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1.
Pyrite is a common and abundant sulfidic mineral subject to oxidation. The weathering characteristics of rock-bearing pyrite sometimes impose serious influences on the surrounding environment as the oxidation of pyrite (FeS2) generates acid drainage that results in the acceleration of rock weathering and the discharge of heavy metals into the environment. Such an accelerated weathering of rocks can reduce its mechanical properties and therefore menace the stability of rock structures, such as excavated slopes and tunnels. The evolution of physical properties of rocks and the chemical composition of drainage were evaluated in this study by a weathering test using a double Soxhlet extractor for 1 month in a laboratory setting. Three groups of biotite gneiss classified according to their pyrite content were used for the Soxhlet extraction experiment (group A with less than 0.1 wt% of pyrite; group B with about 3.3 wt% of disseminated pyrite; group C with about 5.65 wt% of vein type pyrite). The massive groups A and B had limited weathering on the surface; however, group C with the pyrite vein experienced weathering on the surface as well as along the pyrite vein. The weathering type regulated by the occurrence of pyrite apparently controlled the mechanical properties of the rock samples and the chemistry of the drainage. Groups A and B showed no significant quick absorption ratio after the 1-month experiment; however, group C had about 10 % increase in value. The uniaxial compressive strength of the three groups decreased about 20, 10 and 45 % for groups A, B and C, respectively. The mechanical properties of the samples and the chemical compositions of the drainage indicate that the oxidation of pyrite contained in the samples accelerated weathering, resulting in deterioration of mechanical properties of the rocks, and could result in the discharge of heavy metals and acid into the environment with the drainage.  相似文献   

2.
Black shale oxidative weathering plays a significant role in a variety of processes including acid mine drainage and atmospheric CO2 control. The modeling of weathering is highly dependent on reactive surface area. In this study it is shown that black shale oxidative weathering is regulated mainly by the external, geometrical surface area of rock polyhedrons and the organic matter’s (OM) internal surface area. The internal rock surface area decreases dramatically during OM dissolution from ∼15 m2/g to ∼5 m2/g. A linear relationship was found between the decrease of internal rock surface area and quantity of OM dissolved. Optical roughness analyses of black and bleached shale surface area reveal the formation of macropores due to the dissolution of mesoporous and probably microporous OM. However, due to deconsolidation, the geometrical external rock polyhedron surface area increases during weathering. Black shale polyhedrons show a doubling of their external surface area as OM decreases. This provokes an increase of the shale volume which is easily accessible by fluids. The increase of the external rock surface area seems to be self-accelerating during weathering. The upscaling of external and internal rock surface area evolution during weathering presented in this study demonstrates the possible application of these results to the improved understanding of a chemical transport in a variety of natural systems.  相似文献   

3.
Subaerial exposure and oxidation of organic carbon (Corg)-rich rocks is believed to be a key mechanism for the recycling of buried C and S back to Earth's surface. Importantly, processes coupled to microbial Corg oxidation are expected to shift new biomass δ13Corg composition towards more negative values relative to source. However, there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting δ13Corg at the rock-atmosphere interface. This is particularly pertinent to the sulfidic, Corg-rich alum shale units of the Baltoscandian Basin believed to constitute a strong source of metal contaminants to the natural environment, following subaerial exposure and weathering. Consistent with independent support, we show that atmospheric oxidation of the sulfidic, Corg-rich alum shale sequence of the Cambrian-Devonian Baltoscandian Basin induces intense acid rock drainage at the expense of progressive oxidation of Fe sulfides. Sulfide oxidation takes priority over microbial organic matter decomposition, enabling quantitative massive erosion of Corg without producing a δ13C shift between acid rock drainage precipitates and shale. Moreover, 13C enrichment in inorganic carbon of precipitates does not support microbial Corg oxidation as the predominant mechanism of rock weathering upon exposure. Instead, a Δ34S = δ34Sshale − δ34Sprecipitates ≈ 0, accompanied by elevated S levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales, provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering. Slight δ15N depletion in the new surface precipitates relative to shale, coincides with dramatic loss of N from shales. Collectively, the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock-atmosphere interface, indicating that quantitative mass release of Corg, N, S, and key metals to the environment is a response to intense sulfide oxidation. Consequently, large-scale acidic weathering of the sulfide-rich alum shale units is suggested to influence the fate and redistribution of the isotopes of C, N, and S from shale to the immediate environment.  相似文献   

4.
A base line study involving analysis of groundwater samples from the Jhansi district were carried out to determine the major and trace element chemistry and to assess the hydrogeochemical processes and water quality for domestic and irrigation uses. Study results show that groundwater is mildly acidic to alkaline in nature and HCO3 ?, Cl?, Ca2+, Na+ and Mg2+ are the major contributing ions for the dissolved loads. The data plotted on the Gibbs and Piper diagrams reveal that the groundwater chemistry is mainly controlled by rock weathering with secondary contribution from anthropogenic sources. In a majority of the groundwater samples, alkaline earth metals exceed alkalies and weak acid dominate over strong acids. Ca–Mg–HCO3 is the dominant hydrogeochemical facies in the majority of the groundwater samples. The computed saturation indices demonstrate that groundwater is oversaturated with respect to dolomite and calcite. Kaolinite is the possible mineral that is in equilibrium with the water, implying that the groundwater chemistry favors kaolinite formation. A comparison of groundwater quality parameters in relation to specified limits for drinking water shows that the concentrations of TDS, F?, NO3 ?, total hardness and Fe are exceeding the desirable limits in many water samples. Quality assessment for irrigation uses reveal that the groundwater is of good to suitable category. Higher salinity and residual sodium carbonate values at some sites restrict the suitability of groundwater and need an adequate drainage and water management plan for the area.  相似文献   

5.
Twenty-seven samples of black weathering crust and host carbonates were studied from seven European countries (Germany, Hungary, Belgium, Czech Republic, France, Italy and Poland) representing 11 different sites. The samples were collected for sites for which long-term air pollution records are available. The mineralogical analyses (XRD, polarizing microscopy, SEM) have shown that despite decreasing SO2 emissions crust samples are still very rich in gypsum. Further, in all host rock samples gypsum was also detected. Good correlations (R2 > 0.9) were also found between water-soluble calcium and gypsum content and between sulphate and gypsum content both for black crusts and host rocks. The black gypsum crusts are four or five times richer in sulphate than the host rock. The conductivity of dissolved crust and host rock samples also shows a positive correlation with gypsum content of the samples. LA-ICP-MS analyses allowed the detection of high Pb-levels in black crusts and a negative shift in lead concentration at the crust/host rock transition. The lead content of the host rock is 2–5 mg/kg, while that of the crust is 3–25 mg/kg in the sample collected from Germany, while in the Belgian sample these values are 2–14 mg/kg and 80–870 mg/kg for the host rock and crust, respectively. The GC–MS technique allowed to detect the PAH content of black crusts and host rocks. The former one contains 0.6–15.6 (102.5) mg/kg, while in the host rock values between 0.2 and 2.4 mg/kg were found. The present study suggests that still large amounts of air pollution-related minerals and organic pollutants are found in the black weathering crusts of European carbonate buildings despite decreasing trends in air pollution.  相似文献   

6.
A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO2 consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ34S values of 16 to 20‰CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS2. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO2 fluxes from silicate weathering range from ~ 0.012 × 106 to 0.039 × 106 mol/km2/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO2 consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO2 consumption rates in particular, based on major ion abundances, should be treated with skepticism.  相似文献   

7.
Chemical weathering in the Three Rivers region of Eastern Tibet   总被引：2，自引：0，他引：2  
Three large rivers - the Chang Jiang (Yangtze), Mekong (Lancang Jiang) and Salween (Nu Jiang) - originate in eastern Tibet and run in close parallel over 300 km near the eastern Himalayan syntaxis. Seventy-four river water samples were collected mostly during the summer season from 1999 to 2004. Their major element compositions vary widely, with total dissolved solids (TDS) ranging from 31 to 3037 mg/l, reflecting the complex geologic makeup of the vast drainage basins. The major ion distribution of the main channel samples primarily reflects the weathering of carbonates. Evaporite dissolution prevails in the headwater samples of the Chang Jiang in the Tibetan Plateau interior, as evidenced by the high TDS (928 and 3037 mg/l) and the Na-Cl dominant major element composition. Local tributary samples of the Mekong and Salween, draining the Lincang Batholith and the Tengchong Volcano, show distinctive silicate weathering signatures. We used five reservoirs - rain, halite, sulfate, carbonate, and silicate - in a forward model to calculate the contribution from silicate weathering to the total dissolved load and to estimate the consumption rate of atmospheric CO2 by silicate weathering. Carbonate weathering accounts for about 50% of the total cationic charge (TZ+) in the samples of the Mekong and the Salween exiting the Tibetan Plateau. In the “exit” sample of the Chang Jiang, 45% of TZ+ is from halite dissolution inherited from the extreme headwater tributaries in the interior of the plateau, and carbonates contribute only 26% to the TZ+. The net rate of CO2 consumption by silicate weathering is (103-121) × 103 mol km−2 year−1, lower than the rivers draining the Himalayan front. GIS-based analyses indicate that runoff and relief can explain 52% of the spread in the rate of atmospheric CO2 drawdown by silicate weathering, but other climatic (temperature, precipitation, potential evapotranspiration) and geomorphic (elevation, slope) factors also show collinearity. Only qualitative conclusions can be drawn for the significance of lithology due to lack of digitized lithologic information. The effect of the peculiar drainage pattern due to tectonic forcing is not readily apparent in the major element composition or in increased chemical weathering rates. The 87Sr/86Sr ratios and the silicate weathering rates are in general lower in the Three Rivers than in the rivers draining the Himalayan front.  相似文献   

8.
鄱阳湖流域岩石化学风化特征及CO2消耗量估算   总被引：1，自引：0，他引：1       下载免费PDF全文
岩石风化过程中所产生的碳汇是全球碳循环的重要组成部分,该领域受到研究全球变化科学家们的普遍关注。文中通过对鄱阳湖流域河水系统的样品采集和化学成分分析,研究了河水化学成分来源及流域岩石风化所产生的碳汇效应。以大气降水、蒸发岩、硅酸岩和碳酸盐岩为4个端员,计算出它们对河水中溶解质的贡献率分别为10．4％、21．9％、30．...  相似文献   

9.
Aji-Chay River is one of the most important surface reservoirs of northwest of Iran, because it passes through Tabriz city and discharges to Urmia Lake, one of the largest permanent salty lakes in the world. The main objectives of the present study are to evaluate its overall water quality and to explore its hydrogeochemical characteristics, including the potential contamination from heavy metals and metalloids such as Co, Pb, Zn, Cd, Cu, Cr, Al and As. For this purpose, 12 water samples were collected from the main river body and its tributaries within Tabriz plain. The Piper diagram classified water samples mainly into Na–Cl and secondary into Ca–HCO3 and mixed Ca–Mg–Cl types, denoting a profound salinization effect. The cross-plots showed that natural geochemical processes including dissolution of minerals (e.g., carbonates, evaporites and silicates), as well as ion exchange, are the predominant factors that contribute to fluvial hydrogeochemistry, while anthropogenic activities (industrial and agricultural) impose supplementary effects. Cluster analysis classified samples into two distinct clusters; samples of cluster B appear to have elevated electrical conductivity (EC) values and trace metals concentrations such as Co, Pb and Cd, while SiO2 and Zn are low in comparison with the samples of the cluster A. The main processes controlling Aji-Chay River hydrogeochemistry and water quality were identified to be salinization and rock weathering. Both are related with geogenic sources which enrich river system with elevated values of Na+, Cl?, Ca2+, Mg2+, K+, SO4 2? and EC as a direct effect of evaporites leaching and elevated values of Pb and Cd as an impact from the weathering process of volcanic formations. According to the US salinity diagram, all of the water samples are unsuitable for irrigation as having moderate to bad quality.  相似文献   

10.
The aim of this paper was to evaluate the chemical weathering rate at Alto Sorocaba basin, São Paulo State, Brazil, as it is an important parameter on geomorphological characterization of continents. Several tools may be used for such evaluation, and this work compares the results obtained from analytical data involving the uranium concentration, the 234U/238U activity ratio and the major cation concentration for samples of waters, soils, and rocks from that basin. The use of the natural uranium as a tracer in weathering rate rocks studies is advantageous since this element is virtually absent in rainwater and also allows obtaining the dissolution rocks coefficient by the 234U/238U activity ratio. The Alto Sorocaba basin has serious environmental problems in terms of the quality of the rainwater and river waters, and, consequently, the geochemical balance using the main cations presented some difficulties. However, measuring of the weathering rate utilizing the U-isotopes method corresponded to 14 μm/year that allows estimating a time of 72,000 years to weather 1 m of rock at Sorocaba River upstream from Itupararanga Reservoir, under the present climatic conditions.  相似文献   

11.
Surface water resources play a crucial role in the domestic water delivery system in Ghana. In addition, sustainable food production is based on the quality and quantity of water resources available for irrigation purposes to supplement rain-fed agricultural activities in the country. The objective of this research was to determine the main controls on the hydrochemistry of surface water resources in the southern part of Ghana and assess the quality of water from these basins for irrigation activities in the area. R-mode factor and cluster analyses were applied to 625 data points from 6 river basins in southern Ghana after the data had been log transformed and standardized for homogeneity. This study finds that surface water chemistry in the south is controlled by the chemistry of silicate mineral weathering, chemistry of rainfall, fertilizers from agricultural activities in the area, as well as the weathering of carbonate minerals. A Gibb’s diagram plotted with total dissolved solids (TDS) on the vertical axis against (Na+ + K+)/(Ca2+ + K+ + Na+) on the horizontal axis indicates that rock weathering plays a significant role in the hydrochemistry. Activity diagrams for the CaO–Na2O–Al2O–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems suggest that kaolinite is the most stable clay mineral phase in the system. In addition, an assessment of the irrigation quality of water from these basins suggests that the basins are largely low sodium—low to medium salinity basins, delivering water of acceptable quality for irrigation purposes.  相似文献   

12.
While it is critical to accurately understand the sources and transformation of sulfate based on time-series analysis, there are limited studies on temporal variation of sulfate in rivers and on rock weathering by sulfuric acids. We conducted a monthly sampling campaign in the Beipan, Nanpan, and Hongshui Rivers over the course of one hydrological year. This study examined seasonal variations in riverine sulfate impacted by the monsoon climate in the upper reaches of the Xijiang River basin. In general, the SO4 2? contents in these rivers dropped from relatively high levels to low values during the high-flow season, in response to increasing discharge. The sulfate was generally enriched in heavy isotopes during the low-flow season compared to the high-flow season. The calculated results indicate that the riverine sulfate was mainly derived from sulfide oxidation, but that evaporite dissolution could be an important source during the low-flow season, based on isotopic evidence. Mine drainage is likely an important source of sulfate to these rivers during the high-flow season due to contributions from fast surface flow, which responds to frequent heavy rain in monsoonal climate regions. A relatively high proportion of HCO3 ? was found to be derived from rock weathering by sulfuric acid during the high-flow season when compared to that observed during the low-flow season. The results suggest that approximately one quarter of the HCO3 ? in the Hongshui River originated from carbonate weathering by sulfuric acid derived from the oxidation of sulfide. Such information on the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal variability of sulfate concentrations and weathering processes in rivers.  相似文献   

13.
In order to study the major ion chemistry and controls of groundwater, 65 groundwater samples were collected and their major ions measured from wells within Lhasa River Basin. Groundwater has the characteristics of slightly alkaline and moderate total dissolved solid (TDS). TDS concentration ranged from 122.0 to 489.9 mg/L with a median value of 271.2 mg/L. Almost all the groundwater samples suited for drinking and irrigation. The major cations of groundwater are Ca2+ and Mg2+, accounting for 59.6 and 31.3% of the cations, respectively. Meanwhile, HCO3? and SO42? constituted about 56.7 and 36.9% of the anions, respectively, in Lhasa River Basin. The hydrochemical type of groundwater is HCO3-SO4-Ca-Mg. The chemical composition of groundwater samples located in the middle of Gibbs model, which indicates that the major chemical process of groundwater is controlled by rock weathering. Carbonate weathering was the dominant hydro-geochemical process controlling the concentration of major ions in groundwater within Lhasa River Basin, but silicate weathering also plays an important role.  相似文献   

14.
Acid rock drainage (ARD) is a longstanding problem often associated with the resulting corrosion due to the acidity generated from sulfidic oxidation. To evaluate characteristics of ARD and corrosion, samples from the road side rock mass of Boeun and Mujoo were analysed using X-ray diffraction, acid/base accounting and Leaching tests. The results indicated that many samples had a pyritic origin and can be regarded as acid-generating rocks. The Leaching test showed that the average pH of the leachates of samples from both Boeun and Mujoo were moderately acidic, ranging from 3 to 4. Interestingly, as acidity increases from pH 4, the SO4−, Fe, Al and Mg concentrations increase abnormally. Samples from roadside slope of Mujoo showed high corrosive potential. Maximum sulfide oxidation rate of a sample taken from Mujoo was as high as 5,166 mg/kg/week.  相似文献   

15.
Hydrochemical studies were carried out in parts of Oban Massif and Mamfe Embayment, Southeastern Nigeria to examine the contributions of barite mining activities on water quality. Pond and stream water samples were collected from six abandoned barite mines and adjoining streams areas during dry and wet seasons. These samples were analysed for physicochemical parameters using standard techniques. The results showed that the quality of water samples in the vicinity of barite mine sites was characterized by low pH, low mineralization, high suspended solids and deep colour. Sodium (Na+) and calcium (Ca2+) are the dominant cations and bicarbonate (HCO3 ?) and sulphate (SO4 2?) the dominant anions. The low concentration of dissolved silica, relatively high concentrations of Na+, HCO3 ? and SO4 2? suggest a combined influence of silicate, carbonate and weathering of mine spoils. The total dissolved solids, total suspended solids, electrical conductivity and major ions (except Na+ and K+) are higher for water draining barite mines from Cretaceous sediments, while Na+, K+, Ba2+ are higher in basement rock areas. Correlation and factor analyses suggest that the main components of the water samples are related to anthropogenic, geogenic, mineralization and environmental controls, while Gibbs diagram plots indicate weathering as the main process controlling the chemistry of water. Computed values of hardness and some irrigational parameters showed that the pond and stream water samples are generally soft and can be used for irrigation. Generally, the water quality satisfied the WHO (2008) and NIS (2007) standards for domestic, agriculture and industrial. However, there is the need to assess the impact of the pollution on the ecosystem and human health while remediation measures are being considered.  相似文献   

16.
Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (δ7Li) have been identified as a tool for investigating water–rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial δ7Li values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with δ7Li values of the possible sources. In samples from the playas, δ7Li values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial δ7Li value in water flowing into each playa. However, all δ7Li values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water–rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that δ7Li may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.  相似文献   

17.
A geochemical investigation was carried out mainly in a 14-km thick sedimentary sequence of late Precambrian age. The project included analysis of rock samples and drainage samples for Cu, Co, and Zn. The drainage samples were heavy-mineral concentrates and stream sediment samples.The results for the drainage sediments distinguish three well defined cupriferous horizons in the sedimentary column. Lithogeochemical studies of quartzites, pelites, and carbonates confirm that the anomalous values obtained in the drainage sediments arise from several Cu-mineralized horizons.The study has shown that geochemical prospecting appears to be well suited to Cu prospecting in Central East Greenland under arctic climate conditions even with a low sampling density of one sample per 5–10 km2 for the drainage samples, and one sample per 10–100 m of the sedimentary stratigraphic column for the rock samples.  相似文献   

18.
Hydrogeochemistry of groundwater is important for sustainable development and effective management of the groundwater resource. Fifty-six groundwater samples were collected from shallow tube wells of the intensively cultivated southern part of district Bathinda of Punjab, India, during pre- and post-monsoon seasons. Conventional graphical plots were used to define the geochemical evaluation of aquifer system based on the ionic constituents, water types, hydrochemical facies and factors controlling groundwater quality. Negative values of chloroalkaline indices suggest the prevalence of reverse ion exchange process irrespective of the seasons. A significant effect of monsoon is observed in terms chemical facies as a considerable amount of area with temporary hardness of Ca2+–Mg2+–HCO3 ? type in the pre-monsoon switched to Ca2+–Mg2+–Cl? type (18%) followed by Na+–HCO3 ? type (14%) in the post-monsoon. Evaporation is the major geochemical process controlling the chemistry of groundwater process in pre-monsoon; however, in post-monsoon ion exchange reaction dominates over evaporation. Carbonate weathering is the major hydrogeochemical process operating in this part of the district, irrespective of the season. The abundance of Ca2+ + Mg2+ in groundwater of Bathinda can be attributed mainly to gypsum and carbonate weathering. Silicate weathering also occurs in a few samples in the post-monsoon in addition to the carbonate dissolution. Water chemistry is deteriorated by land-use activities, especially irrigation return flow and synthetic fertilisers (urea, gypsum, etc.) as indicted by concentrations of nitrate, sulphate and chlorides. Overall, results indicate that different natural hydrogeochemical processes such as simple dissolution, mixing, weathering of carbonate minerals locally known as ‘‘kankar’’ and silicate weathering are the key factors in both seasons.  相似文献   

19.
Filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) over the 2011 melt season at the Athabasca Glacier (Alberta, Canada) and analyzed for strontium-87/strontium-86 (87Sr/86Sr) isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock composition and subglacial water–rock interaction time are the primary influences on meltwater 87Sr/86Sr. The Athabasca Glacier is situated atop Middle Cambrian carbonate bedrock that also contains silicate minerals. The length of time that subglacial meltwater interacts with the underlying bedrock and substrate is a predominant determining factor in solute concentration. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in 87Sr/86Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.Early in the melt season, rates of carbonate dissolution slow as meltwater approaches saturation with respect to calcite and dolomite, corresponding to an increase in silicate weathering that includes Sr-rich silicate minerals, and an increase in meltwater 87Sr/86Sr. However, carbonate minerals are preferentially weathered in unsaturated waters. During the warmest part of a melt season the discharged meltwater is under saturated, causing an increase in carbonate weathering and a decrease in the radiogenic Sr signal. Likewise, larger fraction contributions of meltwater from glacial ice corresponds to lower 87Sr/86Sr values, as the meltwater has lower water–rock interaction times in the subglacial system. These results indicate that although weathering of Sr-containing silicate minerals occurs in carbonate dominated glaciated terrains, the continual contribution of new meltwater permits the carbonate weathering signal to dominate.  相似文献   

20.
Isotopic compositions of He (27 samples) and Ar (7 samples) as well as major and trace elements have been determined in Cenozoic effusive rocks and hosted olivine and pyroxene phenocrysts from northern and central Mongolia. The R = 3He/4He values are within (0.1–13)⋅10−6. Abnormally high R value, 13⋅10−6, atypical of Cenozoic basaltic-volcanism areas in Mongolia, has been first revealed at one of the sites of the Hangayn upland. In composition the rocks under study correspond to tephrites, trachybasalts, and subalkalic andesite-basalts. Analysis of their REE patterns and spidergrams shows that the elements participating in the formation of basalt fields of the Hangayn upland were supplied from the enriched mantle (EM1). These patterns are similar to the OIB ones; (La/Yb)n = 9–53. The R values in the olivine phenocrysts are higher as compared with the pyroxene phenocrysts and the bulk rock compositions of the same samples. Based on the elemental composition of the rocks, their contents of radiogenic 4He and 3He were calculated. The rate of 3He formation is 5.65⋅10−2 at/(g⋅year). The calculated and measured R values in the rock samples point to the presence of trapped mantle helium.  相似文献   

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