Ab Initio Calculation of Equilibrium Isotopic Fractionations of Potassium and Rubidium in Minerals and Water

We
used first-principle approaches to calculate the equilibrium
isotopic fractionation factors of potassium (K) and rubidium (Rb)
in a variety of minerals of geological relevance (orthoclase, albite,
muscovite, illite, sylvite, and phlogopite). We also used molecular
dynamics simulations to calculate the equilibrium isotopic fractionation
factors of K in water. Our results indicate that K and Rb form bonds
of similar strengths and that the ratio between the equilibrium fractionations
of K and Rb is approximately 3–4. Under low-temperature conditions
relevant to weathering of continents or alteration of seafloor basalts
(∼25 °C), the K isotopic fractionation between solvated
K+ and illite (a proxy for K-bearing clays) is +0.24‰,
exceeding the current analytical precision, so equilibrium isotopic
fractionation can induce measurable isotopic fractionations for this
system at low temperature. These findings, however, cannot easily
explain why the δ41K value of seawater is shifted
by +0.6‰ relative to igneous rocks. Our results indicate that
part of the observed fractionation is most likely due to kinetic effects.
The narrow range of mean force constants for K and Rb in silicate
minerals suggests that phase equilibrium is unlikely to create large
K and Rb isotopic fractionations at magmatic temperatures (at least
in silicate systems). Kinetic effects associated with diffusion can,
however, produce large K and Rb isotopic fractionations in igneous
rocks.