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.