The role of pH on Cr(VI) partitioning and isotopic fractionation during its incorporation in calcite

• Published in: Geochimica et cosmochimica acta 2019-11, Vol.265, p.520-532
• Main Authors: Füger, A., Bruggmann, S., Frei, R., Leis, A., Dietzel, M., Mavromatis, V.
• Format: Article
• Language: English
• Subjects: Aqueous speciation; Calcite; Chromium isotopes; Fractionation; Sciences of the Universe
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2019.07.047

The Cr(VI) incorporation and chromium stable isotope composition in calcite has been studied in experiments performed in the pH range between 8.0 and 10.6 at constant temperature (T = 25 ± 1 °C), precipitation rate (rp = 10−7.7±0.2 mol m−2 s−1) and total aqueous Cr(VI) concentration (Cr(VI) = 49.6 ± 1.3 mM). The obtained results indicate that Cr(VI) incorporation in calcite is pH-dependent and it is likely significantly affected by the formation of the aqueous CaCrO40 species. The experimental findings suggest that during calcite growth at pH  9.4 the smaller contributions of CaCrO40 in the total concentration of Cr(VI) in calcite yields in a Δ53Crcalcite-solution value close to 0‰. Our results imply that the chromium isotope tracer system applied to calcite, as an environmental proxy for the reconstruction of ocean redox conditions, is not solely a mirror of redox effects in the aqueous fluid from which the carbonates precipitate, but additionally is controlled by the pH of the forming fluid and consequently by the relative stability and the distribution of aquo-complexes. Speciation calculations for calcite precipitated from seawater and include the presence of CaCrO0 complex, predict isotope fractionation values that lay within −0.67‰ < Δ53Crcalcite-solution