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1.
Moisture samples obtained from unsaturated-zone profiles in sands from northern Nigeria were used to obtain recharge estimates
using the chloride (Cl) mass-balance method and to produce records of past recharge and climatic events. Recharge rates range
from 14–49 mm/year, on the basis of unsaturated-zone Cl values and rainfall chemistry measured over eight years at three local
stations. The unsaturated-zone results also provide a record of the changing recharge and climatic events of the past 80 years;
this record compares quite well with modelling results using precipitation data from Maiduguri, especially for the late 20th-century
period of drought. The best fit for the model is made, however, by using a lower mean rainfall Cl (0.65 mg/l) than that obtained
from the mean of the field results (1.77 mg/l Cl). This result implies that the measured rainfall Cl probably overestimates
the depositional flux of Cl, although the lower value is comparable to the minimum of the measured rainfall Cl values (0.6 mg/l
Cl). Recharge estimates made using these lower Cl values range from 16–30 mm/year. The spatial variability was then determined
using results from 360 regional shallow wells over 18,000 km2.
Using the revised rainfall estimate, the Cl balance indicates a value of 43 mm for the regional recharge, suggesting that
either additional preferential flow is taking place over and above that from the vadose one, or that the regional recharge
represents inputs from earlier wetter periods. These recharge estimates compare favourably with those from hydraulic modelling
in the same area and suggest that the recharge rates are much higher than values previously published for this area. High
nitrate (NO3) concentrations (NO3-N>Cl) preserved under aerobic conditions in the vadose zone reflect secondary enrichment from N-fixing vegetation, as occurs
elsewhere in the Sahel.
Electronic Publication 相似文献
2.
L. Ya. Aranovich E. O. Dubinina A. S. Avdeenko Yu. M. Lebedeva S. A. Bushmin D. D. Dolivo-Dobrovol’skii 《Geochemistry International》2010,48(8):739-751
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
3.
Amor Ben Moussa Sarra Bel Haj Salem Kamel Zouari Vincent Marc Fayçal Jlassi 《Environmental Earth Sciences》2011,62(6):1287-1300
Detailed hydrogeochemical and isotopic data of groundwaters from the Hammamet–Nabeul unconfined aquifer are used to provide
a better understanding of the natural and anthropogenic processes that control the groundwater mineralization as well as the
sources of different groundwater bodies. It has been demonstrated that groundwaters, which show Na–Cl and Ca–SO4–Cl water facies, are mainly influenced by the dissolution of evaporates, the dedolomitization and the cation-exchange process;
and supplementary by anthropogenic process in relation with return flow of irrigation waters. The isotopic signatures permit
to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted
δ
18O and δ
2H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents
reflecting the significance infiltration of return flow irrigation waters. Tritium data in the studied groundwaters lend support
to the existence of pre-1950 and post-1960 recharge. Carbon-14 activities in shallow wells that provide evidence to the large
contamination by organic 14C corroborate the recent origin of the groundwaters in the study area. 相似文献
4.
The isotopic composition of dissolved boron, in combination with the elemental concentrations of B, Cl and salinities in freshwater-seawater mixed samples taken from the estuary of the Changjiang River, the largest one in China, was investigated in detail in this study. Brackish water and seawater samples from the estuary of the Changjiang River were collected during low water season in November, 1998. Boron isotopic compositions were determined by the Cs2BO^+2-graphite technique with a analytical uncertainty of 0.2‰ for NIST SRM 951 and an average analytical uncertainty of 0.8‰ for the samples. The isotopic compositions of boron, expressed in δ^11B, and boron concentrations in the Changjiang River at Nanjing and seawater from the open marine East Sea, China, are characterized by δ^11B values of -5.4‰ and 40.0‰, as well as 0.0272 and 4.43 mg B/L, respectively. Well-defined correlations between δ^11B values, B concentrations and Cl concentrations are interpreted in terms of binary mixing between fiver input water and East Sea seawater by a process of straightforward dilution. The offsets of δ^11B values are not related to the contents of clastic sediment and to the addition of boron. These relationships favor a conservative behavior of boron at the estuarine of the Changjiang River. 相似文献
5.
The need for more agricultural or residential land has encouraged reclamation at the coastal areas of Korea since 1200 ad (approximately). The groundwaters of these reclaimed areas could be expected to reveal hydrogeochemical properties different
from those of areas directly affected by seawater intrusion. The purpose of this study, therefore, was to examine the salinization
of shallow groundwater in a coastal reclaimed area and to identify the effect of land reclamation on groundwater quality.
Major cations and anions, iodide, total organic carbon, δD, δ
18O and δ
13C were measured to assist the hydrogeochemical analysis. Chloride, δD and δ
18O data clearly show that the Na–Cl type water results from mixing of groundwater with seawater. In particular, the δD and δ
18O of Ca+Mg–Cl+NO3 type groundwaters are close to the meteoric water line, but Na–Cl type waters enriched in chloride are 18O-enriched with respect to the meteoric water line. Meanwhile, carbon isotopic data and I/Cl ratios strongly suggest that
there are various sources of salinity. The δ
13C values of Na–Cl type groundwaters are generally similar to those of Ca+Mg–Cl+NO3 type waters, which are depleted in 13C with respect to seawater. I/Cl ratios of Na–Cl type groundwater are 10–100 times higher than that of seawater. Because the
reclamation has incorporated a large amount of organic matter, it provides optimum conditions for the occurrence of redox
processes in the groundwater system. Therefore, the salinization of groundwater in the study area seems to be controlled not
only by saltwater intrusion but also by other effects, such as those caused by residual salts and organic matter in the reclaimed
sediments. 相似文献
6.
R. Abart 《Contributions to Mineralogy and Petrology》1995,122(1-2):116-133
Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation
in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello
batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite,
which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C.
The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered
contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly
shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The
isotopic compositions of calcite (cc) in the unaltered marble are about δ18O (SMOW; Standard mean Ocean Water)=21.0‰ and δ13C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ18Occ is 15.0 to 16.0‰ and δ13Ccc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which
occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation
between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ18Occ-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the
infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite
from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability
was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration.
Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite
was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled
calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid
isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid
exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid
exchange.
Received: 16 June 1994/Accepted: 20 May 1995 相似文献
7.
EL H. Talbi J. Honnorez N. Clauer F. Gauthier-Lafaye P. Stille 《Contributions to Mineralogy and Petrology》1999,137(3):246-266
Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the
Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight
hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic
deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between
300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which
characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite).
Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These
exchanges lead to decrease of gabbroic δ18O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (87Sr/86Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in
Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization
and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean
geochemistry.
Received: 8 March 1999 / Accepted: 12 July 1999 相似文献
8.
9.
D. L. Kirschner H. Masson Z. D. Sharp 《Contributions to Mineralogy and Petrology》1999,136(1-2):169-183
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
10.
Quantifying regional groundwater flow between Continental Intercalaire and Djeffara aquifers in southern Tunisia using isotope methods 总被引:4,自引:0,他引:4
Large-scale interaction between the Continental Intercalaire and the Djeffara aquifer systems in the southeast of Tunisia
has been investigated with the aid of chemical and isotopic tracers. Two distinct groundwater types have been identified:
(1) the Continental Intercalaire groundwater characterized by elevated temperatures (50–61.4°C), low δ18O (−8.4 to−7.87) and δ2H (−67.2 to−59) values and negligible radiocarbon content, both testifying its great age dating from the late Pleistocene
period, and (2) the Djeffara groundwater with distinctly heavier isotopic composition (δ18O = −8.31 to −5.80, δ2H = −65.9 to −31.9). The Djeffara groundwaters reveal a distinct changes of physico-chemical and isotopic parameters near
El Hamma Faults in the northwestern part of the Djeffara basin. These changes could possibly be explained by a vertical leakage
from the Continental Intercalaire aquifer through El Hamma Faults. The mixing proportions inferred from stable isotope mass
balance prove that the contribution of the Continental Intercalaire to the recharge of Djeffara aquifer is very significant
and may reach 100% in the El Hamma region and in the northern part of Gabes. Isotope tracers strongly suggest that recent
recharge to the Djeffara aquifer system is very limited. Its current yield, particularly in its central and northern parts
can be maintained only thanks to large-scale underground inflow from the Continental Intercalaire aquifer system, which carries
late Pleistocene palaeowater. Consequently, current exploitation of groundwater resources of the Djeffara aquifer has non-sustainable
character. 相似文献
11.
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
12.
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
13.
Oxygen isotope mapping of the Archean Sturgeon Lake caldera complex and VMS-related hydrothermal system, Northwestern Ontario, Canada 总被引:1,自引:0,他引:1
The hydrothermal and magmatic evolution of the Sturgeon Lake caldera complex is graphically documented by a regional-scale
(525 km2) analysis of oxygen isotopes. Spatial variations in whole-rock oxygen isotope compositions provide a thermal map of the cumulative
effects of multiple stages of hydrothermal metasomatism before, during, and after volcanogenic massive sulfide (VMS) mineralization.
There is a progressive, upward increase in δ18O from less than 2‰ to greater than 15‰ through a 5-km-thick section above the Biedelman Bay subvolcanic intrusive complex.
This isotopic trend makes it clear that at least the earlier phases of this intrusive complex were coeval with the overlying
VMS-hosting cauldron succession and provided thermal energy to drive a convective hydrothermal circulation system. The sharp
contrast in δ18O values between late stage phases of the Biedelman Bay intrusion and immediate hanging wall strata indicates that the main
phase of VMS-related hydrothermal activity took place before late-stage resurgence in the cauldron-related magmatic activity.
Mineralogical and isotopic evidence indicates the presence of both syn- and postmineralization hydrothermal activity defined
by the presence of widespread semiconformable and more restricted discordant alteration zones that affect the pre- and syncauldron
strata. The semiconformable alteration zones formed during early stages of hydrothermal circulation and are defined by widespread
silicification and carbonatization in association with relatively high δ18O values. The discordant alteration assemblages, containing Al-silicate minerals with chloritoid and/or Fe-rich carbonate
or chlorite, centered on synvolcanic faults represent restricted zones of both seawater inflow and hydrothermal fluid upflow.
A rapid increase in δ18O values (∼7–9‰) over a short distance (<200 m) suggests marked cooling of hydrothermal fluid from ∼350°C to less than 130°C
either just before or during discharge onto the seafloor. Late emplacement of diorite sills and a dacite dome disrupted the
isotopic imprint of cauldron-stage hydrothermal activity. The abrupt lowering of δ18O values at the transition from explosive to passive volcanism (andesite flows and dacite domes) indicates postcauldron emergence.
Subsequently, thrust faulting disrupted the older synvolcanic hydrothermal isotopic patterns. 相似文献
14.
Amber Hardison Craig Tobias Jennifer Stanhope Elizabeth Canuel Iris Anderson 《Estuaries and Coasts》2011,34(2):243-255
The water–sediment interface is a dynamic zone where the benthic and pelagic environments are linked through exchange and
recycling of organic matter and nutrients. However, it is often difficult to measure rate processes in this zone. To that
end, we designed an experimental apparatus for continuous and homogeneous perfusion of sediment porewater with dissolved conservative
(SF6, Rhodamine WT dye) and isotopic (H13CO3− and 15NH4+) tracers to study nitrogen and carbon cycling by the sediment microbial community of shallow illuminated sediments. The perfusionator
consists of a 60-cm ID × 60-cm high cylinder that includes a reservoir for porewater at the base of the sediment column. Porewater
amended with conservative and stable isotopic tracers was pumped through a mixing reservoir and upward through the overlying
sediments. We tested the perfusionator in a laboratory setting, as part of an outdoor mesocosm array, and buried in coastal
sediments. Conservative and isotopic tracers demonstrated that the porewater tracers were distributed homogeneously through
the sediment column in all settings. The perfusionator was designed to introduce dissolved stable isotope tracers but is capable
of delivering any dissolved ionic, organic, or gaseous constituent. We see a potentially wide application of this technique
in the aquatic and marine sciences in laboratory and field settings. 相似文献
15.
Stable carbon isotopic composition of soil organic matter in the karst areas of Southwest China 总被引:1,自引:1,他引:0
This study dealt with the distribution characteristics of soil organic carbon (SOC) and the variation of stable carbon isotopic composition (δ^13C values) with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ^13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ^13C values of soil organic matter (SOM) changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical pattems of organic carbon and δ^13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas. 相似文献
16.
A. I. Grabezhev B. G. Pokrovskii F. P. Buslaev V. V. Zaikov G. N. Pshenichnyi 《Lithology and Mineral Resources》2000,35(1):70-75
Substantial differences in isotopic compositions of micas and pyrophyllites from metasomatites related to various stages of
the process that formed the giant Gai massive sulfide deposit have been established. The illite from the earliest and predominant
chlorite-illite-quartz metasomatite is characterized by the least δD values of −(50–85)‰ and δ18O=7–11‰. The pyrophyllite-quartz metasomatite as well as illite and pyrophyllite schists developed locally in the southern
part of the deposit that likely correspond to the site of discharge of late geothermal paleosystem, contain pyrophyllite and
illite with much higher values of δD=−(25–45)‰ and δ18O=4–9‰. Local zones of illite-paragonite schist complete the mineral formation and are characterized by the transitional δD values of −(30–55)‰ and elevated δ18O of 10–11‰. The most plausible model of isotopic evolution in the hydrothermal system, with an initial temperature of mica
formation at 250°C, assumes the mixing of transformed sea water with a magmatic (metamorphic) water at the initial stage when
the background metasomatites and massive sulfide orebodies of the northern lode have been formed. Subsequently, after the
burial of the northern lode beneath basaltic andesite flows, the repeated sea water invasions took place in the southern discharge
site of the system. As a result, the pyrophyllite-quartz metasomatite was formed; the pyrophyllite and illite schists originated
in tectonic compression zones. The interaction of this water with silicate rocks was completed by a formation of illite-paragonite
schist. In general, the substantial contribution of sea water to the formation of metasomatic halo of the deposit casts no
doubt. 相似文献
17.
The present work was conducted in the Sinai Peninsula (1) to identify the recharge and flow characteristics and to evaluate
the continuity of the Lower Cretaceous Nubian Sandstone aquifer; and (2) to provide information for the aquifer's rational
appraisal. Isotopic and hydrochemical compositions combined with the geological and hydrogeological settings were used for
this purpose.
A considerable depletion in isotopic content (oxygen-18 and deuterium) and low d-excess values exist in the studied groundwater,
reflecting the contribution of old meteoric water that recharged the aquifer in pluvial times. Modern recharge also occurs
from precipitation that falls on the aquifer outcrops. The wide scatter of the data points around the two meteoric lines,
the global meteoric water line (GMWL) and Mediterranean meteoric water line (MMWL), in the δ18O–δD diagram indicates considerable variation in recharge conditions (amount, altitude, temperature, air masses, distances
from catchment, overland flow, etc.). The isotopic composition in the El-Bruk area is minimum (18O=–9.53‰), very close to the average value of the Western Desert Nubian Sandstone (18O=–10‰), where the local structural and lithologic conditions retard groundwater flow and the main bulk of water becomes noncyclic.
The continuity of the aquifer in northern and central Sinai is evidenced by the isotopic similarity between samples taken
from above and below the central Sinai Ragabet El-Naam fault, the distribution of potentiometric head, and hydrogeological
cross sections.
The combination of isotopic composition in terms of 18O and chemical composition in terms of TDS and salt contents is the basis for separating the studied groundwater into groups
that reflect the recharge sources and isotopic and chemical modifications during flow.
Electronic Publication 相似文献
18.
Tiziano Boschetti Lorenzo Toscani Orfan Shouakar-Stash Paola Iacumin Giampiero Venturelli Claudio Mucchino Shaun K. Frape 《Aquatic Geochemistry》2011,17(1):71-108
The salt waters from the Emilia-Romagna sector of the Northern Apennine Foredeep have been investigated using major and trace
element and stable isotope (δ2H, δ18O, δ37Cl, δ81Br and 87Sr/86Sr ratio). Ca, Mg, Na, K, Sr, Li, B, I, Br and SO4 vs. Cl diagrams suggest the subaerial evaporation of seawater beyond gypsum and before halite precipitation as primary process
to explain the brine’s salinity, whereas saline to brackish waters were formed by mixing of evaporated seawater and water
of meteoric origin. A diagenetic end-member may be a third component for mud volcanoes and some brackish waters. Salinization
by dissolution of (Triassic) evaporites has been detected only in samples from the Tuscan side of the Apennines and/or interacting
with the Tuscan Nappe. In comparison with the seawater evaporation path, Ca–Sr enrichment and Na–K–Mg depletion of the foredeep
waters reveal the presence of secondary processes such as dolomitization–chloritization, zeolitization–albitization and illitization.
Sulfate concentration, formerly buffered by gypsum-anhydrite deposition, is heavily lowered by bacterial and locally by thermochemical
reduction during burial diagenesis. From an isotopic point of view, data of the water molecule confirm mixing between seawater
and meteoric end-members. Local 18O-shift up to +11‰ at Salsomaggiore is related to water–rock interaction at high temperature (≈150°C) as confirmed by chemical
(Mg, Li, Ca distribution) and isotopic (SO4–H2O) geothermometers. 37Cl/35Cl and 81Br/79Br ratios corroborate the marine origin of the brines and evidence the diffusion of halogens from the deepest and most saline
aquifers toward the surface. The 87Sr/86Sr ratio suggests a Miocene origin of Sr and rule out the hypothesis of a Triassic provenance of the dissolved components
for the analyzed waters issuing from the Emilia-Romagna sector of the foredeep. Waters issuing from the Tuscan side of the
Apennines and from the Marche sector of the foredeep show higher 87Sr/86Sr ratios because of the interaction with siliciclastic rocks. 相似文献
19.
Whole rock oxygen isotope data are presented for the Panorama district, in the Archean Pilbara Craton of Western Australia,
where near-perfect exposure reveals a cross section through a complete volcanogenic massive sulfide (VMS) hydrothermal alteration
system. The δ18O values decrease with depth in the volcanic pile, across semi-conformable alteration zones, to values below 6‰ immediately
above a large (180 km2) subvolcanic intrusion. Altered rocks in the upper parts of the subvolcanic intrusion have lower δ18O values (6–8‰) than least altered granite (8‰), apart from sericite–quartz altered zones, which are slightly higher (8–10‰).
Corridors of low δ18O values crosscut this regional zonation, and are coincident with transgressive feldspar-destructive alteration zones, which
underlie VMS mineralization. The whole rock oxygen isotope distribution patterns are interpreted to represent alteration temperature,
where high δ18O values correspond to low temperature alteration and low δ18O values correspond to high temperature alteration. Alteration temperatures, which were calculated using modal alteration
mineral abundances and an assumed fluid δ18O, are consistent with this interpretation. Increasing temperatures with depth in the volcanic pile and high temperatures
in transgressive corridors leading up to VMS deposits, are consistent with a convective hydrothermal model, in which heat
from the subvolcanic intrusion drove seawater through the volcanic pile. Granite-hosted sericite–quartz alteration zones are
18O-enriched, and are tentatively interpreted to have formed from a mixed magmatic-evolved seawater fluid.
Received: 12 April 1999 / Accepted: 6 April 2000 相似文献
20.
Reinhard Aesse 《中国地球化学学报》1987,6(2):99-114
The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant
portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation
rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic
pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which
are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition
of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light
sulfide (δ
34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ
34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted,
semi-stagnant basins (Baltic Sea).
pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the
nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-,
Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in
the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited
or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals
carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate
concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation,
called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including
the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ
13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing
concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations.
Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides
apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the
alkalinity maximum.
Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and
non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to
Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic
stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier
bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite
to illite transformation) to start. 相似文献