The equilibrium fractionation factor between CaCO3 and Ca2+ (aq): Fractionation mechanisms and diagenetic Ca isotope effects

The Ca isotopic compositions (δ44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau), combined with reactive transport modeling, are used to determine the equilibrium Ca isotope fractionation factor (αs-f) between calcite and dissolved Ca2+. The value of αs-f at equilibrium is inferred to be 1.0000 ± 0.0001, which is significantly different from the value (0.9985) inferred from abiotic calcite precipitation in the laboratory. Since calcite precipitation rates at 807A (constrained by Ca and Sr elemental and isotopic data) are ~10--14 orders of magnitude slower than laboratory rates, and the pore fluids are close to calcite saturation, the Ca isotopic composition of calcite should reflect precipitation at isotopic equilibrium. Consequent modeling of diagenesis produces a maximum shift in δ44Ca of +0.15‰ at 807A; however, diagenesis will have a larger impact in sections where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses. We propose that adsorption and diffusion control Ca isotopic fractionation when precipitation rates and oversaturation levels are high, such as in biogenic calcite formation and in laboratory precipitation experiments. At high rates of precipitation, calcite growth may be limited by diffusion to the crystal surface; this may result in no isotopic fractionation due to the hypothesized equality of the aqueous diffusivities for 44Ca2+ and 40Ca2+. At intermediate precipitation rates, growth is controlled primarily by adsorption, producing a surface layer that has low 44Ca/40Ca via preferential adsorption of 40Ca2+. Rapid precipitation incorporates these layers into the crystal before they equilibrate with the solution, resulting in observed isotope fractionation between crystal and solution. At very low precipitation rates, the adsorbed surface layer is in contact with the bulk solution for a relatively long time. As a result, the surface layer can equilibrate with the bulk solution before it is removed from the reacting system by the deposition of successive layers.