Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation:: II.: Ca isotopes

Ca isotope fractionation during inorganic calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 < logR < 4.4 mu mol/m(2)/h) and temperatures (5, 25, and 40 degrees C) with traces of Sr using the CO2 diffusion technique. Results show that in analogy to Sr/Ca [see Tang J., Kohler S. J. and Dietzel M. (2008) Sr2+/Ca2+ and Ca-44/Ca-40 fractionation during inorganic calcite formation: I. Sr incorporation. Geochim. Cosinochim. 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 alpha(calcite-aq) (Ca-44/Ca-40)/(Ca-44/Ca-40)(aq) = 1 and Delta Ca-44/40(calcite-aq) = 0 parts per thousand), whereas at disequilibriurn Ca-44 is fractionated in a primary surface layer (i.e., the surface entrapment factor of Ca-44, F-44Ca < 1). As a crystal grows at disequilibrium, the surface-depleted Ca-44 is entrapped into the newly formed crystal lattice. Ca-44 depletion in calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated Ca-44 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 Ca-44 ion diffusion and less Ca-44 fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less Ca-44 fractionation and the impact of precipitation rate on Ca-44 fractionation is reduced. A highly significant positive linear relationship between absolute Ca-44/Ca-40 fractionation and the apparent Sr distribution coefficient during calcite formation according to the equation Delta Ca-44/40(calcite-aq) = (-1.90 +/- 0.26) . logD(Sr) -2.83 +/- 0.28 is obtained from the experimental results at 5, 25, and 40 degrees 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, Delta Ca-44/41(calcite-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 Delta Ca-44/40(calcite-aq) = (-0.000940 +/- 0.000108) . Sr (mg/kg CaCO3) - 0.07 +/- 0.09. 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. (C) 2008 Elsevier Ltd. All rights reserved.