Oxygen isotope fractionation between synthetic aragonite and water: Influence of temperature and Mg2+ concentration

Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO2 degassing, passive CO2 degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite–water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg2+ ions is required to nucleate and precipitate aragonite from Na–Ca–Cl–HCO3 solutions under these experimental conditions, the influence of the total Mg2+ concentration (up to ∼0.9 molal) on the aragonite–water oxygen isotope fractionation was examined at 25 °C. No significant Mg2+ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg2+ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg2+.On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite–water fractionation is proposed over the temperature range of 0–40 °C:1000lnαaragonite-water=17.88±0.13(103/T)-31.14±0.461000lnαaragonite-water=17.88±0.13(103/T)-31.14±0.46where αaragonite–water is the fractionation factor between aragonite and water, and T is in kelvins. Given the analytical and statistical errors associated with this and previous determinations, the new relation reveals that many biogenic aragonites are precipitated at and or very near oxygen isotope equilibrium with their ambient water. When the new aragonite–water expression is combined with the calcite–water calibration published by Kim and O ’Neil [Kim S. -T., and O’Neil J. R. (1997) Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. Geochim. Cosmochim. Acta61, 3461–3475], a positive aragonite–calcite fractionation (∼0.8‰ at 25 °C) is obtained over the temperature range investigated.