Barium isotope fractionation in barite–fluid systems at chemical equilibrium

Abstract

The barium isotope composition of sedimentary barite (BaSO4, barium sulfate) is emerging as a powerful tracer of the sources and cycling of Ba in modern and ancient marine environments. To reliably use Ba isotopes to interrogate the marine Ba cycle, it is important to identify and constrain processes that fractionate the isotope composition of Ba in BaSO4. Of particular interest is ion exchange: micro-scale dissolution and precipitation that occurs in mineral–fluid systems at chemical equilibrium. This process is often important in systems where minerals, such as BaSO4, and a fluid remain in contact for prolonged periods of time; however, the impact of ion exchange on Ba isotope compositions in BaSO4 is unknown. To constrain the rate and isotopic effect associated with ion exchange in BaSO4–fluid systems, we conducted a series of experiments under marine-relevant conditions and interpreted the results using a multi-phase time-dependent numerical reactor model. From a series of isotope-tracer experiments, we find that BaSO4–fluid ion exchange progresses at a rate between 5 and 53 pmol m−2 s−1. In a parallel set of experiments used to assess mass-dependent isotope fractionation of Ba, the combined effect of BaSO4 dissolution and precipitation while at chemical equilibrium was found to result in the continued evolution of Ba isotopes and produced a modeled offset of Δ138Babarite–dBa = −0.10 ± 0.05 ‰ at isotopic equilibrium. We then constrained the magnitude of isotopic fractionation during BaSO4 dissolution by fitting our data in the numerical reactor model and using previous estimates of Ba isotope fractionation during BaSO4 precipitation (⍺precipitation = 0.99968 ± 0.00002). At chemical equilibrium, we find our data are best explained by an ⍺dissolution = 0.99978 ± 0.00006, implying that BaSO4 dissolution releases isotopically ‘light’ Ba to solution. Since the magnitude of the isotope effects associated with BaSO4 precipitation and dissolution are imbalanced, ion exchange will tend to alter the isotope composition of co-located BaSO4 and fluids until the two phases are offset by ≈0.10 ‰. The importance of this effect on sedimentary BaSO4 likely depends on several factors and we suggest multiple site-screening criteria to maximize the utility of this emerging proxy.