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Frank M Richter - One of the best experts on this subject based on the ideXlab platform.
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Isotope Fractionation of Li and K in silicate liquids by Soret diffusion
Geochimica et Cosmochimica Acta, 2014Co-Authors: Frank M Richter, E. Bruce Watson, Marc Chaussidon, Ruslan A. Mendybaev, John N ChristensenAbstract:Laboratory experiments were used to determine the thermal (Soret) isotopic Fractionation of lithium and potassium in a basalt melt, which adds elements with ionic charge +1 to the list of elements for which thermal isotopic Fractionations in silicate liquids have been previously reported (i.e., Ca, Mg, Fe, Si, O, Sr, Hf, and U). The new experiments were run at a moderate pressure of about 1.5 GPa in a piston cylinder apparatus in order to avoid gas bubbles once the sample was melted. The samples were displaced slightly below the hot spot of the piston cylinder assembly graphite furnace so that there would be a temperature difference of about 125° C across the samples while molten. The thermal isotopic Fractionation factor Ω (per mil Fractionation per 100° C per one atomic mass unit difference) was found to be 6.0 for lithium isotopes and 1.1 for potassium isotopes. The isotopic Fractionation in both cases resulted in the heavy isotopes becoming enriched at the cold end. The expanded data set of thermal isotopic Fractionation in silicate liquids is used to evaluate the degree to which recently proposed parameterizations are able to reproduce the experimental data.
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Lithium isotope Fractionation by diffusion in minerals. Part 1: Pyroxenes
Geochimica et Cosmochimica Acta, 2014Co-Authors: Frank M Richter, Marc Chaussidon, Ruslan A. Mendybaev, Bruce Watson, Dan RuscittoAbstract:Abstract Several recent studies found large lithium isotopic Fractionations correlated with concentration gradients in pyroxene minerals from lava flows and mantle nodules that were interpreted as indicating diffusion of lithium into the grains. Motivated by these findings experiments were undertaken in which powdered spodumene (LiAlSi2O6) or Li2SiO3 was used to diffuse lithium into Templeton augite or Dekalb diopside grains at 900 °C and oxygen fugacity ranging from log fO2 = −17 to log fO2 = −12. The purpose of these experiments was to determine the diffusion coefficient of lithium in pyroxene minerals and to measure the isotopic Fractionation of lithium in the diffusion boundary layer due to the relative mobility of 6Li compared to 7Li. The diffusion profiles of lithium that had not yet reached the center of Templeton augite grains were in most cases sharp steps propagating in from each boundary. In one case a more usual profile with smoothly decreasing lithium concentration with distance from the grain boundary was found. A model in which lithium occupies two different sites – one being fast diffusing interstitial lithium, the other much less mobile lithium in a metal site, reproduced both types of profiles. The step-like profiles arise in the model when interstitial lithium diffusing into the grain is strongly partitioned into abundant metal sites and thus does not penetrate further into the grain until all the metal sites at a given distance become filled. While the rate of propagation of the concentration step can be used to calculate an effective diffusivity for the penetration of lithium into the augite grains, the multiple speciation of lithium precludes making a separate precise determination of the diffusion coefficient of the interstitial lithium. Isotopic Fractionations of 7Li/6Li of about 30‰ were found in the step-like diffusion boundary layers, which translate into a ratio of the isotope diffusion coefficients D 7 Li / D 6 Li = 0.9592 (i.e., (6/7)β with β = 0.27). The same value of β = 0.27 was also able to fit the isotopic Fractionation data from the experiment with the smoothly decreasing lithium concentration profile. The laboratory experiments confirmed that diffusion of lithium produces large kinetic isotopic Fractionations and thus highlight the importance of isotopic measurements for discriminating when a particular instance of chemical zoning in minerals was the result of diffusion or some other process such crystal growth from an evolving melt. The experiments also showed that, contrary to conventional wisdom, isotopic gradients do not dissipate faster than gradients in the parent element, indeed the contrary is seen in several of the Templeton experiments where very large lithium isotopic Fractionations persisted after the lithium concentration had become effectively homogenized. Published data of lithium isotopic Fractionation of zoned augite grains from a Martian meteorite and in clinopyroxene grains from both lava flows on the Solomon Islands and from a San Carlos mantle xenolith were modeled in terms of lithium diffusion with an isotope Fractionation parameter β between 0.25 and 0.30, which is very similar to that derived from the laboratory experiments.
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Magnesium isotope Fractionation by chemical diffusion in natural settings and in laboratory analogues
Geochimica et Cosmochimica Acta, 2012Co-Authors: Rahul Chopra, E. Bruce Watson, Frank M Richter, Christian R. ScullardAbstract:Abstract Laboratory experiments are used to document isotopic Fractionation of magnesium by chemical diffusion in a silicate melt and the results compared to the magnesium isotopic composition across contacts between igneous rocks of different composition in natural settings. The natural samples are from transects from felsic to mafic rocks at Vinal Cove in the Vinalhaven Intrusive Complex, Maine and from the Aztec Wash pluton in Nevada. Two laboratory diffusion couples made by juxtaposing melts made from powders of the felsic and mafic compositions sampled at Vinal Cove were annealed at about 1500 °C for 22.5 and 10 h, respectively. The transport of magnesium in the diffusion couples resulted in easily measured magnesium isotopic Fractionations at the interface (δ26Mg∼1.5‰). These isotopic Fractionations provide a distinctive isotopic “fingerprint” that we use to determine whether chemical gradients in natural settings where melts of different composition were juxtaposed were due to chemical diffusion. The magnesium isotopic Fractionation along one profile at Vinal Cove is exactly what one would expect based on the Fractionations found in the laboratory experiments. This is an important result in that it shows that the isotope Fractionation by chemical diffusion found in highly controlled laboratory experiments can be found in a natural setting. This correspondence implies that chemical diffusion was the dominant process responsible for the transport of magnesium across this particular contact at Vinal Cove. A second Vinal Cove profile has a very similar gradient in magnesium concentration but with significantly less magnesium isotopic Fractionation than expected. This suggests that mass transport at this location was only partly by diffusion and that some other mass transport mechanism such as mechanical mixing must have also played a role. The magnesium isotopic composition of samples from Aztec Wash shows no resolvable isotopic Fractionation across the contact between the mafic and felsic rocks. The different degrees of magnesium isotopic Fractionation associated with otherwise similar composition gradients in natural settings show that kinetic isotope Fractionations provide a key discriminator for establishing whether or not molecular diffusion was the process responsible for an observed elemental gradient. In the one case of a contact at Vinal Cove where we are confident that the magnesium elemental and isotopic gradients were produced by diffusion, we deduced a cooling rate of about 1.5 °C per day.
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Isotope Fractionation in silicate melts by thermal diffusion
Nature, 2011Co-Authors: Frank M RichterAbstract:Arising from F. Huang, P. et al. Nature 464 , 396–400 (2010)10.1038/nature08840 ; Huang et al. reply It was recently shown that relatively large (compared to analytical precision) steady state thermal isotope Fractionations are produced in silicate melts whenever temperature differences are maintained for a sufficiently long time^ 1 , 2 . Huang et al. ^ 3 reported new data on thermal isotopic Fractionation of magnesium, calcium, and iron in silicate liquids, and claimed (1) that thermal isotopic Fractionations in silicate liquids are independent of composition and temperature, and (2) that their “results lead to a simple and robust framework for characterizing isotope Fractionations by thermal diffusion in natural and synthetic systems”. Here I consider whether the data and arguments presented by Huang et al. ^ 3 support their claims. In summary, I caution against assuming (on the basis of the data presented by Huang et al. ^ 3 ) that the thermal isotopic Fractionations are independent of temperature and composition, or that a framework of the type claimed has been found.
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Isotope Fractionation in silicate melts by thermal diffusion.
Nature, 2011Co-Authors: Frank M RichterAbstract:Arising from F. Huang, P. et al. , 396–400 (2010)10.1038/nature08840 ; Huang et al. reply It was recently shown that relatively large (compared to analytical precision) steady state thermal isotope Fractionations are produced in silicate melts whenever temperature differences are maintained for a sufficiently long time1,2. Huang et al.3 reported new data on thermal isotopic Fractionation of magnesium, calcium, and iron in silicate liquids, and claimed (1) that thermal isotopic Fractionations in silicate liquids are independent of composition and temperature, and (2) that their “results lead to a simple and robust framework for characterizing isotope Fractionations by thermal diffusion in natural and synthetic systems”. Here I consider whether the data and arguments presented by Huang et al.3 support their claims. In summary, I caution against assuming (on the basis of the data presented by Huang et al.3) that the thermal isotopic Fractionations are independent of temperature and composition, or that a framework of the type claimed has been found.
Edwin A Schauble - One of the best experts on this subject based on the ideXlab platform.
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silicon isotope Fractionation in silicate minerals insights from first principles models of phyllosilicates albite and pyrope
Geochimica et Cosmochimica Acta, 2014Co-Authors: Merlin Meheut, Edwin A SchaubleAbstract:Abstract Isotopic Fractionation factors for oxygen and silicon in phyllosilicates (pyrophyllite, talc), albite and pyrope have been calculated using first-principles methods based on density functional theory. Based on exhaustive analysis of numerical convergence, we also update our previous calculations on enstatite and forsterite silicon Fractionation properties. Calculated oxygen isotope Fractionations agree well with existing estimates for talc and albite. In the case of silicon, qualitative agreement is found with natural data. For phyllosilicates (kaolinite, lizardite, pyrophyllite, talc), Si isotope Fractionation properties appear to be correlated with stoichiometry: (1) 1000 ln α 30 Si phyllosilicate – quartz = a Mg ( T ) · Mg eq . + a Al ( T ) · Al eq . Si eq . , where Si eq. =#Si, Al eq . = 3 4 # Al and Mg eq . = 1 2 # Mg (cation equivalents) are the charge-weighed stoichiometric coefficients of each cation, normalized to the charge of the silicon atom, and a X ( T ) are proportionality coefficients depending on temperature. It is suggested that the effect of cation X on Si isotope Fractionation (i.e. a X ( T ) ) will increase with decreasing electronegativity of X. Si isotope Fractionation is further correlated with Si–O distances, suggesting a crystal chemical explanation for relation (1) in terms of electron donation effects. This relationship appears valid for quartz, pyrope and enstatite ( R 2 = 0.99 , n = 7 ) , but forsterite is strongly anomalous (error of 0.7‰ at 600 °C). These models indicate that attention should be given to chemical compositions in Si isotope studies. Relation (1) would explain the enrichment in heavy silicon isotopes accompanying magmatic differentiation.
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first principles estimates of equilibrium magnesium isotope Fractionation in silicate oxide carbonate and hexaaquamagnesium 2 crystals
Geochimica et Cosmochimica Acta, 2011Co-Authors: Edwin A SchaubleAbstract:Equilibrium mass-dependent magnesium isotope Fractionation factors are estimated for a range of crystalline compounds including oxides, silicates, carbonates, and salts containing the Mg(H2O)62+ solvation complex. Fractionation factors for the gas-phase species Mg and MgO are also presented. Fractionation factors are calculated with density functional perturbation theory (DFPT), using norm-conserving pseudopotentials. The results suggest that there will be substantial inter-mineral Fractionation, particularly between tetrahedrally coordinated Mg2+ in spinel (MgAl2O4) and the more common octahedrally coordinated Mg2+-sites in silicate and carbonate minerals. Isotope Fractionations calculated for Mg2+ in hexaaquamagnesium(2+) salts are in good agreement with previous Fractionation models of Mgaq2+ based on large molecular clusters (Black et al., 2007), but show possibly more significant disagreement with a more recent study (Rustad et al., 2010). These models further suggest that solvated Mgaq2+, in the form of Mg(H2O)62+, will have higher 26Mg/24Mg than coexisting magnesite and dolomite. Calculated Fractionations are consistent with Mg-isotope Fractionations observed in peridotite mineral separates and inorganic carbonate precipitates. Predicted large, temperature-sensitive spinel-silicate Fractionations, in particular, may find use in determining equilibration temperatures of peridotites and other high-temperature rock types.
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first principles estimates of equilibrium magnesium isotope Fractionation in silicate oxide carbonate and hexaaquamagnesium 2 crystals
Geochimica et Cosmochimica Acta, 2011Co-Authors: Edwin A SchaubleAbstract:Equilibrium mass-dependent magnesium isotope Fractionation factors are estimated for a range of crystalline compounds including oxides, silicates, carbonates, and salts containing the Mg(H2O)62+ solvation complex. Fractionation factors for the gas-phase species Mg and MgO are also presented. Fractionation factors are calculated with density functional perturbation theory (DFPT), using norm-conserving pseudopotentials. The results suggest that there will be substantial inter-mineral Fractionation, particularly between tetrahedrally coordinated Mg2+ in spinel (MgAl2O4) and the more common octahedrally coordinated Mg2+-sites in silicate and carbonate minerals. Isotope Fractionations calculated for Mg2+ in hexaaquamagnesium(2+) salts are in good agreement with previous Fractionation models of Mgaq2+ based on large molecular clusters (Black et al., 2007), but show possibly more significant disagreement with a more recent study (Rustad et al., 2010). These models further suggest that solvated Mgaq2+, in the form of Mg(H2O)62+, will have higher 26Mg/24Mg than coexisting magnesite and dolomite. Calculated Fractionations are consistent with Mg-isotope Fractionations observed in peridotite mineral separates and inorganic carbonate precipitates. Predicted large, temperature-sensitive spinel-silicate Fractionations, in particular, may find use in determining equilibration temperatures of peridotites and other high-temperature rock types.
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role of nuclear volume in driving equilibrium stable isotope Fractionation of mercury thallium and other very heavy elements
Geochimica et Cosmochimica Acta, 2007Co-Authors: Edwin A SchaubleAbstract:Equilibrium stable isotope Fractionations of mercury and thallium are estimated for molecules, atoms and ions using first-principles vibrational frequency and electronic structure calculations. These calculations suggest that isotopic variation in nuclear volume is the dominant cause of equilibrium Fractionation, driving 205Tl/203Tl and 202Hg/198Hg Fractionations of up to 3‰ at room temperature. Mass-dependent Fractionations are smaller, ca. 0.5–1‰ for the same isotopes. Both Fractionation mechanisms tend to enrich the neutron-rich isotopes in oxidized mercury- and thallium-bearing phases (Tl3+ and Hg2+) relative to reduced phases (Tl+ and Hg0). Among Hg2+-bearing species, inorganic molecules and complexes like HgCl2, HgCl42- and Hg(H2O)62+ will have higher 202Hg/198Hg than coexisting methylmercury species, suggesting a possible application of Hg-isotope measurements to understanding mercury methylation and increasing methylmercury concentrations at the top of the food chain. Estimated 205Tl/203Tl Fractionation between Tl(H2O)63+ and Tl(H2O)3+ is in reasonable agreement with the Fractionations previously observed between seawater and Fe–Mn crusts, supporting an equilibrium-like reduction/oxidation Fractionation mechanism. More generally, nuclear-volume isotope Fractionation will concentrate larger (heavier) nuclei in species where the electron density at the nucleus is small—due to lack of s-electrons (e.g., Hg2+—[Xe]4f145d106s0 vs. Hg0—[Xe]4f145d106s2) or enhanced s-electron screening by extra p, d, or f electrons (e.g., Tl0—[Xe]4f145d106s26p1 vs. Tl+—[Xe]4f145d106s26p0). Nuclear-volume Fractionations become much smaller for lighter elements, declining from ∼1‰/amu for thallium and mercury to ∼0.2‰/amu for ruthenium and ∼0.02‰/amu for sulfur.
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Applying stable isotope Fractionation theory to new systems
Reviews in Mineralogy & Geochemistry, 2004Co-Authors: Edwin A SchaubleAbstract:A basic theoretical understanding of stable isotope Fractionations can help researczzzhers plan and interpret both laboratory experiments and measurements on natural samples. The goal of this chapter is to provide an introduction to stable isotope Fractionation theory, particularly as it applies to mass-dependent Fractionations of non-traditional elements and materials. Concepts are illustrated using a number of worked examples. For most elements, and typical terrestrial temperature and pressure conditions, equilibrium isotopic Fractionations are caused by the sensitivities of molecular and condensed-phase vibrational frequencies to isotopic substitution. This is explained using the concepts of vibrational zero-point energy and the partition function, leading to Urey’s (1947) simplified equation for calculating isotopic partition function ratios for molecules, and Kieffer’s (1982) extension to condensed phases. Discussion will focus on methods of obtaining the necessary input data (vibrational frequencies) for partition function calculations. Vibrational spectra have not been measured or are incomplete for most of the substances that Earth scientists are interested in studying, making it necessary to estimate unknown frequencies, or to measure them directly. Techniques for estimating unknown frequencies range from simple analogies to well-studied materials to more complex empirical force-field calculations and ab initio quantum chemistry. Mossbauer spectroscopy has also been used to obtain the vibrational properties of some elements, particularly iron, in a variety of compounds. Some kinetic isotopic Fractionations are controlled by molecular or atomic translational velocities; this class includes many diffusive and evaporative Fractionations. These Fractionations can be modeled using classical statistical mechanics. Other kinetic Fractionations may result from the isotopic sensitivity of the activation energy required to achieve a transition state, a process that (in its simplest form) can be modeled using a modification of Urey’s equation (Bigeleisen 1949). Theoretical estimates of isotopic Fractionations are particularly powerful in systems that are difficult to characterize experimentally, or when empirical …
Ruslan A. Mendybaev - One of the best experts on this subject based on the ideXlab platform.
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Isotope Fractionation of Li and K in silicate liquids by Soret diffusion
Geochimica et Cosmochimica Acta, 2014Co-Authors: Frank M Richter, E. Bruce Watson, Marc Chaussidon, Ruslan A. Mendybaev, John N ChristensenAbstract:Laboratory experiments were used to determine the thermal (Soret) isotopic Fractionation of lithium and potassium in a basalt melt, which adds elements with ionic charge +1 to the list of elements for which thermal isotopic Fractionations in silicate liquids have been previously reported (i.e., Ca, Mg, Fe, Si, O, Sr, Hf, and U). The new experiments were run at a moderate pressure of about 1.5 GPa in a piston cylinder apparatus in order to avoid gas bubbles once the sample was melted. The samples were displaced slightly below the hot spot of the piston cylinder assembly graphite furnace so that there would be a temperature difference of about 125° C across the samples while molten. The thermal isotopic Fractionation factor Ω (per mil Fractionation per 100° C per one atomic mass unit difference) was found to be 6.0 for lithium isotopes and 1.1 for potassium isotopes. The isotopic Fractionation in both cases resulted in the heavy isotopes becoming enriched at the cold end. The expanded data set of thermal isotopic Fractionation in silicate liquids is used to evaluate the degree to which recently proposed parameterizations are able to reproduce the experimental data.
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Lithium isotope Fractionation by diffusion in minerals. Part 1: Pyroxenes
Geochimica et Cosmochimica Acta, 2014Co-Authors: Frank Richter, Marc Chaussidon, Ruslan A. Mendybaev, Bruce Watson, Dan RuscittoAbstract:Several recent studies found large lithium isotopic Fractionations correlated with concentration gradients in pyroxene minerals from lava flows and mantle nodules that were interpreted as indicating diffusion of lithium into the grains. Motivated by these findings experiments were undertaken in which powdered spodumene (LiAlSi2O6) or Li2SiO3 was used to diffuse lithium into Templeton augite or Dekalb diopside grains at 900 °C and oxygen fugacity ranging from log fO2 = −17 to log fO2 = −12. The purpose of these experiments was to determine the diffusion coefficient of lithium in pyroxene minerals and to measure the isotopic Fractionation of lithium in the diffusion boundary layer due to the relative mobility of 6Li compared to 7Li. The diffusion profiles of lithium that had not yet reached the center of Templeton augite grains were in most cases sharp steps propagating in from each boundary. In one case a more usual profile with smoothly decreasing lithium concentration with distance from the grain boundary was found. A model in which lithium occupies two different sites – one being fast diffusing interstitial lithium, the other much less mobile lithium in a metal site, reproduced both types of profiles. The step-like profiles arise in the model when interstitial lithium diffusing into the grain is strongly partitioned into abundant metal sites and thus does not penetrate further into the grain until all the metal sites at a given distance become filled. While the rate of propagation of the concentration step can be used to calculate an effective diffusivity for the penetration of lithium into the augite grains, the multiple speciation of lithium precludes making a separate precise determination of the diffusion coefficient of the interstitial lithium. Isotopic Fractionations of 7Li/6Li of about 30‰ were found in the step-like diffusion boundary layers, which translate into a ratio of the isotope diffusion coefficients D7 Li/De Li = 0.9592 (i.e., (6/7)β with β = 0.27). The same value of β = 0.27 was also able to fit the isotopic Fractionation data from the experiment with the smoothly decreasing lithium concentration profile. The laboratory experiments confirmed that diffusion of lithium produces large kinetic isotopic Fractionations and thus highlight the importance of isotopic measurements for discriminating when a particular instance of chemical zoning in minerals was the result of diffusion or some other process such crystal growth from an evolving melt. The experiments also showed that, contrary to conventional wisdom, isotopic gradients do not dissipate faster than gradients in the parent element, indeed the contrary is seen in several of the Templeton experiments where very large lithium isotopic Fractionations persisted after the lithium concentration had become effectively homogenized. Published data of lithium isotopic Fractionation of zoned augite grains from a Martian meteorite and in clinopyroxene grains from both lava flows on the Solomon Islands and from a San Carlos mantle xenolith were modeled in terms of lithium diffusion with an isotope Fractionation parameter β between 0.25 and 0.30, which is very similar to that derived from the laboratory experiments.
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Lithium isotope Fractionation by diffusion in minerals. Part 1: Pyroxenes
Geochimica et Cosmochimica Acta, 2014Co-Authors: Frank M Richter, Marc Chaussidon, Ruslan A. Mendybaev, Bruce Watson, Dan RuscittoAbstract:Abstract Several recent studies found large lithium isotopic Fractionations correlated with concentration gradients in pyroxene minerals from lava flows and mantle nodules that were interpreted as indicating diffusion of lithium into the grains. Motivated by these findings experiments were undertaken in which powdered spodumene (LiAlSi2O6) or Li2SiO3 was used to diffuse lithium into Templeton augite or Dekalb diopside grains at 900 °C and oxygen fugacity ranging from log fO2 = −17 to log fO2 = −12. The purpose of these experiments was to determine the diffusion coefficient of lithium in pyroxene minerals and to measure the isotopic Fractionation of lithium in the diffusion boundary layer due to the relative mobility of 6Li compared to 7Li. The diffusion profiles of lithium that had not yet reached the center of Templeton augite grains were in most cases sharp steps propagating in from each boundary. In one case a more usual profile with smoothly decreasing lithium concentration with distance from the grain boundary was found. A model in which lithium occupies two different sites – one being fast diffusing interstitial lithium, the other much less mobile lithium in a metal site, reproduced both types of profiles. The step-like profiles arise in the model when interstitial lithium diffusing into the grain is strongly partitioned into abundant metal sites and thus does not penetrate further into the grain until all the metal sites at a given distance become filled. While the rate of propagation of the concentration step can be used to calculate an effective diffusivity for the penetration of lithium into the augite grains, the multiple speciation of lithium precludes making a separate precise determination of the diffusion coefficient of the interstitial lithium. Isotopic Fractionations of 7Li/6Li of about 30‰ were found in the step-like diffusion boundary layers, which translate into a ratio of the isotope diffusion coefficients D 7 Li / D 6 Li = 0.9592 (i.e., (6/7)β with β = 0.27). The same value of β = 0.27 was also able to fit the isotopic Fractionation data from the experiment with the smoothly decreasing lithium concentration profile. The laboratory experiments confirmed that diffusion of lithium produces large kinetic isotopic Fractionations and thus highlight the importance of isotopic measurements for discriminating when a particular instance of chemical zoning in minerals was the result of diffusion or some other process such crystal growth from an evolving melt. The experiments also showed that, contrary to conventional wisdom, isotopic gradients do not dissipate faster than gradients in the parent element, indeed the contrary is seen in several of the Templeton experiments where very large lithium isotopic Fractionations persisted after the lithium concentration had become effectively homogenized. Published data of lithium isotopic Fractionation of zoned augite grains from a Martian meteorite and in clinopyroxene grains from both lava flows on the Solomon Islands and from a San Carlos mantle xenolith were modeled in terms of lithium diffusion with an isotope Fractionation parameter β between 0.25 and 0.30, which is very similar to that derived from the laboratory experiments.
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isotopic Fractionation of the major elements of molten basalt by chemical and thermal diffusion
Geochimica et Cosmochimica Acta, 2009Co-Authors: Frank M Richter, Bastian Georg, James M Watkins, Bruce E Watson, Ruslan A. Mendybaev, Nicolas Dauphas, John W ValleyAbstract:Abstract Samples produced in piston cylinder experiments were used to document the thermal isotopic Fractionation of all the major elements of basalt except for aluminum and the Fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic Fractionations are summarized in terms of a parameter Ωi defined as the Fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report ΩCa = 1.6, ΩFe = 1.1, ΩSi = 0.6, ΩO = 1.5. In an earlier paper we reported ΩMg = 3.6. These Fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt–rhyolite diffusion couple showed significant Fractionation that are parameterized in terms of a parameter βFe = 0.03 when the ratio of the diffusion coefficients D54 and D56 of 54Fe and 56Fe is expressed in terms of the atomic mass as D54/D56 = ( 56 / 54 ) β Fe . This value of βFe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., βLi = 0.215, βCa = 0.05, βMg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope Fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic Fractionations driven by either or both chemical and thermal gradients.
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magnesium isotope Fractionation in silicate melts by chemical and thermal diffusion
Geochimica et Cosmochimica Acta, 2008Co-Authors: Frank M Richter, Bruce E Watson, Ruslan A. Mendybaev, Fangzhen Teng, P E JanneyAbstract:Two types of laboratory experiments were used to quantify magnesium isotopic Fractionations associated with chemical and thermal (Soret) diffusion in silicate liquids. Chemical diffusion couples juxtaposing a molten natural basalt (SUNY MORB) and a molten natural rhyolite (Lake County Obsidian) were run in a piston cylinder apparatus and used to determine the isotopic Fractionation of magnesium as it diffused from molten basalt to molten rhyolite. The thermal diffusion experiments were also run in a piston cylinder apparatus but with a sample made entirely of molten SUNY MORB displaced from the hotspot of the assembly furnace so that the sample would have a temperature difference of about 100–200 °C from one end to the other. The chemical diffusion experiments showed Fractionations of 26Mg/24Mg by as much as 7‰, which resulted in an estimate for the mass dependence of the self-diffusion coefficients of the magnesium isotopes corresponding to D26Mg/D24Mg=(24/26)β with β = 0.05. The thermal diffusion experiments showed that a temperature difference of about 100 °C resulted in the MgO, CaO, and FeO components of the basalt becoming slightly enriched by about 1 wt% in the colder end while SiO2 was enriched by several wt% in the hotter end. The temperature gradient also fractionated the magnesium isotopes. A temperature difference of about 150 °C produced an 8‰ enrichment of 26Mg/24Mg at the colder end relative to the hotter end. The magnesium isotopic Fractionation as a function of temperature in molten basalt corresponds to 3.6 × 10−2‰/°C/amu.
Nicolas Dauphas - One of the best experts on this subject based on the ideXlab platform.
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Calcium and titanium isotopic Fractionations during evaporation
Geochimica et Cosmochimica Acta, 2014Co-Authors: Junjun Zhang, Akihiko Hashimoto, Nicolas Dauphas, Shichun Huang, Andrew M. Davis, Stein B. JacobsenAbstract:Isotope Fractionations associated with high temperature evaporation provide important constraints on the physicochemical processes that affected planetary materials at the birth of the solar system. Previous evaporation experiments have focused on isotopic Fractionation of moderately to highly volatile elements. Here, we investigate the isotope Fractionation behavior of two highly refractory elements, calcium and titanium, during evaporation of perovskite (CaTiO3) in a vacuum furnace. In our experiments, isotope Fractionation during evaporation follows the Rayleigh law, but not the commonly used exponential law, with the dominant evaporating species being Ca(g) and TiO2(g). If isotope Fractionations in early solar system materials did follow the Rayleigh law, the common practice of using an exponential Fractionation law to correct for mass-dependent Fractionation in the study of mass-independent Fractionations may introduce significant artificial isotope anomalies.
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isotopic Fractionation of the major elements of molten basalt by chemical and thermal diffusion
Geochimica et Cosmochimica Acta, 2009Co-Authors: Frank M Richter, Bastian Georg, James M Watkins, Bruce E Watson, Ruslan A. Mendybaev, Nicolas Dauphas, John W ValleyAbstract:Abstract Samples produced in piston cylinder experiments were used to document the thermal isotopic Fractionation of all the major elements of basalt except for aluminum and the Fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic Fractionations are summarized in terms of a parameter Ωi defined as the Fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report ΩCa = 1.6, ΩFe = 1.1, ΩSi = 0.6, ΩO = 1.5. In an earlier paper we reported ΩMg = 3.6. These Fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt–rhyolite diffusion couple showed significant Fractionation that are parameterized in terms of a parameter βFe = 0.03 when the ratio of the diffusion coefficients D54 and D56 of 54Fe and 56Fe is expressed in terms of the atomic mass as D54/D56 = ( 56 / 54 ) β Fe . This value of βFe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., βLi = 0.215, βCa = 0.05, βMg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope Fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic Fractionations driven by either or both chemical and thermal gradients.
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diffusion driven kinetic isotope effect of fe and ni during formation of the widmanstatten pattern
Meteoritics & Planetary Science, 2007Co-Authors: Nicolas DauphasAbstract:Iron meteorites show resolvable Fe and Ni isotopic Fractionation between taenite and kamacite. For Toluca (IAB), the isotopic Fractionations between the two phases are around +0.1‰/amu for Fe and −0.4‰/amu for Ni. These variations may be due to i) equilibrium Fractionation, ii) differences in the diffusivities of the different isotopes, or iii) a combination of both processes. A computer algorithm was developed in order to follow the growth of kamacite out of taenite during the formation of the Widmanstatten pattern as well as calculate the Fractionation of Fe and Ni isotopes for a set of cooling rates ranging from 25 to 500 °C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4‰/amu for Fe and Ni (β = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 °C/Myr. This value agrees with earlier cooling rate estimates based on Ni concentration profiles. This supports the idea that the Fractionation measured for Fe and Ni in iron meteorites is driven by differences in diffusivities of isotopes. It also supports the validity of the value of 0.25 adopted for β for diffusion of Fe and Ni in Fe-Ni alloy in the temperature range of 400-700 °C.
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Diffusion-driven Kinetic Isotope Fractionation of Fe and Ni in Iron Meteorites: A New Dimension to the Analysis of Cooling Rates
2007Co-Authors: Nicolas DauphasAbstract:Introduction: Widmanstatten patterns in iron meteorites show concentration gradients indicating that growth of kamacite out of taenite occurred in a diffusion-limited regime. This feature can be used to estimate cooling rates [1,2]. Formation of Widmanstatten pattern is complex to model because many parameters are involved (e.g., degree of undercooling when kamacite nucleation occurs, positions of the phase boundaries as a function of temperature and composition of the alloy, interdiffusion coefficients also as a function of temperature and composition of the alloy). Isotopes can be fractionated during diffusive transport (light isotopes diffuse faster than heavy ones). Recently, Roskosz et al. [3] reported Fe isotopic Fractionation during diffusion in Pt at 1,500 °C. They concluded that the diffusivities of adjacent isotopes of Fe differed by ~4 ‰/amu. If the same parameter applies to Fe and Ni diffusion in Fe-Ni alloy during growth of Widmanstatten pattern (~400-700 °C), then measurable kinetic isotope Fractionation may be present in iron meteorites. Several research groups have reported Fe and Ni isotopic Fractionation in adjacent taenite and kamacite [4-8], which can indeed be explained by diffusion-driven kinetic isotope Fractionation [8,9]. Observations: The Toluca iron meteorite (IAB) is the most extensively studied sample for Fe and Ni isotopic Fractionation [4-7] and cooling rate estimate [e.g., 10]. For this reason, it is used here as a case example to compare modeling and observations. Measured Fe and Ni isotopic Fractionations published to this day [47] are shown in Fig. 1. Taenite has similar or slightly heavy Fe isotopic composition relative to kamacite (0 to 0.1‰/amu). In contrast, taenite has significantly light Ni isotopic composition relative to kamacite (-0.4 ‰/amu). Both the directions and relative magnitudes of Fe and Ni isotopic Fractionations are consistent with diffusion-driven kinetic isotope Fractionation. Because taenite and kamacite have higher concentrations of Fe compared to Ni (by factors of 4 to 10), the diffusive signal should be diluted to a greater extent by normal (0 ‰/amu) background for Fe than for Ni. So a larger absolute Fractionation is expected for Ni compared to Fe, which is observed. Growth of kamacite out of taenite is mainly limited by diffusion in taenite. In all studies reporting Fe and Ni isotopic analyses of iron meteorites, the sampling location was either not specified or of insufficient spatial resolution to be useful. Most likely, the samples analyzed came from the center regions of the phases. Because the center of taenite has low Ni concentration compared to the border, the net diffusive flux is from the border to the center and one would expect taenite analyses to show light Ni/heavy Fe isotopic compositions relative to kamacite. Again, this is exactly what is observed.
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Fractionation of silicon isotopes in liquids: The importance of configurational disorder
Chemical Geology, 2015Co-Authors: Romain Dupuis, Magali Benoit, Elise Nardin, Merlin MeheutAbstract:Silicon isotopes are a promising tool to assess low-temperature geochemical processes such as weathering or chert precipitation. However, their use is hampered by an insufficient understanding of the Fractionation associated with elementary processes such as precipitation or dissolution. In particular, the respective contributions of kinetic and equilibrium processes remain to be determined. In this work, equilibrium Fractionation factors for silicon isotopes have been calculated using first-principles methods for quartz, kaolinite, and dissolved silicic acid (H4SiO4 and H3SiO4 -) at 300K.The two liquid systems are treated both as realistically as possible, and as consistently with the solids as possible. They are first simulated by ab initio molecular dynamics, then individual snapshots are extracted from the trajectories and relaxed, giving inherent structures (IS). The Fractionation properties of these IS are then calculated. A significant variability of the Fractionation properties (σ= 0.4‰) is observed between the independent snapshots, emphasizing the importance of configurational disorder on the Fractionation properties of solutions. Furthermore, a correlation is observed between the Fractionation properties of these snapshots and the mean Si-O distances, consistent with calculations on minerals. This correlation is used to identify other parameters influencing the Fractionation, such as the solvation layer. It is also used to reduce the number of configurations to be computed, and therefore the computational cost.At 300K, we find a Fractionation factor of +2.1±0.2‰ between quartz and H4SiO4, +0.4±0.2‰ between kaolinite and H4SiO4, and -1.6±0.3‰ between H3SiO4 - and H4SiO4. These calculated solid-solution Fractionations show important disagreement with natural observations in low-temperature systems, arguing against isotopic equilibration during silicon precipitation in these environments. On the other hand, the large Fractionation associated with the de-protonation of silicic acid suggests the importance of speciation, and in particular pH, for the Fractionation of silicon isotopes at low temperature.
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silicon isotope Fractionation in silicate minerals insights from first principles models of phyllosilicates albite and pyrope
Geochimica et Cosmochimica Acta, 2014Co-Authors: Merlin Meheut, Edwin A SchaubleAbstract:Abstract Isotopic Fractionation factors for oxygen and silicon in phyllosilicates (pyrophyllite, talc), albite and pyrope have been calculated using first-principles methods based on density functional theory. Based on exhaustive analysis of numerical convergence, we also update our previous calculations on enstatite and forsterite silicon Fractionation properties. Calculated oxygen isotope Fractionations agree well with existing estimates for talc and albite. In the case of silicon, qualitative agreement is found with natural data. For phyllosilicates (kaolinite, lizardite, pyrophyllite, talc), Si isotope Fractionation properties appear to be correlated with stoichiometry: (1) 1000 ln α 30 Si phyllosilicate – quartz = a Mg ( T ) · Mg eq . + a Al ( T ) · Al eq . Si eq . , where Si eq. =#Si, Al eq . = 3 4 # Al and Mg eq . = 1 2 # Mg (cation equivalents) are the charge-weighed stoichiometric coefficients of each cation, normalized to the charge of the silicon atom, and a X ( T ) are proportionality coefficients depending on temperature. It is suggested that the effect of cation X on Si isotope Fractionation (i.e. a X ( T ) ) will increase with decreasing electronegativity of X. Si isotope Fractionation is further correlated with Si–O distances, suggesting a crystal chemical explanation for relation (1) in terms of electron donation effects. This relationship appears valid for quartz, pyrope and enstatite ( R 2 = 0.99 , n = 7 ) , but forsterite is strongly anomalous (error of 0.7‰ at 600 °C). These models indicate that attention should be given to chemical compositions in Si isotope studies. Relation (1) would explain the enrichment in heavy silicon isotopes accompanying magmatic differentiation.
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First-principles calculation of H/D isotopic Fractionation between hydrous minerals and water
Geochimica et Cosmochimica Acta, 2010Co-Authors: Merlin Meheut, Etienne Balan, Michele Lazzeri, Francesco MauriAbstract:Abstract Hydrogen Fractionation laws between selected hydrous minerals (brucite, kaolinite, lizardite, and gibbsite) and perfect water gas have been computed from first-principles quantum-mechanical calculations. The β -factor of each phase was calculated using the harmonic phonon dispersion curves obtained within density functional theory. All the Fractionation laws show the same shape, with a minimum between 200 °C (brucite) and 500 °C (gibbsite). At low temperatures, the mineral/liquid water Fractionation laws have been obtained using the experimental gas/liquid water Fractionation laws. The resulting Fractionation laws systematically overestimate measurements by 15‰ at low temperatures to 8‰ at ≈400 °C. Based on this general agreement, all calculated laws were empirically corrected with reference to brucite/water data. These considerations suggest that the experimental or natural calibrations by Xu and Zheng (1999) and Horita et al. (2002) (brucite/water), Gilg and Sheppard (1996) (kaolinite/water), Wenner and Taylor (1973) (lizardite/water), and in some extents Vitali et al. (2001) (gibbsite/water) are representative of equilibrium Fractionations. Besides, internal isotopic Fractionation of hydrogen between inner-surface and inner hydroxyl groups has been computed for kaolinite and lizardite. The obtained Fractionation is large, of opposite sign for the two systems (respectively, −23‰ and +63‰ at 25 °C) and is linear in T - 2 . Internal Fractionation of hydrogen in TO phyllosilicates might thus be used in geothermometry.
