| 1. |
- Krall, Lindsay, et al.
(författare)
-
Natural uranium in Forsmark, Sweden : The solid phase
- 2015
-
Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 59, s. 178-188
-
Tidskriftsartikel (refereegranskat)abstract
- U-bearing solid phases from Forsmark, Sweden, a proposed host for radioactive waste repositories, havebeen identified and characterized. Elevated dissolved U was found in some groundwater samples duringthe site investigations, prompting a need to study the local U geochemistry. Previous hydrochemical andwhole-rock geochemical studies indicated that U was derived from local pegmatites, and mobilized andre-deposited during several geological events. In this study, down-hole gamma logs guided sampling oflocal pegmatites, cataclasites, and fracture fillings. Back-scattered electron-imaging, petrographic microscopy,and electron microprobe analyses were used to find and analyze U phases in thin sections. Theresults show that the principal U sources at Forsmark include pegmatitic uraninite (PbO up to22 wt%) and metamict uranothorite. These primary minerals show variable degrees of alteration suchas enrichment in Ca and Al and/or replacement by secondary Ca–U(VI)-silicates, haiweeite and uranophane.The haiweeite contains up to 5 wt% Al2O3, a chemical signature reflecting early (Proterozoic)events of hydrothermal fluid migration. Coffinitized, secondary uraninite is found in association withFeAl-silicates or Palaeozoic sulfide/sulfate minerals, indicating remobilization-precipitation and/or a secondary,sedimentary source of U. It is inferred that U was oxidized during geologically early periods.Later, U(IV) phases formed in fractures open to fluid circulation during the Palaeozoic. This study establishesthe phases available as local U sources and/or sinks, and which will be considered in future isotopicand hydrochemical studies aimed to constrain the mechanisms and timing of water–U phase interaction.
|
|
| 2. |
- Krall, Lindsay, et al.
(författare)
-
Tracing the palaeoredox conditions at Forsmark, Sweden, using uranium mineral geochronology
- 2019
-
Ingår i: Chemical Geology. - 0009-2541 .- 1872-6836. ; 506, s. 68-78
-
Tidskriftsartikel (refereegranskat)abstract
- U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (<100 Ma) alteration or precipitation of these minerals in connection to reaction with carbonate-rich fluids. The results support a geologically early oxidation of U(IV) to U(VI) and provide insight into the palaeoredox conditions that may impart an on-going influence on the mobility of natural U in the Forsmark fracture network.
|
|
| 3. |
- Drake, Henrik, et al.
(författare)
-
Extreme fractionation and micro-scale variation of sulphur isotopes during bacterial sulphate reduction in deep groundwater systems
- 2015
-
Ingår i: Geochimica Et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 161, s. 1-18
-
Tidskriftsartikel (refereegranskat)abstract
- This study conducted at the Aspo Hard Rock Laboratory, SE Sweden, determines the extent and mechanisms of sulphur-isotope fractionation in permanently reducing groundwater in fractured crystalline rock. Two boreholes > 400 m below the ground surface were investigated. In the 17-year-old boreholes, the Al instrumentation pipes had corroded locally (i.e., Al[oxy] hydroxides had formed) and minerals (i.e., pyrite, iron monosulphide, and calcite) had precipitated on various parts on the equipment. By chemically and isotopically comparing the precipitates on the withdrawn instrumentation and the borehole waters, we gained new insight into the dynamics of sulphate reduction, sulphide precipitation, and sulphur-isotope fractionation in deep-seated crystalline-rock settings. An astonishing feature of the pyrite is its huge variability in delta S-34, which can exceed 100 parts per thousand in total (i.e., -47.2 to +53.3 parts per thousand) and 60 parts per thousand over 50 mu m of growth in a single crystal. The values at the low end of the range are up to 71 parts per thousand lower than measured in the dissolved sulphate in the water (20-30 parts per thousand), which is larger than the maximum difference reported between sulphate and sulphide in pure-culture experiments (66 parts per thousand) but within the range reported from natural sedimentary settings. Although single-step reduction seems likely, further studies are needed to rule out the effects of possible S disproportionation. The values at the high end of the range (i.e., high delta S-34(py)) are much higher than could be produced from the measured sulphate under any biogeochemical conditions. This strongly suggests the development of closed-system conditions near the growing pyrite, i.e., the rate of sulphate reduction exceeds the rate of sulphate diffusion in the local fluid near the pyrite, causing the local aqueous phase (and thus the forming pyrite) to become successively enriched in heavy S (S-34). Consequently, the delta S-34 values of the forming pyrite become exceptionally high and strongly decoupled from the delta S-34 values of the sulphate in the bulk fluid. The Al-(oxy) hydroxide and calcite precipitates are explained by a combination of deposit and galvanic corrosion initiated by Al corrosion by H2S produced by sulphate-reducing microorganisms. (C) 2015 Elsevier Ltd. All rights reserved.
|
|
| 4. |
- Drake, Henrik, et al.
(författare)
-
Quaternary redox transitions in deep crystalline rock fractures at the western margin of the Greenland ice sheet
- 2017
-
Ingår i: Applied Geochemistry. - : Elsevier. - 0883-2927 .- 1872-9134. ; 76, s. 196-209
-
Tidskriftsartikel (refereegranskat)abstract
- When planning for long term deep geological repositories for spent nuclear fuel knowledge of processes that will influence and change the sub-surface environment is crucial. For repositories in northern Europe and similar areas, influence from advancing and retreating continental ice sheets must be planned for. Rapid transport of meltwater into the bedrock may introduce oxic conditions at great depth, which may affect the copper canisters planned to encapsulate the spent fuel. The lack of direct observations has led to simplified modelling assumptions not reflecting the complexity of natural systems. As part of a unique field and modelling study, The Greenland Analogue Project, of a continental ice sheet and related sub-surface conditions, we here present mineralogical and U-series data unravelling the Quaternary redox history in the deep bedrock fracture system close to the margin of the Greenland ice sheet. The aim was to increase the understanding of circulation of potentially oxygenated glacial meltwater from the surface down to 650 m depth. Secondary mineral coatings were sampled from open fractures in cored boreholes down to 650 m, within and below the current permafrost. Despite continental ice sheet coverage and/or prevailing permafrost during large parts of the last 1 Ma, measured disequilibrium in the U-238-U-234-Th-230 system shows that water has circulated in the bedrock fracture system at various occasions during this time span. In fractures of the upper 60 m, infiltration of oxygenated surface water has resulted in a prominent near-surface "oxidised zone" with abundant FeOOH precipitation. However, this zone must be relict because it is currently within permafrost and the U-series disequilibrium signatures of most fracture coatings show evidence of deposition of U prior to the Holocene and even prior to the last glaciation maximum which occurred less than 100 ka ago. This U deposition is found both within and below the near surface "oxidised zone" indicating temporal redox variation within this zone during the last 1 Ma. Potential Holocene leaching of U is indicated by Th-230/U-238 >> 1 and close to secular equilibrium for U-234/U-238 in some of the near surface fractures and also in a couple of deeper fractures. Indicated U-leaching in the talik within the last 200 ka is proposed to be the result of talik-related discharge of water with a capability of keeping U in solution. Circulation of oxidative water in the deep system beneath the permafrost is indicated only in a few fractures and solely by U-series disequilibrium (Th-230/U-238 activity ratios up to 2.97 at 431 m depth), probably due to restricted, perhaps sporadic infiltration of oxidative water, potentially during the Holocene. In these fractures, the conditions have in general been more reducing than in the near surface system where oxidising conditions have prevailed and penetration of oxygenated waters may have been continuous. (C) 2016 Elsevier Ltd. All rights reserved.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Larson, Sven Åke, 1941, et al.
(författare)
-
Sveriges berggrund
- 2017
-
Bok (övrigt vetenskapligt/konstnärligt)
|
|
| 8. |
|
|
