V42A-01 INVITED
Nuclear field shift effect in chemical exchange reactions
The classic theory of stable isotope fractionation in chemical exchange reactions has been established by Bigeleisen, Mayer, and Urey in 1947. The theory was based on the difference of molecular vibrational energies of isotopomers that are proportional to the respective masses, and hence, results in mass-dependent isotope effect only. In 1996, this conventional mass-dependent theory has been expanded by Bigeleisen to include a mass-independent term named the nuclear field shift effect. The nuclear field shift is an isotope shift in orbital electrons, which results from the isotopic difference in nuclear size and shape. The new equation defined by Bigeleisen (at a constant temperature) can be simply expressed as, ln α = δ<r2> A + (δ m/mm') B, where α is the isotope separation factor, δ<r2> isotopic difference in mean-square nuclear charge radius, δm difference between isotopic masses m and m'. A and B are scaling factors of the nuclear field shift effect and the conventional mass effect, respectively. Since this new theory was presented, the mass-independent isotope fractionation of various elements, e.g, Ti, Cr, Ni, Zn, Sr, Zr, Mo, Ru, Cd, Te, Ba, Nd, Sm, Gd, Yb, and U, found in chemical exchange systems has been successfully explained as the nuclear field shift effect. In our most recent studies, the nuclear field shift effect of Cr, Mo, Ru, Cd, and Te isotopes has been found in laboratory scale experiments. The isotopes of these elements were fractionated by using a liquid-liquid extraction system (a ligand exchange system) at room temperature. The isotopic analysis was performed by the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with a typical precision of <100 ppm (at ENS Lyon or UC Davis). Isotope enrichment factors did not show mass-dependent trend, but possessed a similar variation of their nuclear charge radii. For Cr, we tested a different chemical exchange system (a redox system): at high temperature (723-1023 K), an eutectic melt was contacted with a liquid metal. In this system, the nuclear field shift effect of Cr was also found. All these experimental results suggest that the nuclear field shift effect may occur in every chemical exchange reaction at various temperatures to various degrees. Therefore, isotopic anomalies found in a natural system might be partly or largely affected by the nuclear field shift effect via chemical reactions occurred in the nature. In order to clarify the degree and significance of its contribution, we may need to pay more attention to the nuclear field shift effect created chemically.
V42A-02
A Photochemical Mechanism for Sulfur Mass-independent Fractionation in Ancient Rocks
A quantitative understanding of the origin of sulfur isotope mass-independent fractionation (MIF) is essential to a full interpretation of the sulfur geochemical record in Archean and Paleoproterozoic rocks. However the chemical mechanism responsible for the MIF remains unknown. Several possible sources of sulfur MIF can be identified: 1) symmetry-dependent non-statistical reactions during formation of poly-sulfur compounds (analogous to oxygen MIF during ozone formation); 2) MIF during photodissociation of sulfur gases; 3) hyperfine effects (nuclear- spin electron-spin interactions) which preferentially produce MIF in 33S; and 4) nuclear field shift effects. I will briefly review the implications of laboratory experiments in distinguishing between these possible MIF mechanisms. I will then report the results of atmospheric chemistry modeling of isotope-selective photodissociation of SO2 in the key vibronic band system from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Utilizing the results of ab initio methods to determine the band shifts for SO2 isotopologues, I will show that photodissociation of SO2 in the atmosphere yields MIF signatures (both Δ33S and Δ36S) that are comparable to values measured in ancient rocks. The sulfur MIF arises in part from self- shielding due to photon absorption in the rotationally-resolved lines of 32SO2. In addition to SO2 photolysis, dissociation of SO may also contribute to the sulfur MIF record. Field shift effects are too small to be a significant source of sulfur MIF. Hyperfine effects remain to be evaluated. A photolytic origin for sulfur MIF demonstrates that MIF in terrestrial rocks (and meteorites) can be derived from photochemistry independent of molecular symmetry.
V42A-03 INVITED
Kinetic Isotope Fractionation by Transport Prosesses in Geological Materails
Recent experimental results on kinetic isotope fractionations associated with mass transport within and between phases are showing that these fractionations can be very large compared to the analytical precision of modern isotope measurements. Five examples of kinetic isotope fractionations will be presented: 1. Isotope fractionation of Si, Mg and Fe by evaporation from a molten silicate liquid. 2. Isotope fractionation of Li, Ca, Mg and Fe by chemical diffusion between molten basalt and rhyolite. 3. Isotope fractionation of Li and Mg, and Cl by diffusion of dissolved salts in water. 4. Li isotopic fractionation by grain boundary diffusion. 5. Steady state fractionation of Ca, Mg, Fe, and Si by a 100 ?C temperature difference across molten basalt. The common theme that emerges from these studies is that isotopes can be used as fingerprints of diffusive transport processes and distinguish these from transport by advection or physical mixing processes. Another common theme is that while the experimental evidence for kinetic isotope fractionation is rapidly expanding, the same is not true of the theoretical understanding of these fractionations. For example, why is the observed isotopic fractionation of isotopes by evaporation significantly less than the often-assumed result that the relative evaporation rates should be proportional to the inverse square root of the mass of the evaporating species? Why are the kinetic isotope fractionations in molten silicates so very much larger than what is found for diffusion in water? Why is the thermal isotopic sensitivity factor (i.e., per mil fractionation per ?C) so much larger in molten basalt than in gases? Hopefully, experimental results of the sort presented here will be sufficiently interesting to stimulate molecular modeling that will begin to answer some of these questions.
V42A-04
Isotopic approach for determining the structural components of silicate liquids
The main structural units in silicate materials are silica and alumina tetrahedra that are linked together by bridging oxygen atoms to form complex chains, sheets, and three-dimensional networks. Most studies of silicate structures focus on these (Al,Si)Ox polymer units, and the degree of interlinking between them. Another important aspect, however, that is more difficult to determine, is the degree of association of other cations with the polymer units. The phase diagrams of many simple binary and ternary oxide systems seem to suggest that this association can be substantial. Diffusive isotopic fractionation of major cations in silicate liquids may also provide information on the association of cations with the polymer units, and perhaps on the effective size of the polymer units. The isotopic species should diffuse at different rates depending on the mass of the diffusing species, and the extent of isotopic fractionation by diffusion may indicate the size of the polymer unit associated with each ion and/or the degree of association between the cations and the polymer structure. We are approaching this problem using binary diffusion couples with rhyolite liquid on one side and mafic liquid on the other, run in piston cylinder apparatus for several hours at 1450°C and 1 GPa. This follows the experiments of Richter et al. (2003), who demonstrated significant isotopic fractionation for Ca and Li isotopes in silicate liquids with natural chemical compositions. Those experiments show that diffusive fractionation of Ca isotopes is small when considering the elemental mass ratio (44/40), as if Ca is associated with larger polymer units. In contrast, Li isotopes exhibit a much greater diffusive fractionation that suggests the diffusing species are comparable in size to the elemental masses. As noted by Richter et al., the bulk diffusivity of Li is very high and similar to that of hydrogen. Presumably Li, like H, is not strongly bound to the silicate polymer units and is readily exchanged between units, allowing for both fast diffusion and greater mass discrimination. We have reproduced the results of Richter et al. (2003) for Ca isotopes using rhyolite and tholeiitic basalt. As in their experiments, we see significant Ca isotope fractionation (ca. 6 per mil) that can be reproduced in models of chemical diffusion using different diffusivities for the 40Ca and 44Ca species. However, we also observe isotopic gradients in the charges that are not accounted for in our model of chemical diffusion. We have repeated the experiment with a mafic liquid (ugandite) of different composition, which has lower silica activity, higher Mg and alkalis, and is presumably less polymerized than tholeiitic basalt. Preliminary results indicate that the degree of Ca isotopic fractionation varies with composition, and that there are large isotopic effects in our experimental charges that may be due to temperature gradients and/or tracer diffusion in addition to simple chemical diffusion.
V42A-05
Lattice Boltzmann Simulation of Kinetic Isotope Effect During Snow Crystal Formation
The isotopic composition of precipitation, especially that of snow, plays a special role in the global hydrological cycle and in reconstruction of past climates using polar ice cores. The fractionation of the major water isotope species (HHO, HDO, HHO-18) during ice crystal formation is critical to understanding the global distribution of isotopes in precipitation. Ice crystal growth in clouds is traditionally treated with a spherically-symmetric steady state diffusion model, with semi-empirical modifications added to account for ventilation and for complex crystal morphology. Although it is known that crystal growth rate, which depends largely on the degree of vapor over- saturation, determines crystal morphology, there are no quantitative models that relate morphology to the vapor saturation factor. Since kinetic (vapor phase diffusion-controlled) isotopic fractionation also depends on growth rate, there should be direct relationships between vapor saturation, crystal morphology, and crystal isotopic composition. We use a 2D lattice Boltzmann model to simulate diffusion-controlled ice crystal growth from vapor- oversaturated air. In the model, crystals grow solely according to the diffusive fluxes just above the crystal surfaces, and hence crystal morphology arises from the initial and boundary conditions in the model and does not need to be specified a priori. Crystal growth patterns can be varied between random growth and deterministic growth (along the maximum concentration gradient for example). The input parameters needed are the isotope- dependent vapor deposition rate constant (k) and the water vapor diffusivity in air (D). The values of both k and D can be computed from kinetic theory, and there are also experimentally determined values of D. The deduced values of k are uncertain to the extent that the condensation coefficient for ice is uncertain. The ratio D/k is a length (order 1 micron) that determines the minimum scale of dendritic growth features and allows us to scale the numerical calculations to atmospheric conditions. Our calculations confirm that the crystal/vapor isotopic fractionation approaches the equilibrium value, and the crystals are compact (circular in 2D) as the saturation factor approaches unity (S= 1.0). However, few natural crystals form under such conditions. At higher oversaturation (e.g. S = 1.2), dendritic crystals of millimeter size develop on timescales appropriate to cloud processes, and kinetic effects control isotopic fractionation. Fractionation factors for dendritic crystals are similar to those predicted by the spherical diffusion model, but the model also gives estimates of crystal heterogeneity. Dendritic crystals are constrained to be relatively large, with dimension much greater than about 20D/k. The most difficult aspect of the modeling is to account for the large density difference between air and ice, which requires us to use a fictitious higher density for the vapor-oversaturated air and scale the crystal growth time accordingly. An approach using a larger scale simulation and the domain decomposition method can provide a vapor flux for a nested smaller scale calculation. The results clarify the controls on crystal growth, and the relationships between saturation state, growth rate, crystal morphology and isotopic fractionation.
V42A-06 INVITED
Vibrational properties of H-D substituted brucite from experiments and ab initio calculations: a step towards the prediction of D/H fractionation for complex structures
D/H partitioning between water and minerals are often not well constrained from equilibrium experiments, except for simple hydroxides such as brucite. Vibrational models and experiments can be used to constrain more accurately isotopic fractionation factors, provided they are first calibrated and tested on relatively simple systems such as MgO-SiO2-H2O (MSH) starting with brucite Mg(OH)2, then going towards increasing complexity in minerals like talc Mg3Si4O10(OH)2 and antigorite Mg48Si34O85(OH)62. We present here first results from combining Raman and IR spectroscopic and ab initio calculations on brucite. Raman measurements were carried out on significantly isotopically doped phases, with several intermediate compositions between end-members. Various mode behaviors are observed, with low frequency modes shifting smoothly in frequency with increasing isotopic substitution while high frequency modes present a two-mode behavior. The good agreement between predicted and observed frequencies allows to identify the modes corresponding to those associated to OH-OD vibrations. Ab initio calculations can thus be used as a guide for deciphering the more complex vibrational spectra such as those obtained on antigorite; and will help constructing reliable vibrational models of D/H partitioning.
V42A-07 INVITED
Silicate-metal fractionation of silicon isotopes at high pressure and temperature
Equilibrium 30Si/28Si fractionations between magnesium-silicate perovskite (MgSiO3) and silicon-bearing iron metal (Fe3Si) are estimated with first principles lattice dynamical models. The purpose of this study is to investigate possible silicon-isotope fractionation at high pressure during core formation in the Earth-Moon system, recently inferred from high-precision Si-isotope measurements of terrestrial, lunar, and meteorite samples (1). Models use plane-wave basis sets and a combination of norm-conserving and ultrasoft pseudopotentials, with a gradient-correct density functional (PBE). Pressure effects on isotopic fractionation are modeled quasiharmonically, by optimizing each crystal structure at a series of pressures (at 0 K), and applying a thermal pressure correction based on modal Grüneisen parameters. The resulting equations of state are in good agreement with previous measurements and density functional theory models (2). Results indicate that silicate-metal fractionation increases with pressure (with 30Si/28Si 0.5‰ higher in perovskite relative to metal at 25 GPa, 2500 K, and 1.0‰ higher at 140 GPa, 2500 K) but decreases strongly with increasing temperature (from 1.7‰ at 30 GPa, 1500 K to 0.4‰ at 30 GPa, 3000 K). Along the adiabatic portion of the modern lower mantle geotherm, pressure and temperature effects roughly cancel, yielding a nearly constant 1‰ fractionation. A smaller fractionation of ~0.5‰ is expected at liquidus conditions at the base of a deep magma ocean. This fractionation is of the same order of magnitude, but somewhat smaller than the ~1.5‰ fractionation inferred from the Si-isotope composition and mantle Mg/Si ratio of the Earth-Moon system. The presence of IV- and V-coordinated Si in high-pressure silicate magma equilibrated with metal (3) (as opposed to VI-fold coordination in perovskite) might increase 30Si/28Si fractionation. References: 1) Georg et al. (2007) Nature 447:1102-1106. 2) Karki (2000) Am. Mineral. 85:1447-1451; Hirao et al. (2004) Phys. Chem. Min. 31:329-336. 3) Stixrude and Karki (2005) Science 310:297-299.