Oxygen isotope fractionation in synthetic magnesian calcite
CONCEPCION JIMENEZ-LOPEZ,1,2,* CHRISTOPHER S. ROMANEK,2,3
F. JAVIER HUERTAS,1 HIROSHI OHMOTO,4 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, P A 16802, USA
(Received October 16, 2002; accepted in revisedform November 7, 2003)
Abstract-Mg-bearing calcite was precipitated at 25°C
in closed system free-drift experiments from solutions containing NaHC03,
CaCl2 and MgCI2. 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 103lnaMg-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 also values
for the bulk solid, 103lnaMg-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 103lnaMg-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 103lnaMg-cl-H2O
due to the heterogeneous nature of the solid.
The results of this study suggest that paleotemperatures inferred from
the s180 values of high magnesian calcite (> 10 mo1% 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.
