Mechanisms of equilibrium and kinetic oxygen isotope effects in synthetic aragonite at 25 °C

Abstract

Aragonite was precipitated in the laboratory at 25 °C in isotopic equilibrium with Na-Ca-Mg-Cl-CO 3 solutions at two different pH values (i.e., pH = ∼8.2 and ∼10.8) by the constant addition method. On the basis of the oxygen isotope composition of the aragonite precipitates, it was demonstrated that the equilibrium aragonite–water fractionation factor is independent of the pH of the parent solution and equal to: 1000 ln α ( aragonite – H 2 O ) = 29.12 ± 0.09 To elucidate the mechanism(s) of aragonite precipitation, the equilibrium oxygen isotope fractionations between HCO 3 - as well as CO 3 2 - and H 2O at 25 °C were also determined experimentally to estimate the oxygen isotope compositions of the HCO 3 - and CO 3 2 - ions in the parent solutions. The isotopic composition of BaCO 3 (witherite) precipitated quantitatively from solutions of various pH values and the speciation of carbonic acid species in these solutions were combined to yield the following oxygen isotope fractionation factors: 1000 ln α ( HCO 3 - – H 2 O ) = 30.53 ± 0.08 1000 ln α ( CO 3 2 - – H 2 O ) = 23.71 ± 0.08 The oxygen isotope composition of witherite obtained from two types of fractional precipitation experiments revealed that CO 3 2 - ions are preferentially incorporated into the growing crystal. In addition, preferential deprotonation of isotopically light HCO 3 - ions and the incorporation of the light CO 3 2 - isotopologues are proposed to account for the kinetic isotope effects observed in the course of aragonite and witherite precipitations.