Oxygen isotope fractionation in synthetic magnesian calcite

Mg-bearing calcite was precipitated at 25degreesC in closed system free-drift experiments from solutions containing NaHCO3, CaCl2 and MgCl2. The chemical and isotope composition of the solution and precipitate were investigated during time course experiments of 24-h duration. Monohydrocalcite and calcite precipitated early in the experiments (<8 h), while Mg-calcite was the predominant precipitate (>95%) thereafter. Solid collected at the end of the experiments displayed compositional zoning from pure calcite in crystal cores to up to 23 mol% MgCO3 in the rims. Smaller excursions in Mg were superimposed on this chemical record, which is characteristic of oscillatory zoning observed in synthetic and natural solid-solution carbonates of differing solubility. Magnesium also altered the predominant morphology of crystals over time from the 11041 to {100} and {110} growth forms. The oxygen isotope fractionation factor for the magnesian-calcite-water system (as 10(3)Inalpha(Mg-cl-H2O)) displayed a strong dependence on the mol% MgCO3 in the solid phase, but quantification of the relationship was difficult due to the heterogeneous nature of the precipitate. Considering only the Mg-content and delta(18)O values for the bulk solid, 10(3)Inalpha(Mg-cl-H2O) increased at a rate of 0.17 +/- 0.02 per mol% MgCO3; this value is a factor of three higher than the single previous estimate (Tarutani T., Clayton R.N., and Mayeda T. K. (1969) The effect of polymorphims and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta 33, 987-996). Nevertheless, extrapolation of our relationship to the pure calcite end member yielded a value of 27.9 +/- 0.02, which is similar in magnitude to published values for the calcite-water system. Although no kinetic effect was observed on 10(3)Inalpha(Mg-cl-H2O) for precipitation rates that ranged from 10(3.21) to 10(4.60) mumol (.) m(-2) (.) h(-1), it was impossible to disentangle the potential effect(s) of precipitation rate and Mg-content on 10(3)Inalpha(Mg-cl-H2O) due to the heterogeneous nature of the solid. The results of this study suggest that paleotemperatures inferred from the delta(18)O values of high magnesian calcite (>10 mol% MgCO3) may be significantly underestimated. Also, the results underscore the need for additional experiments to accurately characterize the effect of Mg coprecipitation on the isotope systematics of calcite from a chemically homogeneous precipitate or a heterogeneous material that is analyzed at the scale of chemical and isotopic zonation. Copyright (C) 2004 Elsevier Ltd.