Barite reactivity at solubility equilibrium as a function of [Ba2+]/[SO42-] ratios

We carried out ¹³⁷Ba and ³⁴S-spiked experiments and measured barium attachment and detachment fluxes from and to barite crystal surfaces in solutions at solubility equilibrium with barite. The [Ba]/[SO₄] ratios in solutions varied from 0.06 to 52. Both attachment and detachment fluxes increase with [Ba]/[SO₄] ratios. As expected, since the solutions were near solubility equilibrium ( ), the attachment and detachment fluxes were nearly equal and net fluxes or reaction rates were zero.

The isotope flux data together with step velocity data from AFM studies by Kowacz et al. (2007) were simultaneously fit into the Zhang and Nancollas (1998) process-based AB crystal growth model, which describes crystal growth and dissolution through nucleation and propagation of kink sites. The Newton Conjugate Gradient Trust Region algorithm was used for simultaneously and optimally regressing both attachment and detachment rate coefficients. Simultaneous fitting step velocity data of Kowacz et al. (2007) significantly reduced the number of non-unique solutions. The excellent agreement indicates that attachment and detachment fluxes and step velocity are consistent and complement each other.

The results of this study demonstrate significant isotopic changes in solutions and solids from mineral-fluid interactions at solubility equilibrium. The Zhang and Nancollas (1998) model has been used as a foundation for interpreting isotopes and trace element data. Our results therefore have significant implications for extending it to the understanding in diagenetic and low temperature metamorphic processes.