Experimental Studies of Oxygen Isotope Fractionation Factors between Aragonite and Water at Low Temperatures

Aragonite at 0 to 70℃ was synthesized by slow decomposition and “two step” overgrowth approaches. The phase compositions and morphology were detected by XRD and SEM techniques. The effect of aragonite precipitation rate on oxygen isotope fractionation between aragonite and water was studied by combining XRD and SEM techniques with oxygen isotope analysis. For the calcium carbonates synthesized in the temperature range of 0 to 50℃, the XRD and SEM analyses show that with increasing temperature the rate of aragonite precipitation increases and oxygen isotope fractionations between aragonite and water are progressively out of equilibrium. As a result, the experimentally measured oxygen isotope fractionations for the aragonite water system at 50℃ is at disequilibrium, and the lower fractionation values obtained at 0℃ and 25℃ are proxy for equilibrium fractionations. Taking the lower values at 0℃ and 25℃ together with the data obtained from the “two step” overgrowth technique at 50℃ and 70℃, it yields the following fractionation equation for the aragonite water system: 10 3lnα=20 41×10 3/T-41 42 This equation not only agrees well with the theoretic calculation by the increment method, but is close to the previous experimental results for the aragonite water and aragonite/calcite mixture water systems as well as the empirical estimates on the biogenic aragonite water system. It provides the first experimental calibration of oxygen isotope fractionation between inorganically precipitated aragonite and water at the low temperatures and is thus responsible for thermodynamic equilibrium fractionation.[KH2D][WT5HZ]