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

Abstract

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...

The influence of temperature, pH, and growth rate on the δ18O composition of inorganically precipitated calcite

“…This is in contrast to the behavior of both calcium and oxygen isotopes, which exhibit light isotope enrichment in the solid relative to the predominant dissolved species under non-equilibrium conditions (Tang et al, 2008). For calcium isotopes, this light isotope enrichment has previously been attributed to mass-dependent ion desolvation rates (Nielsen et al, 2012;Hofmann et al, 2012), which can successfully explain the observed sign and magnitude of kinetic Ca 2+ isotope effects in calcite (Hofmann et al, 2012). However, there are important differences among cations and anions that must be considered before extending the same conclusions to either oxygen or carbon isotope effects in calcite.…”

Barium-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation

“…A handful of recent studies have begun examining Ba-isotopic variability in nature, and have identified significant Ba-isotopic fractionation during mineral precipitation experiments (Von Allmen et al, 2010;Böttcher et al, 2012), between various CaCO 3 minerals (Pretet, 2014), and most recently in igneous rocks (Miyazaki et al, 2014). Theoretical predictions support the direction, though not the magnitude, of Ba-isotopic fractionation observed during low-temperature mineral precipitation, which may be related to Ba 2+ ion desolvation (Hofmann et al, 2012). However, examination of the Ba-isotopic systematics of seawater have hitherto escaped detailed study, largely because of the analytical difficulties associated with the isolation of sufficient quantities of chemically pure Ba from complex matrices such as seawater.…”

Theoretical and Experimental Principles

“…Kinetic fractionations have been observed for instance during diffusive mass transfer for elements like Li, Mg and Fe, which in the case of Li can be large and which occur an spatial scales from µm to m. Kinetic isotope fractionations are also observed during the precipitation of solid metal compounds (i.e. carbonates), opposite to equilibrium isotope fractionation which depend on bond energies (Hofmann et al 2013).…”