Stable strontium isotope fractionation in synthetic barite

Abstract

The mineral barite (BaSO4) accommodates strontium (Sr) in its crystal structure, providing an archive of Sr-isotopes (87Sr/86Sr and δ88/86Sr) in the highly stable sulfate mineral. We investigated mass dependent stable Sr-isotope fractionation (Δ88/86Sr=δ88/86Srsolid−δ88/86Srsolution) during inorganic precipitation of barite from a barium-rich solution by addition of sulfate under controlled conditions and compared this to equilibrium isotopic fractionation calculated using Density Functional Theory modeling. Sr-substituted barite is predicted to have lower 88Sr/86Sr than any other studied species, and at 25°C will be about 0.6–0.7‰ lower than the two modeled Sr(H2O)82+-bearing salts that could approximate aqueous Sr2+. This agrees in direction and order of magnitude with experimental results that estimate equilibrium Sr-isotope fractionation in barite to be 0.3‰ lower than aqueous Sr2+ at ∼20°C. The high ionic strength of some of the precipitating solutions (up to 1M) and potential differences in the average coordination number of aqueous Sr2+ add to uncertainty in a direct comparison of the calculated equilibrium isotopic fractionation values with the experimental results.Stable Sr-isotope fractionation varied along with the distribution coefficient of Sr [Kd(Sr)=[Sr/Ba]barite/[Sr/Ba]solution], which is a function of both temperature and barite saturation state. However the relationship between mass dependent isotopic fractionation and Kd(Sr) is different for conditions of changing temperature versus barite saturation state. With increasing temperature (from 5 to 40°C), the barite phase became isotopically lighter (Δ88/86Sr=−0.29‰ to −0.41‰). Conversely, with increasing saturation state (saturation index of barite=3.0–4.3) the barite phase became isotopically heavier (Δ88/86Sr=−0.25‰ to −0.10‰). These observations suggest chemical kinetic effects control isotopic fractionation rather than equilibrium temperature effects. The relationship with saturation state indicates the potential presence of a diffusive boundary layer. Barite crystal morphology appears to be affected by the diffusion rate of solute (sulfate) to the growing crystal surface relative to the overall growth rate of barite crystals during precipitation.