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1.
Rare earth element (REE) concentrations in alkaline lakes, circumneutral pH groundwaters, and an acidic freshwater lake were determined along with the free carbonate, free phosphate, and free sulfate ion concentrations. These parameters were used to evaluate the saturation state of these waters with respect to REE phosphate and carbonate precipitates. Our activity product estimates indicate that the alkaline lake waters and groundwaters are approximately saturated with respect to the REE phosphate precipitates but are significantly undersaturated with respect to REE carbonate and sulfate precipitates. On the other hand, the acidic lake waters are undersaturated with respect to REE sulfate, carbonate, and phosphate precipitates. Although carbonate complexes tend to dominate the speciation of the REEs in neutral and alkaline waters, our results indicate that REE phosphate precipitates are also important in controlling REE behavior. More specifically, elevated carbonate ion concentrations in neutral to alkaline natural waters tend to enhance dissolved REE concentrations through the formation of stable REE-carbonate complexes whereas phosphate ions tend to lead to the removal of the REEs from solution in these waters by the formation of REE-phosphate salts. Removal of REEs by precipitation as phosphate phases in the acid lake (pH=3.6) is inconsequential, however, due to extremely low [PO 4 3– ] F concentrations (i.e., 10–23 mol/kg).  相似文献   

2.
Experiments were conducted to evaluate the impact of organic complexation on the development of Ce anomalies and the lanthanide tetrad effect during the adsorption of rare-earth elements (REE) onto MnO2. Two types of aqueous solutions—NaCl and NaNO3—were tested at pH 5 and 7.5. Time-series experiments indicate that a steady-state is reached within less than 10 h when REE occur as free inorganic species, whereas steady state is not reached before 10 d when REE occur as REE-humate complexes. The distribution coefficients (KdREE) between suspended MnO2 and solution show no or only very weak positive Ce anomaly or lanthanide tetrad effect when REE occur as humate complexes, unlike the results obtained in experiments with REE occurring as free inorganic species. Monitoring of dissolved organic carbon (DOC) concentrations show that log KdREEorganic/KdDOC ratios are close to 1.0, implying that the REE and humate remain bound to each other upon adsorption. Most likely, the Ce anomaly reduction/suppression in the organic experiments arises from a combination of two processes: (i) inability of MnO2 to oxidize Ce(III) because of shielding of MnO2 surfaces by humate molecules and (ii) Ce(IV) cannot be preferentially removed from solution due to quantitative complexation of the REE by organic matter. We suggest that the lack of lanthanide tetrad effect arises because the adsorption of REE-humate complexes onto MnO2 occurs dominantly via the humate side of the complexes (anionic adsorption), thereby preventing expression of the differences in Racah parameters for 4f electron repulsion between REE and the oxide surface. The results presented here explain why, despite the development of strongly oxidizing conditions and the presence of MnO2 in the aquifer, no (or insignificant) negative Ce anomalies are observed in organic-rich waters. The present study demonstrates experimentally that the Ce anomaly cannot be used as a reliable proxy of redox conditions in organic-rich waters or in precipitates formed at equilibrium with organic-rich waters.  相似文献   

3.
Carbonate, largely in the form of dolomite, is found throughout the host rocks and ores of the Nchanga mine of the Zambian Copperbelt. Dolomite samples from the hanging wall of the mineralization show low concentrations of rare-earth elements (REE) and roof-shaped, upward convex, shale-normalized REE patterns, with positive Eu*SN anomalies (1.54 and 1.39) and marginally negative Ce anomalies (Ce*SN 0.98,0.93). In contrast, dolomite samples associated with copper and cobalt mineralization show a significant rotation of the REE profile, with HREE enrichment, and La/LuSN ratios <1 (0.06–0.42). These samples also tend to show variable but predominantly negative Eu*SN and positive cerium anomalies and an upwardly concave MREE distribution (Gd-Er). Malachite samples from the Lower Orebody show roof-tile-normalized REE patterns with negative europium anomalies (Eu*SN 0.65–0.80) and negative cerium anomalies (Ce*SN 0.86–0.9). The carbonate 87Sr/86Sr signature correlates with the associated REE values. The uppermost dolomite samples show Neoproterozoic seawater-like 87Sr/86Sr ratios ranging from 0.7111 to 0.7116, whereas carbonate from Cu–Co mineralized samples show relatively low concentrations of strontium and more radiogenic 87Sr/86Sr, ranging between 0.7136–0.7469. The malachite samples show low concentrations of strontium, but give a highly radiogenic 87Sr/86Sr of 0.7735, the most radiogenic 87Sr/86Sr ratio. These new data suggest that the origin and timing of carbonate precipitation at Nchanga is reflected in the REE and Sr isotope chemistry. The upper dolomite samples show a modified, but essentially seawater-like signature, whereas the rotation of the REE profile, the MREE enrichment, the development of a negative Eu*SN anomaly and more radiogenic 87Sr/86Sr suggests the dolomite in the Cu–Co mineralized samples precipitated from basinal brines which had undergone significant fluid–rock interaction. Petrographic, REE, and 87Sr/86Sr data for malachite are consistent with the original sulfide Lower Orebody being subject to a later oxidizing event.  相似文献   

4.
Fifty-seven shallow groundwater samples were collected from Guiyang karst basin, China, to analyze the aqueous rare-earth elements in low-water seasons and it is shown that the total amount of rare-earth elements (ΣREE) in karst groundwater is exceedingly low compared with that in carbonate rocks or weathering crusts of carbonate rocks, and ranges from 0.01 to 0.43, from 0.03 to 0.27, from 0.03 to 0.19 and from 0.05 to 1.38 μg·L-1 for dolomite, dolomitic & limestone, limestone and clastic rock aquifer, respectively. Both distributions and contents of rare-earth elements (REE) in karst groundwater reflect the lithology of host rocks or weathering crusts of carbonate rocks through which groundwater flows. The chondrite-normalized patterns show a non-flat profile with higher enrichment of slightly light rare-earth elements (LREE) than heavy rare-earth elements (HREE), prominent fractionation between LREE and HREE, negative Ce anomalies and negative or positive Eu anomalies. There is more obvious fractionation between LREE and HREE in groundwater than that in carbonate rocks and their weathering crusts due to high contents of HCO3? and PH in groundwater. In shallow karst groundwater, REE(CO3)n2n-3 (n=1 and 2) is the main inorganic species of REE. But for a clastic rock aquifer, both REESO4+ and REECO3+ are the main inorganic species of REE. Species of REE in groundwater is closely associated with the hydrochemical type of groundwater which is predominated by the lithology of host rocks, groundwater-rock interaction and weathering-pedogenesis of carbonate rocks.  相似文献   

5.
Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002-0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH2+, LnSO4+, and LnCO3+, but not that of Ln(CO3)2- or LnPO4o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln3+) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln3+) but also as aqueous complexes (e.g., as Ln(CO3)2- in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.  相似文献   

6.
Potential-based molecular dynamics simulations of aqueous uranyl carbonate species (MxUO2(CO3)y2+2x−2y with M = Mg, Ca, or Sr) were carried out to gain molecular-level insight into the hydration properties of these species. The simulation results were used to estimate the self-diffusion coefficients of these uranyl carbonate species, which often dominate uranyl speciation in groundwater systems. The diffusion coefficients obtained for the monoatomic alkaline-earth cations and polyatomic ions (uranyl, carbonate, and uranyl tri-carbonate) were compared with those calculated from the Stokes-Einstein (SE) equation and its variant formulation by Impey et al. (1983). Our results show that the equation of Impey et al. (1983), originally formulated for monovalent monoatomic ions, can be extended to divalent monoatomic ions, with some success in reproducing the absolute values and the overall trend determined from the molecular dynamics simulations, but not to polyatomic ions, for which the hydration shell is not spherically symmetrical. Despite the quantitative failure of both SE formulations, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed that a general linear dependence is observed for these complexes as expected from the SE equation. The nature of the alkaline-earth cation in the uranyl carbonate complexes was not found to have a significant effect on the ion’s diffusion coefficient, which suggests that the use of a single diffusion coefficient for different alkaline-earth uranyl carbonate complexes in microscopic diffusion models is appropriate.The potential model reproduced well published quantum mechanical and experimental data of and of the individual constituent ions, and therefore is expected to offer reliable predictions of the structure of magnesium and strontium uranyl carbonate aqueous species, for which there is no structural data available to date. In addition, the interatomic distances reported for could help with the refinement of the interpretation of EXAFS data of these species, which is made difficult by the similar uranium-distant carbonate oxygen and uranium-calcium distances.An analysis of the dynamics of water exchange around the alkaline-earth cations revealed that the presence of the uranyl tri-carbonate molecule has a strong influence on the geometry of the cation’s first hydration shell, which, in turn, can considerably affect the water exchange kinetics depending on whether the imposed geometry matches that around the isolated alkaline-earth cation. This result shows that the alkaline-earth uranyl carbonate complexes have distinct water exchange dynamics, which may lead to different reactivities. Finally, significant changes in water residence time were also predicted when replacing carbonate for water ligands in the uranyl coordination shell.  相似文献   

7.
舒小超  刘琰  李德良  贾玉衡 《岩石学报》2019,35(5):1372-1388
霓长岩化作用是指碳酸岩(或碱性岩)流体对围岩的交代蚀变,它是碳酸岩型稀土(REE)矿床常见的蚀变类型,其所形成的岩石即为霓长岩。对霓长岩的深入研究可以鉴别碳酸岩体的存在,厘定碳酸岩岩浆(或流体)的地球化学性质及源区特征,这对于找寻碳酸岩相关的矿产资源(尤其是REE)以及剖析矿床成因机制有着重要的地质意义。川西冕宁-德昌稀土矿带是中国最重要的轻稀土矿带之一,包括牦牛坪超大型、大陆槽大型、木落寨和里庄中小型REE矿床以及一系列矿点。REE矿化与碳酸岩-碱性岩杂岩体密切相关,受一系列新生代走滑断裂的控制。该矿带广泛发育霓长岩化蚀变带,尤以大陆槽及里庄矿床为显著。岩相学分析表明,大陆槽和里庄霓长岩中的矿物多呈他形粒状结构,主要由长石、黑云母、霓辉石以及少量副矿物组成;主微量元素分析表明,霓长岩的碱质(K_2O+Na_2O)、MgO、Fe_2O_3T含量较高,且富集REE、Sr、Ba等微量元素;电子探针分析表明,霓长岩中的霓辉石Fe OT含量较高,长石Na_2O及K_2O含量较高,Ca O含量极低。An-Ab-Or三角图解显示长石主要为透长石和钠长石,属碱性长石系列;黑云母的地球化学成分图解表明云母的成因类型为交代型且具有相对富镁、贫铁等特征,属镁质黑云母。霓长岩化作用的交代流体含有较高的CO_2组分,且富含碱质、Mg、Fe及REE、Sr、Ba等元素。对比霓长岩与原岩的主微量元素发现:相比于正长岩原岩,在主量元素中,霓长岩的Fe、Mg、Ca等元素含量增加,Si、Al等元素含量降低;微量元素中,霓长岩的REE及Sr、Ba等元素显著增加。这意味着交代流体含有的Fe_2O_3T、MgO、CaO等组分在霓长岩化过程中被带进了围岩,而SiO_2和Al_2O_3等从围岩中被逐出。大陆槽及里庄矿区发育的角砾岩指示了矿区曾经历过频繁的角砾岩化事件,这提高了霓长岩作用的强度,并且为矿脉的穿插及REE矿物的沉淀提供了空间。在霓长岩化过程中,流体-围岩的组分交换反复发生,这削弱了REE络合物的稳定性,伴随多期次的热液活动及构造事件,最终完成REE活化→迁移→沉淀的过程。  相似文献   

8.
A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin.Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO3)?54), LnSO+4, LnCl+2 and LnF+2. Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general.  相似文献   

9.
Humic Ion-Binding Model V, which focuses on metal complexation with humic and fulvic acids, was modified to assess the role of dissolved natural organic matter in the speciation of rare earth elements (REEs) in natural terrestrial waters. Intrinsic equilibrium constants for cation-proton exchange with humic substances (i.e., pKMHA for type A sites, consisting mainly of carboxylic acids), required by the model for each REE, were initially estimated using linear free-energy relationships between the first hydrolysis constants and stability constants for REE metal complexation with lactic and acetic acid. pKMHA values were further refined by comparison of calculated Model V “fits” to published data sets describing complexation of Eu, Tb, and Dy with humic substances. A subroutine that allows for the simultaneous evaluation of REE complexation with inorganic ligands (e.g., Cl−, F−, OH−, SO42−, CO32−, PO43−), incorporating recently determined stability constants for REE complexes with these ligands, was also linked to Model V. Humic Ion-Binding Model V’s ability to predict REE speciation with natural organic matter in natural waters was evaluated by comparing model results to “speciation” data determined previously with ultrafiltration techniques (i.e., organic acid-rich waters of the Nsimi-Zoetele catchment, Cameroon; dilute, circumneutral-pH waters of the Tamagawa River, Japan, and the Kalix River, northern Sweden). The model predictions compare well with the ultrafiltration studies, especially for the heavy REEs in circumneutral-pH river waters. Subsequent application of the model to world average river water predicts that organic matter complexes are the dominant form of dissolved REEs in bulk river waters draining the continents. Holding major solute, minor solute, and REE concentrations of world average river water constant while varying pH, the model suggests that organic matter complexes would dominate La, Eu, and Lu speciation within the pH ranges of 5.4 to 7.9, 4.8 to 7.3, and 4.9 to 6.9, respectively. For acidic waters, the model predicts that the free metal ion (Ln3+) and sulfate complexes (LnSO4+) dominate, whereas in alkaline waters, carbonate complexes (LnCO3+ + Ln[CO3]2−) are predicted to out-compete humic substances for dissolved REEs. Application of the modified Model V to a “model” groundwater suggests that natural organic matter complexes of REEs are insignificant. However, groundwaters with higher dissolved organic carbon concentrations than the “model” groundwater (i.e., >0.7 mg/L) would exhibit greater fractions of each REE complexed with organic matter. Sensitively analysis indicates that increasing ionic strength can weaken humate-REE interactions, and increasing the concentration of competitive cations such as Fe(III) and Al can lead to a decrease in the amount of REEs bound to dissolved organic matter.  相似文献   

10.
对东秦岭地区河南嵩县一带进行地质调查,发现了一系列具有一定规模的含稀土碱性碳酸岩矿脉并在其中发现一种特殊的钡解石矿物.依据该钡解石主量元素组成,计算分子式为Ba104Cao81Sral4(CO3)2,为锶钡解石,LA-ICP-MS分析表明其富Na、K、Fe、Mn、Pb、REE、Y等元素,稀土元素总量最高为4 080× ...  相似文献   

11.
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12.
Anomalously high Au concentrations (2.5 to 50 ppb) in regolith carbonate accumulations, such as calcrete and calcareous sands in aeolian sand dunes overlying Au mineralisation of the Gawler Craton, South Australia, show a marked covariance of Au with K, Mg and most notably Ca. This relationship appears to be linked to the authigenic formation of smectites and carbonates within the aeolian dunes in the region. However, little is known about the processes involved in the formation of carbonates under semi-arid and arid conditions. In this study the geochemical properties of aeolian dunes along several depth profiles of 2 to 4 m are investigated in order to assess the relationship between Au mobility and calcrete formation. In the profiles a strongly systematic relationship between Au and the increasing Ca–Mg contents at depth highlights the close association between the enrichment of Au in the calcrete and the underlying hydrothermal mineralisation. Intense calcrete formation and concurrent Au enrichment also occurs in the vicinity of roots penetrating the dune. Thin section petrography and cathodoluminescence show that most of the calcrete in the regolith profiles is micritic; some sparic crystallites have also been identified. To demonstrate the presence of microbial processes that may mediate the formation of calcrete, samples from a depth profile in the dune were taken under sterile conditions. After amendment with urea and incubation of up to 24 h, up to 18 mg/l of NH4+ were detected in near surface samples. At depth of 2.3 m 1 mg/l NH4+ were detected compared to a control that contained below 0.05 mg/l NH4+. These results suggest that the genesis of calcrete and pedogenic carbonate in the area may be partly biologically mediated via processes such as the metabolic breakdown of urea by resident microbiota which generates a pH and pCO2 environment conducive to the precipitation of carbonate. In the process of urea breakdown organic Au complexes such as Au-amino acid complexes may become destabilised in solution and Au may be co-precipitated, resulting in the fine, non-particulate distribution of Au throughout the micritic calcrete carbonate. In conclusion, this study suggests a coupled mechanism of biologically mediated and inorganic mechanisms that lead to the formation of Au-in-calcrete anomalies.  相似文献   

13.
The mobility of the rare-earth elements(REE)during hydrothermal activities is increasingly documented.Geological and experimental evidence suggests that REE may be mobile in solutions rich in F^-,Cl^-,HCO3^-,CO^2- 3,HPO4^2-,PO4^3-,or in combinations of the above ligands,even though little has been known about which ligand or which combination is most effective in mobilizing REE. The fractionation of REE resulting from hydrothermal activities is inconsistent.One set of field data implies the prererential mobility of the light rare-earth elements(LREE).whereas another set of field observations indicates the dominant mobilization of the heavy rare earth elements(HREE),and some theoretical prediction is comtradictory to the field evidence.The Eu anomalies due to hydrothermal activities are complex and plausible explanation is not available.The existing experimental approaches dealing with REE are not adequate for explanation ofREE behaviour in aqueous solutions.Systematic experimental approaches are suggested.  相似文献   

14.
《Applied Geochemistry》1997,12(5):607-623
The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure.The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO3. The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The87Sr/86Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO3 extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr.The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and FeMn oxides for the REE in the HCl and HNO3 extracts. A part of the HNO3 extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by143Nd/144Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.  相似文献   

15.
Three thin, syn-caldera ash flow tuffs of the Suswa volcano, Kenya, contain pumiceous clasts and globules of trachytic glass, and clasts rich in carbonate globules, in a carbonate ash matrix. Petrographic and textural evidence indicates that the carbonate was magmatic. The trachyte is metaluminous to mildly peralkaline and varies from nepheline- to quartz-normative. The carbonate is calcium-rich, with high REE and F contents. The silicate and carbonate fractions have similar 143Nd/144Nd values, suggesting a common parental magma. Chondrite-normalized REE patterns are consistent with a carbonate liquid being exsolved from a silicate liquid after alkali feldspar fractionation. Sr isotopic and REE data show that the carbonate matrix of even the freshest tuffs interacted to some degree with hydrothermal and/or meteoric water. A liquid immiscibility relationship between the trachyte and carbonate is indicated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass and of fiamme rich in carbonate globules separated by silicate glass, and by the fact that similar phenocryst phases occur in both melts. It is inferred that the carbonate liquid separated from a carbonated trachyte magma prior to, or during, caldera collapse. Viscosity differences segregated the magma into a fraction comprising silicate magma with scattered carbonate globules, and a fraction comprising carbonate globules in a silicate magmatic host.Explosive disruption of the magma generated silicate-and carbonate-rich clasts in a carbonate matrix. The silicate liquid was disaggregated by explosive disruption and texturally appears to have been budding-off into the carbonate matrix. After emplacement, the basal parts of the flows welded slightly and flattened. The Suswa rocks represent a rare and clear example of a liquid immiscibility relationship between trachyte and carbonate melts.  相似文献   

16.
The solubility of REE(III) fluoride solids was determined in fluoride- and chloride-bearing solutions at 150, 200 and 250 °C and saturated water vapor pressure. These experimental data, together with experimental data from previously published studies, were used to evaluate formation constants for chloride- and fluoride-bearing aqueous species of the entire REE(III) group at temperatures up to 300 °C. The data show that the stability of these species differs significantly from that predicted theoretically. For example, contrary to the theoretical predictions, LREEF2+ species are more stable than HREEF2+ species at elevated temperature. The behavior of the chloride-bearing species is similar. Parameters for the Helgeson–Kirkham–Flowers (HKF) equation of state were determined for REEF2+, REECl2+ and REECl2+ complexes using these experimental data and permit calculation of formation constants of these species at conditions not investigated experimentally. These data now permit the mobility of all REE in fluoride- and chloride-bearing hydrothermal systems to be reliably evaluated at intermediate temperatures and pressures.  相似文献   

17.
Using published data and the results of a new study, the main characteristics of seven Angolan carbonatite complexes are here presented. With the exception of the Lupongola complex which intrudes anorthosites, the remaining complexes are hosted by Precambrian silicic rocks. The complexes are of central or dike type and are well exposed. They represent some of the seven morpholithological types present in the province and have some intermediate lithochemical features between those of Brazilian and East African examples.Sovites at Lupongola are the richest in Sr and F, and also have the highest CaO/MgO and La/Y ratios of all sovites studied. Carbonatites from Bonga and Bailundo are the richest in P2O5, while those from Coola and Longonjo are the richest in BaO. Ferrocarbonatites from Bailundo and Virulundo have the highest REE contents. TiO2 contents are usually low. Only Bonga carbonatites show well-defined variation between Ba and Mn contents and the index CaO/(CaO+Fe2O3+FeO+MnO+MgO).The CO2-SiO2-(Al2O3+Na2O+K2O) diagram distinguishes silicified carbonatites, feldspar-bearing carbonatites in which the main silicate mineral is K-feldspar, carbonatites and fenites. Potassic fenitization of country rocks is well developed at Bailundo, Bonga and Virulundo, and probably it also affected cogenetic nepheline syenites at Tchivira and Monte Verde.Fluorcarbonates of Ca and REE are encountered in all chemical varieties of carbonatites, and crystallized during late stages of rock formation. They have a strong influence both on total REE contents and on the slopes of chondrite-normalized patterns. Fluorapatite and pyrochlore are other important potential REE host minerals in the rocks studied. The REE patterns usually have discrete negative Ce anomalies, and sometimes show very discrete negative Eu anomalies. Apart from these anomalies, some rocks have very near-linear patterns, but most show inflections, which may occur between light and middle, and between middle and heavy REE. The origins of these variations are still uncertain.Other aspects of rock geochemistry show that, while some features could be explained by crystal fractionation differentiation processes, late-stage or secondary chemical modifications were widespread.  相似文献   

18.
The speciation of carbonate adsorbed to hematite in air-equilibrated aqueous solutions has been studied using ATR-FTIR spectroscopy. Samples were measured over a range of pH conditions, at 0.1 M NaCl and at low ionic strength, and in H2O and D2O solutions to permit a multispecies analysis of the data. Second-derivative analyses and fits to the spectra indicate the presence of two major and two minor surface-bound carbonate species. The two major complexes coexist at near-neutral pH and low ionic strength. One of these two complexes is relatively sensitive to ionic strength, being displaced at 0.1 M NaCl, whereas the other is not. Comparison of experimental to DFT/MO-calculated frequencies suggest these two major species to be (a) a monodentate binuclear inner-sphere carbonate surface complex, and (b) a fully or partially solvated carbonate (CO32−) species that is symmetry broken and appears to reside in the structured vicinal water layers at the hematite-water interface, retained by hydrogen bonding and/or other forces. Minor carbonate complexes include diffuse layer CO32− and an unidentified inner-sphere species. Both of the dominant species observed here are likely to be significant controls of the surface charge and sorptive properties of Fe-oxides.  相似文献   

19.
与碱性岩有关的碳酸岩型内生稀土矿床在中国乃至世界上轻稀土资源储量中占有极为重要的地位,诸如我国内蒙古的白云鄂博稀土矿床、川西冕宁—德昌稀土成矿带中的牦牛坪、大陆槽等稀土矿床、山东微山县郗山稀土矿床以及美国的Mountain Pass稀土矿床等都属于这种类型的稀土矿床.当前,对于这类稀土矿床的成矿流体演化机制,学界主要存...  相似文献   

20.
Rare earth element (REE) and strontium isotope data (87Sr/86Sr) are presented for hydromagnesite and surface waters that were collected from Dujiali Lake in central Qinghai-Tibet Plateau (QTP), China. The goal of this study is to constrain the solute sources of hydromagnesite deposits in Dujiali Lake. All lake waters from the area exhibit a slight LREE enrichment (average [La/Sm]PAAS = 1.36), clear Eu anomalies (average [Eu/Eu*]PAAS = 1.31), and nearly no Ce anomalies. The recharge waters show a flat pattern (average [La/Sm]PAAS = 1.007), clear Eu anomalies (average [Eu/Eu*] PAAS = 1.83), and nearly no Ce anomalies (average [Ce/Ce*]PAAS = 1.016). The REE+Y data of the surface waters indicate the dissolution of ultramafic rock at depth and change in the hydrogeochemical characteristics through fluid-rock interaction. These data also indicate a significant contribution of paleo-groundwater to the formation of hydromagnesite, which most likely acquired REE and Sr signatures from the interaction with ultramafic rocks. The 87Sr/86Sr data provide additional insight into the geochemical evolution of waters of the Dujiali Lake indicating that the source of Sr in the hydromagnesite does not directly derive from surface water and may have been influenced by both Mg-rich hydrothermal fluids and meteoric water. Additionally, speciation modeling predicts that carbonate complexes are the most abundant dissolved REE species in surface water. This study provides new insights into the origins of hydromagnesite deposits in Dujiali Lake, and contributes to the understanding of hydromagnesite formation in similar modern and ancient environments on Earth.  相似文献   

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