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David L Shuster - One of the best experts on this subject based on the ideXlab platform.

  • neon Diffusion Kinetics in olivine pyroxene and feldspar retentivity of cosmogenic and nucleogenic neon
    Geochimica et Cosmochimica Acta, 2012
    Co-Authors: Loraine Gourbet, David L Shuster, Greg Balco, William S Cassata, Paul R. Renne, Dylan H Rood
    Abstract:

    Abstract We performed stepwise degassing experiments by heating single crystals of neutron- or proton-irradiated olivine, pyroxene and feldspar to study Diffusion Kinetics of neon. This is important in evaluating the utility of these minerals for cosmogenic 21 Ne measurements and, potentially, for Ne thermochronometry. Degassing patterns are only partially explained by simple Arrhenius relationships; most samples do not exhibit a precisely-determined activation energy in an individual Diffusion domain. Regardless, we find clear differences in Diffusion Kinetics among these minerals. Based on sub-selected data, our estimates for neon Diffusion Kinetics (activation energy E a and pre-exponential factor D o , assuming the analyzed fragments approximate the Diffusion domain) in each mineral are as follows: for the feldspars, E a ranges from ∼65 to 115 kJ/mol and D o from 3.9 × 10 −3 to 7.1 × 10 2  cm 2 s −1 ; for the pyroxenes, E a ranges from ∼292 to 480 kJ/mol and D o from 1.6 × 10 2 to 2.9 × 10 11  cm 2 s −1 ; for the olivines, E a ranges from ∼360 to 370 kJ/mol and D o from 1.5 × 10 6 to 5.0 × 10 6  cm 2 s −1 . Differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar, and olivine and pyroxene. These results indicate that cosmogenic 21 Ne will be quantitatively retained within olivine and pyroxene at Earth surface temperatures over geological timescales. The Diffusion Kinetics for feldspars, on the other hand, predicts that 21 Ne retention at Earth surface temperatures will vary significantly with domain size, crystal microtexture, surface temperature, and exposure duration. Quantitative retention is expected only in favorable conditions. This conclusion is reinforced by additional measurements of cosmogenic 21 Ne in coexisting quartz and feldspar from naturally irradiated surface samples; sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention, whereas alkali–feldspar from several granites does not.

  • argon Diffusion in plagioclase and implications for thermochronometry a case study from the bushveld complex south africa
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Paul R. Renne, William S Cassata, David L Shuster
    Abstract:

    Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K–Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon Diffusion Kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision 40 Ar/ 39 Ar data and quantifies Ar Diffusion Kinetics of low-K minerals. We present results of five Diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed Diffusion Kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155–178 kJ/mol and ln(D0/a 2 ) varies between 3.5 and 6.5. These Diffusion parameters correspond to closure temperatures of 225–300 C, for a 10 C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale Diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150–200 C over � 600 Ma. 2009 Elsevier Ltd. All rights reserved.

  • the influence of artificial radiation damage and thermal annealing on helium Diffusion Kinetics in apatite
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: David L Shuster, Kenneth A Farley
    Abstract:

    Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium Diffusion Kinetics in apatite. Earth Planet. Sci. Lett. 249(3–4), 148–161] revealing a correlation between radiogenic 4He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He Diffusion Kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U–Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage. Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic 3He used as a diffusant in step-heating Diffusion experiments. In every sample, irradiation increased the activation energy (Ea) and the frequency factor (Do/a^2) of Diffusion and yielded a higher He closure temperature (Tc) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in Ea and ln(Do/a^2) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites. To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural 4He as the diffusant. At temperatures above 290 °C a systematic change in Tc was observed, with values becoming lower with increasing temperature and time. For example, reduction of Tc from the starting value of 71 to ~52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in Tc are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same Ea − ln(Do/a2) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity. Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He Diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.

  • the influence of natural radiation damage on helium Diffusion Kinetics in apatite
    Earth and Planetary Science Letters, 2006
    Co-Authors: David L Shuster, Rebecca M Flowers, Kenneth A Farley
    Abstract:

    Abstract Stepwise degassing Diffusion experiments on 39 different apatite samples using radiogenic 4He and proton-induced 3He reveal a range in closure temperature (Tc) from ∼ 50 to 115 °C, for a cooling rate of 10 °C/Myr. There is no correlation between helium Diffusion and apatite chemistry including F/Cl ratio, but the closure temperature is positively correlated with the radiogenic 4He concentration ([4He]) in each sample. We argue that [4He] is a proxy for a sample's natural exposure to actinide radioactivity below the closure temperature, and that helium Diffusion in apatite is impeded by radiation-induced damage to the apatite structure. The Kinetics must therefore be an evolving function of time; measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage on helium Diffusion appears to far exceed other known controls on helium diffusivity, including grain size. Our Diffusion data are well described by a previously proposed, quantitative model that consists of two Arrhenius relations, one for volume Diffusion through undamaged mineral structure and one for release of helium from radiation damage “traps.” The unknown parameters in this “trapping model” were determined from the Diffusion experiments, and allow us to develop a tentative mathematical function that relates diffusivity to temperature and [4He]. By inserting this function into a 4He production-Diffusion model we have explored how these results affect the interpretation of apatite (U–Th)/He thermochronometry. The model predicts that the effective 4He closure temperature of apatite will vary with cooling rate and effective U concentration (eU) and may differ from the commonly assumed Tc of 70 °C by up to ± 15 °C. The 4He partial retention zone will look similar to previous expectations, but its depth will depend on accumulation time and on eU. Most notably, samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected. These predictions are consistent with recent observations of unexpected apatite (U–Th)/He ages in some settings.

Abdellah Kharicha - One of the best experts on this subject based on the ideXlab platform.

  • modeling Diffusion governed solidification of ternary alloys part 2 macroscopic transport phenomena and macrosegregation
    Computational Materials Science, 2014
    Co-Authors: Andreas Ludwig, Abdellah Kharicha
    Abstract:

    Abstract Part 1 of this two-part investigation presented a multiphase solidification model incorporating the finite Diffusion Kinetics and ternary phase diagram with the macroscopic transport phenomena (Wu et al., 2013). In Part 2, the importance of proper treatment of the finite Diffusion Kinetics in the calculation of macrosegregation is addressed. Calculations for a two-dimensional (2D) square casting (50 × 50 mm 2 ) of Fe–0.45 wt.%C–1.06 wt.%Mn considering thermo-solutal convection and crystal sedimentation are performed. The modeling result indicates that the infinite liquid mixing Kinetics as assumed by classical models (e.g., the Gulliver–Scheil or lever rule), which cannot properly consider the solute enrichment of the interdendritic or inter-granular melt at the early stage of solidification, might lead to an erroneous estimation of the macrosegregation. To confirm this statement, further theoretical and experimental evaluations are desired. The pattern and intensity of the flow and crystal sedimentation are dependent on the crystal morphologies (columnar or equiaxed); hence, the potential error of the calculated macrosegregation caused by the assumed growth Kinetics depends on the crystal morphology. Finally, an illustrative simulation of an engineering 2.45-ton steel ingot is performed, and the results are compared with experimental results. This example demonstrates the model applicability for engineering castings regarding both the calculation efficiency and functionality.

Andreas Ludwig - One of the best experts on this subject based on the ideXlab platform.

  • modeling Diffusion governed solidification of ternary alloys part 2 macroscopic transport phenomena and macrosegregation
    Computational Materials Science, 2014
    Co-Authors: Andreas Ludwig, Abdellah Kharicha
    Abstract:

    Abstract Part 1 of this two-part investigation presented a multiphase solidification model incorporating the finite Diffusion Kinetics and ternary phase diagram with the macroscopic transport phenomena (Wu et al., 2013). In Part 2, the importance of proper treatment of the finite Diffusion Kinetics in the calculation of macrosegregation is addressed. Calculations for a two-dimensional (2D) square casting (50 × 50 mm 2 ) of Fe–0.45 wt.%C–1.06 wt.%Mn considering thermo-solutal convection and crystal sedimentation are performed. The modeling result indicates that the infinite liquid mixing Kinetics as assumed by classical models (e.g., the Gulliver–Scheil or lever rule), which cannot properly consider the solute enrichment of the interdendritic or inter-granular melt at the early stage of solidification, might lead to an erroneous estimation of the macrosegregation. To confirm this statement, further theoretical and experimental evaluations are desired. The pattern and intensity of the flow and crystal sedimentation are dependent on the crystal morphologies (columnar or equiaxed); hence, the potential error of the calculated macrosegregation caused by the assumed growth Kinetics depends on the crystal morphology. Finally, an illustrative simulation of an engineering 2.45-ton steel ingot is performed, and the results are compared with experimental results. This example demonstrates the model applicability for engineering castings regarding both the calculation efficiency and functionality.

Bilge Yildiz - One of the best experts on this subject based on the ideXlab platform.

  • fast oxygen exchange and Diffusion Kinetics of grain boundaries in sr doped lamno3 thin films
    Physical Chemistry Chemical Physics, 2015
    Co-Authors: Edvinas Navickas, Tobias Huber, Yan Chen, Walid Hetaba, Gerald Holzlechner, Ghislain M Rupp, M Stogerpollach, Gernot Friedbacher, Herbert Hutter, Bilge Yildiz
    Abstract:

    In this study, the contribution of grain boundaries to the oxygen reduction and Diffusion Kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. 18O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from Diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the Diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen Diffusion but also fast oxygen exchange Kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

  • stretching the energy landscape of oxides effects on electrocatalysis and Diffusion
    Mrs Bulletin, 2014
    Co-Authors: Bilge Yildiz
    Abstract:

    Elastic strain engineering offers a new route to enable high-performance catalysts, electrochemical energy conversion devices, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of reactions involved in the material can be altered. This is the so-called mechano-chemical coupling. Here we discuss how elastic strain activates reactions on metals and oxides. We also present analogies to strained polymer reactions. A rich set of investigations have been performed on strained metal surfaces over the last 15 years, and the mechanistic reasons behind strain-induced reactivity are explained by an electronic structure model. On the other hand, the potential of strain engineering of oxides for catalytic and energy applications has been largely underexplored. In oxides, mechanical stress couples to reaction and Diffusion Kinetics by altering the oxygen defect formation enthalpy, migration energy barrier, adsorption energy, dissociation barrier, and charge transfer barrier. A generalization of the principles for stress activated reactions from polymers to metals to oxides is offered, and the prospect of using elastic strain to tune reaction and Diffusion Kinetics in functional oxides is discussed.

Kenneth A Farley - One of the best experts on this subject based on the ideXlab platform.

  • the influence of artificial radiation damage and thermal annealing on helium Diffusion Kinetics in apatite
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: David L Shuster, Kenneth A Farley
    Abstract:

    Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium Diffusion Kinetics in apatite. Earth Planet. Sci. Lett. 249(3–4), 148–161] revealing a correlation between radiogenic 4He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He Diffusion Kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U–Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage. Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic 3He used as a diffusant in step-heating Diffusion experiments. In every sample, irradiation increased the activation energy (Ea) and the frequency factor (Do/a^2) of Diffusion and yielded a higher He closure temperature (Tc) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in Ea and ln(Do/a^2) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites. To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural 4He as the diffusant. At temperatures above 290 °C a systematic change in Tc was observed, with values becoming lower with increasing temperature and time. For example, reduction of Tc from the starting value of 71 to ~52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in Tc are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same Ea − ln(Do/a2) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity. Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He Diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.

  • the influence of natural radiation damage on helium Diffusion Kinetics in apatite
    Earth and Planetary Science Letters, 2006
    Co-Authors: David L Shuster, Rebecca M Flowers, Kenneth A Farley
    Abstract:

    Abstract Stepwise degassing Diffusion experiments on 39 different apatite samples using radiogenic 4He and proton-induced 3He reveal a range in closure temperature (Tc) from ∼ 50 to 115 °C, for a cooling rate of 10 °C/Myr. There is no correlation between helium Diffusion and apatite chemistry including F/Cl ratio, but the closure temperature is positively correlated with the radiogenic 4He concentration ([4He]) in each sample. We argue that [4He] is a proxy for a sample's natural exposure to actinide radioactivity below the closure temperature, and that helium Diffusion in apatite is impeded by radiation-induced damage to the apatite structure. The Kinetics must therefore be an evolving function of time; measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage on helium Diffusion appears to far exceed other known controls on helium diffusivity, including grain size. Our Diffusion data are well described by a previously proposed, quantitative model that consists of two Arrhenius relations, one for volume Diffusion through undamaged mineral structure and one for release of helium from radiation damage “traps.” The unknown parameters in this “trapping model” were determined from the Diffusion experiments, and allow us to develop a tentative mathematical function that relates diffusivity to temperature and [4He]. By inserting this function into a 4He production-Diffusion model we have explored how these results affect the interpretation of apatite (U–Th)/He thermochronometry. The model predicts that the effective 4He closure temperature of apatite will vary with cooling rate and effective U concentration (eU) and may differ from the commonly assumed Tc of 70 °C by up to ± 15 °C. The 4He partial retention zone will look similar to previous expectations, but its depth will depend on accumulation time and on eU. Most notably, samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected. These predictions are consistent with recent observations of unexpected apatite (U–Th)/He ages in some settings.