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Luc André - One of the best experts on this subject based on the ideXlab platform.
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Coupled silicon-oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2011Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, M. G. Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (-0.64‰ < δ30Si < -0.01‰ and + 8.6‰ < δ18O < + 11.9‰) to silicified basalts (δ30Si and δ18O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ30Si < + 1.05‰ and + 10.9 < δ18O < + 17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to + 19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the Silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.
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Coupled silicon–oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2010Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, Matthias Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa. The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (−0.64‰
K. Abraham - One of the best experts on this subject based on the ideXlab platform.
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Coupled silicon-oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2011Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, M. G. Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (-0.64‰ < δ30Si < -0.01‰ and + 8.6‰ < δ18O < + 11.9‰) to silicified basalts (δ30Si and δ18O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ30Si < + 1.05‰ and + 10.9 < δ18O < + 17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to + 19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the Silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.
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Coupled silicon–oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2010Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, Matthias Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa. The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (−0.64‰
Médard Thiry - One of the best experts on this subject based on the ideXlab platform.
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Pleistocene cold climate groundwater Silicification, Jbel Ghassoul region, Missour Basin, Morocco
Journal of the Geological Society, 2015Co-Authors: Médard Thiry, Milnes Antony, Mohamed Ben BrahimAbstract:Surficial Silicifications have been long considered to be indicative of warm and dry climates. Here we describe various forms of supergene Silicification in a Miocene lacustrine sequence in the Missour Basin near Jbel Ghassoul (Morocco) in a landscape with accentuated relief. The Silicification is almost exclusively limited to a 10 - 40 m wide zone from the edges of scarp and mesa landforms. This distribution is interpreted to record the locations where groundwaters which produced the Silicification discharged from a higher level paleolandscape. The main component of the silica was imported late and significantly post-dates the deposition of the sediments. This implies that significant volumes of silica-bearing solutions flowed through these formations in response to an hydraulic gradient generated by relief. Silicification thus occurred only after uplift and incision of the sedimentary fill of the Missour Basin. The zones of Silicification of the Jbel Ghassoul sequence can be linked geomorphically to remnants of high level pediments that have been dated in the literatures as early to middle Pleistocene and interpreted to have been formed during cold climates. Low temperatures in outcrops near the discharge zones during cold periods is considered to be a key factor in silica precipitation from groundwaters.
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Pleistocene cold climate groundwater Silicification, Jbel Ghassoul region, Missour Basin, Morocco
Journal of the Geological Society, 2014Co-Authors: Médard Thiry, A. R. Milnes, Mohamed Ben BrahimAbstract:Surficial Silicifications have been long considered to be indicative of warm and dry climates. Here we describe various forms of supergene Silicification in a Miocene lacustrine sequence in the Missour Basin near Jbel Ghassoul (Morocco) in a landscape with accentuated relief. The Silicification is almost exclusively limited to a 10–40 m wide zone from the edges of scarp and mesa landforms. This distribution is interpreted to record the locations where groundwaters which produced the Silicification discharged from a higher level palaeolandscape. The main component of the silica was imported late and significantly post-dates the deposition of the sediments. This implies that significant volumes of silica-bearing solutions flowed through these formations in response to a hydraulic gradient generated by relief. Silicification thus occurred only after uplift and incision of the sedimentary fill of the Missour Basin. The zones of Silicification of the Jbel Ghassoul sequence can be linked geomorphologically to remnants of high-level pediments that have been dated in the literature to the early to middle Pleistocene and interpreted to have been formed during cold climates. Low temperatures in outcrops near the discharge zones during cold periods are considered to be a key factor in silica precipitation from groundwaters.
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Datation des calcites et des grès de Fontainebleau : une remise à plat de l'âge des Silicifications des terrains tertiaires du Bassin.de Paris
2014Co-Authors: Médard Thiry, Christophe Innocent, François Ménillet, Jean-michel SchmittAbstract:La mise en lumière du rôle des périodes froides dans les mécanismes de précipitation de la silice permet de proposer un modèle de Silicification unique, homogène et cohérent pour toutes les Silicifications du Bassin de Paris : grès, calcaires silicifiés et meumières. Ce modèle doit être envisagé bien au-delà du Bassin de Paris, nombre de Silicifications des formations tertiaires d’Europe, d’Amérique du nord et de régions altitudinales plus méridionales ( ?) méritent d’être revisitées avec un œil neuf.
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Les poudingues, les calcaires et les polissoirs de la vallée du Loing.
2006Co-Authors: Médard Thiry, Jean-pierre HofstetterAbstract:Guide pour une excursion géologique dans la région de Nemours (77) - Vallée du Loing. Conglomérat et Silicification du Sparnacien. Polissoirs préhistoriques.
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Altération bauxitique associée aux argiles à chailles sur la bordure sud-est du Bassin de Paris.
Bulletin de la Société Géologique de France, 2005Co-Authors: Médard Thiry, Régine Simon-coinçon, Florence Quesnel, Robert WynsAbstract:On the southeastern margin of the Paris Basin, Nivernais region, the marine deposits have been continuous all along the Jurassic period. The sea withdrew a first time during the Upper Tithonian. This regression coincided with erosion and weathering. Apparently, the sea invaded again the Nivernais region only during the Albian, to which are attributed a few relicts of glauconitic sandstone with quartz pebbles. The youngest marine occurrences are of Santonian-Campanian age, made of silicified urchins and microfauna preserved in flint flows. The Tertiary deposits are limited to small grabens and made of nodular lacustrine limestones with charophytes ascribed to the Middle Eocene. In this southeastern margin of the basin, palaeoweathering products of the Jurassic and Cretaceous, i.e. Clay-with-Jurassic cherts and Clay-with-Cretaceous flints, are widespread. Their study allow to reconstruct the long continental evolution of the area. A red silty clay formation, with scattered cherts and abundant ferruginous gravels, containing typically from 10 to 20% of gibbsite, has been identified at Beaumont-la-Ferrière. Besides the lack of granulometric sorting of the quartz grains and of the ferruginous nodules and gravels, the micromorphologic analysis of the thin sections shows well developed illuviation structures, pointing clearly out pedogenetic features. The red silty clay formation in Beaumont-la-Ferrière relates to an in situ bauxitic palaeoprofile developed above Callovo-Oxfordian limestones. Near Beaumont-la-Ferrière, at St-Benin-d'Azy and at Ste Colombe, outliers display several weathering formations with cherts and flints that total over 50 to 80 m thickness. At the base, there is in-situ Clay-with-cherts, in which occurs the siderolithic iron ore deposits and which locally contains also gibbsite. In places, this Clay-with-cherts is topped by rounded cherts, from 3 to 10 cm in diameter, and sand lenses containing quartz gravels. The Clay-with-flints of lustreous break, translucent, with branchy morphologies and porous cortex, typical of the Clay-with-flints known elsewhere, occurs at the top of the buttes. The reworked cherts, the sand and the Clay-with-flints Formation are often silicified by typical pedogenic Silicification, with illuviation features and titania-rich matrix. The palaeoweathering formations occurring in the Nivernais region allow to specify at least three successive weathering stages in the southeastern Paris Basin. (1) Development during the early Cretaceous of a thick blanket of Clay-with-cherts by weathering of the Jurassic chert-bearing limestones after withdrawal of the sea at the end of the Tithonian. Locally this weathering lead to the development of true bauxitic profiles, with gibbsite, pisolites, nodules, wide illuviation structures, etc. The development of the siderolithic iron ores relates to this weathering stage. (2) A second weathering stage, after the withdrawal of the sea during the Campanian, lead to development of Clay-with-flints from leaching of the chalk deposits. (3) Silicification affected the Clay-with-cherts and the Clay-with-flints, as well as the reworked deposits that today armour the reliefs. These Silicifications display the same micromorphologic and geochemic characters as the silcretes that developed during Middle/Upper Eocene in the central parts of the Paris Basin. This is the first time that in situ Cretaceous bauxitic profiles are described north of the Massif Central. It seems likely that these bauxitic formations have been initially much more widespread, but a number of profiles may probably have been resilicified, with alteration of the gibbsite into kaolinite along the Silicification of the siderolithic piedmont during the Eocene. The old pisolithic iron ore workings of the nineteenth century are located in this Clay-with-cherts Formation. In this way, the siderolithic iron ore deposits, typical from the southern margins of the Paris Basin, are of early Cretaceous age and correlative to the bauxite deposits of southern France.
Stephen Edge - One of the best experts on this subject based on the ideXlab platform.
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physicochemical comparison between microcrystalline cellulose and silicified microcrystalline cellulose
International Journal of Pharmaceutics, 1998Co-Authors: Mike Tobyn, John N. Staniforth, Gerard P Mccarthy, Stephen EdgeAbstract:Abstract Silicified microcrystalline cellulose (SMCC) has been compared with a standard grade of microcrystalline cellulose (MCC) using several physicochemical techniques in order to elucidate any chemical or polymorphic changes in the material that could be attributed to the Silicification process. Samples of SMCC, MCC and dry and wet mixes of MCC and silicon dioxide were analysed using FT-IR, 13 C NMR, powder X-ray diffraction, mercury porosimetry, helium pycnometry and scanning electron microscopy together with particle size analysis and deaggregation studies. Analysis of the data obtained from these methods suggested that there were no discernible chemical or polymorphic differences between the samples, indicating that the `Silicification' process produces a material which is chemically and physically very similar to standard MCC.
C. Harris - One of the best experts on this subject based on the ideXlab platform.
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Coupled silicon-oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2011Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, M. G. Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (-0.64‰ < δ30Si < -0.01‰ and + 8.6‰ < δ18O < + 11.9‰) to silicified basalts (δ30Si and δ18O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ30Si < + 1.05‰ and + 10.9 < δ18O < + 17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to + 19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the Silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.
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Coupled silicon–oxygen isotope fractionation traces Archaean Silicification
Earth and Planetary Science Letters, 2010Co-Authors: K. Abraham, Axel Hofmann, S. F. Foley, Damien Cardinal, C. Harris, Matthias Barth, Luc AndréAbstract:Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of Silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa. The δ30Si and δ18O values of bulk rock increase with increasing amount of Silicification from unsilicified basalts (−0.64‰
