Experimental and theoretical investigations of stable Sr isotope fractionation during its incorporation in aragonite

Strontium partitioning and isotope fractionation between aragonite and fluid have been determined experimentally at low values of the fluid saturation state (Ω) with respect to this mineral (1.1 ≤ Ω_aragonite ≤ 2.2), and the measured isotope fractionation has been compared with the results of first-principles simulations. For the latter, Density Functional Theory (DFT) was used for estimation of the equilibrium Sr isotope fractionation between aragonite and Sr²⁺(aq). The obtained results suggest that, for values of Ω_aragonite close to unity, the apparent distribution coefficient of Sr in aragonite ([Formula presented]) exhibits values higher than one that rapidly decrease at increasing aragonite growth rate. Under equilibrium conditions (i.e. Ω_aragonite=1) a D_{Sr,aragoinite} value of 2.7 can be extrapolated. Additionally, for aragonite growth rates r_p ≤ 10^−8.0±0.2 (mol/m²/s) the Sr isotope fractionation between aragonite and the fluid (i.e. Δ^88/86Sr_{aragonite-fluid}) shows a constant value of −0.1±0.05‰, whereas it decreases to −0.40‰ when the growth rate increases to 10^−7.7(mol/m²/s). The surface reaction kinetic model (SRKM) developed by DePaolo (2011) has been used to describe the dependence of D_Sr,aragonite and Δ^88/86Sr_{aragonite-fluid} on mineral growth rate. In this model the best fit for D_Sr,aragonite and Δ^88/86Sr_{aragonite-fluid} were 4 and −0.01‰, respectively, whereas the kinetic isotope fractionation factor for Δ^88/86Sr_{aragonite-fluid} was −0.6‰. The results of first-principles calculations yield an equilibrium Sr isotope fractionation factor of −0.04‰ which is in excellent agreement with the experimental value of the present study. These results are the first experimental measurements of Sr isotope fractionation during inorganic aragonite precipitation as a function of growth rate and the first DFT calculations of Sr equilibrium fractionation in the aragonite-fluid system. The results of this study provide new insight into the mechanisms controlling stable Sr isotope composition in aragonite, which has implications for using Sr isotopes for paleo-reconstructions of natural archives, particularly those of abiogenic origin.