Ca isotope fractionation during inorganic calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 〈 log R 〈 4.4 μmol/m2/h) and temperatures (5, 25, and 40 °C) with traces of Sr using the CO2 diffusion technique.
Results show that in analogy to Sr/Ca [see Tang J., Köhler S. J. and Dietzel M. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: I. Sr incorporation. Geochim. Cosmochim. Acta] the 44Ca/40Ca fractionation during calcite formation can be followed by the Surface Entrapment Model (SEMO). According to the SEMO calculations at isotopic equilibrium no fractionation occurs (i.e., the fractionation coefficient αcalcite-aq = (44Ca/40Ca)s/(44Ca/40Ca)aq = 1 and Δ44/40Cacalcite-aq = 0‰), whereas at disequilibrium 44Ca is fractionated in a primary surface layer (i.e., the surface entrapment factor of 44Ca, F44Ca 〈 1). As a crystal grows at disequilibrium, the surface-depleted 44Ca is entrapped into the newly formed crystal lattice. 44Ca depletion in calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated 44Ca fractionation in calcite grown at high precipitation rates due to limited time for Ca isotope re-equilibration by ion diffusion. Elevated temperature results in an increase of 44Ca ion diffusion and less 44Ca fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less 44Ca fractionation and the impact of precipitation rate on 44Ca fractionation is reduced.
A highly significant positive linear relationship between absolute 44Ca/40Ca fractionation and the apparent Sr distribution coefficient during calcite formation according to the equation Δ44/40Cacalcite-aq=(−1.90±0.26)·logDSr−2.83±0.28is obtained from the experimental results at 5, 25, and 40 °C. Thus, Sr partitioning during calcite formation directly reflects Ca isotopic fractionation, independent of temperature, precipitation rate, and molar (Sr/Ca)aq ratio of the aqueous solution. If the (Sr/Ca)aq ratio is constant, Δ44/40Cacalcite-aq values can be directly followed by the Sr content of the precipitated calcite. A (Sr/Ca)aq ratio close to that of modern seawater yields the equation
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Our experimental results indicate that neither precipitation rate nor temperature dominantly controls Ca isotope fractionation. However, Ca isotopes and Sr content of inorganic calcite comprise an excellent environmental multi-proxy in natural and applied systems.