Oxygen isotope fractionation in synthetic magnesian calcite

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doi: 10.1016/j.gca.2003.11.033
Authors:Jiménez-López, Concepción; Romanek, Christopher S.; Huertas, F. Javier; Ohmoto, Hiroshi; Caballero, Emilia
Author Affiliations:Primary:
CSIC, Estacion Experimental del Zaidin, Granada, Spain
Other:
University of Georgia, United States
Pennsylvania State University, United States
Volume Title:Geochimica et Cosmochimica Acta
Source:Geochimica et Cosmochimica Acta, 68(16), p.3367-3377. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037
Publication Date:2004
Note:In English. 75 refs.; illus., incl. 2 tables
Summary:Mg-bearing calcite was precipitated at 25°C 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 {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% 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 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% 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. Abstract Copyright (2004) Elsevier, B.V.
Sections:Geochemistry
Subsections:Minerals and ores
Subjects:Alkaline earth metals; Applications; Carbonates; Closed systems; Crystal chemistry; Crystal form; Crystal growth; Experimental studies; Geochemistry; Isotope fractionation; Isotope ratios; Isotopes; Laboratory studies; Magnesian calcite; Magnesium; Metals; Nucleation; O-18/O-16; Oxygen; Paleotemperature; Precipitation; Stable isotopes; Synthetic materials
Abstract Numbers:04M/3530
Record ID:2005024871
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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