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1.
Titanite (sphene, CaTiSiO5) is sensitive to changes in temperature, oxygen and water fugacity, and fluid composition. In order to understand formation processes and the nature of hydrothermal fluids, various types of titanite from Cu ores at the Baogutu reduced porphyry Cu deposit were chosen for detailed study. Magmatic titanite is associated with biotite, plagioclase and K-feldspar, whereas hydrothermal titanite occurs with K-feldspar, chlorite, actinolite and calcite. The formation of hydrothermal titanite was related to hydration of igneous minerals under high fH2O, whereas the widespread replacement of ilmenite by titanite (without magnetite) indicates a relatively low oxygen fugacity. Magmatic titanite has low Al, high Fe, Y, Sn, Zr, Nb and REE contents, relative to hydrothermal titanite. On the basis of the Zr-in-titanite and Al-in-chlorite geothermometers, formation temperatures for magmatic and hydrothermal titanite are estimated to be 687–739 °C and 250–670 °C, respectively. The gradual decrease in REE, Y and Sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of REE-bearing minerals. Magmatic and hydrothermal titanites have similar chondrite-normalized REE patterns with negative Eu anomalies and relatively flat HREE. Randomly selected titanites have Nd isotopic compositions similar to the host rocks. Thus, both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source.  相似文献   

2.
Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO2. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg2+/Ca2+ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca2+ and Mg2+ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg2+ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.  相似文献   

3.
It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO2 drawdown by means of precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO2 consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO2 sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477 Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO2 sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO2 sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change.  相似文献   

4.
Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO2 consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO2 consumption to silicate weathering than methods utilizing Ca/Sr and 87Sr/86Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and 87Sr/86Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (PCO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca2+-Mg2+-HCO3− waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca2+ and Sr2+, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and PCO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO2. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg2+, Ca2+, and HCO3− mass balances support interpretations of preferential Ca2+ removal by calcite precipitation. On the basis of patterns of saturation state and PCO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca2+ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca2+ during riverine transport brings the Ca/Sr and 87Sr/86Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr2+ budget and suggest that the conventional application of two-component Ca/Sr and 87Sr/86Sr mixing equations has overestimated silicate-derived Sr2+ and HCO3− fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.  相似文献   

5.
Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO2) through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals. The hydromagnesite [Mg5(CO3)4(OH)2·4H2O] playas of Atlin, British Columbia, Canada provide a natural model to examine mineral carbonation on a watershed scale. At near surface conditions, CO2 is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals. The purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for CO2 sequestration with emphasis on microbial processes that accelerate mineral carbonation.Regions with ultramafic bedrock, such as Atlin, represent the best potential sources of feedstocks for mineral carbonation. Elemental compositions of a soil profile show significant depletion of MgO and enrichment of SiO2 in comparison to underlying ultramafic parent material. Polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years. Upon retrieval, the cube surfaces, as seen using scanning electron microscopy, had been colonized by bacteria that were associated with surface pitting. Degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium-rich bedrock at Atlin. Stable carbon isotopes of groundwater from a well, situated near a wetland in the southeastern playa, indicate that  12% of the dissolved inorganic carbon has a modern origin from soil CO2.The mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments: (1) the wetland, characterized by biologically-aided precipitation of carbonate minerals from waters concentrated by evaporation, (2) isolated wetland sections that lead to the formation of consolidated aragonite sediments, and (3) the emerged grassland environment where evaporation produces mounds of hydromagnesite. Examination of sediments within the southeastern playa–wetland suggests that cyanobacteria, sulphate reducing bacteria, and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals.The Atlin site, as a biogeochemical model, has implications for creating carbon sinks that utilize passive microbial, geochemical and physical processes that aid in mineral carbonation of magnesium silicates. These processes could be exploited for the purposes of CO2 sequestration by creating conditions similar to those of the Atlin site in environments, artificial or natural, where the precipitation of magnesium carbonates would be suitable. Given the vast quantities of Mg-rich bedrock that exist throughout the world, this study has significant implications for reducing atmospheric CO2 concentrations and combating global climate change.  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.
A.T. Brasier 《Earth》2011,(4):213-239
Common models for modern calcite precipitation in and around caves, soils, springs and streams involve CO2 supplied by thick, high pCO2 biogenic soils which were probably thin or non-existent before vascular plants. Indeed plant-influenced chemical weathering might have caused accelerated terrestrial carbonate production from the Devonian onwards. However terrestrial carbonates have also been documented from the Archaean, Proterozoic, Cambrian, Ordovician and Silurian. Mechanisms which could have caused non-marine carbonates to precipitate without organic-rich soils are described, and some geological events likely to have influenced non-marine carbonate precipitation up to the origin of vascular plants are highlighted. As organisms have evolved, so have the petrographic characteristics of non-marine carbonates; some examples of this are also given here.  相似文献   

8.
In a mid-continental North American grassland, solute concentrations in shallow, limestone-hosted groundwater and adjacent surface water cycle annually and have increased steadily over the 15-year study period, 1991-2005, inclusive. Modeled groundwater CO2, verified by measurements of recent samples, increased from 10−2.05 atm to 10−1.94 atm, about a 20% increase, from 1991 to 2005. The measured groundwater alkalinity and alkaline-earth element concentrations also increased over that time period. We propose that carbonate minerals dissolve in response to lowered pH that occurs during an annual carbonate-mineral saturation cycle. The cycle starts with low saturation during late summer and autumn when dissolved CO2 is high. As dissolved CO2 decreases in the spring and early summer, carbonates become oversaturated, but oversaturation does not exceed the threshold for precipitation. We propose that groundwater is a CO2 sink through weathering of limestone: soil-generated CO2 is transformed to alkalinity through dissolution of calcite or dolomite. The annual cycle and long-term increase in shallow groundwater CO2 is similar to, but greater than, atmospheric CO2.  相似文献   

9.
The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO2. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO2 in the fluid equilibrated with the minerals.  相似文献   

10.
Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km2 in size. The δ18OPDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ13CPDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The 87Sr/86Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca2+) ions with relatively minor amounts of Ca2+ derived from the chemical weathering of bedrock.  相似文献   

11.
The draw down of CO2 from the atmosphere during mineral weathering plays a major role in the global budget of this greenhouse gas. Silicate minerals remove twice the CO2 of carbonate minerals per mole of calcium in runoff during weathering. Bedrock weathering chemistry was investigated in the White River watershed of northeastern USA to investigate whether there are seasonal differences in carbonate and silicate weathering chemistry. Geographic Information Systems analyses of bedrock geology were combined with major element concentrations in river waters to gain an understanding of the consistency of mineral weathering during three seasons. The percent of carbonate mineralogy comprising the bedrock in tributaries of the White River varied from less than 5% to 45% by area. A mass balance calculation using major element concentrations in waters was applied to estimate the seasonal relationships between bedrock geology and bicarbonate flux. In all tributaries and the main stem of the White River the highest calculated percent of bicarbonate from carbonate mineral weathering was measured in the late fall. The results suggest that carbonate and silicate bedrock weathering processes are seasonally controlled. Thus single season sampling could not accurately represent an entire year's geochemical budget. In the White River, water samples obtained solely during the summer would consistently underestimate the total yearly source of bicarbonate from carbonate bedrock weathering. The same sample set would also provide data that would lead to an underestimation of the yearly atmospheric CO2 draw down by bedrock weathering in the watershed. For example at four of the seven locations studied there was an almost two-fold difference between summer and spring calculated atmospheric CO2 consumption rates.  相似文献   

12.
<正>REE fractionation during the weathering of dolomite has been recognized for decades.A regolith profile on dolomite in southwest Yunnan of China was selected to investigate the behaviors of REE during weathering.The weathering of dolomite is divided into two stages:the pedogenesis stage and soil evolution stage,corresponding to the saprolites and soils respectively in the regolith profile. SiO_2,TiO_2,P_2O_5,Zr,Hf,Nb and Ta were immobile components during the weathering by and large, while Al_2O_3,K_2O and Fe_2O_3 were lost during the soil evolution stage in the physical form(clay minerals probably).REE were fractionated during the whole weathering of dolomite.The field weathering profile and the lab acid-leaching experiments on dolomite indicate that MREE were enriched clearly relative to other REE during the pedogenesis stage in a "capillary ascending-adsorption" mechanism, but they did not fractionate clearly in the soil evolution stage.REE were lost and accumulated in the weathering front of dolomite during the soil evolution stage in a "physical-chemical leaching" mechanism.  相似文献   

13.
This study focused on typical injection layers of deep saline aquifers in the Shiqianfeng Formation used in the Carbon Capture and Sequestration Demonstration Projects in the Ordos Basin, Northwest China. The study employed experiments and numerical simulations to investigate the mechanism of CO2 mineral sequestration in these deep saline aquifers. The experimental results showed that the dissolved minerals are plagioclase, hematite, illite–smectite mixed layer clay and illite, whereas the precipitated minerals are quartz (at 55, and 70 °C) and kaolinite (at 70 °C). There are rare carbonate mineral precipitations at the experimental time scale, while the precipitation of quartz as a product of the dissolution of silicate minerals and some intermediate minerals rich in K and Mg that transform to clay minerals, reveals the possibility of carbonate precipitation at the longer time scale. These results are consistent with some results previously reported in the literature. We calibrated the kinetic parameters of mineral dissolution and precipitation by these experimental results and then simulated the CO2 mineral sequestration under deep saline aquifer conditions. The simulation results showed that the dissolved minerals are albite, anorthite and minor hematite, whereas the precipitated minerals are calcite, kaolinite and smectite at 55 and 70 °C. The geochemical reaction of illite is more complex. At 55 °C, illite is dissolved at the relatively lag time and transformed to dawsonite; at 70 °C, illite is precipitated in the early reaction period and then transformed to kaolinite. Based on this research, sequestrated CO2 minerals, which are mainly related to the temperature of deep saline aquifers in Shiqianfeng Fm., are calcite and dawsonite at lower temperature, and calcite at higher temperature. The simulation results also establish that calcite could precipitate over a time scale of thousands of years, and the higher the temperature the sooner such a process would occur due to increased reaction rates. These characteristics are conducive, not only to the earlier occurrence of mineral sequestration, but also increase the sequestration capacity of the same mineral components. For a sequestration period of 10,000 years, we determined that the mineral sequestration capacity is 0.786 kg/m3 at 55 °C, and 2.180 kg/m3 at 70 °C. Furthermore, the occurrence of mineral sequestration indirectly increases the solubility of CO2 in the early reaction period, but this decreases with the increase in temperature.  相似文献   

14.
The injection of CO2 into deep saline aquifers is being considered as an option for greenhouse gas mitigation. However, the response of an aquifer to the injected CO2 is largely unknown. Experiments involving the reaction of Navajo Sandstone with acidic brine were conducted at 200°C and 25 or 30 MPa to evaluate the extent of fluid–rock interactions. The first experiment examined sandstone interaction with CO2-impregnated brine; the second experiment examined sandstone dissolution in CO2-free acidic brine; the third one is carried out in a mixed-flow reactor and designed to measure sandstone dissolution rates based on time-series Si concentrations. The solution chemistry data indicate that the SiO2(aq) increases gradually and pH increases slowly with reaction progress. Silicate minerals in the sandstone display textures (dissolution features, secondary mineralization), indicating that these phases are reacting strongly with the fluid. Dissolution of feldspars and conversion of smectite to illite are likely to be the two reactions that contribute to the release of SiO2(aq). The product minerals present at the end of the experiments are illite, illite/smectite, allophane, and carbonate minerals (for the CO2-charged system). Dissolved CO2 is likely to acidify the brine and to provide a source of carbon for the precipitation of carbonate minerals. Mineral trapping through the precipitation of carbonate minerals is favored thermodynamically and was observed in the experiments. The chemical reactions likely increase the bulk porosity of the sandstone due to dissolution of silicate minerals. However, allophane and illite/smectite fill voids in sandstone grains. There is no evidence for the removal of clay coatings due to chemical reactions. It is uncertain whether the mechanical forces near an injection well would mobilize the smectite and allophane and clog pore throats. Trace amounts of metals, including Cu, Zn, and Ba, were mobilized.  相似文献   

15.
The chemical composition of water from three streamsflowing through a carbonate watershed wasinvestigated. Although the study area is not spatiallyvery large (a few km2), local inhomogeneitieswithin the lithology appreciably affect the chemicalcomposition of the water and the geochemical gradientas a function of elevation. The main chemical processwhich leads to the observed water chemistry is thedissolution of calcium carbonate by atmospheric andmetabolic CO2. In the stream La Sigouste, thewater dissolves nearly pure calcite and this reactionproceeds until an equilibrium with respect to calciteis reached. In Le Lauzon stream, local inhomogeneitiesin the lithologic composition prevent theestablishment of an unambiguous weathering budgetwhile, for Le Rif de l'Arc a stoichiometric modelshows that the weathering process is adequatelyrepresented by the dissolution of calcite associatedwith a minor incongruent dissolution of chloritepresent in marls. For the two last streamssupersaturation with respect to calcite is observed.For both streams, supersaturation with respect toatmospheric CO2 is nearly permanent. The observedaltitudinal gradients of sulfate are interpretedthrough a set of biogeochemical redox reactions.  相似文献   

16.
Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO2, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients.When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.  相似文献   

17.
The systematic sampling of the chemical composition of the groundwater from five karst springs (including an overflow spring) and one outflowing borehole have permitted to determine distinctive chemical changes in the waters that reflect the geochemical processes occurring in a carbonate aquifer system from southern Spain. The analysis of the dissolution parameters revealed that geochemical evolution of the karst waters basically depends on the availability of the minerals forming aquifer rocks and the residence time within the aquifers. In the three proposed scenarios in the aquifers, which include the preferential flow routines, the more important geochemical processes taking place during the groundwater flow from the recharge to the discharge zones are: CO2 dissolution and exsolution (outgassing), calcite net dissolution, calcite and dolomite sequential dissolution, gypsum/anhydrite and halite dissolution, de-dolomitization and calcite precipitation. A detailed analysis of the hydrochemical data set, saturation indices of the minerals and partial pressure of CO2 in the waters joined to the application of geochemical modelling methods allowed the elaboration of a hydrogeochemical model of the studied aquifers. The developed approach contributes to a better understanding of the karstification processes and the hydrogeological functioning of carbonate aquifers, the latter being a crucial aspect for the suitable management of the water resources.  相似文献   

18.
Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO2 flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO2 fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ13C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K2O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K2O, Rb, and Ba, and are depleted in Na2O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO2 enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO2 in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO2 ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 1013 mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 1011 cm2/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO2 in the Early Archean.  相似文献   

19.
Physiochemical controls on the carbonate geochemistry of large river systems are important regulators of carbon exchange between terrestrial and marine reservoirs on human time scales. Although many studies have focused on large-scale river carbon fluxes, there are few investigations of mechanistic aspects of carbonate mass balance and transport at the catchment scale. We determined elemental and carbonate geochemistry and mass balances for net carbonate dissolution fluxes from the forested, mid-latitude Huron River watershed, established on carbonate-rich unconfined glacial drift aquifers. Shallow groundwaters are near equilibrium with respect to calcite at pCO2 values up to 25 times atmospheric values. Surface waters are largely groundwater fed and exhibit chemical evolution due to CO2 degassing, carbonate precipitation in lakes and wetlands, and anthropogenic introduction of road salts (NaCl and CaCl2). Because the source groundwater Mg2+/HCO3 ? ratio is fairly constant, this parameter permits mass balances to be made between carbonate dissolution and back precipitation after groundwater discharge. Typically, precipitation does not occur until IAP/K calcite values exceed 10 times supersaturation. Stream chemistry changes little thereafter even though streams remain highly supersaturated for calcite. Our data taken together with historical United States Geological Survey (USGS) data show that alkalinity losses to carbonate precipitation are most significant during periods of lowest discharge. Thus, on an annual basis, the large carbon flux from carbonate dissolution in soil zones is only decreased by a relatively small amount by the back precipitation of calcium carbonate.  相似文献   

20.
《International Geology Review》2012,54(14):1792-1812
Abundant crude oil and CO2 gas coexist in the fourth member of the Upper Cretaceous Quantou reservoir in the Huazijing Step of the southern Songliao Basin, China. Here, we present results of a petrographic characterization of this reservoir based on polarizing microscope, X-ray diffraction, fluid inclusion, and carbon–oxygen isotopic data. These data were used to identify whether CO2 might be trapped in minerals after the termination of a CO2-enhanced oil recovery (EOR) project, and to determine what effects might the presence of CO2 have on the properties of crude oil in the reservoir. The crude oil reservoir in the study area, which coexists with mantle-derived CO2, is hosted by dawsonite-bearing lithic arkoses and feldspathic litharenites. These sediments are characterized by a paragenetic sequence of clay, quartz overgrowth, first-generation calcite, dawsonite, second-generation calcite, and ankerite. The dawsonite analysed during this study exhibits δ13 C (Peedee Belemnite, PDB) values of ?4.97‰ to 0.67‰, which is indicative for the formation of magmatic–mantle CO2. The paragenesis and compositions of fluid inclusions in the dawsonite-bearing sandstones record a sequence of two separate filling events, the first involving crude oil and the second involving magmatic–mantle CO2. The presence of prolate primary hydrocarbon inclusions within the dawsonite indicates that these minerals precipitated from oil-bearing pore fluids at temperatures of 94–97°C, in turn suggesting that CO2 could be stored as carbonate minerals after the termination of a CO2-EOR project. In addition, the crude oil in the basin would become less dense after deposition of bitumen by deasphalting the injection of CO2 gas into the oil pool.  相似文献   

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