carbon isotopic composition of lead carbonates. It can be concluded that soil organic matters play an important role in the process of bullets weathering and controls migration of the elevated lead in the soil.  相似文献   

10.
The oxygen-isotopic composition of lacustrine authigenic carbonates from a core taken from Lake Tianshuihai in western Qinghai-Tibet Plateau shows an overall increasing δ18O trend (OIT) from about 130 to 15 ka bp. Based on the argument by Wei and Gasse (1999) that the δ18O of lacustrine carbonates from lakes in west China is controlled mainly by the δ18O of the local precipitation, we propose that this OIT may have resulted from a similar OIT of δ18O of precipitation over the drainage area of the Tianshuihai Lake. This OIT suggests that from about 130 to 15 ka bp, variations of δ18O of precipitation on the Qinghai-Tibet Plateau were controlled mainly by changes, not in temperature, but in moisture sources, especially in the ratio of precipitation of moisture from marine sources (MMS) to that from continental evaporation (MCE). The changes in the sources of the moisture and in the MMS/MCE ratio may be mainly a function of the uplift of the Qinghai-Tibet Plateau.  相似文献   

11.
Stratigraphic relations, detailed petrography, microthermometry of fluid inclusions, and fine-scale isotopic analysis of diagenetic phases indicate a complex thermal history in Tithonian fluvial sandstones and lacustrine limestones of the Tera Group (North Spain). Two different thermal events have been recognized and characterized, which are likely associated with hydrothermal events that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartz cementation were identified using scanning electron microscope cathodoluminescence on sandstones and fracture fills. Primary fluid inclusions reveal homogenization temperatures (Th) from 195 to 350°C in the quartz cements of extensional fracture fillings. The high variability of Th data in each particular fluid inclusion assemblage is related to natural reequilibration of the fluid inclusions, probably due to Cretaceous hydrothermal metamorphism. Some secondary fluid inclusion assemblages show very consistent data (Th = 281–305°C) and are considered not to have reequilibrated. They are likely related to an Eocene hydrothermal event or to a retrograde stage of the Cretaceous hydrothermalism. This approach shows how multiple thermal events can be discriminated. A very steep thermal gradient of 97–214°C/km can be deduced from δ18O values of ferroan calcites (δ18O −14.2/−11.8‰ V-PDB) that postdate quartz cements in fracture fillings. Furthermore, illite crystallinity data (anchizone–epizone boundary) are out of equilibrium with high fluid inclusion Th. These observations are consistent with heat-flux related to short-lived events of hydrothermal alteration focused by permeability contrasts, rather than to regional heat-flux associated with dynamo-thermal metamorphism. These results illustrate how thermal data from fracture systems can yield thermal histories markedly different from host-rock values, a finding indicative of hydrothermal fluid flow.  相似文献   

12.
The relationships between the δ18O of quartz veins and veinlets pertaining to the main stage of gold mineralization at the Sukhoi Log deposit and metasomatically altered host slates are estimated. The oxygen isotopic composition of veined quartz and host slates is not uniform. The δ18O of quartz veins from the Western, Central, and Sukhoi Log areas of the deposit vary from +16 to + 18 ‰. The δ18O range of metasomatically altered slates in the Western and Sukhoi Log areas attains 6 ‰. The δ18O of quartz veins are always higher than those of host slates by 3–7‰. The regular difference in the δ18O between quartz veins and host slates indicates that the oxygen isotopic composition of the ore-bearing fluid forming the system of quartz veins and veinlets at the Sukhoi Log deposit could have formed as a result of interaction with silicate rocks, for instance, terrigenous slates enriched in δ18O. Such interaction, however, took place at deeper levels of the Sukhoi Log deposit. It is suggested that the fluid phase participating in the formation of the vein and veinlet system had initially high δ18O(>+10‰) due to interaction with the rocks enriched in δ18O at a low fluid/rock ratio. The oxygen isotope data indicate that the fluid participating in the formation of gold mineralization at the Sukhoi Log deposit was not in equilibrium with igneous rocks at high temperatures.  相似文献   

13.
Groundwater is often the only water source in semi-arid regions of Turkey. Günyüzü Basin, located in the Sakarya River basin, SW of Eskişehir, exhibits semi-arid conditions. The study area is composed of Paleozoic metamorphic rocks, Eocene granitic rocks, Neogene sedimentary rocks, and Quaternary alluvium. In the basin, Paleozoic Marbles are the main reservoir rocks for hot and cold water, bordered by impermeable diabases dykes at the sides and by impermeable granites and schists. Neogene-aged limestones, conglomerates and alluvium represent the other significant aquifers. Water samples chosen to exemplify the aquifer characteristics, were collected from springs and wells in both the dry and the wet seasons. The cation and anion permutation of the samples show that carbonates are the dominant lithology in the formation of chemical composition. δ18O (−11.2 to −8.9‰) and δ2H (−79 to −60‰) isotopic values show that all waters (thermal and cold) are meteoric in origin. The hydrological, hydrochemical, and isotopic properties of the waters reveal that there exist two main groups of groundwater systems; one of these is deep circulating, while the other one is shallow. Tritium values, 0–4 TU (Tritium Unit) indicate the presence of old, static water in these aquifer systems.  相似文献   

14.
Carbon and oxygen isotopic analyses of silicate and carbonate minerals indicate that isotopic compositions in metasediments of the Wallace Formation (Belt Supergroup) exposed northwest of the Idaho batholith have been affected by both prograde and retrograde fluid-rock interaction. Silicates retain isotopic fractionations that reflect equilibration at peak metamorphic temperatures. In contrast, calcite oxygen isotopic compositions range from δ18O(Calcite)=+2.3 to +18.6‰ SMOW (standard mean oceanic water) and indicate that some calcites have exchanged with low-δ18O meteorichydrothermal fluids. Values of Δ18O (Quartz-Calcite) as large as +15.5 clearly indicate that the isotopic depletion of these calcites postdates the peak of regional metamorphism. Carbon isotopic compositions of 18O-depleted calcites are not significantly shifted relative to δ13C values in undepleted calcites, suggesting that the retrograde fluid was carbon-poor. Petrographically, retrograde fluid-rock interaction is associated with the occurrence of fine-grained, highly-luminescent calcite overgrowths on less-luminescent, metamorphic calcites, slight to moderate argillic alteration, and pseudomorphing of scapolite porphyroblasts by fine-grained albite. Retrograde isotopic depletions may be related to shallow meteoric-hydrothermal fluid flow developed around the Idaho batholith after intrusion and rapid uplift of the terrane. Peak metamorphic isotopic compositions in the Wallace Formation reflect mineralogically heterogeneous protolith compositions and isotopic fractionation due to devolatilization and/or infiltration. Variability in oxygen isotopic compositions on the order of 4–6‰ within the same rock type can be attributed to the combined effects of inherited isotopic compositions and isotopic shifts resulting from prograde devolatilization. Isotopic and compositional heterogeneity on the scale of mm to m precludes generalization of isotopic gradients on a regional scale. The isotopic data presented here, and metamorphic fluid compositions determined in previous studies, are best reconciled with heterogeneous bulk compositions, dominantly channelized prograde and retrograde fluid flow, and locally low fluid-rock ratios.  相似文献   

15.
 Previous stable isotope studies at Lizzies Basin revealed that metasedimentary rocks are 18O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also isotopically homogeneous on a scale of hundreds of meters (quartz δ18O=+9.0 to +9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less 18O-depleted and more heterogeneous in δ18O. In order to understand more fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across various metamorphic and igneous lithologies were completed at Lizzies Basin, and at the structurally higher Angel Lake locality. Traverses in the Lizzies Basin Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitoids, reveal similar degrees of homogeneity, although the average δ18O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate gneiss and very little granitoid have more heterogeneous quartz δ18O values (+11.9 to +13.4 per mil), and also have a higher average δ18O (+12.9 per mil), than observed elsewhere. The scale of 18O/16O homogeneity in quartz observed at Lizzies Basin and Angel Lake (meters to hundreds of meters) requires fluid-mediated isotope exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of 18O-depletion in metasedimentary rocks, 18O/16O homogenization between lithologies, and the proportion of granitoids (leucogranites in particular) within any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-18O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by acting as relatively high-18O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments. Single-pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of 18O-depletion and 18O/16O homogenization during regional metamorphism. Received: 27 July 1993 / Accepted: 1 July 1994  相似文献   

16.
Oxygen isotopic composition was studied in the altered host rocks of the Dal’negorsk borosilicate deposit in order to establish a boron source and the origin of ore-forming fluids responsible for deposition of economic borosilicate ore. The relationships between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal positions in the ore zone were studied, including premineral high-potassium minor intrusions located in the zones of datolite mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as concerns the chemical elements contained in ore-forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ18O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ18O (from −2.9 to +0.1‰) is characteristic of the premineral high-potassium and ultrapotassium minor intrusions from skarn-datolite zone. Igneous rocks from the sedimentary framework of the deposit have δ18O of +2 to −0.9‰ The δ18O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore zone. The oxygen isotopic composition of aqueous fluid evidences its exogenic origin. The geochemical and isotopic characteristics of ore-forming fluid show that it could have been deep-seated subsurface water similar to the contemporary water of the Alpine fold zone, which contain up to 700–1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites were a source of boron as well. The relationships between δ18O, K-Ar age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to suggest that the subsurface B-bearing water discharged through narrow channels controlled by premineral basaltic bodies. The discharge was probably initiated by emplacement of basalt and dolerite dikes.  相似文献   

17.
Isotopic composition of monthly composite precipitation samples from Kozhikode (n = 31), a wet tropic station and Hyderabad (n = 25), a semi-arid station across southern India were studied for a period of four years from 2005 to 2008. During the study period, the Kozhikode station recorded an average rainfall of 3500 mm while the Hyderabad station showed an average rainfall of 790 mm. The average stable isotope values in precipitation at the Kozhikode station were δ 18O = −3.52‰, d-excess = 13.72‰; δ 18O = −2.94‰, d-excess = 10.57‰; and δ 18O = −7.53‰, d-excess = 13.79‰, respectively during the pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–February) seasons. For the Hyderabad station, the average stable isotope values were δ 18O = −5.88‰, d-excess = 2.34‰; δ 18O = −4.39‰, d-excess = 9.21‰; and δ 18O = −8.69‰, d-excess = 14.29‰, respectively for the three seasons. The precipitation at the two stations showed distinctive isotopic signatures. The stable isotopic composition of precipitation at the Hyderabad station showed significant variations from the global trend while the Kozhikode station almost followed the global value. These differences are mainly attributed to the latitudinal differences of the two stations coupled with the differences in climatic conditions.  相似文献   

18.
The δ18O, δ13C and 87Sr/86Sr values of calcite and organic matter were measured for carbonate mylonites from numerous thrusts in the Helvetic Alps. Carbonate mylonites in most of the thrusts retain essentially unaltered protolith δ18O and δ13C values, consistent with there having been little to no advection of isotopically distinct fluid through these faults. Only carbonate mylonites from the basal thrusts of the largest nappes have δ18O and/or δ13C values that differ from those of their protoliths. The zone of isotopic alteration/exchange is confined to c. 10 to 20 meters of these fault contacts. We propose the fluids that migrated through these faults contained variable amounts of organically derived carbon and radiogenic strontium, and were probably derived from dewatering of the sedimentary rocks and prograde metamorphic reactions in the nappes' root zones. Apart from the basal thrusts of the largest nappes that behaved as narrow, laterally extensive conduits for fluids, there is little isotopic evidence that large quantities of fluids passed through most of the carbonate-hosted thrusts in the Helvetic Alps. Received: 25 August 1998 / Accepted: 26 February 1999  相似文献   

19.
The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and oxygen stable isotope analyses from the carbonates show non-marine values with ranges of δ13 from ? 7 to ? 11‰and δ18 from ? 3 to ? 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil-derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates.  相似文献   

20.
The Ouenza siderite deposit is located proximal to evaporitic diapirs of Triassic age. Mineralization occurs mainly in Aptian neritic limestones which host important iron concentrations (120–150 MT) and minor Pb, Zn, Cu, Ba and F occurrences. The iron ore consists of iron carbonate minerals which have been oxidized partially to hematite. Fine-grained ankerite and siderite replace limestones, whereas sparry ankerite and siderite were emplaced in veins. Limited variation in the chemical and isotopic compositions of ankerite and siderite were observed, which indicate that they precipitated from the same fluid. Stable isotope compositions (δ18O and δ13C) of iron carbonates and limestones allow estimation of the isotopic composition of the mineralizing fluid and precipitation temperature: δ18O = 7.5‰ SMOW, T = 100–120 °C. Later deposition of Pb, Zn, Cu, Ba and F minerals is controlled by fractures oriented NE–SW and SE–NW. Fluid inclusion studies of quartz yield salinities of 18–22 wt.% equivalent NaCl and homogenization temperatures between 150 and 180 °C. These values are similar to those of Mississippi Valley type deposits which are associated with basinal brines. Received: 4 January 1996 / Accepted: 17 July 1996  相似文献   

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