Conceptual model for radionuclide release,Yucca Mountain,Nevada, USA     

John C. Walton  Lubna HamdanArturo Woocay 《Applied Geochemistry》2010

A new conceptual model for release rate of radionuclides from the proposed repository for high level nuclear waste located at Yucca Mountain, Nevada is developed. The model predicts that heat generated from radioactive decay combined with the unsaturated environment will lead to an inward flow system that, under many relevant conditions, will slow the release of and sometimes sequester radionuclides at locations of higher heat release and lower water percolation. The amount of protection will be greatest for more concentrated waste forms such as spent fuel and less for glass waste forms. The redistribution and concentration of the radionuclides is anticipated to significantly delay radionuclide release and create a tendency towards gradual release over time that is independent of localized penetrations of metallic barriers.  相似文献   

Martian regolith in Elephant Moraine 79001 shock melts? Evidence from major element composition and sulfur speciation     

E.L. Walton  P.J. Jugo  M. Wilke 《Geochimica et cosmochimica acta》2010,74(16):4829-4843

Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S²⁻ versus S⁶⁺). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt - pyroxene (En_72-75Fs_20-21Wo_5-7) and olivine (Fo_75-80). Reconstruction of glass + crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite.Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.  相似文献   

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319     

A.H. Peslier  D. Hnatyshin  E.L. Walton  T.J. Lapen 《Geochimica et cosmochimica acta》2010,74(15):4543-6820

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.  相似文献   

Crystallization rates of shock melts in three martian basalts: Experimental simulation with implications for meteoroid dimensions     

Erin Walton  Cliff Shaw  John Spray 《Geochimica et cosmochimica acta》2006,70(4):1059-1075

Dynamic crystallization experiments have been performed on synthetic glasses representative of shock-generated melts observed in Los Angeles, Sayh al Uhaymir 150 and Dar al Gani 476 martian basalts. On the basis of qualitative (texture) and quantitative (fractal analysis) results, we show that melt pockets in Los Angeles cooled at a rate of 1040-1560 °C/h. Sayh al Uhaymir 150 and Dar al Gani 476 melt pockets cooled at 780 °C/h. Conductive cooling models, for a range of meteoroid diameters (10-50 cm), indicate that the minimum meteoroid diameter was small, on the order of 10-15 cm and that melt pockets cooled from post-shock temperatures within minutes. Our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases. Modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization ∼4-60% of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport.  相似文献   

Future Water-level Declines in Deep Sandstone Wells in Chicago Region   总被引：2，自引：0，他引：2  

W. C. Walton 《Ground water》1964,2(1):13-20

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Educational Facilities in Ground-Water Geology and Hydrology in the United States and Canada, 1963     

W. C. Walton Member MWWA   《Ground water》1964,2(3):21-25

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Spatial and temporal variations of sediment size on a mixed sand and gravel beach     

Diane P. Horn  Susan M. Walton 《Sedimentary Geology》2007,202(3):509-528

Mixed sand and gravel beaches have been the subject of comparatively few studies in the UK. This paper describes the sediment distribution before, during and after a programme of beach nourishment along a section of mixed sand and gravel beach forming part of the Pevensey Bay Coastal Defences, in East Sussex, UK. The beach was recharged in September 2002, and beach profiles were measured along three cross-shore transects from August 2002 to February 2003. Sediment samples were taken along the transects between August and November 2002, and a total of 147 sediment samples were analysed, 40 before nourishment and 107 after nourishment. The majority of the sediment samples were strongly bimodal, with mean sizes varying between a minimum of 0.18 mm (2.48 ?) for the sand fraction and a maximum of 27 mm (− 4.74 ?) for the gravel. The recharge material was also bimodal but contained more fine sediment than the natural beach material, particularly on the upper beach. The recharge sediment had grain sizes and sorting similar to some of the natural material but lower bimodality parameters than any of the natural sediment. The sediment distributions after recharge contained significantly more fine sediment, particularly on the upper beach. Over time, the beach profile lowered and fine sediment appeared to be selectively transported seawards from the beachface.  相似文献   

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe Ar/Ar investigation     

Erin L. Walton  Simon P. Kelley 《Geochimica et cosmochimica acta》2007,71(2):497-520

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, ⁴⁰Ar/³⁹Ar ratios and K₂O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb ⁴⁰Ar) than shock veins and matrix minerals (0.3-3 ppb ⁴⁰Ar). The ⁴⁰Ar/³⁶Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic ³⁶Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (⁴⁰Ar/³⁶Ar: 480-490) in matrix minerals. Average apparent ⁴⁰Ar/³⁹Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These ⁴⁰Ar/³⁹Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped ⁴⁰Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic ³⁶Ar and ³⁸Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.  相似文献   

Solar Feature Identification using Contrasts and Contiguity     

Preminger  Dora G.  Walton  Stephen R.  Chapman  Gary A. 《Solar physics》2001,202(1):53-62

We present a new technique for the rapid, automatic identification of solar features on full-disk photometric images. The technique permits the detection of features whose contrasts are only slightly above the noise level. Contrast and contiguity criteria are used to identify pixels belonging to an individual feature. The criteria used are simple and objective, and do not require one to guess at the contrast distribution of the features. Comparison of Caii K images with magnetograms shows excellent agreement between the identified features and observed magnetic features. In addition, we can now reliably identify faculae on continuum images. Since this technique can be rapidly applied to a large set of images, it allows us to compile a database of the physical and photometric properties of individual solar features.  相似文献   

Arsenic in glacial aquifers: sources and geochemical controls   总被引：1，自引：0，他引：1  

Kelly WR  Holm TR  Wilson SD  Roadcap GS 《Ground water》2005,43(4):500-510

A total of 176 wells in sand-and-gravel glacial aquifers in central Illinois were sampled for arsenic (As) and other chemical parameters. The results were combined with archived and published data from several hundred well samples to determine potential sources of As and the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the As concentrations. High concentrations were confined to areas smaller than 1 km in diameter. Arsenic and well depth were uncorrelated. Arsenic solubility appeared to be controlled by oxidation-reduction (redox) conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing water does As accumulate in solution. In wells in which total organic carbon (TOC) was below 2 mg/L and sulfate (SO4(2-)) was present, As concentrations were low or below the detection limit (0.5 microg/L). Arsenic concentrations >10 microg/L were almost always found in wells where TOC was >2 mg/L and SO4(2-) was absent or at low concentrations, indicating post-SO4 (2-)reducing conditions. Iron (Fe) is common in the aquifer sediments, and Fe oxide reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as Fe oxide is reduced.  相似文献