Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance

Young, Edward; Galy, Αlbert; Nagahara, H.

doi:10.1016/s0016-7037(01)00832-8

Cited by 783 publications

(768 citation statements)

References 28 publications

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“…4) may provide a clue into the fundamental basis of the isotopic mass dependence of solute diffusion coefficients in liquid water (D i ∝ m i −β ) (14-17), because solute D values are positively correlated with k wex , at least in the case of Li + and Na + in liquid water (33). Second, our simulations, in which isotopes have identical interatomic interaction parameters and differ only by their masses, show that-in addition to the expected quantum mechanical effects-classical mechanical effects can play an important role in causing kinetic isotope fractionations in geochemical systems, as pointed out by Young et al (51). This finding is consistent with studies showing that Newtonian mechanics are the primary cause of carbon kinetic isotope effects associated with the dimerization of cyclopentadiene (52) and may contribute significantly to a range of oxygen kinetic isotope effects (53).…”

supporting

confidence: 70%

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Hofmann

Bourg

DePaolo

2012

Proc. Natl. Acad. Sci. U.S.A.

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Molecular dynamics simulations show that the desolvation rates of isotopes of Li + , K + , Rb + , Ca 2+ , Sr 2+ , and Ba 2+ may have a relatively strong dependence on the metal cation mass. This inference is based on the observation that the exchange rate constant, k wex , for water molecules in the first hydration shell follows an inverse power-law mass dependence (k wex ∝ m −γ ), where the coefficient γ is 0.05 ± 0.01 on average for all cations studied. Simulated waterexchange rates increase with temperature and decrease with increasing isotopic mass for each element. The magnitude of the water-exchange rate is different for simulations run using different water models [i.e., extended simple point charge (SPC/E) vs. four-site transferrable intermolecular potential (TIP4P)]; however, the value of the mass exponent γ is the same. Reaction rate theory calculations predict mass exponents consistent with those determined via molecular dynamics simulations. The simulation-derived mass dependences imply that solids precipitating from aqueous solution under kinetically controlled conditions should be enriched in the light isotopes of the metal cations relative to the solutions, consistent with measured isotopic signatures in natural materials and laboratory experiments. Desolvation effects are large enough that they may be a primary determinant of the observed isotopic fractionation during precipitation.aqueous geochemistry | kinetic isotope effect | ligand exchange N onequilibrium processes are generally recognized as important influences on isotopic fractionation during mineral precipitation, especially at temperatures below a few hundred degrees Celsius (1, 2). Recent work in "nontraditional" stable isotopes (Li, Mg, Ca, Fe, Cd, Cu, etc.) has confirmed this inference and shown that nonequilibrium fractionation must be accounted for to understand global biogeochemical cycles involving these elements (2-5). A key observation is that light isotopes are preferentially incorporated into precipitating solids (6-8)-the opposite direction of enrichment expected for equilibrium fractionation, which depends on bond energetics (9, 10). The origin of this nonequilibrium light-isotope enrichment is a key unknown in the isotopic geochemistry of mineral growth.Calcite grown from aqueous solutions provides an excellent illustration of light-isotope enrichment during precipitation. Synthetically precipitated and natural samples of calcite and aragonite are fractionated relative to aqueous Ca 2+ such that δ 44/40 Ca in calcite is lower by 0.5-2‰ (7, 11). Equilibrium Ca isotope fractionation between Ca 2+ (aq) and calcite has not been measured in the laboratory, but studies of deep-sea sedimentary pore fluids suggest that the equilibrium fractionation is negligible: ε eq ∼ 0.0 ± 0.1‰ (12). Growth of calcite from seawater-like aqueous solutions is not likely to be diffusion-limited for Ca, so the isotopic effects are inferred to be due to kinetic effects occurring at the solid/fluid interface (11, 13). Diffusion in liquid water can r...

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supporting

confidence: 70%

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Hofmann

Bourg

DePaolo

2012

Proc. Natl. Acad. Sci. U.S.A.

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“…These results thus document the presence of mass-independent Ni isotopic heterogeneity in bulk meteoritic samples, as previously proposed by Regelous et al 2008 (EPSL 272, 330-338), but our new analyses are more precise and include determination of 64 Ni. Intriguingly, we find that terrestrial materials do not yield homogenous internally-normalised Ni isotope compositions, which, as pointed out by Young 2002 (GCA 66, 1095-1104, may be the expected result of using the exponential (kinetic) law and atomic masses to normalise all fractionation processes. The certified Ni isotope reference material NIST SRM 986 defines zero in this study, while appropriate ratios for the bulk silicate Earth are given by the peridotites JP-1 and DTS-2 and, relative to NIST SRM 986, yield devi- …”

Section: Introductionmentioning

confidence: 46%

Confirmation of mass-independent Ni isotopic variability in iron meteorites

Steele

Elliott

Coath

et al. 2011

Geochimica et Cosmochimica Acta

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We report high-precision analyses of internally-normalised Ni isotope ratios in 12 bulk iron meteorites. and E 64 Ni 58 61 , respectively, which is comparable to our sample reproducibility. We show that this 'massindependent' Ni isotope variability cannot be ascribed to interferences, inaccurate correction of instrumental or natural mass-dependent fractionation, fractionation controlled by nuclear field shift effects, nor the influence of cosmic ray spallation. These results thus document the presence of mass-independent Ni isotopic heterogeneity in bulk meteoritic samples, as previously proposed by Regelous et al. 2008 (EPSL 272, 330-338), but our new analyses are more precise and include determination of 64 Ni. Intriguingly, we find that terrestrial materials do not yield homogenous internally-normalised Ni isotope compositions, which, as pointed out by Young 2002 (GCA 66, 1095-1104, may be the expected result of using the exponential (kinetic) law and atomic masses to normalise all fractionation processes. The certified Ni isotope reference material NIST SRM 986 defines zero in this study, while appropriate ratios for the bulk silicate Earth are given by the peridotites JP-1 and DTS-2 and, relative to NIST SRM 986, yield devi-

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“…Furthermore, the magnitude of the observed ∆ 17 O anomalies, even in NWA 1240, which has the smallest anomaly of the six isotopically aberrant eucrites, seems far beyond the reach of mass-dependent processes with different slope factors without resorting to a sequence of large scale processes, probably requiring other phases like water (e.g. Young et al, 2002).…”

Section: Source Of Eucrites With Anomalous Oxygen Isotopic Compositionsmentioning

confidence: 94%

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Scott

Greenwood

Franchi

et al. 2009

Geochimica et Cosmochimica Acta

147

176

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A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have ∆ 17 O values that lie within 2σ of their mean value viz., -0.242±0.016‰, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites-NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira-have ∆ 17 O values that lie respectively 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ 18 O value that is 5σ below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like many unbrecciated eucrites from Vesta, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (~10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta's crust and the lack of deeply buried samples from the hypothesized Vestalike bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisonal grinding.

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Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance

Cited by 783 publications

References 28 publications

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Confirmation of mass-independent Ni isotopic variability in iron meteorites

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

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