V51E-0827
Mass Independent Fractionation of Mercury Isotopes in the Environment
The toxicity of mercury's methylated species, biomagnification in aquatic food chains and global dispersion by the atmosphere makes it a worldwide health problem. Recent reports have observed natural mass dependent fractionation in mercury isotopes, and recent theoretical work has emphasized that isotopic separation in mercury is due primarily to nuclear field shifts (nuclear volume effect) or a magnetic isotope effect. Both effects produce mass-independent fractionation (MIF). Separation factors are closely proportional to mass number for even neutron number isotopes. Odd mass number mercury isotopes depart from this relationship, giving rise to a component of mass-independent fractionation. During equilibrium exchange reactions, the nuclear volume effect is a function of the oxidation state of the Hg chemical species: Hg0, and methylmercury, exhibit positive deviations, and oxidized species (e.g., Hg+2) exhibit negative deviations. We present analytical evidence of mass independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope effects in a wide variety of natural environmental samples – peat, sediment, soil and moss. Even mass number isotopes exhibit a pattern indistinguishable from that produced by mass-dependent fractionation, with both positive and negative 199Hg and 201Hg anomalies. For majority of the samples analyzed, magnetic isotope effect seems to be more significant than nuclear volume effect. MIF is more accurately determined in Hg isotopes than mass dependent fractionation alone, and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment.
V51E-0828
First measurements on how pressure affects the half-life of 22Na: Comparison to theory and analog to 40K
Radioactive decay plays a central role in planetary sciences as appropriate decay schemes are used to date geological and astronomical processes and radioactivity provides an important source of heat in planetary bodies, both in their early history during accretion and differentiation and also over geological times. The most important isotopes that currently heat the Earth are 40K, 232Th, 235U and 238U. As radioactive decay is a nuclear process it is considered to be insensitive to external factors such as pressure or chemical environment. This has been shown to be true for α, β+ and β- processes, however, electron capture decay is dependent on the electron charge density at the nucleus of a compound, which is sensitive to the external environment. Using high-resolution Ge gamma-ray detectors to make relative measurements with 137Cs and the positron decay of 22Na, we measure how pressure affects the half-life of 22Na due to electron-capture decay. Our systematics look favorable for observing this small effect. We will compare our preliminary measurements with complementary ab-initio all-electron computations using the linearized augmented plane wave method (LAPW). Using 22Na as an analog for 40K, our results suggest that the pressure effect for 40K, combined with the opposing effects of high temperatures, will have little, discernible effect on the heat production in the deep Earth as our predicted changes are smaller than the uncertainties in the total decay constant for 40K. This work was supported in part by the Carnegie/DOE Alliance Center (CDAC), through the Stewardship Science Academic Alliances Program of the U.S. Department of Energy. The LANSCE facility is operated, and portions of this work were performed, by Los Alamos National Security, LLC, funded by the U.S. Department of Energy under Contract No. DE-AC52- 06NA25396.
V51E-0829
A Model of Fe Isotope Fractionation Within a High-Temperature Sulfide Structure from the Mothra Field, Juan de Fuca Ridge
Significant variations in δ56Fe and δ65Cu have been documented for seafloor hydrothermal sulfides, and may provide insights to the development of mid-ocean ridge sulfide structures and evolution of hydrothermal fluids. We present δ56Fe and δ65Cu data from concentric mineralogical zones within the wall of "Finn" – a high temperature sulfide structure recovered from the Juan de Fuca Ridge – and a model of Fe isotope fractionation within the chimney wall. The mineralogical zones range from a high temperature, chalcopyrite lined inner wall, to low temperature, oxidized rinds of the outer surface. We analyzed two samples from each of seven mineralogical zones, and samples from two transects (6 and 9 cm) that cross all zones. This suite is the most thorough sampling of a single structure for transition metal isotope analysis to date. δ56Fe values measured in Finn vary from 0‰ to -1.2‰ (relative to the IRMM-014 standard) and show strong negative correlations with some trace element abundances, namely As, Ag, Au and Sb. δ65Cu values range from 0.9‰ to -0.6‰ (relative to NIST-976). The lack of correlation between δ56Fe and δ65Cu suggests they are decoupled in this environment; the mechanism of Cu isotope fractionation remains unclear. Observed trace element patterns and δ56Fe variations in the two transects of Finn are reproduced with a simple equilibrium thermodynamic box model. The model simulates interactions between the sulfide matrix of the chimney wall and the percolating fluid by continuously introducing discrete fluid packets of a single isotopic composition into the walls of the chimney, which are divided into a series of reaction volumes. Each packet of fluid equilibrates with each volume of sulfide matrix according to the partition coefficient or fractionation factor, then the modified fluid packet moves into the adjacent volume. δ56Fe variations of the transects are reproduced using an initial hydrothermal fluid value of - 0.6‰ and fractionation factors from 1.0005 in high temperature regions to 1.0017 in the outer rind. These values are compatible with those of previous studies. A single mechanism in which Fe isotope fractionation occurs as fluids and sulfides interact within chimney walls can produce both 54Fe and trace element enrichments in the outer, cooler part of Finn chimney walls.
V51E-0830
Pressure-dependent boron isotopic fractionation observed by column chromatography
Boron isotopic fractionation factor ( S ) between boron taken up in strongly basic anion exchange resin and boron in aqueous solution was determined by breakthrough column chromatography at 5 and 17 MPa at 25°C, using 0.1 mmol/L boric acid solution as feed solution. The S values obtained were 1.018 and 1.012, respectively, which were smaller than the value reported by using the same chromatographic method at atmospheric pressure at 25°C with the boron concentration of 10 mmol/L, but were larger than the values at the same condition with much higher concentration of 100 and 501 mmol/L, indicating that borate-polymerization reducing the isotopic fractionation was negligible. However, calculations based on the theory of isotope distribution between two phases estimated that 21% (5MPa) and 47% (17MPa) of boron taken up in the resin phase was in the three-coordinated B(OH)3-form, instead of in the four-coordinated B(OH)4--form, at high pressures even with the very diluted solution. We discussed this discrepancy by introducing (1) hydration or (2) a partial molar volume difference between isotopic molecules. It was inferred that borate ions were partially dehydrated upon transfer from the solution phase to the resin phase at high pressures, which resulted in smaller S values compared with those at the atmospheric pressure. Alternatively, it was likely that the S value decreased with increasing pressure, because the difference of the partial isotopic molar volumes between 10B(OH)3 and 11B(OH)3 was larger than that between 10B(OH)4- and 11B(OH)4-. If either will be the case, the influence of a pressure upon the isotope effect may not be negligible for boron isotopic exchange equilibrium. This knowledge is crucial for the principle of the boron isotopic pH-metry reconstructing a chemical variation at the paleo-deep oceanic environment where the early life may have been evolved.
V51E-0831
Experimental determination of Fe isotope fractionation between liquid metal, silicate and sulfide at high pressures and temperatures
There is evidence for significant equilibrium Fe isotope fractionation (≤0.26‰/amu) between metal and troilite (FeS) in iron meteorites (Williams et al., EPSL (250) 2006) and a smaller fractionation (<0.1‰/amu) between metal and olivine in pallasites (Zhu et al., EPSL (200) 2002; Weyer et al., EPSL (240) 2005). Theory suggests that differences in iron oxidation state and coordination between metal, silicate and FeS will result in stable isotope fractionation (Polyakov and Mineev, GCA (64) 2000; Schauble et al., GCA (65) 2001). However, it is not yet clear if the apparent observed fractionations can be extrapolated to the pressure and temperature conditions of planetary core formation. We have investigated Fe isotope fractionation between silicate melt and liquid Fe-S alloys and between liquid iron and basaltic melt at pressure and temperature conditions of 2-2.5GPa and 1920-2150K using piston-cylinder partitioning experiments from previous studies (Kilburn and Wood EPSL (152) 1997; Gessmann and Wood, EPSL (200) 2002; Wood et al., EPSL (in revision) 2007). Metal, sulfide and silicate fractions were separated from mounted and sectioned experimental charges using a computer-controlled micromill (New Wave-Merchantek). Sample dissolution, Fe purification and isotopic analysis followed established procedures (Williams et al., EPSL (235) 2005). In agreement with another preliminary high-pressure experimental study (Poitrasson and Roskosz, LPSC XXXVIII 2007) we find no appreciable fractionation between liquid iron metal and basaltic melt. However, there is a resolvable Fe isotope fractionation between silicate melt and Fe-S alloy which ranges from 0.12±0.04 to 0.15±0.04‰/amu for separate experiments (errors are propagated based on the 2 SD errors of replicate analyses). The Fe isotope compositions of coexisting phases from these experiments define a positive linear relationship with a slope that is, within error, equal to unity, implying isotopic equilibrium. No relationship between apparent fractionation factor and pressure or temperature is detectable within the range covered by the experiments. The fractionation factors determined from our experiments overlap with the average equilibrium fractionation factor obtained between silicate melt and pyrrhotite (Fe1-xS) of 0.18±0.02‰/amu at 0.5GPa and 1114-1274K (Schuessler et al., GCA (71) 2007) and are also broadly consistent with silicate-FeS fractionation factors inferred indirectly from iron meteorites and pallasites which range from ~0.16 to 0.24‰/amu. Taken together these observations suggest that resolvable stable isotope fractionation between Fe-S alloys and silicate melts can take place at extreme pressure and temperature conditions and that isotopically light Fe can be sequestered into the S-bearing parts of planetary cores.
V51E-0832
The Determination of Kinetic Nitrogen-Isotope Fractionation During Partial Decomposition of NH3 at 700 ° C And Its Potential Applications
Nitrogen (N) isotopes are potential proxies to trace the early Earth's evolution and detect the biosphere-deep Earth interactions. Yet their application is limited due to our poor knowledge of N-isotope fractionation (equilibrium and kinetic). To better understand the large variations of N isotopic compositions (δ15N from <-25‰ to >35‰) in Earth's materials, we experimentally studied the N-isotope behavior during partial decomposition of NH3 (2NH3 --> N2 + 3H2) at 700 ° C and low pressure (ranging between 3 and 9 bar) using sealed quartz tubes. The δ15N values of remaining NH3 (1.4 to 34.7‰) and produced N2 (-17.8 to -5.7‰) at different decomposition ratios (26 to 99%) reflect Rayleigh fractionation. We verified that there was no N loss during the experiments, thus the N- isotope fractionation factor was determined from isotope measurements of either NH3 or N2. The best fitted N-isotope fractionation factor between NH3 and N2 at 700 ° C (αNH3-N2) is 0.9829 ± 0.0006 (2σ). Following this kinetic effect, N2 with negative δ15N values can be easily produced from the sediments after limited/partial decomposition. This process may help understand the large N isotopic variations, especially the extremely low δ15N values (as low as -22‰) of N2 in the oil field, and also draw attention to the possibility of overestimating the contribution of mantle N2 in the volcanic gas.
V51E-0833
Experimental determination of germanium isotopic fractionation during metal-silicate segregation
Germanium is a moderately siderophile, moderately volatile element, whose concentration has long been used for the classification of iron meteorites. Despite a range in elemental abundance covering four orders of magnitude, it has been recently shown that the isotopic composition of Ge in different classes of magmatic irons is constant within analytical error (Luais, 2007). In order to constrain the processes which have led to these elemental and isotopic characteristics, we have undertaken a series of preliminary experiments to study the isotopic consequences of Ge transfer from an oxidized silicate liquid to a metallic phase. Particular attention has been paid to the role of oxygen fugacity (fO2), inferred to be an important factor controlling the bulk element chemistry of magmatic irons (e.g. Fe/Ni ratio), and known to affect metal-silicate partition coefficients of Ge. Experiments were performed at 1 atm. in a vertical drop quench furnace. A glass corresponding to the 1bar anorthite - diopside eutectic was synthesized and doped with ~4,000 ppm Ge, added in the form of an ICP Aldrich standard solution. Powders of this glass were placed in pure Ni capsules and heated to 1355°C for 2 to 60 hours over a range of fO2 from 4 log units below, to 2.5 log units above, the IW buffer. At this temperature, Ni is below its melting point, but the silicate was above its liquidus. Metal and silicate phases were mechanically separated. The metal phase was dissolved in nitric acid, and Ge separated from the Ni matrix using chromatographic methods of Luais (2007). Isotopic measurements were performed on the Isoprobe MC-ICPMS. At the IW buffer, time series experiments show that there is a clear continuous increase in δ74Ge in the metal as a function of time, values being isotopically lighter than the reference Aldrich standard after 2 hours (δ74Ge ~-0.5‰), reaching a δ74Ge of ~+3‰ after 60 hours. For experiments at 24 hours, an increase in δ74Ge (from ~-0.3‰ to +10‰) is observed with increasing fO2. The enrichment of light isotopes in the metallic phase most likely reflects the consequence of Ge diffusion from the silicate to the metal (c.f. case for iron, Roskosz et al., 2006). On the other hand, enrichment in heavy isotopes is indicative of loss of Ge, most probably due to evaporation. More experiments are planned to constrain these processes and to quantify the relative time-scales of Ge transport between silicate, metal and gas reservoirs. Luais B (2007) EPSL In press: Roskosz et al. (2006) EPSL v. 248 p. 851.
V51E-0834
Molecular Dynamics Simulation of Isotope Fractionation in a Temperature Gradient
Recent experimental work on silicate melts has shown that isotopes may be fractionated by Soret diffusion along a temperature gradient. The magnitude of the fractionation is surprisingly large and not well understood. We carried out non-equilibrium molecular dynamics simulations on silicate melts with an imposed steady-state temperature gradient, with the goal of gaining insight into the physical mechanisms that underlie these experimental results. Two isotopes of each element are included, and a steady-state spatial distribution of elements and isotopes is achieved. In agreement with experiments, we find that heavy isotopes are concentrated in the cold region, as are Mg and other network-modifying cations. The detailed physical origins of these behaviors will be discussed.
V51E-0835
A Distinct Magnetic Isotope Effect Measured in Atmospheric Mercury in Epiphytes
Due to the importance of Mercury as an environmental contaminant, mercury cycling in the atmosphere has been extensively studied. However, there still remain uncertainties in the relative amounts of natural and anthropogenic emissions, atmospheric deposition rates as well as the spatial variation of atmospheric mercury. Part of a study to determine the isotopic composition of mercury deposited from the atmosphere has involved the use of epiphytes as monitors. The greatest advantage of such natural monitors is that a widespread, high-density network is possible at low cost. One of the disadvantages at present is that these monitors likely contain different mercury species (for example both gaseous, elemental mercury trapped by adsorption and Hg (II) by wet deposition). The project began with the understanding that biochemical reactions involving metallothioneins within the epiphytes might have produced an isotopic effect. One such regional network was composed of samples of Tillandsia usenoides (common name: Spanish moss) collected along the eastern Coastal Plain of the U.S. from northern Florida to North Carolina. The isotopic composition of a sample is expressed as permil deviations from a standard. The deviations are defined as δAHg = \left(\frac{Rsample}{Rstd}-1 \right)1000 ‰ , where A represents the atomic mass number. R=\frac{AHg}{202Hg} were measured for the isotopes 198Hg, 199Hg, 200Hg, 201Hg, 202Hg and 204Hg relative to the mercury standard SRM NIST 3133, by a standard-sample bracketing technique. For all samples, the delta values of the even-N plotted against atomic mass numbers define a linear curve. For the odd-N isotopes, δ199Hg and δ201Hg deviate from this mass-dependent fractionation (MDF) relationship and indicate a mass-independent fractionation (MIF) effect and a negative anomaly, i.e. a depletion in 199Hg and 201Hg relative to the even-N isotopes. These deviations are expressed as Δ199Hg = δ199Hgtotal - δ199HgMDF. A Δ201Hg/Δ199Hg ratio of 1.11 is predicted by isotope fractionation due to the Magnetic Isotope Effect (MIE), because 1.11 is the ratio of the magnetic moments of the two odd-N isotopes. A plot of Δ199Hg versus Δ201Hg values obtained reveals a striking pattern. All samples plot well within analytical uncertainly along a straight line passing through zero and having a slope of 1.11. Based on thermodynamic principles, some have argued that nuclear spin effects are quite insignificant in producing isotopic fractionation. However the MIE is a kinetic one in which those isotopes with non-zero magnetic moments effect the rates of recombination of free radical pairs by nuclear-electron hyperfine interaction and can become enriched or depleted in either reactants or products. In the samples studied here, the nuclear spin is far more important than either nuclear mass or nuclear volume in effecting isotopic fractionation of Mercury.
V51E-0836
Redox-controlled iron isotope fractionation during basalt differentiation?
Theoretical studies suggest that the scale of equilibrium isotope fractionation decreases as the temperature increases and is likely to be negligible during high-temperature processes such as mantle melting and basaltic differentiation. This is confirmed by studies of Li and Mg isotopes, which show no measurable Li and Mg isotopic fractionation during basaltic differentiation at temperatures greater than 1050oC. However, iron chemistry is unique in that two oxidation states are present in magmas. During crystallization, they partition differently between melts and solids, which can potentially produce measurable isotopic fractionation. We present high-precision Fe isotopic data measured by MC-ICP-MS for a set of well-characterized samples from Kilauea Iki lava lake, Hawaii. The Kilauea Iki lava lake has a relatively simple geological setting and experienced extensive closed-system differentiation that makes it an ideal field laboratory for studying the effect of basaltic differentiation on isotopes. The samples range from olivine-rich cumulates to andesitic segregation veins. The δ56Fe values correlate negatively with MgO contents, and correlate positively with Fe3+/Fe2+ ratios. Olivine cumulates have higher MgO contents (up to 26.87 wt%) and lower δ56Fe (down to -0.03‰) and Fe3+/Fe2+ ratios while late-stage veins have lower MgO contents (down to 2.37 wt%) and higher δ56Fe (up to +0.22‰) and Fe3+/Fe2+ ratios. The Fe isotopic variations (> 0.2‰) in Kilauea Iki lava lake were most likely produced by fractional crystallization, with an estimated isotopic fractionation factor between minerals and melts of ~ - 0.1‰. The oxidation state of Fe affects key chemical properties such as the coordination number. This can explain why Fe isotopes are fractionated during basalt differentiation while Li and Mg are not. Our study suggests that the Fe isotopic fractionation during magmatic differentiation is largely controlled by the oxidation state of iron. Therefore, Fe isotopes can potentially be used to constrain the redox history of terrestrial and exterrestrial magmas
V51E-0837
Fractionated Mercury Isotopes in Fish: The Effects of Nuclear Mass, Spin, and Volume
Mercury is long known as a common environmental contaminant. In methylated form it is even more toxic and the methylation process is facilitated by microbial activities. Methyl mercury easily crosses cell membrane and accumulates in soft tissues of fishes and finally biomagnifies with increasing trophic levels. Natural variations in the isotopic composition of mercury have been reported and such variations have emphasized mass dependent fractionations, while theory and laboratory experiments indicate that mass-independent isotopic fractionation (MIF) effects are likely to be found as well. This study focuses on the MIF of mercury isotopes in the soft tissues of fishes. Samples include both fresh water and marine fish, from different continents and oceans. Approximately 1 gm of fish soft tissue was dissolved in 5 ml of conc. aqua regia for 24 hrs and filtered through a ¬¬¬100 μm filter paper and diluted with DI water. Hg is measured as a gaseous phase generated by reduction of the sample with SnCl2 in a continuous- flow cold-vapor generator connected to a Thermo-Finnigan Neptune MC-ICPMS. To minimize instrumental fractionation isotope ratios were measured by sample standard bracketing and reported as δ‰ relative to NIST SRM 3133 Hg standard where δAHg = [(A Hg/202Hg)sample/(A Hg/202Hg)NIST313] -1 ×1000‰. In this study we have measured the isotope ratios 198Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, 201Hg/202Hg and 204Hg/202Hg. In all the fish samples δ198Hg, δ200Hg, δ202Hg, δ204Hg define a mass- dependent fractionation sequence, where as the δ199Hg and δ201Hg depart from the mass- dependent fractionation line and indicate an excess of the odd-N isotopes. The magnitude of the deviation (ΔAHg where A=199 or 201) as obtained by difference between the measured δ199Hg and δ201Hg of the samples and the value obtained by linear scaling defined by the even-N isotopes ranges from approximately 0.2 ‰ to 3‰. The ratios of Δ199Hg /Δ201Hg range from 0.8 to 1.3, and thus more than one mass-independent isotope effect is inferred. MIF of mercury can be caused by the nuclear volume effect. Schauble, 2007 has calculated nuclear volume fractionation scaling factors for a number of common mercury chemical species in equilibrium with Hgo vapor. From his calculations the nuclear field shift effect is larger in Δ199Hg than in Δ201Hg by approximately a factor of two. The predominant mercury chemical species in fish is methylmercury cysteine. From the experimental studies of Buchachenko and others (2004) on the reaction of methylmercury chloride with creatine kinase it seems reasonable to predicted that the thiol functional groups of cysteine gets enriched in 199Hg and 201Hg. Here the magnetic isotope effect (MIE) produces a kinetic partial separation of isotopes with non-zero nuclear spin quantum numbers from the even-N isotopes. The ratio of enrichment of Δ201Hg /Δ199Hg is predicted from theory to be 1.11, which is the ratio of the magnetic moments of 199Hg and 201Hg. Because mercury possesses two odd-N isotopes, it is possible to detect and evaluate the effects of two distinct, mass-independent isotope fractionating processes. From the data obtained on fish samples, we can deconvolute the contributions of the isotope effects of nuclear mass, spin and volume. For these samples the role of spin or the magnetic isotope effect is the most dominant.
V51E-0838
Bulk Diffusion and Isotopic Fractionation of Lithium in Olivine: an Experimental Study
Lithium is the lightest of the lithophile elements, with high solid-state diffusivity (e.g. Giletti and Shanahan, 1997). Recent studies in synthetic and natural systems suggest that isotopic fractionation of lithium accompanies diffusion (Richter et al., 2003; Lundstrom et al., 2005; Beck et al., 2006; Teng et al., 2007; Jeffcoate et al., 2007). Lithium isotope variations therefore have the potential to constrain time scales of geological processes. We have performed a series of experiments to quantitatively assess the bulk diffusion of lithium in olivine and the degree of isotopic fractionation that can occur during diffusive transport. Single crystals of San Carlos olivine were surrounded by LiF and then held at temperature (800°C to 1000°C) for variable lengths of times (up to 20 hours at 800°C and as short as 2 hours at 1000°C). Measurements of the lithium concentration and isotopic composition of the experimental charges were conducted using the Cameca IMS 3f at Arizona State University. The lowest temperature experiments show no discernable diffusion of lithium into the olivine crystal, while in the highest temperature experiment, up to 650 ppm lithium is incorporated into the olivine. The lithium content in all experiments above 1000°C decreases smoothly from the crystal rim to the core; indicating diffusive transport of Li into the crystal. Lithium concentrations in the crystal cores range from background in the lowest temperature experiment, to ~100 ppm in the highest temperature experiment. Rim values, in contrast, vary from 650 ppm in the high temperature experiments to core concentrations in the low temperature experiment. All experiments in which diffusive transport into the olivine crystal occurred also display a fractionation of 6Li from 7Li. In the most extreme case, a variation of 50‰ in δ7Li was observed across the diffusion profile, which was generated in just under 2 hours. As lithium content decreases toward the crystal center, the δ7Li values become increasingly negative, reaching values of -45‰ in one experiment. These results confirm that solid state Li diffusion can be a major influence on the isotopic composition of Li in olivine, as shown previously for silicate melts (Richter et al., 2003). In one experiment, low δ7Li values were observed ahead of detectable changes in Li concentration, suggesting that isotopic exchange of Li in olivine may be more rapid than bulk Li diffusion. With experimentally determined values of Li diffusivity, it should be possible to place strict time constraints on geochemical processes that affect the lithium abundance and isotopic ratios in olivine.
V51E-0839
High-precision multiple-sulfur isotope analysis as a new tool to decouple isotope mixing and fractionation: application to subsurface sulfur cycles
Isotope mixing and fractionation are often two important processes that govern isotope ratios of geomaterials. Single isotope ratio analysis, however, is one-dimensional such that it constrains either isotope mixing or fractionation. Multiple-stable isotope ratios (e.g., 32S/33S/34S/36S) were thought to carry no additional information because they were thought to follow mass-dependent relationships such that isotope fractionation in 33S/32S, for example, is about a half of that of 34S/32S. Mass-dependent equilibrium isotope fractionation, in theory, follows a power law (e.g., 33α = 34α 0.515) function, ultimately due to exponential function of the Boltzmann energy distribution. This power law function of mass-dependent isotope fractionation has important implication because it is expressed non-linearly in conventional delta-delta plots (e.g., δ33S versus δ34S). This contrasts to isotope mixing that is expressed as a linear function in delta-delta plots. This functional difference produces small but measurable variations in multiple-stable isotope systems, and can be used to decouple isotope mixing and fractionation. I will discuss application into natural systems, in particular, how high-precision multiple-sulfur isotope analysis is used to trace subsurface sulfur cycles, where both sulfur isotope mixing and fractionation is occurring. The same approach may be used to other multiple- stable isotope systems if required analytical precision can be achieved.
V51E-0840
Molecular Dynamics Simulations of Kinetic Isotope Fractionation During the Diffusion of Ionic Solutes and Noble Gases in Liquid Water
Interpretation of isotope ratios, a powerful tool in geochemical investigations of fluid-rock systems, requires an understanding of all relevant processes that fractionate isotopes. One such process, diffusion in liquid water, has remained problematic despite its potential significance as a major cause of kinetic isotope fractionation. Recent laboratory experiments have shown clearly for the first time that lithium and chloride isotopes are fractionated by diffusion in liquid water, whereas magnesium isotopes are not. We present the results of molecular dynamics simulations of chloride, magnesium, alkali metal cations (Li+, Na+, K+, Cs+) and noble gases (He, Ne, Ar, Xe) in liquid water that were designed to determine the isotopic mass dependence of solute diffusion coefficients. Our results indicate that the self-diffusion coefficients of all solutes follow an inverse power-law dependence on isotopic mass (Di \propto {mi}-β, where Di is the self-diffusion coefficient of a solute with isotopic mass {mi}). The power-law exponents (β) deduced for lithium, chloride, and magnesium from recent diffusivity data are consistent with the mass dependencies found in our simulations. The isotopic mass-dependence of noble gas diffusion coefficients in liquid water found in our simulations is much smaller than assumed in recent groundwater hydrology and paleoclimate reconstruction studies.
V51E-0841
Fluid speciation controls of low-temperature copper isotope fractionation
Since the classic study of Ohmoto (1972) on the sulfur isotope system, it has been clear that mass-balance among reduced and oxidized solution species can exert strong control on the isotopic composition of sulphide ore minerals. Similarly, experimental studies on copper isotope fractionation suggest that fluid speciation among reduced and oxidized copper species could control the Cu-isotopic composition of copper sulfides. We explore this issue for sedimentary copper sulfide deposits through speciation calculations based on thermodynamic stability of reduced and oxidized copper aqueous solutions species. The calculations are made as a function of Eh, pH and Cu-isotopic fractionation factors at sedimentary temperatures. Speciation was calculated using The Geochemists Workbench v4.03, Cu isotopic fractionations were taken from experimental calibrations and diagrams were calculated by Matlab programming. The calculations show that isotopically-light Cu-sulfide minerals (with respect to the bulk δ65Cu value) will only form in Eh-pH conditions where oxidized species dominate the mass balance, whereas more reducing conditions will give Cu-sulfide minerals with bulk δ65Cu values similar to the bulk solution value. Applying these speciation diagrams to stratiform sedimentary hosted copper deposits of the Timna Valley in Israel and the Kupferschiefer deposits of SW Poland, points to notable differences between the two deposits in terms of the ore formation conditions. Whereas the Timna valley copper sulfides have light Cu-isotopic compositions with bulk δ65Cu = -2.04 ± 0.4 ‰, the Kupferschiefer Cu-sulfides show significantly higher δ65Cu values = -0.42 ± 0.38 ‰. Assuming that the source solution had a bulk δ65Cu = 0 at both locations (i.e., an igneous copper porphyry source), the speciation diagrams indicate that the Kupferschiefer deposits precipitated at more reducing conditions and higher Cu(I)aq/Cu(II)aq values than the Timna deposits. These observations are in accord with field relations showing that Cu(II) minerals dominate the Timna system but that Cu-sulfides are the major minerals of the Kupferschiefer deposits. * Ohmoto, H. 1972. Systematics of sulfur and carbon isotopes in hydrothermal ore deposits. Econ. Geol. 67: 551- 578.