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Reiner Dohrmann - One of the best experts on this subject based on the ideXlab platform.
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Determining the extent of Bentonite alteration at the Bentonite/cement interface
Applied Clay Science, 2020Co-Authors: Stephan Kaufhold, Reiner Dohrmann, Kristian UferAbstract:Abstract Bentonite is investigated as a geotechnical barrier material in repositories for high-level radioactive waste (HLRW). One of the most important research topics concerns the stability of Bentonite at interfaces such as Bentonite/metal or Bentonite/cement. Alkaline cements are particularly hazardous because they may dissolve at least part of the Bentonite. This reaction is, therefore, investigated extensively. Bentonite properties and performances in different applications vary from one deposit to another and even within some deposits. The hypothesis, therefore, was, that the stability of different Bentonites in contact with cement could be different. Hence, the aim of the study was to compare the reactivity of a significant set of well characterized Bentonites with Portland cement and to understand the reason for the differences. For Bentonite comparison standard reaction parameters were fixed: 3 months, 80 °C, 25% cement (or 33%). The resulting degree of degradation was compared with basic Bentonite parameters. A fairly good correlation (R2 = 0.7) of the extent of alteration with the amount of soda soluble silica (reactive silica which to variable amounts is present in natural Bentonites) was obtained which proved, that soda soluble silica was able to buffer the reaction. Such Bentonites with a higher amount of soda soluble silica showed less smectite degradation and hence will perform better at the cement/Bentonite interface than others with less soda soluble silica. Varying the clay/cement ratio showed that smectite degradation requires 10–20% cement. No reaction was observed below this value. Linear curves were observed which allow to conclude that 1 g cement is able to degrade about 0.6 g of Bentonite (smectite) at the selected solution/solid ratios. The reaction was almost complete after 1 month and no difference was observed between 60 and 80 °C.
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Characterization of the Second Parcel of the Alternative Buffer Material (ABM) Experiment — I Mineralogical Reactions
Clays and Clay Minerals, 2017Co-Authors: Stephan Kaufhold, N. Götze, Reiner Dohrmann, D. SvenssonAbstract:The performance of Bentonite barriers for high level radioactive waste (HLRW) disposal is currently being tested in various real-and up-scale disposal tests. One of the disposal tests, the ABM test (ABM = alternative buffer material), was conducted by SKB (Svensk Kärnbränslehantering) as a mediumscale experiment at the Äspö hard rock laboratory in Sweden. The present study deals with the second parcel (ABM-II), which was retrieved after 6.5 years with 2.5 years of water saturation and 3–4 years of heating up to 141°C. Nine different Bentonites and two marine clays were tested to investigate the performance. The aim of the study was to provide a detailed characterization of the mineralogical and chemical changes that took place in ABM-II, compare the findings with ABM-I (the first of the six test parcels), and try to draw some general conclusions concerning the use of Bentonites in such geotechnical barriers. The ABM-II test parcel revealed a set of reactions that a HLRW Bentonite might undergo. The most prominent reaction was the rather complete exchange of cations, which was discussed in a second part to this publication (II — cation exchange; Dohrmann and Kaufhold, 2017). The corrosion of the Fe in metal canisters was observed, but no discrete corrosion product was identified. At the interface of Bentonite and the metal canister, the formation of smectite-type trioctahedral clay minerals was observed. In contrast to the ABM-I test, anhydrite was present in many of the Bentonite blocks of the ABM-II test. In most concepts used for HLRW disposal in crystalline rocks, a temperature below 100°C at the canister surface was applied to avoid boiling. In the ABM-II test, boiling of water was possibly observed. Throughout the experiment, a pressure/water loss was recorded in the upper part of the geotechnical barrier and water was added to maintain pressure in the Bentonite. As a result of evaporation, NaCl crusts might have formed and the barrier was partly disintegrated. These results demonstrated that a reasonable assumption is that no boiling of water occurs in disposal concepts in which a pressure loss can occur.
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about differences of swelling pressure dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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About differences of swelling pressure — dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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Cation Exchange and Mineral Reactions Observed in MX 80 Buffer Samples of the Prototype Repository In Situ Experiment in Äspö, Sweden
Clays and Clay Minerals, 2014Co-Authors: Reiner Dohrmann, Stephan KaufholdAbstract:Bentonites are candidate materials for high-level radioactive waste (HLRW) repositories and, therefore, are investigated with respect to long-term stability. In order to identify possible Bentonite alteration processes, long-term in situ tests are conducted in rock laboratories. The prototype repository in situ experiment (PR) is one of the best examples of this kind of test due to the size of the installation as well as the duration. In the present study, chemical and mineralogical alteration processes of the Bentonite MX 80 after an 8 y heating period were investigated. The water content of all samples increased following inflowing Na-Ca-Cl-type granitic groundwater causing cation exchange in the Bentonite buffer materials. Exchangeable magnesium was desorbed in the buffer and MgO concentration increased at the Bentonite-Cu canister interface; the Mg sink could not be detected, however. CaO also accumulated at this interface mainly as Ca carbonate and Ca sulfate. Cu corrosion products were identified at the Bentonite-canister interface by chemical analysis, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), and differential thermal analysis. Up to 0.5 mm into the Bentonites Cu could be detected by SEM-EDX. No cristobalite dissolution was observed in contrast to other in situ tests in which iron heaters were used. The corrosion products and the lubricant which was added during manufacturing of the Bentonite blocks were mixed with the Bentonite at the Bentonite-canister interface. A quantitative measure of that mixture was the decrease in the cation exchange capacity (CEC). The CEC also reduced in all other samples, however, compared to the CECs of the reference samples, particularly in the warmer deposition hole 5 compared to the colder deposition hole 6. Overall, the PR in situ experiment proved that cation exchange reactions occurred in full-scale Bentonite buffer experiments in all Bentonite blocks but structural degradation of smectite could not be identified.
Stephan Kaufhold - One of the best experts on this subject based on the ideXlab platform.
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Determining the extent of Bentonite alteration at the Bentonite/cement interface
Applied Clay Science, 2020Co-Authors: Stephan Kaufhold, Reiner Dohrmann, Kristian UferAbstract:Abstract Bentonite is investigated as a geotechnical barrier material in repositories for high-level radioactive waste (HLRW). One of the most important research topics concerns the stability of Bentonite at interfaces such as Bentonite/metal or Bentonite/cement. Alkaline cements are particularly hazardous because they may dissolve at least part of the Bentonite. This reaction is, therefore, investigated extensively. Bentonite properties and performances in different applications vary from one deposit to another and even within some deposits. The hypothesis, therefore, was, that the stability of different Bentonites in contact with cement could be different. Hence, the aim of the study was to compare the reactivity of a significant set of well characterized Bentonites with Portland cement and to understand the reason for the differences. For Bentonite comparison standard reaction parameters were fixed: 3 months, 80 °C, 25% cement (or 33%). The resulting degree of degradation was compared with basic Bentonite parameters. A fairly good correlation (R2 = 0.7) of the extent of alteration with the amount of soda soluble silica (reactive silica which to variable amounts is present in natural Bentonites) was obtained which proved, that soda soluble silica was able to buffer the reaction. Such Bentonites with a higher amount of soda soluble silica showed less smectite degradation and hence will perform better at the cement/Bentonite interface than others with less soda soluble silica. Varying the clay/cement ratio showed that smectite degradation requires 10–20% cement. No reaction was observed below this value. Linear curves were observed which allow to conclude that 1 g cement is able to degrade about 0.6 g of Bentonite (smectite) at the selected solution/solid ratios. The reaction was almost complete after 1 month and no difference was observed between 60 and 80 °C.
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Characterization of the Second Parcel of the Alternative Buffer Material (ABM) Experiment — I Mineralogical Reactions
Clays and Clay Minerals, 2017Co-Authors: Stephan Kaufhold, N. Götze, Reiner Dohrmann, D. SvenssonAbstract:The performance of Bentonite barriers for high level radioactive waste (HLRW) disposal is currently being tested in various real-and up-scale disposal tests. One of the disposal tests, the ABM test (ABM = alternative buffer material), was conducted by SKB (Svensk Kärnbränslehantering) as a mediumscale experiment at the Äspö hard rock laboratory in Sweden. The present study deals with the second parcel (ABM-II), which was retrieved after 6.5 years with 2.5 years of water saturation and 3–4 years of heating up to 141°C. Nine different Bentonites and two marine clays were tested to investigate the performance. The aim of the study was to provide a detailed characterization of the mineralogical and chemical changes that took place in ABM-II, compare the findings with ABM-I (the first of the six test parcels), and try to draw some general conclusions concerning the use of Bentonites in such geotechnical barriers. The ABM-II test parcel revealed a set of reactions that a HLRW Bentonite might undergo. The most prominent reaction was the rather complete exchange of cations, which was discussed in a second part to this publication (II — cation exchange; Dohrmann and Kaufhold, 2017). The corrosion of the Fe in metal canisters was observed, but no discrete corrosion product was identified. At the interface of Bentonite and the metal canister, the formation of smectite-type trioctahedral clay minerals was observed. In contrast to the ABM-I test, anhydrite was present in many of the Bentonite blocks of the ABM-II test. In most concepts used for HLRW disposal in crystalline rocks, a temperature below 100°C at the canister surface was applied to avoid boiling. In the ABM-II test, boiling of water was possibly observed. Throughout the experiment, a pressure/water loss was recorded in the upper part of the geotechnical barrier and water was added to maintain pressure in the Bentonite. As a result of evaporation, NaCl crusts might have formed and the barrier was partly disintegrated. These results demonstrated that a reasonable assumption is that no boiling of water occurs in disposal concepts in which a pressure loss can occur.
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about differences of swelling pressure dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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About differences of swelling pressure — dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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Cation Exchange and Mineral Reactions Observed in MX 80 Buffer Samples of the Prototype Repository In Situ Experiment in Äspö, Sweden
Clays and Clay Minerals, 2014Co-Authors: Reiner Dohrmann, Stephan KaufholdAbstract:Bentonites are candidate materials for high-level radioactive waste (HLRW) repositories and, therefore, are investigated with respect to long-term stability. In order to identify possible Bentonite alteration processes, long-term in situ tests are conducted in rock laboratories. The prototype repository in situ experiment (PR) is one of the best examples of this kind of test due to the size of the installation as well as the duration. In the present study, chemical and mineralogical alteration processes of the Bentonite MX 80 after an 8 y heating period were investigated. The water content of all samples increased following inflowing Na-Ca-Cl-type granitic groundwater causing cation exchange in the Bentonite buffer materials. Exchangeable magnesium was desorbed in the buffer and MgO concentration increased at the Bentonite-Cu canister interface; the Mg sink could not be detected, however. CaO also accumulated at this interface mainly as Ca carbonate and Ca sulfate. Cu corrosion products were identified at the Bentonite-canister interface by chemical analysis, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), and differential thermal analysis. Up to 0.5 mm into the Bentonites Cu could be detected by SEM-EDX. No cristobalite dissolution was observed in contrast to other in situ tests in which iron heaters were used. The corrosion products and the lubricant which was added during manufacturing of the Bentonite blocks were mixed with the Bentonite at the Bentonite-canister interface. A quantitative measure of that mixture was the decrease in the cation exchange capacity (CEC). The CEC also reduced in all other samples, however, compared to the CECs of the reference samples, particularly in the warmer deposition hole 5 compared to the colder deposition hole 6. Overall, the PR in situ experiment proved that cation exchange reactions occurred in full-scale Bentonite buffer experiments in all Bentonite blocks but structural degradation of smectite could not be identified.
Wiebke Baille - One of the best experts on this subject based on the ideXlab platform.
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about differences of swelling pressure dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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About differences of swelling pressure — dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
Tom Schanz - One of the best experts on this subject based on the ideXlab platform.
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about differences of swelling pressure dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
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About differences of swelling pressure — dry density relations of compacted Bentonites
Applied Clay Science, 2015Co-Authors: Stephan Kaufhold, Wiebke Baille, Tom Schanz, Reiner DohrmannAbstract:Abstract The swelling capacity is one of the most characteristic properties of Bentonites. This property, along with others, made Bentonite a candidate material for the safe disposal of high-level radioactive waste (HLRW). For HLRW-barrier systems the swelling pressure is of particular interest because it basically controls sealing properties (a large swelling pressure generally results in low permeability). Commonly, the swelling pressure is investigated as a function of compaction and hence plotted against the density, usually the dry density. Different dry density/swelling pressure relations of Bentonites were published. Therefore, the aim of the present study was to identify reasons for these differences. To be able to compare a significant set of different Bentonites, a small scale swelling pressure device was developed which is based on measuring a swelling pressure related value of 500 mg Bentonite powder. This device allowed recording about 200 single values (in duplicate) and hence dry density/swelling pressure relations of thirty six different Bentonites. The differences of the dry density/swelling pressure plots of different Bentonites could be explained by i) different ways to obtain a range of dry densities (either constant load or constant water content), ii) different portions of uncompactable porosity of the different Bentonites, and iii) different smectite contents.
Warren D. Huff - One of the best experts on this subject based on the ideXlab platform.
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K-Bentonites: A review
American Mineralogist, 2016Co-Authors: Warren D. HuffAbstract:Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called Bentonite or K-Bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-Bentonite and Bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-Bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-Bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.
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chemostratigraphy k ar ages and illitization of silurian k Bentonites from the central belt of the southern uplands down longford terrane british isles
Journal of the Geological Society, 1991Co-Authors: Warren D. Huff, T. B. Anderson, C. C. Rundle, G. S. OdinAbstract:The Central Belt of the Southern Uplands Terrane, in both Scotland and Ireland, is a faulted and imbricated sequence of Caradoc to Llandovery shales and Llandovery turbidites with numerous interbedded K-Bentonites. The Bentonites are composed dominantly of R3-ordered mixedlayer illite/smectite (I/S) containing 90–95% illite, and represent the product of illitization during low-grade metamorphism. K-Ar age determination on the μ m size fraction gave a range from 379 ± 10 to 406 ± 10 Ma. The fixed K is thought to have originated in the precursor ash, and was remobilized during the transformation of smectite to I/S. K-Ar ages record a retrograde thermal event that post-dates Wenlock prehnite—pumpellyite facies metamorphism and is contemporaneous with cooling and uplift during the end-Silurian—early Devonian collision of Laurentia with the East Avalonian terrane. Differences in Rb and other trace elements between the K–Bentonite beds are due, in part, to differences in original ash composition, and can be used to group the beds within biostratigraphically-defined boundaries. The chemical identification of groups of K-Bentonite beds offers additional criteria for their stratigraphical correlation on a regional scale.
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Chemostratigraphy, K-Ar ages and illitization of Silurian K–Bentonites from the Central Belt of the Southern Uplands–Down–Longford terrane, British Isles
Journal of the Geological Society, 1991Co-Authors: Warren D. Huff, T. B. Anderson, C. C. Rundle, G. S. OdinAbstract:The Central Belt of the Southern Uplands Terrane, in both Scotland and Ireland, is a faulted and imbricated sequence of Caradoc to Llandovery shales and Llandovery turbidites with numerous interbedded K-Bentonites. The Bentonites are composed dominantly of R3-ordered mixedlayer illite/smectite (I/S) containing 90–95% illite, and represent the product of illitization during low-grade metamorphism. K-Ar age determination on the μ m size fraction gave a range from 379 ± 10 to 406 ± 10 Ma. The fixed K is thought to have originated in the precursor ash, and was remobilized during the transformation of smectite to I/S. K-Ar ages record a retrograde thermal event that post-dates Wenlock prehnite—pumpellyite facies metamorphism and is contemporaneous with cooling and uplift during the end-Silurian—early Devonian collision of Laurentia with the East Avalonian terrane. Differences in Rb and other trace elements between the K–Bentonite beds are due, in part, to differences in original ash composition, and can be used to group the beds within biostratigraphically-defined boundaries. The chemical identification of groups of K-Bentonite beds offers additional criteria for their stratigraphical correlation on a regional scale.
