SREL Reprint #2836




Oxygen isotope fractionation in synthetic magnesian calcite

Concepción Jiménez-López1,2, Christopher S. Romanek2,3, F. Javier Huertas1, Hiroshi Ohmoto4,
and Emilia Caballero1

1Estacion Experimental del Zaidin, CSIC, Profesor Albareda 1, 18008 Granada, Spain
2Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802, USA
3Department of Geology, University of Georgia, Athens, GA 30602, USA
4Astrobiology Research Center and Department of Geoscience, The Pennsylvania State University, University Park, PA 16802, USA

Abstract: Mg-bearing calcite was precipitated at 25°C in closed system free-drift experiments from solutions containing NaHC03, 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% MgC03 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 {104} to {100} and {110} growth forms.
The oxygen isotope fractionation factor for the magnesian-calcite-water system (as 103lnαMg-cl-H2O) displayed a strong dependence on the mol% MgC03 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 δ18O values for the bulk solid, 103lnαMg-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 103lnαMg-cl-H2O for precipitation rates that ranged from 103.21 to 104.60 µmol · m-2 · h-1, it was impossible to disentangle the potential effect(s) of precipitation rate and Mg-content on 103lnαMg-cl-H2O due to the heterogeneous nature of the solid.
The results of this study suggest that paleotemperatures inferred from the δ18O values of high magnesian calcite (> 10 mol% MgC03) 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.

SREL Reprint #2836

Jiménez-López, C., C. S. Romanek, F. J. Huertas, H. Ohmoto, and E. Caballero. 2004. Oxygen isotope fractionation in synthetic magnesian calcite. Geochimica et Cosmochimica Acta 68:3367-3377.

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