Oxygen isotope systematics in carbonate-water systems : influence of temperature, solution chemistry, and kinetic isotope fractionation

• Main Author: Kim, Sang-Tae, 1970-
• Format: Thesis
• Language: English
• Published: McGill University 2006
• Subjects: Geochemistry; Carbonic acid
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=111838

The oxygen isotope fractionation between HCO3-/CO 32- and H2O was determined experimentally in order to elucidate the precipitation mechanisms of orthorhombic carbonate minerals in solutions. The oxygen isotope composition of BaCO3 (witherite), precipitated quantitatively from solutions of various pH and the relative contribution of carbonic acid species to DIC (total dissolved inorganic carbon) at each pH in these solutions were used to generate the following fractionations at 25°C: 1000lnaHCO -3-H2O =30.53+/-0.08 1000lna&parl0;CO 2-3-H2 O&parr0;=23.17+/-0.08 Based upon the above oxygen isotope fractionation and the oxygen isotope systematics of aragonite and witherite obtained from various precipitation experiments, it was discovered that CO32- rather than the more abundant HCO3- ions are preferentially incorporated into the growing carbonate crystal. Furthermore, it was proposed that the faster deprotonation kinetics of isotopically light HCO 3- ions and the preferential incorporation of light CO3 2- isotopologues could account for the kinetic isotope effects observed in these carbonate minerals. Aragonite was inorganically precipitated from Na-Ca-Mg-Cl-HCO3 solutions at 0, 5, 10, 25, and 40°C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between this mineral and the parental solution. To establish a reliable relationship between temperature and oxygen isotope fractionation as well as discount potential experimental artifacts, three different precipitation protocols were employed but statistically identical results were obtained under the conditions investigated. In addition, the extent of kinetic isotope effects induced by variations of the precipitation rate, pH, and Mg2+ concentration of the parent solutions, were thoroughly tested to establish that equilibrium oxygen isotope fractionation occurs. In order to obtain accurate oxygen isotope compositions of the aragonite precipitates by the conventional phosphoric acid dissolution method, a statistically reliable acid fractionation factor for aragonite (and calcite) was also determined from pure natural aragonite (and calcite) specimens. From a large number (i.e., 29 and 60 individual analyses for aragonite and calcite, respectively) of replicate total oxygen isotope analyses and the isotopic composition of the acid-liberated CO2 between 25 to 75°C, the following two new equations are proposed: 1000lnaCO2&parl0;ACID &parr0;-Calcite=3.59&parl0; 103/T&parr0;-1.79 1000lnaCO2&parl0;ACID &parr0;-Aragonite=3.39&parl0; 103/T&parr0;-0.83 By combining the new acid fractionation factor of 1.01063 for aragonite at 25°C with the oxygen isotope data from the synthesis experiments determined in this study, a new relationship is reported for the temperature dependence of the aragonite-water oxygen isotope fractionation over the temperate range of 0-40°C: 1000lnaaragonite-water=17. 88+/-0.13&parl0;103/T&parr0; -31.14+/-0.46 This new experimental calibration of the aragonite-water fractionation yields a positive aragonite-calcite oxygen isotope fractionation over the temperature range investigated, in agreement with theoretical calculations. This result also provides a baseline to quantify vital or kinetic effects that are frequently observed in natural abiogenic or biogenic aragonite minerals.