HR: 0800h
AN: V51C-0589    [Abstracts]
TI: Oxygen Isotope Fractionation between Synthetic Aragonite and Water: Influence of Solution Chemistry
AU: * Kim, S
EM: sangtae@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill Univeristy, 3450 University Street, Montreal, QC H3A 2A7 Canada
AU: O'Neil, J R
EM: jro@umich.edu
AF: Department of Geological Sciences, University of Michigan, 2534 C. C. Little Building, Ann Arbor, MI 48109-1063 United States
AU: Mucci, A
EM: alm@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill Univeristy, 3450 University Street, Montreal, QC H3A 2A7 Canada
AU: Hillaire-Marcel, C
EM: geotop@uqam.ca
AF: GEOTOP, Universit‚ du Qu‚bec … Montr‚al, C.P. 8888, Montreal, QC H3C 3P8 Canada
AB: Oxygen isotope analyses of natural aragonite, of both biogenic and abiogenic origin, are used frequently in studies of paleoclimatology, oceanography and carbonate diagenesis, but few experimental determinations of the temperature dependence of the oxygen isotope fractionation between aragonite and water have been carried out. In an attempt to obtain precise and reliable equilibrium oxygen isotope fractionation factors for the aragonite-water system and to resolve some of the inconsistencies in the literature, we developed a number of methods of slow precipitation of inorganic aragonite from Na$^{+}$-Ca$^{2+}$-Cl$^{-}$-HCO$_{3}$$^{-}$ solutions under laboratory-controlled conditions. On the basis of principles established in previous work (Kim and O'Neil, 1997), precipitation was assumed to occur at isotopic equilibrium and measurements were made of the aragonite-water fractionation factor as a function of temperature (5 - $40\deg$C). In addition, the influence of Mg$^{2+}$ concentration (20 to 100 mM) and pH ($\sim$8.2 to $\sim$10.7) of the precipitating solution on the oxygen isotope fractionation between aragonite and water at $25\deg$C was investigated. Under our experimental conditions, the measured aragonite-water oxygen isotope fractionation factor was independent of both the Mg$^{2+}$ concentration and pH, and the temperature dependence of the aragonite-water fractionation was similar to that of the calcite-water fractionation. More importantly, aragonite was always enriched in $^{18}$O relative to calcite when published aragonite or calcite acid fractionation factors were applied. Based on results of equilibrium experiments and the published acid fractionation factor for aragonite ($\alpha$=1.01034), a new expression is proposed for the oxygen isotope fractionation between aragonite and water at low temperatures: 1000ln$\alpha$(Aragonite-H$_{2}$O) = 17.32 (10$^{3}$T$^{-1}$) - 29.00 {\bf Reference} Kim S.-T. and O'Neil J. R. (1997) Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. {\it Geochim. Cosmochim. Acta} 61, 3461-3475.
DE: 4870 Stable isotopes
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting