Barium isotope fractionation during witherite (BaCO₃) dissolution, precipitation and at equilibrium

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• 作者：Vasileios Mavromatis^a ; ^b ; ^{mavromatis@tugraz.at" class="auth_mail" title="E-mail the corresponding author} ; Kirsten van Zuilen^c ; ¹ ; Bettina Purgstaller^a ; Andre Baldermann^a ; Thomas F. Nä ; gler^c ; Martin Dietzel^a
• 关键词：Witherite ; Ba isotopes ; Kinetic fractionation ; Fractionation at equilibrium
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 October 2016
• 年：2016
• 卷：190
• 期：Complete
• 页码：72-84
• 全文大小：918 K

文摘

This study examines the behavior of Ba isotope fractionation between witherite and fluid during mineral dissolution, precipitation and at chemical equilibrium. Experiments were performed in batch reactors at 25 °C in 10⁻² M NaCl solution where the pH was adjusted by continuous bubbling of a water saturated gas phase of CO₂ or atmospheric air. During witherite dissolution no Ba isotope fractionation was observed between solid and fluid. In contrast, during witherite precipitation, caused by a pH increase, a preferential uptake of the lighter ¹³⁴Ba isotopomer in the solid phase was observed. In this case, the isotope fractionation factor α_{witherite-fluid} is calculated to be 0.99993 ± 0.00004 (or Δ^137/134Ba_{witherite-fluid} ≈ −0.07 ± 0.04‰, 2 sd). The most interesting feature of this study, however, is that after the attainment of chemical equilibrium, the Ba isotope composition of the aqueous phase is progressively becoming lighter, indicating a continuous exchange of Ba²⁺ ions between witherite and fluid. Mass balance calculations indicate that the detachment of Ba from the solid is not only restricted to the outer surface layer of the solid, but affects several (∼7 unit cells) subsurface layers of the crystal. This observation comes in excellent agreement with the concept of a dynamic system at chemical equilibrium in a mineral–fluid system, denoting that the time required for the achievement of isotopic equilibrium in the witherite–fluid system is longer compared to that observed for chemical equilibrium. Overall, these results indicate that the isotopic composition of Ba bearing carbonates in natural environments may be altered due to changes in fluid composition without a net dissolution/precipitation to be observed.