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1.
This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of these studies was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, as applicable to high-temperature radwaste isolation (particularly in the U.S. nuclear waste program). Research efforts were focused on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite.The regional rocks studies have a vertically elongated δD, δ18O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb---Sr whole-rock isotopic data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral-separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial ratio of approximately 0.7051. The initial ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ18O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm---Nd isotopic data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated reference isochron for 78 Ma and a fixed initial 143Nd/144Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This is probably due to the phonolite samples having different initial 143Nd/144Nd values. All regional samples lie within the “Mantle Array” trend. Their location within Nd–Sr space indicates as asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian gneiss surrounding the Poços de Caldas plateau).The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ18O shift. The δD-δ18O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow, mineralogical alteration and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.Regression of Rb---Sr whole-rock isotopic data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial ratio of approximately 0.7053. Due to the marked hydrothermal alteration and metasomatism of this sample, the Rb---Sr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering. Again, the F4 nepheline syenite samples do not define an Sm---Nd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial 143Nd/144Nd of 0.512365. The Sm---Nd isotopic data also exhibit a possible disturbance by the hydrothermal, metasomatic alteration. A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the Rb---Sr age of the hydrothermally altered nepheline syenite subsamples.  相似文献   

2.
Groundwaters and surface water in the Shihongtan sandstone-hosted U ore district, Xinjiang, NW China, were sampled and analyzed for their major-, and trace element concentrations and oxygen, hydrogen, boron and strontium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The waters in the study district have been grouped into three hydrochemical facies: Facies 1, potable spring-water, is a pH neutral (7.0), Na–Ca–HCO3 type water with low total dissolved solids (TDS; 0.2 g/l, fresh) and has δ18O of − 8.3‰, δD of − 48.2‰,δ11B of 1.5‰, and 87Sr/86Sr of 0.70627. Facies 2 groundwaters are mildly acidic to mildly alkaline (pH of 6.5–8.0, mean 7.3), Na–Ca–Mg–Cl–SO4 type waters with moderate TDS (8.2 g/l–17.2 g/l, mean 9.3 g/l, brackish) and haveδ18O values in the − 5.8‰ to − 9.3‰ range (mean − 8.1‰), δD values in the − 20.8‰ to − 85.5‰ range (mean − 47.0‰),δ11B values in the + 9.5‰ to + 39.1‰ range (mean + 17.1‰), and 87Sr/86Sr values in the 0.70595 to 0.70975 range (mean 0.70826). Facies 3, Aiting Lake water, is a mildly alkaline (pH = 7.4), Na–Ca–Mg–Cl–SO4 type water with the highest TDS (249.1 g/l, brine) and has δ18O of − 2.8‰, δD of − 45.8‰,δ11B of 21.2‰, and 87Sr/86Sr of 0.70840. The waters from the study district show a systematic increase in major, trace element and TDS concentrations and δ11B values along the pathway of groundwater migration which can only be interpreted in terms of water–rock interaction at depth and strong surface evaporation. The hydrochemical and isotopic data presented here confirm that the groundwaters in the Shihongtan ore district are the combined result of migration, water–rock interaction and mixing of meteoric water with connate waters contained in sediments.  相似文献   

3.
《Applied Geochemistry》2002,17(3):163-183
The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO3 type with high CO2 content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na–SO4 type that is also associated with metamorphic rocks of the Menderes Massif with H2S addition; and (4) a Ca–Mg–HCO3–SO4 type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Çanakkale–Tuzla (Br/Cl<sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water–rock interactions; (2) intensification of reactions due to high dissolved CO2 and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ11B=2.3‰ to 18.7‰; n=6) can indicate either leaching of B from the rocks, or B(OH)3 degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. 87Sr/86Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (87Sr/86Sr ratio as high as 0.719479) and carbonate rocks (low 87Sr/86Sr ratio of 0.707864).  相似文献   

4.
Determinations of δ18O values from 100 outcrops of Belt Supergroup (Wallace Fm.) metasedimentary rocks in the Idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange. Low grade argillites at large distances (60 km) from the Idaho batholith have high δ18O values +15, compatible with their probable primary values. Pelitic rocks with anomalously low δ18O values of + 8.7 to + 12.7‰ occur in the following zones: (1) in a 5000 km2 zone of schist and gneiss peripheral to the Idaho batholith, generally coincident with high-grade (sillimanite-bearing) assemblages; (2) in high-grade metasedimentary roof pendants within the Idaho batholith; (3) peripheral to small Cretaceous stocks; and (4) within and near the scapolite-bearing zone south-west of St. Regis. On δ18O−δ18O plots, data from coexisting minerals define trends with unit slopes, indicating that the reductions in 18O occurred under high-grade metamorphic conditions. This metamorphism culminated in the emplacement of the Idaho batholith, probably as a consequence of profound crustal thickening associated with the Cretaceous accretion of the Wallowa-Seven Devils arc terranes with North America. The huge low-18O region is bounded by a “steep” δ18O gradient (0.1–0.5‰/km) that occurs in low-grade rocks along and near the Lewis and Clark Line, well below the biotite isograd. This boundary zone may be analogous to, but is not nearly as sharp as, those of meteoric-hydrothermal systems in many regions. The important ore deposits of the Coeur d'Alene district are located in this peripheral zone, suggesting that the metamorphic-hydrothermal system may have been intimately involved in their formation. In addition, the metamorphic-hydrotermal system in Idaho is larger, deeper and higher in temperature than typical meteoric-hydrothermal systems, and it involved fluids with much higher δ18O values that were probably dominantly derived from formation waters. Accordingly, this system produced rocks with δ18O values similar to those of the Idaho batholith, and mineral assemblages that approach isotopic equilibrium under high temperature conditions.  相似文献   

5.
Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located 50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/86Sr and 143Nd/144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/188Os ratios that range widely from 0.1168 to 0.1288 (γOs = −8.0 to +1.1), averaging 0.1239 (γOs = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/188Os ratios, calculated for 6 Ma, that range from 0.126 (γOs = −1) to as high as 0.561 (γOs = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/188Os between MORB and abyssal peridotites.The collective results of this study suggest that this >500 km3 block of the mantle underwent at least two stages of melting. The first stage occurred at 1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.  相似文献   

6.
Many speleothems show evidence for calcite precipitation under disequilibrium conditions. To improve the understanding of these kinetic processes, several laboratory experiments were performed to study the fractionation of stable oxygen and carbon isotopes during the precipitation of calcite. Carbonate was precipitated under controlled conditions from both a body of standing water (beaker experiments) and a solution flowing along a channel (channel experiments) at a relative humidity of 100%. Slow degassing of CO2, simulated by the beaker experiments, results in δ18O values in equilibrium with the solution. In contrast, the δ13C values show a significant enrichment, inversely proportional to the height of the solution in the beakers. Fast degassing of CO2, simulated by the channel experiments, showed an enrichment of both δ13C and δ18O and a slope of Δδ13C/Δδ18O of 1.4±0.6. These results represent experimental evidence for the Hendy effect, which is manifested in (i) a progressive increase in δ18O and δ13C away from the growth axis and (ii) a positive correlation between δ18O and δ13C along a single growth layer of a stalagmite.  相似文献   

7.
Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high δ13C values of C1–C4 gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the δ13CiC4 value is higher than that of the δ13CnC4, the dominant ranges for the δ13C1, δ13C2, and δ13C3 values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The δ13C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the δ13C1, δ13C2 and δ13C3 values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.  相似文献   

8.
Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal18O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ13C is 1.7‰ (0.0–1.7‰), and δ13C covaries positively and linearly with δ18O. δ13C values are highest when δ18O values indicate the most arid climatic conditions. This is best explained by expansion of C4grasses at the expense of C3plants at low latitudes during glacial periods when atmosphericpCO2was lowered. C4dominance was overridingly influenced by global change in atmosphericpCO2despite the lowered summer rainfall.  相似文献   

9.
Twenty two samples of calcretes from seven depth-profiles in the Menindee catchment, Broken Hill region, Australia were analysed for their inorganic and organic carbon contents and inorganic carbon and oxygen isotopes. The organic carbon content is very low (from 0.06 to 0.31 wt.%) while inorganic carbon (carbonate) is up to 3.9 wt.%. Both δ13C and δ18O become more positive closer to the surface. Carbon isotopes vary from − 8.5‰ to −5.5‰ PDB. Oxygen isotopes vary from − 6‰ to − 1.8‰ V-PDB. Depth-related δ13C and δ18O variations correlate over at least 15 km and show no significant variation along the flow path. δ13C values increase by 3‰ and δ18O values increase by 4‰ with decreasing depth in a 1.40 m thick soil profile. The variation is interpreted to indicate an increasingly elevated air temperature, greater water stress and subsequently an aridification of the area through time. The Broken Hill calcrete data confirm that climatic evolution can be deduced from isotopic series and be applied successfully to the Broken Hill region.  相似文献   

10.
We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ11B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ11B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ11B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ11B≤+8‰) provide a potential third boron source.  相似文献   

11.
Oxygen and carbon isotopes in Jordanian phosphorites and associated fossils   总被引：1，自引：0，他引：1  
Stable isotopes have proven to be efficient tools for paleoenvironmental analysis and interpretation of paleotemperature. Oxygen and carbon isotopes were analyzed in carbonate flourapatite (francolite), oyster shells, tests of foraminifera and ostracods from the Phosphorite Unit throughout Jordan.Isotopic analysis showed δ18O to be enriched in authigenic francolite in Upper Cretaceous in NW Jordan, indicating lower temperatures, a deeper depositional environment and lower salinity than Central Jordan. In Central Jordan, the local basin of Hafira shows enrichment of δ18O indicating a deeper depositional environment than shallower highs in Mutarammil and Wadi El-Hasa. The δ13C shows that the depositional environment was oxic to suboxic and may have reached the suboxic to anoxic interface in the deeper environment in NW Jordan.δ18O values in tests of foraminifera and ostracods are similar to δ18O values of authigenic phosphate, which is enriched in NW Jordan, indicating lower temperature, lower salinity and a deeper environment than Central Jordan. In Central Jordan, δ13C shows more depletion in the Sultani section due to land derived organic carbon (food web supply) carried by terrestrial water draining to the sea.The δ18O in oyster shells show an upward enrichment in the Wadi El-Hasa section, which indicate an increase of intense upwelling, enrichment of nutrients, development of productivity and growth of oyster buildups. Meanwhile, Hafira shows enrichment of δ18O and lower temperature, in agreement with foraminifera and ostracods. The two samples of oysters from SE Jordan, although affected by diagenesis, show heavier oxygen to the north, indicating a deeper water environment and lower salinity in the same basin.  相似文献   

12.
The possible contamination of a groundwater system with industrial wastewater originating from a paper mill factory has been investigated in Piteå, N. Sweden. Six samples were collected from the wastewater in the waste dump and twelve samples from the adjacent groundwater were analyzed for chemistry and sulfur isotopes. The industrial wastewater is a saline water consisting mainly of Na–HCO3–SO4, having a high pH and showing δ34S values between 7‰ and 9‰ affected by bacterial sulfate reduction. The groundwaters are relatively dilute, dominated by Na+, Ca2+ and HCO3−, but with varying concentrations as exemplified by sulfate with concentrations varying between 3 and 69 mg L− 1 while the δ34S values range from − 0.5‰ to 14.3‰. The data suggest that the main S sources in the waters are the bedrock sulfides and/or atmospheric deposition, which, sometimes, are overlapped by bacterial sulfate reduction. Contamination from the waste dump does not occur.  相似文献   

13.
Empirical datasets provide the constraints on the variability and causes of variability in stable isotope compositions (δD or δ18O) of surface water and precipitation that are essential not only for models of modern and past climate but also for investigations of paleoelevation. This study presents stable isotope data for 76 samples from four elevation transects and three IAEA GNIP stations in the Eastern Cordillera of Colombia and the northern Andean foreland. These data are largely consistent with theories of stable isotope variability developed based on a global dataset. On a monthly basis, the precipitation-amount effect exerts the dominant control on δDp and δ18Op values at the IAEA GNIP stations. At the Bogotá station (2547 m), the δDp and δ18Op values vary seasonally, with isotopic minima correlating with maxima in precipitation-amount. Although surface water samples from Eastern Cordilleran streams and rivers fall on the Global Meteoric Water Line, samples from three of four lakes (2842–3459 m) have evaporatively elevated δDsw and δ18Osw values. The IAEA GNIP station data averaged over multiple years, combined with stream and river water data, define vertical lapse rates of −1.8‰ km−1 for Δδ18O and −14.6‰ km−1 for ΔδD, and are a close fit to a common thermodynamically based Rayleigh distillation model. Elevation uncertainties for these relationships are also evaluated. Comparison of this Colombian dataset with the elevation uncertainties generated by the thermodynamically based model shows that the model underestimates uncertainty at high Δδ18O and ΔδD values while overestimating it for low Δδ18O and ΔδD values. This study presents an independent, empirical assessment of stable isotope-based elevation uncertainties for the northern Andes based on a dataset of sufficient size to ensure statistical integrity. These vertical lapse rates and associated uncertainties form the basis for stable isotope paleoelevation studies in the northern Andes.  相似文献   

14.
A number of thermal springs with temperature up to 64°C are found in the Western Cape Province of South Africa. The average δ13C value of gas (CO2+CH4) released at three springs is −22, which is consistent with an entirely biogenic origin for the C and supports previous investigations which showed that the springs are not associated with recent or nascent volcanic activity. Most springs issue from rocks of the Table Mountain Group, where faulted and highly jointed quartzites and sandstones of the Cape Fold Belt act as the main deep aquifer. The δD and δ18O values of the springs range from −46 to −18 and from −7.3 to −3.9, respectively. Although the thermal springs have isotope compositions that plot close to the local meteoric water line, their δD and δ18O values are significantly lower than ambient meteoric water or groundwater. It is, therefore, suggested that the recharge of most of the thermal springs is at a significantly higher altitude than the spring itself. The isotope ratios decrease wuth increasing distance from the west coast of South Africa, which is in part related to the continental effect. However, a negative correlation between the spring water temperature and the δ18O value in the thermal springs closest to the west coast indicates a progressive in increase in the average altitude of recharge away from the coast.  相似文献   

15.
Surface carbonate and land-derived deposits in the sea off southern Chile were investigated for their mineralogical and geochemical composition. The data were related to environmental features and compared with those of similar temperate and polar carbonate deposits from Tasmania, New Zealand, Arctica, and Antarctica. The mineralogy of the siliciclastic fraction is typical of cold areas and is mainly composed of chlorite, mica, quartz, feldspars and amphibole. The CaCO3 content varies from 30 to 90%; carbonate mineralogy is made up of low-Mg calcite, high-Mg calcite and minor amounts of aragonite. The Ca, Mg, Sr, Fe, and Mn contents of bulk carbonates and some selected skeletal hard parts are comparable to those of carbonates from Tasmania. The elemental composition is mainly related to carbonate mineralogy, skeletal components, and seawater conditions. The δ13C and δ18O values of carbonates are positive, and their field falls between the “seafloor diagenesis” and “upwelling water” trend lines, because the sediments are likely to be in equilibrium with waters of Antarctic origin. The mineralogical, elemental, and isotopic compositions of carbonates from southern Chile show better similarities with the “temperate” carbonates from Tasmania and New Zealand than with the “polar” carbonates from Arctica and Antarctica. Carbonate deposition is allowed by the low terrigenous input, the low SPM concentration and, probably, the upwelling of seawater from Antarctica.  相似文献   

16.
An empirical model is presented that derives the initial isotopic composition of water used by terrestrial plants and the relative humidity that prevailed during photosynthesis from the isotopic composition of oxygen and carbon-bound hydrogen in the plant cellulose. The model uses fixed values for the fractionation factors that describe the evapotranspirative isotopic enrichment of leaf waters and the biochemical fractionations occurring during cellulose synthesis. When applied to cellulose extracted from fossil plant matter, the model can be used to generate quantitative estimates of the temperature and moisture regimes of the past. The application of the model is demonstrated by consideration of δ18O and δ2H results on cellulose from fossil wood collected at Brampton, Ontario.The Brampton data yield records of systematic changes in the temperature and moisture regimes of the eastern Great Lakes region over the past 11 500 a. Both mean annual temperature and photosynthetic humidity increased substantially from values much lower than the present at the end of the Wisconsin Stage, to values higher than the present in the mid-Holocene. The phase relation of the paleotemperature and paleohumidity curves engenders differentiation of the climatic history at Brampton into four characteristic zones: (I) a period of ameliorating climate predominantly much colder and drier than now, lasting until about 7400 B.P.; (II) an “early Hypsithermal” period of warm and dry conditions, that transformed after 6000 a B.P. into (III) the “main Hypsithermal” time of warmth and pronounced moistness. Subsequent climatic detrioration led to establishment of (IV) cool, moist conditions like the present sometime within the last few thousand years.  相似文献   

17.
A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ13C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ13Ckerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ13C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO2-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO2” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.  相似文献   

18.
A comparison of a 6450 14C yr δ18O and δ13C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing δ18O and δ13C values from 4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after 2800 BP when δ18O increases again, reflecting more arid conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both δ18Ocalcite and δ13Ccalcite immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. δ18O, δ13C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual–decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past.  相似文献   

19.
Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO2 contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic 207Pb/204Pb(t) (15.41–15.65) and 208Pb/204Pb(t) (37.66–38.51) ratios at given 206Pb/204Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with 87Sr/86Sr(t) ranging from 0.7056 to 0.7071 and εNd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record 87Sr/86Sr(t) ratios of between 0.7044 and 0.7050 and εNd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.  相似文献   

20.
The Changkeng Au and Fuwang Ag deposits represent an economically significant and distinct member of the Au–Ag deposit association in China. The two deposits are immediately adjacent, but the Au and Ag orebodies separated from each other. Ores in the Au deposit, located at the upper stratigraphic section and in the southern parts of the orefield, contain low Ag contents (< 11 ppm); the Ag orebodies, in the lower stratigraphic section, are Au-poor (< 0.2 ppm). Changkeng is hosted in brecciated cherts and jasperoidal quartz and is characterized by disseminated ore minerals. Fuwang, hosted in the Lower Carboniferous Zimenqiao group bioclastic limestone, has vein and veinlet mineralization associated with alteration comprised of quartz, carbonate, sericite, and sulfides. Homogenization temperatures of fluid inclusions from quartz veinlets in the Changkeng and Fuwang deposits are in the range of 210 ± 80 °C and 230 ± 50 °C, respectively. Salinities of fluid inclusions from the two deposits range from 1.6 to 7.3 wt.% and 1.6 to 2.6 wt.% equiv. NaCl, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of the fluid inclusions from the Changkeng deposit range from − 80‰ to − 30‰, − 7.8‰ to − 3.0‰, − 16.6‰ to − 17.0‰ and 0.0100 to 0.0054 Ra, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of fluid inclusions from the Fuwang deposit range from − 59‰ to − 45‰, − 0.9‰ to 4.1‰, − 6.7‰ to − 0.6‰ and 0.5930 to 0.8357 Ra, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of the fluid inclusions suggest the ore fluids of the Changkeng Au-ore come from the meteoric water and the ore fluids of the Fuwang Ag-ore are derived from mixing of magmatic water and meteoric water. The two deposits also show different Pb-isotopic signatures. The Changkeng deposit has Pb isotope ratios (206Pb/204Pb: 18.580 to 19.251, 207Pb/204Pb: 15.672 to 15.801, 208Pb/204Pb: 38.700 to 39.104) similar to those (206Pb/204Pb: 18.578 to 19.433, 207Pb/204Pb: 15.640 to 15.775, 208Pb/204Pb: 38.925 to 39.920) of its host rocks and different from those (206Pb/204Pb: 18.820 to 18.891, 207Pb/204Pb: 15.848 to 15.914, 208Pb/204Pb: 39.579 to 39.786) of the Fuwang deposit. The different signatures indicate different sources of ore-forming material. Rb–Sr isochron age (68 ± 6 Ma) and 40Ar–39Ar age (64.3 ± 0.1 Ma) of the ore-related quartz veins from the Ag deposit indicate that the Fuwang deposit formed during the Cenozoic Himalayan tectonomagmatic event. Crosscutting relationships suggests that Au-ore predates Ag-ore. The adjacent Changkeng and Fuwang deposits could, however, represent a single evolved hydrothermal system. The ore fluids initially deposited Au in the brecciated siliceous rocks, and then mixing with the magmatic water resulted in Ag deposition within fracture zones in the limestone. The deposits are alternatively the product of the superposition of two different geological events. Age evidence for the Fuwang deposit, together with the Xiqiaoshan Tertiary volcanic-hosted Ag deposit in the same area, indicates that the Pacific Coastal Volcanic Belt in the South China Fold Belt has greater potential for Himalayan precious metal mineralization than previous realized.  相似文献   

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