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1.
James P. McKinley John M. Zachara Steven C. Heald Brian W. Kempshall 《Geochimica et cosmochimica acta》2006,70(8):1873-1887
An alkaline brine containing uranyl (UO22+) leaked to the thick unsaturated zone at the Hanford Site. We examined samples from this zone at microscopic scale to determine the mode of uranium occurrence—microprecipitates of uranyl (UO22+) silicate within lithic-clast microfractures—and constructed a conceptual model for its emplacement, which we tested using a model of reactive diffusion at that scale. The study was driven by the need to understand the heterogeneous distribution of uranium and the chemical processes that controlled it. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1-3 μm precipitates were difficult, but indicated a uranyl silicate. These observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a reactive diffusion model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore-space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During this time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated Vadose Zone. 相似文献
2.
Nobuyoshi Miyajima Tommaso Mandolini Florian Heidelbach Caroline Bollinger 《Comptes Rendus Geoscience》2019,351(2-3):295-301
Dislocation microstructures in experimentally deformed single-crystal pyrope-rich garnet, (Mg,Fe)3(Al,Cr)3Si3O12, and polycrystalline forsterite, Mg2SiO4, were investigated by using electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) combined with a focused ion beam (FIB)-microsampling. In the orientation-optimized ECCI method, we successfully observed individual dislocations across subgrain boundaries in a low-atomic-number mineral, pyrope-rich garnet (averaged Z-numbers, AZs ～ 10). Dislocations in a deformed forsterite (iron-free olivine) were also visible in the ECCI. In the ECCI on the single-crystal garnet, deformation bands consisting of dislocations, unusual contrasts in stripes and inhomogeneous distributions of sub-micrometer-sized pores were found. Further site-specific TEM observation on the deformation band revealed a high density of partial dislocations and stacking fault ribbons. The site-specific characterizations from ECCI to TEM, with assistance of FIB, can provide a new approach to investigate dislocation microstructures of deformed materials at high pressure and high temperature. 相似文献
3.
James K. Fredrickson John M. Zachara Steve M. Heald David W. Kennedy Ponnusamy Nachimuthu 《Geochimica et cosmochimica acta》2009,73(8):2299-4913
Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Tc is highly mobile in its most oxidized state and less mobile in the reduced form [Tc(IV)O2·nH2O]. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state; unconsolidated Pliocene-age fluvial sediment from the upper Ringold (RG) Formation at the Hanford Site and a clay-rich saprolite from the Field Research Center (FRC) background site on the Oak Ridge Site. Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe-phyllosilicates of different types. Shewanella putrefaciens CN32 reduced Mn(III/IV) oxides and generated Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. After bioreduction and heat-killing, biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO2-like phase in both sediments. The biogenic redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular scale speciation of Tc compared to RG. X-ray microprobe, electron microprobe, X-ray absorption spectroscopy, and micro X-ray diffraction were applied to the whole sediment and isolated Tc-containing particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50-100 μm-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O2 into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)-Fe(III) molecular species. 相似文献
4.
Yen-Hong Shau Melanie E. Feather Eric J. Essene Donald R. Peacor 《Contributions to Mineralogy and Petrology》1991,106(3):367-378
Electron microbeam techniques have been used to examine submicroscopically intergrown paragonite, phengite and chlorite from
the South Fork Mountain Schist of the Franciscan Terrane of northern California, which was subjected to blueschist facies
metamorphism. The sample also contains quartz, albite, lawsonite, and rutile. The subassemblage albite-lawsonite-rutile requires
metamorphic conditions on the low-temperature side of the equilibrium albite+lawsonite+rutile=paragonite+sphene+quartz+H2O (T<200° C and P<7.4 kbars based on thermodynamic data of Holland and Powell 1990). The white micas appear to be optically
homogeneous, but back-scattered electron images can distinguish two different micas by their slight difference in contrast.
Electron microprobe analyses (EMPA) of micas show Na/(Na+K) ranging from 0.2 to 0.8. The two micas are resolved by transmission
electron microscopy (TEM) as packets of phengite and paragonite that range from 20 to several hundred nm in thickness. The
compositions, determined by analytical electron microscopy (AEM), constrain the limbs of the phengite-paragonite solvus to
values of Na/(Na+K)=<0.02 and 0.97, representing less mutual solid solution than ever reported by EMPA. The textural relations
imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate
stage of textural development. We caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous
data and lead to faulty conclusions using phengite barometry and paragonite-muscovite thermometry, especially in fine-grained
rocks that formed at relatively low temperatures.
Contribution no. 473 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann
Arbor, Michigan, USA 相似文献
5.
Kim N. Dalby Alan J. Anderson Anthony N. Mariano Robert A. Gordon Robert A. Mayanovic Richard Wirth 《Lithos》2010,114(1-2):86-94
Cathodoluminescence (CL) reveals red and blue colors within single, non-turbid albite (Ab98–99) grains from the Georgeville granite, Nova Scotia. A 720 nm X-ray excited optical luminescence (XEOL) peak characterizes red CL regions, while a 280 nm XEOL feature dominates blue CL regions. Synchrotron X-ray fluorescence results indicate that red CL and the 720 nm XEOL peak intensities relate to total Fe concentrations. The relationship between red CL and Fe content is confirmed by electron microprobe (EMPA) and laser ablation-inductively coupled mass spectrometry (LA-ICP-MS). The XEOL technique is used to exclude the Fe K-edge as the cause of red CL. X-ray absorption spectroscopy results indicate that Fe in both the red and blue CL regions is Fe3+, and that red CL activation may relate to the Si–Al order of the feldspar and to the distribution of Fe on tetrahedral sites. The CL textures, combined with EMPA and LA-ICPMS analyses, indicate that blue CL albite (Ab98) regions contain higher concentrations of Ca, Ti, Pb and rare earth elements, and were replaced, in part, by a more Fe-rich, trace element depleted albite (Ab99) which displays red CL. Complex diffraction contrasts and amorphous deposits identified in transmission electron microscope images suggest that aqueous fluids have reacted with both red and blue CL regions. Fluid inclusion homogenization temperatures of up to 430 °C provide a lower estimate of the fluid temperature. 相似文献
6.
John M Zachara Steven C SmithChongxuan Liu James P McKinleyR.Jeffrey Serne Paul L Gassman 《Geochimica et cosmochimica acta》2002,66(2):193-211
The sorption of Cs+ was investigated over a large concentration range (10−9−10−2 mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO3 brine) is the carrier. Cs+ sorption was measured on homoionic sediments (Na+, K+, Ca2+) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na+ electrolyte, concentrations were extended to near saturation with NaNO3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs+ for both high- and low-affinity sites according to the trend K+ >> Na+ ≥ Ca2+. At high salt concentration, Cs+ adsorption occurred only on high-affinity sites. Na+ was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs+(aq), and analyzed by electron microprobe to identify phases and features important to Cs+ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na+ in HLW and indigenous K+ displaced from the sediments may act to expedite the migration of strongly sorbing Cs+ in subsurface environments. 相似文献
7.
通过野外采样、化学分析、电子探针(EPMA)和X射线衍射(XRD)分析等手段,研究了贵州织金地区黑色页岩矿物成分、化学组分、微量元素、稀土元素特征。研究区矿样化学成分以SiO_2和Al_2O_3为主,且具有高K低Na的特征。电子探针和X射线衍射分析表明,研究区黑色页岩主要矿物组成有石英、粘土矿物、白云石及黄铁矿等。织金黑色页岩中Pb、Ni、U、V、Cr等金属元素存在不同程度的富集,稀土元素总量为153.2×10~(-6)~224.89×10~(-6),属轻稀土元素富集型。同时从多金属层、页岩气、页岩提钾及近底部含磷铀矿资源等方面讨论了织金黑色页岩资源化利用。织金黑色页岩多金属层含有Mo、V、Ni、Ag及U等多金属元素,具综合利用价值;其中有机碳含量达到页岩气开发大于2%的条件,可进一步开展研究;页岩中伊利石含量较多,可提取黑色页岩中的钾制备含钾复合肥;黑色页岩底部与磷矿层接触带产出磷铀矿,主要为胶状磷铀矿,接触带可作为铀矿找矿的标志层。 相似文献
8.
紫云山岩体是赣中地区与钨铀成矿关系极为密切的过铝质花岗岩体,但目前该岩体的成岩时代尚不明确.通过偏光显微镜、扫描电镜、电子探针等手段,首次开展了紫云山花岗岩中赋存晶质铀矿的精细矿物学研究.结果表明:晶质铀矿主要赋存于黑云母之中,少数被黄铁矿包裹,部分晶质铀矿被不同程度溶蚀和交代,表明晶质铀矿是本区花岗岩型铀矿的主要铀源矿物之一.利用电子探针U-Th-Pb化学定年法测得蕉坑单元 (J3J)5颗晶质铀矿年龄为154.5~168.9 Ma,加权平均年龄为161.8±2.4 Ma (MSWD=0.26,n=26),庙前单元 (J3M) 三颗晶质铀矿年龄为152.8~164.7 Ma,加权平均年龄为159.7±3.2 Ma (MSWD=0.2,n=15).获得的年龄与南岭地区主要含钨花岗岩的侵入时间高度一致,对应华南中生代大规模岩浆活动的第二阶段.晶质铀矿年龄与华南含钨花岗岩锆石U-Pb年龄非常一致,验证了过铝质富铀花岗岩中晶质铀矿电子探针定年方法的可行性. 相似文献
9.
Chongxuan Liu John M. Zachara James P. McKinley Zheming Wang 《Geochimica et cosmochimica acta》2004,68(22):4519-4537
The dissolution of uranium was investigated from contaminated sediments obtained at the US. Department of Energy (U.S. DOE) Hanford site. The uranium existed in the sediments as uranyl silicate microprecipitates in fractures, cleavages, and cavities within sediment grains. Uranium dissolution was studied in Na, Na-Ca, and NH4 electrolytes with pH ranging from 7.0 to 9.5 under ambient CO2 pressure. The rate and extent of uranium dissolution was influenced by uranyl mineral solubility, carbonate concentration, and mass transfer rate from intraparticle regions. Dissolved uranium concentration reached constant values within a month in electrolytes below pH 8.2, whereas concentrations continued to rise for over 200 d at pH 9.0 and above. The steady-state concentrations were consistent with the solubility of Na-boltwoodite and/or uranophane, which exhibit similar solubility under the experimental conditions. The uranium dissolution rate increased with increasing carbonate concentration, and was initially fast. It decreased with time as solubility equilibrium was attained, or dissolution kinetics or mass transfer rate from intraparticle regions became rate-limiting. Microscopic observations indicated that uranium precipitates were distributed in intragrain microfractures with variable sizes and connectivity to particle surfaces. Laser-induced fluorescence spectroscopic change of the uranyl microprecipitates was negligible during the long-term equilibration, indicating that uranyl speciation was not changed by dissolution. A kinetic model that incorporated mineral dissolution kinetics and grain-scale, fracture-matrix diffusion was developed to describe uranium release rate from the sediment. Model calculations indicated that 50-95% of the precipitated uranium was associated with fractures that were in close contact with the aqueous phase. The remainder of the uranium was deeply imbedded in particle interiors and exhibited effective diffusivities that were over three orders of magnitude lower than those in the fractures. 相似文献
10.
In order to determine how As speciation in lacustrine sediment changes as a function of local conditions, sediment cores were taken from three lakes with differing hydrologic regimes and subjected to extensive chemical and TEM analysis. The lakes (Killarney, Thompson and Swan Lakes) are located within the Coeur d’ Alene River system (northern Idaho, USA), which has been contaminated with trace metals and As, from over 100 a of sulfide mining. Previous analyses of these lakebed sediments have shown an extensive amount of contaminant metals and As associated with sub-μm grains, making them extremely difficult to analyze using standard methods (scanning electron microscopy, X-ray diffraction). Transmission electron microscopy offers great advantages in spatial resolution and can be invaluable in determining As speciation when combined with other techniques. Data indicate that because of differences in local redox conditions, As speciation and stability is dramatically different in these lakes. Killarney and Thompson Lakes experience seasonal water-level fluctuations due to drawdown on a downstream dam, causing changes in O2 content in sediment exposed during drawdown. Swan Lake has relatively constant water levels as its only inlet is dammed. Consequently, Killarney and Thompson Lakes show an increase in labile As-bearing phases with depth, while Swan Lake data indicate stable As hosts throughout the sediment profile. Based on these observations it can be stated that As in lakebed sediments is much less mobile, and therefore less bioavailable, when water is kept at a constant level. 相似文献
11.
《Chemie der Erde / Geochemistry》2021,81(1):125678
A small-scale hydrothermal uranium mineralization hosted within the siderite deposit at Hüttenberg (Eastern Alps, Austria) was re-investigated using modern scanning electron microscope (SEM) and microprobe (EMPA) methods. The uranium mineralization comprises brannerite, coffinite and uraninite, and is spatially associated with Fe-, Ni- and Co-arsenides (loellingite, rammelsbergite, safflorite), bismuth and bismuthinite, as well as rare accessory silver-bearing minerals and gold locally. The U-bearing fluids also carried P, Y and REEs, which precipitated as xenotime and P–Y coffinite. The uranium mineralization paragenetically postdates the metasomatic formation of the host siderite ore.Uraninite allows for precise single spot EMPA ages to be calculated (±2 Ma) due to its high radiogenic lead content. There is an excellent internal consistency in the uraninite data with calculated dates ranging between 77 and 84 Ma. We interpret that these record the crystallization age of the uraninite, and that any influences of lead loss or common Pb are minimal. Brannerite was also analyzed but U-total Pb microprobe data scatter (c.30–80 Ma) with evidence of lead loss, particularly in altered domains. It does, however, provide a minimum constraint on the timing of mineralization that is consistent with the crystallization age of the uraninite. Coffinite proved completely unsuitable for U-total Pb geochronology due to extreme lead loss. However, hydrothermal xenotime yields a U-total Pb age of 78 ± 5 Ma, consistent with the uraninite age. Therefore, the U-total Pb geochronological data support a late Cretaceous age for the U-mineralization of c. 80 Ma. Considering this timing constraint together with other geological and indirect thermochronological aspects, we conclude that the siderite deposit of Hüttenberg is older and formed at c. 90−80 Ma. 相似文献
12.
Wooyong Um Jonathan P. Icenhower R. Jeffery Serne Cleveland J. Dodge Arokiasamy J. Francis 《Geochimica et cosmochimica acta》2010,74(4):1363-6031
The concentration and distribution of uranium (U) in sediment samples from three boreholes recovered near radioactive waste storage tanks at Hanford, Washington, USA, were determined in detail using bulk and micro-analytical techniques. The source of contamination was a plume that contained an estimated 7000 kg of dissolved U that seeped into the subsurface as a result of an accident that occurred during filling of tank BX-102. The desorption character and kinetics of U were also determined by experiment in order to assess the mobility of U in the vadose zone. Most samples contained too little moisture to obtain quantitative information on pore water compositions. Concentrations of U (and contaminant phosphate—P) in pore waters were therefore estimated by performing 1:1 sediment-to-water extractions and the data indicated concentrations of these elements were above that of uncontaminated “background” sediments. Further extraction of U by 8 N nitric acid indicated that a significant fraction of the total U is relatively immobile and may be sequestered in mobilization-resistant phases. Fine- and coarse-grained samples in sharp contact with one another were sub-sampled for further scrutiny and identification of U reservoirs. Segregation of the samples into their constituent size fractions coupled with microwave-assisted digestion of bulk samples showed that most of the U contamination was sequestered within the fine-grained fraction. Isotope exchange (233U) tests revealed that ∼51% to 63% of the U is labile, indicating that the remaining fund of U is locked up in mobilization-resistant phases. Analysis by Micro-X-ray Fluorescence and Micro-X-ray Absorption Near-Edge Spectroscopy (μ-XRF and μ-XANES) showed that U is primarily associated with Ca and is predominately U(VI). The spectra obtained on U-enriched “hot spots” using Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) provide strong evidence for uranophane-type [Ca(UO2)2(SiO3OH)2(H2O)5] and uranyl phosphate [Ca(UO2)2(PO4)2(H2O)10-12] phases. These data show that disseminated micro-precipitates can form in narrow pore spaces within the finer-grained matrix and that these objects are likely not restricted to lithic fragment environments. Uranium mobility may therefore be curtailed by precipitation of uranyl silicate and phosphate phases, with additional possible influence exerted by capillary barriers. Consequently, equilibrium-based desorption models that predict the concentrations and mobility of U in the subsurface matrix at Hanford are unnecessarily conservative. 相似文献
13.
Sorption of U(VI) on Hanford fine sand (HFS) with varying Fe-oxide (especially ferrihydrite) contents showed that U(VI) sorption increased with the incremental addition of synthetic ferrihydrite into HFS, consistent with ferrihydrite being one of the most reactive U(VI) sorbents present in natural sediments. Surface complexation model (SCM) calculations for U(VI) sorption, using only U(VI) surface-reaction constants obtained from U(VI) sorption data on freshly synthesized ferrihydrite at different pHs, were similar to the measured U(VI) sorption results on pure synthetic ferrihydrite and on HFS with high contents of ferrihydrite (5 wt%) added. However, the SCM prediction using only U(VI) sorption reactions and constants for synthetic ferrihydrite overestimated U(VI) sorption on the natural HFS or HFS with addition of low amounts of added ferrihydrite (1 wt% added). Over-predicted U(VI) sorption was attributed to reduced reactivity of natural ferrihydrite present in Hanford Site sediments, compared to freshly prepared synthetic ferrihydrite. Even though the SCM general composite (GC) approach is considered to be a semi-quantitative estimation technique for contaminant sorption, which requires systematic experimental data on the sorbent–sorbate system being studied to obtain credible SCM parameters, the general composite SCM model was still found to be a useful technique for describing U(VI) sorption on natural sediments. Based on U(VI) batch sorption results, two simple U(VI) monodentate surface species, SO_UO2HCO3 and SO_UO2OH on ferrihydrite and phyllosillicate in HFS, respectively, can be successfully used to describe U(VI) sorption onto Hanford Site sediment contacting varying geochemical solutions. 相似文献
14.
Pyrite (FeS2) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O2 after transport by bioturbation to the sediment surface. FeS2 and FeS may also be oxidized within the anoxic sediment in which NO3−, Fe(III) oxides, or MnO2 are available as potential electron acceptors. In chemical experiments, FeS2 and FeS were oxidized by MnO2 but not with NO3− or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked 55FeS2 occurred with MnO2 but not with NO3− or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS2 and FeS oxidation with NO3− or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS2 as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO3−, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe2+ and S0 as substrates and NO3− as electron acceptor, in the presence of 55FeS2, to test for co-oxidation of FeS2, but an anaerobic microbial dissolution of 55FeS2 could not been detected. FeS2 and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS2 could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO3− as electron acceptor. O2 and MnO2, but not NO3− or amorphous Fe(III) oxide, are chemical oxidants for both FeS2 and FeS. 相似文献
15.
《Chemical Geology》2007,236(1-2):92-111
Complexly zoned zircons (19 grains, ∼ 3.3 Ga) from a porphyritic granite in the Jack Hills, Western Australia, have been investigated using electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) in order to examine the effects of radiation as a function of dose, as well as the nano-scale microstructure and composition of impurities and secondary alteration phases. In back-scattered electron (BSE) images, zones with bright contrast consist of an almost unaltered zircon with limited amounts of impurity elements. In contrast, the dark zones contain higher concentrations of trace elements: U, Th, Pb, Fe, Y, Ce, Ca and Al. The cumulative doses due to alpha-decay in the dark zones are calculated to be 0.21–1.0 × 1017 (alpha-decay events/mg), equivalent to 1.0–4.7 dpa (displacements per atom). These doses are much higher than the dose required for radiation-induced amorphization, as determined by ion-beam irradiation of synthetic zircon, 0.3–1.0 dpa (0–600 °C). However, based on high-resolution TEM observations, none of the zircons are fully amorphous, to the result of annealing under ambient conditions. The concentrations of Ca and Al, which was considered to indicate alteration, increase dramatically at a cumulative dose of 1.6 × 1016 (alpha-decay events/mg). This is the dose at which the first percolation point occurs, as amorphous domains overlap and form an interconnected network. In the altered zones, nanocrystallites of zircon are present with a random orientation, and the amorphous matrix contains the impurity elements. Although the Ce-concentration is extremely high, 1–2 wt.%, the Ce anomaly, Ce/Ce⁎, does not vary significantly as a function of dose or degree of alteration, indicating that the LREE patterns were overprinted by the fluids with a similar Ce-anomaly. The valence of Ce analyzed by EELS (electron energy loss spectroscopy) is tetravalent in the altered zone, suggesting that the altering fluids were oxidizing. 相似文献
16.
砂岩型铀矿微区原位U-Pb同位素定年技术方法研究 总被引：2，自引：2，他引：0
铀矿物定年一直是成矿年代学中的难点,随着微区原位U-Pb同位素定年技术的发展,可以直接针对矿石矿物(铀矿物)进行同位素定年;但是其中的砂岩型铀矿由于其存在状态复杂,在原位定年中剥蚀要求高,也缺乏合适的外部校正标准物质,所以定年准确度有待提高。本文研究了两种微区原位U-Pb同位素测年的方法,对砂岩型铀矿定年进行了尝试,试图解决铀矿测年中的无基体匹配问题并提高砂岩型铀矿定年水平。一是建立了一种激光剥蚀多接收电感耦合等离子体质谱仪联合电子探针进行微区原位U-Pb同位素测年的技术(LA-MC-ICP-MS&EMPA)。通过优化实验方法,对秦岭陈家庄花岗岩型铀矿进行了测试,获得与同位素稀释热电离质谱法(ID-TIMS)一致的年龄结果,证明了微区原位U-Pb同位素测年无基体匹配标准物质分析的可行性;并利用此法获得鄂尔多斯盆地红庆河和塔然高勒砂岩型铀矿的微区原位U-Pb同位素年龄信息。二是尝试了利用飞秒激光剥蚀多接收电感耦合等离子体质谱法(fsLA-MC-ICP-MS)对红庆河和宁夏宁东砂岩型铀矿样品进行微区原位U-Pb同位素定年,并获得了微区原位U-Pb同位素年龄,表明飞秒激光剥蚀技术在砂岩型铀矿定年中有很好的应用前景。本文提出,比较单一且年龄偏老的单矿物样品可以选择LA-MC-ICP-MS&EMPA联合法进行分析,需要高空间分辨率的样品建议使用fsLA-MC-ICP-MS法。 相似文献
17.
N. R. Khisina S. N. Teplyakova R. Wirth V. G. Senin A. A. Averin A. A. Shiryaev 《Geochemistry International》2017,55(4):317-329
The mineralogy and texture of shock-induced melt veinlets and melt pockets in silicate inclusions in the Elga IIE iron meteorite have been studied by reflected-light optical microscopy, EMPA, SEM, Raman spectroscopy and TEM. The results suggest that Elga experienced two discrete impact events. The earlier event involved the collision of a metallic projectile with a silicate target and resulted in partial melting and recrystallization of the silicate material, forming schreibersite and oxide rims between the metal and silicate. The later impact event resulted in melt pockets in the silicate inclusions and was associated with fragmentation, melting, and brecciation of the rims and displacement of some fragments into the melt pockets. These fragments are shown to contain carbon-bearing phases: siderite and amorphous sp 2 carbon, which form carbon–oxide, siderite–oxide, and siderite–schreibersite associations. The fact that the carbon-bearing fragments are spatially constrained to shock breccia and melt zones indicates that these fragments are genetically related to the impact process and that their carbon-bearing phases are of cosmic origin. 相似文献
18.
Viktoria K. Kis Tatyana Shumilova Victor Masaitis 《Physics and Chemistry of Minerals》2016,43(9):661-670
High-resolution transmission electron microscopy was applied for the detailed nanostructural investigation of Popigai impact diamonds with the aim of revealing the nature of the amorphous carbon of the matrix. The successful application of two complementary specimen preparation methods, focused ion beam (FIB) milling and mechanical cleavage, allowed direct imaging of nanotwinned nanodiamond crystals embedded in a native amorphous carbon matrix for the first time. Based on its stability under the electron beam, native amorphous carbon can be easily distinguished from the amorphous carbon layer produced by FIB milling during specimen preparation. Electron energy loss spectroscopy of the native amorphous carbon revealed the dominance of sp 2-bonded carbon and the presence of a small amount of oxygen. The heterogeneous size distribution and twin density of the nanodiamond crystals and the structural properties of the native amorphous carbon are presumably related to non-graphitic (organic) carbon precursor material. 相似文献
19.
Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits 总被引：1，自引：0，他引：1
Anne-Magali Seydoux-Guillaume Richard Wirth Urs Schärer 《Geochimica et cosmochimica acta》2004,68(11):2517-2527
The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO2, and ∼0.94 wt. % UO2. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation. 相似文献
20.
James K Fredrickson John M Zachara Ravi K Kukkadapu Steve M Heald Chongxuan Liu 《Geochimica et cosmochimica acta》2004,68(15):3171-3187
The potential for reduction of 99TcO4−(aq) to poorly soluble 99TcO2 · nH2O(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO2 · nH2O(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO4−(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h−1), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and 57Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O4− were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO4−(aq) associated with groundwater contaminant plumes. 相似文献