| 1. |
- Drake, Henrik, 1979-, et al.
(författare)
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Activity of sulfur reducing bacteria in deep bedrock fractures revealed by variability of δ34S in pyrite and dissolved sulphate
- 2013
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Ingår i: Procedia Earth and Planetary Science. - : Elsevier. - 1878-5220. ; 7, s. 228-231
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Tidskriftsartikel (refereegranskat)abstract
- Euhedral pyrite crystals coating 46 granite fractures at depths down to nearly 1 km at Laxemar, Sweden, were analysed for sulfur isotopes (δ34Spyr) by in situ SIMS (secondary ion mass spectrometry) analysis. Most of these fractures had corresponding chemical and isotopic groundwater data, providing a unique opportunity of pyrite-sulfate comparison within the same fracture network. Comparison of the isotopic ratios (δ18O, δ13C, 87Sr/86Sr) of co-genetic calcite with the groundwater showed that the sampled fractures carried pyrite and calcite that are of low-temperature origin, and with some exceptions, possibly precipitated from the present groundwater, or similar pre-modern fluids.The δ34Spyr showed huge variations across individual crystals (such as -32 to +73‰) and an extreme overall range (-50‰ to +91‰), which can only be explained by the activity of sulfur reducing bacteria (SRB). The most common sub-grain features were an increase in δ34Spyr with crystal growth related to successively higher δ34SSO4 caused by ongoing SRB activity and Rayleigh fractionation in fractures with low flow. The groundwater δ34SSO4 values (+9 to +37‰) are, in particular in the sulfate-poor waters down to -400 m, higher than the anticipated initial values, and this can also be explained by SRB-related Rayleigh distillation. The δ34SSO4 of the groundwater is, however, lower than that required to produce the highest δ34Spyr values, which may be due to the signal of groundwater in low-flow fractures (carrying 34S-rich sulfate and pyrite) being masked in the water data by high-flow parts of the water-flowing structure carrying abundant and less fractionated sulfate.
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| 2. |
- Drake, Henrik, 1979-, et al.
(författare)
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Trace metal distribution and isotope variations in low-temperature calcite and groundwaters in granitoid fractures down to 1 km depth
- 2012
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533. ; 84, s. 217-238
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Tidskriftsartikel (refereegranskat)abstract
- Studies of low-temperature fracture calcite in Proterozoic or Archaean crystalline rocks are very limited, mainly becausethis calcite usually is, first, not very abundant and second, very fine-grained or forms rims on older (and much more abundant)hydrothermal calcite and is thus difficult to distinguish. Knowledge of chemical characteristics and the correlation withgroundwater chemistry is thus scarce for low-temperature calcite in these settings, and consequently, knowledge of the recentpalaeohydrogeological history is limited. Boreholes drilled with triple-tube technique in the upper 1 km of the Palaeoproterozoiccrystalline crust at Laxemar, SE Sweden, have enabled preservation of fragile and potentially recently formed fractureminerals. Earlier investigations of these boreholes have resulted in an extensive set of groundwater chemistry data from variousdepths, and in detailed knowledge of the fracture mineral assemblages (ranging from 1.8 Ga to present). This has made itpossible to identify and sample low-temperature, potentially recently formed, calcite from water-flowing fractures for whichrepresentative groundwater chemical data exists. This, in turn, provides an opportunity to detailed comparisons of fracturecalcite (age span in the order of million years, with possibility of post-glacial contributes) and groundwater (age in the order ofdecades to more than a million year depending on depth) in terms of both isotopic and geochemical properties, giving input tothe understanding of groundwater history, partition coefficients derived in laboratory experiment, and reliability of calcitegeochemistry in terms of representing the actual source fluid composition. In this study, the focus is on trace elements (Fe,Mg, Mn and Sr), stable isotopes and Sr isotopes and, for the groundwater data set, also aquatic speciation with Visual MINTEQ.An optimised step-by-step sample specific analytical procedure was used for the collection of calcite coatings. The methodsused depended on the crystal homogeneity (one or several calcite generations), discerned by detailed SEM-investigations(back-scatter and cathodo-luminescence). 87Sr/86Sr ratios as well as d18O signatures in calcite are in the range expected for theprecipitates from present-day groundwater, or older groundwater with similar composition (except in sections with a considerableportion of glacial water, where calcite definitely is older than the latest glaciation). Stable carbon isotopes in calcitegenerally show values typically associated with HCO3 originating from soil organic matter but at intermediate depth frequentlywith HCO3 originating from in situ microbial anaerobic oxidation of methane (highly depleted d13C). For one ofthe studied metals – manganese – there was a strong correlation between the sampled calcite coatings and hypothetical calcitepredicted by applying laboratory-based partition coefficients (literature data) on groundwater chemistry for sections correspondingto those where the calcites were sampled. This points to temporal and spatial stability in groundwater Mn/Ca ratiosover millions of years, or even more, and show that it is possible to assess, based on laboratory-derived data on Mn partitioning,past groundwater Mn-composition from fracture calcites. For other metals – Fe, Sr, and Mg – which are expected to interact with co-precipitating minerals to a higher degree than Mn, the correlations between measured and predicted calcitewere weaker for various reasons.
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| 3. |
- Drake, Henrik, et al.
(författare)
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Variability of sulphur isotope ratios in pyrite and dissolved sulphate in granitoid fractures down to 1km depth - Evidence for widespread activity of sulphur reducing bacteria
- 2013
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 102, s. 143-161
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Tidskriftsartikel (refereegranskat)abstract
- Euhedral pyrite crystals in 46 open bedrock (granitoid) fractures at depths down to nearly 1 km were analysed for sulphur isotope ratios (delta S-34) by the in situ secondary ion mass spectrometry (SIMS) technique and by conventional bulk-grain analysis, and were compared with groundwater data. Twenty nine of the fractures sampled for pyrite had corresponding data for groundwater, including chemistry and isotopic ratios of sulphate, which provided a unique opportunity to compare the sulphur-isotopic ratios of pyrite and dissolved sulphate both at site and fracture-specific scales. Assessment of pyrite age and formation conditions were based on the geological evolution of the area (Laxemar, SE Sweden), and on data on co-genetic calcite as follows: (1) the isotopic ratios of the calcite crystals (delta O-18, delta C-13, Sr-87/Sr-86) were compared with previously defined isotopic features of fracture mineral assemblages precipitated during various geological periods, and (2) the delta O-18 of the calcites were compared with the delta O-18 of groundwater in fractures corresponding to those where the calcite/pyrite assemblages were sampled. Taken together, the data show that all the sampled fractures carried pyrite/calcite that are low-temperature and precipitated from the current groundwater or similar pre-existing groundwater, except at depths of -300 to -600 m where water with a glacial component dominates and the crystals are from pre-modern fluids. An age of <10 Ma are anticipated for the pre-modern fluids. The delta S-34(pyr) showed huge variations across individual crystals (such as -32 to +73 parts per thousand) and extreme minimum (-50 parts per thousand) and maximum (+91 parts per thousand) values. For this kind of extreme S-isotopic variation at earth-surface conditions there is no other explanation than activity of sulphur reducing bacteria coupled with sulphate-limited conditions. Indeed, the most common subgrain feature was an increase in delta S-34(pyr) values from interior to rim of the crystal, which we interpret are related to successively higher delta S-34 values of the dissolved source SO42- caused by ongoing bacterial sulphate reduction in fractures with low-flow or stagnant waters. The measured groundwater had delta S-34(SO4) values of +9 parts per thousand to +37 parts per thousand, with the highest values associated with low sulphate concentrations. These values are overall, and especially in the sulphate-poor waters down to -400 m, somewhat higher than the anticipated initial values, and can thus, like for the S-34-enriched pyrites, be explained by a Rayleigh distillation process driven by microbial sulphate reduction. An intriguing feature was that the delta S-34(SO4) values of the groundwater were in no case reaching up to the values required to produce biogenic pyrite with delta S-34 values of +40 parts per thousand to +91 parts per thousand. To explain this feature, we suggest that groundwater in low-flow fractures with near-stagnant water (carrying sulphate and pyrite with high delta S-34) is masked by high-flow parts of the fracture system carrying groundwater that often contains sulphate in abundance and considerably less fractionated with respect to S-34 and S-32. In order to gain detailed knowledge of chemical processes and patterns in groundwater in fractured rock, fracture-mineral investigations are a powerful tool, as we have shown here for the sulphur system. (C) 2012 Elsevier Ltd. All rights reserved.
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| 4. |
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| 5. |
- Mathurin, Frédéric A., et al.
(författare)
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Effect of tunnel excavation on source and mixing of groundwater in a coastal granitoidic fracture network.
- 2012
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Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 46:23, s. 12779-12786
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to assess how the excavation of the Äspö Hard Rock Laboratory tunnel has impacted on sources and mixing of groundwater in fractured crystalline (granitoidic) bedrock. The tunnel is 3600 m long and extends to a depth of 460 m at a coastal site in Boreal Europe. The study builds on a unique data set consisting of 1117 observations on chloride and δ(18)O of groundwater collected from a total of 356 packed-off fractures between 1987 and 2011. On the basis of the values of these two variables in selected source waters, a classification system was developed to relate the groundwater observations to source and postinfiltration mixing phenomena. The results show that the groundwater has multiple sources and a complex history of transport and mixing, and is composed of at least glacial water, marine water, recent meteoric water, and an old saline water. The tunnel excavation has had a large impact on flow, sources, and mixing of the groundwater. Important phenomena include upflow of deep-lying saline water, extensive intrusion of current Baltic Sea water, and substantial temporal variability of chloride and δ(18)O in many fractures.
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| 6. |
- Mathurin, Frédéric A., et al.
(författare)
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High cesium concentrations in groundwater in the upper 1.2km of fractured crystalline rock - Influence of groundwater origin and secondary minerals
- 2014
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier Ltd. - 0016-7037 .- 1872-9533. ; 132, s. 187-213
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Tidskriftsartikel (refereegranskat)abstract
- Dissolved and solid phase cesium (Cs) was studied in the upper 1.2km of a coastal granitoid fracture network on the Baltic Shield (Äspö Hard Rock Laboratory and Laxemar area, SE Sweden). There unusually high Cs concentrations (up to 5-6μgL-1) occur in the low-temperature (<20°C) groundwater. The material includes water collected in earlier hydrochemical monitoring programs and secondary precipitates (fracture coatings) collected on the fracture walls, as follows: (a) hydraulically pristine fracture groundwater sampled through 23 surface boreholes equipped for the retrieval of representative groundwater at controlled depths (Laxemar area), (b) fracture groundwater affected by artificial drainage collected through 80 boreholes drilled mostly along the Äspö Hard Rock Laboratory (underground research facility), (c) surface water collected in local streams, a lake and sea bay, and shallow groundwater collected in 8 regolith boreholes, and (d) 84 new specimens of fracture coatings sampled in cores from the Äspö HRL and Laxemar areas. The groundwater in each area is different, which affects Cs concentrations. The highest Cs concentrations occurred in deep-seated saline groundwater (median Äspö HRL: 4.1μgL-1; median Laxemar: 3.7μgL-1) and groundwater with marine origin (Äspö HRL: 4.2μgL-1). Overall lower, but variable, Cs concentrations were found in other types of groundwater. The similar concentrations of Cs in the saline groundwater, which had a residence time in the order of millions of years, and in the marine groundwater, which had residence times in the order of years, shows that duration of water-rock interactions is not the single and primary control of dissolved Cs in these systems. The high Cs concentrations in the saline groundwater is ascribed to long-term weathering of minerals, primarily Cs-enriched fracture coatings dominated by illite and mixed-layer clays and possibly wall rock micaceous minerals. The high Cs concentrations in the groundwater of marine origin are, in contrast, explained by relatively fast cation exchange reactions. As indicated by the field data and predicted by 1D solute transport modeling, alkali cations with low-energy hydration carried by intruding marine water are capable of (NH4 + in particular and K+ to some extent) replacing Cs+ on frayed edge (FES) sites on illite in the fracture coatings. The result is a rapid and persistent (at least in the order of decades) buildup of dissolved Cs concentrations in fractures where marine water flows downward. The identification of high Cs concentrations in young groundwater of marine origin and the predicted capacity of NH4 + to displace Cs from fracture solids are of particular relevance in the disposal of radioactive nuclear waste deep underground in crystalline rock. © 2014 Elsevier Ltd.
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| 7. |
- Sandström, Björn, 1976, et al.
(författare)
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Fracture-related hydrothermal alteration of metagranitic rock and associated changes in mineralogy, geochemistry and degree of oxidation: a case study at Forsmark, central Sweden
- 2010
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Ingår i: International Journal of Earth Sciences. - 1437-3254 .- 1437-3262. ; 99:1, s. 1-25
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Tidskriftsartikel (refereegranskat)abstract
- Red-staining of rocks due to fluid–rock interaction during hydrothermal circulation in fractures is a common feature in crystalline sequences. In this study, redstained metagranitic rock adjacent to fractures in Forsmark, central Sweden, has been studied with emphasis on the mineral reactions and associated element mobility occurring during the alteration. The main mineral reactions associated with the hydrothermal alteration are an almost complete saussuritization of plagioclase accompanied by total chloritization of biotite. Magnetite has been partly replaced by hematite whereas quartz and K-feldspar were relatively unaffected by the hydrothermal alteration. We show that redistribution of elements on the whole rock scale was very limited and is mainly manifested by enrichment of Na2O and volatiles and depletion of CaO, FeO and SiO2 in the red-stained rock. However, on the microscale, element redistribution was more extensive, with both intragranular and intergranular migration of e.g. Ca, K, Na, Al, Si, Fe, Ba, Cs, Rb, Sr, Ti and REEs. The altered rock shows a shift towards higher total oxidation factors, but the change is smaller than 1r and the redstaining of the rock is due to hematite dissemination rather than a significant oxidation of the rock. An increase in the connected porosity is also observed in the altered rock.
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