The Experts below are selected from a list of 207 Experts worldwide ranked by ideXlab platform
Quentin J. Fisher - One of the best experts on this subject based on the ideXlab platform.
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U-Pb systematics of an Upper Carboniferous black shale from South Yorkshire, UK
Chemical Geology, 2003Co-Authors: Quentin J. Fisher, Robert A. Cliff, M.h. DodsonAbstract:Abstract Open-system behaviour of uraniferous shales, which has been known for many years, has discouraged attempts to use U–Pb geochronology to date sedimentary systems. Techniques now available can facilitate better understanding of their geochemical evolution and their possible use in geochronometry. For the U-rich Alton ( G. listeri ) Marine Band, a combined fission track mapping, electron optical and sequential chemical extraction study confirms that uranium is incorporated into Francolite, an early diagenetic precipitate. U–Pb analyses of uranium-rich (>1000 ppm) Francolite nodules are discordant and imply ages ∼50–150 Ma younger than the date of sedimentation. Pb isotopic analysis suggests that uranium daughters continually leaked from the Francolite, 238 U daughters being released more efficiently than those of 235 U. Extrapolation of the U–Pb data to concordia produces an age consistent with the time of sedimentation. These features are also displayed by other uranium-rich shales such as the Swedish Kolm Measures, despite uranium being incorporated into different phases. Preferential loss of 238 U daughters from fine-grained particles due to alpha recoil could explain the unusual U–Pb isotopic composition, in both examples. Further work would be justified to investigate the application of U–Pb isotopic analysis of such material to date sedimentary sequences.
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Palaeoenvironmental controls on the uranium distribution in an Upper Carboniferous black shale (Gastrioceras listeri Marine Band) and associated strata; England
Chemical Geology, 2001Co-Authors: Quentin J. Fisher, Paul B. WignallAbstract:The mudrocks associated with the Gastrioceras listeri (G. listeri) Marine Band contain 0 to 200 ppm authigenic uranium. Several geochemical (degree of pyritisation (DOP), C/S ratios and framboidal pyrite size distribution) and palaeontological indicators (oxygen-restricted biofacies (ORB) analysis) suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen-restricted biofacies (ORBs 2 and 3). The uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals. These horizons may have been deposited when the oxygen minimum zone impinged on the seafloor. Brief oxygenation of an otherwise anoxic environment was conducive to Francolite precipitation, which then scavenged dissolved uranium. Slow sedimentation rates are also important in concentrating uranium and Francolite. Thus, enriched U values are only encountered under specific conditions of low, but fluctuating, oxygen regime and extremely slow sedimentation rates. Truly euxinic facies, lacking any fossils, and a uniformly small framboid population do not contain high concentrations of Francolite and are not U-rich.
M.h. Dodson - One of the best experts on this subject based on the ideXlab platform.
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U-Pb systematics of an Upper Carboniferous black shale from South Yorkshire, UK
Chemical Geology, 2003Co-Authors: Quentin J. Fisher, Robert A. Cliff, M.h. DodsonAbstract:Abstract Open-system behaviour of uraniferous shales, which has been known for many years, has discouraged attempts to use U–Pb geochronology to date sedimentary systems. Techniques now available can facilitate better understanding of their geochemical evolution and their possible use in geochronometry. For the U-rich Alton ( G. listeri ) Marine Band, a combined fission track mapping, electron optical and sequential chemical extraction study confirms that uranium is incorporated into Francolite, an early diagenetic precipitate. U–Pb analyses of uranium-rich (>1000 ppm) Francolite nodules are discordant and imply ages ∼50–150 Ma younger than the date of sedimentation. Pb isotopic analysis suggests that uranium daughters continually leaked from the Francolite, 238 U daughters being released more efficiently than those of 235 U. Extrapolation of the U–Pb data to concordia produces an age consistent with the time of sedimentation. These features are also displayed by other uranium-rich shales such as the Swedish Kolm Measures, despite uranium being incorporated into different phases. Preferential loss of 238 U daughters from fine-grained particles due to alpha recoil could explain the unusual U–Pb isotopic composition, in both examples. Further work would be justified to investigate the application of U–Pb isotopic analysis of such material to date sedimentary sequences.
Xabier Pereda-suberbiola - One of the best experts on this subject based on the ideXlab platform.
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Francolite as a diagenetic mineral in dinosaur and other Upper Cretaceous reptile bones (Laño, Iberian Peninsula): microstructural, petrological and geochemical features
Cretaceous Research, 1999Co-Authors: Javier Elorza, Humberto Astibia, Xabier Murelaga, Xabier Pereda-suberbiolaAbstract:Abstract Dinosaur and other Upper Cretaceous reptile (crocodile and turtle) bone samples from the Lano quarry, in the northern Iberian Peninsula, are composed of well-crystallized Francolite (carbonate fluorapatite). The replacement of biogenic hydroxyapatite by Francolite is related to diagenetic changes. Moreover, the crystallinity index (CI) of the Lano bone fossils is lower (0.2 to 0.3) than that of recent bones (0.39–0.48) and, compared to fresh bones (0.41–0.54), the Francolite has higher concentrations of trace elements and rare earth elements (ΣREE 500 to 900 ppm). The geochemical composition and trends of the fossil reptile remains suggest that the diagenetic processes were relatively uniform at the Lano site. The main fossil accumulations of Lano are associated with ferruginous crust surfaces. The reptile bones are usually covered with nodule-like iron oxides which are mainly composed of goethite and small detrital quartz grains. The diagenetic processes that led to the development of the iron oxides could have been a consequence of a hydromorphic process owing to seasonal variations of the phreatic water level. This suggests a climate with dry and wet seasonal periods.
Paul B. Wignall - One of the best experts on this subject based on the ideXlab platform.
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Palaeoenvironmental controls on the uranium distribution in an Upper Carboniferous black shale (Gastrioceras listeri Marine Band) and associated strata; England
Chemical Geology, 2001Co-Authors: Quentin J. Fisher, Paul B. WignallAbstract:The mudrocks associated with the Gastrioceras listeri (G. listeri) Marine Band contain 0 to 200 ppm authigenic uranium. Several geochemical (degree of pyritisation (DOP), C/S ratios and framboidal pyrite size distribution) and palaeontological indicators (oxygen-restricted biofacies (ORB) analysis) suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen-restricted biofacies (ORBs 2 and 3). The uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals. These horizons may have been deposited when the oxygen minimum zone impinged on the seafloor. Brief oxygenation of an otherwise anoxic environment was conducive to Francolite precipitation, which then scavenged dissolved uranium. Slow sedimentation rates are also important in concentrating uranium and Francolite. Thus, enriched U values are only encountered under specific conditions of low, but fluctuating, oxygen regime and extremely slow sedimentation rates. Truly euxinic facies, lacking any fossils, and a uniformly small framboid population do not contain high concentrations of Francolite and are not U-rich.
Ihsan S. Al-aasm - One of the best experts on this subject based on the ideXlab platform.
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Conditions of formation and diagenetic evolution of Upper Proterozoic phosphate nodules from southern Sweden: evidence from petrology, mineral chemistry and isotopes
Sedimentary Geology, 1994Co-Authors: Sadoon Morad, Ihsan S. Al-aasmAbstract:Abstract Dark-gray to black cryptocrystalline Francolite occurs as nodules (∼ 2–8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingso Group (Upper Proterozoic, southern Sweden). The δ13CPDB values (−18.4 to + 0.98‰) of structural-CO2, complemented by studies of the petrographic character and strontium isotopic composition of Francolite, indicate precipitation in the suboxic and sulfate-reduction zones. Francolite is intergrown with berthierine that has formed in the suboxic Fe-reduction zone. Calcite enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of Francolite. Mn- and Mg-rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis. The carbon ( δ 13 C PDB =−0.75 to +7.04‰ ) and oxygen ( δ 18 O PDB =−14.21 to −10.58‰ ) isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60° to 80°C. Some of the void- and fracture-filling calcites ( δ 13 C = −20.8 to −18.63‰ δ 18 O = −5.46 to −5.21‰ ) have, however, precipitated at lower temperatures (∼ 25–30°C) in the sulfate-reduction zone.
