The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Graeme T Swindles - One of the best experts on this subject based on the ideXlab platform.
-
standard Chemical based tephra extraction methods significantly alter the geochemistry of volcanic glass shards
Journal of Quaternary Science, 2019Co-Authors: Claire L Cooper, Ivan P Savov, Graeme T SwindlesAbstract:The Chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible Chemical Alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well-characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to Chemical Alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K2O, Na2O, CaO, MgO) and SiO2, and the most stable oxides are Al2O3 and FeO. Felsic glasses are considerably less susceptible to Alteration by both acidic (HCl, HNO3, H2SO4) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses.
-
Standard Chemical‐based tephra extraction methods significantly alter the geochemistry of volcanic glass shards
'Wiley', 2019Co-Authors: Cl Cooper, Ip Savov, Graeme T SwindlesAbstract:The Chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible Chemical Alteration of microscopic (
Michael L Everett - One of the best experts on this subject based on the ideXlab platform.
-
Chemical Alteration of poly vinyl fluoride tedlar induced by exposure to vacuum ultraviolet radiation
Applied Surface Science, 2006Co-Authors: Michael L Everett, Gar B HoflundAbstract:Abstract In this study the Chemical Alteration of poly(vinyl fluoride) Tedlar® by vacuum ultraviolet radiation (VUV) (115–400 nm) has been examined using X-ray photoelectron spectroscopy (XPS). The initial F/C atom ratio of 0.34 decreases to 0.17 after a 2-h exposure. The F/C atom ratio is further reduced to a steady-state value of approximately 0.04 after a 24-h exposure. Similarly, the O/C atom ratio is reduced from 0.08 to 0.05 and then to 0.02 during these two exposures. As the F and O are removed by VUV exposure, the C concentration increases from 70.5 to 82.0 and then to 94.6 at.% thus forming a graphitic or amorphous carbon-like layer which erodes more slowly than the virgin Tedlar surface. Exposure of the VUV-damaged surface to O2 results in chemisorption of O, indicating that reactive sites are formed during the Chemical erosion by VUV. Further exposure to VUV removes this chemisorbed oxygen but a subsequent exposure to air at atmospheric conditions causes a three-fold increase in O chemisorbed at the surface. Comparison of XPS data indicates that the mechanisms of Chemical Alteration by VUV radiation and hyperthermal (∼5 eV) atomic oxygen (AO) are similar.
-
Chemical Alteration of poly vinyl fluoride tedlar by hyperthermal atomic oxygen
Applied Surface Science, 2005Co-Authors: Gar B Hoflund, Michael L EverettAbstract:Abstract In this study the erosion of poly(vinyl fluoride) Tedlar by hyperthermal atomic oxygen (AO) has been examined using X-ray photoelectron spectroscopy (XPS). Initially the Tedlar film had F/C and O/C atom ratios of 0.45 and 0.11, which decrease to 0.018 and 0.04, respectively, after a 2-h exposure to a flux of 2 × 10 15 atoms/cm 2 s AO with an average kinetic energy of 5 eV. This exposure essentially produced a graphitic or amorphous carbon-like layer with a carbon content greater than 90 at.%. Longer AO exposures do not alter the composition of this layer significantly. Exposure to O 2 or air nearly doubles the oxygen content in the near-surface region. This is due to dissociative oxygen adsorption at reactive sites formed at the polymer surface during AO exposure. Further exposure to AO removes this chemisorbed oxygen.
Gar B Hoflund - One of the best experts on this subject based on the ideXlab platform.
-
Chemical Alteration of poly vinyl fluoride tedlar induced by exposure to vacuum ultraviolet radiation
Applied Surface Science, 2006Co-Authors: Michael L Everett, Gar B HoflundAbstract:Abstract In this study the Chemical Alteration of poly(vinyl fluoride) Tedlar® by vacuum ultraviolet radiation (VUV) (115–400 nm) has been examined using X-ray photoelectron spectroscopy (XPS). The initial F/C atom ratio of 0.34 decreases to 0.17 after a 2-h exposure. The F/C atom ratio is further reduced to a steady-state value of approximately 0.04 after a 24-h exposure. Similarly, the O/C atom ratio is reduced from 0.08 to 0.05 and then to 0.02 during these two exposures. As the F and O are removed by VUV exposure, the C concentration increases from 70.5 to 82.0 and then to 94.6 at.% thus forming a graphitic or amorphous carbon-like layer which erodes more slowly than the virgin Tedlar surface. Exposure of the VUV-damaged surface to O2 results in chemisorption of O, indicating that reactive sites are formed during the Chemical erosion by VUV. Further exposure to VUV removes this chemisorbed oxygen but a subsequent exposure to air at atmospheric conditions causes a three-fold increase in O chemisorbed at the surface. Comparison of XPS data indicates that the mechanisms of Chemical Alteration by VUV radiation and hyperthermal (∼5 eV) atomic oxygen (AO) are similar.
-
Chemical Alteration of poly vinyl fluoride tedlar by hyperthermal atomic oxygen
Applied Surface Science, 2005Co-Authors: Gar B Hoflund, Michael L EverettAbstract:Abstract In this study the erosion of poly(vinyl fluoride) Tedlar by hyperthermal atomic oxygen (AO) has been examined using X-ray photoelectron spectroscopy (XPS). Initially the Tedlar film had F/C and O/C atom ratios of 0.45 and 0.11, which decrease to 0.018 and 0.04, respectively, after a 2-h exposure to a flux of 2 × 10 15 atoms/cm 2 s AO with an average kinetic energy of 5 eV. This exposure essentially produced a graphitic or amorphous carbon-like layer with a carbon content greater than 90 at.%. Longer AO exposures do not alter the composition of this layer significantly. Exposure to O 2 or air nearly doubles the oxygen content in the near-surface region. This is due to dissociative oxygen adsorption at reactive sites formed at the polymer surface during AO exposure. Further exposure to AO removes this chemisorbed oxygen.
Claire L Cooper - One of the best experts on this subject based on the ideXlab platform.
-
standard Chemical based tephra extraction methods significantly alter the geochemistry of volcanic glass shards
Journal of Quaternary Science, 2019Co-Authors: Claire L Cooper, Ivan P Savov, Graeme T SwindlesAbstract:The Chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible Chemical Alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well-characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to Chemical Alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K2O, Na2O, CaO, MgO) and SiO2, and the most stable oxides are Al2O3 and FeO. Felsic glasses are considerably less susceptible to Alteration by both acidic (HCl, HNO3, H2SO4) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses.
Geoffrey S Ellis - One of the best experts on this subject based on the ideXlab platform.
-
kinetics of elemental sulfur reduction by petroleum hydrocarbons and the implications for hydrocarbon thermal Chemical Alteration
Geochimica et Cosmochimica Acta, 2019Co-Authors: Geoffrey S Ellis, Tongwei Zhang, Paul G Kralert, Yongchun TangAbstract:Abstract Although sulfur-containing compounds are known to play a significant role in the diagenic and catagenic processes that generate oil and gas, relatively little is known about the kinetics of reactions between elemental S and petroleum hydrocarbons. To investigate this subject, a series of closed-system pyrolysis experiments using paraffin, a low-sulfur oil, and a high-sulfur oil with and without elemental S were conducted, and first-order Chemical kinetics were fit to the experimental results. The average value for the activation energy required to reduce elemental S to H2S and to thermoChemically alter higher molecular weight hydrocarbons to methane was calculated to be 193 kJ mol−1 (46 kcal mol−1). The results of this study demonstrate that under typical geologic conditions the rate of reduction of elemental S to H2S by petroleum hydrocarbons is quite rapid. The maximum time for substantial amounts of elemental S to persist in contact with petroleum hydrocarbons is estimated to be no more than a few million years in cool reservoirs (e.g., 120 °C) the half-life of elemental S may be as short as hundreds of years. Additionally, the presence of elemental S substantially lowers the onset temperature of hydrocarbon thermal Chemical Alteration (TCA). The activation energy for TCA of a low-sulfur oil to generate methane is estimated to be lowered by 92 kJ mol−1 (22 kcal mol−1) due to the presence of elemental sulfur. Consequently, the presence of elemental S in petroleum reservoirs is expected to lower the thermal stability of oil and decrease the maximum depth at which oil occurs within a basin (thermal deadline). The observed acceleration of hydrocarbon TCA is possibly due to organic sulfur compounds (e.g., thiols and sulfides) that form through the reaction of H2S or polysulfides with hydrocarbons and subsequently thermally degrade leading to the formation of sulfur radicals that in turn enhance TCA reactions.
-
distinguishing solid bitumens formed by thermoChemical sulfate reduction and thermal Chemical Alteration
Organic Geochemistry, 2008Co-Authors: Simon R Kelemen, Clifford C Walters, P J Kwiatek, Mobae Afeworki, Michael Sansone, Howard Freund, Robert J Pottorf, Hans G Machel, Tongwei Zhang, Geoffrey S EllisAbstract:Insoluble solid bitumens are organic residues that can form by the thermal Chemical Alteration (TCA) or thermoChemical sulfate reduction (TSR) of migrated petroleum. TCA may actually encompass several low temperature processes, such as biodegradation and asphaltene precipitation, followed by thermal Alteration. TSR is an abiotic redox reaction where petroleum is oxidized by sulfate. It is difficult to distinguish solid bitumens associated with TCA of petroleum from those associated with TSR when both processes occur at relatively high temperature. The focus of the present work was to characterize solid bitumen samples associated with TCA or TSR using X-ray photoelectron spectroscopy (XPS). XPS is a surface analysis conducted on either isolated or in situ (>25 μm diameter) solid bitumen that can provide the relative abundance and Chemical speciation of carbon, organic and inorganic heteroatoms (NSO). In this study, naturally occurring solid bitumens from three locations, Nisku Fm. Brazeau River area (TSR-related), LaBarge Field Madison Fm. (TSR-related), and the Alaskan Brooks range (TCA-related), are compared to organic solids generated during laboratory simulation of the TSR and TCA processes. The abundance and Chemical nature of organic nitrogen and sulfur in solid bitumens can be understood in terms of the nature of (1) petroleum precursor molecules, (2) the concentration of nitrogen by way of thermal stress and (3) the mode of sulfur incorporation. TCA solid bitumens originate from polar materials that are initially rich in sulfur and nitrogen. Aromaticity and nitrogen increase as thermal stress cleaves aliphatic moieties and condensation reactions take place. Organic sulfur in TCA organic solids remains fairly constant with increasing maturation ( 3.5 to not, vert, similar17 sulfur per 100 carbons) into aromatic structures and to the low levels of nitrogen in their hydrocarbon precursors. Hence, XPS results provide organic Chemical composition information that helps to distinguish whether solid bitumen, either in situ or removed and concentrated from the rock matrix, was formed via the TCA or TRS process.